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claims | 1. A protection apparatus, comprising:a garment that substantially contours to an operator's body wherein the garment is operable to protect the operator from a substantial portion of radiation;a frame that substantially contours to the operator's body wherein the frame has an open back to permit unimpeded rear entry without having to manipulate the frame and is operable to support the garment separate from the operator and is operable to substantially maintain its substantially contoured shape in the absence of the operator; and,a suspension assembly operable to support the weight of the frame and garment relative to the operator. 2. The apparatus of claim 1 wherein the suspension assembly is one selected from a group of components consisting of:a) a trolley;b) an articulating arm;c) a balancer;d) a reaction arm;e) an extension arm;f) a jib crane;g) a wire rope; and,h) a bridge crane. 3. The apparatus of claim 1, further comprising a light source detachably secured to the frame. 4. The apparatus of claim 1, further comprising a face shield movably secured to the frame. 5. A protection apparatus, comprising:a garment that substantially contours to an operator's body wherein the garment is operable to protect the operator from a substantial portion of radiation;a frame that substantially contours to the operator's body wherein the frame is operable to support the garment separate from the operator and is operable to substantially maintain its substantially contoured shape in the absence of the operator;a suspension assembly operable to support the weight of the frame and garment relative to the operator; and,a binding assembly operable to provide for engaging the operator and the protection apparatus wherein the binding assembly includes a binding component attached on or about at least one of the operator's person or the garment. 6. The apparatus of claim 5, wherein the binding assembly includes a first another binding component attached on or about the frame. 7. The apparatus of claim 5, wherein the binding assembly includes a male-female connection operable to detachably secure the operator to the protection apparatus. 8. The apparatus of claim 5, wherein the binding assembly includes a friction connection operable to detachably secure the operator to the protection apparatus. 9. The apparatus of claim 5, wherein the binding assembly includes a magnetic connection operable to detachably secure the operator to the protection apparatus. 10. The apparatus of claim 5, wherein the binding assembly includes hook and loop fasteners to detachably secure the operator to the protection apparatus. 11. A protection apparatus comprising:a garment that substantially contours to an operator's body wherein the garment is operable to protect the operator from a substantial portion of radiation;a frame that substantially contours to the operator's body wherein the frame is operable to support the garment separate from the operator and is operable to substantially maintain its substantially contoured shape in the absence of the operator;a suspension assembly operable to support the weight of the frame and garment relative to the operator; and,a binding assembly operable to provide for engaging the operator and the protection apparatus wherein the binding assembly includes a binding component attached on or about at least one of the operator's person or the garment; and,a locking mechanism operable to substantially immobilize at least one component of the suspension assembly. 12. A method, comprising:suspending with a suspension assembly a radiation protection system which includes a garment and a face shield secured to a frame wherein the frame has an open back to permit unimpeded rear entry to the radiation protection system without having to manipulate the frame and is operable to protect an operator from a substantial portion of radiation;contouring a portion of the frame to the operator's body; and,engaging the operator with the protection system. 13. The method of claim 12 wherein the protection system includes a binding assembly wherein the binding assembly includes a binding component attached on or about at least one of the operator's person or the garment. 14. The method of claim 13 wherein the binding assembly includes another binding component attached on or about the protection system. 15. The method of claim 13 wherein the binding assembly is one selected from the group consisting of:a) hook and loop fasteners;b) friction fasteners;c) mechanical fasteners;d) magnetic fasteners; and,e) electromagnetic fasteners. 16. The method of claim 12, further comprising:immobilizing the protection system with a locking mechanism. 17. The method of claim 12 wherein the suspension assembly is one selected from a group of components consisting of:a) a trolley;b) an articulating arm;c) a balancer;d) a reaction arm;e) an extension arm;f) a jib crane;g) a wire rope; and,h) a bridge crane. 18. The method of claim 12 wherein the operator engages with the radiation protection system by moving into proximity with the protection system. 19. The method of claim 12 wherein the operator disengages with the radiation protection system by pushing away from the frame. 20. The method of claim 12 further comprising:providing a lamp detachably secured to the protection system. 21. The method of claim 12 further comprising:providing an environmental control detachably secured to the protection system. 22. A method, comprising:suspending with a suspension assembly a radiation protection system which includes a garment and a face shield secured to a frame operable to protect an operator from a substantial portion of radiation;contouring a portion of the frame to the operator's body; and,engaging the operator assembly wherein the binding assembly forms a rigid connection between a first binding component and a second binding component. 23. A method, comprising:suspending with a suspension assembly a radiation protection system which includes a garment and a face shield secured to a frame operable to protect an operator from a substantial portion of radiation;contouring a portion of the frame to the operator's body; and,engaging the operator with the protection with a binding assembly wherein the binding assembly forms a flexible connection between a first binding component and a second binding component. 24. A method, comprising:suspending with a suspension assembly a radiation protection system which includes a garment and a face shield secured to a frame operable to protect an operator from a substantial portion of radiation;contouring a portion of the frame to the operator's body; and,engaging the operator with the protection system wherein the operator engages with the radiation protection system by activating or deactivating an electromagnet. 25. The method of claim 23 wherein the operator disengages with the radiation protection system by activating or deactivating the electromagnet. 26. A method, comprising:suspending with a suspension assembly a radiation protection system which includes a garment and a face shield secured to a frame operable to protect an operator from a substantial portion of radiation;contouring a portion of the frame to the operator's body; and,engaging the operator with the protection system wherein the operator engages with the radiation protection system by coupling a male binding component and a female binding component. 27. The method of claim 26 wherein the operator disengages with the radiation protection system by decoupling the male binding component and the female binding component. 28. A protection apparatus, comprising:a frame that substantially contours to an operator's body wherein the frame has an open back to permit unimpeded rear entry by the operator without having to manipulate the frame;a face shield supported by the frame; and,a suspension assembly operable to support the protection apparatus. 29. The apparatus of claim 28 wherein the suspension assembly is one selected from a group of components consisting of:a) a trolley;b) an articulating arm;c) a balancer;d) a reaction arm;e) an extension arm;f) a jib crane;g) a wire rope; and,h) a bridge crane. 30. The apparatus of claim 28, further comprising:a binding assembly operable to provide for engaging the operator and the protection apparatus. 31. The apparatus of claim 28, further comprising a light source detachably secured to the frame. 32. The apparatus of claim 28, wherein the face shield is movably attached to the frame. |
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abstract | A two-dimensional (2D) magneto-optical trap (MOT) for alkali neutral atoms establishes a zero magnetic field along the longitudinal symmetry axis. Two of three pairs of trapping laser beams do not follow the symmetry axes of the quadruple magnetic field and are aligned with a large non-zero degree angles to the longitudinal axis. In a dark-line 2D MOT configuration, there are two orthogonal repumping beams. In each repumping beam, an opaque line is imaged to the longitudinal axis, and the overlap of these two line images creates a dark line volume in the longitudinal axis where there is no repumping light. The zero magnetic field along the longitudinal axis allows the cold atoms maintain a long ground-state coherence time without switching off the MOT magnetic field, which makes it possible to operate the MOT at a high repetition rate and a high duty cycle. |
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abstract | A decay heat removal system for a liquid metal reactor, in which a decay heat exchanger (DHX) is installed concentrically with an intermediate heat-exchanger (IHX) in the same cylinder which separates the DHX and IHX from the reactor pool fluid, and serves to remove the reactor core decay heat. The cylinder surrounds the IHX and the DHX, and has an opened top portion protruded out of the level of the fluid in a hot pool, a bottom portion connected to a cold pool and a guide pipe for allowing the passage of the fluid from the hot pool into the IHX. The decay heat removal system can remove decay heat immediately after occurrence of an accident, thereby improves the safety of a nuclear plant. |
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claims | 1. A filter which includes a stack of foils which are locally attached to one another including a pair of oppositely and substantially parallel disposed rigid members, forming top and bottom filter surfaces, between which pair of rigid members a stack of foils are disposed, wherein movement of the rigid members away from each other enables the foils to be moved away from one another in a rain direction which extends transversely to the surfaces, ducts being formed between the foils including walls coated with electrically conductive material in order to control an amount of x-ray absorbing liquid to be contained within the ducts, wherein at least one of the rigid members is attached to an outer surface of the stack of toils by way of a buffer member, wherein the buffer member is contractible mainly in a direction extending parallel to the surface of the foils and transversely to the ducts, and wherein the buffer member includes a number of laminations, each of which is rigidly connected, near a first edge, to a plate which constitutes the rigid member, each lamination being connected to one of the outer surfaces of the stack of foils by way of a second edge which is remote from the first edge, the second edge extending parallel to the ducts and being movable towards the first edge while the foils move away from one another in the main direction. 2. A filter as claimed in claim 1 , wherein the laminations extend parallel to one another after the foils have moved away from one another over a given distance. claim 1 3. A filter as claimed in claim 1 , wherein the buffer member is connected to one of the outer surfaces of the stack of foils by means of an elastic means. claim 1 4. A filter as claimed in claim 3 , wherein the elastic material is a two-side adhesive layer. claim 3 5. An X-ray apparatus which includes a control device, an X-ray source, an X-ray detector, a filter as set forth in claim 1 which is arranged between the X-ray source and the X-ray detector and includes ducts and an X-ray absorbing liquid which is contained in the ducts, the quantity of X-ray absorbing liquid variable in the individual ducts to vary the X-ray absorptivity of the ducts, which quantity and consequential absorptivity being adjustable by means of the control device. claim 1 6. A filter which includes a stack of foils which are locally attached to one another including a pair of oppositely and substantially parallel disposed rigid members, forming top and bottom filter surfaces, between which pair of rigid members a stack of foils are disposed, wherein movement of the rigid members away from each other enables the foils to be moved away from one another in a main direction which extends transversely to the surfaces, ducts being formed between the foils including walls coated with electrically conductive material in order to control an amount of x-ray absorbing liquid to be contained within the ducts, wherein at least one of the rigid members is attached to an outer surface of the stack of foils by way of a buffer member, wherein the buffer member is contractible mainly in a direction extending parallel to the surface of the foils and transversely to the ducts, and wherein the buffer member is provided with a spring which includes turns, the rigid member extending through the turns and the turns being connected to one of the outer surfaces of the stack at a side which is remote from the rigid member. 7. A filter as claimed in claim 6 , wherein the rigid member includes two rods extending parallel to one another. claim 6 8. A filter as claimed in claim 1 , wherein the rods are made of metal. claim 1 9. An X-ray apparatus which includes a control device, an X-ray source, an X-ray detector, a filter as set forth in claim 6 which is arranged between the X-ray source and the X-ray detector and includes ducts and an X-ray absorbing liquid which is contained in the ducts, the quantity of X-ray absorbing liquid variable in the individual ducts to vary the X-ray absorptivity of the ducts, which quantity and consequential absorptivity being adjustable by means of the control device. claim 6 |
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claims | 1. A radiation therapy system comprising:a gantry comprising a stationary frame and a rotatable ring configured to rotate up to 70 RPM;a slip-ring located between the stationary frame and the rotatable ring and configured to communicate electrical signals therebetween while the rotatable ring rotates up to 70 RPM;a therapeutic radiation source mounted on the gantry; andone or more positron emission tomography (PET) detectors mounted on the gantry. 2. The system of claim 1, further comprising a first controller located on the rotatable ring and a second controller on the stationary frame, where the first controller generates control commands for the therapeutic radiation source and the one or more PET detectors, the second controller generates control commands for a gantry motion system, and synchronization data between the first controller and the second controller is transferred via the slip-ring. 3. The system of claim 2, wherein the first controller is configured to generate a signal for activating the therapeutic radiation source and acquiring PET data, wherein the second controller is configured to generate a signal for rotating the ring, and a synchronization signal is transmitted between the first and second controllers via the slip-ring to synchronize activation of the therapeutic radiation source, acquisition of the PET data and gantry motion. 4. The system of claim 1, wherein the slip-ring comprises a data brush block and a power brush block. 5. The system of claim 1, wherein the rotatable ring comprises a drum having a first ring-shaped end surface, a second ring-shaped end surface opposite the first end surface, and a length therebetween such that deflection of the first and second end surfaces is less than about 0.5 mm when the ring rotates up to 70 RPM. 6. The system of claim 5, further comprising a housing that defines a volume that encloses the gantry, the housing comprising one or more lateral hatches along the length of the drum that are configured to allow access to the therapeutic radiation source and the one or more PET detectors. 7. The system of claim 5, wherein the therapeutic radiation source comprises a linear accelerator (linac) and a magnetron, wherein the linac is attached along the length of the drum by a first mounting assembly and enclosed in a radiation shield that is separate from the linac and first mounting assembly, and wherein the magnetron is radially mounted along the length of the drum such that a cathode support of the magnetron is aligned with a direction of a centripetal force that is generated while the rotatable ring rotates up to 70 RPM. 8. The system of claim 7, wherein the one or more PET detectors are mounted along the length of the drum. 9. The system of claim 7, wherein the radiation shield is mounted to the gantry using a second mounting assembly that is separate from the first mounting assembly. 10. The system of claim 9, wherein the second mounting assembly does not directly contact the first mounting assembly. 11. The system of claim 9, wherein the first mounting assembly and the second mounting assembly are separated by an air gap. 12. The system of claim 9, wherein the radiation shield and the second mounting assembly do not contact the linac. 13. The system of claim 9, wherein the linac and the radiation shield are separated by an air gap. 14. The system of claim 9, further comprising an actuator coupled to the linac and the first mounting assembly using a ball screw, and wherein a location of the linac is configured to be adjusted by the actuator. 15. The system of claim 14, wherein the actuator is removable. 16. The system of claim 14, wherein the actuator is controllable from a remote location. 17. The system of claim 16, wherein the rotatable gantry is located in a room and the remote location is outside of the room. 18. The system of claim 1, further comprising a motion system comprising a plurality of rotor elements around the rotatable ring, a stator element enclosed within the stationary frame across from the rotor elements, and ball bearings located adjacent to the plurality of rotor elements. 19. The system of claim 18, wherein the plurality of rotor elements comprise one or more magnetic or inductive elements, and the stator element comprises a coil. 20. The system of claim 1, further comprising a first communication interface comprising a first receiver element mounted to the rotatable ring and a first transmitter element mounted to the stationary frame that is configured to transmit a first plurality of signals to the first receiver element while the rotatable ring is moving; anda second communication interface comprising a second transmitter element mounted to the rotatable ring and a second receiver element mounted to the stationary frame, wherein the second transmitter element is configured to transmit a second plurality of signals to the second receiver element while the rotatable ring is moving. 21. The system of claim 20, wherein the first plurality of signals are transmitted across the first communication interface and the second plurality signals are transmitted across the second communication interface concurrently. 22. The system of claim 20, further comprising a multi-leaf collimator disposed in front of the therapeutic radiation source, wherein the multi-leaf collimator is configured to transmit position data of individual leaves of the multi-leaf collimator to the second transmitter element for transmission to the second receiver element. 23. The system of claim 20, wherein the second plurality of signals comprises gantry rotation speed data. 24. The system of claim 20, wherein the second plurality of signals comprises positron emission data from the one or more PET detectors. 25. The system of claim 20, further comprising a radiation detector mounted on the rotatable ring across from the therapeutic radiation source and wherein the second plurality of signals comprises radiation data from the radiation detector. 26. The system of claim 20, further comprising a first controller located on the rotatable ring and a second controller on the stationary frame, wherein the second controller is in communication with the first transmitter element, wherein the first plurality of signals comprises radiation source commands from the second controller. 27. The system of claim 26, further comprising a multi-leaf collimator disposed in front of the therapeutic radiation source, wherein the first plurality of signals comprises multi-leaf collimator commands from the second controller. 28. The system of claim 26, wherein the first plurality of signals comprises gantry rotation commands from the second controller. 29. The system of claim 20, wherein the first communication interface and the second communication interface transmit signals using inductive signal transfer methods. 30. The system of claim 20, wherein the first communication interface and the second communication interface transmit signals using capacitive signal transfer methods. 31. The system of claim 20, further comprising a first position sensor mounted to the rotatable ring and in communication with the first receiver element, and a second position sensor mounted to the stationary frame and in communication with the second receiver element. 32. The system of claim 31, wherein the rotatable ring comprises a plurality of index markers located around the circumference of the ring and detectable by the second position sensor, and the stationary frame comprises a plurality of index markers located around a circumference of the frame and detectable by the first position sensor. 33. The system of claim 32, wherein the first plurality of signals comprises index marker data from the first position sensor and the second plurality of signals comprises index marker data from the second position sensor, and wherein the system further comprises a controller configured to receive and compare the first and second plurality of signals to identify a difference in the first and second plurality of signals. 34. The system of claim 33, wherein the controller is configured to generate a signal to indicate a difference between the first and second plurality of signals. 35. The system of claim 31, wherein the first plurality of signals comprises angular position data of the rotatable ring from the first position sensor and the second plurality of signals comprises angular position data of the rotatable ring from the second position sensor, and wherein the system further comprises a controller configured to receive and compare the first and second plurality of signals to identify a difference in the first and second plurality of signals, wherein identifying the difference between the first plurality of signals and second plurality of signals comprises:calculating a derivative of the first plurality of signals over time;calculating a derivative of the second plurality of signals over time;determining a difference between the calculated derivatives; andif the difference exceeds a predetermined threshold, generating a position sensor fault signal. 36. The system of claim 1, wherein the therapeutic radiation source is configured to generate a radiation beam emitted along a beam path, the radiation beam having a two-dimensional projection having a x-axis aspect and a y-axis aspect; and wherein the system further comprises a beam-limiting assembly disposed in the beam path, the beam-limiting assembly comprising:upper jaws configured to shape the y-axis aspect of the radiation beam;a multi-leaf collimator configured to shape the x-axis aspect of the radiation beam; andlower jaws configured to shape the y-axis aspect of the radiation beam, wherein the multi-leaf collimator is located between the upper jaws and the lower jaws. 37. The system of claim 36, wherein the upper jaws are located closer to the therapeutic radiation source than the multi-leaf collimator and the lower jaws, and the lower jaws are located further from the therapeutic radiation source than the multi-leaf collimator and the upper jaws. 38. The system of claim 36, wherein the upper jaws comprise inward faces that are angled at a first angle with respect to a vertical axis, and the lower jaws comprise inward faces that are angled at a second angle with respect to the vertical axis, and wherein the first angle is greater than the second angle. 39. The system of claim 36, wherein the radiation beam has a beam spread and beam boundary defined by a focal line, and wherein the upper jaws comprise inward faces that are not aligned along the focal line, and the lower jaws comprise inward faces that are aligned along the focal line. 40. The system of claim 39, wherein the inward faces of the upper jaws are angled at a first angle with respect to a vertical axis, the inward faces of the lower jaws are angled at a second angle with respect to the vertical axis, and the focal line is angled at a third angle with respect to the vertical axis. 41. The system of claim 40, wherein the first angle is greater than the second angle. 42. The system of claim 1, wherein the therapeutic radiation source comprises a linear accelerator (linac) and a magnetron. 43. The system of claim 42, wherein the magnetron is configured to provide RF energy for accelerating electrons in the linac, the magnetron further comprising:a ring anode having one or more cavities including a central cavity; anda cathode located in the central cavity of the ring anode;wherein a cathode support couples the cathode to the ring anode, wherein a longitudinal axis of the cathode support is aligned along a radial axis of the gantry. 44. The system of claim 1, further comprising a temperature management system having one or more heat exchangers that transfers heat from the rotatable ring to the stationary frame. 45. The system of claim 44, wherein the one or more heat exchangers comprises a first set of heat exchangers configured to transfer heat generated from the rotatable ring to the stationary frame and a second set of heat exchangers configured to transfer the heat from the stationary frame to an external heat sink. 46. The system of claim 45, wherein the external heat sink is a closed-loop, facility liquid system. 47. The system of claim 44, wherein the temperature management system transfers heat from the rotatable ring to a cooling fluid on the stationary frame. 48. The system of claim 1, further comprising a second gantry mounted to the rotatable ring, and a kV system mounted on the second gantry. 49. The system of claim 48, wherein the kV system comprises a kV radiation source configured to generate a beam emitted along a beam path, and a collimator mounted to the second gantry disposed in the beam path of the kV radiation source, the collimator having a first configuration that blocks the beam and a second configuration that transmits the beam. 50. The system of claim 49, wherein the collimator rotates to transition between the first and second configurations. 51. The system of claim 50, wherein the collimator comprises a cylinder made of a radiation-blocking material and an aperture that is transverse to a longitudinal axis of the cylinder, wherein in the first configuration, the aperture is not aligned along the beam path and in the second configuration, the aperture is aligned along the beam path. 52. The system of claim 1, wherein the gantry comprises a bore, wherein the bore comprises a first portion and a second portion, and wherein a second portion diameter is greater than a first portion diameter. 53. The system of claim 52, further comprising an image projector configured to illuminate at least a region of the second portion. 54. The system of claim 53, wherein illumination from the image projector comprises one or more of an image and video. 55. The system of claim 52, further comprising a flexible display disposed along a surface of the bore. 56. The system of claim 55, wherein the flexible display is an organic light-emitting diode (OLED) display. 57. The device of claim 55, further comprising an optical eye tracker configured to detect one or more of an eye position and eye gaze of a patient in the bore, and a processor configured to change illumination from the image projector using the eye position and the eye gaze. 58. The device of claim 52, further comprising an audio device configured to output sound within the bore. 59. The device of claim 52, further comprising an airflow device configured to direct airflow through the second portion of the bore. 60. A radiation therapy system comprising:a gantry comprising a stationary frame and a rotatable ring configured to rotate up to 70 RPM, wherein the rotatable ring comprises a drum having a first ring-shaped end surface, a second ring-shaped end surface opposite the first end surface, and a length therebetween such that deflection of the first and second end surfaces is less than 0.5 mm when the ring rotates up to 70 RPM;a slip-ring located between the stationary frame and the rotatable ring and configured to communicate electrical signals therebetween while the rotatable ring rotates up to 70 RPM;a therapeutic radiation source comprising a linear accelerator (linac) and a magnetron, wherein the linac is attached along the length of the drum by a first mounting assembly and enclosed in a radiation shield that is separate from the linac and first mounting assembly, and wherein the magnetron is radially mounted along the length of the drum such that a cathode support of the magnetron is aligned with a direction of a centripetal force that is generated while the rotatable ring rotates up to 70 RPM; andone or more positron annihilation emission (PET) detectors mounted along the length of the drum. 61. The system of claim 60, further comprising a temperature management system having one or more heat exchangers that transfers heat from the rotatable ring to cooling fluid on the stationary frame. |
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052232107 | claims | 1. A dual passive cooling system for liquid metal cooled nuclear fission reactors, comprising the combination of: a reactor vessel for containing a pool of liquid metal coolant with a core of heat generating fissionable fuel substantially submerged therein, a side wall of the reactor vessel forming an innermost first partition; a containment vessel substantially surrounding the reactor vessel in spaced apart relation having a side wall forming a second partition; a first baffle cylinder substantially encircling the containment vessel in spaced apart relation having an encircling wall forming a third partition; a guard vessel substantially surrounding the containment vessel and first baffle cylinder in spaced apart relation having a side wall forming a forth partition; a sliding seal at the top of the guard vessel edge to isolate the dual cooling system air streams; a second baffle cylinder substantially encircling the guard vessel in spaced part relationship having an encircling wall forming a fifth partition; a concrete silo substantially surrounding the guard vessel and the second baffle cylinder in spaced apart relation providing a sixth partition; a first fluid coolant circulating flow course open to the ambient atmosphere for circulating air coolant comprising at least one downcomer duct having an opening to the atmosphere in an upper area thereof and making fluid communication with the space between the guard vessel and the first baffle cylinder and at least one riser duct having an opening to the atmosphere in the upper area thereof and making fluid communication with the space between the first baffle cylinder and the containment vessel whereby cooling fluid air can flow from the atmosphere down through the downcomer duct and space between the forth and third partitions and up through the space between the third and second partition and the riser duct then out into the atmosphere; and a second fluid coolant circulating flow course open to the ambient atmosphere for circulating air coolant comprising at least one downcomer duct having an opening to the atmosphere in an upper area thereof and making fluid communication with the space between the concrete silo and the second baffle cylinder and at least one riser duct having an opening to the atmosphere in the upper area thereof and making fluid communication with the space between the second baffle cylinder and the guard vessel whereby cooling fluid air can flow from the atmosphere down through the downcomer duct and space between the sixth partition and the fifth partition and up through the space between the fifth partition and the forth partition and up the riser duct and out into the atmosphere. a reactor vessel for containing a pool of liquid metal coolant with a core of heat generating fissionable fuel substantially submerged therein, a side wall of the reactor vessel forming an innermost first partition; a containment vessel substantially surrounding the reactor vessel in spaced apart relation having a side wall forming a second partition; a first baffle cylinder substantially surrounding the length of the containment vessel in spaced apart relation having an encircling wall forming a third partition; a guard vessel substantially surrounding the containment vessel and first baffle cylinder in spaced apart relation having a side wall forming a forth partition; a sliding seal at the top of the guard vessel to isolate the area within the guard vessel; a second baffle cylinder substantially surrounding the length of the guard vessel in spaced part relationship having an encircling wall forming a fifth partition; a concrete silo substantially surrounding the guard vessel and the second baffle cylinder in spaced apart relation providing a sixth partition; a first fluid coolant circulating flow course open to the ambient atmosphere for circulating air coolant comprising at least one downcomer duct having an opening to the atmosphere in an upper area thereof and making fluid communication with the space between the guard vessel and the first baffle cylinder and at least one riser duct having an opening to the atmosphere in the upper area thereof with a closing valve and making fluid communication with the space between the first baffle cylinder and the containment vessel whereby cooling fluid air can flow from the atmosphere down through the downcomer duct and space between the forth and third partitions and up through the space between the third and second partition and the riser duct then out into the atmosphere; and a second fluid coolant circulating flow course open to the ambient atmosphere for circulating air coolant comprising at least one downcomer duct having an opening to the atmosphere in an upper area thereof and making fluid communication with the space between the concrete silo and the second baffle cylinder and at least one riser duct having an opening to the atmosphere in the upper area thereof and making fluid communication with space between the second baffle cylinder and the guard vessel whereby cooling fluid air can flow from the atmosphere down through the downcomer duct and space between the sixth partition and the fifth partition and up through the space between the fifth partition and up the riser duct and out into the atmosphere. a reactor vessel for containing a pool of liquid metal coolant with a core of heat generating fissionable fuel substantially submerged therein, a side wall of the reactor vessel forming an innermost first partition; a containment vessel substantially surrounding the reactor vessel in spaced apart relation having a side wall forming a second partition; a first baffle cylinder substantially surrounding the length of the containment vessel in spaced apart relation having an encircling wall forming a third partition; a guard vessel substantially surrounding the containment vessel and first baffle cylinder in spaced apart relation having a side wall forming a forth partition; a second baffle cylinder substantially surrounding the length of the guard vessel in spaced part relationship having an encircling wall forming a fifth partition; said reactor vessel, containment vessel, first baffle cylinder, guard vessel and second baffle cylinder each being substantially circular in cross-section and of respectively increasing diameter and concentrically arranged with their side walls providing spaced apart partitions forming annular intermediate area therebetween; a concrete silo substantially buried below ground level and substantially surrounding the guard vessel and the second baffle cylinder in spaced apart relation providing a sixth partition; a superstructure bridging across and supported by the concrete silo with said reactor vessel and containment vessel being suspended from the superstructure; a first fluid coolant circulating flow course open to the ambient atmosphere for circulating air coolant comprising at least one downcomer duct having an opening to the atmosphere in an upper area thereof with a closing valve and making fluid communication with the space between the guard vessel and the first baffle cylinder, and at least one riser duct having an opening to the atmosphere in the upper area with a closing valve and making fluid communication with the space between the first baffle cylinder and containment vessel whereby cooling fluid air can flow from the atmosphere down through the downcomer duct and space between the forth and third partitions and up through the space between the third and second partition and the riser duct then out into the atmosphere; and a second fluid coolant circulating flow course open to the ambient atmosphere for circulating air coolant comprising at least one downcomer duct having an opening to the atmosphere in an upper area thereof and making fluid communication with the space between the concrete silo and the second baffle cylinder and at least one riser duct having an opening to the atmosphere in the upper area thereof and making fluid communication with the space between the second baffle cylinder and the guard vessel whereby cooling fluid air can flow from the atmosphere down through the downcomer duct and space between the sixth partition and the fifth partition and up between the fifth partition and the forth partition and up the riser duct and out into the atmosphere. 2. The dual passive cooling system for liquid metal cooled nuclear reactors of claim 1, wherein the reactor vessel containing the fuel core submerged within liquid metal coolant is located substantially buried below ground level. 3. The dual passive cooling system for liquid metal cooled nuclear reactors of claim 1, wherein the reactor vessel, the containment vessel, the first baffle cylinder, the guard vessel, and the second baffle cylinder are each circular in cross-section, of respectively increasing diameter and concentrically arranged with their side walls providing spaced apart partitions forming annular intermediate areas therebetween. 4. The dual passive cooling system for liquid metal cooled nuclear reactors of claim 1, wherein closing valves are provided in the downcomer duct and the riser duct of the first fluid coolant circulating flow course. 5. A dual passive cooling system for liquid metal cooled nuclear fission reactors, comprising the combination of: 6. The dual passive cooling system for liquid metal cooled nuclear reactor of claim 5, wherein the reactor vessel containing the fuel core substantially submerged within the liquid metal coolant is located substantially buried below ground level. 7. The dual passive cooling system for liquid metal cooled nuclear reactor of claim 5, wherein the reactor vessel, the containment vessel, the first baffle cylinder, the guard vessel, and the second baffle cylinder are each circular in cross-section, of respectively increasing diameter and concentrically arranged with their side walls providing spaced apart partitions forming annular intermediate areas therebetween. 8. The dual passive cooling system for liquid metal cooled nuclear reactor of claim 5, wherein a superstructure bridges across and is supported by the concrete silo with the reactor vessel and the containment vessel being suspended from the superstructure. 9. The dual passive cooling system for liquid metal cooled nuclear reactor of claim 8, wherein the superstructure is separated from and supported upon the concrete silo by means of seismic shockabsorbers. 10. A dual passive cooling system for liquid metal cooled nuclear fission reactors, comprising the combination of: |
description | The present invention relates to the field of the treatment of industrial waste. More particularly, the invention relates to a process for the solidification and stabilization of concentrated aqueous sodium hydroxide solution. Concentrated sodium hydroxide solution can be radioactive when it originates from nuclear reactors. Concentrated sodium hydroxide solution can also be contaminated by other pollutants. The nuclear industry has designed power stations that are capable of producing large amounts of energy from a small amount of nuclear fuel. It has thus developed steam generators requiring advanced heat transfer systems, which can be characterized as pressurized water systems or, for breeder reactors or fast neutron reactors, systems based on molten sodium metal. The liquid sodium used as heat transfer fluid in the primary and secondary circuits of fast neutron reactors has to be treated when the circuits are dismantled. To reduce the chemical risk of storing the sodium in its liquid metallic form, it is converted to concentrated sodium hydroxide solution. The approach adopted hitherto consists of a two-step conversion of the liquid sodium metal potentially contaminated by radioactive isotopes: a hydrolysis step to convert said sodium to sodium hydroxide, and a solidification/stabilization step to convert the sodium hydroxide to a solid whose stability is compatible with storage at an appropriate central point. The process according to the present invention can be applied to this second step. It is also desirable to be able to solidify and stabilize concentrated aqueous sodium hydroxide solution contaminated by other pollutants. Possible examples of such pollutants which may be mentioned are organic products and heavy metals (zinc, lead, arsenic, etc.). The problem therefore consists in incorporating aqueous sodium hydroxide solution into a solid matrix with a high loading rate of the aqueous solution. Patent FR 2804103 has already disclosed a process for the conditioning of aqueous sodium hydroxide solution to give solid compounds of the “nepheline” type. These are obtained by reacting the sodium hydroxide with compounds that provide silica and alumina, such as metakaolin, bentonite, dickite, halloysite and pyrophillite. After a primary reaction, which takes place at ambient temperature to form a precipitate of the zeolite type (cancrinite), a second treatment phase at a temperature between 1000 and 1500° C. makes it possible to convert this zeolite to very sparingly soluble nepheline (sodium aluminosilicate). It is clearly understood that such a process is very efficient since it enables all the sodium present to be converted to a practically insoluble mineral. However, this process has the disadvantage of requiring two treatment steps, the second of which consists of a high-temperature heating phase. The main object of the invention is to provide a process in which concentrated aqueous sodium hydroxide solution can be solidified directly at ambient temperature. The object of the present invention is therefore to propose a one-step process that makes it possible, at ambient temperature and in advantageous manner, to solidify concentrated aqueous sodium hydroxide solution to give a stable, massive solid block with a very high sodium incorporation rate. The process according to the invention comprises mixing concentrated aqueous sodium hydroxide solution with a hydraulic binder containing blast furnace slag, in the presence of at least one additional source of calcium ions and/or magnesium ions and/or silica, to form a slurry. At least one of these three components (additional source of calcium ions, additional source of magnesium ions, additional source of silica) is necessary for carrying out the invention. Within the framework of the invention, it is also possible to use combinations of two components selected from these three components, or to use all three. The present invention therefore consists of a process for the solidification and stabilization of concentrated aqueous sodium hydroxide solution, characterized in that the following steps are carried out: a) a hydraulic binder containing blast furnace slag is mixed with said sodium hydroxide solution, in the presence of at least one additional source of calcium ions and/or magnesium ions and/or silica, to form a slurry, and b) the slurry is left to set to a solid product. “Hydraulic binder” is understood as meaning any compound capable of developing hydraulic properties, i.e. of forming hydrates, and capable of developing setting and hardening properties. “Blast furnace slag” or “slag” is understood as meaning a material obtained by rapid cooling of the scoria originating from the fusion of iron ore in a blast furnace, which has been ground to a particle size below 200 μm and preferably below 100 μm. “Slag cement” is understood as meaning a hydraulic binder containing slag and “clinker”, the latter being obtained by burning mixtures of limestone (predominantly) and clay. “Additional source” is understood as meaning a source other than that provided by the constituents already present in the hydraulic binder. In fact, hydraulic binders themselves constitute sources of Ca, Mg or SiO2 and, within the framework of the present invention, that which is provided by the hydraulic binder is supplemented by adding a source of at least one of these species. “Slurry” is understood as meaning a suspension of mineral particles in water. In the context of the use of hydraulic binders, the “slurry” will consist of a mixture of water, particles of hydraulic binder(s) and other optional mineral or organic components. More particularly, the present invention relates to a process for the solidification and stabilization of radioactive or non-radioactive concentrated aqueous sodium hydroxide solution by the production of a slurry that is easy to use. In one preferred embodiment of the invention, the slurry composition will be chosen such that the slurry has a good fluidity and retains its flow properties for an appropriate period of time in view of the industrial conditions under which the invention is carried out. In order to be able to transfer the slurry from the mixer to a container that can be used as a final storage means, it can be of particular value to ensure a workability time of at least about 30 minutes, and tests have demonstrated the possibility of maintaining the flow properties for about 45 or even 60 minutes. In another preferred embodiment of the invention, it is possible to envisage working under conditions where the workability is maintained for a restricted period, limiting the transfer of the constituted slurry to a container. In this embodiment, the container serves as a mixer. Once the slurry has been prepared by the mixing of its constituent components, it is left to set so as to develop its mechanical properties, thereby ensuring a limited diffusion of the sodium hydroxide into the external medium. The hardened slurries obtained by carrying out the process according to the invention exhibit neither bleeding nor exudation phenomena. The process according to the invention is particularly simple to carry out since it merely amounts to the production of a slurry, which is an operation well known to those skilled in the art. In addition to the sodium hydroxide solution, the hydraulic binder containing blast furnace slag, and at least one additional source of calcium ions and/or magnesium ions and/or silica, the slurry according to the invention can contain an adjuvant of the “plasticizer” type and/or of the “retarder” type. Preferably, when the slurry according to the invention is prepared, the hydraulic binder containing blast furnace slag, the additional source(s) of calcium ions and/or magnesium ions and/or silica, and optional adjuvants, are added successively to the sodium hydroxide solution to be treated. The constituents of the slurry can be added in any order to produce a slurry according to the invention. However, in one currently preferred embodiment, the adjuvant of the “plasticizer” type and/or of the “retarder” type is added to the sodium hydroxide first (if an adjuvant is used), followed by the hydraulic binder containing slag, and then by the additional source(s) of calcium ions and/or magnesium ions and/or silica. It is also possible to mix two, three or more solid constituents together before adding them to the sodium hydroxide solution to be treated. In the context of treating the concentrated sodium hydroxide solution by incorporation into a matrix based on hydraulic binder containing blast furnace slag, it is optionally possible to envisage adding water to dilute the slurry. This is not recommended, however, because one of the desired objectives is to maximize the sodium incorporation rate per unit volume of slurry. In one preferred embodiment of the invention, the slurry obtained by mixing its constituent components in a mixer can then be poured into a container that can be used as a packaging means. The containers used will preferably be flexible and leaktight (commonly called “big bags”). The volume of these leaktight flexible containers is generally in the order of 1 m3, especially greater than or equal to 0.4 m3 and preferably between at least about 0.5 m3 and about 2 m3. In another preferred embodiment of the invention, the slurry is left to set directly without being transferred to a new receptacle after preparation. In the case in point, the packaging container also serves as a mixer. This embodiment will be of particular value when the rapid rise in viscosity of the slurry means that it cannot flow easily, if at all, after preparation. In such cases it may also be noted that the moving part of the stirrer used to mix the constituents of the slurry will generally have to be permanently incorporated into the slurry. The volume of the container in this embodiment, without transfer of the slurry after constitution, will generally be less than 1 m3, preferably less than or equal to 0.4 m3 and particularly preferably less than or equal to 0.2 m3. According to the invention, the slurry (optionally packaged) is left to set to a solid block. The present invention further relates to the solid product obtained by carrying out the process described above. The solid product according to the present invention is characterized in that it contains waste consisting of radioactive or non-radioactive concentrated aqueous sodium hydroxide solution, and products resulting from the chemical reaction between the concentrated aqueous sodium hydroxide solution, the hydraulic binder containing slag, and the source(s) of calcium and/or magnesium and/or silica. The solid product according to the present invention is characterized by a sodium incorporation rate that is preferably greater than 100 kg/m3 of slurry. Advantageously, the amount of sodium incorporated in the slurry is between 100 and 400 kg/m3 of slurry and particularly preferably between 120 and 220 kg/m3 of slurry. This sodium incorporation rate is measured at the time of preparation of the slurry, taking into account the density of the slurry and the amount of sodium hydroxide used. It has been found that the incorporation rate does not vary greatly after preparation of the slurry, even though a variation in the sodium incorporation rate over a long storage period cannot be excluded, at least in those parts of the solid block of hardened slurry which are exposed to the external medium. The concentration of the sodium hydroxide solution to be treated within the framework of the process according to the invention is preferably between 8 N and 18 N and particularly preferably between 8 N and 14 N. The hydraulic binder according to the invention advantageously consists of a blast furnace slag cement (CEM III A, B or C). The compositions of the blast furnace slag cements designated as “CEM III A”, “CEM III B” and “CEM III C” are defined by industrial standards in the field of cements, especially by standard NF EN 197-1. It is also possible to use pure slag ground to a particle size preferably below 100 μm. A customary average composition of pure slag is as follows: Al2O3-7.5 to 12.5%; Fe2O3-0.35 to 1.75%; CaO-37 to 47%; MgO-5 to 8%; SiO2-33 to 37%. In general, and regardless of its nature, the hydraulic binder which can be used within the framework of the invention preferably comprises particles with a size predominantly below 200 μm and preferably below 100 μm. Advantageously, the weight ratio of the amount of hydraulic binder containing blast furnace slag to the amount of aqueous sodium hydroxide solution (weight/weight) is between 0.3 and 2 and preferably between 0.4 and 1.5. The amount of water relative to the hydraulic binder will therefore depend on this ratio (weight of hydraulic binder)/(weight of aqueous sodium hydroxide solution) and on the concentration of the aqueous sodium hydroxide solution used. The Applicant proposes, without this possible theoretical interpretation of the process implying a limitation of the invention, that the incorporation of an additional source of calcium ions and/or magnesium ions would assist in controlling the viscosity of the slurry by neutralizing part of the concentrated aqueous sodium hydroxide solution, allowing the formation of Ca(OH)2 and Mg(OH)2 respectively. For example, it is considered that the calcium chloride used in the process of the present invention would probably be converted very rapidly to lime (with the production of sodium chloride). The calcium sulfate, including anhydrite, would be converted to lime (with the production of sodium sulfate) at a significant rate, but more slowly than in the case of calcium chloride. The calcite or calcium carbonate would be converted to lime (with the production of sodium carbonate). It is considered that the flow properties of the slurry depend on the properties of the charged mineral particles that will flocculate and increase the viscosity of the slurry. The partial neutralization of the concentrated aqueous sodium hydroxide solution in a first stage, and then the action of the silica to allow the formation of hydrated calcium silicates in a second stage, will improve the homogeneous setting of the slurry. The additional source of calcium and/or magnesium is preferably selected from the nitrate, sulfate, chloride and carbonate salts of calcium or magnesium or a calcium-rich thermal power station ash such as a sulfo-calcium ash, ground dolomite (CaMg(CO3)2) or ground calcite (calcium carbonate). The sources of calcium sulfate which can be used within the framework of the present invention as an additional calcium source include especially plaster and gypsum (hydrated calcium sulfates) and anhydrite (anhydrous calcium sulfate). According to the invention, the ratio of the number of mol of calcium and/or magnesium mixed with the slurry as an additional source (additional sources) of calcium and/or magnesium ions, to the number of mol of sodium in the aqueous NaOH solution, is preferably between 0.01 and 0.6 and particularly preferably between 0.05 and 0.45. According to the invention, a source of silica, selected from ground silica, thermal power station silico-aluminous fly ash and/or fluidized bed ash, can also be incorporated into the mixture at a rate of 25 to 500 kg of ash per m3 of slurry, preferably of 50 to 350 kg per m3 of slurry, the volume of the slurry being determined at the time of its preparation by mixing of the solid constituents with the liquid sodium hydroxide. Within the framework of the present invention, the viscosity of the slurry increases very rapidly after mixing of the hydraulic binder containing blast furnace slag and the concentrated aqueous sodium hydroxide solution. It may therefore be desirable, or even essential under certain operating conditions, to add an adjuvant of the “plasticizer” or “retarder” type in order to control the viscosity of the mixture and its workability time. It may be desirable to have a sufficient time, if appropriate, to transfer the slurry to a container that can be used as a packaging means. In particular, it may be desirable for the slurry not to set prematurely after it has been constituted by the mixing of its constituent components. It may also be desirable to slow down the setting in order to avoid an excessive evolution of heat. In particular, it will be preferable to prevent the core of a 1 m3 block from reaching a temperature above 95° C. when the binders set. It may therefore be desirable, or even essential under certain operating conditions, to add an adjuvant of the “retarder” type in order to avoid excessively rapid setting. As far as adjuvants are concerned, it should be noted that some products known in particular for their plasticizing function also have retarding properties, and vice-versa. The plasticizers within the framework of the present invention disperse all the aggregates which may be present in the slurry so as to produce a homogeneous mixture and reduce the force required to mix the cement with the concentrated aqueous sodium hydroxide solution. Suitable plasticizers can be selected from the group comprising naphthalenesulfonate polymers, melamine/formaldehyde polymers, water-soluble acrylic polymers (such as those of the range marketed under the name Bentocryl®) and polyoxyethylene-polycarboxylate polymers (such as those of the Chrysofluid® range). The slurry used in the process according to the invention can also comprise a setting retarder of the lignosulfonate or gluconate type, e.g. sodium gluconate (such as those of the Résitard P®, Cimaxtard® or Chrysotard® ranges) or a mixture of sodium gluconate and sodium phosphate (such as those of the SIKA retarders range). The adjuvants of the “plasticizer” and/or “retarder” type which are most preferred in the present invention are selected from gluconate-based products and more particularly from those of the Cimaxtard® and SIKA retarders ranges. Within the framework of the present invention, if the slurry is produced with a sodium hydroxide concentration towards the bottom of the range of interest (8 to 18 N), particularly of around 10 N, it is desirable to add an adjuvant of the “plasticizer” and/or “retarder” type in order to maintain a sufficient workability time, especially if it is desired to transfer the slurry, after preparation, from a mixer to another container. Thus, according to a first preferred feature of the invention, the latter relates to a process for the solidification and stabilization of aqueous sodium hydroxide solution having a concentration of between 8 N and 14 N, characterized in that the following steps are carried out: a) a hydraulic binder containing blast furnace slag is mixed with said sodium hydroxide solution, in the presence of at least one additional source of calcium ions and/or magnesium ions and/or silica, and in the presence of at least one adjuvant of the “plasticizer” and/or “retarder” type, to form a slurry, and b) the slurry is left to set to a solid product. As regards the concentration of the adjuvants, the slurry used in the process according to the invention, when at least one adjuvant is used, contains preferably from 0.05 to 5% and particularly preferably from 0.05 to 2% of adjuvant(s) of the “plasticizer” and/or “retarder” type, expressed by dry weight of adjuvant, based on the dry weight of hydraulic binder. Within the framework of the present invention, if the slurry is produced with a sodium hydroxide concentration towards the top of the range of interest (8 to 18 N), particularly of around 18 N, it is not necessary to add an adjuvant of the “plasticizer” and/or “retarder” type in order to maintain a sufficient workability time to allow the transfer of the slurry after preparation. On the other hand, it has been discovered, totally surprisingly, that only the additional calcium sources containing calcium sulfate make it possible to regulate the initial setting and thus to have a sufficient workability time to transfer the slurry by allowing it to flow. The additional calcium sources containing calcium sulfate which are particularly preferred within the framework of this embodiment are selected especially from anhydrite, gypsum and plaster, anhydrite being the most preferred source. Thus, according to a second preferred feature of the invention, the latter relates to a process for the solidification and stabilization of aqueous sodium hydroxide solution having a concentration of between 14 N and 18 N, characterized in that the following steps are carried out: a) a hydraulic binder containing blast furnace slag is mixed with said sodium hydroxide solution, in the presence of an additional source of calcium ions containing calcium sulfate, and optionally of at least one other additional source of calcium ions and/or magnesium ions and/or silica, to form a slurry, and b) the slurry is left to set to a solid product. In another preferred embodiment of the invention, it can be envisaged to work under rapid setting conditions, limiting the transfer of the constituted slurry to a container. In this embodiment, the container serves as a mixer. This embodiment will be of particular value when the rapid rise in viscosity of the slurry means that it cannot flow easily, if at all, after preparation, this phenomenon being observed in the absence of retarder and especially at sodium hydroxide concentrations of between 8 N and 14 N. Thus, according to a third preferred feature of the invention, the latter relates to a process for the solidification and stabilization of aqueous sodium hydroxide solution having a concentration of between 8 N and 18 N, preferably of between 8 N and 14 N, characterized in that the following steps are carried out: a) a hydraulic binder containing blast furnace slag is mixed with said sodium hydroxide solution, in the presence of at least one additional source of calcium ions and/or magnesium ions and/or silica, and in the absence of an adjuvant of the “plasticizer” and/or “retarder” type, to form a slurry, and b) the slurry is left to set to a solid product. This last embodiment will preferably be carried out using a container with a volume less than 1 m3, preferably less than or equal to 0.4 m3 and particularly preferably less than or equal to 0.2 m3. Restricted container volumes are preferable, especially because of the sharp temperature rise associated with rapid setting. For the purpose of clarifying the invention, several modes of carrying it out will now be described; they are given as Examples that do not limit the scope of the invention. The following are introduced successively into a high-turbulence mixer in order to solidify/stabilize 867 kg of 10 N aqueous sodium hydroxide solution: CaCl2•2H2O280 kgRetarder (Résitard P ® 608A)*3.66 kg Fluidized bed ash 75 kgCEM III C475 kg*Résitard P ® 608A retarder is a sodium gluconate. This composition is mixed for 3 min 30 sec and then poured into a “big bag”. Test pieces are made up in plastic moulds (Ø=4 cm, h=8 cm) for evaluation of the performance characteristics. The slurry prepared in this way retained its flow properties for 60 min. After ageing for 28 days to allow the hydraulic binder to set and develop its mechanical properties, this test piece is subjected to a compressive strength test according to standard NF P 18-406, the result of which, given in megapascals (MPa), is indicated below (Rc MPa value), and to a leaching test according to standard XP X 31-211. In fact, it is desirable to obtain an Rc MPA value of at least 3 MPa. Inter alia, by allowing stacking, a high compressive strength will facilitate storage of the massive blocks of solidified slurry containing sodium hydroxide. The eluate is the result of contact between a solid product and 10 times its dry weight of water for 24 hours, with agitation. The eluate obtained is then analysed to determine the fraction that is still soluble. The results of these tests are as follows: Rc MPa5.37 MPaSodium content 147 kg/m3Result on eluate according to standard XP X 31-211 (1×24 h): pH12.55Soluble fraction7.94% The following are introduced successively into a high-turbulence mixer in order to solidify/stabilize 867 kg of 10 N aqueous sodium hydroxide solution: Ground dolomite100 kgRetarder (Résitard P ® 608A) 3.5 kgCEM III C500 kg This composition is mixed for 3 min 30 sec and then poured into a “big bag”. Test pieces are made up for evaluation of the performance characteristics. The slurry prepared in this way retained its flow properties for 60 min. After ageing for 28 days, the solid product has the following characteristics: Rc MPa4.36 MPaSodium content 171 kg/m3Result on eluate according to standard XP X 31-211 (1×24 h): pH12.91Soluble fraction10% The following are introduced successively into a high-turbulence mixer in order to solidify/stabilize 867 kg of 10 N aqueous sodium hydroxide solution: Ground calcite190 kgRetarder (Résitard P ® 608A) 3.7 kgFluidized bed ash 75 kgCEM III C475 kg This composition is mixed for 3 min 30 sec and then poured into a “big bag”. Test pieces are made up for evaluation of the performance characteristics. The slurry prepared in this way retained its flow properties for 60 min. After ageing for 28 days, the solid product has the following characteristics: Rc MPa6.38 MPaSodium content 161 kg/m3Result on eluate according to standard XP X 31-211 (1×24 h): pH12.97Soluble fraction6.2% The following are introduced successively into a high-turbulence mixer in order to solidify/stabilize 1125 kg of 18 N aqueous sodium hydroxide solution (50%): Anhydrite135 kgGround slag powder520 kg This composition is mixed for 3 min 30 sec and then poured into a “big bag”. Test pieces are made up for evaluation of the performance characteristics. The slurry prepared in this way retained its flow properties for 45 min. After ageing for 28 days, the solid product has the following characteristics: Rc MPa 5.8 MPaSodium content320 kg/m3Result on eluate according to standard XP X 31-211 (1×24 ): pH13.23Soluble fraction13.45% The following are introduced successively into a high-turbulence mixer in order to solidify/stabilize 977 kg of 10 N aqueous sodium hydroxide solution: Ground dolomite84.5 kgChrysotard ® retarder16.9 kgSilico-aluminous ash84.5 kgCEM III C 507 kg This composition is mixed for 3 min 30 sec and then poured into a “big bag”. Test pieces are made up for evaluation of the performance characteristics. The slurry prepared in this way retained its flow properties for 60 min. After ageing for 28 days, the solid product has the following characteristics: Rc MPa5.38 MPaSodium content 170 kg/m3Result on eluate according to standard XP X 31-211 (1×24 h): pH13.04Soluble fraction8.45% The following are introduced successively into a high-turbulence mixer in order to solidify/stabilize 977 kg of 10 N aqueous sodium hydroxide solution: Ground silica192 kgRetarder (Résitard P ® 608A) 7.7 kgCEM III C769 kg This composition is mixed for 3 min 30 sec and then poured into a “big bag”. Test pieces are made up for evaluation of the performance characteristics. The slurry prepared in this way retained its flow properties for 60 min. After ageing for 28 days, the solid product has the following characteristics: Rc MPa16.9 MPaSodium content 150 kg/m3Result on eluate according to standard XP X 31-211 (1×24 h): pH12.76Soluble fraction5.54% |
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description | FIG. 1a shows an embodiment of a fuel assembly according to the invention. During operation, the fuel assembly is arranged vertically in the reactor core. FIG. 1b shows a vertical section Ib-Ibxe2x80x2 through the fuel assembly, and FIG. 1c shows a vertical section Ic-Icxe2x80x2 through the fuel assembly. The fuel assembly comprises an upper handle 1, a lower end portion 2 and a plurality of fuel units 3a, 3b and 3c stacked on top of each other. Each fuel unit comprises a plurality of fuel rods 4 arranged between a top tie plate 5 and a bottom tie plate 6. The fuel units are stacked on top of each other in the longitudinal direction of the fuel assembly and they are stacked in such a way that the top tie plate 5 in one fuel unit is facing the bottom tie plate 6 in the next fuel unit in the stack a fuel rod 4 comprises fuel in the form of a stack of uranium pellets arranged in a cladding tube 7. The fuel assembly is enclosed in a fuel channel 9 of substantially square cross section. In this embodiment, the fuel assembly comprises eight fuel units. A fuel unit has 100 fuel rod positions in an orthogonal 10xc3x9710 lattice. A fuel rod position is a position in the lattice in which it is possible to arrange a fuel rod. All the positions in the lattice need not be occupied by fuel rods. The fuel unit is divided into four sub-bundles with 25 fuel rod positions in an orthogonal 5xc3x975 lattice. The lattice in one sub-bundle comprises a fuel rod position in the centre of the sub-bundle, and around this an inner square ring is arranged consisting of 8 fuel rod positions. Outside the inner ring, there is an outer square ring consisting of 16 fuel rod positions. The fuel assembly comprises three different types of fuel units 3a, 3b, 3c. In the two lowermost fuel units 3a, all the fuel rod positions are occupied by fuel rods which are arranged in parallel with the longitudinal axis of the fuel assembly. The fuel rods in the fuel units 3b and 3c are arranged so as to be inclined between the bottom tie plate and the top tie plate. In one sub-bundle, all the fuel rods in the two rings are inclined in the same direction, that is, either clockwise or counterclockwise around the centre of the sub-bundle. The purpose of inclining the fuel rods around the centre of the sub-bundle is to set water and steam, which flow upwards through the fuel assembly, into rotation, thus achieving a separation of water and steam. Such a fuel assembly is known from Swedish patent specification No. 96024476. The fuel unit 3b has 96 fuel rods distributed among four sub-bundles. Each one of the sub-bundles comprises 24 fuel rods arranged in an inner ring 11a and an outer ring 11b. The fuel rods are inclined in the direction of the arrows, that is, around the centre of the sub-bundle. The fuel rod position in the centre of the sub-bundle is unoccupied. In this way an empty volume is created in the centre of the fuel bundle. The empty volume constitutes the lower part of a vertical channel which extends through the six uppermost fuel units in the fuel assembly. There are four channels 12a, 12b, 12c, 12d extending through the fuel assembly, one channel in each sub-bundle. The inclined fuel rods in the sub-bundle bring about an eddy of water and steam around the channel. The direction of the eddies is marked by arrows in the channel. In this eddy, the water and the steam are separated from each other by the water being thrown outwards while at the same time the steam is pressed against the centre of the eddy. The four uppermost fuel units 3c each have 80 fuel rods distributed among four sub-bundles. In each sub-bundle, the fuel rod position in the centre and four positions in the inner ring are unoccupied, thus forming an empty volume which extends through the sub-bundle. The empty volumes contribute to the four channels 12a-12d which extend through the fuel assembly. In each one of the channels, four steam pipes 10a, 10b, 10c and 10d are arranged one above the other such that their longitudinal axes coincide with one another. Through the steam pipes, the steam is conducted towards the outlet of the fuel assembly. In this embodiment, all the steam pipes are designed identically. Each fuel unit 3c comprises four steam pipes arranged between the bottom tie plate and the top tie plate. FIG. 2 shows in more detail how a steam pipe may be designed. The lower end 14 of the steam pipe, hereinafter referred to as the inlet end of the steam pipe, has an opening which constitutes an inlet 14b for the steam. The upper end 13 of the steam pipe, hereinafter referred to as the outlet end of the steam pipe, has an opening which constitutes an outlet 13b for steam and water which have accumulated on the inner side of the steam pipe. The outlet end of the first steam pipe 10a is arranged at a distance from the inlet end of the next steam pipe 10b. The inlet end of the steam pipe has an outer diameter D2 which is smaller than the diameter D3 of the opening in the outlet end. In this way, the water 15 emanating from the inside of the first steam pipe is thrown out at a distance from the inlet of the next pipe, thus separating the water from the steam which continues up through the next steam pipe. The distance between two steam pipes shall be so large as to provide a sufficient inflow area for the steam while at the same time it must not be so large that the separated water has time to be deflected to such an extent that it follows the steam up into the next steam pipe. Preferably, the opening between the steam pipes shall have an area which is of an order of magnitude near the cross-section area of the steam pipe, which is determined by the diameter D1 of the steam pipe. The arrows in the figure show the direction of the steam flow. The inflow of the steam is facilitated by designing the outlet ends and inlet ends of the steam pipes such that venturi effect is obtained. For this purpose, the inlet ends and the outlet ends are arranged such that they are tapering towards the opening. The diameter D2 of the inlet and the diameter D3 of the outlet are to be smaller than the diameter D1 of the steam pipe. The inside of the outlet end 13 is provided with slightly angularly adjusted grooves 16 which open out into the outlet. The task of these grooves is to collect water from the water film 17 which covers the inside of the steam pipe and to concentrate the water to the orifices 16b of the grooves. In this way, large water drops are formed in localized paths. One advantage of this is that large water drops are not deflected as easily as smaller water drops and hence the risk of the water accompanying the steam into the next steam pipe is reduced. In addition, the localized paths with water cause formation of water-free paths between these first-mentioned paths, through which steam may flow into the next steam pipe without being obstructed. The inlet end 14 is provided with a rejection edge 18 and the outlet end 13 is provided with a rejection edge 19 for scraping off the water which is transported along the outside of the steam pipe. To reinforce the formation of the water film on the inside of the steam pipe, the inside of the steam pipe may be provided with oblique vanes 20. Alternatively, vanes may be arranged in the bottom tie plate 6. The inlet end of the steam pipe is attached to the bottom tie plate 6 with a plurality of attachment means 21 and the outlet end of the steam pipe is attached to the top tie plate 5 with a plurality of attachment means 22. FIG. 3 shows another example of how the outlet end 25 and the inlet end 26 of the steam pipe may be designed. The opening edge of the outlet end is provided with lugs 27. The lugs have the same function as the grooves in the preceding example, namely to form large water drops in localized paths. The figure shows the difference between the shape of the water drops when the opening edge is provided with lugs 27 and when the opening edge is straight 28. At the straight opening edge, a curtain with smaller water drops is formed which risk penetrating into the opening between the steam pipes. The steam pipe is provided with a rejection ring 29 on its inlet end and with an additional rejection ring 30 at its outlet end. The task of the rejection rings is to remove the water which is accumulated on the outside of the steam pipe. The outlet end of the steam pipe is attached to the top tie plate 5 with attachment means 32 and the inlet end of the steam pipe is attached to the bottom tie plate 6 with attachment means 31. Swedish patent document 9604720-4 shows a fuel assembly with fuel units stacked on top of each other, in which the fuel rods are arranged in a polar lattice comprising a number of concentric rings. In such a fuel assembly, the invention may be advantageously applied. FIG. 4 shows part of a fuel assembly according to a second embodiment of the invention. The fuel assembly comprises a number of fuel units 40a-40c which all have fuel rods arranged in a polar lattice comprising three concentric rings. In the lower part of the fuel assembly, fuel units 40a are arranged. All the fuel rod positions in the fuel unit 40a are occupied. In the fuel unit 40b, all the fuel rod positions are also occupied. The fuel rods 42 have a smaller diameter than the fuel rods 41 in the fuel unit 40a. The fuel rods in the inner rings are inclined outwards from the centre of the rings so as to form an empty volume in the centre. The fuel rods are not inclined in the direction of the ring, as is the case in the first embodiment. In this way, no eddy is formed which separates the steam from the water. One advantage of inclining the fuel rods outwards is that the water accompanies the fuel rods, and in this way a certain separation of water and steam occurs. In the fuel units 40c, the fuel rod positions in the inner ring are unoccupied and, instead, a steam pipe 10a, 10b is arranged in the centre. The steam pipes have the same design as has been described above. The advantage of this embodiment is that it is simpler to manufacture than the first-embodiment. FIG. 5 shows part of a fuel assembly according to a third embodiment of the invention. The fuel assembly comprises a top tie plate 37, a bottom tie plate 38 and a plurality of full-length fuel rods 36 which are arranged between the top tie plate and the bottom tie plate. Further, the fuel assembly comprises a number of part-length fuel rods 39 which extend from the bottom tie plate and terminate far below the top tie plate. Above these part-length fuel rods, a plurality of steam pipes 10a, 10b are arranged. The steam pipes are arranged one above the other. Between the steam pipes, spacer elements 44 are arranged to keep the steam pipes in spaced relationship to each other and to fix the steam pipes to each other. To keep the fuel rods in spaced relationship to each other, a number of spacers 45 are arranged in spaced relationship to each other along the fuel assembly in the longitudinal direction thereof. The steam pipes are attached to the spacers. The steam pipes are designed in the same way as has been described in the preceding embodiments. |
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054992770 | claims | 1. A liquid metal-cooled nuclear reactor comprising a containment vessel, a reactor vessel surrounded by said containment vessel with an inert gas-filled gap space therebetween, a nuclear fuel core arranged inside said reactor vessel, a heat collector cylinder surrounding said containment vessel with a space therebetween, a silo surrounding said heat collector cylinder, an air inlet duct and an air outlet duct in flow communication with atmospheric air external to said reactor, a cold air downcomer gap in flow communication with said air inlet duct and extending between said heat collector cylinder and said silo, a hot air riser gap in flow communication with said cold air downcomer gap and said air outlet duct and extending between said heat collector cylinder and said containment vessel, an inert gas inlet duct and an inert gas outlet duct in flow communication with said inert gas-filled gap space, an inert gas downcomer duct in flow communication with said inert gas inlet duct and an inert gas riser duct in flow communication with said inert gas outlet duct and with said inert gas downcomer duct, wherein said inert gas downcomer duct and said air outlet duct share a common wall made of heat conductive material for removing heat from said inert gas by heat exchange with atmospheric air, wherein said inert gas downcomer duct and said inert gas riser duct are not annular. 2. The liquid metal-cooled nuclear reactor as defined in claim 1, further comprising thermal insulation applied to at least a portion of the outer surface of said inert gas riser duct. 3. The liquid metal-cooled nuclear reactor as defined in claim 1, further comprising an electromagnetic pump and a heat exchanger arranged inside said reactor vessel, and first, second and third baffles arranged vertically in said insert gas-filled gap space, said first and second baffles, in conjunction with said reactor vessel and said containment vessel, defining a first channel for the flow of inert gas, and said second and third baffles, in conjunction with said reactor vessel and said containment vessel, defining a second channel for the flow of inert gas, said first channel being in flow communication with said inert gas outlet duct and located radially outside said heat exchanger, and said second channel being in flow communication with said inert gas inlet duct and said first channel and located radially outside said electromagnetic pump. 4. The liquid metal-cooled nuclear reactor as defined in claim 1, further comprising a stack which surrounds said air inlet duct, said air outlet duct and said inert gas downcomer duct. 5. The liquid metal-cooled nuclear reactor as defined in claim 4, wherein said inert gas downcomer duct communicates with said inert gas riser duct via a horizontal duct which penetrates said stack. 6. The liquid metal-cooled nuclear reactor as defined in claim 1, wherein said inert gas downcomer duct and said air inlet duct share a common wall. 7. A system for removing heat from a liquid metal-cooled nuclear reactor in which a reactor vessel is surrounded by a containment vessel with a fluid-filled gap space therebetween, comprising: a fluid outlet duct in flow communication with said fluid-filled gap space; a fluid inlet duct in flow communication with said fluid-filled gap space; a fluid riser duct in flow communication with said fluid outlet duct; a fluid downcomer duct in flow communication with said fluid inlet duct and said fluid riser duct; and air circulation flowpath means in flow communication with atmospheric air external to said reactor, wherein said air circulation flowpath means has a first section in heat exchange relationship with said containment vessel and a second section in heat exchange relationship with said fluid downcomer duct, whereby heat is removed from fluid in said fluid downcomer duct by heat exchange with atmospheric air in said air circulation flowpath means, wherein said fluid gas downcomer duct and said fluid riser duct are not annular. 8. The heat removal system as defined in claim 7, further comprising thermal insulation applied to at least a portion of the outer surface of said inert gas riser duct. 9. The heat removal system as defined in claim 7, further comprising a stack which surrounds said air inlet duct, said air outlet duct and said inert gas downcomer duct. 10. The heat removal system as defined in claim 9, wherein said inert gas downcomer duct communicates with said inert gas riser duct via a horizontal duct which penetrates said stack. 11. The heat removal system as defined in claim 9, wherein said stack is made of thermally insulating material. 12. The heat removal system as defined in claim 7, wherein said fluid is an inert gas. 13. In a liquid metal-cooled nuclear reactor comprising a containment vessel, a reactor vessel surrounded by said containment vessel with an inert gas-filled gap space therebetween, a nuclear fuel core arranged inside said reactor vessel, a heat collector cylinder surrounding said containment vessel with a space therebetween, a silo surrounding said heat collector cylinder, first and second air inlet ducts in flow communication with atmospheric air external to said reactor, first and second air outlet ducts in flow communication with atmospheric air external to said reactor, a cold air downcomer gap in flow communication with said first and second air inlet ducts and extending between said heat collector cylinder and said silo, a hot air riser gap in flow communication with said cold air downcomer gap and said first and second air outlet ducts and extending between said heat collector cylinder and said containment vessel, the improvement comprising first and second inert gas circulation loops in flow communication with said inert gas-filled gap space, said first inert gas circulation loop being in heat exchange relationship with said first air outlet duct and said second inert gas circulation loop being in heat exchange relationship with said second air outlet duct, and first through fourth baffles arranged vertically in said insert gas-filled gap space, said first through fourth baffles, in conjunction with said reactor vessel and said containment vessel, defining first through fourth channels for the flow of inert gas, said first and second channels being in flow communication with respective ends of said first inert gas circulation loop, and said third and fourth channels being in flow communication with respective ends of said second inert gas circulation loop. 14. The liquid metal-cooled nuclear reactor as defined in claim 13, further comprising first and second electromagnetic pumps arranged inside said reactor vessel at generally diametrally opposed first and second azimuthal positions, and first and second heat exchangers arranged inside said reactor vessel at generally diametrally opposed third and fourth azimuthal positions intermediate said first and second azimuthal positions, wherein said first and third channels are located radially outside said first and second heat exchangers respectively, and said second and fourth channels are located radially outside said first and second electromagnetic pumps. 15. The liquid metal-cooled nuclear reactor as defined in claim 13, wherein each of said first through fourth baffles extends from a highest elevation of said fluid-filled gap space to an elevation above a lowest elevation of said fluid-filled space so that inert gas may flow from one of said first through fourth channels to an adjacent one of said first through fourth channels around a bottom of a respective one of said first through fourth baffles therebetween. 16. The liquid metal-cooled nuclear reactor as defined in claim 13, wherein each of said first and second inert gas circulation loops comprises an inert gas inlet duct and an inert gas outlet duct in flow communication with said inert gas-filled gap space, an inert gas downcomer duct in flow communication with said inert gas inlet duct and an inert gas riser duct in flow communication with said inert gas outlet duct and with said inert gas downcomer duct, wherein said inert gas downcomer duct of said first inert gas circulation loop and said first air outlet duct share a common wall made of heat conductive material, and said inert gas downcomer duct of said second inert gas circulation loop and said second air outlet duct share a common wall made of heat conductive material. 17. The liquid metal-cooled nuclear reactor as defined in claim 16, further comprising a first stack which surrounds said first air inlet duct, said first air outlet duct and said inert gas downcomer duct of said first inert gas circulation loop, and a second stack which surrounds said second air inlet duct, said second air outlet duct and said inert gas downcomer duct of said second inert gas circulation loop. 18. The liquid metal-cooled nuclear reactor as defined in claim 17, wherein said inert gas downcomer duct of said first inert gas circulation loop communicates with said inert gas riser duct of said first inert gas circulation loop via a horizontal duct which penetrates said first stack. 19. The liquid metal-cooled nuclear reactor as defined in claim 16, wherein said inert gas downcomer duct of said first inert gas circulation loop and said first air inlet duct share a common wall. |
description | The present patent application is a non-provisional application claiming the priority of a provisional application of Application No. 60/781,040 filed Mar. 10, 2006. The present invention relates to a probe forming lithography system for projecting an image pattern on to a target surface such as a wafer, using an “on” and “off” writing strategy, thereby dividing said pattern over a grid comprising grid cells, in each of which cells said probe is switched “on” or “off”. Such systems having a so called black and white writing strategy are widely known in the art, and may e.g. be laser based, and feature the use of direct writing means, so called maskless systems. By switching said probe on or of, each grid cell is either written or not respectively. Such probes are characterised by the probe effect in the target surface, which in turn is often described by a so-called point spread function. The point spread function (PSF) generally has a Gaussian distribution. Probe size from such a distribution is generally taken as the size of the distribution in which 50% of the energy of said probe is present. One particular kind of such probe based lithographic system is known from the international patent publication WO2004038509 in the name of Applicant and involves a massive multiplicity of charged particle beams generated in a charged particle beams column, for writing said pattern on to said target, which writing beams are for that purpose scanned over said target, said target being capable of moving in a direction transverse to a scanning direction of said beams, and said writing beams being modulated for that purpose, based in which system pattern features are positioned on said target using a virtual grid over said target, and using writing information for modulating one or more charged beams. Writing of a pattern by the known lithography system is thus effected by the combination of relative movement of the target surface and a timed “on” and “off” switching of a writing beam by said blanker optics upon signalling by said control unit, more in particular by a so-called pattern streamer thereof. By the operational use of a virtual grid, the known systems are able to determine whether a writing beam is to be turned into “on” or “off” modus, which in the particular, exemplified system means whether the beam is to be blanked or not to blanked respectively. The size of such used grid is e.g. in the particular known embodiment determined by the question whether an accidental, i.e. unwanted fall out of a spot, the likelihood of which is considerable in nowadays multi writing beam systems, would be disturbing to the pattern to be written on to the substrate. Thus, a tendency exists to choose the grid as small as possible. This tendency is fed by the desire of designers to have a virtual infinite choice in designing a line- or object width or at determining a positioning location. The latter would, in accordance with an insight underlying the present invention, mean an additional possibility for correcting proximity effects at writing. On the other hand it is, particularly in massive multi writing beam systems, desired to have the grid as large as possible so as to limit the amount of data to be processed and transferred to the writing tool part of the litho system, and so as to enable the blanker to timely switch for correct writing of a feature, without the need for highly complex and/or relatively expensive blanker structures capable of swiftly switching. As a balance between the above described conditions and as a reflection of the state of contemporary technology, the known litho machine discriminates so-called critical dimension cells, typically e.g. 45 nm, which are written by writing beams having a probe size of corresponding order, e.g. 30 nm, and which are divided into a multiplicity of grid cells, e.g. in 20 by 20 grid cells, thus of a small dimension relative to the probe size, e.g. of 2.25 nm. In such a setup an accidental blanking or not blanking of only one grid cell would only have a minor effect on a deposited dose of electrons, e.g. of only 0.25%, which effect is in practice to be considered negligible. The present invention now deals with the problem of how to even more accurately position pattern features on a surface of a substrate to be patterned, in a raster based lithography system without the above said drawbacks thereof. Apart from the pre-mentioned reasons, particularly in multi beam writing beam systems, accurate positioning of features or edges thereof is extremely important, in particular since different parts of the substrate to be exposed may be patterned by different probes, such as different charged particle beams as in multiple beam systems, and since such would provide another instrument for correcting proximity effects, within the blanking tool, by sub-pixel re-location of features. In other prior art, a widely used method to pattern a substrate with beams also is a raster scan. In order to accurately write the pattern on the substrate, the pattern is rasterized. Each charged particle beam performs its writing operation on a substrate to be patterned, which is positioned on a motor driven stage that is moved in a continuous way. At the same time the beam is scanned in a direction perpendicular to the stage motion. By supplying the writing information to the beam at right times, a pattern is written on a grid, which does not necessarily needs to be a Cartesian grid. A major problem in this art is that a feature can only be positioned within the dimension of a single grid cell. Issuing pattern design rules does not solve this problem, since before exposure, a pattern design needs to be corrected for several resolution-disturbing phenomena, like the proximity effect. These corrections can shift the edges of a feature away from a grid line. A method towards improved accuracy of writing developed in the prior art of multi beam lithography systems is known from U.S. Pat. No. 5,103,101, in which a pattern is written by employing multiple passes. The pattern is first rasterized. After rasterization the pixels are separated in a selectable number of “phases”. Each phase is printed in a separate raster scan. This results in the selected number of raster scans to construct the feature. Since the pitch between pixels is enlarged in two directions each scan can be performed at a higher speed. Thus, in this approach a pattern is partly written during a first exposure. The entire grid is shifted within the dimensions of a single grid cell and then a second part of the pattern is written. In this manner a feature edge can thus be positioned twice as accurate as before. By employing even more passes, a more accurate pattern placement can be obtained. A rather important disadvantage of this approach is the considerable loss of throughput implied by the multiple passes, especially at increased levels of accuracy. Another known technique applying a raster, is known as grey writing, and for instance described in early U.S. Pat. No. 5,393,987. In this approach a relatively small number of grid cells is used. However, the applied dose within each grid cell is varied, e.g. to 0%, 30%, 70% and 100% by varying a duration of illumination. The 30% and 70% pixels are used along the edge of a feature, so as to locate the edge when written between the lines of a Cartesian raster. As a result, the position of a feature can be accurately tuned “without the need for multiple passes”. Moreover, less data is needed to provide the same result. This known technique and system however, goes along with several disadvantages. E.g., the dose levels are created either by partial blanking or by controlling the exposure time. In such a set up, the required control of the discrete steps needs to operate in an extremely accurate way. Especially for high throughput applications such a requirement results in highly difficult designs for the lithography system, and correspondingly high costs thereof. Additionally the yield of such a system may suffer considerably. A single bit error in the pattern control data has a relatively large impact on the relevant exposure in the system, due to the relatively large grid dimensions. As a result a relatively high degree of processed substrates like masks or wafers, runs a risk of requiring to be repaired, or worse, of becoming destroyed, i.e. becoming irreparable. Yet another method and machine designed towards overcoming the limitations of a raster or grid system is referred to by the term virtual addressing, and is for instance disclosed in U.S. Pat. No. 4,498,010. According to this system, in which the dimension of a grid cell is equalised to that of the probe size, an edge of a feature can be positioned halfway between two grid lines by writing additional pixels either before or after the selected feature, thus blanking the beam in alternating probe positions. This method, be it to the detriment of edge smoothness of a written feature, reduces the positioning error of the system, and favourably maintains throughput thereof. However, the system is limited to one particular distance of displacement of an edge, namely half the dimension of a grid cell, which in this system corresponds to half of a probe size, which in practice not only means that extension of positioning locations is limited to a single location halfway a pixel, but also, as the publication indicates, that such a shifted edge will be rather crude in shape. Not being related to a contemporary system with probe sizes significantly larger than pixel size, this known system neither teaches how to realise virtually infinite sub-pixel placement of smooth edges. In a combination of a so-called vector scan writing strategy and a raster scan strategy as disclosed in US2002/0104970, the pattern to be written is rasterized and a group of pixels is combined, thereby forming a cell. Within this cell a finite number of possible pattern configurations is available. Each available configuration is given a shape code. The pattern is subsequently written by flashing each cell at a desired location, while moving from position to position in a raster scan way. The position of the edges can be tuned with respect to the grid of the rasterization by applying the right shape codes. Thus it could be stated that this particular patent publication discloses a form of grey writing using patterned cells. Apart from the fact that only a limited number of cell-configurations is used, it may be evident that such concept is not based on the contribution of single pixels as is dealt with in the lithography system under consideration. The present invention aims at overcoming the restrictions imposed by a rastering method on positioning features on a substrate as in the prior art or, alternatively posed is directed to virtually independently from such raster or grid, positioning the edge of a feature at a desired location. More in particular such is aimed for at least virtually without loss in throughput of the lithography system, with maintenance of a relatively very fine grid structure, while employing a conventional black and white writing strategy, and virtually without reduction in edge smoothness of a feature. With such a probe forming lithography system in which in fact positioning of a feature is no longer, at least hardly limited to the finite size of a grid cell of to the half size thereof, a highly advanced critical dimension control is to be attained, more in particular even in a relatively economic manner, which critical dimension control renders a virtually unhampered placement possibility of edges on a target, virtually independent of an applied grid. The present invention enhances the possibilities of contemporary lithography systems showing grid size significantly smaller than a probe size by virtually even further reducing pixel size through virtual infinite sub-pixel positioning of features and/or edges thereof, without loss of edge smoothness and, where applicable such as in maskless, massive multi beam systems, without the disadvantages associated with further reduction of grid size, i.e. without increase of data to be processed and transferred within the system, and without a need for technically complex and relatively expensive blanking systems as required for swift switching associated with small grid sizes. The above effect is achieved by a system for projecting an image pattern on to a target surface such as a wafer, in which lithography system pattern features are positioned on said target using a virtual grid, in which system the point spread function of a writing beam probe on said target surface is significantly larger than the size of a grid cell in that it covers at least a multiplicity of grid cells, and in which system the edge of a feature to be written is positioned by the measure of modulating at least one set of at least a number of grid cells within said feature. With the present invention it is now possible to position features and/or edges thereof with an unprecedented level of precision and smoothness of such edges. It may be evident that the scope of the invention extends to various kinds of raster based lithography systems. It is further remarked that the invention is directed to writing features of a size larger than that of the grid cell. With such a lithography system of contemporary nature and accordingly following specifications, highly accurate patterning of features is attained without the need for multiple passages, i.e. with maintenance of throughput, with relatively small grid measures, and without a requirement of changing a conventional black- and white writing tool. A particular advantage of the measures according to the invention is the new possibility to control features of a lithography system both in size and position thereof. A specific merit of the present invention is that it enables such refined positioning under the application of a point spread function that is significantly larger than the applied grid size. The latter is according to an insight further underlying the invention, desired, at least preferred for decreasing sensitivity to inaccuracy in line width and line width roughness. Also, with the present invention, costs and technical complexity of a blanking part of the system need not be pushed to undue remote limits. For instance, at a typical contemporary lithography system, leaving out an entire line oriented in a mechanical or stage movement direction Mm rather than in a scanning direction Sd, would now require that the blanker can be switched on and off with a 1 pixel frequency and thus would require a rise/fall time that will in practice not be realized due to associated costs and structural complexity of a blanker complying to such requirement. Thus, departing from a limited, though economic operating frequency of a blanker, desired with a view to said reduction in costs and structural complexity, the pattern to be projected may favourably be adjusted according to a combination of two measures according to the invention, in such a way that an individual blanker of the system does not have to switch faster than such limiting frequency. These measures, which may also be applied separately, in short include addition of written pixels beyond a projected pattern edge and leaving out, i.e. not writing pixels near such border, within the projected boundary of the pattern. In accordance with a further aspect of the present invention however, also a local dose modification may be performed within the range of several features up to large parts of the complete pattern so as to overcome an otherwise present proximity effect. For patterns with a large difference in feature density within them it is possible that different parts of said pattern do not share a common development level, which means that parts of the exposured pattern will not show any features after development. By applying dose modulation between regions with different pattern density, i.e. less dose for dense parts of the pattern, a common development level is thus created by the invention. For the exact positioning of the feature edges this dose modulation is combined with writing or leaving out pixels at the edges or leaving out complete lines parallel to said edge. In fact the invention also can be denoted as a lithography system of the described contemporary nature, which uses at least one of a set of measures for the purpose of shifting the position of a feature or an edge thereof, comprising: writing additional pixels near an edge of a feature at the outside of a feature, in a manner having more than one consecutive on/off pixels between each on/off switch; leaving out, i.e. not writing pixels or entire lines near the edge of said feature within the boundaries thereof; lowering the energy dose within a pattern, for one or more entire features to be written by not writing part of the pixels encompassed by a feature, including the cells adjoining the projected edge there-of; applying a combination of two or all of the three preceding measures; The invention may also be typified in that it provides a solution, in particular a high quality solution, to a problem that could not have been solved solely by instructing designers that features may only be designed within a number of times a grid size. Because of proximity effects generally known in the art, features in the proximity of a structure to be written may shift the edge of a feature by arbitrary distances. Thus, a high need exists to economically and favourably locate edges of a feature at pre-viewed positions, i.e. at least virtually independent from an actually applied grid. In the preceding respect, an insight underlying the present invention provides that the probe size of a beam, in particular an electron beam, is much larger than the grid size generally applied in contemporary lithography systems. Thus, when pixels are added immediately near the edge of a feature, e.g. as according to preference with an interruption of two or more blank grid cells between each pair of added cells, a raggedness of the edge thus created, will at least virtually not be noticeable in the final feature. Yet, the edge of the feature has been shifted in the final pattern towards the outside of the feature by a value of the number of written pixels divided by the modulus at which such number of pixels is added. For instance if line 1 is extended in width to n+1 pixels, line two also, and line three is maintained at a width n, defined by the applied grid, than the actual edge of the relevant feature will actually become located at grid location n+⅔. It is remarked that additional pixels may easily be located at a grid distance corresponding to around half of an applied probe size, without significant effect of raggedness of the finally attained feature shape. However, in this manner a significant subdivision pattern location within the dimension of grid cells, e.g. by a factor of 10 may be attained. In this respect, at a grid size of 2.25 nm, and with nine written lines left without an additionally written cell, a sub division of the edge definition of 0.22 nm is attained, while critical dimension control is contributed with 0.11 nm. It is further remarked that an alternative application and effect of the present measures, in accordance with the invention, is to increase the cell width of the applied raster, so as to reduce the required amount of electronic memory as is generally included in a pattern streamer subsystem. This effect of the present invention may e.g. in particular become interesting at so called massive multibeam lithography systems, e.g. with 10.000 writing beams or more. A second, above mentioned solution according to the invention, and part thereof relates to leaving out, i.e. not to write grid points inside a feature, i.e. written structure. It is in accordance with yet further insight underlying the present invention acknowledged, that the point spread function of a probe is much larger than the address grid, and that positive use can be made of this condition. In particular the invention claims the idea and measure to leave out points inside the structure for the purpose of shifting the edge position of a feature, since the influence of a grid point near an edge, however within a feature spreads out to such edge. Such shift distance depends on how far away from a relevant feature edge grid points are deleted, i.e. not written. The effect of a deleted grid point is that at the edge it locally effectively lowers the dose and thus shifts the edge position of a feature. The method of leaving out illumination, i.e. not writing on grid points inside a feature, alternatively denoted structure, is in accordance with a further aspect of the invention also used as an extra correction for proximity effects by dose correction. It is relatively easy to calculate because it effectively leaves exposure dose away, where neighbouring structures have already deposited dose. Effectively it is the use of grey levels. At e.g. a 20 by 20 grid, a half critical dimension square has 100 grid points, so effectively 100 gray levels plus the freedom to choose which grid points will be left unwritten. According to a further aspect of the present invention, edge positioning can be favourably effected by combining the two in the preceding explained manners of positioning an edge, i.e. by adding pixels or by leaving pixels away, each in a specified manner. In the sub-pixel placement of the features and/or edges thereof, the maximum number of possible steps is according to further insight underlying the invention limited, not only due to the limited range of the probe size, but also to a blanker constraint, i.e. with a view to maintain economically achievable blanking systems of technically limited complexity. This constraint is only set in the fastest scan direction in the case of a rasterscan such as the deflection scan in maskless, massively multi beam systems. This means that for feature edges lying in a direction of mechanical movement of a stage comprising said target to be written on, various types of “ragged edge” can be written, however, not writing of only a single line or part thereof in the same direction inside the border of the relevant feature will in practice not be feasible due to said blanker constraint. Therefore slightly different strategies for sub-pixel location of an edge apply in positioning edges lying in the direction of mechanical movement than for edges predominantly oriented in an electronic scanning direction. From practical use of a lithography apparatus, it has in the field become apparent that the requirements set on a design grid for chips is not wanted. A contribution to the critical dimension control (CDC) and overlay is thus budgeted for this effect. With the mentioned ragged-edge method this is easily possible within less than a tenth of the grid size. It is remarked that in accordance with the scope and aim of the invention, not only edges of a feature are positioned at sub-pixel level of accuracy, but also entire features can be located, dislocated or adapted in size accurately. In practice this means that more flexibility is achieved at favorably patterning, which image amongst others includes that a new manner of preventing line width errors due to proximity effects between features to be located on a target. The invention further encompasses an idea and measure of limiting the overall energy dose for an image to be projected, such that the above-described purpose of placing features and edges thereof is effected. This is according to the invention realized by not writing part of the cells falling within the boundaries of the feature to be written, including arrays of cells directly adjacent an edge of a feature. Preferably but not necessarily such not writing of cells is performed in a predominantly regular pattern within the boundaries of a feature. In this manner features and the edges thereof may not only be located or displaced, but also the dimension, e.g. width of a feature may according to the invention be manipulated. All of the three measures described in the above are according to the invention applied in combination, however may also be applied individually. One often-applied combination is the measure of adding written cells adjacent an edge, and outside a feature, and the measure not writing cells adjacent an edge inside a feature. In the computer programs used for positioning or manipulating features as described above, the entire range of possible combinations of the three measures is made available. In the figures, corresponding structural features, i.e. at least functionally, are referred to by identical reference numbers. FIG. 1 represents an overall side view of one prior art lithography system that can be improved by the current invention. In this system, here described as an example where the present invention may be applied, at light emitter, or modulation means ends 2 of a light carrier Fb, in case embodied by optical fibers Fb, light beams 8 are projected on modulator array 24 using an optical system, represented by lenses 54. Modulated light beams 8 from each optical fiber end are projected on a light sensitive element, i.e. light sensitive part of a modulator of said modulator array 24. In particular, ends of the fibers Fb are projected on the modulator array. Each light beam 8 holds a part of the pattern data for controlling one or more modulators, the modulation thereof forming a signaling system for transferring pattern data based modulator array instructions for realizing a desired image on said target surface. FIG. 1 also shows a beam generator 50, which generates a diverging charged particle beam 51, in this example an electron beam. Using an optical system 52, in case an electron optical system, this beam 51 is shaped into a parallel beam. The parallel beam 51 impinges on aperture plate 53, resulting in a plurality of substantially parallel writing beams 22, directed to modulation array 24, alternatively denoted blanker array. Using modulators in the modulation array 24, comprising electrostatic deflector elements, writing beams 27 are deflected away from the optical axis of the litho system, and writing beams 28 pass the modulators undeflected. Using a beam stop array 25, the deflected writing beams 27 are stopped. The writing beams 28 passing stop array 25 are deflected at deflector array 56 in a first writing direction, and the cross section of each beamlet is reduced using projection lenses 55. During writing, the target surface 49 moves with respect to the rest of the system in a second writing direction. The lithography system furthermore comprises a control unit 60 comprising data storage 61, a read out unit 62 and data converter 63, including a so-called pattern streamer. The control unit 60 is located remote from the rest of the system, for instance outside the inner part of a clean room. Using optical fibers Fb, modulated light beams 8 holding pattern data are transmitted to a projector 54 which projects the ends of the fibers on to the modulation array 24. The subject of the present invention by which e.g. an above kind of lithography system is improved will first be illustrated in general lines, after which the invention will be discussed more in detail. FIGS. 2A and 2B illustrate a feature designed according to grid constraints and a feature designed unhampered hereby, as is desired by users of the system, and as is now made possible the invention. FIG. 2A shows a pattern perfectly aligned with an applied grid. Each grid cell is either fully exposed or not exposed at all. FIG. 2A shows the same pattern, which is now misaligned with the applied grid. The edges of the pattern do not fall on the grid lines corresponding to the writing grid of the exposure apparatus, however still show a virtually identical smoothness. This is the effect that is desired to be attained by the present invention, however which can not be attained by a conventional black and white writing strategy, which is limited to realising feature edges that co-inside with grid lines. By FIG. 3 it is a.o. illustrated that a grid cell is exposed on a target when the center of the writing beamlet is positioned on top of the center of the cell. The diameter of the probe generated at incidence on a desired lithography target is much larger than the cell dimensions. For sake of clarity, the width of a gaussian probe as depicted in FIG. 3A is rather small compared to reality. In reality the probe is generally at least smeared out over a length of about ten grid cells. A full exposure of a certain cell thus also causes a partial exposure, an exposure with less intensity, in the cells adjacent to the exposed cell. So when a number of adjacent grid cells are fully exposed, the number of electrons deposited within an individual grid cell, the dose, constitutes of the sum of the dose received directly from exposure of the cell itself and indirectly via the exposure of adjacent cells. As a result the total dose exceeds the dose per grid cell when it is solely exposed, thus forming a broader and larger structure, as can be taken from the overall, top hat like graph line in FIG. 3A. By choosing the right cut-off dose level however, in FIG. 3A denoted by the dashed line, the desired feature dimensions can be reconstructed. The result of such a reconstruction is shown in FIG. 3B. FIG. 4 shows a first effect of an embodiment of the present invention. In FIG. 4B the desired pattern to be drawn is shown in bold (lines Ee). The top edge of the feature does not align with the grid lines of the rasterized pattern. The bold line Ee drawn through the modulated top array of cells shows the rasterized pattern and edge thereof that is effectively written by the beamlets according to a measure of the present invention. The desired edges that did not align with the grid lines at writing, have a ragged shape. In FIG. 4A, the probe Ss effected by a writing beam is represented within a square raster portion of a size concurring to the typical width for a critical dimension CD of 45 nm. Since the probe size Ss of the writing beamlet is much larger than the size of a single grid cell Ps, alternatively denoted pixel size, the shape of the edge is invisible after developing the exposed pattern. The hatched, i.e. ragged edge in the rasterized pattern effectively shifts the eventual feature edge outwards to its desired position. The possible accuracy of this placement technique depends on the size of the ragged edge pixels length compared to the probe size Ss of the beamlet. By careful selection, an accuracy of less than 1/10 of the grid cell dimensions is obtained. One solution for sub-pixel modulation is to use ragged edges. The sub-pixel placement of the feature edge is achieved by having some pixels on and some off within the outer pixel line of the feature. For example for an outer line with a series 2 on, 3 off, 2 on 3 off the edge in the resist, i.e. the target for writing the pattern on to, will be at ⅖ of a pixel. By choosing the correct ratio on/off the edge of the feature is effectively placed much finer than an applied pixel size Ps of e.g. 2.25 nm. Because the probe size Ss of the e-beam is much larger than the grid size, the raggedness of the line is virtually invisible in the final pattern. The maximum number of possible steps in the sub-pixel placement of the feature edge is according to insight underlying the invention limited, not only to probe size but also due to a blanker constraint, i.e. with a view to maintain economically achievable blanking systems of technically limited complexity. This constraint is however only set in the direction of the deflection scan. In FIG. 6B such impossible modulations are indicated by surrounding circles Np. This can be overcome by also leaving out pixels inside the feature, e.g. as in FIG. 6A. FIG. 5 illustrates a second measure and effect of a lithography system according to a further measure according to the present invention, based on leaving out, i.e. not writing pixels near a rastered edge. FIG. 5B in this respect shows the desired pattern to be written, while FIG. 5A shows the adapted rasterized pattern to achieve the exact placement of the feature. Instead of an adaptation of the edges of the pattern, in this embodiment the content of the grid cells within the feature are altered. The removal of a number of interior grid cells, i.e. in the deflection lines preceding the last deflection line, is for the purpose of positioning the edge of a feature performed adjacent the feature edge in the rasterized pattern, and effectively shifts the edge of the feature inwards as may be taken from the figure. Another method to shift the edge of a feature with sub-pixel resolution is to leave out, i.e. not to write complete lines near such edge, inside the feature. This measure is illustrated in FIG. 7. Because the point spread function PSF of a written probe Ss is much larger than the address grid, the influence of deleting lines or part thereof inside a structure spreads out over the edges. The point spread function PSF is here defined as the inclination angle of the slope of the Gaussian curve that represents the impact of a writing beam on a litho target. At the edge of a feature, such deletion effectively lowers the dose and thus shifts the edge position. The distance over which the edge is shifted depends on how far from the edge the deleted pixel is located and on the number of deleted pixels. Leaving out an entire line in the mechanical movement Mm direction rather than in the scanning direction Sd however, would require that the blanker of the litho system can be switched on and off with 1 pixel frequency and thus would require a very fast rise/fall time of typically within tenths of nanoseconds, which will in practice not be realized due to associated costs and structural complexity of a blanker complying to such requirement. Yet, in principle also edges oriented in the direction of mechanical movement may be shifted e.g. by deleting, i.e. not writing pixels over a width of one or two cells as is illustrated by FIG. 7. In practice according to the invention, a pattern to be exposed will not only be adjusted using each measure separately, but is favourably also used in a combination of the said two, e.g. as represented in FIG. 6A. In the blanking strategy as used in many lithography tools, the exposure of each grid cell is controlled by shielding the writing beamlet at the right moment in time using a blanker. When the operating frequency of a blanker is limited, which often is desired with a view to reducing costs and structural complexity of the blanker, the pattern is adjusted according to a suitable combination of said two measures in such a way that each individual blanker does not have to switch faster than said limiting frequency. In a further particular embodiment an entire deflection line interior to a ragged edge is blanked for the purpose of shifting the position said feature edge. In the examples illustrated by FIGS. 6 and 7, the writing strategy for writing a pattern on to a wafer is a raster scan with a pixels size of 2.25 nm. A constraint for electing grid size 2.25 nm grid is the imposition of a design grid. Two reasons that may affect the elected pixels size are that the size of 2.25 nm is not realistic for chip manufacturers because of a desired design freedom, and secondly, that at a proximity-effect for which a correction is needed with layout biasing, a need for sub-pixel placement arises, a solution to which need is supported by the present invention. An advantage of increasing grid size is the fact that less memory will be required in a so-called pattern streamer sub system (PSS) of a maskless lithography apparatus, which may significantly reduce costs of the apparatus. Also the pixel rate, i.e. the rate at which pixel information is fed to the system may remain relatively low. An advantage of the present invention therefore also resides in increasing grid size while maintaining design freedom by the use of the above explained ragged edge modulation. FIG. 6 illustrates that because of a for economic reasons present blanker constraint, the probe modulation as taken in the direction of mechanical movement Mm of a stage carrying the wafer or target to be processed, is slightly different from the type of modulation applied in the direction of the deflection scan Sd. Encircled positions Np indicate “off” modulations that, depending a specific lay-out of the system, in particular the available rate of blanker switching, could be impossible to perform. The hatched area Dl indicate the area as according to the lay-out data, while Fr denotes the area of the desired feature. FIG. 7 by a hatched area LO illustrates a feature or image layout, while the grey area REQ reflects the required feature after exposure. It shows that with the measures according to the invention, the position of the edges of the written features shifts inwards at leaving out internal lines. Such shift is related to the distance of the left out line from the parallel edge to be shifted. It is understood through the present invention, that for proximity-effect correction (PEC) with layout biasing, i.e. adjusting the feature size based on local background dose level, a finer grid would be required which is not desired for economic reasons. With the ragged edges measure according to the invention, a grid of less than half a tenth of the grid size can be obtained with a maximum error in CD or line width roughness of a nominal value in the same order. In the light of this result, there is a basis to increase the grid size provided in the above example. The advantage is that such reduces the number of pixels and thus also the amount of uncompressed memory in a pattern streamer subsystem (PSS) of a litho machine. In FIG. 8, the results of calculations on on/off combinations for a single line ragged edge modulation is provided for several grid sizes, as expressed in number of pixels fitting a critical dimension (CD) applied in the system. It shows the largest modulation step-size as a result for a range of such grid sizes. It can be seen that from 15 pixels and up per CD, rather than 20 as in the above example, the error in location of parts of an edge positioned with the above described ragged edge method is below 0.1 nm, i.e. is virtually absent, at least fully acceptable, even in respect of the highly refined dimensions of projected features in contemporary lithography. Apart from the concepts and all pertaining details as described in the preceding the invention also relates to all features as defined in the following set of claims as well as to all details as may be directly and unambiguously be derived by one skilled in the art from the above mentioned figures, related to the invention. In the following set of claims, rather than fixating the meaning of a preceding term, any reference numbers corresponding to structures in the figures are for reason of support at reading the claim, included solely as an exemplary meaning of said preceding term. |
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052356272 | description | DETAILED DESCRIPTION OF THE EMBODIMENT FIG. 1 is a schematic block diagram of the first embodiment of the present invention. In the figure, an X-ray diagnostic system of the present invention comprises a catheter table 13 on which a patient P is laid, a first supporting system 15 installed on the floor closed to the top end of the catheter table 13, a second supporting system 17 which can travel over the catheter table 13 along a guide rail 21, a measuring system 25 for measuring elements which causes offsets in an X-ray irradiated field, a compensation controller 23 with a memory 31 for compensating the offsets, and a system controller 27 with a I/O terminal device 29 for controlling all systems. FIG. 2 is a front view of the first supporting system 15. As shown in the figure, the first supporting system comprises a stand 41 installed on the floor, an arm holder 43 rotatably attached to the stand 41, a C-shaped holding arm 45 swingably attached to the arm holder 43. An X-ray tube 51 used as a X-ray source is attached to the end of the C-shaped holding arm 45. An X-ray diaphragm apparatus 53 for limiting an irradiated field is mounted at the front of the X-ray tube 51. An X-ray camera unit 55 is movably attached to the other end of the C-shaped holding arm opposing the X-ray tube, which is provided with an image intensifier 57 for intensifying an X-ray image, TV camera and a film changing unit 59 for radiographing. FIG. 3 is a front view of the second supporting system 17. The second supporting system comprises, as shown in the figure, a vehicle 71 movably suspended from the ceiling, an arm holder 73 rotatably attached to the vehicle 71, and a .OMEGA.-shaped holding arm 75 swingably attached to the arm holder 73. An X-ray tube 77 is attached movably in the vertical direction to an end of the .OMEGA.-shaped holding arm 75. An X-ray diaphragm apparatus 79 is mounted on the front of the X-ray tube 77. A camera unit 81 is attached to the other end of the .OMEGA.-shaped holding arm 75 opposing the X-ray tube 77, freely movable in the vertical direction and in the opposing direction. The camera unit 81 is also provided with an image intensifier 83, TV camera and a film changing unit (not shown in the figure). FIGS. 4A and 4B show examples of the movable wings of the X-ray diaphragm apparatus 53, 79. An X-ray diaphragm apparatus comprises four movable wings 871, 872, 873, 874 and their drive unit (not shown in the figure). Each of the movable wings shown in FIG. 4A can be moved in two directions as shown in the figure so as to form a rectangular aperture 89. These wings can also be rotated together. A wing is made of material such as lead, impervious prevents X-rays. Each of the movable wings 911, 912, 913, 914 shown in FIG. 4B can be moved in two directions as shown in the figure so as to form a substantially circular aperture 93. A pair of wings opposing each other are moved together in an usual operation and each of the wings is moved independently for compensation. The movements of these wings are controlled by the drive unit and control signals are supplied from the system controller 27. An advantage of the former wings is ease of manufacture. An advantage of the latter wings is that the camera units 55, 81 can be used efficiently because the front face of the camera tube of the unit is circular and unwanted X-rays are stopped. The system of the embodiment, preferably includes two types of the X-ray diaphragm above mentioned. FIG. 5 is a total control system of the first embodiment. In the figure, the first and the second supporting systems 15, 17 are driven by drive units 101, 103. These drive units 101, 103, the X-ray tubes 51, 77 the X-ray camera units 55, 81 and the X-ray diaphragm apparatus 53, 79 are sequentially controlled by the system controller according to instruction signals input from the I/O terminal. The X-ray diaphragm is collimated according to a distance SID described below. Image data output from the TV camera units 55, 81 are displayed on the terminal display 107. If necessary, the image data are displayed on the display after being processed by the image processing means 105. The image processing means 105 is also controlled by the system controller 27. The measuring system 25 comprises a first angle measuring device 251, a second angle measuring device 253, an aperture measuring device 255, and a SID measuring device 257. The first angle measuring device 251 is connected to the first and the second supporting systems 15, 17 for measuring angles of rotation angle .theta.1 and .phi.1 of the arm holders 43, 73. The second angle measuring device is connected to the supporting systems 15, 17 for measuring angles of rotation angle .theta.2 and .phi.2 of the arms 45, 75. The aperture measuring device 255 is connected to the X-ray diaphragm apparatus 53, 79 for measuring the positions of the movable wings. The SID measuring device 257 is connected to the arms for measuring the distance between the focus of the X-ray tube and the image intensifier. This distance will be referred to SID hereinafter. The SID is a distance between the focus and a film surface when the film unit is used. The X-ray diaphragm apparatus 53 is compensated according to the data: .theta.1, .theta.2 and its SID, and the diaphragm apparatus 79 according to the data: .phi.1, .phi.2 and its SID. Data items measured by the measuring device 251, 253, 255, 257 are considered to be quantities which affect the offset of the X-ray irradiated field. The data output from these measuring devices is supplied to the compensation controller 23. This compensation controller 23 is connected to the compensation data memory 31 and the system controller 27. The compensation data memory 31 includes a compensating table with which the compensation amounts can be calculated based on the data measured by the measuring devices. FIGS. 6A to 6F show examples of the offsets of the the X-ray irradiated field. In the figures, the solid rectangle is a X-ray irradiated field and the solid circle is the image receiving range of the X-ray camera unit. FIG. 6A shows that the field coincides with the range. FIGS. 6B to 6F show various cases of the offsets. A seeking method of compensation amounts is explained in a case where .theta.1, .theta.2 and SID are given values .theta.1', .theta.2' and SID'. First, the arm and the arm holder are rotated until .theta.1 and .theta.2 equal to zero. The X-ray irradiated field "F0" at this case is displayed on the display unit and the positions P0 of the X-ray diaphragm are memorized. Second, the arm and the arm holder are rotated back to the angles .theta.1' and .theta.2'. Then, the X-ray diaphragm is adjusted watching the display until a X-ray irradiated field "F" coincides with the the field "F0". The compensation amounts at the case of .theta.1', .theta.2' and SID' are given by the difference of the positions between P0 and adjusted positions P. In some case, compensation amounts are calculated upon the interpolation method with data measured already. The compensation amounts are preferably sought at both a cases where TV camera is used and where the film unit is used. A compensation process is explained in the following. First, the case in which the compensating table has enough data for compensation will be described. In this case, the compensation amounts are calculated based on the measured data by the interpolation method or like, utilizing the table. The system controller 27 supplies the compensation instruction signal to the X-ray diaphragm apparatus 53, 79 based upon the data output from the compensating controller 23, and the movable wings of the X-ray diaphragm apparatus are adjusted to eliminate the offsets according to the instruction signal. Second, the case which the compensating table has insufficient data to produce the compensation amount in all cases will be described. In this case, the compensation amounts are sought according the above mentioned seeking method if the compensation table can not be used. The sought compensation amounts are supplied to the system controller and stored in the compensating table. Therefore, the compensating table is gradually completed. If the table is available, the interpolation method is used. FIG. 7 shows the total system of the second embodiment of the invention. In the figure, an X-ray diagnostic system comprises a catheter table 121 swingably mounted on the base, a supporting system 123 attached to the catheter table 121 movable in the lateral and longitudinal directions of the table, a drive unit 125 for driving the table 121 and the supporting system 123, and a system controller 127 for controlling these mechanical devices. The supporting system 123 is formed of a flat lower part which is unbendable and a inverse L-shaped upper part which can be bent. An X-ray tube 131 is attached to the end of the L-shaped upper part. An X-ray camera unit 135 is attached to the flat lower part opposing the X-ray tube under the catheter table 121. An X-ray diaphragm apparatus 137 is mounted on the front of the X-ray tube. When the catheter table 121 is inclined, the support system is inclined together with the table so that offsets of the X-ray irradiated field are changed. A control system for compensating the offsets comprises an angle measuring device 141 for measuring the angle of rotation of the catheter table, an aperture measuring device 143 for measuring the positions of the movable wings of the X-ray diaphragm apparatus 137, a compensation controller 145 with a compensation data memory 147 which computes compensation amounts based on the data from the angle measuring device and the aperture measuring device, utilizing a data table stored in the compensation data memory 147, and a system controller 127 with an I/O terminal device 149. In order to display radiographed pictures, the control system is provided with an image processing unit 151, which is connected to output terminal of the X-ray camera unit 135 and the display of the I/O terminal device 149 under control of the system controller 127. The compensating process for the X-ray diagnostic system of the second embodiment constructed as mentioned above will be as follows. First, the angle of rotation of the catheter table is measured by the angle measuring device 141 and the positions of the movable wings of the X-ray diaphragm apparatus are measured by the aperture measuring device 143. The output data are supplied to the compensation controller 145. Second, the compensation controller 145 calculates the compensation amounts based upon the input data using the table stored in the compensation data memory 147 and sends these amounts to the system controller 127. The system controller 127 sends an instruction signal to the X-ray diaphragm apparatus 137 to adjust the positions of the movable wings according to the amount. As explained above, the present invention makes it possible to minimize the offsets of the X-ray irradiated field caused by flexing of the holding arm and to utilize a larger X-ray diagnostic system. Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof. |
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claims | 1. A method for operating a nuclear power plant including a boiling water reactor, lines flowing to the boiling water reactor, a hydrogen injector for injecting hydrogen into at least one of the lines and an alcohol injector for injecting an alcohol into at least one of the lines, the method comprising: injecting hydrogen from the hydrogen injector into at least one of the lines flowing to the boiling water reactor and injecting the alcohol from the alcohol injector into at least one of the lines flowing to the boiling water reactor during normal operation of the power plant; and compensating for a loss of hydrogen by injecting an amount of alcohol stoichiometrically equivalent to the amount of the loss of hydrogen. 2. The method as recited in claim 1 wherein the alcohol is methanol. 3. The method as recited in claim 1 wherein the alcohol includes at least one selected from a group including methanol, ethanol and propanol. 4. The method as recited in claim 1 wherein the alcohol and hydrogen are injected at a same time. 5. The method as recited in claim 1 further comprising providing a reductive nitrogen compound to the boiling water reactor. 6. The method as recited in claim 1 wherein the alcohol is injected into one of a reactor water cleanup system, an emergency core cooling system, a control rod driving system or a primary loop recirculation system. 7. The method as recited in claim 1 wherein the compensating is controlled by a controller. 8. The method as recited in claim 1 wherein the alcohol injected establishes an alcohol concentration from 0.1 to 300 μmol/kg in a downcomer of the boiling water reactor. 9. The method as recited in claim 8 wherein the alcohol concentration is from 0.1 to 10 μmol/kg in the downcomer of the boiling water reactor. 10. The method as recited in claim 5 wherein the reductive nitrogen compound injected is hydrazine, the hydrazine providing up to a maximum hydrazine concentration of 300 μmol/kg in a downcomer of the boiling water reactor. 11. A method for operating a nuclear power plant including a boiling water reactor, lines flowing to the boiling water reactor, a hydrogen injector for injecting hydrogen into at least one of the lines, an alcohol injector for injecting an alcohol into at least one of the lines and a reductive nitrogen compound injector for injecting a reductive nitrogen compound into at least one of the lines, the method comprising, during shut-down: injecting the reductive nitrogen compound from the reductive nitrogen compound injector into at least one of the lines flowing to the boiling water reactor at an increasing quantity; injecting the hydrogen from the hydrogen injector into at least one of lines flowing to the boiling water reactor at a decreasing quantity; and injecting the alcohol from the alcohol injector into at least one of the lines flowing to the boiling water reactor at a constant quantity; wherein the reductive nitrogen compound, the hydrogen and the alcohol are injected during shut-down such that a reducing environment characterized by an electrochemical potential higher than −300 mV is provided in the boiling water reactor. 12. A method for operating a nuclear power plant including a boiling water reactor, lines flowing to the boiling water reactor, a hydrogen injector for injecting hydrogen into at least one of the lines, an alcohol injector for injecting an alcohol into at least one of the lines and a reductive nitrogen compound injector for injecting a reductive nitrogen compound into at least one of the lines, the method comprising, during a beginning of the start-up: injecting the reductive nitrogen compound from the reductive nitrogen compound injector into at least one of the lines flowing to the boiling water reactor at an increasing quantity; and injecting the alcohol from the alcohol injector into at least one of the lines flowing to the boiling water reactor at an increasing quantity; wherein no hydrogen is provided from the hydrogen injector into any of the lines; wherein the reductive nitrogen compound and the alcohol are injected during the beginning of start-up such that a reducing environment characterized by an electrochemical potential higher than −300 mV is provided in the boiling water reactor. 13. A method for operating a nuclear power plant including a boiling water reactor, lines flowing to the boiling water reactor, a hydrogen injector for injecting hydrogen into at least one of the lines, an alcohol injector for injecting an alcohol into at least one of the lines and a reductive nitrogen compound injector for injecting a reductive nitrogen compound into at least one of the lines, the method, during an end of the start-up, comprising: injecting the reductive nitrogen compound from the reductive nitrogen compound injector into at least one of the lines flowing to the boiling water reactor at an increasing quantity; injecting the alcohol from the alcohol injector into at least one of the lines flowing to the boiling water reactor at a constant quantity; wherein no hydrogen is provided from the hydrogen injector into any of the lines; wherein the reductive nitrogen compound and the alcohol are injected during the end of start-up such that a reducing environment characterized by an electrochemical potential higher than −300 mV is provided in the boiling water reactor. 14. A method for operating a nuclear power plant including a boiling water reactor, lines flowing to the boiling water reactor, a hydrogen injector for injecting hydrogen into at least one of the lines, an alcohol injector for injecting an alcohol into at least one of the lines and a reductive nitrogen compound injector for injecting a reductive nitrogen compound into at least one of the lines, the method, during a beginning of a normal operation of a hydrogen water chemistry or noble metals chemistry addition, comprising: injecting hydrogen from the hydrogen injector into at least one of the lines flowing to the boiling water reactor, injecting a reductive nitrogen compound from the reductive nitrogen compound injector into at least one of the lines flowing to the boiling water reactor and injecting an alcohol from the alcohol injector into at least one of the lines flowing to a boiling water reactor, wherein the alcohol is injected at a constant quantity as the hydrogen is injected at an increasing quantity and as the reductive nitrogen compound is injected at a decreasing quantity; wherein the hydrogen, reductive nitrogen compound and the alcohol are injected during the beginning of the normal operation of the hydrogen water chemistry or noble metals chemistry addition such that a reducing environment characterized by an electrochemical potential higher than −300 mV is provided in the boiling water reactor. 15. The method as recited in claim 1 further comprising:compensating for effects on electrochemical potential during on-line noble metals chemistry addition injection by increasing the alcohol injected. 16. The method as recited in claim 1 further comprising maintaining an electrochemical potential higher than −300 mV. |
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048329010 | abstract | A method for reparing bent mixing vanes in a nuclear reactor fuel assembly is described, including the steps of introducing an apparatus having movable blades below a grid with a mixing vane; opening the blades; withdrawing the apparatus so that a blade abuts and straightens the bent mixing vane; closing the blades; and fully extracting the apparatus from the fuel cell. |
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description | This application is a continuation of U.S. patent application Ser. No. 14/550,421, filed Nov. 21, 2014, which continuation of U.S. patent application Ser. No. 14/310,972, filed Jun. 20, 2014, now granted as U.S. Pat. No. 8,912,514, issued on Dec. 16, 2014, which is a continuation of U.S. patent application Ser. No. 13/830,380, filed Mar. 14, 2013, now granted as U.S. Pat. No. 8,791,440, issued on Jul. 29, 2014, each of which is titled TARGET FOR EXTREME ULTRAVIOLET LIGHT SOURCE, and each of which is incorporated herein by reference in its entirety. The disclosed subject matter relates to a target for an extreme ultraviolet (EUV) light source. Extreme ultraviolet (EUV) light, for example, electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, can be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers. Methods to produce EUV light include, but are not necessarily limited to, converting a material that has an element, for example, xenon, lithium, or tin, with an emission line in the EUV range into a plasma state. In one such method, often termed laser produced plasma (LPP), the plasma can be produced by irradiating a target material, for example, in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam that can be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment. In one general aspect, a method includes releasing an initial target material toward a target location, the target material including a material that emits extreme ultraviolet (EUV) light when converted to plasma; directing a first amplified light beam toward the initial target material, the first amplified light beam having an energy sufficient to form a collection of pieces of target material from the initial target material, each of the pieces being smaller than the initial target material and being spatially distributed throughout a hemisphere shaped volume; and directing a second amplified light beam toward the collection of pieces to convert the pieces of target material to plasma that emits EUV light. Implementations can include one or more of the following features. The EUV light can be emitted from the hemisphere shaped volume in all directions. The EUV light can be emitted from the hemisphere shaped volume isotropically. The initial target material can include a metal, and the collection of pieces can include pieces of the metal. The metal can be tin. The hemisphere shaped volume can define a longitudinal axis along a direction that is parallel to a direction of propagation of the second amplified light beam and a transverse axis along a direction that is transverse to the direction of propagation of the second amplified light beam, and directing the second amplified light beam toward the collection of pieces can include penetrating into the hemisphere shaped volume along the longitudinal axis. The majority of the pieces in the collection of pieces can be converted to plasma. The first amplified light beam can be a pulse of light having a duration of 150 ps and a wavelength of 1 μm. The first amplified light beam can be a pulse of light having a duration of less than 150 ps and a wavelength of 1 μm. The first amplified light beam can include two pulses of light that are temporally separated from each other. The two pulses can include a first pulse of light and a second pulse of light, the first pulse of light having a duration of 1 ns to 10 ns, and the second pulse of light having a duration of less than 1 ns. The first and second amplified light beams can be beams of pulses. The first amplified light beam can have an energy that is insufficient to convert the target material to plasma, and the second amplified light beam have an energy that is sufficient to convert the target material to plasma. A density of the pieces of target material can increase along a direction that is parallel to a direction of propagation of the second amplified light beam. The pieces of target material in the hemisphere shaped volume can have a diameter of 1-10 μm. In another general aspect, a target system for an extreme ultraviolet (EUV) light source includes pieces of a target material distributed throughout a hemisphere shaped volume, the target material including a material that emits EUV light when converted to plasma; and a plane surface adjacent to the hemisphere shaped volume and defining a front boundary of the hemisphere shaped volume, the front boundary being positioned to face a source of an amplified light beam. The hemisphere shaped volume faces away from the source of the amplified light beam. Implementations can include one or more of the following features. The hemisphere shaped volume can have a cross-sectional diameter in a direction that is transverse to a direction of propagation of the amplified light beam, and a maximum of the cross-sectional diameter can be at the plane surface. A density of the pieces of the target material in the hemisphere shaped volume can increase along a direction that is parallel to a direction of propagation of the amplified light beam. At least some of the pieces can be individual pieces that are physically separated from each other. The hemisphere shaped volume can be irradiated with an amplified light beam having sufficient energy to convert the individual pieces of the target material to plasma, and the hemisphere shaped target can emit EUV light in all directions. The target material droplet can be part of a stream of target material droplets that are released from a nozzle, and the target system also can include a second target material droplet that is separate from the target material droplet and released from the nozzle after the target material droplet. The target system also can include the nozzle. The source of the amplified light beam can be an opening in a chamber that receives the target material droplet. In another general aspect, an extreme ultraviolet (EUV) light source includes a first source that produces a pulse of light; a second source that produces an amplified light beam; a target material delivery system; a chamber coupled to the target material delivery system; and a steering system that steers the amplified light beam toward a target location in the chamber that receives a target material droplet from the target material delivery system, the target material droplet including a material that emits EUV light after being converted to plasma. The target material droplet forms a target when struck by the pulse of light, the target including a hemisphere shaped volume having pieces of the target material throughout the volume, and a plane surface positioned between the hemisphere shaped volume and the second source. Implementations can include the following feature. The pulse of light can be 150 ps or less in duration. Implementations of any of the techniques described above may include a method, a process, a target, an assembly for generating a hemisphere shaped target, a device for generating a hemisphere shaped target, a kit or pre-assembled system for retrofitting an existing EUV light source, or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. Referring to FIG. 1A, a perspective view of an exemplary target 5 is shown. The hemisphere shape and gently sloped density profile of the target 5 enables the target 5 to provide additional EUV light, increased conversion efficiency, and EUV light that is radially emitted outward from the target in all directions. The hemisphere shape can be a half of a sphere or any other portion of a sphere. However, the hemisphere shape can take other forms. For example, the hemisphere shape can be a partial oblate or prolate spheroid. The target 5 can be used in a laser produced plasma (LPP) extreme ultraviolet (EUV) light source. The target 5 includes a target material that emits EUV light when in a plasma state. The target material can be a target mixture that includes a target substance and impurities such as non-target particles. The target substance is the substance that is converted to a plasma state that has an emission line in the EUV range. The target substance can be, for example, a droplet of liquid or molten metal, a portion of a liquid stream, solid particles or clusters, solid particles contained within liquid droplets, a foam of target material, or solid particles contained within a portion of a liquid stream. The target substance, can be, for example, water, tin, lithium, xenon, or any material that, when converted to a plasma state, has an emission line in the EUV range. For example, the target substance can be the element tin, which can be used as pure tin (Sn); as a tin compound, for example, SnBr4, SnBr2, SnH4; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. Moreover, in the situation in which there are no impurities, the target material includes only the target substance. The discussion below provides examples in which the target material is a target material droplet made of molten metal. In these examples, the target material is referred to as the target material droplet. However, the target material can take other forms. Irradiating the target material with an amplified light beam of sufficient energy (a “main pulse” or a “main beam”) converts the target material to plasma, thereby causing the target 5 to emit EUV light. FIG. 1B is a side view of the target 5. FIG. 1C is a front cross-sectional view of the target 5 along the line 1C-1C of FIG. 1A. The target 5 is a collection of pieces of target material 20 distributed in a hemisphere shaped volume 10. The target 5 is formed by striking a target material with one or more pulses of radiation (a “pre-pulse”) that precede (in time) the main pulse to transform the target material into a collection of pieces of target material. The pre-pulse is incident on a surface of the target material and the interaction between the initial leading edge of the pre-pulse and the target material can produce a plasma (that does not necessarily emit EUV light) at the surface of the target material. The pre-pulse continues to be incident on the created plasma and is absorbed by the plasma over a period that is similar to the temporal duration of the pre-pulse, about 150 picoseconds (ps). The created plasma expands as time passes. An interaction between the expanding plasma and the remaining portion of the target material can generate a shock wave that can act on the target material non-uniformly, with the center of the target material receiving the brunt of the shock wave. The shock wave can cause the center part of the target material to break into particles that expand in three dimensions. However, because the center part also experiences force in an opposite direction from the expanding plasma, a hemisphere of particles can be formed instead of a sphere. The pieces of target material 20 in the collection can be non-ionized pieces or segments of target material. That is, the pieces of target material 20 are not in a plasma state when the main pulse strikes the target 5. The pieces or segments of target material 20 can be, for example, a mist of nano- or micro-particles, separate pieces or segments of molten metal, or a cloud of atomic vapor. The pieces of target material 20 are bits of material that are distributed in a hemisphere shaped volume, but the pieces of target material 20 are not formed as a single piece that fills the hemisphere shaped volume. There can be voids between the pieces of target material 20. The pieces of target material 20 can also include non-target material, such as impurities, that are not converted to EUV light emitting plasma. The pieces of target material 20 are referred to as the particles 20. Individual particles 20 can be 1-10 μm in diameter. The particles 20 can be separated from each other. Some or all of the particles 20 can have physical contact with another particle. The hemisphere shaped volume 10 has a plane surface 12 that defines a front boundary of the hemisphere shaped volume 10, and a hemisphere shaped portion 14 that extends away from the plane surface in a direction “z.” When used in a EUV light source, a normal 15 of the plane surface 12 faces an oncoming amplified light beam 18 that propagates in the “z” direction. The plane surface 12 can be transverse to direction of propagation of the oncoming amplified light beam 18, as shown in FIGS. 1A and 1B, or the plane surface 12 can be angled relative to the oncoming beam 18. Referring also to FIG. 1D, the particles 20 are distributed in the hemisphere shaped volume 10 with an exemplary density gradient 25 that has a minimum at the plane surface 12 of the target 5. The density gradient 25 is a measure of the density of particles in a unit volume as a function of position within the hemisphere shaped volume 10. The density gradient 25 increases within the target 5 in the direction of propagation (“z”) of the main pulse, and the maximum density is on a side of the target 5 opposite from the side of the plane surface 12. The placement of the minimum density at the plane surface 12 and the gradual increase in the density of the particles 20 results in more of the main pulse being absorbed by the target 5, thereby producing more EUV light and providing a higher conversion efficiency (CE) for a light source that uses the target 5. In effect, this means that enough energy is provided to the target 5 by the main pulse to ionize the target 5 efficiently to produce ionized gas. Having the minimum density at or near the plane surface 12 can increase the absorption of main beam by the target 5 in at least two ways. First, the minimum density of the target 5 is lower than the density of a target that is a continuous piece of target material (such as a target material droplet made of molten tin or a disk shaped target of molten tin). Second, the density gradient 25 places the lowest density portions of the target 5 at the plane surface 12, which is the plane where the amplified light beam 18 enters the target 5. Because the density of the particles 20 increases in the “z” direction, most, or all, of the amplified light beam 18 is absorbed by particles 20 that are closer to the plane surface 12 before the beam 18 reaches and is reflected from a region of high density within the target 5. Therefore, compared to a target that has a region of high density closer to the point of impact with the amplified light beam 18, the target 5 absorbs a higher portion of the energy in the amplified light beam 18. The absorbed light beam 18 is used to convert the particles 20 to plasma by ionization. Thus, the density gradient 25 also enables more EUV light to be generated. Second, the target 5 presents a larger area or volume of particles to the main pulse, enabling increased interaction between the particles 20 and the main pulse. Referring to FIGS. 1B and 1C, the target 5 defines a length 30 and a cross-section width 32. The length 30 is the distance in the “z” direction along which the hemisphere portion 14 extends. The length 30 is longer than a similar length in a target that is a continuous piece of target material because the hemisphere shaped volume 10 has a longer extent in the “z” direction. A continuous piece of target material is one that has a uniform, or nearly uniform, density in the direction of propagation of the amplified light beam 18. Additionally, because of the gradient 25, the amplified light beam 18 propagates further into the target 5 in the “z” direction while reflections are kept low. The relatively longer length 30 provides a longer plasma scale length. The plasma scale length for the target 5 can be, for example, 200 μm, which can be twice the value of the plasma scale length for a disk shaped target made from a continuous piece of target material. A longer plasma scale length allows more of the amplified light beam 18 to be absorbed by the target 5. The cross-section width 32 is the width of the plane surface 12 of the target 5. The cross-section interaction width 32 can be, for example, about 200 μm, when the target 5 is generated with a pre-pulse that occurs 1000 ns prior to the main pulse, and the pre-pulse has a duration of 150 ps and a wavelength of 1 μm. The cross-section interaction width 32 can be about 300 μm when the target 5 is generated with a 50 ns duration CO2 laser pulse. A pulse of light or radiation has a temporal duration for an amount of time during which a single pulse has an intensity of 50% or more of the maximum intensity of the pulse. This duration can also be referred to as the full width at half maximum (FWHM). Like the length 30, the cross-section width 32 is larger than a similar dimension in a target that is made of a continuous, coalesced piece of target material (such as a target material droplet made of coalesced molten metal). Because both the interaction length 30 and the interaction width 32 are relatively larger than other targets, the target 5 also has a larger EUV light emitting volume. The light emitting volume is the volume in which the particles 20 are distributed and can be irradiated by the amplified light beam 18. For example, the target 5 can have a light emitting volume that is twice that of a disk shaped target of molten metal. The larger light emitting volume of the target 5 results in generation of greater amounts of EUV light and a higher conversion efficiency (CE) because a higher portion of the target material (the particles 20) in the target 5 is presented to and irradiated by the amplified light beam 18 and subsequently converted to plasma. Further, the target 5 does not have a wall or high density region at a back side 4 that could prevent EUV light from being emitted in the direction of propagation of the main pulse. Thus, the target 5 emits EUV radially outward in all directions, allowing more EUV light to be collected and further increasing the collection efficiency. Moreover, radially isotropic EUV light or substantially isotropic EUV light can provide improved performance for a lithography tool (not shown) that uses the EUV light emitted from the target 5 by reducing the amount of calibration needed for the tool. For example, if uncorrected, unexpected spatial variations in EUV intensity can cause overexposure to a wafer imaged by the lithography tool. The target 5 can minimize such calibration concerns by emitting EUV light uniformly in all directions. Moreover, because the EUV light is radially uniform, errors in alignment and fluctuations in alignment within the lithography tool or upstream from the lithography tool do not also cause variations in intensity. FIGS. 2A, 2B, and 3A-3C show exemplary LPP EUV light sources in which the target 5 can be used. Referring to FIG. 2A, an LPP EUV light source 100 is formed by irradiating a target mixture 114 at a target location 105 with an amplified light beam 110 that travels along a beam path toward the target mixture 114. The target location 105, which is also referred to as the irradiation site, is within an interior 107 of a vacuum chamber 130. When the amplified light beam 110 strikes the target mixture 114, a target material within the target mixture 114 is converted into a plasma state that has an element with an emission line in the EUV range to produce EUV light 106. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture 114. These characteristics can include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma. The light source 100 also includes a target material delivery system 125 that delivers, controls, and directs the target mixture 114 in the form of liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. The target mixture 114 can also include impurities such as non-target particles. The target mixture 114 is delivered by the target material delivery system 125 into the interior 107 of the chamber 130 and to the target location 105. The light source 100 includes a drive laser system 115 that produces the amplified light beam 110 due to a population inversion within the gain medium or mediums of the laser system 115. The light source 100 includes a beam delivery system between the laser system 115 and the target location 105, the beam delivery system including a beam transport system 120 and a focus assembly 122. The beam transport system 120 receives the amplified light beam 110 from the laser system 115, and steers and modifies the amplified light beam 110 as needed and outputs the amplified light beam 110 to the focus assembly 122. The focus assembly 122 receives the amplified light beam 110 and focuses the beam 110 to the target location 105. In some implementations, the laser system 115 can include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre-pulses. Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the laser system 115 produces an amplified light beam 110 due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system 115 can produce an amplified light beam 110 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system 115. The term “amplified light beam” encompasses one or more of: light from the laser system 115 that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system 115 that is amplified (externally or within a gain medium in the oscillator) and is also a coherent laser oscillation. The optical amplifiers in the laser system 115 can include as a gain medium a filling gas that includes CO2 and can amplify light at a wavelength of between about 9100 and about 11000 nm, and in particular, at about 10.6 μm, at a gain greater than or equal to 1000. In some examples, the optical amplifiers amplify light at a wavelength of 10.59 μm. Suitable amplifiers and lasers for use in the laser system 115 can include a pulsed laser device, for example, a pulsed, gas-discharge CO2 laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, 10 kW or higher and high pulse repetition rate, for example, 50 kHz or more. The optical amplifiers in the laser system 115 can also include a cooling system such as water that can be used when operating the laser system 115 at higher powers. FIG. 2B shows a block diagram of an example drive laser system 180. The drive laser system 180 can be used as the drive laser system 115 in the source 100. The drive laser system 180 includes three power amplifiers 181, 182, and 183. Any or all of the power amplifiers 181, 182, and 183 can include internal optical elements (not shown). The power amplifiers 181, 182, and 183 each include a gain medium in which amplification occurs when pumped with an external electrical or optical source. Light 184 exits from the power amplifier 181 through an output window 185 and is reflected off a curved mirror 186. After reflection, the light 184 passes through a spatial filter 187, is reflected off of a curved mirror 188, and enters the power amplifier 182 through an input window 189. The light 184 is amplified in the power amplifier 182 and redirected out of the power amplifier 182 through an output window 190 as light 191. The light 191 is directed toward the amplifier 183 with fold mirrors 192 and enters the amplifier 183 through an input window 193. The amplifier 183 amplifies the light 191 and directs the light 191 out of the amplifier 183 through an output window 194 as an output beam 195. A fold mirror 196 directs the output beam 195 upwards (out of the page) and toward the beam transport system 120. The spatial filter 187 defines an aperture 197, which can be, for example, a circular opening through which the light 184 passes. The curved mirrors 186 and 188 can be, for example, off-axis parabola mirrors with focal lengths of about 1.7 m and 2.3 m, respectively. The spatial filter 187 can be positioned such that the aperture 197 coincides with a focal point of the drive laser system 180. The example of FIG. 2B shows three power amplifiers. However, more or fewer power amplifiers can be used. Referring again to FIG. 2A, the light source 100 includes a collector mirror 135 having an aperture 140 to allow the amplified light beam 110 to pass through and reach the target location 105. The collector mirror 135 can be, for example, an ellipsoidal mirror that has a primary focus at the target location 105 and a secondary focus at an intermediate location 145 (also called an intermediate focus) where the EUV light can be output from the light source 100 and can be input to, for example, an integrated circuit beam positioning system tool (not shown). The light source 100 can also include an open-ended, hollow conical shroud 150 (for example, a gas cone) that tapers toward the target location 105 from the collector mirror 135 to reduce the amount of plasma-generated debris that enters the focus assembly 122 and/or the beam transport system 120 while allowing the amplified light beam 110 to reach the target location 105. For this purpose, a gas flow can be provided in the shroud that is directed toward the target location 105. The light source 100 can also include a master controller 155 that is connected to a droplet position detection feedback system 156, a laser control system 157, and a beam control system 158. The light source 100 can include one or more target or droplet imagers 160 that provide an output indicative of the position of a droplet, for example, relative to the target location 105 and provide this output to the droplet position detection feedback system 156, which can, for example, compute a droplet position and trajectory from which a droplet position error can be computed either on a droplet by droplet basis or on average. The droplet position detection feedback system 156 thus provides the droplet position error as an input to the master controller 155. The master controller 155 can therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system 157 that can be used, for example, to control the laser timing circuit and/or to the beam control system 158 to control an amplified light beam position and shaping of the beam transport system 120 to change the location and/or focal power of the beam focal spot within the chamber 130. The target material delivery system 125 includes a target material delivery control system 126 that is operable in response to a signal from the master controller 155, for example, to modify the release point of the droplets as released by a target material supply apparatus 127 to correct for errors in the droplets arriving at the desired target location 105. Additionally, the light source 100 can include a light source detector 165 that measures one or more EUV light parameters, including but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside of a particular band of wavelengths, and angular distribution of EUV intensity and/or average power. The light source detector 165 generates a feedback signal for use by the master controller 155. The feedback signal can be, for example, indicative of the errors in parameters such as the timing and focus of the laser pulses to properly intercept the droplets in the right place and time for effective and efficient EUV light production. The light source 100 can also include a guide laser 175 that can be used to align various sections of the light source 100 or to assist in steering the amplified light beam 110 to the target location 105. In connection with the guide laser 175, the light source 100 includes a metrology system 124 that is placed within the focus assembly 122 to sample a portion of light from the guide laser 175 and the amplified light beam 110. In other implementations, the metrology system 124 is placed within the beam transport system 120. The metrology system 124 can include an optical element that samples or re-directs a subset of the light, such optical element being made out of any material that can withstand the powers of the guide laser beam and the amplified light beam 110. A beam analysis system is formed from the metrology system 124 and the master controller 155 since the master controller 155 analyzes the sampled light from the guide laser 175 and uses this information to adjust components within the focus assembly 122 through the beam control system 158. Thus, in summary, the light source 100 produces an amplified light beam 110 that is directed along the beam path to irradiate the target mixture 114 at the target location 105 to convert the target material within the mixture 114 into plasma that emits light in the EUV range. The amplified light beam 110 operates at a particular wavelength (that is also referred to as a source wavelength) that is determined based on the design and properties of the laser system 115. Additionally, the amplified light beam 110 can be a laser beam when the target material provides enough feedback back into the laser system 115 to produce coherent laser light or if the drive laser system 115 includes suitable optical feedback to form a laser cavity. Referring to FIG. 3A, a top plan view of an exemplary optical imaging system 300 is shown. The optical imaging system 300 includes an LPP EUV light source 305 that provides EUV light to a lithography tool 310. The light source 305 can be similar to, and/or include some or all of the components of, the light source 100 of FIGS. 2A and 2B. As discussed below, the target 5 can be used in the light source 305 to increase the amount of light emitted by the light source 305. The light source 305 includes a drive laser system 315, an optical element 322, a pre-pulse source 324, a focusing assembly 326, a vacuum chamber 340, and an EUV collecting optic 346. The EUV collecting optic 346 directs the EUV light emitted by converting the target 5 to plasma to the lithography tool 310. The EUV collection optic 346 can be the mirror 135 (FIG. 2A). Referring also to FIGS. 3B-3E, the light source 305 also includes a target material delivery apparatus 347 that produces a stream of target material 348. The stream of target material 348 can include target material in any form, such as liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. In the discussion below, the target material stream 348 includes target material droplets 348. In other examples, the target material stream can include target material of other forms. The target material droplets travel along the “x” direction from the target material delivery apparatus 347 to a target location 342 in the vacuum chamber 340. The drive laser system 315 produces an amplified light beam 316. The amplified light beam 316 can be similar to the amplified light beam 18 of FIGS. 1A-1C, or the amplified light beam 110 of FIGS. 2A and 2B, and can be referred to as a main pulse or a main beam. The amplified light beam 316 has an energy sufficient to convert the particles 20 in the target 5 into plasma that emits EUV light. In some implementations, the drive laser system 315 can be a dual-stage master oscillator and power amplifier (MOPA) system that uses carbon dioxide (CO2) amplifiers within the master oscillator and power amplifier, and the amplified light beam 316 can be a 130 ns duration, 10.6 μm wavelength CO2 laser light pulse generated by the MOPA. In other implementations, the amplified light beam 316 can be a CO2 laser light pulse that has a duration of less than 50 ns. The pre-pulse source 324 emits a pulse of radiation 317. The pre-pulse source 324 can be, for example, a Q-switched Nd:YAG laser, and the pulse of radiation 317 can be a pulse from the Nd.YAG laser. The pulse of radiation 317 can have a duration of 10 ns and a wavelength of 1.06 μm, for example. In the example shown in FIG. 3A, the drive laser system 315 and the pre-pulse source 324 are separate sources. In other implementations, they can be a part of the same source. For example, both the pulse of radiation 317 and the amplified light beam 316 can be generated by the drive laser system 315. In such an implementation, the drive laser system 315 can include two CO2 seed laser subsystems and one amplifier. One of the seed laser subsystems can produce an amplified light beam having a wavelength of 10.26 μm, and the other seed laser subsystem can produce an amplified light beam having a wavelength of 10.59 μm. These two wavelengths can come from different lines of the CO2 laser. Both amplified light beams from the two seed laser subsystems are amplified in the same power amplifier chain and then angularly dispersed to reach different locations within the chamber 340. In one example, the amplified light beam with the wavelength of 10.26 μm is used as the pre-pulse 317, and the amplified light beam with the wavelength of 10.59 μm is used as the amplified light beam 316. In other examples, other lines of the CO2 laser, which can generate different wavelengths, can be used to generate the two amplified light beams (one of which is the pulse of radiation 317 and the other of which is the amplified light beam 316). Referring again to FIG. 3A, the optical element 322 directs the amplified light beam 316 and the pulse of radiation 317 from the pre-pulse source 324 to the chamber 340. The optical element 322 is any element that can direct the amplified light beam 316 and the pulse of radiation 317 along similar paths and deliver the amplified light beam 316 and the pulse of radiation 317 to the chamber 340. In the example shown in FIG. 3A, the optical element 322 is a dichroic beamsplitter that receives the amplified light beam 316 and reflects it toward the chamber 340. The optical element 322 receives the pulse of radiation 317 and transmits the pulses toward the chamber 340. The dichroic beamsplitter has a coating that reflects the wavelength(s) of the amplified light beam 316 and transmits the wavelength(s) of the pulse of radiation 317. The dichroic beamsplitter can be made of, for example, diamond. In other implementations, the optical element 322 is a mirror that defines an aperture (not shown). In this implementation, the amplified light beam 316 is reflected from the mirror surface and directed toward the chamber 340, and the pulses of radiation pass through the aperture and propagate toward the chamber 340. In still other implementations, a wedge-shaped optic (for example, a prism) can be used to separate the main pulse 316, the pre-pulse 317, and the pre-pulse 318 into different angles, according to their wavelengths. The wedge-shaped optic can be used in addition to the optical element 322, or it can be used as the optical element 322. The wedge-shaped optic can be positioned just upstream (in the “−z” direction) of the focusing assembly 326. Additionally, the pulse of radiation 317 can be delivered to the chamber 340 in other ways. For example, the pulse 317 can travel through optical fibers that deliver the pulses 317 and 318 to the chamber 340 and/or the focusing assembly 326 without the use of the optical element 322 or other directing elements. In these implementations, the fiber can bring the pulse of radiation 317 directly to an interior of the chamber 340 through an opening formed in a wall of the chamber 340. Regardless of how the amplified light beam 316 and the pulses of radiation 317 and 318 are directed toward the chamber 340, the amplified light beam 316 is directed to a target location 342 in the chamber 340. The pulse of radiation 317 is directed to a location 341. The location 341 is displaced from the target location 342 in the “−x” direction. The amplified light beam 316 from the drive laser system 315 is reflected by the optical element 322 and propagates through the focusing assembly 326. The focusing assembly 326 focuses the amplified light beam 316 onto the target location 342. The pulse of radiation 317 from the pre-pulse source 324 passes through the optical element 322 and through the focusing assembly 216 to the chamber 340. The pulse of radiation 317 propagates to the location 341 in the chamber 340 that is in the “−x” direction relative to the target location 342. The displacement between the location 342 and the location 341 allows the pulse of radiation 317 to irradiate a target material droplet to convert the droplet to the hemisphere shaped target 5 before the target 5 reaches the target location 342 without substantially ionizing the target 5. In this manner, the hemisphere shaped target 5 can be a pre-formed target that is formed at a time before the target 5 enters the target location 342. In greater detail and referring also to FIGS. 3B and 3C, the target location 342 is a location inside of the chamber 340 that receives the amplified light beam 316 and a droplet in the stream of target material droplets 348. The target location 342 is also a location that is positioned to maximize an amount of EUV light delivered to the EUV collecting optic 346. For example, the target location 342 can be at a focal point of the EUV collecting optic 346. FIGS. 3B and 3C show top views of the chamber 340 at times t1 and t2, respectively, with time=t1 occurring before time=t2. In the example shown in FIGS. 3B and 3C, the amplified light beam 316 and the pulsed beam of radiation 317 occur at different times and are directed toward different locations within the chamber 340. The stream 348 travels in the “x” direction from the target material supply apparatus 347 to the target location 342. The stream of target material droplets 348 includes the target material droplets 348a, 348b, and 348c. At a time=t1 (FIG. 3B), the target material droplets 348a and 348b travel in the “x” direction from the target material supply apparatus 347 to the target location 342. The pulsed beam of radiation 317 irradiates the target material droplet 348a at the time “t1” at the location 341, which is displaced in the “−x” direction from the target location 342. The pulsed beam of radiation 317 transforms the target material droplet 348b into the hemisphere target 5. At the time=t2 (FIG. 3C), the amplified light beam 316 irradiates the target 5 and converts the particles 20 of target material into EUV light. Referring to FIG. 4, an exemplary process 400 for generating the hemisphere shaped target 5 is shown. The process 400 can be performed using the target material supply apparatus 127 (FIG. 2A) or the target material supply apparatus 347 (FIGS. 3B-3E). An initial target material is released toward a target location (410). Referring also to FIGS. 3B and 3C, the target material droplet 348a is released from the target material supply apparatus 347 and travels toward the target location 342. The initial target material is a target material droplet that emerges or is released from the target material supply apparatus 347 as a droplet. The initial target material droplet is a droplet that has not been transformed or altered by a pre-pulse. The initial target material droplet can be a coalesced sphere or substantially spherical piece of molten metal that can be considered as a continuous piece of target material. The target material droplet 348a prior to the time “t1” is an example of an initial target material in this example. A first amplified light beam is directed toward the initial target material to generate a collection of pieces of target material distributed in a hemisphere shaped volume (420) without substantially ionizing the initial target material. The collection of pieces of target material can be the particles 20 (FIGS. 1A-1C), which are distributed in the hemisphere shaped volume 10. The first amplified light beam can be the pulsed light beam 317 emitted from the source 324 (FIGS. 3A, 3D, and 3E). The first amplified light beam can be referred to as the “pre-pulse.” The first amplified light beam is a pulse of light that has an energy and/or pulse duration sufficient to transform the target material droplet 348a from a droplet that is a continuous or coalesced segment or piece of molten target material into the target 5, which is a hemisphere shaped distribution of particles 20. The first amplified light beam can be, for example, a pulse of light that has a duration of 130 ns and a wavelength of 1 μm. In another example, first amplified light beam can be a pulse of light that has a duration of 150 ps, a wavelength of 1 μm, an energy of 10 milliJoules (mJ), a 60 μm focal spot, and an intensity of 2×1012 W/cm2. The energy, wavelength, and/or duration of the first amplified light beam are selected to transform the target material droplet into the hemisphere shaped target 5. In some implementations, the first amplified light beam includes more than one pulse. For example, the first amplified light beam can include two pulses, separated from each other in time, and having different energies and durations. FIG. 9 shows an example in which the first amplified light beam includes more than one pulse. Further, the first amplified light beam can be a single pulse that has a shape (energy or intensity as a function of time) to provide an effect that is similar to that achieved by multiple pre-pulses. The second amplified light beam has energy sufficient to convert the target material droplet into a collection of pieces. A second amplified light beam is directed toward the collection of pieces to convert the particles 20 to plasma that emits EUV light (430). The second amplified light beam can be referred to as the “main pulse.” The amplified light beam 316 of FIG. 3A is an example of a second amplified light beam. The amplified light beam 316 has sufficient energy to convert all or most of the particles 20 of the target 5 into plasma that emits EUV light. Referring to FIG. 5, an example of a waveform 500 that can be used to transform a target material droplet into a hemisphere shaped target is shown. FIG. 5 shows the amplitude of the waveform 500 as a function of time. The waveform 500 shows a representation of the collection of amplified light beams that strike a particular target material droplet in a single cycle of operation of the EUV light source. A cycle of operation is a cycle that emits a pulse or burst of EUV light. The waveform 500 also can be referred to as a laser train 500 or a pulse train 500. In the waveform 500, the collection of amplified light beams includes a pre-pulse 502 and a main pulse 504. The pre-pulse 502 begins at time t=0, and the main pulse 504 begins at a time t=1000 ns. In other words, the main pulse 504 occurs 1000 ns after the pre-pulse 502. In the waveform 500, the pre-pulse 502 can have a wavelength of 1.0 μm, a duration of 150 ps, an energy of 10 mJ, a focal spot 60 μm in diameter, and an intensity of 2×1012 W/cm2. This is an example of one implementation of the waveform 500. Other parameter values can be used, and the parameter values of the pre-pulse 502 can vary by a factor of 5 as compared to this example. For example, in some implementations, the pre-pulse 502 can have a duration of 5-20 ps, and an energy of 1-20 mJ. The main pulse 504 can have a wavelength of 5-11 μm, a pulse duration of 15-200 ns, a focus spot size of 50-300 μm, and an intensity of 3×109 to 8×1010 W/cm2. For example, the main pulse 504 can have a wavelength of 10.59 μm and a pulse duration of 130 ns. In another example, the main pulse can have a wavelength of 10.59 μm and a pulse duration of 50 ns or less. In addition to the times t=0 and t=1000 ns, the times t1-t4 are also shown on the time axis. The time t1 is shortly before the pre-pulse 502 occurs. The time t2 is after the pre-pulse 502 ends and before the main pulse 504 begins. The time t3 occurs shortly before the main pulse 504, and the time t4 occurs after the main pulse 504. The times t1-t4 are used in the discussion below, with respect to FIGS. 6A-6D, of a transformation of a target material droplet to a hemisphere shaped target using the waveform 500. Although the waveform 500 is shown as a continuous waveform in time, the pre-pulse 502 and main pulse 504 that make up the waveform 500 can be generated by different sources. For example, the pre-pulse 502 can be a pulse of light generated by the pre-pulse source 324, and the main pulse 504 can be generated by the drive laser system 315. When the pre-pulse 502 and the main pulse 504 are generated by separate sources that are in different locations relative to the chamber 340 (FIG. 3A), the pre-pulse 502 and the main pulse 504 can be directed to the chamber 340 with the optical element 322. Referring also to FIGS. 6A-6D, interactions between a target material droplet 610 and the waveform 500 that transform the target material droplet 610 into a hemisphere shaped target 614 are shown. A target supply apparatus 620 releases a stream of target material droplets 622 from an orifice 624. The target material droplets 622 travel in the “x” direction toward a target location 626. FIGS. 6A-6D show the target supply apparatus 620 and the droplet stream 622 at the times t=t1, t=t2, t=t3, and t=t4, respectively. FIG. 5 also shows the times t=t1 through t=t4 relative to the waveform 500. Referring to FIG. 6A, the pre-pulse 502 approaches the target material droplet 610. The target material droplet 610 is a droplet of target material. The target material can be molten metal, such as molten tin. The target material droplet 610 is a continuous segment or piece of target material that has a uniform density in the “z” direction (the direction of propagation of the waveform 500). The cross-sectional size of a target material droplet can be, for example, between 20-40 μm. FIG. 7A shows the density of the target material droplet 610 as a function of position along the “z” direction. As shown in FIG. 7A, compared to free space, the target material droplet 610 presents a steep increase in density to the waveform 500. The interaction between the pre-pulse 502 and the target material droplet 610 forms a collection of pieces of target material 612 that are arranged in a geometric distribution. The pieces of target material 612 are distributed in a hemisphere shaped volume that extends outward from a plane surface 613 in the “x” and “z” direction. The pieces of target material 612 can be a mist of nano- or micro-particles, separate pieces of molten metal, or a cloud of atomic vapor. The pieces of target material can be 1-10 μm in diameter. A purpose of the interaction between the pre-pulse 502 and the target material droplet 610 is to form a target that has a spatial extent that is larger than the diameter of the main pulse 504 but without substantially ionizing the target. In this manner, as compared to a smaller target, the created target presents more target material to the main beam and can use more of the energy in the main pulse 504. The pieces of target material 612 have a spatial extent in the x-y and x-z planes that is larger than the extent of the target material droplet 610 in the x-y and x-z planes. As time passes, the collection of pieces 612 travels in the “x” direction toward the target location 626. The collection of pieces 612 also expands in the “x” and “z” directions while moving toward the target location 626. The amount of spatial expansion depends on the duration and intensity of the pre-pulse 502, as well as the amount of time over which the collection of pieces 612 is allowed to expand. The density of the collection of pieces 612 decreases as time passes, because the pieces spread out. A lower density generally allows an oncoming light beam to be absorbed by more of the material in a volume, and a high density can prevent or reduce the amount of light absorbed and the amount of EUV light produced. A wall of high density through which light cannot pass or be absorbed and is instead reflected is the “critical density.” However, the most efficient absorption by a material can occur near but below the critical density. Thus, it can be beneficial to for the target 614 to be formed by allowing the collection of pieces 614 to expand over a finite time period that is long enough to allow the collection of pieces 613 to expand spatially without being so long that the density of the pieces decreases to a point where the efficiency of laser absorption decreases. The finite time period can be the time between the pre-pulse 502 and the main pulse 504 and can be, for example, about 1000 ns. Referring also to FIGS. 8A and 8B, examples of the spatial expansion of the collection of pieces 612 as a function of time after the pre-pulse strikes a target material droplet for two different pre-pulses are shown, with FIG. 8A showing an example for a pre-pulse similar to the pre-pulse 502. The time after the pre-pulse strikes a target material droplet can be referred to as the delay time. FIG. 8A shows the size of the collection of pieces 612 as a function of delay time when the pre-pulse has a wavelength of 1.0 μm, a duration of 150 ps, an energy of 10 mJ, a focal spot 60 μm in diameter, and an intensity of 2×1012 W/cm2. FIG. 8B shows the size of the collection of pieces 612 as a function of delay time when the pre-pulse has a wavelength of 1.0 μm, a duration of 150 ps, an energy of 5 mJ, a focal spot 60 μm in diameter, and an intensity of 1×1012 W/cm2. Comparing FIG. 8A to FIG. 8B shows that the collection of pieces 612 expands more rapidly in the vertical directions (x/y) when struck by the more energetic and more intense pre-pulse of FIG. 8A. Referring again to, FIG. 6C the target material droplet 610 and the stream of droplets 622 are shown at the time=t3. At the time=t3, the collection of target material pieces 612 has expanded into the hemisphere shaped target 614 and arrives at the target location 626. The mail pulse 504 approaches the hemisphere shaped target 614. FIG. 7B shows the density of the hemisphere shaped target 614 just before the main pulse 504 reaches the target 614. The density is expressed as density gradient 705 that is density of particles 612 in the target 614 a function of position in the “z” direction, with z=0 being the plane surface 613. As shown, the density is minimum at the plane surface 613 and increases in the “z” direction. Because the density is at a minimum at the plane surface 613, and the minimum density is lower than that of the target material droplet 610, compared to the target material droplet 610, the main pulse 504 enters the target 614 relatively easily (less of the main pulse 504 is absorbed). As the main beam 504 travels in the target 614, the particles 612 absorb the energy in the main beam 504 and are converted to plasma that emits EUV light. The density of the target 614 increases in the direction of propagation “z” and can increase to an amount where the main beam 504 cannot penetrate and is instead reflected. The location in the target 614 with such a density is the critical surface (not shown). However, because the density of the target 614 is initially relatively low, a majority, most, or all, of the main beam 504 is absorbed by the particles 615 prior to reaching the critical surface. Thus, the density gradient provides a target that is favorable for EUV light generation. Additionally, because the hemisphere shaped target 614 does not have a wall of high density, the EUV light 618 is radially emitted from the target 614 in all directions. This is unlike a disk shaped target or other target with a higher density, where the interaction between the main pulse and the target generates plasma and a shock wave that blows off some of the target as dense target material in the direction of propagation of the main pulse 504. The blown off material reduces the amount of material available for conversion to plasma and also absorbs some of the EUV light emitted in the forward (“z”) direction. As a result, the EUV light is emitted over 2π steradians, and only half of the EUV light is available for collection. However, the hemisphere shaped target 614 allows collection of EUV light in all directions (4π steradians). After the main pulse 504 irradiates the hemisphere shaped target 614, there is negligible or no dense target material left in the hemisphere shaped target 614, and the EUV light 618 is able to escape the hemisphere shaped target 614 radially in all directions. In effect, there is very little matter present to block or absorb the EUV light 618 and prevent it from escaping. In some implementations, the EUV light 618 can be isotropic (uniform intensity) in all directions. Thus, the hemisphere shaped target 614 provides additional EUV light by allowing EUV light 619, which is generated in the forward direction, to escape the hemisphere shaped target 614. Because the hemisphere shaped target 614 emits EUV light in all directions, a light source that uses the hemisphere shaped target 614 can have increased conversion efficiency (CE) as compared to a light source that uses a target that emits light over only 2π steradians. For example, when measured over 2π steradians, a hemisphere shaped target that is irradiated with a MOPA CO2 main pulse having a duration of 130 ns can have a conversion efficiency of 3.2%, meaning that 3.2% of the CO2 main pulse is converted into EUV light. When the hemisphere shaped target is irradiated with a MOPA CO2 main pulse having a duration of 50 ns, the conversion efficiency is 5% based on measuring the EUV light emitted over 2π steradians. When the EUV light is measured over 4π steradians, the conversion efficiency is doubled because the amount of EUV light emitted from the target is doubled. Thus, the conversion efficiency for the two main pulses becomes 6.4% and 10%, respectively. In the example of FIGS. 6A-6D, the waveform 500, which has a delay time of 1000 ns between the pre-pulse 502 and the main pulse 504, is used to transform the target material droplet 610 into the hemisphere shaped target 614. However, other waveforms with other delay times can be used for the transformation. For example, the delay between the pre-pulse 502 and the main pulse 504 can be between 200 ns and 1600 ns. A longer delay time provides a target with a larger spatial extent (volume) and a lower density of target material. A shorter delay time provides a target with a smaller spatial extent (volume) and a higher density of target material. FIG. 9 shows another exemplary waveform 900 that, when applied to a target material droplet, transforms the target material droplet to a hemisphere shaped target. The waveform 900 includes a first pre-pulse 902, a second pre-pulse 904, and a main pulse 906. The first pre-pulse 902 and the second pre-pulse 904 can be collectively considered as the first amplified light beam, and the main pulse 906 can be considered as the second amplified light beam. The first pre-pulse 902 occurs at time t=0, the second pre-pulse 904 occurs 200 ns later at time t=200 ns, and the main pulse 906 occurs at time t=1200 ns, 1200 ns after the first pre-pulse 902. In the example of FIG. 9, the first pre-pulse 502 has a duration of 1-10 ns, and the second pre-pulse 504 has a duration of less than 1 ns. For example, the duration of the second pre-pulse 504 can be 150 ps. The first pre-pulse 502 and the second pre-pulse 504 can have a wavelength of 1 μm. The main pulse 506 can be a pulse from a CO2 laser that has a wavelength of 10.6 μm and a duration of 130 ns or 50 ns. FIGS. 10A-10D show the waveform 900 interacting with a target material droplet 1010 to transform the target material droplet 1010 into a hemisphere shaped target 1018. FIGS. 10A-10D show times t=t1 to t4, respectively. Times t=t1 to t4 are shown relative to the waveform 900 on FIG. 9. The time t=t1 occurs just before the first pre-pulse 502, and the time t=t2 occurs just before the second pre-pulse 504. The time t=t3 occurs just before the main pulse 506, and the time t=t4 occurs just after the main pulse 506. Referring to FIG. 10A, a target material supply apparatus 1020 releases a stream of target material droplets 1022. The stream 1022 travels from the target material supply apparatus 1020 to a target location 1026. The stream 1022 includes target material droplets 1010 and 1011. The first pre-pulse 502 approaches and strikes the target material droplet 1010. The cross-sectional size of a target material droplet can be, for example, between 20-40 μm. Referring also to FIG. 11A, the density profile 1100 of the target material droplet 1010 is uniform in the direction of propagation “z” of the pre-pulse 502, and the target material droplet 1010 presents a steep density transition to the pre-pulse 502. The interaction between the first pre-pulse 502 and the target material droplet 1010 produces a short-scale plume 1012 (FIG. 10B) on a side of the target material droplet 1010 that faces the oncoming first pre-pulse 902. The plume 1012 can be a cloud of particles of the target material that is formed on or adjacent to the surface of the target material droplet 1010. As the target material droplet 1010 travels toward the target location 1026, the target material droplet 1010 can increase in size in the vertical “x” direction and decrease in size in the “z” direction. Together, the plume 1012 and the target material droplet 1010 can be considered as an intermediate target 1014. The intermediate target 1014 receives the second pre-pulse 504. Referring also to FIG. 11B, at the time t=t2, the intermediate target 1014 has a density profile 1102. The density profile includes a density gradient 1105 that corresponds to the portion of the intermediate target 1014 that is the plume 1012. The density gradient 1105 is minimum at a location 1013 (FIG. 10B) where the second pre-pulse 504 initially interacts with the plume 1012. The density gradient 1105 increases in the direction “z” until the plume 1012 ends and the target material 1010 is reached. Thus, the first pre-pulse 502 acts to create an initial density gradient that includes densities that are lower than those present in the target material droplet 1010, thereby enabling the intermediate target 1014 to absorb the second pre-pulse 504 more readily than the target material droplet 1010. The second pre-pulse 504 strikes the intermediate target 1014 and generates a collection of pieces of target material 1015. The interaction between the intermediate target 1014 and the second pre-pulse 504 generates the collection of pieces 1015, as shown in FIG. 10C. As time passes, the collection of pieces of target material 1015 continues to travel in the “x” direction toward the target location 1026. The collection of pieces of target material 1015 forms a volume, and the volume increases as the pieces expand with the passage of time. Referring to FIG. 10D, the collection of pieces expands for 1000 ns after the second pre-pulse 502 strikes the intermediate target 1014, and the expanded collection of pieces forms the hemisphere shaped target 1018. The hemisphere shaped target 1018 enters the target location 1016 at time t=t4. The hemisphere shaped target 1018 has density that is at a minimum at a surface plane 1019, which receives the main pulse 506, and increases in the “z” direction. The density profile 1110 of the hemisphere shaped target 1018 at a time just before the main pulse 506 strikes the target 1018 is shown in FIG. 11C. The hemisphere shaped target 1018 has a gentle gradient that is at a minimum at the surface plane 1019 that receives the main pulse 506. Thus, like the hemisphere target 614, the hemisphere target 1018 absorbs the main pulse 506 readily and emits EUV light 1030 in all directions. As compared to the hemisphere target 614, the maximum density of the target 1018 is lower and the gradient is less steep. Other implementations are within the scope of the following claims. For example, the shape of the target can vary from a hemisphere that has a rounded surface. The hemisphere shaped portion 14 of the hemisphere shaped target 5 can have one or more sides that are flattened instead of being rounded. In addition to, or alternatively to, being dispersed throughout the hemisphere shaped target 5, the particles 20 can be dispersed on a surface of the hemisphere shaped target 5. |
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048256471 | summary | FIELD OF THE INVENTION This invention relates to thrusters intended to be used for low-power applications such as in the orbital positioning of spacecraft. The thruster disclosed is of the type using propellants such as hydrazine or hydrogen in which the propellant is heated to a desired temperature prior to exiting through a rocket propulsion nozzle. The heating provides a high specific impulse and facilitates decomposition and/or combustion. These thrusters are normally used during the lifetime of a three axis stabilized or spin stabilized satellite (presently 8-10 years for synchronous orbit satellites) in order to place in, to change or to maintain orbit station. Satellite on-board propulsion is frequently required to finalize and, in some instances, make major corrections to achieve final orbit circularization and/or orbit station. When this is accomplished with a typical hydrazine-fueled engine, large quantities of propellant may be expended. Use of a performance augmented engine (using electric energy to extend the nominal chemical reaction performance level) for this function would conserve and retain more fuel for on-orbit functions. Typically, excess electric power is available on a spacecraft even during orbit/station insertion maneuvers. Correction firings are time-spaced, with off periods between firings so as to permit battery recharge for subsequent firings. By this augmentation process, fuel usage can be reduced by as much as 30 percent or more. The thruster may also be used for correcting a satellite orbit which has decayed, or to maintain the orbit of a satellite which experiences some significant atmosphere drag or for repositioning the satellite to another location or station. Such thrusters can also be used for propelling satellites which follow other satellites or for evading tracking satellites. Another application of the performance-enhanced engine is to change the orbital path of a satellite in order to evade ground tracking or to make ground tracking more difficult. An application of this would be in satellite maneuvering for the sole purpose of decoying or saturating would-be tracking capabilities. In usage, this engine could be ground-controlled by the spacecraft operating agency, or in some instances of covert operation, may be preprogrammed for on-orbit automatic control. BRIEF DESCRIPTION OF THE PRIOR ART Liquid propellant fueled spacecraft engines operate at performance levels limited by the chemical reaction energy of the propellant. Performance is generally maximum for steady state operating periods of more than a minute and reduced for pulsing operation. For a monopropellant fueled engine using a propellant such as hydrazine, either a catalyst bed or a thermal decomposer is used to initiate the exothermic reaction process. Of these processes, the catalyst bed is the most common. The thermal decomposer is typically brought to operating temperature by means of an electrical resistance heater. There decomposers serve only to initiate the chemical reaction, but do not add to or augment the chemical performance level. To extend the performance level, investigators have suggested use of electrical resistance heaters to boost the chemical performance level by exposing the chemically reacted or reacting propellant to a high temperature heater, thereby increasing the propellant temperature prior to expansion of the propellant through a nozzle. The inherent problem of such a device is the fact that direct contact between the heater and the flow requires that the propellant be as free as possible of contaminants (standards in excess of those typical of most rocket engine usage specification levels). This also precludes use of the more complex propellant formulations such as any that would contain carbon or oxygen, due to possible chemical interactions with the heating element. In the prior art, the heaters were designed for maximum output during propellant contact. Without heat removal by the propellant the heater would attain excessive temperature and heater burn-out could occur. Accordingly, the power could only be switched on when propellant was flowing, and this meant that the successful transfer of energy from the heating filament to the propellant could be accomplished only when a sufficient amount of electric power was available for heating the propellant at the rate that the propellant was being utilized for thrust. This not only prevented operation during times when battery power was substantially low, but also precluded preheating the thruster with the heater before propellant flow was initiated. This also places limitations upon attainable temperatures. Such an engine cannot be off-flow modulated or pulsed with off periods greater than a few milliseconds and as such is limited in its usefulness. A further characteristic of flow-coupled devices is that the heater is subjected to all pressure fluctuations of the propellant supply and reaction process. Gas dynamic forces from any propellant reaction instability will be transmitted to the heater and may cause a heater distortion failure. Since the flow is circulating through the heater region there is constant flow impacting and washing of the heater. Further, the heat transfer area is limited by the finite surface area of the heater. This prior art design also requires use of high temperature sealed electrical feed-through(s) into the chamber. This places restrictions on the overall engine design as to operating temperature and power. The prior art thrusters used an outer shield having a low emissivity surface in order to reflect as much heat as possible back to within the heating portion of the thruster. This minimized energy losses by maintaining a higher temperature within the heating section of the thruster. Because the minimization of heat loss was accomplished in part by minimizing exterior surface cooling, the exterior tended to remain hot causing heat to be transferred through the thruster's supporting structure to the satellite proper. A further disadvantage of having a high exterior temperature was that infrared sensors could easily distinguish a warm satellite's components from the surrounding space. The rocket nozzle section of thrusters also presented a source of high temperature emissions. This resulted from the high temperatures present at the nozzle's throat and internal expansion chamber areas, which high temperatures were conducted as heat to the outer portions of the rocket nozzle. In prior augmentation designs, the liquid propellant is first decomposed, vaporized and reacted in an uninsulated, thermally separate chamber allowing some of the reaction energy to be lost. Nozzle expansion area ratios of most rocket engines including prior augmentation thrusters are characteristically several hundred or less. Rocket engine test facilities characteristically have limited capability to simulate a space environment, therefore actual spaceflight is typically required to fully evaluate nozzle performance for large expansion ratio nozzles of engines at thrust levels greater than a few tenths of pounds thrust. Ground test data as it exists for these engines due primarily to the inadequacy of space simulation in the test facilities suggests ineffectiveness for expansion ratio nozzles of greater than several hundred. Further, state-of-the-art analytical projections are not definitive as "universal agreement does not exist regarding the correct procedures and assumptions for calculating the propulsive performance for nozzles with these high area ratios," as stated in the following reference: Cooper, L. P. (NASA LeRC), Advanced Propulsion Concepts for Orbital Transfer Vehicles, AIAA paper-83-1243, June 1983. BRIEF DISCUSSION OF THE STATE OF THE ART IMMEDIATELY PRECEDING THE PRESENT INVENTION Use of the electrical power supply of a space vehicle to augment and/or to induce propellant dissociation can result in achievement of more thrust per unit mass of gas as the gas temperature is raised to increasingly higher values. Since satellite launch capabilities limit the mass of material that can be carried as propellant, the higher the temperature of the propellant outflow, generally the longer the useful lifetime of the space vehicle. For communication, navigation, weather or surveillance satellites, space stations, space platforms and space probes, great gains can usually be obtained by increasing the stagnation temperature of the propellant flow. In one aspect of the invention disclosed in parent application Ser. No. 517,265, a thruster is provided which permits propellant to be heated without directly contacting a heater filament. The heater may have single or multiple elements to permit operation at different power levels and/or to have element redundancy. It is a further objective to increase the efficiency of heat transfer from the heater element by increasing the ratio of transfer of thermal energy to the propellant over thermal energy loss. In a further aspect thereof, an increase in propulsion performance is provided by permitting significant amounts of thermal energy to be stored within a heat exchanger for fractions of seconds or for considerable periods of time, such as several minutes. This enables the thruster to operate with reduced amounts of electrical power when necessary or advantageous. This thermal capacity of the heat exchanger also permits the engine to operate in a periodically modulated (interrupted) flow mode with constant heater power to accommodate specific flight operations or circumstances for either balancing or unbalancing the thrust vector. Typically, control engines are operated as matched thrust level pairs mounted on a vehicle to provide parallel thrust vectors which when summed together provide a resultant vector that would generally extend through the center of mass of the vehicle. In the event of an unbalanced disturbance torque on the vehicle or of inadvertent single engine performance degradation, i.e., a fractional reduction of thrust pressure in one engine, the opposite engine could be flow modulated at a rate that would maintain a thrust vector sum that would have the desired orientation. The option is also available to input a torque into the vehicle by this means. For a spinning spacecraft with engines located on either side of the spin axis if one engine were to fail, the second, if flow modulated during each revolution, could maintain a desired thrust vector. In a further aspect of the invention disclosed in the parent application, a thruster is provided which may be operated at maximum efficiency by off-modulating either its heater filament or the propellant as necessary to match thruster pairs and/or to achieve an optimum performance balance under typical spacecraft conditions of a reducing propellant flow rate due to blow down of the propellant tank pressurant over mission life and changing spacecraft power supply due to a let down of battery voltage during a firing sequence and/or power supply capability degradation over mission life. In a further aspect of the invention disclosed in the parent application, a thruster assembly is provided which is efficient in transfer of heat energy from an electrical heating element by means of effective utilization of heat shielding. It is a further object to provide such a thruster which, despite an effective means of maintaining heat in the thruster, maintains a relatively cool exterior surface and thereby presents a cool attachment point for a supporting structure. In a yet further aspect of the invention disclosed in the parent application, a thruster is provided which, despite maintaining heat within the thruster, presents a cool exterior surface which is difficult to track with infrared scanning devices prior to actual ignition of the thruster, thereby decreasing the possibility that an enemy could detect an intended ignition of the thruster, even though the thruster may have a pre-heat capability. In a further aspect, a thruster is provided which has as an option to transfer as little heat as possible from its nozzle throat section to the outside portions of its nozzle. In a further aspect, a thruster is provided which operates with a cool exterior surface so as to make the thruster more difficult to track with infrared sensing devices when the thruster is expelling propellant into space. In one further more specific aspect of the invention disclosed in the parent application, a thruster assembly is provided with a heating filament located within a heater cavity that communicates to space and which has a propellant guiding structure surrounding the heater cavity. This propellant guiding structure provides for the propellant inflow (injection) to occur at a location near the engine supporting structure, by design the coolest zone of the heat exchanger, and the propellant is then guided or channeled to flow through the inlet to the heat exchanger to the hottest zone which is adjacent to the nozzle throat. In this manner, the flow is heated by acquiring some of the heat that would otherwise be lost from the heat exchanger due to conduction into the supporting structure. This regenerative heating of the propellant both increases the efficiency of the heat exchanger and helps achieve higher propulsive performances for a given amount of available electric power. The heater located within the heater cavity of the invention disclosed in the parent application can be assembled and tested separately from the heat exchanger. This modular construction feature also permits the heat exchanger to be assembled and tested separately from the heater, using a test heater or heater simulator. For flight applications, this permits operational testing of a flight heater in a non-degrading environment typical of the used for preflight checkout of rocket engines or spacecraft. A flight heater may be joined with the flight heat exchanger for a preflight vibration test, then removed and replaced with a ground-test-only heater (1) for preflight validation of the heat exchanger-nozzle and engine operating characteristics and (2) calibration. Subsequently the flight heater which has been checked out and calibrated in a separate test series in the non-degrading environment is reinstalled into the heat exchanger/engine in readiness for flight. Another feature of this arrangement permits a heater replacement, if desired, subsequent to engine installation on a vehicle or in a test facility without needing to remove the heat exchanger/engine. The heater disclosed in the parent application may be comprised of one or more radiating heating elements or a combination of heaters and/or a thermionic emitter. A preferred configuration if a thermionic emitter is used would be to energize a heater to bring the cathode emitter to emission temperature. An embodiment featuring use of a cathode emitter to transfer energy to the heat exchanger requires the heater assembly to be electrically insulated from the heat exchanger and the heat exchanger would then function as an anode to receive the electron transfer from the cathode. The heater filament disclosed in the parent application may have a number of configuration options as to shape, spacing and material selection. The heater may include one or more heating elements. Multiple elements may provide heater redundancy and/or the capability to operate at one or more power levels. The heaters may be free standing (self-supporting) or may be provided with additional electrically insulated mechanical supporting structures. The heater material and size are selected to provide an energy transfer capability to match or nearly as is feasible the spacecraft power supply capability making minimum use of additional power controllers and/or voltage-current regulators. The radiating heater materials will be made primarily of tungsten. Additional materials and processing are used with the tungsten to obtain specific predetermined characteristics. Three percent rhenium is added thereto to create the alloy W3Re to provide (1) ease in forming the element and (2) high vibration resistance. Selected trace elements and processing with the tungsten (without 3% rhenium) are used to make a high temperature resistant (in excess of 1925 degrees K.) wire more "sag" or droop resistant in the presence of gravitational and/or centrifugal force fields than would otherwise be attainable. This type of material combination and processing is typical of that used for filaments in aircraft landing lights. Application of this same type of filament material for the radiating heater(s) in the thruster provides a heater that can operate in a gravitational and/or centrifugal force field with less "sag" or deformation than would typically occur with a W3Re filament. This "sag" resistant wire permits extended periods, in excess of 100 hours, of high-g flight time or ground (one g) test time without resorting to heater rotation at rates of one rotation or more per minute; (such rotation rates are required to prevent "sag" of a high temperature self-supporting W3Re heater filament in a typical thruster configuration.) This "sag" resistant wire makes it possible to use a radiating high temperature filament on a spin stabilized spacecraft (characteristically having a rotation rate of 40 to 80 revolutions pr minute) with the engine being mounted away from the spin axis and exposed to centrifugal forces of 2 to 6 g's. A further aspect of the invention disclosed in the parent application is the option of sealing the heater cavity containing a non-reactive gas, such as nitrogen, to enable gas pressurization of the filament. This pressurization will reduce heater filament vaporization rates. Conduction through the gas and gas convection induced by a "g" field will also transfer significant amounts of power from the heater element to the heat exchanger, resulting in a lowered temperature (as much as 220 degrees K. lower) of the coil for the transfer of a given amount of power. This combination of a reduced evaporation rate and a lower coil temperature to transfer a given power from the coil to the heat exchanger can increase the lifetime of a coil by over one order of magnitude, e.g. from 60 hours to over 600 hours. Pressurant gas dynamic forces in the heater cavity may also be used to counteract distorting g forces. That is, the heater filament may be configured in relationship to the cavity so as to interact with the pressurant gas to cause a gas convection force to oppose the "sag" forces. The heating filament may be switched "on" for significant periods of time when propellant is not flowing through the passageway and a heat-sinking capability of the propellant guiding structure permits heat to be transferred to propellant when the filament is switched "off". The propellant guiding structure may be formed in multiple layers to provide plural thermal zones of increasing temperature for the propellant as the propellant is passed through the structure. In order to retain heat within the structure, the shields will be separated by means of physical indention or preformed to specific configurations with thermal processing. Multiple radiation shields may be used internally within the heat exchanger, surrounding and at the base of the heater assembly and external of the heat exchanger. While interior shields have low emissivity in order to reflect and hold heat inwardly, the exterior surface of the thruster may have a coating having a high emissivity in order to present as cool an exterior surface as possible. The tendency of the exterior surface to remain cool by emitting heat enables operation of the thruster with higher internal temperatures, hence more efficiently. The heat exchanger/engine supporting structure, typically designated in the art as a barrier tube, connects and mechanically couples the engine to the spacecraft mount. The preferred embodiment of this barrier tube as disclosed in the parent application, uses a thin tube of extended length, with material cut-outs, formed of a low thermal conductivity material such a titanium to minimize the heat loss through this thermal conductivity path. This extended length barrier tube may be configured as concentric cylinders connected at alternating ends to minimize packaging volume with acceptable engine structural support to meet typical spacecraft launch vibration load requirements. The heating filament and the interior surface of the heater cavity as disclosed in the parent application, may also be provided with high emissivity coatings by means of surface treatment and/or coatings in order to promote a rapid transfer of energy from the heating filament to the materials surrounding the heater cavity with minimum temperature differentials between the wire and the cavity. The heating filament may operate in either a vacuum environment or may be pressurized with an inert or non-reactive gas or with reacted propellant gases in order to prolong the life of the filament. Reacted and/or energized propellant gases may be introduced into the heater cavity directly from a heat exchanger bleed for moderate level pressurization, (40 to 150 psia), or from a bleed from the expansion nozzle wall for less than one atmosphere (as low as 10.sup.-3 psia) pressurization. In that the cavity would be moderately well sealed (low leak of several cubic centimeters of gas per hour or less permitted), the gas is essentially stagnant in the absence of a "g" field. No significant measure of propellant is lost during this pressurization process. The filament itself may be provided with a bifilar helix configuration. In this mode, electromagnetic forces resulting from current flowing through each filament half will cause the filament to maintain a desired central position relative to the other, thereby axially stabilizing the filament when it is hot. The construction is such that the fuel passageways are formed as helix threads or as grooved passageways extending in one or more plural layers along the length of the thruster housing coaxial with the heater cavity. The concentric relationship of the fuel passageways and associated structure, including the shield, permits the thruster to be assembled with a minimum of weldments or other fastening devices. The thruster assembly as disclosed in the parent application may be provided with an injection passage such that the propellant can be introduced as a liquid and heat from the performance augmentation section will thermally decompose it without the use of a dissociation catalyst. The fluid passageways may be coated or plated with a material that is resistant to chemical interaction or, when desired, to enhance the dissociation process of the propellant, permitting use of less costly materials for the passageway such as TZM molybdenum alloy. In a further aspect of the invention as disclosed in the parent application, a thruster assembly such as described above can be formed with a nozzle having a nozzle throat insert. The nozzle throat insert has a high temperature capability, whereas the remainder of the expansion area of the nozzle is not required to have the same high temperature properties. The insert construction also provides a means to reduce thermal emissions from the thruster's nozzle expansion portion. While the thruster assembly disclosed in parent application Ser. No. 517,265 has met with commercial success, potential for improvement exists in its structure and operation in the following particulars: (a) Firstly, the thruster assembly disclosed in the parent application decomposes hydrazine in a typical rocket engine catalytic bed in a separate chamber and feeds the reaction products into the thruster through a feed tube. This technique, however, is found to result in the loss of a significant percentage (up to 30%) of the hydrazine decomposition chemical energy from the gas flow while retaining high fractions of undissociated ammonia. For separate reasons then, these approaches to propellant (hydrazine) decomposition and injection have disadvantages. (b) The heat exchanger surrounding the heater coil will be typically made from Mo4ORe bar stock. This material is presently available in diameters of 11/8 inch or less, and at the maximum size there is some porosity in the material within the inner 1/2 inch core. This material costs about $2,700/kg (1983) and most of the purchased weight has to be machined out and discarded. This present limit at to maximum diameter available of Mo4ORe imposes restrictions on engine thrust level due to heat exchange surface area limitations. This material size restriction also limits the amount of power that can be transferred by radiation to the limited diameter cavity walls, as there are packaging, operating condition and lifetime limits of current heater technology, as demonstrated in FIG. 22. The power radiated from a heater coil with a fixed major helix diameter can be increased somewhat higher than 750 watts and still maintain adequate life by any or all of the following techniques: (i) Enhance the emissivity of the coil by surface roughing or by using a surface coating such as hafnium carbide (maximum increase of power transfer is about a factor of four). (ii) Pressurize the cavity to reduce the surface loss rate and permit the coil to operate at a higher temperature and thus radiate more power-per-unit surface area (see FIG. 27) (maximum increase of power transfer is about a factor of two and one-half). (This feature would also permit some power to be conducted from the heater to the heat exchanger wall by convection through the gas so that the total power increase factor by means of pressurization could be three or three and one-half.) (iii) Lengthen the cavity and simultaneously lengthen the coil. This results in longer unsupported heater coil lengths and weights, and thereby approaches a maximum length limit where the coil will eventually fail either in vibration or in "creep" or "squirm". An estimate on the potential power increase available by lengthening is a factor of about two. (c) Stresses in the heat exchanger section surrounding the inner heater coil, caused by the internal gas pressure, lead to creep rupture distortion and/or failure in lifetimes up to about 500 hours. If wall thicknesses are increased to extend lifetimes, undesirable weight is added. To help minimize the material stresses and wall thickness, the internal pressure is reduced by external pressure drop mechanisms from a typical propellant or fluid supply pressure range of 350-100 psia to values ranging from 100-40 psia over life. This reduces the Reynold's number of the flow through the nozzle and consequently results in a lower thrust coefficient of the nozzle. Hence, the specific impulse of the augmenter is lower than what it could be by utilizing the full pressure available. (d) In order to reduce energy loss out the open end of the heat exchanger cavity, three short (one-inch length) radiation shields (FIG. 16) are placed between the coil and the cavity surface. All of the cavity can be emissivity-enhanced by roughening or by thermo-chemical treatment of or plating the surface. Such processing can increase the effective emissivity by several factors. These arrangements maximize the amount of heater radiation absorbed by the cavity. However, as the surface area of the cavity and the surface emissivity (enhanced or not) is uniform over the cavity length, the radiative heat transfer into the wall is relatively uniform. This arrangement did not permit any significant concentration of radiation transfer near the engine nozzle throat inlet section thereby limiting the peak specific impulse (propellant temperature) operating conditions. (e) Thermal shielding of the heat exchanger components can effectively be accomplished only by using radiation shielding. In existing designs, scrolls of molybdenum foil are used, together with some discs. Both are usually spaced or separated by surface indentations and preforming or by wires. If the ratio of the contact area to the shield areas exceeds 10.sup.-5, the effectiveness of the shields is significantly reduced. It is difficult to obtain an area ratio lower than 10.sup.-4 by using surface indentations and preforming or wire separations. Good shielding at the nozzle end is extremely important to ensure that the heat exchanger in this region attains its highest possible temperature value. If gas from the nozzle exit is permitted to expand and enter the outer shield cover and surrounding the shields, the shields' effectiveness is reduced. In this regard, reference is made to Tables 1A and 1B. If the shield cover is not effectively sealed to the nozzle exit, then the radiation shields will have degraded performance. TABLE 1A ______________________________________ POWER LOST IN WATTS FROM RADIATION SHIELDED HEAT EXCHANGER AS FUNCTION OF AMBIENT PRESSURE AND SHIELD CONTACT SURFACE RATIO A.sub.c /A P(Torr) 10.sup.-3 10.sup.-4 10.sup.-5 10.sup.-6 10.sup.-7 ______________________________________ 1.0E 01 296.86 269.29 266.16 265.84 265.88 16 1.0E 00 227.41 174.28 167.47 166.70 100.73 SHIELDS 1.0E-01 189.87 108.53 95.52 94.08 93.96 T.c = 1900 1.0E-02 184.06 95.45 78.63 76.47 76.25 deg K. = 1.0E-03 183.46 93.94 76.32 73.96 73.71 (2961 1.0E-04 183.40 93.77 76.07 73.69 73.44 deg F.) 1.0E-05 183.40 93.76 76.05 73.66 73.42 .010 in. Spacing 1.0E 01 291.03 259.50 255.88 255.52 255.53 22 1.0E 00 215.77 152.50 144.07 143.20 143.14 SHIELDS 1.0E-01 183.98 92.66 77.45 75.74 75.59 T.c = 1900 1.0E-02 179.52 82.11 63.48 61.00 60.74 deg K. = 1.0E-03 179.05 80.93 61.62 58.92 58.63 (2961 1.0E-04 179.00 80.81 61.43 58.70 58.41 deg F.) .008 in. Spacing 1.0E 01 305.64 282.26 279.64 279.39 279.4 10 1.0E 00 249.37 209.85 205.06 204.58 204.56 SHIELDS 1.0E-01 203.30 138.12 128.38 127.33 127.25 T.c = 1900 1.0E-02 194.83 120.10 106.45 104.83 104.65 deg K. = 1.0E-03 193.87 117.83 103.33 101.55 101.37 (2961 1.0E-04 193.77 117.60 103.00 101.20 101.02 deg F.) 1.0E-05 193.76 117.57 102.97 101.17 100.99 .018 in. 1.0E-06 193.76 117.57 102.96 101.17 100.99 Spacing 1.0E-07 193.76 117.57 102.96 101.17 100.99 1.0E-08 193.76 117.57 102.96 101.17 100.99 ______________________________________ O.D. 1.5 L = 2.75 TABLE 1B ______________________________________ POWER LOST IN WATTS FROM RADIATION SHIELDED HEAT EXCHANGER AS FUNCTION OF AMBIENT PRES- SURE AND SHIELD CONTACT EFFECT OF COATING SHIELD WITH RHODIUM RADIATION SHIELD PERFORMANCE COMPARISON 16 Shields T.c = 1900 deg K. = (2961 deg F.) .010 in. Spacing A.sub.c /A P(Torr) 10.sup.-3 10.sup.-4 10.sup.-5 10.sup.-6 10.sup.-7 ______________________________________ No Coating 1.0E 01 300.90 274.05 271.00 270.70 270.73 1.0E 00 233.06 181.24 174.62 173.93 173.89 1.0E-01 196.54 117.93 105.68 104.34 104.23 1.0E-02 190.91 105.75 90.12 88.09 87.88 1.0E-03 190.32 104.36 87.98 85.75 85.52 1.0E-04 190.26 104.21 87.75 85.50 85.27 Rh Coating 1.0E 01 291.86 263.58 260.28 259.95 259.98 1.0E 00 219.31 162.27 154.90 154.14 154.09 1.0E-01 179.05 90.91 77.15 75.66 75.53 1.0E-02 172.80 77.12 60.64 58.71 58.51 1.0E-03 172.13 75.56 58.60 56.56 56.35 1.0E-04 172.07 75.40 58.38 56.34 56.12 ______________________________________ SUMMARY OF THE INVENTION The present invention improves upon the prior art as discussed hereinabove in the following particulars: (a) The inner and outer heat exchanger components are brazed together along the lands of the threads or the tops of the grooves thereof (FIG. 16). Braze material could be a material such as, for example, molybdenum or iridium. One method of several options to apply the braze material may be by vapor-deposition on the inner diameter of the outer heat exchanger component, the outer diameter of the inner heat exchanger component, or both. Another method of putting the braze in position is by layering between the parts a thin foil of the braze material. A third method is to provide a channel in the meshing parts which contains the braze material, i.e. in the form of a wire (see FIG. 46). The braze can be accomplished by any one of several methods: (i) Heating the whole structure in a vacuum furnace. (ii) Placing a heater assembly in the heat exchange cavity and heating in a vacuum environment (preferably ion-pumped to under 10.sup.-4 Torr). This is a preferential brazing technique, since the inner component will be somewhat hotter than the outer component. The extra thermal expansion of the inner component will close any gaps that may exist between the components over the surface that is to be brazed. Effecting this braze reduces the tension load on the outer component and the compression load on the inner component to negligible values so that the wall thickness (and weight) can be substantially reduced. Also, the creep-rupture problem is almost completely eliminated, since each flow passage is now equivalent to a tube and all of the metal becomes structural. This improvement will permit lower specific weight for the heat exchanger while giving simultaneously a life extension of over 500 hours as compared to an unbrazed structure. (b) Further, a coiled tube replaces the outer heat exchanger passage of the prior art configuration discussed hereinabove. This tube can be made of molybdenum-rhenium, rhenium or other high-temperature materials. Use of centrifugal flow passages for either the inner or outer heat exchanger passage and the resulting effects of the induced secondary flow which is generated into the flowing propellant tends to greatly increase the heat transfer rates over those usually obtainable in the straight flow channel. Also, the gas is heated more uniformly because of the mixing induced by the secondary flows. For better performance, the coiled tube can also have an inside coating of iridium, tungsten or rhenium. Iridium is an advantageous coating material since it would enhance catalyzation of the reaction of converting hydrazine (N.sub.2 H.sub.4) into only hydrogen (H.sub.2) and nitrogen (N.sub.2). It would also help transform any ammonia (NH.sub.3) that might be injected into the tube at the inlet, into hydrogen and nitrogen molecules and thus ensure that this would not occur in the hotter components where some damage to the material could result from the intermediate reaction products (e.g., N or N.sup.+) reacting with some component of the material. Useful thicknesses of the coating will be discussed hereinafter. Some of the advantages of using this injection tube heat exchanger component to duct the propellant (typically hydrazine products) into the inner heat exchanger are as follows: (i) The outer diameter of the "effective" heat exchanger is now not limited by the present manufacturing size limits of molybdenum-rhenium; (ii) The weight and cost of this component are much lower than they would be by machining it from bar stock, as compared even to present available bar stock; and (iii) The stresses are comparatively low in the tube, thus resulting in an extension of typical lifetimes. The further feature of coating the inner diameter of the tube will provide additional lifetime in excess of 500 hours. Several methods may be used to coat the inner diameter of the tube. One method is to vapor-deposit the coating by typical chemical vapor deposition (CVD) techniques. A second method is to vapor-deposit the coating from a wire strung on the axis through the tube. This concept and apparatus are considered to be an integral part of the invention and are described in detail hereinafter. (c) The power radiated from the heater coil is reflected from the inner shield into the preferentially placed optimum (high absorbent) energy absorber structure. In order to maximize this effect, the heater enclosing radiation shields internal to the cavity are lengthened to extend the full distance of the cavity instead of covering only the first approximately one-third as in prior designs. An energy absorber component is brazed or welded into the nozzle end of the cavity wall in order to maximize the power transfer into this region. Power is transferred to this energy absorber component by direct radiation from the heater coil and by reflection from the inner surface of one of the shields. By making the ratio of the gap length to gap spacing in the energy absorber component high enough, impinging photons will undergo enough multiple reflections in the energy absorber component to be absorbed, thus giving the energy absorber component an effective emissivity of unity. The cross-sectional area of each cylinder of the energy absorber component must be adequate to ensure that the temperature difference between the braze joint and the far edge of each cycle is less than 20.degree. C. with maximum energy flux. The energy absorber component may be fabricated from any of the following materials: tungsten, tungsten-rhenium, molybdenum, rhenium, or molybdenum-rhenium. It can be fabricated as a series of cylinders brazed to a disc, or as a scroll brazed to a disc. The combined advantage of the full length shields and the energy absorber component is to have over 50% of the power radiated from the heat coil transferred to the energy absorber component. In this way, the peak temperature of the nozzle inlet end of the heat exchanger can be raised over 200.degree. C. above what it might be without these improvements. (d) A nozzle inlet area heat exchanger component may also be brazed or welded into the cavity wall face opposite the energy absorber component. This nozzle inlet area component can be fabricated as a spiral, attached at both ends to a housing, or as a series of cylinders with gaps at alternate ends. The cross-sectional area of the metal needs to be large enough to ensure that axial temperature differences between the cavity end and the nozzle inlet area end are less than 20.degree. C. This component can be fabricated from any of the following materials: tungsten, rhenium, tungsten-rhenium, molybdenum or molybdenum-rhenium. The advantage of this component is that it will ensure that the gas temperature is heated to within 20.degree. C. of the peak temperature attained anywhere in the heat exchanger and that this peak heat exchanger temperature is adjacent to the inlet sonic orifice of the nozzle. Over one-half of the electrical power input can be transferred to the propellant on this nozzle heat exchanger component. (e) In order to obtain the best performance of the radiation shields, wherever possible they are made as disc-cylinder combinations. They may, if desired, be made of 0.001" thick tungsten foil formed and annealed into cylinders and welded to discs that are 0.005" or 0.010" thick tungsten. These shields can then be nested and held together with a means that mimimizes the contact area between successive shields. The advantage of this method of shield fabrication and support is that the contact-area to shield-area ratio can be kept to values of under 10.sup.-5, making the shield effectiveness close to the theoretical maximum as will be described hereinafter. Where appropriate, these shields can be coated with a low-emissivity metal to enhance their performance. The case and the nozzle exit can be joined together in such a manner as to eliminate propellant leakage or back flow to ensure that the pressure inside of the enclosure which holds the shields is always under 10.sup.-3 Torr at operational conditions. In order to help maintain this internal enclosure pressure low, the enclosure can also be vented at the end where the heater assembly is attached, at a location remote from the nozzle. (f) In order to additionally concentrate the energy flux into the absorber and nozzle heat exchanger, the heat exchanger components may be replaced by a support tube and a second coiled tube heat exchanger, as will be described in greater detail hereinafter. Since the support tube now only serves to support the structure of the energy absorber component and nozzle heat exchanger component, its cross-sectional area for heat conduction away from these components can be much smaller than that of the components described in (a) above (see FIG. 20). This will permit a further increase in the temperature of these components for a given input power and mass flow rate, thus further increasing the specific impulse. It will also reduce the weight and cost of the thruster. This tube may also be internally coated with tungsten or rhenium in the same manner as the coiled tube described hereinabove with reference to FIGS. 16 and 20. With this dual-coiled tube configuration, if the nozzle heat exchanger components, as well as the pressure vessel are made out of rhenium or tungsten-rhenium, operating temperatures can be increased to over 2475.degree. K. (4000.degree. F.). For hydrazine propellant this could allow the mission average specific impulse (I.sub.sp) to approach 340 seconds as explained by the graph in FIG. 21. In order to get this value of I.sub.sp, it may also be necessary to increase the number of shields in the cavity and also around the nozzle heat exchanger. (g) If more of the chemical power of hydrazine decomposition could be retained (and/or used for ammonia dissociation), considerable savings of electrical power would result, especially at the higher thrust levels. For example, at a thrust level of 0.50 lb.sub.f and a flow rate of 5.8 lb.sub.m /hr (I.sub.sp =310 sec), the loss of chemical power could be as high as 230 watts (with a gas temperature of 900.degree. F. at the injection point of fully decomposed hydrazine into the augmenter). The methods of saving most of this power involve adequately insulating the catalytic bed chamber with radiation shields to reduce the heat loss and provide a longer bed (more propellant dwell time and catalyst contact) to maximize ammonia dissociation. Radiation shielding may be accomplished using ultra-light, low-emissivity disc-cylinder radiation shields; preferably micro-arc welded thereto using, for example, the process and equipment described in U.S. Pat. No. 4,404,456 to Cann. This shielding may also be coated to achieve optimum emissivity properties in manner to be described hereinafter. As the augmenter design is improved to permit both higher operating power levels and increased propellant flow rate, an additional advantage can be realized. This higher flow rate through the injector into the decomposer will result in a higher level of absorption of energy before boiling. For example, if the flow rate is 7 lb/hr, by heating all of the liquid from 298.degree. K. to 398.degree. K., the liquid would absorb 206 watts. Additionally, at this flow rate, the liquid flow is likely to be turbulent, thus promoting mixing and better cooling capabilities by turbulent heat transfer from the tube to the fluid (see Table 2). TABLE 2 __________________________________________________________________________ HEAT TRANSFER AND HEAT ABSORPTION PROPERTIES OF LIQUID HYDRAZINE FLOWING THRU THE INJECTION TUBE UPSTREAM OF THE DECOMPOSER T .degree.C. T .degree.F. p.sup.psi .mu. c.sub.p K .rho. Pr ##STR1## ##STR2## w.sub.Lm/sec. w.sub.Lm/sec. P.sub.liqWatts P.sub.liqWatts __________________________________________________________________________ 0 32 5.80.sup.-2 1.28.sup.-3 3,650 .921 1,080 4.95 308 925 3.60 10.8 -5.6 -16.8 50 122 3.48 0.65.sup.-3 3,560 .850 980 2.72 607 1,822 3.96 11.9 5.6 16.8 100 212 9.28 0.40.sup.-3 3,770 .783 925 1.93 987 2,961 4.20 12.6 17.1 51.4 150 302 40.6 0.31.sup.-3 4,390 .703 882 1.94 1,270 3,820 4.41 13.2 30.0 90.0 200 392 149.0 0.24.sup.-3 6,490 .624 818 2.50 1,650 4,930 4.75 14.3 47.1 141.0 250 482 387.0 0.18.sup.-3 10,900 .544 755 3.61 2,193 6,580 5.15 15.4 74.5 228.0 300 572 890.0 0.12.sup.-3 24,300 .452 675 6.45 3,290 9,870 5.76 17.3 350 662 1,335.0 0.08.sup.-3 .368 535 4,930 14,800 7.26 21.8 __________________________________________________________________________ This feature makes the utilization of propellant thermal decomposition eminently feasible with minimum problem from NVR build-up due to boiling and/or three-phase flow in the liquid injection tube. In order to ensure good fluid mixing and minimum power input in the injection tube near the outlet, a mixer and shield assembly may be incorporated into the fluid injection system downstream of the valve, and just upstream of the injection point. This incorporation of a thermal decomposer made integral with the augmenter will permit retention of close to 100% of the chemical power in the gas flow. (h) Using a further configuration, the limitation on power imposed by the maximum presently available material stock diameter of molybdenum-rhenium is removed and the diameter can now be increased to any optimum value. This makes it possible to design radiation transfer augmenters, using combinations of the power enhancing features of this disclosure, to operate over a broad range of power levels, even in excess of 20 kw. Removal of these size limitations makes it possible to optimize performance with H.sub.2 propellant as compared to the performance available with devices of the prior art. Hydrogen typically requires longer dwell time or higher heat transfer rate to achieve a given temperature increase as compared to a hydrazine augmenter. (i) As the power input to the augmenter increases, the coil size and weight increases. On a spinning spacecraft, this coil weight will almost certainly lead to unacceptably high sag rates of the coil when it is hot, thus reducing the lifetime below desired values. To overcome this problem, a technique of combining radiative and thermionic heating may be employed. A thermionic emitter element is mounted in the cavity of the heat exchanger with cylinders interleaving those of the thermal absorber. This emitter element is electrically insulated from the heat exchanger and is connected electrically to the negative terminal of the power bus. The heat exchanger is electrically connected to the positive terminal of the power bus and the operational sequence for this heater is as follows: (1) Power is supplied to the coil by closing a switch. (2) The power radiated from the coil heats the thermionic element and the heat exchanger. (3) Once the temperature of the emitter element gets above about 1650.degree. K. (2500.degree. F.), great numbers of electrons are emitted by the thermionic emitting material, such as thoriated tungsten, from which the emitter is fabricated. The electric field between the emitter and the heat exchanger accelerates these electrons toward the heat exchanger where they impact and are absorbed. (4) This electron flow constitutes an electric current which flows across a potential drop equal to that of the power source, such as a battery, delivering energy to the heat exchanger at a rate given by: EQU P=IV (1) where: P=power; PA1 I=current; PA1 V=potential drop. PA1 I=current; PA1 A=active emitter area; PA1 .epsilon..sub.o =capacitivity of vacuum; PA1 .vertline.e.vertline.=charge on the electron; PA1 m.sub.e =mass of the electron; PA1 x=gap; PA1 V=potential drop between the emitter and the heat exchanger. PA1 1. Propellant characteristics PA1 2. Stagnation pressure and throat area PA1 3. Nozzle contour PA1 4. Nozzle exit area PA1 5. Nozzle wall temperature distribution PA1 thermal diffusion (species diffusion in a temperature gradient) PA1 pressure diffusion (species diffusion in a pressure gradient) PA1 interspecies energy transfer PA1 T.sub.c =stagnation temperature in chamber PA1 .gamma.=ratio of specific heats of gas PA1 M=free stream Mach number in flow PA1 P.sub.r =Prandtl number PA1 r.sub.c =radius of curvature of throat PA1 r*=throat radius PA1 f(.gamma.)=a function of the specific heat ratio, .gamma..perspectiveto.0.97+0.86 PA1 Re*=flow Reynold's number based on throat diameter PA1 .gamma.=ratio of specific heats of the gas PA1 .DELTA.C.sub.F =increment to thrust coefficient from supersonic portion of the nozzle PA1 p.sub.c =chamber pressure PA1 A*=throat flow area PA1 p*=gas pressure at the throat PA1 .rho.*=gas density at the throat PA1 w*=gas velocity at the throat PA1 p.sub.w =gas pressure at the wall PA1 .tau..sub.w =shear stress at the gas-wall interface ##EQU5## .mu..sub.w =viscosity of the gas at the wall W.sub.f.s. =free stream velocity of the gas PA1 .delta.=momentum thickness of the boundary layer PA1 r=radial coordinate in cylindrical coordinate system PA1 R=radial coordinate in spherical coordinate system PA1 .theta.=half angle of the nozzle PA1 r.sub.e =radius at nozzle exit at the nozzle exit, or PA1 F=thrust PA1 p=gas pressure PA1 .rho.=gas density PA1 w=gas velocity PA1 r=radial variable PA1 r.sub.e =radius at nozzle exit PA1 .theta.=half angle of the conical nozzle PA1 M.sub.e =Mach number at the exit of the nozzle PA1 .gamma.=ratio of specific heats of the gas PA1 .theta.=half angle of conical nozzles PA1 Re*=Reynold's number based on the throat diameter PA1 .varies.=a "variable" constant Almost all of this energy is deposited in the thermal absorber, adjacent to the nozzle. (5) This power (P) heats the thermal absorber to temperatures above that of the emitter. Most of this power is transferred to the gas in the heat exchanger near the nozzle. However, some small fraction is radiated back to the emitter, supplying the work function energy to maintain the electron emission and temperature of the emitter, which in turn keeps the electric current flowing. (6) The gaps between the emitter and the absorber are designed to values which control the level of current at the space-charge-limited level, given by the Child-Langmuir equation: ##EQU1## where: (7) Once the design current is flowing and the steady-state operational temperature with propellant flowing is established, a switch can be opened, permitting the coil to cool down to the heat exchanger temperature. Since the coil is used essentially as an initiator, being hot for only a few minutes each firing, the coil lifetime can be many thousands of augmenter operational hours before it would fail due to factors of evaporation and/or distortion or sag. (8) When the firing is to be terminated (after 40-60 minutes), a switch is opened and the propellant flow rate is stopped by closing the valve. The advantages of this type of heating over pure radiative heating are as follows: (a) Unlimited power can be transferred by increasing the voltage, , the area of the emitter, , or by decreasing the gap, . (b) The energy is deposited exactly where it is most useful, near the nozzle throat entrance, thus permitting this final and most thermally isolated section of the heat exchanger to operate at the highest temperature possible and with the highest thermal efficiency. (c) The structure of the emitter can be very rigid and thus not move or deform appreciably under the "g" loading in a spinning spacecraft. (d) The cylindrical structure of the emitting and absorbing elements give great rigidity to the surfaces so that electro-thermo-mechanical instability leading to hot-spot development cannot readily occur. (e) The emission characteristics of thoriated tungsten match exceptionally well to the temperature range to which the heat exchanger (absorber) and propellant gases need to be heated to get specific impulses of 300 to 340 sec. (FIG. 32 is explanatory in this regard). By designing for current densities of under 0.2 amps/cm.sup.2, the emitter temperature can vary from 1650.degree. to 2200.degree. K. (2500.degree. to 3500.degree. F.) before the current flow becomes "emission" limited, rather than "space charge" limited. With the collector part of the heat exchanger operating 38.degree. to 93.degree. C. (100.degree. to 200.degree. F.) hotter than the emitter, an ideal temperature is produced for operating a high-performance augmenter using hydrazine decomposition products, hydrogen or ammonia, as the propellant gases. (f) The intermeshed emitter-collector structure disclosed hereinafter with reference to FIGS. 30-31 represents a near-ideal configuration for implementation in a thruster. The large surface area of both components permits low current density operation. This has the following advantages: (i) A wide operating temperature range at constant power input is available. (ii) The large active emitting surface area permits gaps of over 0.020 inches between the emitter and the collector with conventional spacecraft voltages of about 40 volts (see FIG. 32). These size gaps can be maintained at an adequately constant spacing, even with significant differential thermal expansion between the components. (iii) With low current density, the power density to the collector is low (.perspectiveto.8 watts/cm.sup.2). Thermal conduction in the metal can hence overpower any tendency for hot-spot development due to small changes that may occur locally in the gap spacing. (g) During the lifetime of the spacecraft on which the engine is being used, the average voltage of a typical space vehicle power source during a firing decreases. For most current communication spacecraft this is usually from 41 to 36 volts. Simultaneously, the storage pressure feeding the propellant will decay over life from, typically, 300 psia to 100 psia. Since the power output varies as the voltage to the 5/2 power, thermionic heating will have less change in specific impulse over life than will pure radiation heating, where the power input varies approximately as the 8/5 power of the voltage. In order to further improve upon the performance of thrusters such as that which is disclosed in the parent application, modifications in the nozzle assembly are desirable and the present invention includes aspects of nozzle design and analysis resulting in improved thruster performance. The objective of the nozzle analysis and design optimization is to determine the nozzle configuration which will deliver the maximum I.sub.sp at the specified thrust and to predict the off-design performance over a range of operational parameters likely to be encountered during operations. A further objective is to investigate options for minimizing the back flow contamination of the space vehicle by the exhaust plume, plume interaction with vehicle structure that could be located in the flow path such as solar panel array and plume loss mechanisms in satellite configurations where the exhaust gases must pass through a long large-diameter duct. This last problem area is presently postulated as limiting the usefulness of augmented engines on some spinning spacecraft, the coil "sag" problem in the g-field having been successfully solved recently by the applicant. Nozzle and engine operating and configuration variables to be considered are: The importance of working for a high thrust coefficient is described in greater detail hereinafter and indications are that at a specific impulse of 310 sec., the stagnation gas temperature can be reduced from 2077.degree. K. (3280.degree. F.) to 1950.degree. K. (3050.degree. F.) if the thrust coefficient is increased from 1.60 to 1.65. Alternatively, the specific impulse could be raised to 320 sec. at a gas temperature of 2077.degree. K. (3280.degree. F.) by increasing the thrust coefficient from 1.60 to 1.65 (see in this regard FIG. 33). Since the procedures for analytically and experimentally determining the thrust coefficient were generated for high thrust rockets exhausting to an ambient atmosphere, some of the physical processes that can affect the thrust coefficient in low thrust rockets, whose laminar boundary layers may encompass a significant fraction of the flow, exhausting to the vacuum of space, have not been given due consideration. A curious anomaly appears to have occurred in the design of nozzles for rockets used to control space vehicles. Although rocket nozzle designers realize that they operate only in the vacuum of space, the area ratios of most nozzles are designed to give best performance in the steady-state vacuum achievable in the ground test facility where the engine will be tested. No significant attempt has been made in the prior art to utilize the thrust available, through proper design, from the low pressure expansion region. Nozzles also have traditionally been cooled to minimize chemical and physical erosion, to reduce radiative power loss and to minimize nozzle weight by taking advantage of the higher strength of the material at low temperatures. This design approach needs to be reexamined for low thrust rockets that operate only in a space environment. This is especially true for the high performance augmented engines in which new mechanisms occur when the gas consists of a mixture of atoms or molecules of very different molecular weight, such as is found in the decomposition products, hydrogen and nitrogen, of hydrazine at high temperature. These new mechanisms can be identified by the processes of: It may be possible to utilize one or all of these effects to enhance the thrust coefficient over what it would be for a lower operating temperature engine or as compared to a propellant of a single species gas of the same temperature and molecular weight. The phenomena to be considered are: 1. Thermal diffusion tends to separate the species when a temperature gradient exists. For a mixture of hydrogen and nitrogen, the hydrogen is concentrated in the higher temperature regions. The computed and measured thermal diffusion coefficient is plotted as a function of the species number density ratio in FIG. 34. The amount of species separation that could be achieved is plotted in FIG. 35 as a function of the temperature ratio, T.sub.hydrogen /T.sub.nitrogen. If the nozzle were run hot, then the gas in the boundary layer would have a higher percentage of hydrogen than average (i.e., greater than 67%). This reduces the viscosity in the boundary layer as well as the mass flux through the boundary layer, both of which can reduce viscous momentum losses and may increase the thrust coefficient. 2. Pressure diffusion permits the lighter molecular weight component (in this case hydrogen) to have a higher velocity than the nitrogen as the gas expands through the pressure gradient in the nozzle. This effect becomes more pronounced as the static pressure drops below one Torr. This is shown graphically in FIGS. 36 and 37 where the velocity separation achievable with a fixed pressure gradient is plotted as a function of the pressure. In the "core" flow one wants to discourage this from occurring by having the hydrogen molecules accelerate the nitrogen molecules by way of collisions; this leads to the highest values of the thrust coefficient. A design criterion becomes: The stagnation pressure should be as high as possible so that when the two species do "uncouple", at low pressure, the static temperature of the gas is as low as possible. 3. Interspecies Energy Transfer. For a given mass of gas in the chamber, two-thirds of the energy resides in the hydrogen and one-third in the nitrogen. If this gas is now expanded through a perfect nozzle with no pressure diffusion so that the hydrogen and nitrogen molecules form a univelocity beam, one-eighth of the energy is carried by the hydrogen, and seven-eighths is carried by the nitrogen. For this "massive" energy transfer to occur (more than one-half of the energy in the gas), the temperature of the hydrogen, as well as its directed velocity, must be higher than that of the nitrogen through the nozzle. An estimate of the temperature difference needed to keep the two species moving at the same velocity may be determined as a function of pressure. Again, to ensure that this energy transfer takes place, the pressure should be as high as possible. An enhancement mechanism may be available by considering the following model: (a) Operate the nozzle wall, both upstream and downstream of the throat, above the recovery temperature of the hydrogen. (b) The high thermal conductivity of the hydrogen may transfer energy into the hydrogen in the core, where collisions with the nitrogen will transfer this added energy to the nitrogen. (c) This addition to the stagnation enthalpy of the gas in the nozzle will tend to increase the thrust coefficient by a factor that is proportional to the square root of the ratio of the enhanced stagnation temperature over the unenhanced stagnation temperature. As a result of the above considerations, several important design criteria result: (a) The stagnation pressure in the heat exchanger should be as high as possible. (b) The nozzle wall temperature should be operated at a temperature equal to or greater than the recovery temperature in the gas. This must be "optimized" by including considerations of radiation power loss from the nozzle. The recovery temperature, T.sub.r, is defined as: ##EQU2## where T.sub.r =recovery temperature The curvature of the nozzle at the throat, r.sub.c, is another important parameter of the nozzle design. It appears prominently in the expression for the discharge coefficients C.sub.d in the following form: ##EQU3## where C.sub.d =discharge coefficient How the value of the discharge coefficient affects the thrust coefficient is not immediately obvious. This is investigated by developing a novel method of computing the thrust coefficient. The thrust of a rocket (F), operating in a vacuum, can be computed by two methods: 1. Evaluating the integral: ##EQU4## where C.sub.F =thrust coefficient for subsonic portion of the nozzle (2) Integrating the stress tensor over the axial projection of all interior and exterior surfaces. The approach adopted here is to compute the thrust that is generated up to the nozzle throat using method 1 above, and then to compute the additional thrust in the expanding section using method 2 above. The two components of the thrust coefficient, C.sub.F, are identified as follows: ##EQU6## where EQU C.sub.F =C.sub.F *+.DELTA.C.sub.F and, Since the velocity w* is purely axial at the throat, cylindrical coordinates are used in computing C.sub.F *. For invisid gas, accelerated at constant enthalpy and with a conical diverging nozzle, the integrals can be evaluated. The results are: ##EQU7## where Note: On the above calculations, the flow at the throat has been made spherically symmetric for convenience. Equations 9 through 11 represent the results of the classical approach to computing the thrust coefficient. Assuming that the pressure is independent of the radius at the throat, the viscous effect on C.sub.F * can be computed. The result is: ##EQU8## where EQU C.sub.D =discharge coefficient The expression indicates that the radius of curvature at the throat should be small so that C.sub.D is kept as high as possible. This conclusion may be somewhat modified by the desire to continue heating the gas as it accelerates through the throat. Some indication of the optimum nozzle shape can be determined by using equation 7. Immediately downstream of the throat there will be a negative increment to C.sub.F since p.sub.w sin .theta.<.tau..sub.w cos .theta.. Once the expansion angle is increased to make the expression in brackets positive, the angle .theta. must be adjusted throughout the expansion to ensure that: EQU p.sub.w sin .theta.-.tau..sub.w cos .theta.>0 (13) Eventually the nozzle half-angle will approach 90.degree., becoming a disc perpendicular to the axis of the throat. When the disc is extended out sufficiently far radially, such that substantially no collisions are occurring between propellant particles at the periphery thereof, at that circumferential location, a conical end piece may be provided having an angle with respect to the longitudinal axis of the nozzle designed to maximize deflection of propellant particles in the direction parallel with the nozzle axis. Since the disc part and conical end piece of the nozzle can be fabricated from extremely thin sheet material, the conical end piece should extend to the maximum diameter permitted by space vehicle constraints. The pressure at the wall, p.sub.w, is a strong function of .theta. and .tau..sub.w, a weak function of .theta.. Both decrease as R is increased. An analysis of the nozzle in accordance with the teachings of the present invention should permit an optimization of the nozzle contour and determine the exit area for the range of operational Reynold's numbers. In order to obtain the most accurate results for the nozzle design downstream of the throat, equation 7 or 35 may be calculated for spaced nozzle wall increments as low as one millimeter or less. Such calculations may be done by computer for greater efficiency and accuracy. Taking into consideration the test data and the design implications of the mechanisms discussed earlier, nozzle configurations that have analytical interest are sketched in FIGS. 38, 39 and 40. The rationale for each design feature is indicated on the figures. Some predicted and test data is available for estimating the values of the thrust coefficient for various nozzle shapes and gases (Murch, C. K., Broadwell, J. E., Silver, A. H. and Marcisz, T. J. "Performance Losses in Low-Reynolds-Number Nozzles", J. Spacecraft, Vol. 5, #9; Potter, J. Leith and Carden, William H., "Design of Axisymmetric Contoured Nozzles for Laminar Hypersonic Flow", J. Spacecraft, Vol. 5, #9; and Kinslow, Max and Miller, John T., "Nonequilibrium Expansion of a Diatomic Gas Through a Convergent-Divergent Nozzle", The Physics of Fluids, Vol. 9, #9. The first reference gives throat Reynold's numbers in data for nitrogen and hydrogen flows which are comparable throat Reynold's numbers to those found in "EPAT" and "ACT" in FIG. 41. This data has been examined so that comparisons with the existing test data from the two augmented engines, "EPAT" and "ACT", could be made, and also to determine the feasibility of scaling to higher throat Reynold's numbers. In this comparison a surprising difference in performance between nitrogen and hydrogen is seen, FIG. 41. This may be because of the different rates of freezing the vibrational and perhaps even the rotational energy as the pressure drops during the expansion. Using the definition of nozzle efficiency contained in the first reference, the efficiency values can be converted to thrust coefficients by multiplying the ordinate by 1.7498, the thrust coefficient for a perfect gas expanding through an area ratio of 100 and turned so that the velocity is only in the thrust direction. Scaling curves may be drawn based on the following relation: ##EQU9## where C.sub.F (.gamma., A/A*)=the functional dependence of the thrust coefficient on the ratio of specific heats, and the area ratio Since most of the data for various area ratios and for the two gases passes through the point ##EQU10## where .eta..sub.n =nozzle efficiency Extrapolated performance predictions using this relation are plotted in FIG. 41 out to Reynold's numbers of 20,000 using a value of .gamma.=1.40 with data from the first reference also being shown on this plot. When a higher gas pressure is used it may be possible to recover more of the energy from vibrational and rotational de-excitation. The maximum available thrust coefficient from a gas with a value of .gamma.=1.31 has been calculated assuming an area ratio of 100, and that profile losses scale in the same manner as computed previously. This curve is also displayed in FIG. 41. Finally, the best estimates of the thrust coefficients from several thruster implementations are shown in FIG. 41. These implementation are described in FIG. 41 with abbreviations defined as follows: ______________________________________ HiPEHT a TRW electrothermal hydrazine augmented disclosed in U.S. Pat. No. 4,322,946 ACT a Rocket Research Corporation implementation of a catalytic hydrazine augmenter as disclosed in parent application serial number 517,265 filed July 26, 1983 EPAT TRW test data from applicant's implementation of the catalytic hydrazine augmenter disclosed in the parent application Lord(a) resistojet disclosed in a publication by J. A. Donovan, W. T. Lord and P. J. Sherwood entitled "Fabrication and Preliminary Testing of a 3 KW Hydrogen Resistojet" given at the AIAA 9th Electric Propulsion Conference, April 17-19, 1972. Lord(b) resistojet disclosed in a publication by J. A. Donovan and W. T. Lord entitled "Performance Testing of a 3 KW Hydrogen Resistojet" Yoshida resistojet disclosed in a publication by R. Y. Yoshida, C. R. Halbach and C. R. Hill entitled "Life Test Summary and High Vaccum Tests of 10 MLB Resistojets" ______________________________________ It should be emphasized that the predicted performance, as well as the measured performance from the first reference is based on an area ratio of 100. HiPEHT and ACT have area ratios of 250 to 300, but were tested with ambient pressure of 0.1 to 0.5 Torr. EPAT has an area ratio of 700 and a flat plate nozzle continuation out to a diameter of 2 inches. The test was also conducted with an ambient pressure of 10.sup.-5 to 10.sup.-2 Torr during the test. The large area ratio and the low background pressure in the test chamber accounts for the higher thrust coefficients shown for EPAT. Further data on the effect of the ambient pressure on thrust has been found in papers (by Lord and Yoshida) presenting results from a 3KW hydrogen resistojet and a 10 MLB resistojet. In two cases, thrust was measured as a function of pressure and the thrust coefficient increased from 1.14 to 1.40 in one case as the pressure was decreased from 20 microns to 2.6 microns (1000 microns=1 Torr) and in the other case, the thrust coefficient increased from 1.25 to 1.51 as the pressure was decreased from 1000 micron to 1 micron. This and other data from these papers are plotted in FIG. 41 and are identified as data points by Lord and Yoshida. |
abstract | This is a new technique of producing high energy X-rays for radiation therapy at a patient's level. The dose delivery system uses a linear accelerator with no flattening filter. The technique improves patient radiation therapy by reducing radiation scattered to surrounding normal tissue and reducing electron contamination. It increases dose rate to shorten treatment time. The flattening filter reduces the efficiency of the beam by reducing the fluence and increasing scattered radiation. This technique involves removal of the flattening filter. It uses inverse planning to shape the dose distribution. |
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abstract | A spacer grid includes intersecting straps defining cells with springs and dimples arranged to hold fuel rods passing through the cells. The direction of the springs switches at a switch point in the spacer grid that is not at the center of the spacer grid. The intersecting straps may include a first set of mutually parallel straps including a first transition strap and a second set of mutually parallel straps including a second transition strap, with the second set intersecting the first set. The springs of the first set of mutually parallel straps face away from the first transition strap, and the springs of the second set of mutually parallel straps face away from the second transition strap. The outer straps in some embodiments include dimples but not springs. |
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042254671 | summary | This invention relates to neutron absorbing articles and to assemblies incorporating a plurality of such articles in a container for nuclear material. More particularly, the invention relates to such articles which comprise boron carbide particles in a matrix of cured phenolic polymer and which are in a form suitable for use in assemblies for absorbing neutrons from spent nuclear fuel. Periodically nuclear fuels employed in nuclear reactors to produce power diminish in activity to such an extent that they have to be replaced so that the reactor in which they were employed may operate at specification rate. In the past the spent fuel was temporarily stored in pools of water in which the neutrons and other radiation emitted from the fuel were sufficiently absorbed so as to prevent harm to human life and the environment about the storage facility. Normally, the facility was only temporary and therefore did not have to have a capacity for a very large amount of spent fuel (one or two annual refuelings plus room for a full reactor fuel charge in case of an emergency or complete reactor recharging was usually sufficient capacity) because the spent fuel would be removed from storage periodically for reprocessing or disposal. However, with the moratorium on such reprocessing and the limitations imposed on such disposal operations, many utilities have found it necessary to increase the fuel storage capacities of their existing spent fuel pools. Increasing the capacity of a spent fuel storage pool may be effected in the obvious way, by increasing the size of the pool, but a more efficient and practicable method is to increase the neutron and radiation absorbing ability of the pool itself. Various materials are known to be effective neutron absorbers and of these boron has previously been recognized as exceptionally effective. The B.sup.10 content of the normally occurring isotopic mixture of B.sup.10 and B.sup.11 is the highly effective neutron absorbing component of boron carbide and has a neutron absorbing capability many times that of B.sup.11. Although boron is metalloidal in character it is not generally suitable for manufacturing sufficiently strong thin articles, such as long thin plates and therefore if it is to be employed as a neutron absorber it is usually in the form of its compounds or alloys. A particularly useful compound, having a large percentage of boron in it, is boron carbide, which is in the form of hard black crystals having a Mohs scale hardness of 9.3 to 9.5, a melting point of about 2,350.degree. C. and a specific gravity of about 2.6. Boron carbide has previously been fabricated into neutron absorbing articles by various high temperature methods. It also has been formulated with other materials to form neutron absorbers of improved physical properties and such absorbers can often be produced at lower temperatures. Boron carbide in aluminum (boral) has been employed as a useful neutron absorber and it has been suggested that particulate boron carbide be dispersed in polymeric matrices. Thus, U.S. Pat. No. 2,796,411 mentions the use of acrylate resins suitably impregnated or admixed with various boron compounds, including boron carbide, and made into sheets or various other structures. U.S. Pat. No. 2,796,529 describes a shield for radiation which has boron or its compounds included in a laminated structure with synthetic resinous materials such as polytetrafluoroethylene and polyethylene. Also mentioned therein as possible useful polymeric materials are urea formaldehyde, melamine formaldehyde and melamine-urea-formaldehyde synthetic resins. U.S. Pat. No.2,942,116 mentions neutron shields wherein resins attenuate fast moving neutrons, and boron compounds, such as borax, boric acid and boron carbide, are employed to absorb slow moving neutrons. U.S. Pat. No. 3,133,887 describes boron carbide particles in epoxy resin making a product which is useful as a neutron absorber. Additionally, various other patents refer generally to the employment of resinous materials as bonding agents for neutron absorbers but none appears to be more relevant to the subject matter of the present invention than those already mentioned. Because of the danger of radiation and the possibility that the spent fuel items, usually in vertical rod form, if stored too close together without effective absorption of slow moving neutrons, could exceed a "critical mass" and/or carry out a highly exothermic nuclear reaction, standards for neutron absorbing articles are extremely high. In addition to being effective absorbers, articles must be shown to be stable under usual storage conditions and resistant to physical shocks, temperature variations, radiation and contact with the aqueous medium in the pool in the event that leakage occurs. Also, resistance to galvanic corrosion, such as in the presence of stainless steel, which is often employed as a container for the absorbing article to keep it separate from the pool medium, is an important quality often required of the neutron absorber. Furthermore, it is not enough for the absorber only to be effective and stable but the absorbing power thereof should be accurately controllable so that desirably effective absorption to a pre-calculated extent is obtained. Finally, it is desirable for the absorbing article to be capable of being made by relatively simple and inexpensive techniques so that the cost of the absorber makes it competitive with similar articles made from boral and other such absorbing compositions. In accordance with the present invention a neutron absorbing article comprises boron carbide particles in which the boron carbide content is at least 90% by weight, preferably at least 94%, and which are substantially all of a size to pass through a No. 20 U.S. Sieve Series screen and a solid, irreversibly cured phenolic polymer, e.g., a phenol aldehyde condensation polymer cured to a continuous matrix about the boron carbide particles, with the proportion of boron carbide in the article being such that it contains at least 6%, preferably at least 7% by weight of B.sup.10 from the boron carbide content thereof. In preferred embodiments of the invention the neutron absorbing article is in long, comparatively thin plate form, the boron carbide contains little or no B.sub.2 O.sub.3 and iron and the phenolic resin of the polymer contains essentially no halogen, mercury, lead and sulfur or compounds thereof. Likewise, the boron carbide and phenolic resin do not come into contact with any halogens, mercury, lead or sulfur or compounds thereof in the fabrication processes. Also within the invention is an assembly of a plurality of the described neutron absorbing articles, preferably in long and comparatively thin plate form, in a container for nuclear fuel, positioned so as to absorb neutrons emitted by the nuclear fuel. Additionally a part of this invention is the use of the articles to absorb neutrons from nuclear material, which is preferably spent nuclear fuel. |
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abstract | A radiation shielding panel includes a tungsten powder and a polyurea material. The tungsten powder includes tungsten particles having three different specific diameters. The tungsten powder is mixed and dispersed into the polyurea material. The mixture of the polyurea material and the tungsten powder shields radiation greater than about 6 MeV. |
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description | The present application is a continuation application of U.S. patent application Ser. No. 12/235,086, which is herein incorporated by reference and which was filed on Sep. 22, 2008, which is in turn a divisional application of U.S. patent application Ser. No. 11/196,929, which was filed on Aug. 4, 2005, and is herein incorporated by reference. This application is a continuation-in-part of U.S. patent application Ser. No. 10/950,248 entitled “Manufacture of Silicon-Based Devices Having Disordered Sulfur-Doped Surface Layers,” filed on Sep. 24, 2004, and U.S. patent application Ser. No. 10/950,230, entitled “Silicon-Based Visible And Near-Infrared Optoelectric Devices,” filed on Sep. 24, 2004, both of which are herein incorporated by reference in their entirety. The present application is also related to U.S. patent application Ser. No. 10/155,429, entitled “Systems and Methods for Light Absorption and Field Emission Using Microstructured Silicon,” filed on May 24, 2002, which is also herein incorporated by reference in its entirety. The invention was made with Government Support under contract DE-FC36-01GO11053 awarded by Department of Energy and under grant NSF-PHY-0117795 awarded by National Science Foundation and. The Government has certain rights in the invention. The present invention is generally directed to methods for processing semiconductor substrates and the resultant processed substrates, and more particularly to such methods for modifying the topography of a substrate's surface. A number of techniques are known for generating micrometer-sized structures on silicon surfaces. For example, quasi-ordered arrays of conical spikes can be formed on a silicon surface by irradiating it with high fluence laser pulses by employing, for example, the methods disclosed in the above U.S. patent applications. There is, however, still a need for enhanced methods that allow generating even smaller structures on semiconductor surfaces, and particularly on silicon surfaces. The present invention is directed generally to methods for generating submicron-sized features on a semiconductor surface by irradiating the surface with short laser pulses. The methods allow modulating the sizes of these features by selecting the irradiation wavelength and/or placing a surface portion to be irradiated in contact with a fluid. The invention can provide formation of features that are substantially smaller in size than those generated by previous techniques. The generated features can be, for example, in the form of substantially columnar spikes, each of which extends from a base to a tip, that protrude above the surface. In many embodiments, the average height of the spikes (i.e., the average separation between the base and the tip) can be less than about 1 micron, and the spikes can have an average width—defined, for example, as the average of the largest dimensions of cross-sections of the spikes at half way between the base and the tip—that ranges from about 100 nm to about 500 nm (e.g., in a range of about 100 nm to about 300 nm). In one aspect, the present invention provides a method of processing a semiconductor substrate that includes placing at least a portion of a surface of the substrate in contact with a fluid, and exposing that portion to one or more short laser pulses so as to modify its topography. The laser pulses can be selected to have pulse widths in a range of about 50 femtoseconds to about a few nanoseconds, and more preferably in a range of about 100 femtoseconds to about 500 femtoseconds. In a related aspect, the laser pulses are selected to have energies in a range of about 10 microjoules to about 400 microjoules (e.g., 60 microjoules), and fluences in a range about 1 kJ/m2 to about 30 kJ/m2, or from about 3 kJ/m2 to about 15 kJ/m2, or from about 3 to about 8 kJ/m2. The central wavelength of the pulses can be selected to be less than about 800 nm, and preferably in a range of about 400 nm to less than about 800 nm. The number of pulses applied to each location of the surface can be, e.g., in a range of about 1 to about 2500. In many embodiments, utilizing irradiation wavelengths that are less than about 800 nm, e.g., 400 nm, and/or placing the irradiated portion in contact with the liquid (e.g., water) can lead to formation of sub-micron-sized features over the substrate's surface. In further aspects, in the above method, the fluid can be selected to be any suitable polar or non-polar liquid. Some examples of such liquids include, without limitation, water, alcohol and silicon oil. Further, the semiconductor substrate can be selected to suit a particular application. By way of example, in some embodiments, the substrate can be an undoped or doped silicon wafer (e.g., an n-doped silicon wafer). In another aspect, the invention provides a semiconductor substrate that includes a surface layer having at least a portion that exhibits an undulating topography characterized by a plurality of submicron-sized features having an average height less than about 1 micrometer and an average width in a range of about 100 nm to about 500 nm, and preferably in a range of about 100 nm to about 300 nm. The substrate can be any suitable semiconductor substrate, e.g., silicon. In related aspects, the surface layer has a thickness in a range of about 100 nm to about 1 micrometer and the submicron-sized features comprise spikes each of which extends from a base to tip separated from the base by a distance that is less than about 1 micron. For example, the spikes can protrude above the semiconductor surface by a distance in a range of about 100 nm to about 300 nm. In another aspect, a method of processing a silicon substrate is disclosed that includes irradiating a portion of a semiconductor surface with one or more femto-second laser pulses having a center wavelength in a range of about 400 nm to less than about 800 nm so as to generate a plurality of submicron-sized spikes within an upper layer of that surface. The spikes can have an average height less than about 1 micrometer and an average width in a range of about 100 nm to about 500 nm. In a related aspect, in the above method, the irradiation of the surface portion is performed while that portion is in contact with a fluid. By way of example, the fluid can include a polar or non-polar liquid, or a gas, e.g., one having an electron-donating constituent. In a related aspect, the above method further calls for disposing a solid substance having an electron-donating constituent on the surface portion that is in contact with the fluid prior to its irradiation by the laser pulses. For example, sulfur powder can be applied to the surface followed by disposing a layer of fluid (e.g., having a thickness in a range of about 1 mm to about 20 mm) on the surface. Subsequently, the surface can be irradiated by the laser pulses so as to generate the spikes within an upper layer of the surface and also generate sulfur inclusions in that layer. In another aspect, the fluid that is in contact with the substrate's surface comprises an aqueous solution, e.g., one containing an electron-donating constituent. By way of example, the liquid can comprise sulfuric acid. In a further aspect, a method of processing a semiconductor substrate is disclosed that includes disposing a solid substance having an electron-donating constituent on at least a portion of a surface of the substrate, and irradiating that surface portion with one or more pulses having pulse widths in a range of about 50 fs to about 500 fs so as to generate a plurality of inclusions containing the electron-donating constituent in a surface layer of the substrate. The electron-donating constituent can be, for example, a sulfur-containing substance. Further understanding of various aspects of the invention can be obtained by reference to the following detailed description in conjunction with the associated drawings, which are described briefly below. The present invention generally provides semiconductor substrates having surfaces that exhibit submicron-sized structures, and methods for generating such structures. In many embodiments, the submicron-sized structures are generated by irradiating a semiconductor substrate's surface with ultra short laser pulses (e.g., femtosecond pulses) while maintaining the surface in contact with a fluid (e.g., water). Exemplary embodiments of the invention are discussed below. With reference to a flow chart 10 of FIG. 1, in one exemplar embodiment of a method according to the teachings of the invention for processing a semiconductor substrate, in a step 12, at least a portion of the substrate surface is placed in contact with a fluid, for example, by disposing a layer of the fluid over that portion. In another step 14, the substrate portion that is in contact with the fluid is exposed to one or more short laser pulses so as to modify its surface topography. The laser pulses can have pulse widths in a range of about 100 fs to about a few ns, and more preferably in a range of about 100 fs to about 500 fs. In this exemplary embodiment, the center wavelength of the pulses is chosen to be about 400 nm. More generally, wavelengths in a range of about 400 nm to less than about 800 nm can be employed. The pulse energies can be in a range of about 10 microjoules to about 400 microjoules, and preferably in a range of about 60 microjoules to about 100 microjoules. The modification of the surface topography can include generating submicron-sized features in an upper surface layer of the substrate. For example, the submicron-sized features can include a plurality of microstructured spikes, e.g., columnar structures extending from the surface to a height above the surface. FIG. 2 schematically depicts a plurality of such features (also referred to herein as spikes) 16 formed on a semiconductor substrate surface 18. Each spike can be characterized by a height and a width (the spikes are shown only for illustrative purposes and are not intended to indicate actual density, size or shape). For example, a spike 16a has a height H defined as the separation between its base 20 and its tip 22, and a width defined by a diameter D of a cross-section, e.g., one substantially parallel to the substrate surface, at a location half way between the base and the tip. In case of irregularly shaped spikes, the width can correspond, e.g., to the largest linear dimension of such a cross-section of the spike. In many embodiments, the submicron-sized features exhibit an average height of about 1 micrometer (e.g., a height in a range of about 200 nm to about 1 micrometer) and an average width in a range of about 100 nm to about 500 nm, or in a range of about 100 nm to about 300 nm. In general, the fluid is selected to be substantially transparent to radiation having wavelength components in a range of about 400 nm to about 800 nm. Further, the thickness of the fluid layer is preferably chosen so as to ensure that it would not interfere with the laser pulses (e.g., via excessive self-focusing of the pulses) in a manner that would inhibit irradiation of the substrate surface. While in this embodiment water is selected as the fluid, in other embodiments other fluids, such as alcohol or silicon oil, can be employed. In some embodiments, at least a portion of the substrate can be placed in contact with an aqueous solution having an electron-donating constituent. For example, a solution of sulfuric acid can be applied to at least a portion of the substrate followed by irradiating that portion with short pulses (e.g., femto-second pulses) to not only cause a change in surface topography in a manner described above but also generate sulfur inclusions within a surface layer of the substrate. Referring to a flow chart 24 of FIG. 3, in another embodiment, initially a solid compound, e.g., sulfur powder, is applied to at least a portion of a semiconductor substrate surface (e.g., a surface of a silicon wafer) (step 26). Subsequently, in a step 28, that surface portion is placed in contact with a fluid (e.g., water) and is irradiated (step 30) by one or more short laser pulses (e.g., pulses with pulse widths in a range of about 100 fs to about a few ns, and preferably in a range of about 100 fs to about 500 fs) so as to cause a change in topography of that portion. Similar to the previous embodiment, the pulse energies can be chosen to be in a range of about 10 microjoules to about 400 microjoules (e.g., 10-150 microjoules). FIG. 4 schematically depicts an exemplary apparatus 32 suitable for performing the above methods of processing a semiconductor substrate. The apparatus 32 includes a Titanium-Saphhire (Ti:Sapphire) laser 34 that generates laser pulses with a pulse width of 80 femtoseconds at 800 nm wavelength having an average power of 300 mW and at a repetition rate of 95 MHz. The pulses generated by the Ti:Sapphire laser are applied to a chirped-pulse regenerative amplifier 36 that, in turn, produces 0.4 millijoule (mJ), 100 femtosecond pulses at a wavelength of 800 nm and at a repetition rate of 1 kilohertz. The apparatus 32 further includes a harmonic generation system 38 that receives the amplified pulses and doubles their frequency to produce 140-microjoule, 100-femtosecond second-harmonic pulses at a wavelength of 400 nanometers. The harmonic generation system can be of the type commonly utilized in the art. For example, it can include a lens 38a for focusing the incoming pulses into a doubling crystal 38b to cause a portion of the incoming radiation to be converted into second-harmonic pulses. A dichroic minor 38c can direct the second-harmonic pulses to a lens 40, and a beam stop 38d can absorb the portion of the radiation that remains at the fundamental frequency. The lens 40 focuses the second-harmonic pulses onto a surface of a sample 42 (e.g., a silicon wafer) disposed on a 3-dimensional translation system 44 within a vacuum chamber 46. A glass liquid cell 48 is coupled to the stage over the sample so as to allow a sample surface to have contact with the fluid (e.g., water) contained within the cell. The three-dimensional stage allows moving the sample relative to the laser pulses for exposing different portions of the surface to radiation. The vacuum chamber can be utilized to pump out air bubbles in the fluid. Alternatively, the processing of the sample can be performed without utilizing a vacuum chamber. To illustrate the efficacy of the teachings of the invention and only for illustrative purposes, submicrometer-sized silicon spikes were generated in surface layers of silicon wafers submerged in water by irradiating those surfaces with 400-nm, 100-fs laser pulses. For example, a Si (111) wafer was initially cleaned with acetone and rinsed in methanol. The wafer was placed in a glass container, such as the container 48 described above, that was filled with distilled water and mounted on a three-axis stage. The silicon surface in contact with the water was irradiated by a 1-KHz train of 100-fs, 60-microjoule pulses at a central wavelength of 400 nm generated by a frequency-doubled, amplified Ti:Sapphire laser, such as that described above. A fast shutter was utilized to control the number of laser pulses incident on the silicon surface. The laser pulses were focused by a 0.25-m focal-length lens to a focal point about 10 mm behind the silicon surface. The pulses traveled through about 10 mm of water before striking the silicon surface at normal incidence. The spatial profile of the laser spot at the sample surface was nearly Gaussian characterized by a fixed beam waist of about 50 microns. To correct for chirping of the laser pulses in the water and to ensure minimum pulse duration at the silicon surface, the pulses were pre-chirped to obtain the lowest possible damage threshold at that surface. The results, however, did not depend strongly on the chirping of the laser pulses. During sample irradiation, the irradiated sample surface was monitored with an optical imaging system having a spatial resolution of about 5 microns. It was observed that irradiation cause formation of micrometer-sized water bubbles at the silicon-water interface. After a single pulse, two or three microbubbles were generated; after irradiation with trains of laser pulses thousands of bubbles were generated. It was also observed that some bubbles at times would coalesce to form larger ones, which would adhere to the silicon surface. These larger bubbles were removed by shaking the cell. FIGS. 5A, 5B, and 5C present electron micrographs of the silicon surface after irradiation with one thousand laser pulses, showing formation of a plurality of spikes on the surface. The spikes have a substantially columnar shape with a typical height of about 500 nm and a typical diameter of about 200 nm. They protrude up to about 100 nm above the original surface of the wafer (FIG. 1C). The shape of the spikes is more columnar than the conical spikes that can be formed in the presence of SF6, as disclosed in the above-referenced patent applications. The chemical composition of the uppermost 10 nm of the silicon surface layer having the spikes was determined by employing X-ray photoelectron spectroscopy (XPS). The XPS spectra showed that this layer is composed of about 83% SiO2 and about 17% silicon. The wafer was etched in 5% HF for about 15 minutes to remove the SiO2 layer (about 20 nm in thickness) while leaving the underlying unoxidized Si intact. A comparison of the electron micrographs of the spikes before etching (FIG. 6A) with those obtained after etching (FIG. 6B) showed that the etching process had reduced the width of the spikes by about 40 nm and had rendered their surfaces smoother. After etching, no SiO2 was detected in the X-ray photoelectron spectra of the sample, thereby indicating that the interior of the spikes consisted of silicon and that the spikes were covered prior to etching by an oxide layer that was at most about 20 nm thick. To study the development of the spikes, silicon samples were irradiated with different numbers of laser pulses. FIG. 7A-7J show a series of scanning electron micrographs of the surface of a silicon substrate irradiated with an increasing number of femto second laser pulses while in contact with water, in a manner described above. The images only show the central portion of the irradiated area. As shown in FIG. 7A, a single laser pulse forms surface structures resembling ripples on a liquid surface with a wavelength of about 500 nm. Lower magnification micrographs (not shown here) indicate that the irradiated region typically contains two or three of these ripple-like structures. Without being limited to any particular theory, each ripple structure is likely to correspond to one of the microbubbles that were observed after irradiation. Referring to FIG. 7B, after two pulses, the surface shows overlapping ripple structures. As the number of laser pulses is increased from 5 to 20 (FIGS. 7C, 7D and 7E), the silicon surface roughens from the interaction of many ripple structures. After exposure to 50 laser pulses (FIG. 7F), the surface is covered with submicrometer bead-like structures, which then evolve into spikes as the number of pulses is further increased as shown in FIGS. 7G, 7H, 71 and 7J corresponding, respectively, to irradiating the surface with 100, 200, 300 and 400 laser pulses. The average separation of the resulting spikes is roughly 500 nm and substantially equal to the wavelength of the initial ripple structures. The above silicon spikes prepared in water are one to two orders of magnitude smaller than spikes that can be generated in a silicon substrate exposed to laser pulses in presence of a gas, such as those described in the above-referenced copending patent applications. This remarkable size difference suggests different formation mechanisms for the two types of spikes. Without being limited to any particular theory, it is noted that when a 400-nm laser pulse interacts with the silicon surface, most of the light is absorbed by a silicon layer tens of nanometers thick near the silicon-water interface. The absorption of intense light in such a thin silicon layer can excite a plasma at the silicon-water interface, which can then equilibrate with the surrounding water and silicon, leaving behind a molten silicon layer on the surface. The molten layer can solidify before the next laser pulse arrives. Due to the high temperature of the plasma, some of the water can vaporize or dissociate, thereby generating bubbles at the silicon-water interface. The large bubbles that were observed after irradiation in the above experiments remain in the water for days, thus suggesting that they consist primarily of gaseous hydrogen and oxygen rather than water vapor. Again, without being limited to any particular theory, several possible mechanisms can be considered by which the bubbles may produce the wave-like structures shown in FIGS. 7A-7J. Diffraction of the laser beam by the bubbles may produce rings of light intensity on the silicon surface, or the heat of vaporization and dissociation required to form a bubble at the silicon-water interface may cool the silicon surface locally, exciting a capillary wave in the molten silicon through Marangoni flow. The latter is the most likely formation mechanism for the structures observed after a single pulse; those structures cannot be formed by diffraction from a laser-induced bubble, as the pulse duration is only 100 fs, and the observed wave-like structures can be several micrometers in diameter. A micrometer-sized bubble requires much longer than 100 fs to form and therefore cannot diffract the first pulse. Roughness on the silicon surface can cause an uneven absorption of the laser pulse energy across the surface. The resulting non-uniform temperature of the surface can produce a random arrangement of bubbles. Silicon-water has a contact angle more than 45°, making a gaseous layer between the silicon and water unstable and leading to the formation of bubbles. The vaporization and dissociation of the bubbles can remove thermal energy from the molten silicon surface just below the bubbles, causing the surface to cool rapidly. Because the surface tension of liquid silicon decreases with increasing temperature, the surrounding hot liquid silicon flows toward the cooled region, deforming the surface. This deformation can then excite a circular capillary wave at the liquid-silicon surface. Superposition of ripple-structures caused by multiple laser pulses can then produce the randomly distributed submicrometer beads that appear after 20 laser pulses (See FIGS. 7F-7J). These beads subsequently sharpen into spikes through preferential removal of material around the beads by laser-assisted etching. As noted above, the morphology and sizes of the above spikes generated in a silicon surface by exposing it to femtosecond laser pulses while in contact with water can be different than those observed for spikes generated by irradiating a silicon surface with femtosecond pulses in presence of a gas, such as SF6. The early stage of submicrometer spike formation in water can be different from that in gaseous SF6, while the later stages can be similar. In SF6, straight submicrometer-sized ripple structures first form on the silicon surface, then coarser, micrometer-scale ridges form on top of (and perpendicular to) the ripples. Next, the coarsened layer breaks up into micrometer-sized beads, and finally the beads evolve into spikes through etching. In both SF6 and water, the length scale of the final structures is set by the arrangement of beadlike structures that form after roughly 10-20 pulses, and this length scale appears to be determined by capillary waves in the molten silicon. The much smaller size of the spikes formed in water is likely to be due to a difference in capillary wavelength in the two cases. The molten silicon layer is expected to solidify much faster in water than in SF6, as the thermal conductivity and heat capacity of liquid water are much greater than those of gaseous SF6. The dispersion relation for capillary waves in a shallow layer of molten silicon indicates that decreasing the lifetime of the molten layer should also decrease the longest allowed capillary wavelength. Using a simple model that neglects the effects of ablation and cooling by heat transfer to the environment to calculate the lifetime and depth of the liquid layer, it was found that the longest allowed capillary wavelength is about 1 micron. Because the lifetime is certainly reduced by the flow of heat to the surrounding water in the experiments presented above, the longest allowed wavelength should be less than 1 micron, which is in agreement with submicrometer spike separation observed here. In some embodiments, rather than utilizing a fluid, a solid substance having an electron-donating constituent (e.g., a sulfur powder) is disposed on at least a portion of a surface of semiconductor substrate, e.g., a silicon wafer. That surface portion is then irradiated with one or more pulses having pulse widths in a range of about 50 fs to about 500 fs so as to generate a plurality of inclusions containing the electron donating constituent in a surface layer of the substrate. Those having ordinary skill in the art will appreciate that various changes can be made to the above embodiments without departing from the scope of the invention. |
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061920983 | abstract | A corrosion and hydride resistant nuclear fuel rod having a highly corrosion resistant outer portion in which hydride precipitation is inhibited and an inner portion in which hydride precipitation is promoted. |
048521418 | abstract | A shielding apparatus for use in conjunction with an X-ray generator is disclosed. The apparatus comprises a shielding cone extending from the X-ray generator toward the object to be X-rayed, said cone comprising an upper cone portion detachably connected to the X-ray generator, a lower cone portion forming the base of the cone and a plurality of panels detachably connected to and extending between the upper and lower cone support portions. Additional shielding components are provided about the X-ray generator and secured to the shielded cone to further reduce the emission of stray X-rays. |
051732505 | abstract | The present invention presents a novel apparatus and a method of demolishing a biological shield wall of a nuclear reactor using a concrete cutter device working on a wire saw, said apparatus comprising a concrete cutter device consisting of a driving part for a wire saw and a concrete cutting part attached on the engaging receiver of the driving part, a core boring machine to be attached interchangeably on the receiver in the place of the cutting part and a carrier truck to carry the wire saw driving part of said cutter device, further the concrete cutting part having a pair of vertical rods supported on a table movable vertically and adjustable in a distance of the rods and, at the bottom end of the rods, supporting pulleys and guide rollers for disposing the wire saw are provided. |
description | This application is a continuation application of U.S. patent application Ser. No. 14/434,682, filed on Apr. 9, 2015, which is a National Stage of International Patent Application No. PCT/US2013/064332 filed on Oct. 10, 2013, which claims priority to U.S. Provisional Application Ser. No. 61/711,807 filed on Oct. 10, 2012, and U.S. Provisional Application Ser. No. 61/711,801 filed on Oct. 10, 2012. The entire disclosures of these applications are incorporated by reference herein. As demand for petroleum increases, so too does interest in renewable feedstocks for manufacturing biofuels and biochemicals. The use of lignocellulosic biomass as a feedstock for such manufacturing processes has been studied since the 1970s. Lignocellulosic biomass is attractive because it is abundant, renewable, domestically produced, and does not compete with food industry uses. Many potential lignocellulosic feedstocks are available today, including agricultural residues, woody biomass, municipal waste, oilseeds/cakes and sea weeds, to name a few. At present these materials are either used as animal feed, biocompost materials are burned in a cogeneration facility or are landfilled. Lignocellulosic biomass comprises crystalline cellulose fibrils embedded in a hemicellulose matrix, surrounded by lignin. This produces a compact matrix that is difficult to access by enzymes and other chemical, biochemical and biological processes. Cellulosic biomass materials (i.e., biomass material from which the lignin has been removed) is more accessible to enzymes and other conversion processes, but even so, naturally-occurring cellulosic materials often have low yields (relative to theoretical yields) when contacted with hydrolyzing enzymes. Lignocellulosic biomass is even more recalcitrant to enzyme attack. Furthermore, each type of lignocellulosic biomass has its own specific composition of cellulose, hemicellulose and lignin. While a number of methods have been tried to extract structural carbohydrates from lignocellulosic biomass, they are either are too expensive, produce too low a yield, leave undesirable chemicals in the resulting product, or simply degrade the sugars. Monosaccharides from renewable biomass sources could become the basis of chemical and fuels industries by replacing, supplementing or substituting petroleum and other fossil feedstocks. However, techniques need to be developed that will make these monosaccharides available in large quantities and at acceptable purities and prices. Described herein are methods for treating a biomass material, where the method includes passing an electron beam through multiple window foils and into the biomass material. The multiple window foils can include a system of cooled window foils. In another implementation the invention pertains to methods and systems for cooling a primary and secondary foil window of a scanning type electron beam accelerator. In one embodiment, the invention pertains to methods and systems for cooling a primary and secondary foil window of a scanning type electron beam accelerator and irradiating a material (e.g., a biomass material). A method is provided for producing a treated biomass material, where the method includes: providing a starting biomass material; and passing an electron beam through multiple window foils into starting biomass material; thereby producing a treated biomass material. The treated biomass material can have a lower level of recalcitrance relative to the starting biomass material. The multiple window foils can include a system of gas cooled window foils. Also provided is a system for cooling multiple single-type window foils of an electron beam accelerator, where the system includes: a first flow path for providing a first cooling gas across a primary single-type window foil and second flow path for providing a second cooling gas across a secondary single-type window foil, where the primary and secondary single-type window foils are positioned with a gap of less than about 9 cm between them. Alternately, if the energy of electron beam accelerator is high, than larger gaps can be used. Gaps as large as 75 cm can be used. Also provided is a method for cooling multiple single-type window foils of an electron beam accelerator, where the methods includes: passing a first cooling gas across a primary single-type window foil and passing a second cooling gas across a secondary single-type window foil, where the primary and secondary single-type window foils are positioned facing each other with a gap of less than about 9 cm between them. The system of gas cooled window foils can include: a primary single-type window foil attached to a scanning horn of an electron beam accelerator; a secondary single-type window foil positioned on an atmospheric side of the scanning horn; a first flow path providing a first cooling gas across the primary single-type window foil; a second flow path providing a second cooling gas across the secondary single-type window foil; and a gap between the primary single-type window foil and the secondary single-type window foil. The system of gas cooled window foils can further include: a cooling chamber having an interior volume defined by one or more walls, the primary single-type window foil and the secondary single-type window foil, wherein the cooling chamber include: a first inlet, which allows a first cooling gas to enter the interior volume; an optional second inlet, which allows optionally a second cooling gas to enter the interior volume; and at least one outlet, which allows the first and the second cooling gasses to exit the interior volume. The cooling chamber can include four walls and the interior volume can be approximately rectangular prism in shape. The system of gas cooled window foils can further include a treatment enclosure with a cover surface, where the enclosure is positioned on a side of the secondary single-type window foil opposite the electron beam accelerator. The secondary single-type window foil can be mounted on the cover surface. The cover surface can be perpendicular to the electron beam accelerator. The treatment enclosure can have a first opening. The methods and systems can also include the steps of: conveying the biomass material through the first opening; positioning the biomass material under the secondary single-type window foil; and irradiating the biomass material; thereby producing a treated biomass material. The treatment enclosure can include a second opening. The method can include the step of conveying the treated biomass material out of the treatment enclosure through the second opening. Positioning the biomass can be instantaneous, that is, the positioning step can include conveying the material on a conveyer belt that is continuously moving. The method can also include purging the treatment enclosure with an inert gas, or a reactive gas. The primary single-type window foil can be made from an element selected from the group consisting of: titanium, scandium, vanadium, chromium, nickel, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, hafnium, tantalum, tungsten, rhenium, platinum, iridium, and alloys or mixtures of any of these. Alternatively, the secondary single-type window foil can be made from an element selected from the group consisting of: titanium, scandium, vanadium, chromium, nickel, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, hafnium, tantalum, tungsten, rhenium, platinum, iridium, beryllium, aluminum, silicon, and alloys or mixtures of any of these. The primary single-type window foil and the secondary single-type window foil can be made of the same element, alloy, or mixture, or they can be made of different elements, alloys, or mixtures. The primary single-type window foil or the secondary single-type window foil or both can be made from a low Z element. The primary single-type window foil can be made from a high Z element and the secondary single-type window foil can be made from a low Z element. The primary single-type window foil can be from 10 to 50 microns thick, from 15 to 40 microns thick, from 20 to 30 microns thick, from 5 to 30 microns thick, from 8 to 25 microns thick, or from 10 to 20 microns thick. The single-type window foils can be the same thickness, or different thickness. The starting biomass material is selected from the group consisting of: cellulosic material, lignocellulosic material, and starchy material. The biomass can be paper, paper products, paper waste, wood, particle board, sawdust, agricultural waste, sewage, silage, grasses, wheat straw, rice hulls, bagasse, cotton, jute, hemp, flax, bamboo, sisal, abaca, straw, corn cobs, corn stover, alfalfa, hay, coconut hair, seaweed, algae, and mixtures thereof. The biomass can be treated with between 10 and 200 Mrad of radiation, between 10 and 75 Mrad of radiation, between 15 and 50 Mrad of radiation, or between 20 and 35 Mrad of radiation. The electron beam can include electrons having an energy of about 0.5-10 MeV, about 0.8-5 MeV, about 0.8-3 MeV, about 1-3 MeV, or about 1 MeV. The electron beam can have a beam current of at least about 50 mA, at least about 60 mA, at least about 70 mA, at least about 80 mA, at least about 90 mA, at least about 100 mA, at least about 125 mA, at least about 150 mA. The electron beam can include electrons having an energy of about 1 MeV, and the spacing between the primary single-type window foil and the secondary single-type window foil can be less than about 30 centimeters. The electron beam can include electrons having an energy of about 1 MeV, and the spacing between the primary single-type window foil and the secondary single-type window foil can be less than 20 centimeters. The electron beam can include electrons having an energy of about 1 MeV, and the spacing between the primary single-type window foil and the secondary single-type window foil can be less than 10 centimeters. Alternatively, the electron beam comprises electrons can have an energy of about 5 MeV, and the spacing between the primary single-type window foil and the secondary single-type window foil can be less than 75 centimeters. The electron beam comprises electrons can have an energy of about 5 MeV, and the spacing between the primary single-type window foil and the secondary single-type window foil can be less than 60 centimeters. The electron beam comprises electrons can have an energy of about 5 MeV, and the spacing between the primary single-type window foil and the secondary single-type window foil can be less than 50 centimeters. The electron beam comprises electrons can have an energy of about 5 MeV, and the spacing between the primary single-type window foil and the secondary single-type window foil can be less than 40 centimeters. The electron beam comprises electrons can have an energy of about 5 MeV, and the spacing between the primary single-type window foil and the secondary single-type window foil can be less than 30 centimeters. The electron beam comprises electrons can have an energy of about 5 MeV, and the spacing between the primary single-type window foil and the secondary single-type window foil can be less than 20 centimeters. The methods and systems described herein can include a beam stop. One advantage of the methods and systems discussed herein is that the processes are more robust and incur less down time from failure of foil windows. In particular, multiple window systems greatly reduce the likelihood of primary window failure/implosion, which can destroy expensive accelerator parts. Another advantage is that there can be a reduction in the production of toxic by-products, relative to some conventional processes. These advantages provide safer and more robust processing, e.g., higher and safer throughput in producing useful products. Yet another advantage of some of the methods and systems described is that cooling of foil windows can done with a high flow rate of cooling gas without disturbing the material targeted for irradiation. Another advantage of some of the methods and systems is that the gap between window foils allows for a beam stop to be removable placed between the windows. Implementations of the invention can optionally include one or more of the following summarized features. In some implementations, the selected features can be applied or utilized in any order while in others implementations a specific selected sequence is applied or utilized. Individual features can be applied or utilized more than once in any sequence. In addition, an entire sequence, or a portion of a sequence, of applied or utilized features can be applied or utilized once or repeatedly in any order. In some optional implementations, the features can be applied or utilized with different, or where applicable the same, set or varied, quantitative or qualitative parameters as determined by a person skilled in the art. For example, parameters of the features such as size, individual dimensions (e.g., length, width, height), location of, degree (e.g., to what extent such as the degree of recalcitrance) duration, frequency of use, density, concentration, intensity and speed can be varied or set, where applicable as determined by a person of skill in the art. A method irradiating a biomass material by passing an electron beam through multiple windows into the biomass material. The recalcitrance of the biomass is reduced by the irradiating. At least one of the multiple windows is a metallic foil. The primary single-type window foil is on the high vacuum side of the scanning horn of the electron beam accelerator and a secondary window is positioned on the atmospheric side of the scanning horn. In one aspect, the primary single type window foil and the secondary window are part of the same electron beam structure and the foils are cooled by cooling gas. In one configuration both the primary and secondary window foil has cooling gas. In another aspect the primary window foil is on the vacuum side of the scanning horn of the electron beam accelerator and there is a treatment enclosure with a cover surface, where the enclosure is positioned on a side of the secondary single-type window foil opposite the electron beam accelerator, and the secondary single-type window foil is mounted on the cover surface, perpendicular to the electron beam accelerator and mechanically integral to the treatment enclosure. A method of processing biomass where the biomass is conveyed into a first opening of the treatment enclosure, positioned under the secondary single type window foil and irradiating it, followed by conveying the irradiated biomass out the second opening of the enclosure. The gaseous space of treatment enclosure can be purged with an inert gas, a reactive gas or mixtures of these. The window foils may be made from an element selected from the group consisting of: titanium, scandium, vanadium, chromium, nickel, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, hafnium, tantalum, tungsten, rhenium, platinum, iridium, beryllium, aluminum, silicon, and alloys or mixtures of any of these. The window foils may be made up the same or different elements, or alloys as listed previously. The window foils can be made of a low Z element and the single-type primary window can be made of a high Z element. The primary single-type window foil is from 10 to 50 microns thick, alternately 15 microns to 40 microns, optionally 20 to 30 microns thick. The secondary single-type window foil is from 5 to 30 microns thick, alternately 8 microns to 25 microns, optionally 10 to 20 microns thick. The window foils may be of different thickness. The starting biomass material is selected from the group consisting of: cellulosic material, lignocellulosic material, and starchy material and can be selected from the group consisting of paper, paper products, paper waste, wood, particle board, sawdust, agricultural waste, sewage, silage, grasses, wheat straw, rice hulls, bagasse, cotton, jute, hemp, flax, bamboo, sisal, abaca, straw, corn cobs, corn stover, alfalfa, hay, coconut hair, seaweed, algae, and mixtures thereof. The biomass is treated with between 10 and 200 Mrad of radiation, optionally 10 to 75 Mrad, alternatively 15 to 50 Mrad and further optionally 20 to 35 Mrad. The biomass is treated where the electron beam has an energy of between 0.5 to 10 MeV, optionally 0.8 to 5 MeV, alternatively 0.8 to 3 MeV and further optionally 1 to 3 MeV. The biomass is treated where the electron beam has a beam current of at least 50 mA, alternatively, at least 60 mA, optionally, at least 70, further optionally at least 80 mA, alternately, at least 90 mA, alternately, at least 100 mA, optionally at least 125 mA and further optionally at least 150 mA. The biomass is treated with an electron beam with electrons about 1 MeV and the spacing between the primary single-type window foil and secondary single-type window foil is less than 30 centimeters, alternately, where the spacing is less than 20 centimeters, and optionally where the spacing is less than 10 centimeters. Alternately, when the an electron beam with electrons about 5 MeV and the spacing between the primary single-type window foil and secondary single-type window foil is less than 75 centimeters, alternately, where the spacing is less than 60 centimeters, and optionally, where the spacing is less than 50 centimeters, and optionally where the spacing is less than 40 centimeters, and alternately 30 and alternately less than 20 centimeters. The method of treating where the electron beam accelerator has a beam stop which can be moveable to absorb different levels of electrons. The beam stop and its configuration can absorb 10%, 20%, 40%, 60% 80% and 96% of the incident electron energy. Other features and advantages of the methods and systems will be apparent from the following detailed description, and from the claims. Described herein is a method for irradiating biomass material, which facilitates the conversion of the material into useful products and improves the yield of those products from the biomass material. The treatment methods described herein are therefore useful in producing a biomass feedstock for use in other processes. The methods disclosed herein can effectively lower the recalcitrance level of the biomass material, improving its utility as a feedstock in the production of useful intermediates and products. The claimed methods make the biomass material easier to process by methods such as bioprocessing (e.g., with any microorganism described herein, such as a homoacetogen or a heteroacetogen, and/or any enzyme described herein), thermal processing (e.g., gasification or pyrolysis) or chemical processing (e.g., acid hydrolysis or oxidation). Biomass material intended for use as a feedstock can be treated or processed using one or more of any of the methods described herein, such as mechanical treatment, chemical treatment, radiation, sonication, oxidation, pyrolysis or steam explosion. The various treatment systems and methods can be used in combinations of two, three, or even four or more of these technologies or others described herein and elsewhere. Saccharified biomass can then be manufactured into various products. For example, FIG. 1, shows a process for manufacturing a sugar and other useful products (e.g., alcohol). The process can include, for example, optionally mechanically treating a feedstock (step 110), before and/or after this treatment, treating the feedstock with another physical treatment, for example irradiation by the methods described herein, to further reduce its recalcitrance (step 112), and saccharifying the feedstock, to form a sugar solution (step 114). Optionally, the method may also include transporting, e.g., by pipeline, railcar, truck or barge, the solution (or the feedstock, enzyme and water, if saccharification is performed en route) to a manufacturing plant (step 116). In some cases the saccharified feedstock is further bioprocessed (e.g., fermented) to produce a desired product (step 118) and byproduct (111). The resulting product may in some implementations be processed further, e.g., by distillation (step 120). If desired, the steps of measuring lignin content (step 122) and setting or adjusting process parameters based on this measurement (step 124) can be performed at various stages of the process, as described in U.S. Pat. App. Pub. 2010/0203495 A1, filed on Feb. 11, 2010, the entire disclosure of which is incorporated herein by reference. FIG. 2 shows an irradiation process. This process can be part of the process described in FIG. 1 or it can be part of a separate process. Initially, biomass can be delivered to a conveyor (150). Optionally, the conveyor can be enclosed. The biomass can be pre-irradiation processed while enclosed in the enclosed conveyor or prior to enclosing the material in the enclosed conveyor. Advantageously, the biomass on the conveyor when in an treatment enclosure, is protected from rapid air currents that can cause the biomass (e.g., fines and dust) to be lofted in the air. This can present an explosion hazard or damage equipment. The biomass can be conveyed through an irradiation zone (e.g., radiation field) (154). After irradiation, the biomass can be post processed (156). The process can be repeated (e.g., dashed arrow A). Finally the irradiated biomass is removed from the conveyor and either collected for later processing or sent directly to make useful products. FIG. 3 shows one embodiment, an enclosed conveying system for irradiating a comminuted biomass. The enclosure has an enclosed distribution system (310), an enclosed conveyor (311), material removal system (318) where the irradiated material exits the conveyor and an irradiation vault and a scan horn (322). The electron window foil (not shown) and enclosure window foil (not shown) have window coolers (320) and (326) respectively for blowing air across the surface of the windows. The enclosed material distribution system (310) distributes the biomass onto the conveyor and brings the biomass from outside of the irradiation vault into the enclosed stainless steel conveyor without generating dust outside of the enclosure (e.g., protecting the biomass from air from the window cooling system). The distribution system can be equipped with a spreading system (not shown) to evenly distribute the biomass on the conveyor to a depth of about 0.25 inches. The enclosed removal system (318) allows the material to fall off of the conveyor belt without generating dust outside of the enclosure, where the material can be collected (e.g., outside the irradiation vault) or directed elsewhere for further processing. The scan horn window and enclosure window can be brought together, or lined up so that the electron beams pass thought the scan horn window, through a small gap of cooling air and then through the enclosure window. For example, the conveyor can be aligned by moving it on casters and then fixing it in place. For example the casters can be blocked with a permanent break, a block, and or a depression. The conveyor can also be aligned by other methods and equipment, for example rails, wheels, pulleys, shims e g, in any combination. In this window arrangement the scan horn window and enclosure window do not touch, so that the remaining gap allows for efficient cooling. The scan horn window is part of the electron beam apparatus and the enclosure window is part of the treatment enclosure system. A cross sectional detailed view of the scan horn and scan horn window of FIG. 3 are shown in FIG. 4A. The scan horn window cooler (426) and enclosure window cooler (420) blow air at high velocity across the windows as indicted by the small arrows. The electrons in the electron beam (430) pass through the vacuum of the scan horn (422) through the scan horn window (428), through the cooling air gap between the scan horn window and enclosure window, through the enclosure window (429) and impinge on and penetrate the biomass material (444) on the conveyor surface (415). The scan horn window is shown as curved towards the vacuum side of the scan horn, for example due to the vacuum. In the embodiment illustrated, the enclosure window is curved towards the conveyed material. The curvature of the windows can help the cooling air path flow past the window for efficient cooling. The enclosure window is mounted on the cover (412) of the enclosed conveyor. The enclosure window is aligned with the cover surface. FIG. 4B shows a different configuration of the detailed cross section view of the enclosed conveyor including a beam stop. A beam stop (440) can be pivotally fixed to the scan horn and is shown in the open position, e.g., allowing the e-beam to impinge on the conveyed material. FIG. 4C shows the cross sectional blowup of the scan horn and scan horn window with a beam stop (440) where the beam stop is in position for blocking the electrons. The cover surface is denoted by 414. Optionally, the conveying system shown in FIG. 3 can be maintained under an atmosphere of an inert or reactive gas by a gentle purge through an inlet connected to a nitrogen gas source. The inlet can be positioned at different locations, for example, close to the zone where the biomass is irradiated to be more effective in reducing ozone formation if purging is with an inert gas; or further and downstream of the irradiation if a reactive gas is used that is designed to reacted with an irradiated material. FIG. 5 is a cross sectional view of another embodiment of two foils window extraction system for a scanning electron beam. The primary foil window (510) in a scanning horn (520) is shown. The region indicated is a high vacuum area (525). Generally, the primary window is concave towards the high vacuum area (525). The secondary foil window (530) is flatter but is also concave in the same direction. This curvature helps provide structural support to the window and is mechanically stronger than a flat window. Alternatively the windows can be flat or curved in any direction. Sidewalls (540) and the primary and secondary windows can define an interior space (550). Since the primary and secondary windows are connected by sidewalls in this configuration both windows are part of the electron beam apparatus. Electrons (560) travel through both windows to impinge on and penetrate the biomass disposed beneath. A first inlet on one sidewall (512) is arranged to allow a cooling fluid (e.g., a liquid or a gas) to impinge on the primary window foil. The cooling fluid runs along the window and then reverses direction on meeting the far (opposite) wall and flows back generally through the center of the interior space as shown and then out through an exhaust port and or outlet (514). A second inlet (516) on the sidewall is arranged to allow cooling fluid to impinge on the secondary window foil in a similar fashion. Optionally more inlets (e.g., 2, 3, 4, 5, 6 or more) can bring cooling fluid to the primary and secondary window surfaces and more than one outlets (e.g., 2, 3, 4, 5, 6 or more) can allow the cooling fluid to exit the interior space. In some embodiments one or more side walls can even be a mesh, screen or grate with many openings through which cooling gas can flow while providing structural support to the windows. The system can include a conveyor, with a conveying surface (570). A material, for example biomass (444), can be conveyed in the direction indicated as a thin pile (574), e.g., about 0.25 inches. Electrons irradiated the material as it is conveyed under the two foil extraction system. The Windows The biomass is irradiated as it passes under a window, which is generally a metallic foil (e.g., titanium, titanium alloy, aluminum and/or silicon). The window is impermeable to gases, yet electrons can pass with low resistance. The foil windows are preferably between about 10 and 100 microns thick (e.g., about 10 microns thick to about 30 microns thick, about 15-40 microns, about 20-30 microns, about 5-30 microns, about 8-25 microns, about 10-20 microns, about 20-25 microns thick, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 microns thick). Thin windows are preferable to thick windows since thin windows dissipate less energy as an electron beam passes through them (e.g., the resistive heating is less since Power is the product of the square of the current and the resistance, P=I2R). Thin windows are also less mechanically strong and more likely to fail which causes increased expense and more downtime for the equipment. The distance between the front surface of the primary window foil and back surface of the secondary window foil is preferably less than 30 cm, more preferably less than 20 cm, and most preferably less than 10 cm. The foil window can be cooled by passing air or an inert gas over the window. When using an enclosure, it is generally preferred to mount the window to the enclosure and to cool the window from the side outside of the enclosed conveying system to avoid lofting up any particulates of the material being irradiated. The system can include more than one window, e.g., a primary window and a secondary window. The two windows may form the enclosure to contain the purging gases and/or the cooling gases. The secondary window may serve a function as a “sacrificial” window, to protect the primary window. The electron beam apparatus includes a vacuum between the electron source and the primary window, and breakage of the primary window is likely to cause biomass material to be sucked up into the electron beam apparatus, resulting in damage, repair costs, and equipment downtime. The window can be polymer, ceramic, coated ceramic, composite or coated composite. The secondary window can be, for instance, a continuous sheet/roll of polymer or coated polymer, which can be advanced continuously or at intervals to provide a clean or new section to serve as the secondary window. The primary window and the secondary window can be made from the same material, or different materials. For instance, the primary window foil can be made from titanium, scandium, vanadium, chromium, nickel, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, hafnium, tantalum, tungsten, rhenium, platinum, iridium, or alloys or mixtures of any of these. The secondary single-type window foil can be made from titanium, scandium, vanadium, chromium, nickel, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, hafnium, tantalum, tungsten, rhenium, platinum, iridium, beryllium, aluminum, silicon, or alloys or mixtures of any of these. The primary and secondary windows can be of the same material, mixture of materials, or alloy, or different materials, mixtures of material or alloys. One or both of the windows can be laminates of the same of different materials, mixtures of materials, or alloys. One of more of the windows can have a support structure across its face. The term “single-type window”, as used herein, means a window with no support structure across its face. The term “double-type window”, as used herein means a window with a support structure across its face, where the support structure effectively divides the surface of the window into two parts. Such a double-type window is shown in U.S. Pat. No. 5,877,582 to Nishimura. Additional support structures can also be used. The primary window foil and the secondary window foil can both be made from low Z element. Alternatively, the primary window foil can be made from a high Z element, and the secondary window foil can be made from a low Z element. The embodiments described herein do not preclude the inclusion of additional windows, which may have a protective function, or may be included to modify the radiation exposure. The windows can be concave, flat or convex. It is generally preferred that the window be slightly convex, in a direction away from the direction of the cooling fluid. This curvature improves the mechanical strength of the window and increases the permitted temperature levels as well as allowing a better flow path for the cooling fluid. On the side of the scanning horn the curvature tends to be towards the vacuum (e.g., away from the cooling fluid) due to the vacuum (e.g., about 10−5 to 10−19 torr, about 10−6 to 10−9 torr, about 10−7 to 10−8 torr). The cooling of the window and/or concave shape of the window become especially important for high beam currents, for example at least about 100 mA electron gun currents (e.g., at least about 110 mA, at least about 120 mA, at least about 130 mA, at least about 140 mA, at least about 150 mA at least about 200 mA, at least about 500 mA, at least about 1000 mA) because resistive heating is approximately related to the square of the current as discussed above. The windows can be any shape but typically are approximately rectangular with a high aspect ratio of the width to the length (where the width direction is the same as the width of the conveying system perpendicular to the conveying direction, and the length is the same as the direction of conveying). The distance of the window to the conveyed material can be less than about 10 cm (e.g., less than about 5 cm) and more than about 0.1 cm (e.g., more than about 1 cm, more than about 2 cm, more than about 3 cm, more than about 4 cm). It is also possible to use multiple windows (e.g., 3, 4, 5, 6 or more) with different and varied shapes and configured in different ways. For example, a primary or secondary foil window can include one, two or more windows in the same plane or layered and can include one or more support structures. For example, support structures can be a bar or a grid in the same plane and contacting the windows. In some embodiments, the window that is mounted on the enclosed conveying system is a secondary foil window of a two foil window extraction system for a scanning electron beam. In other embodiments there is no enclosure for conveying the biomass material, e.g., the biomass is conveyed in air under the irradiation device. Window Spacing Although a large spacing between the windows can be advantageous, for example, for the reasons described above, the large spacing poses some disadvantages. One disadvantage of a large spacing between windows is that the electron beams will pass through a larger volume of cooling gas which can cause energy losses. For example, a 1 MeV beam loses about 0.2 MeV/m of energy, a 5 MeV beam loses about 0.23 MeV/m and a 10 MeV beam loses about 0.26 MeV/m. Therefore with a 1 MeV beam of electrons passing through 1 cm of air, the beam loses only 0.2% of its energy, at 10 cm of air, the beam loses 2% of its energy, at 20 cm this is 4% of its energy, while at 50 cm the energy loss is 10%. Since the electrons also have to travel from the secondary foil window to the biomass through additional air, the gap between the windows must be carefully controlled. Preferably, energy losses are less that about 20% (e.g., less than 10%, less than 5% or even less than 1%). It is therefore advantageous to minimize the spacing between the windows to decrease energy losses. Optimal spacing (e.g., average spacing) between the windows (e.g., between the surface side of the electron window foil and the facing surface of the secondary window foil) for the benefit of cooling as described above and for the benefit of reducing energy loss are less than 30 cm (e.g., between about 2 and 20 cm, between about 3 and 20 cm, between about 4 and 20 cm, between about 5 and 20 cm, between about 6 and 20 cm, between about 7 and 20 cm, between about 8 and 20 cm, between about 3 and 15 cm, between about 4 and 15 cm, between about 5 and 15 cm, between about 6 and 15 cm, between about 7 and 15 cm, between about 8 and 15 cm between about 3 and 10 cm, between about 4 and 10 cm, between about 5 and 10 cm, between about 6 and 10 cm, between about 7 and 10 cm, between about 8 and 10 cm, preferably less than 20 cm, and most preferably less than 10 cm. Alternatively, at higher MeV equipment a greater gap can be tolerated. The higher gap can be as great as 75 cm. In some embodiments support structures for the windows can be used across the windows, although these types of structures are less preferred because of energy losses that can occur to the electron beam as it strikes these kinds of structures. A large spacing between the windows can be advantageous because it defines a larger volume between the windows and allows for rapid flowing of a large volume cooling gasses for very efficient cooling. The inlets and outlets are between 1 mm and 120 mm in diameter (e.g., about 2 mm, about 5 mm about 10 mm, about 20 mm, about 50 mm or even about 100 mm). The cooling gas flow can be at between about 500-2500 CFM (e.g., about 600 to 2500 CFM, about 700-2500 CFM, about 800 to 2500 CFM, about 1000 to 2500 CFM, about 600 to 2000 CFM, about 700-2000 CFM, about 800 to 2000 CFM, about 1000 to 2000 CFM, about 600 to 1500 CFM, about 700-1500 CFM, about 800 to 1500 CFM, about 1000 to 1500 CFM). In some embodiments, about 50% of the gas is exchanged per about 60 seconds or less (e.g., in about 50 sec or less, in about 30 sec or less, in about 10 sec or less, in about 1 sec or less). Cooling and Purging Gases The cooling gas in the two foil window extraction system can be a purge gas or a mixture, for example air, or a pure gas. In some embodiments the gas is an inert gas such as nitrogen, argon, helium and or carbon dioxide. It is preferred to use a gas rather than a liquid since energy losses to the electron beam are minimized. Mixtures of pure gas can also be used, either pre-mixed or mixed in line prior to impinging on the windows or in the space between the windows. The cooling gas can be cooled, for example, by using a heat exchange system (e.g., a chiller) and/or by using boil off from a condensed gas (e.g., liquid nitrogen, liquid helium). When using an enclosure, the enclosed conveyor can also be purged with an inert gas so as to maintain an atmosphere at a reduced oxygen level. Keeping oxygen levels low avoids the formation of ozone which in some instances is undesirable due to its reactive and toxic nature. For example, the oxygen can be less than about 20% (e.g., less than about 10%, less than about 1%, less than about 0.1%, less than about 0.01%, or even less than about 0.001% oxygen). Purging can be done with an inert gas including, but not limited to, nitrogen, argon, helium or carbon dioxide. This can be supplied, for example, from a boil off of a liquid source (e.g., liquid nitrogen or helium), generated or separated from air in situ, or supplied from tanks. The inert gas can be recirculated and any residual oxygen can be removed using a catalyst, such as a copper catalyst bed. Alternatively, combinations of purging, recirculating and oxygen removal can be done to keep the oxygen levels low. The enclosure can also be purged with a reactive gas that can react with the biomass. This can be done before, during or after the irradiation process. The reactive gas can be, but is not limited to, nitrous oxide, ammonia, oxygen, ozone, hydrocarbons, aromatic compounds, amides, peroxides, azides, halides, oxyhalides, phosphides, phosphines, arsines, sulfides, thiols, boranes and/or hydrides. The reactive gas can be activated in the enclosure, e.g., by irradiation (e.g., electron beam, UV irradiation, microwave irradiation, heating, IR radiation), so that it reacts with the biomass. The biomass itself can be activated, for example by irradiation. Preferably the biomass is activated by the electron beam, to produce radicals which then react with the activated or unactivated reactive gas, e.g., by radical coupling or quenching. Purging gases supplied to an enclosed conveyor can also be cooled, for example below about 25° C., below about 0° C., below about −40° C., below about −80° C., below about −120° C. For example, the gas can be boiled off from a compressed gas such as liquid nitrogen or sublimed from solid carbon dioxide. As an alternative example, the gas can be cooled by a chiller or part of or the entire conveyor can be cooled. Beam Stops In some embodiments the systems and methods include a beam stop (e.g., a shutter). For example, the beam stop can be used to quickly stop or reduce the irradiation of material without powering down the electron beam device. Alternatively the beam stop can be used while powering up the electron beam, e.g., the beam stop can stop the electron beam until a beam current of a desired level is achieved. The beam stop can be placed between the primary foil window and secondary foil window. For example, the beam stop can be mounted so that it is movable, that is, so that it can be moved into and out of the beam path. Even partial coverage of the beam can be used, for example, to control the dose of irradiation. The beam stop can be mounted to the floor, to a conveyor for the biomass, to a wall, to the radiation device (e.g., at the scan horn), or to any structural support. Preferably the beam stop is fixed in relation to the scan horn so that the beam can be effectively controlled by the beam stop. The beam stop can incorporate a hinge, a rail, wheels, slots, or other means allowing for its operation in moving into and out of the beam. The beam stop can be made of any material that will stop at least 5% of the electrons, e.g., at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or even about 100% of the electrons. Useful levels of stopping electrons can be 10%, 20%, 40%, 60%, 80% and 96% The beam stop can be made of a metal including, but not limited to, stainless steel, lead, iron, molybdenum, silver, gold, titanium, aluminum, tin, or alloys of these, or laminates (layered materials) made with such metals (e.g., metal-coated ceramic, metal-coated polymer, metal-coated composite, multilayered metal materials). The beam stop can be cooled, for example, with a cooling fluid such as an aqueous solution or a gas. The beam stop can be partially or completely hollow, for example with cavities. Interior spaces of the beam stop can be used for cooling fluids and gases. The beam stop can be of any shape, including flat, curved, round, oval, square, rectangular, beveled and wedged shapes. The beam stop can have perforations so as to allow some electrons through, thus controlling (e.g., reducing) the levels of radiation across the whole area of the window, or in specific regions of the window. The beam stop can be a mesh formed, for example, from fibers or wires. Multiple beam stops can be used, together or independently, to control the irradiation. The beam stop can be remotely controlled, e.g., by radio signal or hard wired to a motor for moving the beam into or out of position. Radiation Sources The type of radiation determines the kinds of radiation sources used as well as the radiation devices and associated equipment. The methods, systems and equipment described herein, for example for treating materials with radiation, can utilized sources as described herein as well as any other useful source. Sources of gamma rays include radioactive nuclei, such as isotopes of cobalt, calcium, technicium, chromium, gallium, indium, iodine, iron, krypton, samarium, selenium, sodium, thalium, and xenon. Sources of X-rays include electron beam collision with metal targets, such as tungsten or molybdenum or alloys, or compact light sources, such as those produced commercially by Lyncean. Alpha particles are identical to the nucleus of a helium atom and are produced by the alpha decay of various radioactive nuclei, such as isotopes of bismuth, polonium, astatine, radon, francium, radium, several actinides, such as actinium, thorium, uranium, neptunium, curium, californium, americium, and plutonium. Sources for ultraviolet radiation include deuterium or cadmium lamps. Sources for infrared radiation include sapphire, zinc, or selenide window ceramic lamps. Sources for microwaves include klystrons, Slevin type RF sources, or atom beam sources that employ hydrogen, oxygen, or nitrogen gases. Accelerators used to accelerate the particles (e.g., electrons or ions) can be electrostatic DC, e. g. electrodynamic DC, RF linear, magnetic induction linear or continuous wave. For example, various irradiating devices may be used in the methods disclosed herein, including field ionization sources, electrostatic ion separators, field ionization generators, thermionic emission sources, microwave discharge ion sources, recirculating or static accelerators, dynamic linear accelerators, van de Graaff accelerators, Cockroft Walton accelerators (e.g., PELLETRON® accelerators), LINACS, Dynamitrons (e.g, DYNAMITRON® accelerators), cyclotrons, synchrotrons, betatrons, transformer-type accelerators, microtrons, plasma generators, cascade accelerators, and folded tandem accelerators. For example, cyclotron type accelerators are available from IBA, Belgium, such as the RHODOTRON™ system, while DC type accelerators are available from RDI, now IBA Industrial, such as the DYNAMITRON®. Other suitable accelerator systems include, for example: DC insulated core transformer (ICT) type systems, available from Nissin High Voltage, Japan; S-band LINACs, available from L3-PSD (USA), Linac Systems (France), Mevex (Canada), and Mitsubishi Heavy Industries (Japan); L-band LINACs, available from Iotron Industries (Canada); and ILU-based accelerators, available from Budker Laboratories (Russia). Ions and ion accelerators are discussed in Introductory Nuclear Physics, Kenneth S. Krane, John Wiley & Sons, Inc. (1988), Krsto Prelec, FIZIKA B 6 (1997) 4, 177-206, Chu, William T., “Overview of Light-Ion Beam Therapy”, Columbus-Ohio, ICRU-IAEA Meeting, 18-20 Mar. 2006, Iwata, Y. et al., “Alternating-Phase-Focused IH-DTL for Heavy-Ion Medical Accelerators”, Proceedings of EPAC 2006, Edinburgh, Scotland, and Leitner, C. M. et al., “Status of the Superconducting ECR Ion Source Venus”, Proceedings of EPAC 2000, Vienna, Austria. Some particle accelerators and their uses are disclosed, for example, in U.S. Pat. No. 7,931,784 to Medoff, the complete disclosure of which is incorporated herein by reference. Electrons may be produced by radioactive nuclei that undergo beta decay, such as isotopes of iodine, cesium, technetium, and iridium. Alternatively, an electron gun can be used as an electron source via thermionic emission and accelerated through an accelerating potential. An electron gun generates electrons, which are then accelerated through a large potential (e.g., greater than about 500 thousand, greater than about 1 million, greater than about 2 million, greater than about 5 million, greater than about 6 million, greater than about 7 million, greater than about 8 million, greater than about 9 million, or even greater than 10 million volts) and then scanned magnetically in the x-y plane, where the electrons are initially accelerated in the z direction down the accelerator tube and extracted through a foil window. Scanning the electron beams is useful for increasing the irradiation surface when irradiating materials, e.g., a biomass, that is conveyed through the scanned beam. Scanning the electron beam also distributes the thermal load homogenously on the window and helps reduce the foil window rupture due to local heating by the electron beam. Window foil rupture is a cause of significant down-time due to subsequent necessary repairs and re-starting the electron gun. A beam of electrons can be used as the radiation source. A beam of electrons has the advantages of high dose rates (e.g., 1, 5, or even 10 Mrad per second), high throughput, less containment, and less confinement equipment. Electron beams can also have high electrical efficiency (e.g., 80%), allowing for lower energy usage relative to other radiation methods, which can translate into a lower cost of operation and lower greenhouse gas emissions corresponding to the smaller amount of energy used. Electron beams can be generated, e.g., by electrostatic generators, cascade generators, transformer generators, low energy accelerators with a scanning system, low energy accelerators with a linear cathode, linear accelerators, and pulsed accelerators. Electrons can also be more efficient at causing changes in the molecular structure of carbohydrate-containing materials, for example, by the mechanism of chain scission. In addition, electrons having energies of 0.5-10 MeV can penetrate low density materials, such as the biomass materials described herein, e.g., materials having a bulk density of less than 0.5 g/cm3, and a depth of 0.3-10 cm. Electrons as an ionizing radiation source can be useful, e.g., for relatively thin piles, layers or beds of materials, e.g., less than about 0.5 inch, e.g., less than about 0.4 inch, 0.3 inch, 0.25 inch, or less than about 0.1 inch. In some embodiments, the energy of each electron of the electron beam is from about 0.3 MeV to about 2.0 MeV (million electron volts), e.g., from about 0.5 MeV to about 1.5 MeV, or from about 0.7 MeV to about 1.25 MeV. Methods of irradiating materials are discussed in U.S. Pat. App. Pub. 2012/0100577 A1, filed Oct. 18, 2011, the entire disclosure of which is herein incorporated by reference. Electron beam irradiation devices may be procured commercially or built. For example elements or components such inductors, capacitors, casings, power sources, cables, wiring, voltage control systems, current control elements, insulating material, microcontrollers and cooling equipment can be purchased and assembled into a device. Optionally, a commercial device can be modified and/or adapted. For example, devices and components can be purchased from any of the commercial sources described herein including Ion Beam Applications (Louvain-la-Neuve, Belgium), NHV Corporation (Japan), the Titan Corporation (San Diego, Calif.), Vivirad High Voltage Corp (Billeric, Mass.) and/or Budker Laboratories (Russia). Typical electron energies can be 0.5 MeV, 1 MeV, 2 MeV, 4.5 MeV, 7.5 MeV, or 10 MeV. Typical electron beam irradiation device power can be 1 kW, 5 kW, 10 kW, 20 kW, 50 kW, 60 kW, 70 kW, 80 kW, 90 kW, 100 kW, 125 kW, 150 kW, 175 kW, 200 kW, 250 kW, 300 kW, 350 kW, 400 kW, 450 kW, 500 kW, 600 kW, 700 kW, 800 kW, 900 kW or even 1000 kW. Accelerators that can be used include NHV irradiators medium energy series EPS-500 (e.g., 500 kV accelerator voltage and 65, 100 or 150 mA beam current), EPS-800 (e.g., 800 kV accelerator voltage and 65 or 100 mA beam current), or EPS-1000 (e.g., 1000 kV accelerator voltage and 65 or 100 mA beam current). Also, accelerators from NHV's high energy series can be used such as EPS-1500 (e.g., 1500 kV accelerator voltage and 65 mA beam current), EPS-2000 (e.g., 2000 kV accelerator voltage and 50 mA beam current), EPS-3000 (e.g., 3000 kV accelerator voltage and 50 mA beam current) and EPS-5000 (e.g., 5000 and 30 mA beam current). Tradeoffs in considering electron beam irradiation device power specifications include cost to operate, capital costs, depreciation, and device footprint. Tradeoffs in considering exposure dose levels of electron beam irradiation would be energy costs and environment, safety, and health (ESH) concerns. Typically, generators are housed in a vault, e.g., of lead or concrete, especially for production from X-rays that are generated in the process. Tradeoffs in considering electron energies include energy costs. The electron beam irradiation device can produce either a fixed beam or a scanning beam. A scanning beam may be advantageous with large scan sweep length and high scan speeds, as this would effectively replace a large, fixed beam width. Further, available sweep widths of 0.5 m, 1 m, 2 m or more are available. The scanning beam is preferred in most embodiments describe herein because of the larger scan width and reduced possibility of local heating and failure of the windows. Subsequent Use of the Feedstocks Using the methods described herein, a starting biomass material (e.g., plant biomass, animal biomass, paper, and municipal waste biomass) can be used as feedstock to produce useful intermediates and products such as organic acids, salts of organic acids, anhydrides, esters of organic acids and fuels, e.g., fuels for internal combustion engines or feedstocks for fuel cells. Systems and processes are described herein that can use as feedstock cellulosic and/or lignocellulosic materials that are readily available, but often can be difficult to process, e.g., municipal waste streams and waste paper streams, such as streams that include newspaper, kraft paper, corrugated paper or mixtures of these. In order to convert the feedstock to a form that can be readily processed, the glucan- or xylan-containing cellulose in the feedstock can be hydrolyzed to low molecular weight carbohydrates, such as sugars, by a saccharifying agent, e.g., an enzyme or acid, a process referred to as saccharification. The low molecular weight carbohydrates can then be used, for example, in an existing manufacturing plant, such as a single cell protein plant, an enzyme manufacturing plant, or a fuel plant, e.g., an ethanol manufacturing facility. The feedstock can be hydrolyzed using an enzyme, e.g., by combining the materials and the enzyme in a solvent, e.g., in an aqueous solution. Alternatively, the enzymes can be supplied by organisms that break down biomass, such as the cellulose and/or the lignin portions of the biomass, contain or manufacture various cellulolytic enzymes (cellulases), ligninases or various small molecule biomass-degrading metabolites. These enzymes may be a complex of enzymes that act synergistically to degrade crystalline cellulose or the lignin portions of biomass. Examples of cellulolytic enzymes include: endoglucanases, cellobiohydrolases, and cellobiases (β-glucosidases). During saccharification a cellulosic substrate can be initially hydrolyzed by endoglucanases at random locations producing oligomeric intermediates. These intermediates are then substrates for exo-splitting glucanases such as cellobiohydrolase to produce cellobiose from the ends of the cellulose polymer. Cellobiose is a water-soluble 1,4-linked dimer of glucose. Finally, cellobiase cleaves cellobiose to yield glucose. The efficiency (e.g., time to hydrolyze and/or completeness of hydrolysis) of this process depends on the recalcitrance of the cellulosic material. Biomass Material Preparation—Mechanical Treatments The biomass can be in a dry form, for example with less than about 35% moisture content (e.g., less than about 20%, less than about 15%, less than about 10% less than about 5%, less than about 4%, less than about 3%, less than about 2% or even less than about 1%). The biomass can also be delivered in a wet state, for example as a wet solid, a slurry or a suspension with at least about 10 wt % solids (e.g., at least about 20 wt. %, at least about 30 wt. %, at least about 40 wt. %, at least about 50 wt. %, at least about 60 wt. %, at least about 70 wt. %). The processes disclosed herein can utilize low bulk density materials, for example cellulosic or lignocellulosic feedstocks that have been physically pretreated to have a bulk density of less than about 0.75 g/cm3, e.g., less than about 0.7, 0.65, 0.60, 0.50, 0.35, 0.25, 0.20, 0.15, 0.10, 0.05 or less, e.g., less than about 0.025 g/cm3. Bulk density is determined using ASTM D1895B. Briefly, the method involves filling a measuring cylinder of known volume with a sample and obtaining a weight of the sample. The bulk density is calculated by dividing the weight of the sample in grams by the known volume of the cylinder in cubic centimeters. If desired, low bulk density materials can be densified, for example, by methods described in U.S. Pat. No. 7,971,809 to Medoff, the full disclosure of which is hereby incorporated by reference. In some cases, the pre-irradiation processing includes screening of the biomass material. Screening can be through a mesh or perforated plate with a desired opening size, for example, less than about 6.35 mm (¼ inch, 0.25 inch), (e.g., less than about 3.18 mm (⅛ inch, 0.125 inch), less than about 1.59 mm ( 1/16 inch, 0.0625 inch), is less than about 0.79 mm ( 1/32 inch, 0.03125 inch), e.g., less than about 0.51 mm ( 1/50 inch, 0.02000 inch), less than about 0.40 mm ( 1/64 inch, 0.015625 inch), less than about 0.23 mm (0.009 inch), less than about 0.20 mm ( 1/128 inch, 0.0078125 inch), less than about 0.18 mm (0.007 inch), less than about 0.13 mm (0.005 inch), or even less than about 0.10 mm ( 1/256 inch, 0.00390625 inch)). In one configuration, the desired biomass falls through the perforations or screen and thus biomass larger than the perforations or screen are not irradiated. These larger materials can be re-processed, for example by comminuting, or they can simply be removed from processing. In another configuration, material that is larger than the perforations is irradiated and the smaller material is removed by the screening process or recycled. In this kind of a configuration, the conveyor itself (for example a part of the conveyor) can be perforated or made with a mesh. For example, in one particular embodiment the biomass material may be wet and the perforations or mesh allow water to drain away from the biomass before irradiation. Screening of material can also be by a manual method, for example by an operator or mechanoid (e.g., a robot equipped with a color, reflectivity or other sensor) that removes unwanted material. Screening can also be by magnetic screening wherein a magnet is disposed near the conveyed material and the magnetic material is removed magnetically. Optional pre-processing can include heating the material. For example a portion of the conveyor can be sent through a heated zone. The heated zone can be created, for example, by IR radiation, microwaves, combustion (e.g., gas, coal, oil, biomass), resistive heating and/or inductive coils. The heat can be applied from at least one side or more than one side, can be continuous or periodic and can be for only a portion of the material or all the material. For example, a portion of the conveying trough can be heated by use of a heating jacket. Heating can be, for example, for the purpose of drying the material. In the case of drying the material, this can also be facilitated, with or without heating, by the movement of a gas (e.g., air, oxygen, nitrogen, He, CO2, Argon) over and/or through the biomass as it is being conveyed. Optionally, pre-irradiation processing can include cooling the material. Cooling material is described in U.S. Pat. No. 7,900,857 to Medoff, the disclosure of which in incorporated herein by reference. For example, cooling can be by supplying a cooling fluid, for example water (e.g., with glycerol), or nitrogen (e.g., liquid nitrogen) to the bottom of the conveying trough. Alternatively, a cooling gas, for example, chilled nitrogen can be blown over the biomass materials or under the conveying system. Another optional pre-irradiation processing can include adding a material to the biomass. The additional material can be added by, for example, by showering, sprinkling and or pouring the material onto the biomass as it is conveyed. Materials that can be added include, for example, metals, ceramics and/or ions as described in U.S. Pat. App. Pub. 2010/0105119 A1 (filed Oct. 26, 2009) and U.S. Pat. App. Pub. 2010/0159569 A1 (filed Dec. 16, 2009), the entire disclosures of which are incorporated herein by reference. Optional materials that can be added include acids and bases. Other materials that can be added are oxidants (e.g., peroxides, chlorates), polymers, polymerizable monomers (e.g., containing unsaturated bonds), water, catalysts, enzymes and/or organisms. Materials can be added, for example, in pure form, as a solution in a solvent (e.g., water or an organic solvent) and/or as a solution. In some cases the solvent is volatile and can be made to evaporate e.g., by heating and/or blowing gas as previously described. The added material may form a uniform coating on the biomass or be a homogeneous mixture of different components (e.g., biomass and additional material). The added material can modulate the subsequent irradiation step by increasing the efficiency of the irradiation, damping the irradiation or changing the effect of the irradiation (e.g., from electron beams to X-rays or heat). The method may have no impact on the irradiation but may be useful for further downstream processing. The added material may help in conveying the material, for example, by lowering dust levels. Biomass can be treated as described herein, e.g. with electron beam radiation, while being conveyed. The biomass can be delivered to the conveyor by using, a belt conveyor, a pneumatic conveyor, a screw conveyor, a hopper, a pipe, manually or by combination of these. The biomass can, for example, be dropped, poured and/or placed onto the conveyor by any of these methods. In some embodiments the material is delivered to the conveyor using an enclosed material distribution system to help maintain a low oxygen atmosphere and/or control dust and fines. Lofted or air suspended biomass fines and dust are undesirable because these can form an explosion hazard or damage the window foils. The material can be leveled to form a uniform thickness between about 0.0312 and 5 inches (e.g., between about 0.0625 and 2.000 inches, between about 0.125 and 1 inches, between about 0.125 and 0.5 inches, between about 0.3 and 0.9 inches, between about 0.2 and 0.5 inches between about 0.25 and 1.0 inches, between about 0.25 and 0.5 inches, 0.100+/−0.025 inches, 0.150+/−0.025 inches, 0.200+/−0.025 inches, 0.250+/−0.025 inches, 0.300+/−0.025 inches, 0.350+/−0.025 inches, 0.400+/−0.025 inches, 0.450+/−0.025 inches, 0.500+/−0.025 inches, 0.550+/−0.025 inches, 0.600+/−0.025 inches, 0.700+/−0.025 inches, 0.750+/−0.025 inches, 0.800+/−0.025 inches, 0.850+/−0.025 inches, 0.900+/−0.025 inches, 0.900+/−0.025 inches. Generally, it is preferred to convey the material as quickly as possible through the electron beam to maximize throughput. For example, the material can be conveyed at rates of at least 1 ft/min, e.g., at least 2 ft/min, at least 3 ft/min, at least 4 ft/min, at least 5 ft/min, at least 10 ft/min, at least 15 ft/min, 20, 25, 30, 35, 40, 45, 50 ft/min. The rate of conveying is related to the beam current, for example, for a ¼ inch thick biomass and 100 mA, the conveyor can move at about 20 ft/min to provide a useful irradiation dosage, at 50 mA the conveyor can move at about 10 ft/min to provide approximately the same irradiation dosage. After the biomass material has been conveyed through a treatment area, e.g., a radiation zone, optional post processing can be done. The optional post processing can, for example, be any process described herein. For example, the biomass can be screened, heated, cooled, and/or combined with additives. Uniquely to post-irradiation, quenching of the radicals can occur, for example, quenching of radicals by the addition of fluids or gases (e.g., oxygen, nitrous oxide, ammonia, liquids), using pressure, heat, and/or the addition of radical scavengers. For example, the biomass can be conveyed out of an enclosed conveyor and exposed to a gas (e.g., oxygen) where it is quenched, forming carboxylated groups. In one embodiment the biomass can be exposed during irradiation to a reactive gas or fluid. Quenching of biomass that has been irradiated is described in U.S. Pat. No. 8,083,906 to Medoff, the entire disclosure of which is incorporate herein by reference. If desired, one or more mechanical treatments can be used in addition to irradiation to further reduce the recalcitrance of the carbohydrate-containing material. These processes can be applied before, during and or after irradiation. In some cases, the mechanical treatment may include an initial preparation of the feedstock as received, e.g., size reduction of materials, such as by comminution, e.g., cutting, grinding, shearing, pulverizing or chopping. For example, in some cases, loose feedstock (e.g., recycled paper, starchy materials, or switchgrass) is prepared by shearing or shredding. Mechanical treatment may reduce the bulk density of the carbohydrate-containing material, increase the surface area of the carbohydrate-containing material and/or decrease one or more dimensions of the carbohydrate-containing material. Alternatively, or in addition, the feedstock material can first be physically treated by one or more of the other physical treatment methods, e.g., chemical treatment, radiation, sonication, oxidation, pyrolysis or steam explosion, and then mechanically treated. This sequence can be advantageous since materials treated by one or more of the other treatments, e.g., irradiation or pyrolysis, tend to be more brittle and, therefore, it may be easier to further change the structure of the material by mechanical treatment. For example, a feedstock material can be conveyed through ionizing radiation using a conveyor as described herein and then mechanically treated. Chemical treatment can remove some or all of the lignin (for example chemical pulping) and can partially or completely hydrolyze the material. The methods also can be used with pre-hydrolyzed material. The methods also can be used with material that has not been pre hydrolyzed. The methods can be used with mixtures of hydrolyzed and non-hydrolyzed materials, for example with about 50% or more non-hydrolyzed material, with about 60% or more non-hydrolyzed material, with about 70% or more non-hydrolyzed material, with about 80% or more non-hydrolyzed material or even with 90% or more non-hydrolyzed material. In addition to size reduction, which can be performed initially and/or later in processing, mechanical treatment can also be advantageous for opening up, stressing, breaking or shattering the carbohydrate-containing materials, making the cellulose of the materials more susceptible to chain scission and/or disruption of crystalline structure during the physical treatment. Methods of mechanically treating the carbohydrate-containing material include, for example, milling or grinding. Milling may be performed using, for example, a hammer mill, ball mill, colloid mill, conical or cone mill, disk mill, edge mill, Wiley mill, grist mill or other mill. Grinding may be performed using, for example, a cutting/impact type grinder. Some exemplary grinders include stone grinders, pin grinders, coffee grinders, and burr grinders. Grinding or milling may be provided, for example, by a reciprocating pin or other element, as is the case in a pin mill. Other mechanical treatment methods include mechanical ripping or tearing, other methods that apply pressure to the fibers, and air attrition milling. Suitable mechanical treatments further include any other technique that continues the disruption of the internal structure of the material that was initiated by the previous processing steps. Mechanical feed preparation systems can be configured to produce streams with specific characteristics such as, for example, specific maximum sizes, specific length-to-width, or specific surface areas ratios. Physical preparation can increase the rate of reactions, improve the movement of material on a conveyor, improve the irradiation profile of the material, improve the radiation uniformity of the material, or reduce the processing time required by opening up the materials and making them more accessible to processes and/or reagents, such as reagents in a solution. The bulk density of feedstocks can be controlled (e.g., increased). In some situations, it can be desirable to prepare a low bulk density material, e.g., by densifying the material (e.g., densification can make it easier and less costly to transport to another site) and then reverting the material to a lower bulk density state (e.g., after transport). The material can be densified, for example from less than about 0.2 g/cc to more than about 0.9 g/cc (e.g., less than about 0.3 to more than about 0.5 g/cc, less than about 0.3 to more than about 0.9 g/cc, less than about 0.5 to more than about 0.9 g/cc, less than about 0.3 to more than about 0.8 g/cc, less than about 0.2 to more than about 0.5 g/cc). For example, the material can be densified by the methods and equipment disclosed in U.S. Pat. No. 7,932,065 to Medoff and International Publication No. WO 2008/073186 (which was filed Oct. 26, 2007, was published in English, and which designated the United States), the full disclosures of which are incorporated herein by reference. Densified materials can be processed by any of the methods described herein, or any material processed by any of the methods described herein can be subsequently densified. In some embodiments, the material to be processed is in the form of a fibrous material that includes fibers provided by shearing a fiber source. For example, the shearing can be performed with a rotary knife cutter. For example, a fiber source, e.g., that is recalcitrant or that has had its recalcitrance level reduced, can be sheared, e.g., in a rotary knife cutter, to provide a first fibrous material. The first fibrous material is passed through a first screen, e.g., having an average opening size of 1.59 mm or less ( 1/16 inch, 0.0625 inch), provide a second fibrous material. If desired, the fiber source can be cut prior to the shearing, e.g., with a shredder. For example, when a paper is used as the fiber source, the paper can be first cut into strips that are, e.g., ¼- to ½-inch wide, using a shredder, e.g., a counter-rotating screw shredder, such as those manufactured by Munson (Utica, N.Y.). As an alternative to shredding, the paper can be reduced in size by cutting to a desired size using a guillotine cutter. For example, the guillotine cutter can be used to cut the paper into sheets that are, e.g., 10 inches wide by 12 inches long. In some embodiments, the shearing of the fiber source and the passing of the resulting first fibrous material through a first screen are performed concurrently. The shearing and the passing can also be performed in a batch-type process. For example, a rotary knife cutter can be used to concurrently shear the fiber source and screen the first fibrous material. A rotary knife cutter includes a hopper that can be loaded with a shredded fiber source prepared by shredding a fiber source. The shredded fiber source. In some implementations, the feedstock is physically treated prior to saccharification and/or fermentation. Physical treatment processes can include one or more of any of those described herein, such as mechanical treatment, chemical treatment, irradiation, sonication, oxidation, pyrolysis or steam explosion. Treatment methods can be used in combinations of two, three, four, or even all of these technologies (in any order). When more than one treatment method is used, the methods can be applied at the same time or at different times. Other processes that change a molecular structure of a biomass feedstock may also be used, alone or in combination with the processes disclosed herein. Mechanical treatments that may be used, and the characteristics of the mechanically treated carbohydrate-containing materials, are described in further detail in U.S. Pat. App. Pub. 2012/01000577 A1, filed Oct. 18, 2011, the full disclosure of which is hereby incorporated herein by reference. Sonication, Pyrolysis, Oxidation, Steam Explosion If desired, one or more sonication, pyrolysis, oxidative, or steam explosion processes can be used in addition to irradiation to further reduce the recalcitrance of the carbohydrate-containing material. These processes can be applied before, during and or after irradiation. These processes are described in detail in U.S. Pat. No. 7,932,065 to Medoff, the full disclosure of which is incorporated herein by reference. Biomass Processing after Irradiation After irradiation the biomass may be transferred to a vessel for saccharification. Alternately, the biomass can be heated after the biomass is irradiated prior to the saccharification step. The biomass can be, for example, by IR radiation, microwaves, combustion (e.g., gas, coal, oil, biomass), resistive heating and/or inductive coils. This heating can be in a liquid, for example, in water or other water-based solvents. The heat can be applied from at least one side or more than one side, can be continuous or periodic and can be for only a portion of the material or all the material. The biomass may be heated to temperatures above 90° C. in an aqueous liquid that may have an acid or a base present. For example, the aqueous biomass slurry may be heated to 90 to 150° C., alternatively, 105 to 145° C., optionally 110 to 140° C. or further optionally from 115 to 135° C. The time that the aqueous biomass mixture is held at the peak temperature is 1 to 12 hours, alternately, 1 to 6 hours, optionally 1 to 4 hours at the peak temperature. In some instances, the aqueous biomass mixture is acidic, and the pH is between 1 and 5, optionally 1 to 4, or alternately, 2 to 3. In other instances, the aqueous biomass mixture is alkaline and the pH is between 6 and 13, alternately, 8 to 12, or optionally, 8 to 11. Saccharification The treated biomass materials can be saccharified, generally by combining the material and a cellulase enzyme in a fluid medium, e.g., an aqueous solution. In some cases, the material is boiled, steeped, or cooked in hot water prior to saccharification, as described in U.S. Pat. App. Pub. 2012/01000577 A1, filed Oct. 18, 2011. The saccharification process can be partially or completely performed in a tank (e.g., a tank having a volume of at least 4000, 40,000, or 500,000 L) in a manufacturing plant, and/or can be partially or completely performed in transit, e.g., in a rail car, tanker truck, or in a supertanker or the hold of a ship. The time required for complete saccharification will depend on the process conditions and the carbohydrate-containing material and enzyme used. If saccharification is performed in a manufacturing plant under controlled conditions, the cellulose may be substantially entirely converted to sugar, e.g., glucose in about 12-96 hours. If saccharification is performed partially or completely in transit, saccharification may take longer. It is generally preferred that the tank contents be mixed during saccharification, e.g., using jet mixing as described in International App. No. PCT/US2010/035331, filed May 18, 2010, which was published in English as WO 2010/135380 and designated the United States, the full disclosure of which is incorporated by reference herein. The addition of surfactants can enhance the rate of saccharification. Examples of surfactants include non-ionic surfactants, such as a Tween® 20 or Tween® 80 polyethylene glycol surfactants, ionic surfactants, or amphoteric surfactants. It is generally preferred that the concentration of the sugar solution resulting from saccharification be relatively high, e.g., greater than 40%, or greater than 50, 60, 70, 80, 90 or even greater than 95% by weight. Water may be removed, e.g., by evaporation, to increase the concentration of the sugar solution. This reduces the volume to be shipped, and also inhibits microbial growth in the solution. Alternatively, sugar solutions of lower concentrations may be used, in which case it may be desirable to add an antimicrobial additive, e.g., a broad spectrum antibiotic, in a low concentration, e.g., 50 to 150 ppm. Other suitable antibiotics include amphotericin B, ampicillin, chloramphenicol, ciprofloxacin, gentamicin, hygromycin B, kanamycin, neomycin, penicillin, puromycin, streptomycin. Antibiotics will inhibit growth of microorganisms during transport and storage, and can be used at appropriate concentrations, e.g., between 15 and 1000 ppm by weight, e.g., between 25 and 500 ppm, or between 50 and 150 ppm. If desired, an antibiotic can be included even if the sugar concentration is relatively high. Alternatively, other additives with anti-microbial of preservative properties may be used. Preferably the antimicrobial additive(s) are food-grade. A relatively high concentration solution can be obtained by limiting the amount of water added to the carbohydrate-containing material with the enzyme. The concentration can be controlled, e.g., by controlling how much saccharification takes place. For example, concentration can be increased by adding more carbohydrate-containing material to the solution. In order to keep the sugar that is being produced in solution, a surfactant can be added, e.g., one of those discussed above. Solubility can also be increased by increasing the temperature of the solution. For example, the solution can be maintained at a temperature of 40-50° C., 60-80° C., or even higher. Sugars In the processes described herein, for example after saccharification, sugars (e.g., glucose and xylose) can be isolated. For example, sugars can be isolated by precipitation, crystallization, chromatography (e.g., simulated moving bed chromatography, high pressure chromatography), centrifugation, extraction, any other isolation method known in the art, and combinations thereof. Hydrogenation and Other Chemical Transformations The processes described herein can include hydrogenation. For example, glucose and xylose can be hydrogenated to sorbitol and xylitol respectively. Hydrogenation can be accomplished by use of a catalyst (e.g., Pt/gamma-Al2O3, Ru/C, Raney Nickel, or other catalysts know in the art) in combination with H2 under high pressure (e.g., 10 to 12000 psi). Other types of chemical transformation of the products from the processes described herein can be used, for example production of organic sugar derived products such (e.g., furfural and furfural-derived products). Chemical transformations of sugar derived products are described in U.S. Prov. App. No. 61/667,481, filed Jul. 3, 2012, the disclosure of which is incorporated herein by reference in its entirety. Fermentation Yeast and Zymomonas bacteria, for example, can be used for fermentation or conversion of sugar(s) to alcohol(s). Other microorganisms are discussed below. The optimum pH for fermentations is about pH 4 to 7. For example, the optimum pH for yeast is from about pH 4 to 5, while the optimum pH for Zymomonas is from about pH 5 to 6. Typical fermentation times are about 24 to 168 hours (e.g., 24 to 96 hrs) with temperatures in the range of 20° C. to 40° C. (e.g., 26° C. to 40° C.), however thermophilic microorganisms prefer higher temperatures. In some embodiments, e.g., when anaerobic organisms are used, at least a portion of the fermentation is conducted in the absence of oxygen, e.g., under a blanket of an inert gas such as N2, Ar, He, CO2 or mixtures thereof. Additionally, the mixture may have a constant purge of an inert gas flowing through the tank during part of or all of the fermentation. In some cases, anaerobic condition, can be achieved or maintained by carbon dioxide production during the fermentation and no additional inert gas is needed. In some embodiments, all or a portion of the fermentation process can be interrupted before the low molecular weight sugar is completely converted to a product (e.g., ethanol). The intermediate fermentation products include sugar and carbohydrates in high concentrations. The sugars and carbohydrates can be isolated via any means known in the art. These intermediate fermentation products can be used in preparation of food for human or animal consumption. Additionally or alternatively, the fermentation products can be ground to a appropriate particle size by comminution. Jet mixing may be used during fermentation, and in some cases saccharification and fermentation are performed in the same tank. Nutrients for the microorganisms may be added during saccharification and/or fermentation, for example the food-based nutrient packages described in U.S. Pat. App. Pub. 2012/0052536, filed Jul. 15, 2011, the complete disclosure of which is incorporated herein by reference. Fermentation includes the methods and products that are disclosed in U.S. Prov. App. No. 61/579,559, filed Dec. 22, 2012, and U.S. Prov. App. No. 61/579,576, filed Dec. 22, 2012, the contents of both of which are incorporated by reference herein in their entirety. Mobile fermenters can be utilized, as described in International App. No. PCT/US2007/074028 (which was filed Jul. 20, 2007, was published in English as WO 2008/011598 and designated the United States), the contents of which is incorporated herein in its entirety. Similarly, the saccharification equipment can be mobile. Further, saccharification and/or fermentation may be performed in part or entirely during transit. Distillation After fermentation, the resulting fluids can be distilled using, for example, a “beer column” to separate ethanol and other alcohols from the majority of water and residual solids. The vapor exiting the beer column can be, e.g., 35% by weight ethanol and can be fed to a rectification column. A mixture of nearly azeotropic (92.5%) ethanol and water from the rectification column can be purified to pure (99.5%) ethanol using vapor-phase molecular sieves. The beer column bottoms can be sent to the first effect of a three-effect evaporator. The rectification column reflux condenser can provide heat for this first effect. After the first effect, solids can be separated using a centrifuge and dried in a rotary dryer. A portion (25%) of the centrifuge effluent can be recycled to fermentation and the rest sent to the second and third evaporator effects. Most of the evaporator condensate can be returned to the process as fairly clean condensate with a small portion split off to waste water treatment to prevent build-up of low-boiling compounds. Intermediates and Products Using the processes described herein, the biomass material can be converted to one or more products, such as energy, fuels, foods and materials. Specific examples of products include, but are not limited to, hydrogen, sugars (e.g., glucose, xylose, arabinose, mannose, galactose, fructose, disaccharides, oligosaccharides and polysaccharides), alcohols (e.g., monohydric alcohols or dihydric alcohols, such as ethanol, n-propanol, isobutanol, sec-butanol, tert-butanol or n-butanol), hydrated or hydrous alcohols (e.g., containing greater than 10%, 20%, 30% or even greater than 40% water), biodiesel, organic acids, hydrocarbons (e.g., methane, ethane, propane, isobutene, pentane, n-hexane, biodiesel, bio-gasoline and mixtures thereof), co-products (e.g., proteins, such as cellulolytic proteins (enzymes) or single cell proteins), and mixtures of any of these in any combination or relative concentration, and optionally in combination with any additives (e.g., fuel additives). Other examples include carboxylic acids, salts of a carboxylic acid, a mixture of carboxylic acids and salts of carboxylic acids and esters of carboxylic acids (e.g., methyl, ethyl and n-propyl esters), ketones (e.g., acetone), aldehydes (e.g., acetaldehyde), alpha and beta unsaturated acids (e.g., acrylic acid) and olefins (e.g., ethylene). Other alcohols and alcohol derivatives include propanol, propylene glycol, 1,4-butanediol, 1,3-propanediol, sugar alcohols (e.g., erythritol, glycol, glycerol, sorbitol threitol, arabitol, ribitol, mannitol, dulcitol, fucitol, iditol, isomalt, maltitol, lactitol, xylitol and other polyols), and methyl or ethyl esters of any of these alcohols. Other products include methyl acrylate, methylmethacrylate, lactic acid, citric acid, formic acid, acetic acid, propionic acid, butyric acid, succinic acid, valeric acid, caproic acid, 3-hydroxypropionic acid, palmitic acid, stearic acid, oxalic acid, malonic acid, glutaric acid, oleic acid, linoleic acid, glycolic acid, gamma-hydroxybutyric acid, and mixtures thereof, salts of any of these acids, mixtures of any of the acids and their respective salts. Many of the products obtained, such as ethanol or n-butanol, can be utilized as a fuel for powering cars, trucks, tractors, ships or trains, e.g., as an internal combustion fuel or as a fuel cell feedstock. Many of the products obtained can also be utilized to power aircraft, such as planes, e.g., having jet engines or helicopters. In addition, the products described herein can be utilized for electrical power generation, e.g., in a conventional steam generating plant or in a fuel cell plant. Other intermediates and products, including food and pharmaceutical products, are described in U.S. application Ser. No. 12/417,900 filed Apr. 3, 2009, the full disclosure of which is hereby incorporated by reference herein. Carbohydrate Containing Materials (Biomass Materials) As used herein, the term “biomass materials” is used interchangeably with the term “carbohydrate-containing materials”, and includes lignocellulosic, cellulosic, starchy, and microbial materials. Any of the methods described herein can be practiced with mixtures of any biomass materials described herein. Lignocellulosic materials include, but are not limited to, wood, particle board, forestry wastes (e.g., sawdust, aspen wood, wood chips), grasses, (e.g., switchgrass, miscanthus, cord grass, reed canary grass), grain residues, (e.g., rice hulls, oat hulls, wheat chaff, barley hulls), agricultural waste (e.g., silage, canola straw, wheat straw, barley straw, oat straw, rice straw, jute, hemp, flax, bamboo, sisal, abaca, corn cobs, corn stover, soybean stover, corn fiber, alfalfa, hay, coconut hair), sugar processing residues (e.g., bagasse, beet pulp, agave bagasse), algae, seaweed, manure, sewage, and mixtures of any of these. In some cases, the lignocellulosic material includes corncobs. Ground or hammermilled corncobs can be spread in a layer of relatively uniform thickness for irradiation, and after irradiation are easy to disperse in the medium for further processing. To facilitate harvest and collection, in some cases the entire corn plant is used, including the corn stalk, corn kernels, and in some cases even the root system of the plant. Advantageously, no additional nutrients (other than a nitrogen source, e.g., urea or ammonia) are required during fermentation of corncobs or cellulosic or lignocellulosic materials containing significant amounts of corncobs. Corncobs, before and after comminution, are also easier to convey and disperse, and have a lesser tendency to form explosive mixtures in air than other cellulosic or lignocellulosic materials such as hay and grasses. Cellulosic materials include, for example, paper, paper products, paper waste, paper pulp, pigmented papers, loaded papers, coated papers, filled papers, magazines, printed matter (e.g., books, catalogs, manuals, labels, calendars, greeting cards, brochures, prospectuses, newsprint), printer paper, polycoated paper, card stock, cardboard, paperboard, materials having a high α-cellulose content such as cotton, and mixtures of any of these. For example paper products as described in U.S. application Ser. No. 13/396,365 (“Magazine Feedstocks” by Medoff et al., filed Feb. 14, 2012), the full disclosure of which is incorporated herein by reference. Cellulosic materials can also include lignocellulosic materials which have been de-lignified. Starchy materials include starch itself, e.g., corn starch, wheat starch, potato starch or rice starch, a derivative of starch, or a material that includes starch, such as an edible food product or a crop. For example, the starchy material can be arracacha, buckwheat, banana, barley, cassava, kudzu, oca, sago, sorghum, regular household potatoes, sweet potato, taro, yams, or one or more beans, such as favas, lentils or peas. Blends of any two or more starchy materials are also starchy materials. Mixtures of starchy, cellulosic and or lignocellulosic materials can also be used. For example, a biomass can be an entire plant, a part of a plant or different parts of a plant, e.g., a wheat plant, cotton plant, a corn plant, rice plant or a tree. The starchy materials can be treated by any of the methods described herein. Microbial materials include, but are not limited to, any naturally occurring or genetically modified microorganism or organism that contains or is capable of providing a source of carbohydrates (e.g., cellulose), for example, protists, e.g., animal protists (e.g., protozoa such as flagellates, amoeboids, ciliates, and sporozoa) and plant protists (e.g., algae such alveolates, chlorarachniophytes, cryptomonads, euglenids, glaucophytes, haptophytes, red algae, stramenopiles, and viridaeplantae). Other examples include seaweed, plankton (e.g., macroplankton, mesoplankton, microplankton, nanoplankton, picoplankton, and femtoplankton), phytoplankton, bacteria (e.g., gram positive bacteria, gram negative bacteria, and extremophiles), yeast and/or mixtures of these. In some instances, microbial biomass can be obtained from natural sources, e.g., the ocean, lakes, bodies of water, e.g., salt water or fresh water, or on land. Alternatively or in addition, microbial biomass can be obtained from culture systems, e.g., large scale dry and wet culture and fermentation systems. In other embodiments, the biomass materials, such as cellulosic, starchy and lignocellulosic feedstock materials, can be obtained from transgenic microorganisms and plants that have been modified with respect to a wild type variety. Such modifications may be, for example, through the iterative steps of selection and breeding to obtain desired traits in a plant. Furthermore, the plants can have had genetic material removed, modified, silenced and/or added with respect to the wild type variety. For example, genetically modified plants can be produced by recombinant DNA methods, where genetic modifications include introducing or modifying specific genes from parental varieties, or, for example, by using transgenic breeding wherein a specific gene or genes are introduced to a plant from a different species of plant and/or bacteria. Another way to create genetic variation is through mutation breeding wherein new alleles are artificially created from endogenous genes. The artificial genes can be created by a variety of ways including treating the plant or seeds with, for example, chemical mutagens (e.g., using alkylating agents, epoxides, alkaloids, peroxides, formaldehyde), irradiation (e.g., X-rays, gamma rays, neutrons, beta particles, alpha particles, protons, deuterons, UV radiation) and temperature shocking or other external stressing and subsequent selection techniques. Other methods of providing modified genes is through error prone PCR and DNA shuffling followed by insertion of the desired modified DNA into the desired plant or seed. Methods of introducing the desired genetic variation in the seed or plant include, for example, the use of a bacterial carrier, biolistics, calcium phosphate precipitation, electroporation, gene splicing, gene silencing, lipofection, microinjection and viral carriers. Additional genetically modified materials have been described in U.S. application Ser. No. 13/396,369 filed Feb. 14, 2012 the full disclosure of which is incorporated herein by reference. Saccharifying Agents Suitable cellulolytic enzymes include cellulases from species in the genera Bacillus, Coprinus, Myceliophthora, Cephalosporium, Scytalidium, Penicillium, Aspergillus, Pseudomonas, Humicola, Fusarium, Thielavia, Acremonium, Chrysosporium and Trichoderma, especially those produced by a strain selected from the species Aspergillus (see, e.g., EP Pub. No. 0 458 162), Humicola insolens (reclassified as Scytalidium thermophilum, see, e.g., U.S. Pat. No. 4,435,307), Coprinus cinereus, Fusarium oxysporum, Myceliophthora thermophila, Meripilus giganteus, Thielavia terrestris, Acremonium sp. (including, but not limited to, A. persicinum, A. acremonium, A. brachypenium, A. dichromosporum, A. obclavatum, A. pinkertoniae, A. roseogriseum, A. incoloratum, and A. furatum). Preferred strains include Humicola insolens DSM 1800, Fusarium oxysporum DSM 2672, Myceliophthora thermophila CBS 117.65, Cephalosporium sp. RYM-202, Acremonium sp. CBS 478.94, Acremonium sp. CBS 265.95, Acremonium persicinum CBS 169.65, Acremonium acremonium AHU 9519, Cephalosporium sp. CBS 535.71, Acremonium brachypenium CBS 866.73, Acremonium dichromosporum CBS 683.73, Acremonium obclavatum CBS 311.74, Acremonium pinkertoniae CBS 157.70, Acremonium roseogriseum CBS 134.56, Acremonium incoloratum CBS 146.62, and Acremonium furatum CBS 299.70H. Cellulolytic enzymes may also be obtained from Chrysosporium, preferably a strain of Chrysosporium lucknowense. Additional strains that can be used include, but are not limited to, Trichoderma (particularly T. viride, T. reesei, and T. koningii), alkalophilic Bacillus (see, for example, U.S. Pat. No. 3,844,890 and EP Pub. No. 0 458 162), and Streptomyces (see, e.g., EP Pub. No. 0 458 162). Fermentation Agents The microorganism(s) used in fermentation can be naturally-occurring microorganisms and/or engineered microorganisms. For example, the microorganism can be a bacterium (including, but not limited to, e.g., a cellulolytic bacterium), a fungus, (including, but not limited to, e.g., a yeast), a plant, a protist, e.g., a protozoa or a fungus-like protest (including, but not limited to, e.g., a slime mold), or an alga. When the organisms are compatible, mixtures of organisms can be utilized. Suitable fermenting microorganisms have the ability to convert carbohydrates, such as glucose, fructose, xylose, arabinose, mannose, galactose, oligosaccharides or polysaccharides into fermentation products. Fermenting microorganisms include strains of the genus Saccharomyces spp. (including, but not limited to, S. cerevisiae (baker's yeast), S. distaticus, S. uvarum), the genus Kluyveromyces, (including, but not limited to, K. marxianus, K. fragilis), the genus Candida (including, but not limited to, C. pseudotropicalis, and C. brassicae), Pichia stipitis (a relative of Candida shehatae), the genus Clavispora (including, but not limited to, C. lusitaniae and C. opuntiae), the genus Pachysolen (including, but not limited to, P. tannophilus), the genus Bretannomyces (including, but not limited to, e.g., B. clausenii (Philippidis, G. P., 1996, Cellulose bioconversion technology, in Handbook on Bioethanol: Production and Utilization, Wyman, C. E., ed., Taylor & Francis, Washington, D.C., 179-212)). Other suitable microorganisms include, for example, Zymomonas mobilis, Clostridium spp. (including, but not limited to, C. thermocellum (Philippidis, 1996, supra), C. saccharobutylacetonicum, C. saccharobutylicum, C. Puniceum, C. beijernckii, and C. acetobutylicum), Moniliella pollinis, Yarrowia lipolytica, Aureobasidium sp., Trichosporonoides sp., Trigonopsis variabilis, Trichosporon sp., Moniliellaacetoabutans sp., Typhula variabilis, Candida magnoliae, Ustilaginomycetes sp., Pseudozyma tsukubaensis, yeast species of genera Zygosaccharomyces, Debaryomyces, Hansenula and Pichia, and fungi of the dematioid genus Torula. Many such microbial strains are publicly available, either commercially or through depositories such as the ATCC (American Type Culture Collection, Manassas, Va., USA), the NRRL (Agricultural Research Sevice Culture Collection, Peoria, Ill., USA), or the DSMZ (Deutsche Sammlung von Mikroorganismen and Zellkulturen GmbH, Braunschweig, Germany), to name a few. Commercially available yeasts include, for example, RED STAR®/Lesaffre Ethanol Red (available from Red Star/Lesaffre, USA), FALI® (available from Fleischmann's Yeast, a division of Burns Philip Food Inc., USA), SUPERSTART® (Lallemand Biofuels and Distilled Spirits, Canada), EAGLE C6 FUEL™ or C6 FUEL™ (available from Lallemand Biofuels and Distilled Spirits, Canada), GERT STRAND® (available from Gert Strand AB, Sweden) and FERMOL® (available from DSM Specialties). Other than in the examples herein, or unless otherwise expressly specified, all of the numerical ranges, amounts, values and percentages, such as those for amounts of materials, elemental contents, times and temperatures of reaction, ratios of amounts, and others, in the following portion of the specification and attached claims may be read as if prefaced by the word “about” even though the term “about” may not expressly appear with the value, amount, or range. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains error necessarily resulting from the standard deviation found in its underlying respective testing measurements. Furthermore, when numerical ranges are set forth herein, these ranges are inclusive of the recited range end points. When percentages by weight are used herein, the numerical values reported are relative to the total weight. Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10. The terms “one,” “a,” or “an” as used herein are intended to include “at least one” or “one or more,” unless otherwise indicated. Any patent, publication, or other disclosure material, in whole or in part, that is said to be incorporated by reference herein is incorporated herein only to the extent that the incorporated material does not conflict with existing definitions, statements, or other disclosure material set forth in this disclosure. As such, and to the extent necessary, the disclosure as explicitly set forth herein supersedes any conflicting material incorporated herein by reference. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material set forth herein will only be incorporated to the extent that no conflict arises between that incorporated material and the existing disclosure material. While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. |
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050283782 | claims | 1. A gas cooled high temperature reactor safety system for use in a reactor housed together with steam generators in a prestressed concrete pressure vessel, and shut down by absorber rods and equipped with a reactor protection system to actuate an emergency shutdown, comprising: accident instrumentation apparatus located in the high temperature reactor to monitor certain characteristic process parameters, said parameters are: electronic evaluation means for interrupting power supply to cooling gas blowers, feed water pumps and absorber rod holding devices when said process parameters exceed a predetermined limiting value; parameter sensor devices located in a primary loop, which physically interrupts the power supply to said cooling gas blowers, feed water pumps and absorber rod holding devices if the predetermined limiting value of the corresponding process parameter is exceeded; and a central emergency switch means for manual interruption of power supply to said cooling gas blowers, feed water pumps and absorber rod holding devices in case of a safety system danger signal. 2. A safety system according to claim 1, further comprising manually actuated means for reinforced concrete pressure vessel pressure reduction in case of a safety system danger signal. 3. A safety system according to claim 2, further comprising a liner cooling system associated with said reinforced concrete pressure vessel and a manual device for feeding cooling water to said liner cooling system for actuation upon receipt of a safety system danger signal. 4. A safety system according to claim 1, wherein said process parameters are hot gas temperature, cold gas temperature and cooling gas pressure. 5. A safety system according to claim 4, wherein said parameter sensor devices comprise temperature sensitive devices which are fuses. 6. A safety system according to claim 4, wherein said parameter sensor devices comprise temperature sensitive devices which are bimetallic strips. 7. A safety system according to claim 6, wherein said temperature sensitive devices are located above and under a reactor core. 8. A safety system according to claim 4, wherein said parameter sensor devices comprise pressure sensitive devices which are pressure transducers or pressure contacts. 9. A safety system according to claim 1, further comprising a liner cooling system associated with said reinforced concrete pressure vessel and a manual device for feeding cooling water to said liner cooling system actuated upon receipt of a safety system danger signal. 10. A safety system according to claim 1, wherein said parameter sensor devices comprise temperature sensitive devices which are fuses. 11. A safety system according to claim 1, wherein said parameter sensor devices comprise temperature sensitive devices which are bimetallic strips. 12. A safety system according to claim 11, wherein said temperature sensitive devices are located above and under a reactor core. 13. A safety system according to claim 1, wherein said parameter sensor devices comprise pressure sensitive devices which are pressure transducers or pressure contacts. 14. A safety system according to claim 10, wherein said temperature sensitive devices are located above and under a reactor core. 15. A safety system according to claim 5, wherein said temperature sensitive devices are located above and under a reactor core. |
abstract | Thrust is provided to a vehicle using a self-contained device for producing the thrust through a preselected shaping of an electric field. The device includes a core carried by a housing, with both the core and the housing formed from a material having a high dielectric constant. Multiple cells are carried by the housing and formed around the core, with each cell having a high dielectric sandwiched between an electrode and a lower dielectric. A channel is formed between each cell with the channel providing a spacing filled with a material having a dielectric property of the lower dielectric. Electric wires are connected between an electrical power source and each electrode of each cell for providing power thereto. A set of cells extends radially outward from a longitudinal axis of a cylindrical core to form a circular plate with each cell uniformly positioned within the circular plate. Multiple plates are stacked along a longitudinal axis of the core with the electric wire carried through the high dielectric for connection with the electrodes of each plate. Positive and negative voltage is provided to adjacent plates at a rapidly changing rate to provide thrust resulting from non-linear electric field paths created through the device as a result of the cell and surrounding dielectric material configuration. |
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053496191 | abstract | A fuel assembly for a light water reactor comprising a plurality of fuel rods which contain plutonium as a primary fissile material when exposure is zero, and a light water reactor including such fuel assemblies. The fuel assembly has a structure in which at least one of moderator rods is provided at least in one of each corner portion of an arrangement of the fuel rods and a position adjacent to the corner portion in such a manner that the moderator rods are located in rotation symmetry, each of the moderator rods being filled with water or a solid coolant over a length at least corresponding to a fuel effective length, and the fuel rods are provided at positions in the second layer from the outermost periphery which are adjacent to those positions at which the moderator rods are located. |
description | The present application claims benefit of U.S. Provisional Application No. 62/534,937, filed on Jul. 20, 2017, all of the contents of which are incorporated herein by reference. This invention was made with government support under Prime Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention. The present invention generally relates to molten salt compositions and their use as coolant and heat transfer materials in high temperature applications, such as molten salt reactors. The use of nuclear power is expected to increase due to growing global population and improved standards of living in emerging economies. Nuclear power remains a mature, low carbon source for non-variable baseload electricity generation, and is, therefore, very appealing. Accident tolerance, however, is a growing concern. Molten salt reactors (MSRs) are a class of generation IV nuclear reactors gaining more attention due to their greater accident tolerance by virtue of their use of a liquid coolant or fuel composed of molten metal halides (e.g., Fray, D., Faraday Discuss. 2016, 190, 11-34; and LeBlanc, D., Nucl. Eng. Des. 2010, 240, 1644-1656). Since high temperatures are required to maintain the liquid phase of the coolant/fuel melt, accident conditions result in passive cooling to ambient temperatures and subsequent coolant/fuel solidification. Moreover, in contrast to the current fleet of light water reactors, water is not required to cool the reactor, which advantageously permits construction of such facilities in areas devoid of extensive aqueous resources. While the inherent safety of MSRs bestows great potential for future use, the conventional metal halide salt presents some significant problems, including high corrosivity and lower than ideal heat capacity. The high corrosivity requires the use of costly corrosion-resistant materials (e.g., nickel-based hastelloy) for piping used in transporting the molten salt, and even so, regular inspection and maintenance of the transporting apparatus are needed. Turning to the deficiency in heat capacity, it is important to consider that effective heat transfer dictates the efficiency of reactor energy production. Molten salt cooling is used in several reactor concepts, particularly those involving fluoride salts, such as “FLiNaK,” a LiF—NaF—KF-based eutectic, or “FLiBe,” a LiF—BeF-based eutectic (R. O. Scarlat et al., Progress in Nuclear Energy 2014, 77, 406-420). A significant challenge with these salts is the heat capacity, which directly relates to the energy conversion efficiency in converting the reactor heat to water-steam through a series of heat exchangers. If the heat capacity of the salt is not efficient in transferring the reactor core heat to the water for steam production, the efficiency of electricity generation is negatively impacted. Improved heat removal from the core should also result in reduced reactor construction cost, such as by permitting the use of alternative materials that can operate under less stringent code-case qualifications. In a first aspect, the present disclosure is directed to compositions useful as molten salt heat transfer materials for high temperature processes, such as encountered in molten salt reactors (MSRs). The compositions disclosed herein have exceptional heat capacities, and thus, exceptional heat transfer abilities, and this, advantageously coupled with a significantly reduced corrosivity compared to conventional molten salts of the art. More particularly, the compositions described herein include a halide salt matrix (e.g., alkali and/or alkaline earth halide salt matrix) having dispersed therein nanoparticles containing elemental carbon (i.e., “carbon nanoparticles”) in the absence of water and surfactants, wherein the halide is fluoride or chloride. Generally, the carbon nanoparticles are homogeneously dispersed in a continuous (i.e., non-particulate) matrix of the alkali halide salt. The carbon nanoparticles may be solid or hollow. In some embodiments, the carbon nanoparticles have a core-shell type of structure in which the core is composed of a metal (such as a transition metal or fissile element, such as U, Th, or Pu), which is encapsulated by a carbon shell. Although the compositions are herein described as molten salt compositions, it should be appreciated that the composition may not always be in a molten state, such as when the composition is not in use. The solid composition can be heated so as to revert to a molten state. Thus, the composition described above includes embodiments in which the halide salt matrix is in solid form or liquefied (molten) form. In a second aspect, the present disclosure is directed to a molten salt reactor (MSR) in which the above-described molten salt composition is incorporated as a heat transfer material. In the MSR, the molten salt may be used as a heat exchange material not in admixture with the fissile material, i.e., the molten salt may be transported in coolant loops that transfer heat from the reactor core in which fissile material is separately located. Alternatively, fissile material may be dispersed within molten salt housed in the reactor core, such that the molten salt can function as both reactor (fissile) material and heat transfer material. In the latter embodiment, the reactor material includes a molten alkali halide salt, or a molten alkaline earth halide salt, or molten combination thereof, and carbon nanoparticles, as described above, along with nanoparticles of the fissile material. The nanoparticles of the fissile material may be uncoated with carbon when in the presence of the carbon nanoparticles, or alternatively, the nanoparticles of the fissile material are coated with a layer of carbon (as described above for core-shell metal-carbon nanoparticles), the latter of which may or may not be in the presence of separate carbon nanoparticles dispersed in the molten salt. The present disclosure is also directed to the operation of an MSR containing the molten salt composition described above. In a first aspect, the present disclosure is directed to a heat transfer composition that contains a halide salt composition as a continuous (i.e., non-particulate) matrix in which nanoparticles containing elemental carbon (i.e., carbon nanoparticles) are dispersed. Generally, the carbon nanoparticles are homogeneously dispersed throughout the halide salt matrix. The carbon nanoparticles should contain carbon at least on the surfaces of the nanoparticles, i.e., the nanoparticles may or may not contain a core other than carbon, along with a carbon shell encapsulating the core. Thus, there should exist a carbon-halide salt interface between the carbon nanoparticles and halide salt matrix. In some embodiments, the halide salt is or includes one or more alkali halide salts. The alkali halide is typically an alkali fluoride or alkali chloride. The alkali metal may be, for example, lithium (Li), sodium (Na), potassium (K), or rubidium (Rb), or some combination thereof. Some examples of alkali fluorides include lithium fluoride, sodium fluoride, potassium fluoride, and rubidium fluoride. Some examples of alkali chlorides include lithium chloride, sodium chloride, potassium chloride, and rubidium chloride. In other embodiments, the halide salt is or includes one or more alkaline earth halide salts. The alkaline earth halide is typically an alkaline earth fluoride or alkaline earth chloride. The alkaline earth metal may be, for example, beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), or some combination thereof. Some examples of alkaline earth fluorides include beryllium fluoride, magnesium fluoride, calcium fluoride, and strontium fluoride. Some examples of alkaline earth chlorides include beryllium chloride, magnesium chloride, calcium chloride, and strontium chloride. In some embodiments, the halide salt composition (matrix) contains one alkali halide salt (e.g., solely LiF, LiCl, NaF, or NaCl), which may or may not be admixed with one or more non-alkali halide salts, such as an alkaline earth metal salt (e.g., BeF2, MgF2, or CaF2), main group metal salt (e.g., AlF3), transition metal salt (e.g., ZrF4), or fissile metal salt (e.g., UF4), or combination thereof. In other embodiments, the halide salt composition (matrix) contains two or more alkali halide salts, typically as a eutectic, e.g., a LiF—NaF, LiF—NaF—KF, LiF—BeF, LiCl—NaCl, or LiCl—NaCl—KCl mixture, which may or may not be admixed with one or more non-alkali halide salts, such as an alkaline earth metal salt, main group metal salt, transition metal salt, or fissile metal salt, or combination thereof. In similar fashion, the halide salt composition may contain one alkaline earth halide salt, or two or more alkaline earth halide salts, and the one or two alkaline earth halide salts may or may not be admixed with one or more non-alkaline earth halide salts, such as an alkali halide salt, main group metal salt, transition metal salt, or fissile metal salt, or combination thereof. The term “main group metal,” as used herein, generally refers to Groups 13, 14, and/or 15 of the Periodic Table. The term “transition metal,” as used herein, generally refers to Groups 3-12 (or sub-grouping therein, e.g., Groups 3-5 or 3-6) of the Periodic Table. The halide salt matrix may or may also include a non-halide salt, such as an alkali nitrate (e.g., LiNO3, NaNO3, or KNO3), alkali carbonate (e.g., Li2CO3, Na2CO3 or K2CO3), or alkaline earth carbonate (e.g., MgCO3 or CaCO3). Generally, the halide salt composition (matrix) has a melting point of at least or above, for example, 600° C., 650° C., 700° C., 750° C., 800° C., or 850° C., or a temperature within a range bounded by any two of the foregoing temperatures. The nanoparticles containing elemental carbon (i.e., carbon nanoparticles) are dispersed, preferably homogeneously, within the alkali halide matrix. The term “elemental carbon,” as used herein, refers either to carbon in a formal zerovalent state or in a metal carbide state. The carbon nanoparticles may be solid or hollow. Some examples of carbon nanoparticles containing carbon in the zerovalent state include exfoliated graphite nanoplatelets, spherical fullerenes (e.g., buckminsterfullerene, e.g., C60 as well as any of the smaller or larger buckyballs, such as C20 or C70), tubular fullerenes (e.g., single-walled, double-walled, or multi-walled carbon nanotubes), carbon black (“CB”), carbon nanodiamonds, carbon onions, carbon nanobuds, carbon nanofibers (e.g., vapor grown), graphene oxide nanoparticles, and reduced graphene oxide nanoparticles. All of the foregoing types of carbon nanoparticles are well known in the art. For example, exfoliated graphite nanoplatelets are described in detail in, e.g., J.-H. Ding et al., Scientific Reports, 8, 5567, 2018; graphene oxide nanoparticles are described in detail in, e.g., J.-L. Li et al., Angew. Chem. Int. Ed., 51(8), 1830-1834, 2012; reduced graphene oxide nanoparticles are described in detail in, e.g., S. N. Alam et al., Graphene, 6(1), January 2017; and carbon nanofibers are described in detail in, e.g., M. H. Al-Saleh et al., Composites Part A: Applied Science and Manufacturing, 42(12) 2126-2142, December 2011. Some examples of metal carbide nanoparticles include nanoparticles of silicon carbide (SiC), titanium carbide (TiC), tungsten carbide (WC), and aluminum carbide (Al4C3). In some embodiments, the carbon nanoparticles are functionalized with one or more types of heteroatoms (e.g., F, Cl, O, and/or N) to make the carbon nanoparticles further compatible with, and hence, further dispersible in, the halide salt matrix. The heteroatoms may be present in the form of functional groups on the surface of the carbon nanoparticles, such as hydroxy, carboxy, and/or amine groups. The carbon nanoparticles generally have a particle size of up to or less than 200, 500, or 1000 nm in at least one or two of its dimensions. In different embodiments, the nanoparticles have a size (or average size) of up to or less than, for example, 500 nm, 250 nm, 200 nm, 150 nm, 100 nm, 50 nm, 20 nm, 10 nm, or 5 nm, or a size within a range bounded by any two of the foregoing particle sizes. In particular embodiments, the carbon nanoparticles are hollow. The hollow carbon nanoparticles may be, for example, spherical fullerenes, such as buckminsterfullerene, which has a diameter of approximately 1 nm. Larger hollow carbon nanoparticles, other than the fullerenes, and typically of 20-500 nm diameter, are also well known in the art. Such hollow nanoparticles are described in, for example, Z.-C. Yang et al., Chem. Mater., 25(5), 704-710, 2013; H. Zhang et al., Chem. Mater., 27(18), 6297-6304, 2015; Q. Wang et al., Carbon, 52, 209-218, February 2013; Z. Han et al., Synthetic Metals, 187, 91-93, January 2014; and J. Wutthiprom et al., ACS Omega, 2(7), 3730-3738, 2017, all of the contents of which are incorporated herein by reference. Generally, the carbon nanoparticles are included in the heat transfer composition in an amount of at least 0.1 and up to 10 wt % (i.e., by weight of all salts and carbon nanoparticles included in the heat transfer composition). In different embodiments, the carbon nanoparticles are included in the heat transfer composition in an amount of about, at least, or above, for example, 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 wt %, or an amount within a range bounded by any two of the foregoing values. The carbon nanoparticles can be alternatively included in corresponding volume amounts, such as an amount of at least or above 0.1, 0.2, 0.5, or 1 vol % and up to 1.5, 2, 2.5, 3, 4, or 5 vol %. As mentioned above, the carbon nanoparticles need not be composed of carbon throughout the nanoparticle, as long as the nanoparticle is composed of carbon or a metal carbide at least on surfaces of the nanoparticles. Thus, in some embodiments, the carbon nanoparticle may have a core-shell structure in which a core portion of the nanoparticle is composed of or includes a non-carbon element (e.g., a transition metal or fissile element) and the core portion is surrounded (encapsulated) by a shell of carbon. The transition metal may be a first row, second row, or third row transition metal, either in its metallic state or a non-metallic state, such as in a transition metal oxide. In some embodiments, a transition metal having catalytic properties at elevated temperatures (e.g., capability of facilitating a petrochemical conversion reaction) is selected to be in the core portion. In the case of the core portion being composed of or including a fissile element, the fissile element may be, for example, U, Th, or Pu. The latter fissile elements are typically in the form of their oxides or halides. A shell of carbon can be deposited on metal-containing nanoparticles by means known in the art, such as by the deposition of dopamine followed by carbonization, such as described in, for example, R. Liu et al., Angew. Chem. Int. Ed., 50, 6799-6802, 2011, the contents of which are herein incorporated by reference. In some embodiments, the heat transfer composition further includes nanoparticles composed of or including a fissile material, wherein the nanoparticles of the fissile material are dispersed within the alkali halide salt matrix. The nanoparticles of the fissile material may contain, for example, U, Th, or Pu, typically as oxides or halides of these elements. In some embodiments, the nanoparticles of fissile material are coated with carbon. In the latter case, the carbon-coated fissile nanoparticles function as carbon nanoparticles; thus, the carbon-coated fissile nanoparticles may or may not be admixed with carbon nanoparticles that are homogeneously carbon. In other embodiments, the nanoparticles of fissile material are not coated with carbon, in which case the nanoparticles of fissile material are necessarily admixed with carbon nanoparticles. The nanoparticles of fissile material can have any of the sizes provided above for the carbon nanoparticles. For purposes of the present invention, the heat transfer composition described above does not include water or surfactants. The surfactants being excluded from the above-described composition include organic and inorganic surfactants, wherein it is understood that surfactants function to modify interactions between two phases (in the present case, carbon nanoparticles and alkali halide matrix) such that one phase (e.g., carbon) is maximally dispersed in stable form in the other phase (e.g., alkali halide). Such compounds that are surface active at interfaces are unnecessary for purposes of the present invention, and in any event, would likely carbonize at the high temperatures at which these heat transfer compositions are used. Some examples of surfactants include various gums (e.g., gum arabic), sodium dodecyl sulfate, fatty acid salts, siloxanes, and polysiloxanes, all of which are excluded from the heat transfer composition described herein. In other aspects, the present disclosure is directed to molten salt reactors (MSRs) containing the above-described heat transfer composition, as well as methods for operating such MSRs. As well known in the art, an MSR contains a reactor core in which fissile material is housed and undergoes fission for energy production. As indicated earlier above, and as further discussed below, the above-described halide salt heat transfer composition can be separate from the fissile material, or alternatively, the heat transfer composition can include fissile material as a component. The design, construction, and operation of MSRs is well known in the art and has a long history, such as detailed in, for example, J. Uhlir, Journal of Nuclear Materials, 360, 6-11, 2007; D. LeBlanc, Nuclear Engineering and Design, 240, 1644-1656, 2010; and D. D. Siemer, Energy Science and Engineering, 3(2), 83-97, 2015, all of the contents of which are herein incorporated by reference. In particular embodiments, a coolant loop (generally, a primary coolant loop) containing any of the above-described heat transfer compositions is positioned within sufficient proximity to the reactor core of an MSR so as to remove (transfer) heat from the reactor core to the coolant loop, thereby maintaining the reactor core within a safe operating temperature. As discussed above, the heat transfer composition contains a matrix composed of at least one alkali halide salt, and nanoparticles containing elemental carbon dispersed within the alkali halide salt matrix, wherein the carbon nanoparticles can have a single phase composition or a core-shell structure in which a metal (e.g., a transition metal or fissile element) is within a core and a carbon shell surrounds the core. As also discussed above, the heat transfer material may or may not additionally include nanoparticles of a fissile material that is uncoated. Often, the primary coolant described above is also within sufficient proximity to a secondary coolant loop so as to permit the secondary coolant loop to remove (transfer) heat from the primary coolant loop to the secondary coolant loop. The secondary coolant loop may contain any flowable material useful as a secondary heat transfer material in a nuclear reactor. Some examples of secondary coolant loop materials include water, supercritical carbon dioxide, and low melting salts, e.g., ionic liquids and quaternary ammonium and phosphonium salts. As the carbon nanoparticles in the heat transfer composition described herein have been found to substantially reduce corrosivity of the halide salts, the MSR may employ a more common and less costly material in place of nickel-based hastelloy materials for inner walls of the coolant loop. For example, stainless steel, which is significantly less costly, may be used as a material for the coolant loop holding the heat transfer material described herein. In embodiments where fissile material is included in the heat transfer material, the heat transfer material also functions as reactor (fissile) material. In the latter case, the primary coolant loop may be flowably connected to (and an extension of) the reactor. Such a reactor design is also herein referred to as a “liquid-fuel reactor”. Liquid-fuel reactors are considered as “accident tolerant” alternatives to the light-water reactors currently in widespread use. This is due to the ability to solidify the fuel and stop the fission reaction by simply lowering the salt temperature below the solidification point. There are significant challenges with the conventional model, however, that have precluded widespread commercial use. As indicated earlier, corrosion of reactor components is the primary challenge. The liquid fuels are typically halide salts, e.g. chloride or fluoride, which are moisture sensitive and highly corrosive when loaded with dissolved uranium halide nuclear fuel. This is a roadblock for reactor development as new corrosion resistant materials must be developed and undergo very expensive code qualification for use in reactors. The present invention has improved on the liquid-fuel reactor concept by either dispersing carbon nanoparticles in a molten halide salt matrix in admixture with fissile particles or by coating fissile particles with carbon. In the foregoing embodiments, the carbon coating (or presence of carbon nanoparticles) substantially reduces or prevents aggregation and sintering. Corrosivity of the molten salt is driven by the reduction of UX4 (X=Cl, F) to UX3, which results in oxidation of the reactor components that it contacts. Most traditional metal oxide layers form flakes or are readily dissolved, such as ferrous oxide, commonly observed as rust. Use of fissile nanoparticles reduces the surface area of the uranium species, which decreases contact of the uranium species with reactor components, thereby impeding the corrosion. The carbon coating serves as a passive layer that further inhibits electron transfer to the uranium species, thereby further inhibiting corrosion. This lack of corrosivity advantageously permits the use of less expensive and more common metals for the reactor components, thereby lowering construction costs and decreasing construction lead-time. The development of porous hard carbon spheres possessing tunable diameters and readily accessible interior void volumes have been reported (R. Liu et al., Angew. Chem. Int. Ed., 50, 6799-6802, 2011). These and other rigid carbon spheres can be added to dissolved uranium (or other fissile) solutions, allowing diffusion of the uranium into the interior of the carbon sphere. Alternatively, uranium oxide nanocrystals can be synthesized (Wu, H. M., et al., Journal of the American Chemical Society 2006, 128, 16522-16523) and then coated with a carbon shell, such as the dopamine-based carbon shell described in Liu et al. (supra). Nanoparticles of other fissile materials (e.g., thorium oxide or plutonium oxide) may be synthesized by similar methods, and similarly coated with carbon, if desired. An added benefit to the development of nanoparticle-based fuels in molten salts is the ability to utilize the readily available uranium(VI) oxidation state without deleterious effect on reactor integrity. As discussed above, the reduction of UX4 to UX3 drives corrosion issues, yet the comparative ease of large-scale preparation of the UX4 molecule from gaseous UF6 required for isotopic enrichment is what induces its preferential use in molten salt reactors. Nevertheless, preparation of UX4 involves formation of gaseous uranium and reaction with highly toxic and corrosive gases. In contrast, uranium (VI) is environmentally abundant as the UO22+ (“uranyl”) cation, but would be rapidly reduced in a molten salt, oxidizing three times as many metals as UX4 and accelerating corrosion in the melt. An overall insolubility of UO22+ in chloride salts has also been demonstrated, which would prevent uniform diffusion and transport. By forming uranyl nanoparticles with carbonaceous coatings, the corrosive properties can be simultaneously inhibited, as discussed above, and the cost and hazard associated with producing nuclear fuel for molten salt reactors can also be decreased. The current discussion has been focused on uranium-based fuels, as uranium is the primary choice for fuels in the United States. Nevertheless, the technology discussed herein is applicable to other nuclear fuels as well. For example, thorium-based fuels are becoming popular in liquid-fuel reactors due to the prevalence of thorium relative to uranium and mixed oxide fuels (MOX), which is a blend of depleted uranium (U-238) and plutonium (Pu-239), used to generate power. Examples have been set forth below for the purpose of illustration and to describe certain specific embodiments of the invention. However, the scope of this invention is not to be in any way limited by the examples set forth herein. Synthesis of Heat Transfer Compositions Containing Hollow Carbon Nanospheres Magnesium chloride, due to its hygroscopic nature, was purified by distillation and kept in oxygen and a water-free glovebox. Distillation was performed under vacuum at 800° C. in a Watlow ceramic fiber heater. Potassium and sodium chlorides were purified by carbochlorination using sparging with CCl4 at elevated temperatures. Carbon particles were used as received. In a standard experiment, 2-5 mg of hollow carbon spheres (i.e., “HCS,” generally, about 200-250 nm in diameter) were loaded into a quartz tube and sealed with a valve with two ACE threads, one connecting to the tube and the other to the Schlenck line, to establish air-free conditions. After air was evacuated, the sealed tube was taken into the glovebox and 1 g of salts was added to it. The salt composition included pure MgCl2, MgCl2 and KCl (with 60 and 30 mol % of MgCl2) mixtures, as well as MgCl2 and NaCl (with 30 mol % MgCl2) mixtures. The tube was then heated to 800° C., and the resulting molten salt/carbon mixture was visually examined. After two hours, the salt was cooled down and transferred into the glovebox to be used in further studies with differential scanning calorimetry (DSC) for heat capacity and melting point analyses. Analysis of the Heat Transfer Compositions The best dispersion of carbon particles in the salt mixture was achieved with hollow carbon spheres. Notably, the carbon particles turned bright red in color in the furnace and then slowly turned black, at which time they were removed from the furnace for inspection. The color change was found to be due to black body radiation of the particles that had a higher local temperature than the salt. This color change works as a very good qualitative tool to determine whether particles are dispersed in the salt. Based on these observations, HCS of various wall thicknesses (20-200 nm) disperse in the molten salt under argon, and disperse even more after applying vacuum, due to displacement of the atmosphere in the particles by the salt. Other particles either sedimented or floated in the salt or reacted with it (OLC/MnOx, i.e., onion-like carbon/mangenese oxide core-shell nanoparticles). After it was established that HCS have better dispersion capabilities, further experiments were conducted with them in mixtures of MgCl2 and KCl (or NaCl). Addition of KCl reduced the concentration of particles in the salt mix under argon atmosphere. However, applying a vacuum helped disperse more particles. It should be noted that mixtures with an excess of KCl had a lower concentration of particles even after evacuation. The same trend was observed in the mixtures with NaCl, which indicates that the dispersivity is dependent on the acidity of the salt and not the ionic size of cations in the molten salt. While there have been shown and described what are at present considered the preferred embodiments of the invention, those skilled in the art may make various changes and modifications which remain within the scope of the invention defined by the appended claims. |
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060211709 | claims | 1. A nuclear reactor plant, comprising: a reactor pressure vessel; a holding structure fixed to said reactor pressure vessel, said holding structure having a lower surface; a core barrel supported on said holding structure, said core barrel including a lower part having at least one elastically resilient segment with a projection engaging under said holding structure and defining a wedge-like gap between said lower surface of said holding structure and said projection; and a wedge braced in said gap. fixing a cylindrical holding structure to the reactor pressure vessel and providing the holding structure with a lower surface; providing the core barrel with a lower part having at least one elastically resilient segment with a projection; placing the lower part onto the cylindrical holding structure with the projection engaging under the holding structure and forming a wedge-like gap between the lower surface of the holding structure and the projection; and bracing a wedge in the gap. 2. The nuclear reactor plant according to claim 1, wherein said projection has a beveled surface facing away from said gap to be led past said holding structure. 3. The nuclear reactor plant according to claim 2, including a screw disposed in said segment and acting on an end surface of said segment, for bracing said wedge in said gap. 4. The nuclear reactor plant according to claim 3, wherein said gap tapers in radial direction toward said reactor pressure vessel and said wedge is braced in said gap by pressure force exerted by said screw. 5. The nuclear reactor plant according to claim 4, wherein said screw has a device for securing against rotation. 6. The nuclear reactor plant according to claim 1, wherein said lower part has a longitudinal direction and an outer periphery with a groove formed therein parallel to the longitudinal direction, and said holding structure is pushed into said groove. 7. In a method for mounting a core barrel in a reactor pressure vessel of a nuclear reactor plant, the improvement which comprises: |
abstract | A method, system, and apparatus for the thermal storage of nuclear reactor generated energy including diverting a selected portion of energy from a portion of a nuclear reactor system to an auxiliary thermal reservoir and, responsive to a shutdown event, supplying a portion of the diverted selected portion of energy to an energy conversion system of the nuclear reactor system. |
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description | This application is a continuation of U.S. patent application Ser. No. 12/500,198, filed Jul. 9, 2009 (that issued as U.S. Pat. No. 8,390,788 on Mar. 5, 2013), which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/079,975, filed Jul. 11, 2008; U.S. Provisional Patent Application No. 61/136,150, filed Aug. 14, 2008; and U.S. Provisional Patent Application No. 61/136,983, filed Oct. 20, 2008, which are incorporated by reference herein in their entireties. 1. Field Embodiments of the present invention relate to spectral purity filters (SPFs), and in particular, although not restricted to, spectral purity filters for use in a lithographic apparatus. 2. Background A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet radiation (EUV) having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that radiation with a wavelength of less than 10 nm could be used, for example 6.7 nm or 6.8 nm. In the context of lithography, wavelengths of less than 10 nm are sometimes referred to as ‘beyond EUV’ or as ‘soft x-rays’. Extreme ultraviolet radiation and beyond EUV radiation may be produced using, for example, a plasma. The plasma may be created for example by directing a laser at particles of a suitable material (e.g., tin), or by directing a laser at a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits extreme ultraviolet radiation (or beyond EUV radiation), which is collected using a collector such as a mirrored grazing incidence collector, which receives the extreme ultraviolet radiation and focuses the radiation into a beam. Practical EUV Sources, such those which generate EUV radiation using a plasma, do not only emit desired ‘in-band’ EUV radiation, but also undesirable ‘out-of-band’ radiation. This out-of-band radiation is most notably in the deep ultraviolet (DUV) radiation range (100-400 nm). Moreover, in the case of some EUV sources, for example laser produced plasma EUV sources, the radiation from the laser, usually at 10.6 μm, presents a significant amount of out-of-band radiation. In a lithographic apparatus, spectral purity is required for several reasons. One reason is that resist is sensitive to out-of-band wavelengths of radiation, and thus the image quality of patterns applied to the resist may be deteriorated if the resist is exposed to such out-of-band radiation. Furthermore, out-of-band infrared radiation, for example the 10.6 μm radiation in some laser produced plasma sources, leads to unwanted and unnecessary heating of the patterning device, substrate and optics within the lithographic apparatus. Such heating may lead to damage of these elements, degradation in their lifetime, and/or defects or distortions in patterns projected onto and applied to a resist-coated substrate. In order to overcome these problems, several different transmissive spectral purity filters have been proposed which substantially prevent the transmission of infrared radiation, whilst simultaneously allowing the transmission of EUV radiation. Some of these proposed spectral purity filters include a thin metal layer or foil which is substantially opaque to, for example, infrared radiation, while at the same time being substantially transparent to EUV radiation. These and other spectral purity filters may also be provided with one or more apertures. The size and spacing of the apertures may be chosen such that infrared radiation is diffracted by the apertures, while EUV radiation is transmitted through the apertures. A spectral purity filter provided with apertures may have a higher EUV transmittance than a spectral purity filter which is not provided with apertures. This is because EUV radiation will be able to pass through an aperture more easily than it would through a given thickness of metal foil or the like. One problem associated with spectral purity filters provided with apertures is that the apertures are so small that the manufacturing options that are available to create the apertures are limited and/or expensive. Furthermore, the small diameter of the apertures reduces the mechanical robustness of the spectral purity filter. In a lithographic apparatus it is desirable to minimize the losses in intensity of radiation which is being used to apply a pattern to a resist coated substrate. One reason for this is that, ideally, as much radiation as possible should be available for applying a pattern to a substrate, for instance to reduce the exposure time and increase throughput. At the same time, it is desirable to minimize the amount of undesirable (e.g., out-of-band) radiation that is passing through the lithographic apparatus and which is incident upon the substrate. It is therefore an object of embodiments of the present invention to provide an improved or alternative spectral purity filter. For example, it is an object of embodiments of the present invention to provide a spectral purity filter provided with at least one aperture, and which is easier to manufacture, and/or is more mechanically robust than known or proposed spectral purity filters. It is also an object of embodiments of the present invention to provide alternative spectral purity filter arrangements. It is a further object of embodiments of the present invention to provide a spectral purity filter with improved suppression of undesirable (e.g., out-of-band) radiation, such as infrared radiation. According to a first aspect of the present invention there is provided a spectral purity filter, including: an aperture, the aperture being arranged to diffract a first wavelength of radiation and to allow at least a portion of a second wavelength of radiation to be transmitted through the aperture, the second wavelength of radiation being shorter than the first wavelength of radiation; wherein the aperture has a diameter greater than 20 μm. According to a second aspect of the present invention there is provided a spectral purity filter, including: an aperture, the aperture being arranged to diffract a first wavelength of radiation and to allow at least a portion of a second wavelength of radiation to be transmitted through the aperture, the second wavelength of radiation being shorter than the first wavelength of radiation; wherein a sidewall of the aperture is provided with a coating. The coating may be arranged to absorb at least a portion of the first wavelength of radiation. The coating may be arranged to inhibit reflection of at least a portion of the first wavelength of radiation. The coating may be arranged to promote reflection of at least a portion of the second wavelength. The coating may be arranged to inhibit degradation or environmental damage to the aperture. The aperture may have a diameter greater than 20 μm. According to the first or second aspects of the present invention, the aperture may have a diameter greater than 20 μm and less than or equal to 200 μm. According to the first or second aspects of the present invention, the spectral purity filter may be provided with a plurality of apertures. The spectral purity filter may be provided with a periodic array of apertures. The spectral purity filter may be provided with an aperiodic array of apertures. According to a third aspect of the present invention there is provided a spectral purity filter, including: a first aperture having a first diameter, the first aperture being arranged to diffract a first wavelength of radiation and to allow at least a portion of a second wavelength of radiation to be transmitted through the aperture, the second wavelength of radiation being shorter than the first wavelength of radiation; wherein the spectral purity filter is also provided with: a second aperture having a second diameter, the second diameter being smaller than the first diameter, the second diameter being small enough to prevent diffraction of the first wavelength of radiation, while allowing transmission of at least a portion of the second wavelength of radiation. The first aperture has a diameter greater than 20 μm. The first aperture may have a diameter greater than 20 μm and less than or equal to 200 μm. The second aperture may have a diameter which is less than or equal to half of the wavelength of the first wavelength of radiation. The spectral purity filter may be provided with a plurality of first apertures. The spectral purity filter may be provided with a periodic array of first apertures. The spectral purity filter may be provided with an aperiodic array of first apertures. The spectral purity filter may be provided with a plurality of second apertures. The spectral purity filter may be provided with a periodic array of second apertures. The spectral purity filter may be provided with an aperiodic array of second apertures. According to the first, second or third aspects of the present invention, material forming the spectral purity filter may be substantially transparent to the transmission of the second wavelength of radiation. Material forming the spectral purity filter may be substantially opaque to the transmission of the first wavelength of radiation. Material forming the spectral purity filter may be arranged to absorb or reflect the first wavelength of radiation. According to the third aspect of the present invention, material forming the spectral purity filter may be substantially transparent to the transmission of the first wavelength of radiation. The spectral purity filter may be arranged such that a phase difference is introduced between radiation of the first wavelength that is arranged to pass through the material and radiation of the first wavelength that is arranged to pass through (and be diffracted by) the first aperture. The spectral purity filter may be configured such that destructive interference of a zero diffraction order of the first wavelength of radiation takes place between radiation of the first wavelength that is arranged to pass through (and be diffracted by) the material and radiation of the first wavelength that is arranged to pass through and be diffracted by the first aperture. The spectral purity filter may have a thickness which causes the destructive interference. According to a fourth aspect of the present invention there is provided a spectral purity arrangement, including: a spectral purity filter provided with one or more apertures, the one or more apertures being arranged to transmit and diffract a first wavelength of radiation and to allow at least a portion of a second wavelength of radiation to be transmitted through the one or more apertures, the second wavelength of radiation being shorter than the first wavelength of radiation; material forming the spectral purity filter being substantially transparent to the transmission of the first wavelength of radiation, the spectral purity filter being configured such that destructive interference of a zero diffraction order of the first wavelength of radiation takes place between radiation of the first wavelength that is arranged to pass through the material and radiation of the first wavelength that is arranged to pass through the one or more apertures; the spectral purity arrangement further including: a structure provided with a further aperture, the spectral purity filter and the structure being arranged relative to one another such that at least a portion of the second wavelength of radiation is able to pass through the spectral purity filter and the further aperture that is provided in the structure, and wherein a spacing or diameter of the one or more apertures of the spectral purity filter is configured to ensure that less than 50% of radiation of the first wavelength of radiation is able to pass through the further aperture that is provided in the structure. The spectral purity filter may be configured such that a phase difference is introduced between radiation of the first wavelength that is arranged to pass through the material and radiation of the first wavelength that is arranged to pass through the one or more apertures, in order to cause the destructive interference. The spectral purity filter may have a thickness which causes the destructive interference. The spacing or diameter of the one or more apertures of the spectral purity filter may be configured to ensure that a first diffraction order of the first wavelength of radiation is incident upon the structure and not transmitted through the further aperture of the structure. The structure may be a plate. The structure may be at least a part of a radiation source, an illumination system, or a projection system. The structure may be at least a part of radiation source, an illumination system, or a projection system of a lithographic apparatus. The structure may be at least a part of a housing of the radiation source, at least a part of a housing of the illumination system, or at least a part of a housing of the projection system. The spectral purity filter may be provided with a plurality of apertures. The spectral purity filter may be provided with a periodic array of apertures or an aperiodic array of apertures. The spacing or diameter of the one or more apertures of the spectral purity filter may be configured to ensure that less than 10% of radiation of the first wavelength of radiation is able to pass through the further aperture that is provided in the structure, or such that less than 5% of radiation of the first wavelength of radiation is able to pass through the further aperture that is provided in the structure. According to the first, second, third or fourth aspects of the present invention, the first wavelength of radiation may have a wavelength that is in the infrared part of the electromagnetic spectrum. The first wavelength of radiation may have a wavelength that is approximately 10.6 μm. The second wavelength of radiation may have a wavelength that is in, or is shorter than, the EUV part of the electromagnetic spectrum. According to a fifth aspect of the present invention, there is provided a lithographic apparatus provided with a spectral purity filter or spectral purity arrangement according to the first, second, third or fourth aspects of the present invention. The lithographic apparatus may further include: an illumination system configured to condition a radiation beam, the radiation beam including the first wavelength of radiation, the second wavelength of radiation, or the first wavelength of radiation and the second wavelength of radiation; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate. According to a sixth aspect of the present invention, there is provided a radiation source provided with a spectral purity filter or spectral purity arrangement according to the first, second, third or fourth aspects of the present invention. According to a seventh aspect of the present invention, there is provided a method of affecting the spectral purity of a radiation beam, the radiation beam including a first wavelength of radiation and a second wavelength of radiation, the method including: directing the radiation beam at a spectral purity filter or spectral purity arrangement according to the first, second, third or fourth aspects of the present invention. According to an eighth aspect of the present invention, there is provided a lithographic method, including: directing a radiation beam including a first wavelength of radiation and a second wavelength of radiation at a spectral purity filter or spectral purity arrangement according to the first, second, third or fourth aspects of the present invention; and using radiation transmitted by the spectral purity filter or spectral purity arrangement to apply a pattern to a substrate coated with radiation sensitive material. In the description of embodiments of the present invention, the terms ‘substantially transparent’ and ‘substantially opaque’ have been used. ‘Substantially transparent’ may be defined as a material or object having a transmission of greater than 50% of the radiation in question, and in particular a transmission of between 80% and 100%. ‘Substantially opaque’ may be defined as a material or object having a transmission of less than 50% of the radiation in question, and in particular a transmission of between 0% and 20%. FIG. 1 schematically depicts a lithographic apparatus 2 according to one embodiment of the invention. The apparatus 2 includes: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation); a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters; a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by a patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W. The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. The support structure supports, i.e., bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus 2, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.” The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit. Examples of patterning devices include masks and programmable mirror arrays. Masks are well known in lithography, and typically in an EUV radiation (or beyond EUV) lithographic apparatus would be reflective. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix. The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system. Usually, in an EUV (or beyond EUV) radiation lithographic apparatus the optical elements will be reflective. However, other types of optical element may be used. The optical elements may be in a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”. As here depicted, the apparatus 2 is of a reflective type (e.g., employing a reflective mask). The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines, the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. Referring to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system including, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus. The source SO and the illuminator IL, together with the beam delivery system if required, may be referred to as a radiation system. The illuminator IL may include an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may include various other components, such as an integrator and a condenser. The illuminator IL may be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross-section. The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device. Having been reflected by the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor IF1 can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies. The depicted apparatus 2 could be used in at least one of the following modes. 1. In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure. 2. In scan mode, the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. 3. In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array of a type as referred to above. Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. FIG. 2 shows the lithographic apparatus 2 in more detail, including a radiation source SO, an illuminator IL (sometimes referred to as an illumination system), and the projection system PS. The radiation source SO includes a radiation emitter 4 which may include a discharge plasma. EUV radiation may be produced by a gas or vapor, such as Xe gas or Li vapor in which very hot plasma is created to emit radiation in the EUV radiation range of the electromagnetic spectrum. The very hot plasma is created by causing partially ionized plasma of an electrical discharge to collapse onto an optical axis 6. Partial pressures of e.g., 10 Pa of Xe or Li vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In some embodiments, tin may be used. FIG. 2 illustrates a discharge produced plasma (DPP) radiation source SO. It will be appreciated that other sources may be used, such as for example a laser produced plasma (LPP) radiation source. The radiation emitted by radiation emitter 4 is passed from a source chamber 8 into a collector chamber 10. The collector chamber 10 includes a contamination trap 12 and grazing incidence collector 14 (shown schematically as a rectangle). Radiation allowed to pass through the collector 14 is reflected off a grating spectral filter 16 to be focused in a virtual source point 18 at an aperture 20 in the collector chamber 10. Before passing through the aperture 20, the radiation passes through a spectral purity filter SPF. The spectral purity filter SPF is described in more detail below. From collector chamber 10, a beam of radiation 21 is reflected in the illuminator IL via first and second reflectors 22, 24 onto a reticle or mask MA positioned on reticule or mask table MT. A patterned beam of radiation 26 is formed which is imaged in projection system PS via first and second reflective elements 28, 30 onto a substrate W held on a substrate table WT. It will be appreciated that more or fewer elements than shown in FIG. 2 may generally be present in the source SO, illumination system IL, and projection system PS. For instance, in some embodiments the illumination system IL and/or projection system PS may contain a greater or lesser number of reflective elements or reflectors. It is known to use a spectral purity filter in a lithographic apparatus to filter out undesirable (e.g., out-of-band) wavelength components of a radiation beam. For instance, it is known to provide a spectral purity filter including one or more apertures. The diameter of each aperture is chosen such that it diffracts one or more undesirable wavelengths of radiation, while allowing one or more different wavelengths of desirable radiation to pass through the apertures. For instance, the undesirable radiation may include infrared radiation, whereas the desirable radiation may include EUV or beyond EUV radiation. Proposed spectral purity filters which include apertures (sometimes referred to as aperture spectral purity filters) are provided with apertures having a diameter of up to 20 μm. The small diameter of these apertures limits the manufacturing options which are available to form these apertures, and also reduces the mechanical robustness of the spectral purity filter. In accordance with an embodiment of the present invention, a spectral purity filter is provided with one or more apertures, the apertures having a diameter greater than 20 μm. Because the apertures are greater than 20 μm in diameter, they may be more easily provided in a spectral purity filter than apertures having a diameter lower than 20 μm. For instance, apertures having a diameter of greater than 20 μm may be provided using well established laser drilling apparatus and techniques. An aperture having a diameter larger than 20 μm is suitable for suppressing, for example, the 10.6 μm out-of-band infrared radiation mentioned above. Increasingly larger diameter apertures may also suppress infrared radiation having a higher wavelength. The apertures may have, for example, a diameter which is greater than 20 μm and less than or equal to 200 μm in order to, for example, cause diffraction of and therefore suppress infrared radiation. The use of apertures having a diameter greater than 20 μm has many advantages associated with it when compared with proposed spectral purity filters with apertures having a diameter less than 20 μm. For instance, laser cutting (sometimes referred to as laser drilling) is very suitable for providing one or more (e.g., an array) of holes in metal plates and plates formed from other materials, in order to form an aperture spectral purity filter. The minimum diameter of currently available laser micro-machining systems (e.g., laser drilling apparatus) is about 20 μm, meaning that such micro-machining systems are particularly suitable for use in conjunction with embodiments of the present invention. In contrast, such micro-machining systems are not suited for use in proposed aperture spectral purity filters, where the apertures have a diameter lower than 20 μm. Another advantage is the fact that the mechanical robustness of the spectral purity filter increases when apertures in the filter are greater than 20 μm in diameter. This is because the wall thickness between adjacent apertures for a given fill ratio (i.e., the ratio of aperture space to plate or non-aperture space) is greater when the apertures are greater than 20 μm in diameter than it is when the apertures are less than 20 μm in diameter. This is especially advantageous when the spectral purity filter must withstand large stress concentrations, for example to prevent crack propagation. Another advantage is that in a spectral purity filter with apertures having a diameter greater than 20 μm, EUV optical losses on the sidewalls of the aperture are smaller than with apertures of a smaller diameter (for the same given thickness of spectral purity filter). Such losses may occur due to small misalignment of the spectral purity filter or the apertures of the filter, or due to thickness variation in the spectral purity filter. Losses are greater when a radiation beam incident on the spectral purity filter, for example a beam of EUV radiation, is divergent. A yet further advantage of using apertures having a diameter greater than 20 μm is that it makes it easier to apply a coating to sidewalls of the apertures. For example, it may be desirable to apply an infrared absorbing and/or anti-reflection coating to the sidewalls in order to maximize the suppression of the infrared radiation, or to provide a coating to promote EUV reflection and therefore transmission through the aperture. The above advantages will now be described in more detail with reference to specific embodiments of the present invention. FIG. 3 schematically depicts a spectral purity filter SPF according to a first embodiment of the present invention. The spectral purity filter SPF includes a plate 50 in which a periodic array of circular apertures 52 is provided. The diameter D of the apertures 52 is selected such that a first wavelength of radiation to be suppressed is substantially diffracted at the entrance of each aperture 52, while radiation of a second, shorter wavelength is transmitted through the apertures 52. The diameter of the apertures 52 is greater than 20 μm. A diameter slightly greater than 20 μm is particularly suited to the diffraction and suppression of 10.6 μm infrared radiation, which is often generated by the radiation sources of EUV lithographic apparatus. The apertures 52 may have a diameter which is greater than 20 μm and less than or equal to 200 μm, in order to suppress by diffraction longer wavelengths of infrared radiation. The plate 50 can be formed from any suitable material. The plate 50 should be substantially opaque to the first wavelength or range of wavelengths which the spectral purity filter SPF is designed to suppress. For instance, the plate 50 may reflect or absorb the first wavelength, for example a wavelength in the infrared range of the electromagnetic spectrum. The plate 50 may also be substantially opaque to one or more second wavelengths of radiation which the spectral purity filter SPF is designed to transmit, for example a wavelength in EUV range of the electromagnetic spectrum. However, the spectral purity filter SPF can also be formed from a plate 50 which is substantially transparent to the one or more wavelengths which the spectral purity filter SPF is designed to transmit. This may increase the transmittance of the spectral purity filter with respect to the one or more wavelengths which the spectral purity filter SPF is designed to transmit. An example of a material which may form the plate 50 of a spectral purity filter SPF is a metal. Another example is a thin foil that is substantially transparent to EUV radiation. The apertures 52 in the spectral purity filter SPF are arranged in a hexagonal pattern. This embodiment gives the closest packing of circular apertures, and therefore the highest transmittance for the spectral purity filter. However, other arrangements of the apertures are also possible, for example square, and rectangular or other periodic or aperiodic arrangements may be used. For instance, in the case of an aperiodic array, a random pattern may be employed. FIG. 4 is a schematic depiction of a spectral purity filter SPF in accordance with another embodiment of the present invention. In this embodiment, it can be seen that apertures 54 are provided in the spectral purity filter SPF. The apertures 54 are not circular, but are instead elongated slots or slits. It can be seen that a shorter dimension SD of the aperture 54 has a length of between greater than 20 μm and less than or equal to 200 μm. A longer dimension LD of the apertures 54 may be any length. It will be appreciated that the elongated apertures 54 shown in FIG. 4 are only suitable for situations where the radiation which is incident upon them, and which it is desired to diffract, is substantially polarized in a direction substantially parallel to the longer dimension LD. FIG. 5 is a graph depicting the geometrical transmittance of the spectral purity filter of FIG. 3, and how this varies as a function of the ratio of the wall thickness between apertures to the diameter of those apertures (or, in other words, holes in the spectral purity filter). The geometrical transmittance is proportional to the area which the apertures in total define when the spectral purity filter is viewed end-on, as shown in FIG. 3. It can be seen from FIG. 5 that for a typical wall thickness of one-tenth of the aperture diameter, the geometrical transmittance of the spectral purity filter is about 75%. It can be seen that the geometrical transmittance can be further increased by increasing the diameter of the apertures, although this will probably lead to a less robust spectral purity filter due to a reduction in the wall thickness between apertures. The minimum thickness of the spectral purity filter will depend on whether it is desired to absorb the diffracted radiation in the sidewalls of the apertures of the spectral purity filter, or whether it is desired to absorb the diffracted radiation downstream of the spectral purity filter. When the spectral purity filter is sufficiently thick, most of the diffracted radiation may be absorbed in the sidewalls of the apertures. Since only a small fraction of the incident power (of the radiation beam) reaches the exit of the apertures, the effect of interference between different apertures may be neglected. Consequently, the suppression of infrared radiation is approximately equal to that of a single aperture of the same dimensions. In the Fraunhofer (far field) approximation of diffraction from a single circular aperture, the intensity distribution as a function of the diffraction angle θ is given by: I ( θ ) = I 0 ( 2 J 1 ( k a sin θ ) k a sin θ ) 2 where k=2π/λ is the wavenumber, a is the aperture radius and J1 is the first-order Bessel function of the first kind. It is desirable to suppress the transmission of infrared radiation, as discussed above. Depending on the application in question, it may be desirable to suppress the infrared radiation by a factor of 100 or more, or in other applications by a factor of around 10. In some applications, a suppression factor of 20 may be desirable. This means that only 5% of the infrared radiation incident on the spectral purity filter will be transmitted by the spectral purity filter. FIG. 6 shows the angle within which 5% of the infrared radiation (in terms of power, or in other words intensity) is contained as a function of the aperture diameter (where the aperture diameter is twice the aperture radius, i.e., 2a). FIG. 6 shows that if apertures having a diameter of 30 μm are used, the angle containing 5% of the transmitted radiation will be 2.9°. In order to achieve the desired suppression factor of 20 (in other words, to achieve a transmission of 5% of the incident infrared radiation), all radiation diffracted by more than 2.9° must be absorbed. Such absorption can take place within the spectral purity filter. The minimum thickness of the spectral purity filter is then determined by the aperture diameter and the minimum diffraction angle outside which all radiation is to be absorbed. For example, with the 30 μm diameter apertures described above, the angle containing 5% of the transmitted radiation was described as being 2.9°. Therefore, in order to ensure that all radiation outside of this diffraction angle is absorbed by the sidewalls of the apertures, the spectral purity filter must have a minimum thickness of:30 μm/tan 2.9°=0.584 mm More infrared radiation having a wavelength of 10.6 μm can be suppressed by increasing the thickness of the spectral purity filter. However, the transmittance of radiation having a shorter wavelength (e.g., EUV radiation) will be reduced due to the higher aspect ratio of the apertures. For a thinner spectral purity filter, a substantial fraction of the incident power is transmitted through the spectral purity filter. The degree of interference between the radiation transmitted through the apertures depends on the coherence of the radiation. For example, when the incoming radiation is very incoherent, there is no substantial interference between the apertures and the diffraction pattern is described by the single-aperture approximation described above. In this case, it is possible to absorb the infrared radiation at a location behind (i.e., downstream of) the spectral purity filter. For example, if the spectral purity filter is located 0.5 m before an intermediate focus of the lithographic apparatus (for example, the virtual source point 18 shown in and described with reference to FIG. 2), the diffracted radiation can be absorbed by a plate or the like located at the intermediate focus. The plate is provided with an aperture having a diameter of 8 mm for transmitting, for example, EUV radiation. In this case, all radiation diffracted by 0.9° is prevented from passing through the aperture. From FIG. 6, it can be seen that the apertures in the spectral purity filter can have a diameter as large as 100 μm in order to ensure that the angle containing 5% of the transmitted radiation is 0.9° (or in other words to ensure that the suppression factor is approximately 20). Since the spectral purity filter in this embodiment would only act to diffract the radiation, and not to absorb it, the spectral purity filter may be as thin as minimally required for mechanical stability and may be, for example, 0.1 mm thick. In FIGS. 3 and 4, the apertures of the spectral purity filters have been shown and described as being part of a periodic array of apertures. A coherent source of infrared radiation incident upon the spectral purity filter combined with a periodic aperture array could give rise to a far field diffraction pattern with a very sharp (e.g., less than 0.5°) central maximum, in which 80-90% of the infrared radiation intensity is contained. If a transmission profile having such a far field diffraction pattern were to be established, it would be difficult or impossible to separate the infrared radiation from the EUV radiation in the far field. Therefore, in the case of a substantially coherent source of radiation, other methods of affecting the diffraction pattern may be used. For example, an aperiodic array of apertures will ensure that the central maximum of the resultant diffraction pattern is broadened, and is not very sharp. This would make it easier to separate the infrared radiation from the EUV radiation in the far field, for example using a plate provided with an aperture as described above. FIG. 7 schematically depicts and summarizes the embodiments shown in and described with reference to FIGS. 3 to 6. A radiation beam 60 including EUV radiation 62 and infrared radiation 64 is incident upon a spectral purity filter SPF as described above in relation to, for example, FIG. 3. Downstream of the spectral purity filter SPF is located a plate 66 which is capable of absorbing infrared radiation 64. The plate 66 is provided with an aperture 70, and is located adjacent to an intermediate focus 68 of the radiation beam 60 such that the intermediate focus 68 is located in the aperture 70. When the radiation beam 60 is incident upon the spectral purity filter SPF, the EUV part of the radiation beam 62 passes through the spectral purity filter SPF without being diffracted. At the same time, the infrared part of the radiation beam 64 is diffracted by apertures of the spectral purity filter SPF, as described above. Thus, at the location of the plate 66, it can be seen that the EUV part of the radiation beam 62 passes through the aperture 70, whereas a majority of the diffracted infrared part of the radiation beam 64 is instead directed toward and absorbed by the plate 66. Downstream of the plate 66, it will be appreciated that the infrared part 64 of the radiation beam has been reduced or substantially eliminated, leaving a radiation beam 72 which includes substantially EUV radiation. In the above embodiments, the apertures have been described as being circular or slot or slit-like. The apertures can be other shapes, such as for example rectangular, hexagonal, pentagonal, etc. The spectral purity filter described above has been described as being formed from a plate. The spectral purity filters described herein can instead be formed from a mesh, or two overlapping perpendicular wire grids. It may be easier to manufacture a mesh than it is to drill holes in a plate. Wire grids may also be easier to manufacture than an array of circular apertures in a plate. For instance, each of the wire grids can be formed, for example, using laser interference lithography. This may be quicker and easier to undertake than providing a spectral purity filter with an array of drilled apertures, where each aperture may need to be individually drilled using a laser machine tool. In the above embodiments, absorption in the sidewalls of apertures of the spectral purity filter has been described. The spectral purity filter may be formed from a material which is suitable for absorption of one or more wavelengths of radiation. For example, the spectral purity filter may be formed from a material with a high infrared radiation absorptivity. For instance, many glasses (e.g., fused silica) and ceramics (e.g., TiO2) fall within this category. As mentioned above, the use of apertures having diameters greater than 20 nm makes it easier to apply a coating to sidewalls of the apertures. This means that the sidewalls of the apertures may be provided with a coating which is or includes a material which absorbs or inhibits reflection of certain wavelengths of radiation. For example, in the case of the absorption of infrared radiation, coatings may be provided which are formed from or include materials such as the glasses or ceramics mentioned above. Alternatively or additionally, an anti-reflection coating may be applied to the sidewalls of the apertures, including for example CO2 laser window materials, such as ZnSe, ZnS, GaAs and Ge, and/or low refractive index halides such as ThF4 and YF3. The coating may additionally or alternatively promote the reflection of one or more wavelengths of radiation, such as for example EUV radiation. FIG. 8 shows an embodiment of a spectral purity filter. In this embodiment, a coating (as described above) has been applied to sidewalls of the apertures of the spectral purity filter. FIG. 8 shows a spectral purity filter SPF in section view. Apertures 80 are shown as being provided in a plate 81 of the spectral purity filter SPF. Sidewalls of the apertures 80 have been provided with an infrared radiation absorbing material 82. EUV radiation 84 and infrared radiation 86 are shown as being incident upon the spectral purity filter SPF and apertures 80 of the spectral purity filter SPF. The infrared radiation 86 is diffracted as it enters the apertures 80, and is absorbed by the coating 82 on the side walls of the apertures 80. It can be seen that radiation transmitted by the spectral purity filter SPF includes mainly EUV radiation 84. The coating 82 provided on the sidewalls of the apertures 80 does not need to promote or suppress reflection of one or more wavelengths of radiation. The coating 82 can instead be used to prevent the apertures 80 from degradation or environmental damage. The spectral purity filters of above embodiments have been described as being formed from plates (or the like) that are substantially opaque to an undesired wavelength of radiation, for example infrared radiation. The spectral purity filters described above are provided with apertures which diffract the undesired wavelength of radiation, while allowing a desired wavelength of radiation to be transmitted by the apertures. Thus, the spectral purity filters described above will reflect or absorb undesired wavelengths of radiation, either in the side walls of the apertures or in or from parts of the spectral purity filters upon which the undesired wavelength is incident (for example, parts of the spectral purity filter that do not define apertures). Absorption of the undesired wavelength can cause the spectral purity filter to heat up. Such heating can result in damage to the spectral purity filter and/or distortion of the spectral purity filter. Such distortion or damage may detrimentally affect the functionality of the spectral purity filter. For the reasons given above, in some applications it may therefore be desirable to reduce the heat load on a spectral purity filter. According to an embodiment of the present invention, this may be achieved by the use of a spectral purity filter that acts as a phase grating for the undesired wavelength or wavelengths of radiation. The phase grating spectral purity filter includes a material that is substantially transparent (and, ideally, fully transparent) to the undesired radiation. Furthermore, the phase grating spectral purity filter is provided with apertures which are constructed (e.g., have a spacing or diameter which is) such that most of the undesired radiation is diffracted into diffraction orders other than the zero order. One advantage of this solution is that the undesired radiation is not absorbed by the special purity filter, and does therefore not heat the spectral purity filter. Another advantage is that, since most of the undesired radiation is not diffracted into the zero order, a structure provided with an aperture placed at a location aligned with the zero order will allow desired (and not diffracted) radiation to pass through the aperture, while the undesired radiation is blocked by material (e.g., a plate) surrounding the aperture. In the case of an aperiodic array of apertures, there may not be any ‘orders’ as such. Therefore, according to a more general embodiment of the present invention, the spacing or diameter of the one or more apertures of the spectral purity filter may be configured to ensure that less than 50% of radiation of a first wavelength of radiation (e.g., infrared radiation) is able to pass through the further aperture that is provided in the structure. The spacing or diameter of the one or more apertures of the spectral purity filter may be configured to ensure that less than 10% of radiation of a first wavelength of radiation (e.g., infrared radiation) is able to pass through the further aperture that is provided in the structure. The spacing or diameter of the one or more apertures of the spectral purity filter may be configured to ensure that less than 5% of radiation of a first wavelength of radiation (e.g., infrared radiation) is able to pass through the further aperture that is provided in the structure FIG. 9 schematically depicts a phase grating spectral purity filter PGSPF in accordance with an embodiment of the present invention. The phase grating spectral purity filter PGSPF includes a plate 90 that is substantially transparent to an undesired (e.g., ‘first’) wavelength of radiation. For instance, if the undesired radiation includes 10.6 μm radiation, the plate 90 may be formed from silicon, ZnSe, ZnS, GaAs, Ge, diamond or diamond-like carbon. The plate 90 is provided with an array of apertures 92. The aperture diameter AD is chosen such that the undesired wavelength of radiation diffracts when it is incident upon the aperture 92. For instance, the apertures 92 may conveniently be greater than 20 μm in diameter, for ease of manufacturing as described above. The array of apertures 92 have a periodicity PE which is chosen such that the first diffraction order of the diffracted undesired wavelength of radiation is substantially separated from the zero diffraction order. The significance of this arrangement will be described in more detail below. Typically, the periodicity PE is of the same order of magnitude as the wavelength of radiation that the phase grating spectral purity filter PGSPF is designed to diffract. However, in other embodiments, the periodicity PE may be up to two orders of magnitude larger than this wavelength (for example, if only a very small degree of diffraction of the undesired wavelength of radiation is required). So far FIG. 9 has been described in relation to the diffraction of an undesired wavelength of radiation as a consequence of the incorporation of the apertures 92. As mentioned above, this spectral purity filter is a phase grating spectral purity filter PGSPF. The significance of the “phase grating” prefix will now be described. FIG. 10 shows the phase grating spectral purity filter PGSPF in section view. FIG. 10 illustrates the apertures 92 and the material forming the plate 90 surrounding these apertures 92. Undesired radiation 94 (e.g., infrared radiation) is shown as being incident upon the phase grating spectral purity filter PGSPF. The phase grating spectral purity filter PGSPF has a thickness H which is chosen such that the phase difference between the radiation passing through the plate 96 and the radiation passing through the apertures is π radians. In other words, radiation emerging from the apertures 98 is 180° out of phase with radiation emerging from the plate 96. If it is assumed that the diameter of the apertures is substantially larger than the wavelength of undesired radiation, diffraction effects can be assumed to be small and therefore the phase shift can be calculated based on a plane wave front. The phase difference Δφ between radiation passing through the plate and radiation passing through the apertures is thus given by: Δ φ = 2 π H ( n 1 - n 0 ) λ where n1 is the refractive index of the material of the plate and n0=1 is the refractive index of vacuum. Thus, the thickness that gives a phase difference of π is given by: H = ( m + 1 2 ) λ n 1 - n 0 where m=0, 1, 2, . . . is a non-negative integer. When silicon is chosen to form the material or the plates 90, the thickness H (in μm) that gives a phase difference of π is given by:H=(2.19+m 4.38) μm. As is known in the art, when the thickness of the phase grating spectral purity filter is such that the phase difference between radiation passing through the plate and radiation passing through the apertures is π radians, destructive interference of the undesired wavelength of radiation will take place. FIGS. 11 and 12 depict the effects of the phase grating spectral purity filter of FIGS. 9 and 10 on an incoming beam of radiation including an undesired wavelength of radiation. An undesired wavelength of radiation 94 is shown as being incident upon the phase grating spectral purity filter PGSPF. The diameter and periodicity of apertures of the phase grating spectral purity filter PGSPF are chosen such that the undesired wavelength of radiation is diffracted, and diffracted such that a first diffraction order 100 is substantially separated from a zero diffraction order 102. FIG. 11 does not, however, represent the effects on the zero order 102 of the destructive interference mentioned above. FIG. 12 more accurately represents the cumulative effects of the separation of the first diffraction order 100 from the zero diffraction order 102, and the effects on the zero diffraction of the destructive interference mentioned above. The zero diffraction order 102 contains only a small fraction of the incident intensity of the undesired wavelength of radiation 94 (typically less than 10%). Most of the undesired radiation is diffracted into higher orders, such as the first order and higher orders. To ensure that most of the incident intensity of the undesired wavelength of radiation 94 is diffracted into higher diffraction orders, the fill factor of the phase grating spectral purity filter PGSPF should have a fill factor of the order of 50% or greater (the fill factor being defined as the fractional area occupied by the material forming the plate, and not the spaces left by the apertures). A plate 104 may be provided to absorb the first and higher diffraction orders. An aperture 106 is provided in the plate 104 to allow, for example, undiffracted radiation to pass through (for example EUV radiation). It can be seen that the zero diffraction order 102 is also able to pass through the aperture, although this order (as described above) will typically be less than 10% of incident intensity. The phase grating spectral purity filter PGSPF and the plate 104 and aperture 106 therefore prevent at least 90% of the undesired radiation from passing any further, for example into or through a part of a lithographic apparatus. Because the fill factor is 50%, this means that the transmittance of radiation which is blocked by the plate (for instance, EUV radiation) is also of the order of 50%. The transmittance of radiation not blocked by the plate can be increased by reducing the fill factor, but this results in a reduced suppression of the undesired radiation (for example, infrared radiation). FIG. 13 shows the increase in infrared radiation transmittance of the phase grating spectral purity filter described above for an EUV transmittance increase from 50% to 100% (which corresponds to the fill factor being reduced from 50% to 0%). The graph represents a relationship for a one-dimensional phase grating, for instance a grating including a single row or column of apertures. It can be seen that, for example, at an EUV transmittance of 60%, the infrared transmittance is 4%, and thus the relative suppression factor of infrared radiation is 15 (i.e., the filter decreases the ratio of infrared radiation to EUV radiation by a factor of 15). FIG. 14 schematically summarizes the principles discussed in relation to FIGS. 9 to 13. A beam of radiation including an EUV component 108 and an infrared component 110 is incident upon a phase grating spectral purity filter PGSPF as described above. The phase grating spectral purity filter PGSPF is shown in relation to a plate 112 provided with an aperture 114. It can be seen that the EUV component 108 passes through apertures (not shown in the Figure) of the phase grating spectral purity filter PGSPF and towards and through the aperture 114 in the plate 112. In contrast, the infrared component 110 is destructively interfered with, as well as being and diffracted by the phase grating spectral purity filter PGSPF. The result is that the first diffraction order of the infrared component 110 is separated from the zero order to such an extent that the first and higher diffraction orders may be blocked by the plate 112, and prevented, for example, from passing on to and through a lithographic apparatus. The zero diffraction order is substantially reduced or eliminated. FIG. 15 schematically depicts an embodiment. The phase grating spectral purity filter PGSPF as described above is placed in front of (i.e., up-stream of) an intermediate focus 120 of a lithographic apparatus, for example the virtual source point 18 shown in and described with reference to FIG. 2. Referring back to FIG. 15, the intermediate focus 120 is located within an aperture 122 of a plate 124. The plate may, for example, form part of a source or illuminator housing. A radiation beam 126 incident upon the phase grating spectral purity filter PGSPF includes an EUV component 128 and an infrared component 130. As described above, the phase grating spectral purity filter PGSPF is configured to diffract and cause destructive interference of the infrared component 130 such that the majority of the infrared component 130 is either destructively interfered with, or diffracted to such an extent that is not able to pass through the aperture 122. For instance, it can be seen that the first and higher diffraction orders 132 of the infrared component 130 are diffracted such that they are unable to pass through the aperture 122. A small percentage of the zero diffraction order 134 of the infrared component 130 is able to pass through the aperture 122. However, this small percentage of the zero diffraction order 134 will be only a fraction of the infrared component 130, and maybe for example, less than 10% of the incident intensity. A typical diameter of the aperture 122 at the intermediate focus 120 (for example, the entrance pupil of an illuminator) is 8 mm. Thus, if the distance from the phase grating spectral purity filter PGSPF to the intermediate focus 120 is 0.1 m, diffraction orders separated by more than 2.3° (arctan (0.004/0.1)) from the zero order are suppressed. For the first diffraction order (and therefore higher diffraction orders) to be diffracted by more than 2.3°, the periodicity of the apertures of the phase grating spectral purity filter PGSPF referred to above should be less than 264 μm (calculated using λ/sin θ=264 μm, where λ=10.6 μm and θ=2.3°). In the embodiments described above in relation to FIGS. 9 to 15, up to 50% of the EUV radiation is blocked by the material which forms the plate of the phase grating spectral purity filter. It is desirable to increase the EUV transmittance whilst still suppressing the infrared radiation by destructive interference and diffraction. In accordance with a further embodiment of the present invention, material which forms the phase grating spectral purity filter contains a first array of apertures, and a second array of apertures, the second array of apertures being distributed around the first array of apertures and having diameters which are less than those of the first array. The first array of apertures have a diameter sufficient to cause diffraction of radiation which is to be suppressed (e.g., infrared radiation). The second (or further) array of apertures have a diameter which is less than the wavelength of the radiation which it is desired to suppress. This means that the second array of apertures (which may be referred to as sub-wavelength apertures) do not affect the diffraction of the radiation which it is desired to suppress, but will allow more shorter wavelength (for example EUV) radiation to pass through the phase grating spectral purity filter. For the purposes of determining phase differences for the radiation that passes through the apertures and the material of the phase grating spectral purity filter PGSPF, etc., the refractive index of the material with the sub-wavelength apertures may be approximated by a so-called effective medium approximation, for example, based on a weighted average of the dielectric constants of the material and vacuum (if the phase grating spectral purity filter is used in a vacuum). Therefore, in order to take into account the incorporation of the sub-wavelength apertures, the thickness of the phase grating spectral purity filter must be changed accordingly so as to still obtain the desired phase shift and subsequent destructive interference of the undesired radiation which passes through the larger apertures and through the material forming the phase grating spectral purity filter. FIG. 16 illustrates an embodiment of a phase grating spectral purity filter which is provided with sub-wavelength apertures. FIG. 16 schematically depicts a phase grating spectral purity filter PGSPF. Material 144 forming the phase grating spectral purity filter PGSPF is provided with a first array of apertures 140. The first array of apertures is arranged (e.g., have a diameter which is sufficient) to cause slight diffraction of a first wavelength of radiation, for example an undesired wavelength of radiation such as, for example, infrared radiation. The first array of apertures is also arranged (e.g., have a diameter which is sufficient) to allow transmission of a second wavelength of radiation, for example a desired wavelength of radiation such as, for example, EUV radiation. The second wavelength of radiation has a shorter wavelength that that of the first wavelength of radiation. A second (or in other words further) array of apertures 142 is also provided in material 144 forming the phase grating spectral purity filter PGSPF. The second array of apertures 142 have a diameter which is less than the diameter of the apertures 140 forming the first array. The diameter of the apertures of the second array 142 is less than the first wavelength of radiation of which the phase grating spectral purity filter is arranged to diffract and cause destructive interference. Preferably the diameter of the apertures of the second array 142 is less than half of the wavelength of the first wavelength of radiation, in order to ensure the validity of an effective medium approximation as described above. The diameter of the apertures of the second array 142 is greater than the second wavelength of radiation which the phase grating spectral purity filter is arranged to transmit. This means that the diffraction of the first wavelength of radiation (e.g., infrared radiation) is not affected, while the transmission of the second wavelength of radiation (e.g., EUV radiation) is increased by provision of the further array of apertures. A silicon phase grating spectral purity filter according to this embodiment of the present invention could include a two dimensional array of larger apertures with a diameter of 100 μm and a fill factor of 50%. The material (i.e., silicon) between the large apertures could be provided with a further array of smaller diameter apertures with a diameter of 4 μm and, for example, a fill factor of 50%. The effective refractive index of the material between the large apertures is calculated according to a weighted average of the dielectric constant (which is approximately equal to the square of the refractive index) resulting in a value of √(0.5×3.422+0.5×12)=2.52. Consequently, in order to achieve a phase shift of it of the first wavelength of radiation as it passes through the phase grating spectral purity filter PGSPF (to achieve destructive interference), the calculation that needs to be performed to determine the thickness of the phase grating spectral purity filter PGSPF will need to be modified to take into account this effective refractive index. The thickness of the silicon phase grating spectral purity filter PGSPF should therefore be calculated using:H=(3.49+m 6.98) μm where m is a positive integer or zero. For example, H may be 6.98 μm. It will be appreciated that this calculation will be different if a material having a refractive index different than that of silicon is used to form the phase grating spectral purity filter PGSPF. In this phase grating spectral purity filter, the infrared transmittance is close to zero (see for example the graph of FIG. 13). With an overall fill factor of 25%, the EUV transmittance of the phase grating spectral purity filter is approximately 75%. It will be appreciated that the use of sub-wavelength (with respect to a wavelength to be diffracted by larger apertures of the spectral purity filter) apertures is not restricted to use in phase grating spectral purity filters. Such sub-wavelength apertures may be provided in any spectral purity filter in order to increase the transmission of the spectral purity filter to one or more wavelengths of radiation. In the above described embodiments, a ‘desired’ wavelength of radiation has been described as being a wavelength of radiation in or below the EUV range of the electromagnetic spectrum. Furthermore, an ‘undesired’ wavelength has been described as a wavelength of radiation in the infrared part of the electromagnetic spectrum. It will be appreciated that the present invention is also applicable to other wavelengths of radiation. For example, the embodiments described above in relation to FIGS. 9 to 15 (where the first diffraction order is separated from the zero order such that the first order does not pass through an aperture provided downstream of the spectral purity filter) may also be applicable to wavelengths of radiation other than EUV radiation and infrared radiation. Similarly, the embodiments described above in relation to FIG. 16 (where sub-wavelength apertures are used to improve the transmittance of a phase grating spectral purity filter to a desired wavelength of radiation) may be used in conjunction with wavelengths of radiation other than EUV radiation. Although the above description of embodiments of the invention relates to a radiation source which generates EUV radiation (e.g., 5-20 nm), the invention may also be embodied in a radiation source which generates ‘beyond EUV’ radiation, that is radiation with a wavelength of less than 10 nm. “Beyond EUV” radiation may for example have a wavelength of 6.7 nm or 6.8 nm. A radiation source which generates “beyond EUV” radiation may operate in the same manner as the radiation sources described above. The invention is also applicable to lithographic apparatus that uses any wavelength of radiation where it is desired to separate, extract, filter, etc. one or more wavelengths of radiation from another one or more wavelengths of radiation. The described spectral purity filter may be used, for example, in a lithographic apparatus or a radiation source (which may be for a lithographic apparatus). The invention may also be applied to fields and apparatus used in fields other than lithography. The description above is intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below. |
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052232077 | abstract | An expert system for online surveillance of nuclear reactor coolant pumps. This system provides a means for early detection of pump or sensor degradation. Degradation is determined through the use of a statistical analysis technique, sequential probability ratio test, applied to information from several sensors which are responsive to differing physical parameters. The results of sequential testing of the data provide the operator with an early warning of possible sensor or pump failure. |
description | The operating environment of the present invention is described with respect of a four-slice computed tomography (CT) system. However, it will be appreciated by those of ordinary skill in the art that the present invention is equally applicable for use with other multi-slice configurations. Moreover, the present invention will be described with respect to the detection and conversion of x-rays. However, one of ordinary skill in the art will further appreciate, that the present invention is equally applicable for the detection, conversion, and convergence of other high frequency electromagnetic energy. Additionally, the present invention will be described with respect to a xe2x80x9cthird generationxe2x80x9d CT scanner, but is applicable with other generation CT scanners as well. Referring to FIGS. 1 and 2, a computed tomography (CT) imaging system 10 is shown as including a gantry 12 representative of a xe2x80x9cthird generationxe2x80x9d CT scanner. Gantry 12 has an x-ray source 14 that projects a beam of x-rays 16 toward a detector array 18 on the opposite side of the gantry 12. A pre-subject filter 15 is disposed between source 14 and patient 22 to filter the x-rays received by patient 22. Detector array 18 is formed by a plurality of detectors 20 which together sense the projected x-rays that pass through the medical patient 22. Each detector 20 produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuated beam as it passes through the patient 22. During a scan to acquire x-ray projection data, gantry 12 and the components mounted thereon rotate about a center of rotation 24. Rotation of gantry 12 and the operation of x-ray source 14 are governed by a control mechanism 26 of CT system 10. Control mechanism 26 includes an x-ray controller 28 that provides power and timing signals to an x-ray source 14 and a gantry motor controller 30 that controls the rotational speed and position of gantry 12. A data acquisition system (DAS) 32 in control mechanism 26 samples analog data from detectors 20 and converts the data to digital signals for subsequent processing. An image reconstructor 34 receives sampled and digitized x-ray data from DAS 32 and performs high speed reconstruction. The reconstructed image is applied as an input to a computer 36 which stores the image in a mass storage device 38. Computer 36 also receives commands and scanning parameters from an operator via console 40 that has a keyboard or other data entry device. An associated cathode ray tube display 42 allows the operator to observe the reconstructed image and other data from computer 36. The operator supplied commands and parameters are used by computer 36 to provide control signals and information to DAS 32, x-ray controller 28 and gantry motor controller 30. In addition, computer 3,6 operates a table motor controller 44 which controls a motorized table 46 to position patient 22 and gantry 12. Particularly, table 46 moves portions of patient 22 through a gantry opening 48. As shown in FIGS. 3 and 4, detector array 18 includes a plurality of detectors 20. Each detector 20 includes a two-dimensional photodiode array 52 and a two-dimensional scintillator array 56 positioned above the photodiode array 52. A collimator (not shown) is positioned above the scintillator array 56 to collimate x-ray beams 16 before such beams impinge upon scintillator array 56. Photodiode array 52 includes a plurality of photodiodes 60, deposited or formed on a silicon chip. Scintillator array 56, as known in the art, is positioned over the photodiode array 52. Photodiodes 60 are optically coupled to scintillator array 56 and are capable of transmitting signals representative of the light output of the scintillator array 56. Each photodiode 60 produces a separate low level analog output signal that is a measurement of the attenuated beam entering a corresponding scintillator 57 of scintillator array 56. Photodiode output lines 76 may, for example, be physically located on one side of detector 20 or on a plurality of sides of detector 20. As shown in FIG. 45, photodiode output lines 76 are located on opposing sides of the photodiode array 52. In one embodiment, as shown in FIG. 3, detector array 18 includes detectors 20. Each detector 20 includes a photodiode array 52 and scintillator array 56, each having an array size of 16xc3x9716. As a result, arrays 52 and 56 have 16 rows and 912 columns (16xc3x9757) detectors each, which allows 16 simultaneous slices of data to be collected with each rotation of gantry 12. The scintillator array 56 is coupled to the photodiode array 52 by a thin film of transparent adhesive (not shown). Switch arrays 80 and 82, FIG. 4 are multi-dimensional semiconductor arrays having similar width as photodiode array 52. In one preferred embodiment, the switch arrays 80 and 82 each include a plurality of field effect transistors (FET). Each FET is electrically connected to a corresponding photodiode 60. The FET array has a number of output leads electrically connected to DAS 32 for transmitting signals via a flexible electrical interface 84. Particularly, about one-half of the photodiode outputs are electrically transmitted to switch array 80 and the other one-half of the photodiode outputs are electrically transmitted to switch array 82. Each detector 20 is secured to a detector frame 77, FIG. 3, by mounting brackets 79. Switch arrays 80 and 82 further include a decoder (not shown) that controls, enables, disables, or combines photodiode output in accordance with a desired number of slices and slice resolutions. In one embodiment defined as a 16-slice mode, decoder instructs switch arrays 80 and 82 so that all rows of the photodiode array 52 are activated, resulting in 16 simultaneous slices of data available for processing by DAS 32. Of course, many other slice combinations are possible. For example, decoder may also enable other slice modes, including one, two, and four-slice modes. Shown in FIG. 5, by transmitting the appropriate decoder instructions, switch arrays 80 and 82 can be configured in the four-slice mode so that the data is collected from four slices of one or more rows of photodiode array 52. Depending upon the specific configuration of switch arrays 80 and 82 as defined by the decoder, various combinations of photodiodes 60 of the photodiode array 52 can be enabled, disabled, or combined so that the slice thickness may consist of one, two, three, or four rows of photodiode array elements 60. Additional examples include a single slice mode including one slice with slices ranging from 1.25 mm thick to 20 mm thick, and a two slice mode including two slices with slices ranging from 1.25 mm thick to 10 mm thick. Additional modes beyond those described are contemplated. Now referring to FIG. 6, a cross-sectional view of the cone angle dependent pre-subject filter 15 is shown. Filter 15 includes a bottom surface 86 and a concave top surface 88. Sidewalls 90 connect the bottom surface and the convex top surface in a single solid structure. Filter 15 is formed from a filtering material 92 that, in one embodiment, has a constant density. Convex Concave top surface 88 is fabricated to have a continuous and smooth face. Preferably, filter 15 is fabricated to have a thickness at a generally end region 94 that exceeds a thickness at a generally center region 96. That is, a maximum thickness is enjoyed at each end of the filter whereas a minimum thickness exists in the center region. As a result, the noise index at each generally end region 94 exceeds the noise index of the general center region 96. In one embodiment, filter 15 may comprise a number of thin slabs of filtering material that are stacked together such that the thickness of the filter at the end regions 94 exceeds the thickness of the center region 96 and vice-versa. Alternately, filter 15 could be equivalently formed from a bulk material having non-uniform density such that the filter has a uniform shape yet non-uniform attenuation. For example, the density of the material forming the end regions may be less than the density of the material forming the center region resulting in a varying attenuation profile of the filter. Moreover, the filter may be fabricated from more than one material with varying degrees of density. In the reconstruction process of multi-slice CT, the measured projection data is first weighted by a set of weighting functions prior to the filtered back-projection. These weighting functions serve the purpose of interpolation to estimate a set of projections at the plane of reconstruction (POR). For multi-slice CT, one of the major sources of image artifacts is the cone beam effect. It should be noted that the projection data collected by the detector row closer to the center of the detector are nearly parallel to the POR and are essentially fan-beam sampling. For the projection data collected by the detector rows further away from the detector center, the samples are significantly non-coplanar with the POR. With two-dimensional back-projection hardware, the discrepancy between the actual x-ray path and the x-ray path assumed by the back-projection process often causes imaging artifacts. This type of artifact is commonly referred to as xe2x80x9ccone beam artifactxe2x80x9d referring to the cone beam nature of the data collection. Helical weighting functions have been implemented such that projection samples with larger cone angles contribute less to the final reconstructed image. This is accomplished by assigning less weight to the data projection samples collected by the outer detector rows. For example, one of the weighting schemes for an eight slice 5:1 pitch helical reconstructions assigns the following relative weights to the eight detector rows: 0.125, 0.25, 0.375, 0.5, 0.5, 0.375, 0.25, 0.125. Different weights could be assigned however depending upon the reconstruction algorithm. It should be noted that the contribution from the outermost rows is only one-fourth of the contribution from the center rows. Because the final reconstructed image is obtained by the summation (back-projection) of signals from all detector rows, variance in the final image is the weighted sum of the variances of the projection samples of all detector rows. Since human anatomies do not change quickly over a short distance along the patient long axis, noise in the samples of all detector rows can be assumed approximately equal. Because the contribution from the outer detector rows is much less than the contribution from the inner detector rows, the efficiency of the sample utilization is not optimized. However, if the noise in the outer detector rows is increased, the impact of the noise on the final reconstructed image is much smaller than if the noise in the inner detector rows is increased. As a result, the x-ray flux to the inner detector rows may be increased and the x-ray flux to the outer detector rows may be reduced to obtain an overall improvement in terms of noise and dosage to the patient. Utilization of a cone angle dependent pre-subject filter similar to that shown in FIG. 6 increases the x-ray flux to the inner detector rows and reduces the x-ray flux to the outer detector rows yielding a reconstructed image with fewer artifacts as well as reduced x-ray to the patient. Referring now to FIG. 7, noise distributions from several filter-shaped designs are shown with respect to detector row number for an eight slice helical scan. The noise level at the innermost detector rows (rows 3 and 4) is assumed to be uniform and the noise levels for the other detector rows are normalized accordingly. To ensure artifact-free image when the x-ray focal spot moves (due to mechanical or thermal expansion), the filter shape should be continuous and smooth along the z axis. The several filter-shaped designs differ from one another in the thickness of the generally end regions. As shown, the noise index increases as the thickness of each end region increases. Referring now to FIG. 8, the relative x-ray dosage to patient for the several filter designs characteristically depicted in FIG. 7 are shown. Specifically, the fraction of total dosage projected to the patient decreases as the thickness of the filter is increased. For example, filter shape 1 provides a relative dose of 0.87 whereas filter shape 6 provides a relative dose of approximately 0.85. That is, the radiation detected by the outer rows of detector array 18, FIG. 3, decreases as thickness of the filter end regions increase. The present invention may be incorporated into a CT medical imaging device similar to that shown in FIG. 1. Alternatively, however, the present invention may also be incorporated into a non-invasive package or baggage inspection system, such as those used by postal inspection and airport security systems. Referring now to FIG. 9, package/baggage inspection system 100 includes a rotatable gantry 102 having an opening 104 therein through which packages or pieces of baggage may pass. The rotatable gantry 102 houses a high frequency electromagnetic energy source 106 as well as a detector assembly 108. A filter 107 similar to that cross-sectionally shown in FIG. 6 is also housed within gantry 102. A conveyor system 110 is also provided and includes a conveyor belt 112 supported by structure 114 to automatically and continuously pass packages or baggage pieces 116 through opening 104 to be scanned. Objects 116 are fed through opening 104 by conveyor belt 112, imaging data is then acquired, and the conveyor belt 112 removes the packages 116 from opening 104 in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages 116 for explosives, knives, guns, contraband, etc. Therefore, in accordance with one embodiment of the present invention, a cone angle dependent pre-subject filter for use with a radiation emitting imaging device is provided. The filter includes a flat surface as well as a convex concave surface. A number of sidewalls connecting the flat surface and the concave surface in a single solid structure are also provided. In accordance with another embodiment of the present invention, a radiation emitting imaging device includes a rotatable gantry having an opening defined therein for receiving a subject to be scanned. The device further includes a subject positioner configured to position the subject within the opening as well as a high frequency electromagnetic energy projection source configured to project high frequency electromagnetic energy to the subject. The imaging device further includes at least one filtering device configured to filter high frequency electromagnetic energy projected to the subject. The filtering device is formed of a bulk of filtering material having a non-uniform attenuation. The imaging device also includes a detector array having a plurality of detectors to detect high frequency electromagnetic energy passing through the subject and to output a plurality of electrical signals indicative of an intensity of the high electromagnetic energy detected. A data acquisition system is provided and connected to the detector array and configured to receive a plurality of electrical signals. An image reconstructor connected to the data acquisition system is provided and configured to reconstruct an image of the subject from the plurality of signals received by the data acquisition system. In accordance with a further embodiment of the present invention, a cone angle dependent pre-subject filter includes means for receiving high frequency electromagnetic energy. The filter further includes means for increasing attenuation of high frequency electromagnetic energy flux in a first region as well as means for decreasing attenuation of high frequency electromagnetic energy flux in a second region. In accordance with yet another embodiment of the present invention, a method of manufacturing a pre-subject filter for use with a radiation emitting imaging device includes the step of defining a block of filtering material. The method further includes shaping the block to have a linear surface and fashioning a block to have a curvilinear surface. The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims. |
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053894731 | description | DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with the method of the present invention a monolithic panel is first produced from a photosensitive glass which has a differential of solubility not less than 25 and has side sizes and a thickness which corresponds to required sizes and ratio of the grid. For example, the glass can have the following content (mass %): SiO.sub.2 78-82; Al.sub.2 O.sub.3 3.2-4.8; Li.sub.2 CO.sub.3 11-14; Ca.sub.2 CO.sub.3 1.5-3.5; CeO.sub.2 0.20-0.40; SnO.sub.2 0.15-0.45; AgCl 0.01-0.035. The panel is then exposed by radiation beams for producing a hidden image of an X-ray to be produced, through the whole thickness of the panel. In order to ensure passage of the specifically shaped beams of radiation through the whole thickness of the monolithic panel without distortion of the formed image (in other words without refraction, reflection and dispersion of the rays) and also in order to prevent formation of semi-shadows at the borders of the image, the exposure is performed by a short wave electromagnetic radiation with a wavelength which is shorter than a wavelength of an ultraviolet radiation, for example, with X-ray radiation having a wavelength 0.6-0.03 A or gamma radiation having a wavelength 0.02-0.01 A. In order to provide the beams of the exposing radiation, which forms the three-dimensional grid image, with a desired three dimensional shape and also in order to prevent falling onto the panel of rays which extend not in the direction of shaping of the image, a volumetric tunnel-shaped mask-like device is utilized, such as for example the device disclosed in my patent application Ser. No. 08/009,976 filed on Jan. 27, 1993, now U.S. Pat. No. 5,307,394. The device is designed so that only those beams can pass through it and fall onto parts of the panel which have a shape and an angle providing the formation of a hidden image of openings (cells or lines), but the beams do not fall on the parts of the panel which must contain the image of partitions between the openings, and also semi-shadows at the borders of the images are not formed. This device can use a radiation source of any size and shape, and it can be located not in a focal point of the grid. After the exposure the panel is subjected to a thermal treatment to a temperature between 450.degree.-700.degree. C., for example 600.degree. C., in order to develop the hidden image of the grid. Then the panel is etched in a 10-20%, for example 15%, aqueous solution of hydrofluoric acid. Thereby a grid is produced in which directional throughgoing openings are located at the exposed parts of the panel and partitions between the openings are formed at the non-exposed parts of the panel. During the time corresponding to the etching of the opening (exposed part of the glass) over the length 2h (since the etching is performed from two sides simultaneously), the partition (non-irradiated part) is etched in direction of increase of the width of the opening d by the value 2.DELTA.d (since the etching is performed in direction of increase of the diameter in two directions in the longitudinal section simultaneously. The differential of solubility is a ratio of speeds of etching of exposed and not exposed parts, as follows: ##EQU1## wherein a is a differential of solubility, 2h=r.times.d in the case of a grid, r.gtoreq.5 is a ratio of the grid from requirements of radiology (ratio of r=5 of a cellular grid corresponds ro r=10 of a linear grid), d is a diameter of the opening; EQU .DELTA.d=0.01.delta..times.d .delta..ltoreq.10% is a maximum permissible relative error of geometrical sizes of the produced opening. Thus the mathematical expression of the differential of solubility for the grid is: ##EQU2## After the calculations ##EQU3## a.gtoreq.25. Then the openings are covered with a thin coating of an X-ray absorbing material, for example, lead or tungsten so as to provide a uniform coating of high density with a thickness of 0.050-0.040 mm over the whole length and width of the partitions, for example with an electrolythic or carbonilic coating. The coated partitions form strips of the grid, while the openings between the partitions form the cells of the grid, so as to form a monolithic grate of the grid. In order to increase the strength of the grid its sides are formed as a monolithic frame without openings, while its surfaces are provided with covers from an X-ray highly transparent material. The new methods increases the efficiency of manufacture and the quality of the grids since it makes possible the production of the grids with maximum X-ray transparency for primary radiation and especially for long wave X-ray radiation, with substantially increased absorbency of scattered radiation especially with the cellular grid so as to increase contrast, sharpness and resolution. This in turn improves the quality of X-ray pictures and reduces radiation action on patients and medical personnel. The present invention is not limited to the details shown since various modifications and structural changes are possible without departing in any way from the spirit of the present invention. What is desired to be protected by Letters Patent is set forth in the appended claims. |
description | This application is a divisional application of U.S. application Ser. No. 09/475,592 filed Dec. 30, 1999, now U.S. Pat. No. 6,697,447, issued Feb. 24, 2004. This invention relates generally to nuclear reactors and more particularly to a design analysis method that permits operation of a boiling water nuclear reactor in an expanded region of the power/core flow map. A typical boiling water reactor (BWR) includes a pressure vessel containing a nuclear fuel core immersed in circulating coolant, i.e., water, which removes heat from the nuclear fuel. The water is boiled to generate steam for driving a steam turbine-generator for generating electric power. The steam is then condensed and the water is returned to the pressure vessel in a closed loop system. Piping circuits carry steam to the turbines and carry recirculated water or feedwater back to the pressure vessel that contains the nuclear fuel. The BWR includes several conventional closed-loop control systems that control various individual operations of the BWR in response to demands. For example a control rod drive control system (CRDCS) controls the position of the control rods within the reactor core and thereby controls the rod density within the core which determines the reactivity therein, and which in turn determines the output power of the reactor core. A recirculation flow control system (RFCS) controls core flow rate, which changes the steam/water relationship in the core and can be used to change the output power of the reactor core. These two control systems work in conjunction with each other to control, at any given point in time, the output power of the reactor core. A turbine control system (TCS) controls steam flow from the BWR to the turbine based on pressure regulation or load demand. The operation of these systems, as well as other BWR control systems, is controlled utilizing various monitoring parameters of the BWR. Some monitoring parameters include core flow and flow rate effected by the RFCS, reactor system pressure, which is the pressure of the steam discharged from the pressure vessel to the turbine that can be measured at the reactor dome or at the inlet to the turbine, neutron flux or core power, feedwater temperature and flow rate, steam flow rate provided to the turbine and various status indications of the BWR systems. Many monitoring parameters are measured directly, while others, such as core thermal power, are calculated using measured parameters. Outputs from the sensors and calculated parameters are input to an emergency protection system to assure safe shutdown of the plant, isolating the reactor from the outside environment if necessary, and preventing the reactor core from overheating during any emergency event. To meet regulatory licensing guidelines, the thermal output of the reactor is limited as the percentage of maximum core flow decreases. A line characterized by this percent of thermal power output versus percent of core flow defines the upper boundary of the reactor safe operating domain. Some reactors have been licensed to operate with increased thermal power output (up-rated) with an upper boundary line characterized by the point of 100 percent original rated power and 75 percent of rated core flow. This upper boundary line constrains operation at the uprated power to a significantly smaller range of core flow and reduces flexibility during startup and at full power. It would be desirable to provide a method of operating an up-rated boiling water nuclear reactor with a wider core flow operating range at full licensed power. A method for expanding the operating domain of a boiling water nuclear reactor that permits safe operation of the reactor at low core flows is described below. The operating domain is characterized by a map of the reactor thermal power and core flow. Typically, reactors are licensed to operate below the flow control/rod line characterized by the operating point defined by 100 percent of the original rated thermal power and 75 percent of rated core flow. In an exemplary embodiment, the method for expanding the operating domain of a boiling water nuclear reactor permits operation of the reactor between about 120 percent of rated thermal power and about 85 percent of rated core flow to about 100 percent of rated thermal power and about 55 percent of rated core flow. The method for expanding the operating domain of a boiling water nuclear reactor includes, in one embodiment, determining an elevated load line characteristic that improves reactor performance, performing safety evaluations at the elevated load line to determine compliance with safety design parameters, and performing operational evaluations up to the elevated load line. The method also includes defining a set of operating conditions for the reactor in an upper operating domain characterized by the elevated load line. Operational evaluations performed upto the elevated load line include, but are not limited to, evaluating plant maneuvers, frequent plant transients, plant fuel operating margins, operator training and plant equipment response and setpoints. Based on the results of the operational evaluations, constraints and requirements are established for plant equipment and procedures. Also, automatic adjustment of the control rod pattern, the flow controls, and the pressure controls based on the detection of a reactor transient is provided. Additionally, the method includes performing a detailed analysis of the performance of the core recirculation system and the system control components. Further, the method provides for modifying the reactor process controls and computers to permit the reactor to operate in the expanded operating domain within predetermined safety parameters. Also, safety mitigation action setpoints are adjusted to permit reactor operation in the expanded operating domain. The above described method provides analyzed limits that permit full power operation of the reactor at a core flow lower than 75 percent of rated core flow, which currently is the lowest permitted core flow for license approval. The lower than 75 percent core flow permits operation of the reactor over a larger core flow range and operating flexibility during startup and at full power. The method further provides savings in fuel cycle costs and faster plant startups due to the increased ability to establish desired full power control rod pattern at partial power conditions. The method still further provides a reduced cycle average recirculation pumping power consumption resulting in an increase in net station output. FIG. 1 is a schematic diagram of the basic components of a power generating system 8. The system includes a boiling water nuclear reactor 10 which contains a reactor core 12. Water 14 is boiled using the thermal power of reactor core 12, passing through a water-steam phase 16 to become steam 18. Steam 18 flows through piping in a steam flow path 20 to a turbine flow control valve 22 which controls the amount of steam 18 entering steam turbine 24. Steam 18 is used to drive turbine 24 which in turn drives electric generator 26 creating electric power. Steam 18 flows to a condenser 28 where it is converted back to water 14. Water 14 is pumped by feedwater pump 30 through piping in a feedwater path 32 back to reactor 10. FIG. 2 is a flow chart of a method 40 for expanding the operating domain of boiling water nuclear reactor 10. In one aspect, method 40 is applicable to boiling water nuclear reactor plants which can operate at higher than the original rated thermal power, where the fuel cycle performance at the higher load line is advantageous and plant performance at the higher power output is justified by appropriate safety analysis. In another aspect, method 40 provides the design concept and the analytical justification to operate boiling water nuclear reactor 10 in a significantly expanded region of the power/flow map. Method 40 includes the steps of determining an elevated load line characteristic that improves reactor performance 42, performing safety evaluations at the elevated load line to determine compliance with safety design parameters 44, and performing operational evaluations at the elevated load line 46. Method 40 also includes the step of defining a set of operating conditions for the reactor in an upper operating domain characterized by the elevated load line 48. Based on the results of the operational evaluations of step 46, constraints and requirements are established for plant equipment and procedures 50. The optimum applicable range of the expanded region of operation is established. Also, automatic adjustment of the control rod pattern, the flow controls, and the pressure controls based on the detection of a reactor transient 52 is provided. Additionally, method 40 includes modification of the reactor process controls and computers to permit reactor operation in the upper operating domain 54. To determine the desired elevated load line characteristic, evaluations at elevated core thermal power are performed. The desired load line increase is based on the thermal power increase and the fuel cycle performance improvement that is obtained at the elevated core thermal power. Calculations are performed to define the operating conditions of the reactor in the new operating region characterized by the elevated load line. Evaluations of the expected performance of the reactor throughout the new operating region are also performed. Operational evaluations performed at the elevated load line include, but are not limited to, evaluating plant maneuvers, frequent plant transients, plant fuel operating margins, operator training and plant equipment response and setpoints. Based on the results of the operational evaluations, constraints and requirements are established for plant equipment and procedures. Safety evaluations typically address the safety analysis Chapter 15 of the Final Safety Analysis Report (FSAR). Additionally, non-Chapter 15 safety issues such as containment integrity, stability and anticipated transient without scram (ATWS) are addressed. Safety analysis include demonstration of compatibility with the previous resolutions of reactor stability monitoring and mitigation of unplanned events. The safety evaluations are performed such that compliance to plant design criteria is demonstrated. Assurance of acceptable protection of the reactor and the public is performed and documented to satisfy regulatory authorities. A safety analysis report is generated to comply with regulatory requirements. To maximize the ability of the boiling water reactor unit to avoid trip during transients that may occur while operating in the extended region, automatic adjustment of some controls is provided. For example automatic adjustment of the control rod pattern, flow controls and pressure controls based on sensing the initiation of a transient, such as a pump trip, are provided. These automatic controls improve plant availability, even in the previous range of reactor operation. An operating domain 58 of reactor 10 is characterized by a map of the reactor thermal power and core flow as illustrated in FIG. 3. Typically, reactors are licensed to operate below a flow control/rod line 60 characterized by an operating point 62 defined by 100 percent of the original rated thermal power and 100 percent of rated core flow. In some circumstances, reactors are licensed to operate with a larger domain, but are restricted to operation below a flow control/rod line 64 characterized by an operating point 66 defined by 100 percent of the original rated thermal power and 75 percent of rated core flow. Some reactors have been licensed to operate at higher power as illustrated by lines 67 in FIG. 3. However, these reactors are constrained by flow control/rod boundary line 64. In an exemplary embodiment of the present invention, method 40 expands operating domain 58 of reactor 10 and permits operation of reactor 10 between about 120 percent of original rated thermal power and about 85 percent of rated core flow to about 100 percent of original rated thermal power and about 55 percent of rated core flow. Lines 68, 70 and 72 represents this new upper boundary of an upper operating region 74 of operating domain 58 of reactor 10. FIG. 4 shows another exemplary embodiment of the present invention where method 40 expands operating domain 58 of reactor 10 to an upper boundary represented by the operation of reactor 10 between about 120 percent of original rated thermal power and about 85 percent of rated core flow to about 60 percent of original rated thermal power and about 60 percent of rated core flow. Lines 70, 76 and 78 represents this new upper boundary of an expanded upper operating region 80 of operating domain 58 of reactor 10. Method 40 provides analyzed limits that permit licensed power operation of reactor 10 at a core flow lower than the constraint on core flow imposed by boundary 64. The increased boundary line 68 permits operation of reactor 10 over a larger core flow range and operating flexibility during startup and at full power. Method 40 further provides savings in fuel cycle costs and faster plant startups due to the increased ability to establish desired full power control rod pattern at partial power conditions. Also provided is reduced cycle average recirculation pumping power consumption resulting in an increase in net station output. Another embodiment of the invention includes providing analyses and evaluations to generate a safety analysis report as describe above. Additionally, licensing support is provided to the owner, or managing entity, of the boiling water nuclear reactor, along with technical consultation during the implementation of reactor analyses and modifications described above. While the invention has been described and illustrated in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims. |
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description | This application is a national phase of International Application Number PCT/US2015/047338 filed Aug. 28, 2015 and claims priority of U.S. Provisional Application No. 62/059,484, filed on Oct. 3, 2014, the contents of which is hereby incorporated by reference in its entirety and for all purposes. Field of the Disclosure This disclosure pertains to a holder for a radioactive source capsule, with improved retention of the capsule during transportation and handling. Description of the Prior Art In the field of transportation and handling of radioactive capsules, the need for reliable retention and confinement of the capsules is well-established and self-evident. Examples of prior art for a reusable shaft mounted radioactive capsule holder include the screw and pin design. This design also uses a top and bottom holder ends, but the bottom end threads into the top end and then the parts are pinned together radially. A typical deficiency of this design is the pin is required to be installed and removed for every use. This process degrades the retaining hole in the both the top and bottom holder ends. The pin could eventually fall out due to excessive wear allowing the threaded holder to unscrew during transport or handling. Additionally, once the holder has arrived on site, the radial pin needs to be removed and then the bottom end of the holder unthreaded. Locating and removing the radial pin in a high radiation field frequently requires accurate and robust tooling. The pin and holder are frequently damaged in the pinning and de-pinning process. A further prior art design is the screw and nut design. This design is similar to the screw and pin but uses a locking nut to lock the top and bottom ends together instead of using a radial pin. This design may unthread axially if the nut were to loosen and ultimately release the capsule while in the transport or handling tube. Further improvements are sought in this art to retain the radioactive capsules more securely and more reliably. It is therefore an object of the present disclosure to provide improvements in the transportation and handling of radioactive capsule, particularly with respect to security and reliability. This and other objects are attained by providing a capsule holder with redundant retaining mechanisms. These retaining mechanisms include a set screw to prevent radial movement of the capsule, a locking shelf to fix the axial orientation of the pivoting bottom end of the capsule holder, and a capture tooth within the capsule to prevent release of the capsule while in the shipping and/or handling tube of the capsule holder. Referring now to the drawings in detail, one sees the capsule holder 10 for a radioactive source capsule 100 in various views in FIGS. 1-5. The capsule holder 10 is generally cylindrical in shape with a top end (or proximal end) 12 and a bottom end (or distal end) 14. The top end 12 includes a cylindrical portion 18 which is configured to attach or affix to a shaft 200, which may be flexible or rigid, thereby allowing the capsule holder 10 to be driven by a remote device (not shown). Cylindrical portion 18 typically includes a blind aperture for receiving the shaft 200, but is not limited thereto. Those skilled in the art, after review of this disclosure, will recognize several methods or configurations for attaching or affixing the shaft 200 to the top end 12 of the capsule holder 10. Top end 12 further includes a partially hemispherical portion 20 thereby presenting a flat transverse partially circular locking shelf 22 (see FIG. 2), as well as an oblique face 24 which is formed opposite around the periphery of the locking shelf 22 from a partially circular longitudinally extending wall which forms a capture tooth 26. As described herein, the junction of the locking shelf 22 and the capture tooth 26 forms a partial seat for radioactive source capsule 100. As shown in FIGS. 1 and 2, bottom end (or distal end) 14 includes a frusto-conical portion 30 forming an end cap through which a threaded aperture 32 is formed for threadably receiving a set screw 34 (shown in the inserted position in FIG. 1 and the partially withdrawn position in FIG. 2). Opposed partially cylindrical walls 36, 38 extend from frusto-conical portion 30. As best shown in FIG. 5, partially cylindrical wall 36 terminates in an oblique wall portion 40 and further includes a pivot aperture 42. Partially cylindrical wall 38 likewise terminates in an oblique wall portion 41 and a pivot aperture 43 as shown in FIG. 4. Pivot pin 50 passes from the pivot aperture 42 of partially cylindrical wall 36 to the pivot aperture 43 on partially cylindrical wall 38 through an unillustrated transverse pivot axis passageway which passes immediately beneath the locking shelf 22. This pivot configuration allows the bottom end 14 to pivot between a closed and locked longitudinal configuration as shown in FIG. 1 and the open transverse or generally perpendicular configuration as shown in FIG. 2, with respect to the top end 12. As shown in FIG. 5, the oblique wall portion 40 allows the bottom end 14 to pivot more fully with respect to top end 12. In order to install the radioactive source capsule 100 in the capsule holder 10, the user typically starts in the open configuration of FIG. 2 with the bottom end 14 pivoted away from a longitudinal alignment with top end 12 thereby resulting in the top and bottom ends 12, 14 being perpendicular to each other. The user inserts radioactive source capsule 100 between the first and second partially cylindrical walls 36, 38 with the recessed lid 108 of second end 106 of the radioactive source capsule 100 aligned with the set screw 34, as shown in FIG. 4. The user then pivots the bottom end 14 into the closed position of FIGS. 1 and 3 (i.e., the top and bottom ends 12, 14 being longitudinally aligned or the longitudinal axes of top and bottom ends 12, 14 aligning with each other). In this configuration, the first end 102 of the radioactive source capsule 100 abuts against the locking shelf 22 and the cylindrical side wall 104 of the radioactive source capsule 100 abuts against the capture tooth 26. In the configuration illustrated in FIGS. 1 and 3, the first and second partially cylindrical walls 36, 38 and the outer surface of capture tooth 26 form a partially cylindrical configuration. The user then uses an Allen wrench or similar tool to engage set screw 34 and drive set screw to engage the recessed lid 108 of second end 106 of radioactive source capsule 100 thereby achieving a locked configuration of FIGS. 1 and 3. These steps may be performed by a remote controlled gripping apparatus. One sees that the illustrated embodiment of the reusable radioactive capsule holder 10 typically has the following characteristics: The redundant retaining mechanisms prevent the cylindrically shaped radioactive capsule 100 from being unintentionally released during transport and handling. That is, the set screw 34 engages into the recessed lid 108 at the second end 108 of the capsule 100 (or similar workpiece) thereby preventing radial movement of the capsule 100 once engaged. When the set screw 34 is installed into the holder 10 with the capsule 100 in place, the capsule 100 is pushed up against a locking shelf 22 at the top end 12 of the capsule holder 10 which fixes the axial orientation of the pivoting bottom end 14 with the top end 12 of the holder 10. This locking mechanism is intended to prevent the holder 10 from pivoting open. The capture tooth 26 of the top end 12 of the capsule holder 10 is intended to prevent the release of the capsule 10 while in the shipping and/or handling tube 300 (see FIG. 5). The tube 300 is sized to restrict the pivoting angle between the top and bottom ends 12, 14, keeping the capsule 100 captured by the capture tooth 26. The holder 10 is intended to present an easy method for releasing the radioactive source capsule 100 from the holder 10 after arriving on site for use. Since handling of the radioactive source capsule 100 is performed with a remote controlled gripping mechanism, the holder 100 typically needs only to be inserted into a rotating tool fitted with the proper Allen wrench to withdraw the set screw 34 and allow the holder 10 to pivot open thereby releasing the radioactive source capsule 100. Thus the several aforementioned objects and advantages are most effectively attained. Although preferred embodiments of the invention have been disclosed and described in detail herein, it should be understood that this invention is in no sense limited thereby. |
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claims | 1. A charged particle apparatus equipped with a charged particle source for emitting a beam of charged particles, downstream of said beam followed by condenser optics, followed by a sample position, followed by an objective lens, followed by imaging optics, and followed by a detector system, in which, between the objective lens and the detector system, a first stigmator is positioned for reducing astigmatism when imaging a sample on the detector system and a second stigmator is positioned for reducing astigmatism when the diffraction plane is imaged on the detector system, characterized in that a third stigmator is positioned between the objective lens and the detector system, as a result of which a third degree of freedom is created for reducing the linear distortion. 2. The apparatus of claim 1 in which the three stigmators are positioned between the objective lens and the imaging optics, as a result of which the excitation of the stigmators need not be changed when the settings of the imaging optics are changed. 3. The apparatus of claim 1, the apparatus equipped with a controller with a user interface, the user interface showing controls for controlling the stigmators, the controls controlling the stigmators in such a manner that image astigmatism, diffraction astigmatism and linear distortion are associated with independent controls. 4. The apparatus of claim 1 in which the apparatus is a Transmission Electron Microscope or a Scanning Transmission Electron Microscope. 5. Method of using the apparatus of claim 1, the method comprising exciting the first stigmator to reduce astigmatism when imaging the sample and exciting the second stigmator to reduce astigmatism when imaging the diffraction plane, characterized in that the method comprises exciting the third stigmator to reduce the linear distortion. 6. A charged particle apparatus, comprising:a charged particle source for emitting a beam of charged particles;a sample holder for holding a sample and positioning the sample with respect to the beam of charged particles;a first stigmator positioned for reducing astigmatism when imaging the sample;a second stigmator positioned for reducing astigmatism when imaging the diffraction plane; anda third stigmator positioned for reducing linear distortion. 7. The apparatus of claim 6 further comprising an objective lens for forming an image and one or more projector lenses to magnify the image. 8. The apparatus of claim 7 in which the three stigmators are positioned between the objective lens and the one or more projector lenses, as a result of which the excitation of the stigmators need not be changed when the settings of the imaging optics are changed. 9. The apparatus of claim 6 further comprising a controller with a user interface, the user interface having controls for controlling the stigmators, the controls controlling the stigmators in such a manner that image astigmatism, diffraction astigmatism and linear distortion comprise independent controls. 10. The apparatus of claim 6 in which the apparatus is a Transmission Electron Microscope or a Scanning Transmission Electron Microscope. 11. The apparatus of claim 10 in which the charged particle source provides a beam of electrons with an adjustable energy of between 50 keV and 400 keV. 12. The apparatus of claim 6 in which the three stigmators are configured to avoid ohmic heating and the associated drift of the apparatus. 13. A method of operating the charged particle beam apparatus of claim 6, comprising:forming a beam of charged particles;positioning the first stigmator such that when in imaging mode, the first stigmator affects astigmatism but does not affect the linear distortion;positioning the second stigmator such that when in diffraction mode, the second stigmator affects astigmatism but does not affect the linear distortion;positioning the third stigmator such that the third stigmator mainly affects the linear distortion and only slightly affects the astigmatism in imaging mode and diffraction mode;exciting the first stigmator to reduce the astigmatism in imaging mode;exciting the second stigmator to reduce the astigmatism in diffraction mode; andexciting the third stigmator to reduce the linear distortion. 14. The method of claim 13 in which no changes in excitation are needed when changing from imaging mode to diffraction mode or from diffraction mode to imaging mode. |
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042085888 | abstract | A hand-held shielding device for holding and viewing a container of radioactive material is provided. The device comprises an elongated holding portion for the container formed with a longitudinal aperture therein and a shielding portion overlapping the aperture for shielding against direct radiation through the aperture. Indirect optical imaging is provided on the shielding portion for viewing the container and material therein at a viewing position away from direct radiations emanating through the aperture. |
049838512 | description | BEST MODE FOR CARRYING OUT THE INVENTION Though the drawings show an example in which the present invention is embodied in a magnetic field generating therapeutical apparatus, it is not limited thereto, and it will be appreciated that the present invention may be applied in other various physical therapeutical apparatus as far as they include an internal heat generating portion. FIGS. 1 and 2 show the entire configuration of the magnetic field generating therapeutical apparatus according to one embodiment of the present invention, in which magnetic field generators 2 are disposed respectively in a plurality of plastic cases 1. Each case body 1 is pivotally connected in a row and on the case bodies 1, 1 positioned at opposite ends, and belts 13, 13 are mounted for interconnection. The case body 1 is formed by a pair of half-body cases 11, 12 whose opening surfaces are brought face to face and secured by screws 6 at several locations. As shown in FIG. 2 and FIG. 3, on side end faces of the case body 1, either pivots 14, 14 or bearing holes 15, 15 are formed vertically to interconnect each case body 1, 1 pivotably by engaging the pivot 14 to the bearing hole 15 between the adjacent case bodies 1, 1. As shown in FIG. 4, the magnetic field generator 2 comprises a laminated iron core 20 including legs 22, 23 at opposite ends of a base 21, and coil bobbins 31, 32 engaged to respective legs 22, 23. The coil bobbins 31, 32 are constructed by winding coils 37, 38 around plastic spools 35, 36 having collars 33, 34 on opposite ends. When an AC current is applied to the coils 37, 38, an alternating field is generated from tips of the legs 22, 23. As shown in FIG. 3, on the inner surface of one half-body case 12, positioning pieces 41, 41 for positioning the magnetic field generator 2 are projected integrally, thereby the base 21 of the laminated iron core 20 is positioned inside the two positioning pieces 41, 41. The other half-body case 11 constitutes fixing means for the magnetic field generator 2 for forming a depression (not shown) on its inner surface for supporting the collars 33, 34 of the spools 35, 36, and the plate surface of the half-case body 11 constitutes a magnetic field active surface 16 which contacts to a human body. On the magnetic field active surface 16 of the half-body case 11, a far infrared radiating material such as zirconia, zircon, titania, alumina, cozilite and silica is coated to form a radiation layer 17. Though the far infrared radiating material is excited by heat to radiate far infrared rays, besides coating the far infrared radiating material on the surface of the case body 1 as the present embodiment, this material may be mixed with forming materials of the case body 1. FIG. 5 shows another embodiment of the present invention, in which a uniformalizing layer 5 for distributing heat uniformly on the magnetic field active surface 16 of the half-body case 11 is formed. The uniformalizing layer 5 shown in the figure is, as shown in FIG. 6, formed by disposing a depression 42 throughout the entire magnetic field active surface 16 on the half-body case 11, filling a convection fluid such as air, water and oil or magnesium oxide having a good heat conductivity therein, and covering the opening with a transparent cover 18 by sticking or welding integrally. FIGS. 7 and 8 show another forming process of the uniformalizing layer 5. In the figure, a mounting depression 44 having slide grooves 43, 43 on both sides thereof is formed on the magnetic field active surface 16 of the half-body case 11, and a mounting case 19 having the uniformalizing layer 5 therein is disposed removably with respect to the mounting depression 44. It is to be understood that the uniformalizing layer 5 is formed with air, water, oil, magnesium oxide and so on as same as aforementioned. In the embodiment shown in FIGS. 5 to 8, although the far infrared radiating material may be mixed with the materials of case body 1 or coating it on the surface thereof, it is not limited thereto, in that the far infrared radiating material may be mixed in the uniformalizing layer 5. FIG. 9 shows the entire configuration of the magnetic field generating therapeutical apparatus, in which a covering member B covers the apparatus A. The apparatus A is constructed as same as those shown in FIGS. 1 to 4, wherein necessary corresponding parts are shown by like reference characters to omit its explanation. The cover member B is constituted by a cylindrical bag having a length responsive to the connected length of a plurality of case bodies 1, 1. On both openings of the cover member B, there are provided fastening straps 71, 71 to cover the therapeutical apparatus A as if wrapping it in the bag. Though the cylindrical bag 7 is made of cloth such as woven or unwoven fabrics, it is not limited thereto, it may be formed with resin or rubber materials as far as they are soft and agreeable to the touch. On the surface of cover member B, powdered materials containing the far infrared radiating material such as zirconia, zircon, titania, alumina, cozilite, silica, etc. are coated in a given pattern by printing as shown in FIG. 10 to form radiating portions 72 which are projected slightly. Though the far infrared radiating material is excited by heat to radiate infrared rays, besides printing on the cloth surface as the present embodiment, the cloth may be impregnated with the far infrared radiating material or woven by threads impregnated or coated with the far infrared radiating material. FIGS. 11 and 12 show another embodiment of the cover member B, wherein a uniformalizing layer 75 for distributing heat uniformly is formed. The uniformalizing layer 75 shown is constituted by overlapping two sheets of synthetic resin 74, 74 to construct the cover member B, forming a number of compartments 73, 73 between the sheets by means of thermal fusion or the like and filling a convection fluid such as air, water, oil, etc. or magnesium oxide having a good heat conductivity into each compartment 73. In the aforesaid embodiment, though the entire therapeutical apparatus A is generally covered by the cover member B, it is not limited thereto, it may be covered partially. FIGS. 13 and 14 show the embodiment wherein the therapeutical apparatus A is covered partially and the uniformalizing layer 75 aforementioned is provided in the cover member B. The cover member B shown comprises a skin-contact portion 77 abutting the side of active surface 16 of the half-body case 11 in series, and mounting portions 76a, 76b which are connected at the rear of each case body 1, on overlapping surfaces of the mounting portions 76a, 76b, face fasteners (not shown) which engage and disengage one another are provided. In the skin-contact portion 77, as same as the embodiment of FIGS. 11 and 12, a number of compartments 73 are formed between the sheets, into which air, water, oil, magnesium oxide, etc. is filled to form the uniformalizing layer 75. INDUSTRIAL APPLICABILITY When using the magnetic field generating therapeutical apparatus, first, it is fastened about and fixed to a human body such that one or a plurality of case bodies 1 are contacted to portions of the body where stiffness or pain are generated or to the portion of fractured bone. Then, when a power is applied, coils 37, 38 of each magnetic field generator 2 are energized and their superposed magnetic fluxes are produced from the end faces of legs 22, 23 of the laminated iron core 20. Thereby, heat is generated at the iron core 20 and coils 37, 38, which is conducted to the case body 1 and cover member B. As a result, the case body 1 and cover member B are warmed and the far infrared radiating material is excited to radiate far infrared rays. The far infrared rays radiated acts on the body to warm therein and improves the warming effect. When the uniformalizing layers 5, 57 are provided on the case body 1 and cover member B, they distribute the heat generated uniformly on the surfaces of the case body 1 and cover member B by the convection and heat conduction. Accordingly, heat is applied to the body evenly in a wide area to improve the warming effect and comfortable feeling. |
claims | 1. A pipeline shut-off device comprising:a pipe section configured to be arranged in a pipeline;a shut-off member disposed in the pipe section; anda device arranged in the region of the pipe section and actuable from outside of the pipe section, the device being operable to destroy the shut-off member when the device is actuated from the outside of the pipe, the device including:a housing on the periphery of the pipe section of the pipeline shut-off device;a striker movably supported in the housing;at least one spring engaging the striker;a tensile element attached to and holding the striker in a preloaded position against a load of the at least one spring; andwherein the tensile element is generally axially aligned to the striker. 2. A pipeline shut-off device in accordance with claim 1, wherein:the shut-off member is a plate. 3. A pipeline shut-off device in accordance with claim 2, wherein:the plate is peripherally let into the pipe section. 4. A pipeline shut-off device in accordance with claim 2, wherein:the plate is sealed to the pipe section by a peripheral seal disposed on a perimeter face of the plate. 5. A pipeline shut-off device in accordance with claim 2, wherein:the plate has at least one peripheral seal at least on one side in the axial direction of the pipe section. 6. A pipeline shut-off device in accordance with claim 1, wherein:the shut-off member is manufactured from a brittle material. 7. A pipeline shut-off device in accordance with claim 6, wherein:the shut-off member is manufactured from a glass, a glass-like material or a ceramic material. 8. A pipeline shut-off device in accordance with claim 1, wherein:the tension element is a rope, a chain or similar. 9. An apparatus for the emergency supply of coolant to the fuel rods arranged in a reactor vessel of a nuclear power plant on an incipient core meltdown, comprising:a vessel for receiving coolant, the vessel being connected to a reactor vessel by at least one pipeline; anda pipeline shut-off device in accordance with claim 1. 10. An apparatus in accordance with claim 9,wherein the tensile element is melted on an incipient core meltdown by the temperature arising in this process. |
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042254671 | description | As shown in FIGS. 1 and 2 spent fuel rack 11 includes sixteen assemblies 13, each of which is of essentially square cross section and each of which includes an outer wall 15, an inner wall 17 and intermediate such walls and enclosed between them, tops 18 and bottoms, not illustrated, which are provided to seal off each of the assemblies about the contents, neutron absorbing plates 19. Inside the assemblies of the rack are stored the spent nuclear fuel rods, not illustrated, which extend vertically through the assemblies and have access at the ends thereof to water or aqueous solution in a pool or suitable container, not illustrated, in which the racks are stored. The assemblies of the rack are welded together and are supported on a bottom member 21, mounted on legs 25, which are adjustable in height to permit leveling of the rack. As is seen in FIG. 2 neutron absorbing plates 19, which are of uniform width, height and thickness, are slid into place between vertical stainless steel rods 20, which are welded to the inner and outer walls 17 and 15, respectively. Usually there is little need to take special steps to prevent escape of radiation through the bottoms of the assemblies and racks but if the aqueous medium in which the racks are stored is insufficient to absorb radiation from the tops thereof additional neutron absorbing articles of this invention, preferably enclosed in stainless steel containers, may be interposed between the racks and the top of the storage container to absorb radiation emissions. In FIG. 3 is shown a typical neutron absorbing article, in the form of a long thin plate. For example, plate 19 may be of a length of about 93 cm., a width of about 22 cm. and a thickness of about 3 mm. or in some other cases, where greater neutron absorption is required, about 5 mm. In the storage racks of FIGS. 1 and 2, in which the total absorbing height is designed to be about 3.7 meters, walls are of four of the plates illustrated in FIG. 3, positioned one above the other in the stainless steel enclosure (in which the steel is about 3 mm. in thickness). In the plate shown in FIG. 3 the presence of the individual particles of boron carbide will be evident and such can be felt when the plates are handled but such particles are well covered by cured polymer on the major surfaces thereof, such as the faces and at the sides, ends, edges and corners, so that accidental loss of boron carbide particles is inhibited and the neutron absorbing properties of the article are maintained constant at a design level. In the diagrammatic illustration of FIG. 4 is shown a preferred method for the manufacture of the present neutron absorber. Initially, mixing, represented at 29, is effected in a paddle mixer type of apparatus wherein weighed quantities of boron carbide powder and phenolic resin, in normally liquid state, are thoroughly mixed together to make a boron carbide-resin mix lower in resin content than the polymer content of the final article but still curable to a formretaining intermediate. After mixing is completed the product is screened at 31 (to break up any lumps and otherwise to increase product uniformity) into drying trays to a desired thickness and in drying operation 33 is allowed to dry to a desired extent, preferably in a controlled environment, so that it is desirably "tacky" for molding, yet not too fluid. Preferably such drying is effected at about room temperature, e.g., 10.degree.-35.degree. C., preferably 20.degree. to 25.degree. C., and at normal relative humidities, e.g., 10 to 75%, preferably 35 to 65%, but other conditions can also be used to produce the same result. Next, it is screened in operation(s) 35 and is passed through a finer screen into a mold in which the correct weight of charge is pressed (37) under high pressure for a short period of time. After pressing, the mold is unloaded and the pressed "green" article is cured, as represented by numeral 39, in a forced air oven at an elevated temperature, in a curing cycle which comparatively slowly increases the temperature to the desired elevated level, maintains it at such level and gradually lowers it. Next in impregnating operation 41 the cured plates are placed in an impregnating tank where they are kept separated from each other, the tank is sealed and a vacuum is drawn on it, after which additional resin in liquid state is drawn into the tank by means of the vacuum and is allowed to cover and impregnate each plate. The resin is then forced from the tank by air pressure, the tank is opened and the basket of impregnated plates is removed. The plates are removed from the basket, placed on their sides on drying racks, separated from each other, and are dried at elevated temperature in operation 43, following which they are further cured at a higher temperature in operation 45, again with a controlled heating cycle. The products made are of desired density, uniformity of neutron absorbing capability, flexibility and other required and desired physical properties, look like that of FIG. 3 and are capable of being incorporated in any of various types of storage racks for spent nuclear fuel, such as are illustrated in FIGS. 1 and 2. An important advantage of the neutron absorbing article of this invention is that it contains a high proportion of boron carbide particles, such that at least 6% of the article is B.sup.10, preferably at least 8% and more preferably 8 or 8.5 to 11.5%, usually most preferably 9 to 11%. Thus, relatively thin walled sections of the article, such as those of thicknesses from about 2 mm. to 1 cm., are effective neutron absorbers. Additionally, due to the uniformity of distribution of the boron carbide particles in the phenolic polymer matrix, the neutron absorbing capability of the articles made may be calculated, enabling engineers to design storage racks to a high degree of precision, thereby allowing planned effective loading of a storage rack for spent nuclear fuel when the present neutron absorbing articles are a part thereof. In addition to being primarily effective as a neutron absorber, the present absorbing article is operable over a temperature range at which the spent nuclear fuel is normally stored in storage racks, withstands thermal cyclings from repeated spent fuel insertions and removals, withstands radiation from the spent nuclear fuel over long periods of time without losing desirable neutron absorbing and physical properties, is sufficiently chemically inert in water or aqueous medium in which the spent fuel is stored so as to retain effective neutron absorbing properties in the event that a leak occurs which allows the entry of water into the enclosure for the neutron absorbing article in the storage rack and into contact with such articles, does not galvanically corrode and is sufficiently flexible so as to withstand operational basis earthquake and safe shutdown earthquake seismic events without losing neutron absorbing capability and other desirable physical properties when installed in a storage rack. While boron compounds and boron carbide in particular have been employed in some applications as neutron absorbers and while it has been suggested that various polymeric materials may be used as binders for particles of boron compounds, such as boron carbide, it is not considered that prior art absorbers were as effective and as stable during use as those of this invention, especially when the articles of this invention are made by the method described herein. Also, the high level of product consistency with design specifications provides a much needed and previously lacking technical validity for such products. The boron carbide employed should be in finely divided particulate form. This is important for several reasons, among which are the production of effective bonds to the phenolic polymer cured about the particles, the production of a continuous coating of polymer over the boron carbide particles at the article surface and the obtaining of a uniformly distributed boron carbide content in the polymeric matrix. It has been found that the particle sizes of the boron carbide should be such that substantially all of it (over 90%, preferably over 95%, more preferably over 99% and most preferably over 99.9%) or all passes through a No. 20 (more preferably No. 35) screen. Preferably, substantially all of such particles, at least 90%, more preferably at least 95% pass through a No. 60 U.S. Sieve Series screen and at least 50% pass through a No. 120 U.S. Sieve Series screen. Although there is no essential lower limit on the particle sizes (effective diameters) usually it will be desirable from a processing viewpoint and to avoid objectionable dusting during manufacture for no more than 25% and preferably less than 15% of the particles to pass through a No. 325 U.S. Sieve Series screen and normally no more than 50% thereof should pass through a No. 200 U.S. Sieve Series screen, preferably less than 40%. In addition to boron carbide particle size being of importance in the making of successful neutron absorbers of the present type it is highly desirable that the boron carbide be essentially B.sub.4 C. While the present inventors are aware that work at their assignee company has shown that materials such as silicon carbide can be partially substituted for boron carbide in neutron absorbers of lower desired absorbing activity without deterioration of the physical properties of the article made, to obtain the high absorption characteristics of the present products, which are often required for satisfactory spent fuel storage, it is considered important that the content of such compatible non-absorbing "fillers," such as silicon carbide, should be kept limited and most of the time it should be nil. Boron carbide often contains impurities, of which iron, (including iron compounds) and B.sub.2 O.sub.3 (or impurities which can readily decompose to B.sub.2 O.sub.3 on heating) are among the more common. Both of such materials, especially B.sub.2 O.sub.3, have been found to have deleterious effects on the present products and therefore contents thereof are desirably limited therein. For example, although as much as 3% of iron (metallic or salt) may be tolerable in the boron carbide particles of the present high boron carbide content absorbers, preferably the iron content is held to 2%, more preferably to 1% and most preferably is less than 0.5%. Similarly, to obtain stable absorbing articles, especially in long, thin plate form, it is important to limit the B.sub.2 O.sub.3 content (including boric acid, etc., as B.sub.2 O.sub.3), usually to no more than 2%, preferably less than 1%, more preferably less than 0.5% and most preferably less than 0.2%. Of course, the lower the iron and B.sub.2 O.sub.3 contents the better. The boron carbide particles utilized will usually contain the normal ratio of B.sup.10 but may also contain more than such proportion to make even more effective neutron absorbers. Of course, it is also possible to use boron carbide with a lower than normal percentage of B.sup.10 (the normal percentage being about 18.3%, weight basis, of the boron present) but such products are rarely encountered and are less advantageous with respect to neutron absorbing activities. Other than the mentioned impurities normally boron carbide particles should not contain other components than boron carbide (nominally B.sub.4 C) in significant amounts. Thus, at least 90% of the boron carbide particles should be boron carbide, preferably at least 94% and more preferably at least 97%, and the B.sup.10 content thereof (from the boron carbide) will be at least 12%, preferably at least 14% (14.3% B.sup.10 in pure B.sub.4 C). To maintain the stability of the boron carbide-phenolic polymer article made it is considered to be important to severely limit the contents of halogen, mercury, lead and sulfur and compounds thereof, such as halides, and so, of course, these materials, sometimes found present in impure phenolic resins, solvents, fillers and plasticizers, will be omitted from those and will also be omitted from the composition of the boron carbide particles to the extent this is feasible. At the most, such particles will contain no more of such materials than would result in the final product just meeting the upper limits thereof, which will be mentioned in more detail in the following discussion with respect to the phenolic polymer and the resin from which it is made. The solid, irreversibly cured phenolic polymer, cured to a continuous matrix about the boron carbide particles in the neutron absorbing articles, is one which is made from a phenolic resin. The phenolic resins constitute a class of well-known thermosetting resins. Those most useful in the practice of the present invention are condensation products of phenolic compounds and aldehydes, of which phenolic compounds phenols and lower alkyl- and hydroxy-lower alkyl substituted phenols are preferred. Thus, the lower alkyl substituted phenols may be of 1 to 3 substituents on the benzene ring, usually in ortho and/or para positions, and the alkyls are of 1 to 3 carbon atoms, preferably methyl, and the hydroxy-lower alkyls present will similarly be 1 to 3 in number and of 1 to 3 carbon atoms each. Mixed lower alkyls and hydroxy-lower alkyls may also be employed but the total of substituent groups, not counting the phenolic hydroxyl, is preferably no more than 3. Although it is possible to make a useful product with the phenol of the phenol aldehyde resin being essentially all substituted phenol, usually it is preferred to have some phenol present therein e.g., 5 to 15%. For ease of expression the terms "phenolic type resins," "phenol-aldehyde type resins" and "phenol-formaldehyde type resins" may be employed in this specification to denote broadly the acceptable types of materials described which have properties equivalent to or similar to those of phenol-formaldehyde resins and trimethylol phenol formaldehyde resin, when employed to produce thermosetting polymers in conjunction with boron carbide particles, as described herein. Specific examples of useful "phenols" which may be employed in the practice of this invention, other than phenol, include cresol, xylenol and mesitol and the hydroxy-lower alkyl compounds preferred include mono-, di- and tri-methylol phenols, preferably with the substitutions at the positions previously mentioned. Of course, ethyl and ethylol substitution, instead of methyl and methylol substitution, and mixed substitutions wherein the lower alkyls are both ethyl and methyl, the alkylols are both methylol and ethylol and wherein the alkyl and alkylol substituents are also mixed, are also useful. In short, with the guidance of this specification and the teaching herein that the presently preferred phenols are phenol and trimethylol phenol, other compounds such as those previously described may also be utilized providing that the effects obtained are similarly acceptable. This also applies to the selection of the aldehydes and sources of aldehyde moiety employed but generally the only aldehyde utilized will be formaldehyde (compounds which decompose to produce formaldehyde may be substituted). The phenolic or phenol formaldehyde type resins are usually employed as either resols or novolaks. The former are generally called one-stage or single-stage resins and the latter are two-stage resins. The major difference is that the single-stage resins include sufficient aldehyde moieties in the partially polymerized lower molecular weight resin to completely cure the hydroxyls of the phenol to a cross-linked and thermoset polymer upon application of sufficient heat for a sufficient curing time. The two-stage resins or novolaks are initially partially polymerized to a low molecular weight resin without sufficient aldehyde present for irreversible cross-linking so that a source of aldehyde, such as hexamethylenetetramine, has to be added to them in order for a complete cure to be obtained by subsequent heating. Either type of resin may be employed to make phenolic polymers such as those described herein but it is much preferred to employ the single-stage resins, especially because there is no need to vent any ammonia or other gaseous byproducts of the decomposition of the hexamethylenetetramine source of formaldehyde normally employed with the two-stage resins. The resin employed will be low molecular weight, usually being the dimer, trimer or tetramer. Generally the molecular weight of the resin will be in the range of 200 to 1,000, preferably 250 to 750 and most preferably 250 to 500. The resin will usually be employed as an aqueous, alcoholic or other solvent solution so as to facilitate "wetting" of the boron carbide particles. While water solutions are preferred lower alkanolic solutions such as methanol, ethanol and isopropanol solutions or aqueous alkanolic solutions or dispersions are also usable. Generally the resin content of the liquid state resin preparation employed will be from 50 to 90%, preferably about 55 to 85%. The solvent content, usually principally water, may be from 5 to 30%, usually being from 7 to 20%, e.g., 8%, 10%, 15%, with the balance of liquid components normally including aldehyde, phenolic compound and often, liquid monomer. Thus, for example, in a liquid unmodified phenolic resin of the single-stage type based principally on the condensation product of trimethylol phenol and formaldehyde, there may be present about 81% of dimer, about 4% of monomer, about 2% of trimethylol phenol, about 4% of formaldehyde and about 8% of water. When two-stage resins are employed the curing agent will also be included with the resin, in sufficient quantity to completely cure (cross-link) it. Such quantity can be 0.02 to 0.2 part per part of resin. To avoid ammonia production during curing a sufficient quantity of an aqueous solution of aldehyde or another suitable source thereof which does not release ammonia may be used for curing novolaks. The resins employed are in the liquid state desirably, usually because of the low molecular weight of the condensation products which are the main components thereof but also sometimes due to the presence of liquid media, such as water, other solvents and other liquids which may be present. Generally the viscosity of such resins at 25.degree. C. will be in the range of 200 to 700 centipoises, preferably 200 to 500 centipoises. Usually the resin will have a comparatively high water tolerance, generally being from 200 to 2,000 or more percent and preferably will have a water tolerance of at least 300%, e.g., at least 1,000%. Among the useful products that may be employed are Arotap 352-W-70 (most preferred); Arotap 352-W-71; Arotap 8082-Me-56; Arotap PB-90; Arofene 536-E-56; and Arofene 72155, all manufactured by Ashland Chemical Company, and phenolic resins B-178; R3; and R3A, all manufactured by The Carborundum Company. All such resins will be modified, when desirable, to omit halides, especially chloride, halogens, mercury, lead and sulfur and compounds thereof or to reduce proportions thereof present to acceptable limits. In some cases the procedure for manufacture of the resin will be changed accordingly. Although the mentioned resins are preferred, a variety of other equivalent phenolic type resins, especially phenol-formaldehydes, of other manufacturers and of other types may also be employed providing that they satisfy the requirements for making the molded neutron absorbing articles set forth in this specification. Among such requirements, for successful use of the articles made in spent fuel storage racks the resins should contain only very limited amounts, if any at all, of halide or halogen, mercury, lead and sulfur, all of which are considered to be harmful to the function and stability of the neutron absorber when employed in a spent fuel storage rack. Additionally B.sub.2 O.sub.3 interferes with curing, causing the "green" molded item to lose its shape during the cure. Generally, less than 0.1% of each of such impurities (except the B.sub.2 O.sub.3) are in the final article, preferably less than 0.01% and most preferably less than 0.005%, and resin contents thereof are limited accordingly, e.g., to 0.4%, preferably 0.04%, etc. To assure the absence of such impurities the phenol and aldehyde employed will initially be free of them, at least to such an extent as to result in less than the limiting quantities recited, and the catalysts, tools and equipment employed in the manufacture of the resins will be free of them, too. To obtain such desired results the tools and materials will preferably be made of stainless steel or aluminum or similarly effective non-adulterating material. The proportions of boron carbide particles and irreversibly cured phenol formaldehyde type polymer in the neutron absorbing article will normally be about 60 to 80% of the former and 20 to 40% of the latter, preferably with no other impurities, such as water, solvent, filler, plasticizer, halide or halogen, mercury, lead and sulfur being present or if any of such is present, the amount thereof will be limited as previously described and otherwise held to no more than 5% total. Preferably, the respective proportions will be 65 to 80% and 20 to 35%, with the presently most preferred proportions being about 70% and 30% or 74% and 26%, and with essentially no other components in the neutron absorber. Within the proportions described the production made has the desirable physical characteristics for use in storage racks for spent nuclear fuel, which characteristics will be detailed later. Also, the described ratios of boron carbide particles and phenolic resin permit manufacture by the simple, inexpensive, yet effective method of this invention. To manufacture the neutron absorbers, such as those in thin plate form, there are first mixed together a portion of the curable phenolic resin in liquid state and all of the boron carbide particles. The proportion of resin employed at this stage, preferably having a molecular weight in the preferred range previously mentioned, will usually be 1/5 to 1/2, preferably 1/4 to 2/5 of the total resin used, e.g., 1/3. Thus, for example, from about 80 to 100 parts, preferably 85 to 95 parts, e.g., 89 parts, of boron carbide particles may be mixed with about 7 to about 15 parts, preferably 7 to 12 parts, e.g., 10 parts of resin, with the resin being accompanied by about 0.3 to about 1.4 or 2 parts, e.g., about 0.8 part, of water or aqueous medium, although solvents, such as previously mentioned, may also be present. Additionally, there may be included with the resin other reactants and byproducts of the process for the manufacture thereof. Mixing is effected in a paddle mixer or other suitable heavy duty mixer capable of dispersing the boron carbide particles in the comparatively thick resin. Mixing times will usually be in the range of ten minutes to two hours, preferably from 20 to 40 minutes. After completion of this operation the mixture is screened, preferably through a coarse screen, such as one of about 2 to 5 mesh, and screening is into drying trays to a depth of about 0.5 to 1.5 cm. The mix is allowed to dry on such trays at about room temperature, 15.degree. to 30.degree. C., usually 20.degree. to 25.degree. C., for a period of about 8 to 24 hours, preferably about 12 to 20 hours, at normal humidity, e.g., 35 to 65% relative humidity, so as to permit the evaporation of a significant proportion, preferably 10 to 90% of the moisture, solvent and readily volatile materials present. Any lumps formed during such drying are removed by screening, usually through a 5 to 20 mesh screen, preferably through an 8 to 12 mesh screen. Of course, during the entire manufacturing procedure materials employed will be such that they will not give up objectionable impurities to the mix. Thus, normally, stainless steel and aluminum will be the materials that come into contact with the mix, the articles made and intermediate products. Next the desired pre-calculated weight of grain-resin mixture is screened into a clean mold cavity of desired shape through a screen of 2 to 10 mesh size openings, preferably having 4 to 8 mesh openings, and is leveled in the mold cavity by sequentially running across the major surface thereof a plurality of graduated strikers. This gently compacts the material in the mold, while leveling it, thereby distributing the boron carbide and resin evenly throughout the mold so that when such mix is compressed it will be of uniform density and B.sup.10 concentration throughout. A sheet of glazed paper is placed on top of the leveled charge and atop this there are placed a top setter plate and a top plunger, after which the mold is inserted in a hydraulic press and is pressed at a pressure of about 20 to 150 kg./sq. cm., preferably 35 to 110 kg./sq. cm., for a period of about 1 to 30 seconds, preferably 2 to 5 seconds. After removal of the mold from the molding press setter plates on both sides of the pressed mixture, together with the pressed mixture, are removed from the mold cavity, the top setter plate is removed and the release paper is stripped from the pressed mixture, leaving it supported by the bottom setter plate. Such combination, together with a plurality of other such combinations from other moldings, is next inserted in a curing oven, preferably by being placed in spaced relationship with other such combinations on an oven cart that is then wheeled into a comparatively large oven. The initial cure, preferably effected in a forced air oven, is carried out in stages, in the earlier parts of which remaining volatile materials are evaporated off without damage to the article structure. This initial curing is effected at a temperature in the range of 130.degree. to 200.degree. C., preferably 140.degree. to 160.degree. C., over a period of about 2 to 10 hours, preferably about 4 hours, and normally the curing temperature is gradually attained, preferably over a period of 1 to 6 hours, more preferably over 2 to 4 hours, at a substantially uniform rate of temperature rise, and a similar cool-down period is employed. Normally, the total cycle will be from 5 to 15 hours, preferably 8 to 12 hours, e.g., 10 hours. After cooling of the cured articles to about room temperature they are ready for impregnation with liquid state resin and curing to final product condition. In one impregnation method the cured articles, usually in thin plate form, are held vertically, separated from each other, as by a wire or openwork separator, placed in an impregnating tank or structure, subjected to vacuum, resin impregnated and cured. The use of vacuum is highly desirable to promote the removal of any gas from the article and to aid replacement of it with resin, which increases the strength of the absorber. However, when previous operations have been carried out successfully and in accordance with the instructions given the use of vacuum may often be obviated (because comparatively little excessive air will be present in the molded article). When vacuum is employed it is desirable that it be a comparatively high vacuum, such as from 500 to 750 mm. of Hg, preferably from 600 to 700 mm. Hg. Normally it will take from two minutes to an hour to draw a satisfactory vacuum on the impregnating tank, usually about ten to thirty minutes. The vacuum will preferably be drawn in the absence of any resin so that the surfaces of the initially cured article will be free to permit withdrawal of any gases therein or at least to permit lowering of the pressure of such gases. After the vacuum is drawn it may be held for an additional period of time, e.g., 1 to 5 minutes. The vacuum may be utilized to draw liquid state resin, preferably but not necessarily of the same composition as that initially employed, into the tank, wherein it is allowed to cover each of the plates. Addition of the resin to the tank by means of the vacuum may take from 1 to 20 minutes and after all of the plates are covered the liquid state resin may be left in the tank for an additional 1 to 10 minutes, preferably 1 to 5 minutes. The excess resin is then removed from the molded articles, preferably by a combination of gravity and air pressure, such as a pressure of 0.2 to 5 Kg./sq. cm. gauge. The add-on of resin after such draining of the excess resinaqueous resin mixture will be about 10 to 40 parts, preferably about 15 to 35 parts, e.g., about 28 or 31 parts, so that the final absorber article made will contain about 60 to 80% of boron carbide and 20 to 40% of irreversibly cured phenol formaldehyde type polymer, preferably 65 to 80% and 20 to 35% and most preferably about 70% and 30% or 74% and 26%, respectively. The phenolic resin in liquid state, which comprises the add-on, may be accompanied by 0.3 to 5 parts of liquid, preferably aqueous medium, with such medium usually being 5 to 20% of the resin, preferably 7 to 15% thereof. The plates containing add-on phenolic resin are removed from the baskets or other containers in which they have been positioned in the impregnating vessel and are placed on drying racks so that the major surfaces thereof, when flat, stand vertical. The plates are suitably separated from each other to allow air circulation, as by wires. Drying is effected at an elevated temperature to remove water, solvent and volatiles which may be present. The drying temperature is usually in the range of 40.degree. to 60.degree. C., preferably about 50.degree. C. and the time of drying is from 10 to 100 hours, preferably 24 to 96 hours. After drying, the absorber articles are cured at an elevated temperature, usually 130.degree. to 200.degree. C., preferably 140.degree. to 160.degree. C., over a period of about 2 to 10 hours, preferably with gradual heating over a 4 to 10 hour heat-up cycle and gradual cooling over a 3 to 8 hour cooling cycle, with the total time in the oven being 8 to 24 hours. Preferably, the heating cycle is about 16 hours, with a 7-hour heat-up cycle, a 4-hour hold at 140.degree. to 160.degree. C. and a 5-hour cooling cycle. Although it is not necessary in carrying out the preferred processes of this invention, pressure may be applied on the articles being cured so as to help to prevent any bleeding of resin and so as to maintain the resin evenly distributed throughout the absorber plate. Such pressure is gas pressure, not compressing or press pressure such as the 20 to 150 kg./sq. cm. compacting pressure employed earlier, and usually the gas pressure, if used, will be about 5 to 10 kg./sq. cm. No compressing or press pressure is employed in either the initial or final cure, thereby simplifying the process, avoiding the use of complex equipment and allowing the simultaneous curings of large numbers of absorber plates. In the final cure it is highly preferable to position the dry, impregnated but not finally cured absorber plates on horizontal setter plates, often with fiberglass cloth or equivalent separators between such setter plates and the absorbers. By stacking the absorber-setter plate sets 3 to 10 high, with such separators preventing plate-setter contacts and facilitating ready removals of the absorbers from the setter plates, good cures of undistorted plates are obtained without excessive bleeding of resin, etc. and the plates made are flat (but use of separators is not usually required). Although the neutron absorbing articles made may be of various shapes, such as arcs, cylinders, tubes (including cylinders and tubes of rectangular cross-section), normally they are preferably made in comparatively thin, flat plates, which may be long plates or which may be used a plurality at a time, preferably erected end to end, to obtain the neutron absorbing properties of a longer plate. Generally, to obtain adequately high neutron absorbing capability the articles will be from 0.2 to 1 cm. thick and plates thereof will have a width which is 10 to 100 times the thickness and a length which is 20 to 500 times such thickness. Preferably, the width will be form 30 to 80 times the thickness and the length will be 100 to 400 times that thickness. The neutron absorbing articles made in accordance with this invention are of a desirable density, normally within the range of about 1.2 g./cc. to about 2.3 g./cc., preferably 1.6 to 2.1 g./cc., e.g., 1.8 g./cc. They are of satisfactory resistance to degradation due to temperature and due to changes in temperature. They withstand radiation from spent nuclear fuel over exceptionally long periods of time without losing their desirable properties. They are designed to be sufficiently chemically inert in water so that a spent fuel storage rack in which they are utilized could continue to operate without untoward incident in the event that water leaked into their stainless steel container. They do not galvanically corrode in the presence of stainless steel and aluminum and are flexible enough to withstand seismic events of the types previously mentioned. Thus, they have a modulus of rupture (flexural) which is at least 100 kg./sq. cm. at room temperature, 38.degree. C. and 149.degree. C., a crush strength which is at least 350 kg./sq. cm. at 38.degree. C. and 149.degree. C., a modulus of elasticity which is less than 3.times.10.sup.5 kg./sq. cm. at 38.degree. C. and a coefficient of thermal expansion at 66.degree. C. which is less than 1.5.times.10.sup.-5 cm./cm. .degree. C. The absorbing articles made, when employed in a storage rack for spent fuel, as in an arrangement like that shown in FIGS. 1-3, with fuel rods stored on centers of squares which are from 15 to 40 cm. apart, is designed to give excellent absorption of slow moving neutrons, prevent creation of a critical mass and active or runaway nuclear reactions and allow an increase in storage capacity of a conventional pool for spent fuel storage up to over 500%. The designed system is one wherein the aqueous medium of the pool is water at an acidic or neutral pH or is an aqueous solution of a boron compound, such as an aqueous solution of boric acid or buffered boric acid, which is in contact with the spent fuel rods although such rods are maintained out of contact with the present boron carbide-phenolic polymer neutron absorber plates. In other words, although the spent fuel is submerged in a pool of water or suitable aqueous medium and although the neutron absorber plates are designed to surround it they are normally intended to be protected by a suitably constructed enclosure from contact with both the pool medium and the spent fuel. The absorber plates made in accordance with this invention by the method described above are subjected to stringent tests to make sure they possess the desired resistances to radiation, galvanic corrosion, temperature changes and physical shocks, as from seismic events. Because canisters in which they may be utilized might leak they also should be inert or substantially inert to long term exposure to storage pool water, which, for example, could have a pH in the range of about 4 to 6, a chloride ion concentration of up to 0.15 part per million, a fluoride ion concentration of up to 0.1 p.p.m., a total suspended solids concentration of up to 1 p.p.m. and a boric acid content in the range of 0 to 2,000 p.p.m. of boron. Also, the "poison plates" of this invention should be capable of operating at normal pool temperatures, which may be about 27.degree. to 93.degree. C., and even in the event of a leak in the canister should be able to operate in such temperature range for relatively long periods of time, which could be up to six months or sometimes, a year. Further the product should be able to withstand 2.times.10.sup.11 rads total radiation and should not be galvanically corroded in use nor should it cause such corrosion of metals or alloys employed. In this respect, while normally ordinary 304 or 316 stainless steel may be used for structural members when seismic events are not contemplated, where such must be taken into consideration in the design of storage racks utilizing the present absorbers, high strength stainless steels will preferably be used. The present absorbers are designed so as not to be galvanically corroded and not to promote galvanic corrosion. The advantages of the present invention over prior art neutron absorbers and spent fuel storage racks are many but principally they reduce to the manufacture by simple, efficient and economical methods of a physically stable, chemically stable and radiation-resistant neutron absorber of high and uniform capacity, which may be employed to significantly increase the storage capacity for both pressurized water reactor and boiling water reactor nuclear fuels, normally in the form of rods. The absorbers may also be made of greater lengths than prior products, e.g., 0.8 to 1.2 meters, so fewer joints between plates result. Of course, in addition to being used in stationary spent fuel storage racks the present invention may be employed in racks or other containers or mechanisms in which nuclear material is being used, stored, transported or manufactured. Thus, the present articles have a more general utility than for employment only in nuclear fuel storage racks of the types described. The following examples illustrate but do not limit the invention. In the examples and in this specification all parts are by weight and all temperatures are in .degree. C., unless otherwise indicated. EXAMPLE 1 183.2 Kilograms (kg.) of boron carbide powder and 20.6 kg. of Ashland Chemical Co. Arotap Resin 358-W-70 are mixed in a paddle mixer at room temperature (25.degree. C.) for thirty minutes to produce a homogeneous mixture in which the resin appears to be substantially uniformly distributed over the surfaces of the particles. The boron carbide powder is one which has been previously washed with hot water and/or appropriate other solvents, e.g., methanol, ethanol, to reduce the boric oxide and any boric acid content thereof to less than 0.5% (actually 0.16%) of boric oxide and/or boric acid, as boric oxide. The powder analyzes 75.5% of boron and 97.5% of boron plus carbon (from the boron carbide) and the isotopic analysis is 18.3 weight percent B.sup.10 and 81.7% B.sup.11. The boron carbide particles contain less than 2% of iron (actually 1.13%) and less than 0.05% each of halogen, mercury, lead and sulfur. The particle size distribution is 0% on a 35 mesh sieve, 0.4% on 60 mesh, 41.3% on 120 mesh and 58.3% through 120 mesh, with less than 15% through 325 mesh. The Arotap resin solution, a thick liquid, having a viscosity of 200 to 500 centipoises at 25.degree. C. and a water tolerance of about 1,000%, is principally a condensation product of trimethylolphenol and formaldehyde and contains about 82% of dimer, about 4% of monomer, about 2% of trimethylolphenol, about 4% of formaldehyde and about 8% of water. The resin contains less than 0.01% of each of halogen, mercury, lead and sulfur, including compounds thereof. After completion of mixing, which is effected in a suitable stainless steel or aluminum paddle mixer, the mix is screened through a 3 mesh sieve into drying trays to a thickness of 1.3 cm. The tray dimensions are approximately 1.2 meter .times.1.2 meter, with the bottom being composition board and the sides being hardwood or pine painted over with a protective coating. The mix is allowed to dry in the drying trays for sixteen hours at room temperature (15.degree. to 30.degree. C.) and at normal humidity (35 to 65%). The loss in weight is about 50-70% of the moisture content, e.g., about 6% of the weight of "resin" added or about 0.6% of the mix. The mix is next screened through a 10 mesh screen and is ready for use. The molds employed comprise four sides of case hardened steel (brake die steel) pinned and tapped at all four corners to form an enclosure, identical top and bottom plungers about 2.5 cm. thick made of T-61 aluminum and 1.2 cm. thick flat top and bottom aluminum tool and jig setter plates. The molds, which had been used previously, are prepared by cleaning of the inside surfaces thereof and insertions of the bottom plunger and the bottom setter plate on top of the plunger. A weighed charge (1450 grams) of the mix is then hand screened through a 5 mesh stainless steel screen and is leveled in the mold cavity with a series of graduated strikers, the dimensions of which are such that they are capable of leveling from about a 23 mm. thickness to the desired 20 mm. final mixture thickness, with steps about every 0.8 mm. A special effort is made to make sure to fill the mold at the ends thereof so as to maintain uniformity of boron carbide distribution throughout. Thus, the strikers are initially pushed toward the ends and then moved toward the more central parts of the molds and they are employed sequentially so that each strike further levels the mix in the mold. A piece of glazed paper is then placed on top of the leveled charge, glazed side down and the top setter plate and top plunger, both of aluminum, are inserted. The mold is then placed in a hydraulic press and the powder-resin mix is pressed to a stop at 5.3 mm. thickness of the molded composition therein. The size of the "green" plate made is about 21 cm. by 85 cm. by 5.4 mm. The pressure employed is about 107 kg./sq. cm. and it is held for three seconds. The pressure may be varied so long as the desired initial "green" article thickness and density are obtained. After completion of pressing the mold is removed from the press and at an unloading station a ram and a fixture force the plungers, setter plates and pressed mixture upwardly and through the mold cavity. The top plunger and top setter plate are then removed and the pressed mixture is movable intact with the bottom setter plate. The release paper is stripped from the pressed mixture and the bottom setter plate plus the pressed mixture are placed flat on an oven cart with other such combinations in spaced apart positions, for wheeling into an oven in which the pressed mixtures are cured. The initial cure of the pressed mix is effected by increasing the temperature gradually by about 40.degree. C. per hour from room temperature to 149.degree. C. over a period of about three hours, holding for four hours at 149.degree. C. and then cooling at a rate of about 40.degree. C./hr. for three hours, back to room temperature. The total cycle is about ten hours and is automatically controlled. At the end of the curing cycle (the initial cure) the pressed plates can be easily removed from the setter plates and are independently form retaining. When weighed it is noted that they have lost additional weight, often losing an average of 40 grams each, e.g., to 1387 grams each. Such weight corresponds to a density of about 1.5 g./cc. and in view of the density of boron carbide particles (about 2.3 to 2.6 g./cc.) and the cured resin (about 1.3 g./cc.) it is evident that despite the use of high pressure pressing and the compacting of the charge by the press the plate is somewhat porous. The plates made are subject to a quality control check of the uniformity of the thickness thereof and any significant variation therefrom, for example, outside the 5.1 to 5.6 mm. range, may be cause for rejection. The quality control check may be made after pressing but before initial curing, when desired, or checks at both points may be utilized. After completion of the initial cure, the pressed plates, removed from the setter plates, are positioned vertically in a basket, 50 plates per unit, standing on ends therein and separated by wires or screening and the basket is installed in an impregnating vessel, which includes connections to a source of vacuum, pressurized air and liquid resin. The stainless steel vessel is then sealed and a vacuum of about 66.0 mm. of mercury is drawn on the tank over a period of about five minutes, after which the valve to the resin supply is opened and liquid resin is drawn into the tank and allowed to completely cover all of the plates therein. Such addition of resin takes place over a period of about 1 to 5 minutes, after which the connection to the vacuum source is closed and the plates, submerged in the liquid resin, are allowed to absorb such resin over a period of 1 to 5 minutes. Then, the resin is forced from the tank by compressed air at a pressure of about 260 mm. Hg gauge. The vessel is then opened and the basket containing the impregnated plates is removed therefrom. The plates are taken out of the baskets, placed on their thin sides on drying racks separated by lengths of stainless steel or aluminum wire or clips and are dried at 52.degree. C. for a period of about 60 hours. During this drying operation there is a weight loss of about 1/12 of the approximately thirty additional percent of liquid state phenolic resin impregnating the plates (about 1.9% of the plates). The resin add-on is about 3/5 to 3/4 of the total resin content. The dried impregnated plates are next placed on setter plates of the type previously described, form-retaining flat aluminum with fiberglass cloth separators covering the impregnated plates, and are stacked six high, flat sides up and down, on carts, which are then placed in a pressurizable oven, which is sealed and pressurized to about 6.4 kg./sq. cm. gauge. The temperature in the pressurized oven is raised to 149.degree. C. gradually over a seven hour period with one hour holds at 79.degree. C., 93.degree. C. and 121.degree. C. After holding for four hours at 149.degree. C. the temperature is gradually decreased to room temperature over a period of five hours, dropping at about 26.degree. C. per hour. Thus, the total pressurized curing cycle takes sixteen hours, after which the cured plates are removed from the carts and are inspected. The finished plates are of about 70% boron carbide particles and 30% phenolic polymer, of a length of 84.8 cm., a width of 21.0 cm. and a thickness of 5.4 mm., like the measurements previously given. However, the weight is 1.754 kg. and the density is 1.82 g./cc. The plates analyze 54.8% of boron, corresponding to 70.8% of boron carbide (boron + carbon) and also to 10.0% of B.sup.10. They have a modulus of rupture (flexural) of at least 100 kg./sq. cm. (actually 484 kg./sq. cm. at room temperature) at 25.degree. C., 38.degree. C. and 149.degree. C., a crush strength of at least 750 kg./sq. cm. at 38.degree. C. and 149.degree. C., a modulus of elasticity less than 3.times.10.sup.5 kg./sq. cm. (actually 1.75.times.10.sup.5 kg./sq. cm. at 25.degree. C.) at 38.degree. C. and a coefficient of thermal expansion at 66.degree. C. which is less than 1.5.times.10.sup.-5 cm./cm. .degree. C. EXAMPLE 2 Following the method described in Example 1, 125 kg. of boron carbide powder and 15.5 kg. of Arotap Resin 358-W-70 are admixed. The boron carbide powder, like that of Example 1, is low in B.sub.2 O.sub.3 content, containing 0.17% thereof. The powder analyzes 71.7% of boron and 97.4% of boron + carbon (from the boron carbide) and the isotopic analysis is 18.3% B.sup.10 and 81.7% B.sup.11. The boron carbide particles contain 0.8% of iron and less than 0.05% each of halogen, mercury, lead and sulfur, including compounds thereof. The particle size distribution is 0% on a 35 mesh sieve, 0.9% on a 60 mesh sieve, 42.2% on a 120 mesh sieve and 56.9% through a 120 mesh sieve, with less than 40% through a 200 mesh sieve and less than 15% through a 325 mesh sieve. The charge of the boron carbide-resin mix weighed out and added to each mold is 950 grams and the size of the green plate made is about 14.9 cm. by 79.2 cm. by 5.33 mm. No fiberglass separators are employed in final curing. The finished plates are of about 63% of boron carbide particles and 37% of phenolic polymer and of the dimensions previously given herein. The weight of the absorber plate is 1.17 kg. and the density thereof is 1.86 g./cc. The plates analyze 48.3% of boron, corresponding to 61.7% of boron carbide and 8.8% of B.sup.10. They have a modulus of rupture (flexural) of 381 kg./sq. cm. at room temperature and over 100 kg./sq. cm. at 38.degree. C. and 149.degree. C., a crush strength of at least 750 kg./sq. cm. at 38.degree. C. and 149.degree. C., a modulus of elasticity of 1.1.times.10.sup.5 kg./sq. cm. at room temperature and less than 3.times.10HU 5 kg./sq. cm. at 38.degree. C. and a coefficient of thermal expansion at 66.degree. C. which is less than 1.5.times.10.sup.-5 cm./cm. .degree. C. EXAMPLE 3 The procedures of Examples 1 and 2 are repeated, utilizing a boron carbide powder containing 79.1 weight percent of boron and 98.5% of boron plus carbon (from the boron carbide). The weights of boron carbide particles and resin employed are 159 kg. and 19.7 kg., respectively and the boron carbide particles contain 0.07% of iron, less than 0.05% each of halogen, mercury, lead and sulfur and 0.1% of boric oxide and/or boric acid, as boric oxide (B.sub.2 O.sub.3). The particle size distribution of the boron carbide particles is 0.1% on 35 mesh (No. 35 screen), 3.4% on 60 mesh, 32.8% on 120 mesh, and 63.7% through 120 mesh, with less than 20% and preferably less than 15% through 325 mesh. The compacted charge made is about 79 cm. long by 14.9 cm. wide by 5.4 mm. thick and weighs about 985 g. before curing and 963 g. thereafter. The final product, after impregnation and final curing, measures 79 cm. by 14.9 cm. by 5.4 mm. and weighs 1170 grams. The density thereof is 1.84 g./cc. It includes 57.9% of boron, 72% of boron carbide, 73.2% of boron carbide particles (the 1.2% difference being largely inerts) and 10.7% of B.sup.10. The modulus of rupture (flexural) is at least 100 kg./sq. cm. at room temperature, 38.degree. C. and 149.degree. C. (actually 517 kg./sq. cm. at room temperature), the crush strength is at least 750 kg./sq. cm. at 38.degree. C. and 149.degree. C., the modulus of elasticity is less than 3.times.10.sup.5 kg./sq. cm. at 38.degree. C., being 1.6.times.10.sup.5 kg./sq. cm. at room temperature and the coefficient of thermal expansion at 66.degree. C. is less than 1.5.times.10.sup.-5 cm./cm. .degree. C. The neutron absorbing plates made are of satisfactory resistance to degradation due to temperature and changes in temperature encountered in normal uses as neutron absorbers. They are designed to withstand radiation from spent nuclear fuel over long periods of time without losing desirable properties and similarly are designed to be sufficiently chemically inert in water so that a spent fuel storage rack could continue to operate without untoward incident in the event that water should leak into a stainless steel container in such a rack in which they are contained. They do not galvanically corrode and are sufficiently flexible, when installed in a spent nuclear fuel rack, to survive seismic events of the types previously mentioned. When installed in a rack for storage of spent nuclear fuel, such as one of a type generally illustrated in FIGS. 1 and 2, with the spent fuel being that from either a pressurized water reactor or a boiling water reactor, immersed in water or an aqueous solution of boric acid, with the plates of this invention being stacked to a sufficient height to contain the spent fuel, e.g., 4 to 5 high on a side of a square about the spent nuclear fuel and inside a hermetically sealed stainless steel chamber, the plates are effective neutron absorbers and allow for significant increases in storage capacity of conventional pools for storing spent nuclear fuel, e.g., 100% to over 500% capacity increases. In variations of this process the resin may be replaced with a phenol formaldehyde resin of similar viscosity and polymerizable resin content (even two-step resins may be used), the boron carbide particles may be replaced by such particles containing less than 0.1% of B.sub.2 O.sub.3 and variations in the manufacturing procedure within the limitations described in the specification may be employed. The product resulting will be a useful neutron absorbing plate. Similarly, instead of pressing into plate form other shapes may be made. Different proportions of resin and boron carbide particles may be utilized within the ranges described and the product made may be of different densities within the ranges given, e.g., 1.6 g./cc. and 2.0 g./cc. When absorbing plates are to be manufactured the dimensions may be varied and in addition to the plate sizes mentioned earlier in this specification they may also be produced in a variety of other dimensions, e.g., 23 cm. by 81 cm. by 3, 4, 5, or 6 mm. Of course, products of other dimensions may also be made as desired, with the length, width and height, if plates are made, being within the ranges previously given. Changes in the manufacturing procedure may be effected providing that the proportions of liquid state resin and boron carbide particles are such as will make workable mixtures capable of holding together satisfactorily after pressing to green plate shape so as to be curable to such shape on a setter plate. In this respect, evaporation of moisture, solvents and other materials which could cause fluidizing of the green plate will be controlled so that such plate will maintain its structural integrity during the initial cure, especially as the uncured resin is being heated and is thereby being decreased in viscosity before being cured. Although it is not as important to control fluidity of the resin during the normal room temperature impregnation of the initially cured plates the use of vacuum prior to impregnation is desirable, as is the employment of pressure thereafter to aid in the discharge of the resin from the impregnating vessel. Similarly, pressure applied during the curing cycle tends to prevent leaking of the resin from the plate as the resin is thinned by heating but it is contemplated that the present plates may be made without the use of vacuum and pressure in the impregnating vessel and without the employment of pressure in curing the impregnated articles. Different liquid state resins may be utilized for the impregnation step than those initially mixed with the powdered boron carbide. Of course, various other changes may be made in proportions of components, types thereof, temperatures, pressures, times and various treatments within the description of this invention and useful neutron absorbers will result. The two-step process utilized allows the manufacture of an absorbing article of uniform neutron absorbing power without the need for complex equipment, high temperature and high pressure pressing operations designed to cure the resin in the mold, and allows quick and economical production of the absorbing articles. Particularly, it is important that the pressing of the resin-boron carbide particle mix into green plates or articles be a fast operation because such articles can be made quickly and then large numbers of them can be cured, impregnated and finally cured together. If the desired final bonding proportion of liquid state resin of the types described, such as Arotap 358-W-70, is mixed with boron carbide particles initially the result is too fluid a mix, which does not form satisfactorily form-retaining green plates and which, on curing, tends to have the resin bleed from the plate. Thus, use of the two-step process results in a product which is capable of being manufactured rapidly and to specifications, utilizing simple, efficient and essentially trouble free equipment and processes. The irreversibly cured thermosetting resins of this invention, whether resols or novolaks are employed, result in form-retaining stable structures considered to be superior to various other synthetic organic polymeric plastics, in uses under the testing conditions in which racks for the storage of spent nuclear fuel are operated. The invention has been described with respect to various illustrations and embodiments thereof but is not to be limited to these because it is evident that one of skill in the art with the present specification before him will be able to utilize substitutes and equivalents without departing from the spirit of the invention. |
051446472 | summary | BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an apparatus for limiting a radiation exposure field in an equipment for the radiation therapy or for the non-destructive inspection using radiations, and more particularly to an apparatus for accurately defining a radiation exposure field while preventing radiation leakage from a gap between radiation shielding members. 2. Description of the Prior Art Conventionally, a radiation exposure field limiting apparatus is incorporated, for example, in a linear electron accelerator for the medical application in which radiations such as, for example, X-rays are generated. An exemplary one of conventional linear electron accelerators for the medical application is schematically shown in FIG. 9. Referring to FIG. 9, the conventional linear electron accelerator shown includes a fixed frame 1, a rotatable frame 2 supported for rotation around a horizontal axis 6 on the fixed frame 1, and a medical table 5 on which a top plate 4 for supporting thereon a patient to undergo radiation therapy is supported. X-rays 8 are emitted from a radiation source 11 and irradiated along a radiation center axis 7 toward an intersecting point between the axis 6 of rotation of the rotatable frame 2 and the radiation center axis 7 of X-rays 8. Such intersecting point is the center of medical treatment at which the affected part of a patient to be treated is normally positioned and will be hereinafter called an iso-center. Meanwhile, a line 10 shown in FIGS. 11 and 12 which passes the iso-center 9 and extends orthogonally to the axis 6 of rotation and the X-ray radiation center axis 7 will be hereinafter called an exposure field center axis. Referring also to FIGS. 10 and 11, the size of an exposure field to be formed by X-rays 8 generated from the radiation source 11 is defined by a pair of radiation shielding devices 12 and 13 disposed along the X-ray radiation center axis 7 for delineating an exposure field in perpendicular directions to each other and each composed of a pair of radiation shielding members or blocks 12a and 12b or 13a and 13b. The radiation shielding blocks 12a, 12b, 13 a and 13b are made of a heavy metal such as lead. Referring now to FIG. 10, an electron beam 50 emitted from an electron beam source not shown is accelerated by an accelerating tube 47 and then guided by a detecting electromagnet 49 in a beam duct 48 maintained in a vacuum condition so that it is introduced to the radiation source 11 at which X-rays 8 are generated in response to the electron beam 50. A primary collimator 43 for defining a maximum extent of X-rays 8 is disposed just below the radiation source 11, and a flattening filter 44 for making the distribution of X-rays 8 in an exposure field uniform is disposed just below the primary collimator 43. Further, a dose monitor 45 for monitoring an amount of X-rays 8 on the real time basis is disposed just below the flattening filter 44, and a mirror 46 is interposed between the dose monitor 45 and the radiation shielding blocks 12a and 12b. The mirror 46 is disposed such that visible rays of light from a light source 51 may be introduced to provide the same extent as X-rays 8 in order to permit visual observation of an X-ray exposure field. The light source 51 is disposed at a position equivalent to that of the radiation source 11 with respect to the mirror 46. Referring also to FIG. 11, the dimension of an exposure field which is projected from the radiation source 11 or the light source 51 by way of the radiation shielding blocks 12a and 12b onto a plane including the iso-center 9 and extending orthogonally to the X-ray radiation center axis 7 (such plane will be hereinafter referred to as an iso-center plane) is represented by a capital letter "L", and the dimension of the exposure field which is projected similarly by way of the radiation shielding blocks 13a and 13b onto the iso-center plane is represented by another capital letter "W". FIG. 12 shows such exposure field as viewed from the radiation source 11. Referring to FIG. 12, a rectangular area having sides of the dimensions L and W is denoted at 41, and the affected part (such as a tumor) to be medically treated in a body of a patient 3 is denoted at 42. Referring to FIG. 13, each of the radiation shielding blocks 13a and 13b is shown formed from a plurality of radiation shielding members or parts 31a to 36a or 31b to 36b such that they may define an X-ray exposure area of a profile approximated to that of the affected part 42. Such radiation shielding parts 31a to 36a and 31b to 36b will be hereinafter called each a leaf, and here, the leaves 31a and 31b are center leaves which define an exposure area portion W1 along the exposure field center axis 10 while the other leaves are sequentially numbered toward the opposite outer sides from the center leaves 31a and 31b beginning with the number 32. While several leaves on the left-hand side in FIG. 13 of the center leaves 31a and 31b are not denoted by any reference character, they should be considered to be numbered similarly as 32a 36a and 32b to 36b. Accordingly, the radiation shielding blocks 13a and 13b include seven pairs of leaves in the arrangement shown in FIG. 13. Naturally, however, they may include any arbitrary plural number of pairs of leaves. A radiation shielding block including a plurality of leaves will be hereinafter referred to as a multi-leaf radiating shielding block. Referring now to FIG. 14, there is shown an exemplary structure of such multi-leaf radiation shielding block as shown in FIG. 13, and each of the radiation shielding blocks 12a and 12b shown in FIG. 10 may be replaced by such multi-leaf radiation shielding block as shown in FIG. 14. Though not shown, the other radiation shielding block 12b or 13b includes leaves 31b to 36b corresponding to the leaves 31a to 36a. In the case of the multi-leaf radiation shielding block shown in FIG. 14, each of the leaves has a rectangular cross section. FIG. 15 shows another exemplary structure of a multi-leaf radiation shielding block. The multi-leaf radiation shielding block shown in FIG. 15 includes a similar number of leaves which are numbered in a similar manner but have different sectional areas from those of the leaves shown in FIG. 14. In particular, each of the leaves has a sectional area of a generally trapezoidal shape as is provided by cutting a circular cone having the apex at the radiation source 11 along a generating line. In order to prevent X-rays from passing through a gap between each adjacent ones of the leaves, each of the leaves has a projection 39 formed thereon, and the projection 39 is fitted for sliding movement in a complementary recess formed in an opposing face of an outer adjacent one of the leaves. FIG. 16 illustrates a principle of construction of each leaf. Referring to FIG. 16, reference numerals 61 and 62 denote generating lines on faces of two circular cones having the apexes commonly at the radiation source 11. Thus, the leaves shown in FIG. 15 are constituted if such a member as is indicated by a hatched portion between the generating lines 61 and 62 in FIG. 16 and similar members as are defined similarly by generating lines are replaced into the individual leaves shown in FIG. 15 and then a projection 39 is provided on each of the leaves while each of the leaves is machined to form a complementary recess in which the projection 39 of an inner adjacent one of the leaves is fitted. FIGS. 17 and 18 show portions of an exposure field in the direction along L which are formed by such leaves. In particular, FIG. 17 shows, as an example, exposure field portions formed by the leaves 32a and 33a of the radiating shielding block shown in FIG. 14. The exposure field portion projected from the light source 51 is such as denoted by La where the leaf 32a is positioned farther from the axis 6 of rotation than the leaf 33a, but is such as denoted by Lb where the leaf 32a is positioned nearer to the axis 6 of rotation than the leaf 33a. Meanwhile, FIG. 18 shows exposure field portions formed by the leaves 32a and 33a of the radiating shielding block shown in FIG. 15. Similarly, where the leaf 32a is farther from the axis 6 of rotation than the leaf 33a, such an exposure field portion as denoted by Lc is formed, but where the leaf 32a is nearer than the leaf 33a, such another exposure field portion as denoted by Ld is formed. Subsequently, operation of the linear electron accelerator will be described. Referring back to FIG. 9, a patient 3 to undergo radiation therapy will first lie on the top plate 4. In order to permit X-rays 8 for the radiation therapy generated from the radiation source 11 to be irradiated from any position around the patient 3, the radiation source 11 can be circularly moved over an angular range greater than 360 degrees around the axis 6 of rotation by means of the rotatable frame 2. Further, the patient 3 can be moved in leftward and rightward directions and in forward and backward directions by means of the top plate 4. Since the top plate 4 can be moved also in upward and downward directions by means of the medical table 5, the affected part 42 of the patient 3 to be irradiated can be positioned in an area including the iso-center 9 to perform radiation therapy. Referring now to FIGS. 10 and 11 which illustrate a manner of generating X-rays and determining an exposure field, an electron beam 50 is accelerated to a high energy, for example, to 3 to 20 MeV by the accelerating tube 47 and then runs in the beam duct 48 in a vacuum condition while it is deflected by the deflecting electromagnet 49 so that it may be directed toward the patient 3. Consequently, the electron beam 50 is introduced to the radiation source 11 which is made of a metal material which generates X-rays when it is irradiated by an electron beam. The radiation source 11 thus generates intense X-rays 8 in the advancing direction of the electron beam 50. While the X-rays 8 have a forward directivity, the primary collimator 43 is disposed in order to absorb unnecessary X-rays outside a required exposure field. Since the X-rays 8 are spread radially from the radiation source 11 which serves as a point radiation source, the primary collimator 43 is scooped out to form a hole having the profile of a truncated cone having the apex at the radiation source 11, and the X-rays 8 can pass only through the area of the hole. Generally, an exposure field defined by such truncated conical hole of the primary collimator 43 has a maximum dimension specified with the arrangement. Further, since X-rays generally have a directivity, the flattening filter 44 is disposed to make the distribution in intensity of the X-rays 48 uniform, and while such X-rays 48 continue to be generated, the intensity of the X-rays 48 is monitored on the real time basis by means of the dose monitor 45. The X-rays 8 are then stopped by the radiation shielding blocks 12a, 12b and 13a, 13b so that they may be irradiated upon the affected part 42 of the patient 3 in such an exposure field area 41 as seen in FIG. 12. In order to allow visual observation of such exposure field, the mirror 46 is disposed such that visible light which is emitted from the light source 41 disposed at the equivalent position to that of the radiation source 11 with respect to the mirror 46 is reflected by the mirror 46 so that an exposure field of the light may be produced on a face of the skin of the patient 3. Thus, the size of the exposure field, that is, the dimensions L and W can be confirmed from the exposure field of the visible light. While the linear electron accelerator having a rectangular exposure field operates in such a manner as described above, since X-rays are irradiated also upon normal structure of the patient other than the affected part as seen in FIG. 12, there is the possibility of radiation hazard to such normal structure. Accordingly, a linear electron accelerator in recent years is constituted such that an exposure field is defined by radiation shielding blocks of such multi-leaf construction as shown FIG. 13 so as to assure protection of normal structure. Thus, the radiation shielding blocks 13a and 13b are replaced by those which are formed from leaves 31a to 36a and 31b to 36b, respectively, so that an exposure field in the W direction in FIG. 12 may have several dimensions W1, W2, . . . while the exposure field in the L direction is defined by the radiation shielding blocks 12a and 12b. Where such multi-leaf construction is employed, radiation therapy can be achieved with a higher degree of accuracy. Such multi-leaf radiation shielding blocks as shown in FIG. 14 or 15 have been proposed and produced with an intention to obtain such an exposure field as shown in FIG. 13. In particular, in place of the radiation shielding blocks 13a and 13b, multi-leaf radiation shielding blocks are disposed at the locations of the radiation shielding blocks 13a and 13b which are normally positioned at substantially mid locations between the radiation source 11 and the iso-center 9. In the case of the arrangement shown in FIG. 14 in which a multi-leaf radiation shielding block is viewed from a similar point of view to that in FIG. 10, each leaf has a rectangular cross section. A radiation shielding block is constituted in most cases from an odd plural number of leaves because it is normally necessary to determine the dimension W1 of an exposure field portion along the exposure field center axis and consequently the radiation shielding block includes a center leaf and a pair or pairs of leaves on the opposite sides of the center leaf. In the case of the arrangement of FIG. 14, since the radiation source 11 is not included in a plane of opposing faces of each adjacent ones of the leaves, no X-rays from the radiation source 11 pass through the gaps between the leaves, and accordingly, there is no possibility of direct leakage of X-rays to any locations outside an exposure field. Besides, since each leaf has a profile of a rectangular parallelepiped, it can be worked readily at a low cost. On the other hand, the exposure field defined by such multi-leaf radiation shielding blocks presents such dimensional relationship in the L direction as seen in FIG. 17. In case the dimension of the exposure field in the W direction is greater at the leaf 32a than at the leaf 33a, the dimension of the exposure field in the L direction is such as indicated by La, but on the contrary in case the dimension of the exposure field in the W direction is smaller at the leaf 32a than at the leaf 33a, the dimension of the exposure field in the L direction is such as indicated by Lb, and those dimensions look different from each other. Since X-rays pass more or less obliquely through edge portions of the leaves 32a and 33a, the X-ray exposure field will be dim at portions of the dimensions La and Lb thereof. While the difference between the dimensions La and Lb is small near the center of the exposure field, it may be increased to 1 cm or so at a location of outer leaves. On the other hand, in the case of the arrangement of FIG. 15, opposing faces of each adjacent ones of the leaves extend along an outer face of a circular cone having the apex at the radiation source 11. Such manner is illustrated in FIG. 16. Referring to FIG. 16, each of leaves has a section in an area between a generating line 61 of a face of a circular cone having the apex at the radiation source 11 and another generating line 62 of a face of another similar circular cone and makes part of a zone defined by and between the two generating lines. The leaves are defined by successive ones of faces of such similar circular cones so that they may be driven to slide along each other without mutually interfering with each other. In this instance, since opposing faces of each adjacent ones of the leaves pass the radiation source 11, if there is only a small gap between adjacent ones of the leaves, X-rays will pass through the gap and reach the patient 3 without being attenuated at all. Accordingly, X-rays may directly leak to a location outside an exposure field, which will make trouble to radiation therapy. Therefore, in the arrangement which employs such multi-leaf shielding blocks as described above, each of the leaves has a pawl 39 formed thereon so as to have such a section as shown in FIG. 15 in order to reduce or prevent such possible leakage of X-rays through gaps between adjacent ones of the leaves. In this instance, the dimension of an exposure field portion or a visually observable exposure field portion in the L direction is such as indicated by Lc in FIG. 18 when the dimension of the exposure field in the W direction is greater at the leaf 32a than at the leaf 33a, but on the contrary when the dimension of the exposure field in the W direction is smaller at the leaf 32a than at the leaf 33a, the dimension of the exposure field portion in the L direction is such as indicated by Ld in FIG. 18. Normally, the difference between the dimensions Lc and Ld is 3 to 4 mm, and since the thickness of each leaf is smaller at a portion thereof adjacent a leaf gap than at any other portion thereof, the X-ray shielding effect at such portion is lower than any other portion. Besides, the production cost of such leaves is high. In this manner, radiation therapy is conventionally performed with a linear electron accelerator for the medical application in which such a radiation exposure field limiting apparatus of the multi-leaf type as described above is incorporated. Since a conventional radiation exposure field limiting apparatus of the multi-leaf type is constructed in such a manner as described above, where such structure as shown in FIG. 14 is employed, the difference between the dimensions La and Lb is significantly great at an end portion of an exposure field, and a medical treatment project must necessarily be put into operation taking such difference into consideration. On the other hand, where such structure as shown in FIG. 15 is employed, since such difference between Lc and Ld exists for each leaf although it is small, it is similarly difficult to make a medical treatment project, and besides inadvertent undesirable linear leakage of X-rays to a location outside an exposure field cannot be avoided. In addition, since the profile of each leaf is complicated, the production cost is high. SUMMARY OF THE INVENTION It is an object of the present invention to provide a radiation exposure field limiting apparatus which prevents leakage of X-rays through a gap between each adjacent ones of leaves of radiation shielding blocks. It is another object of the present invention to provide a radiation exposure field limiting apparatus which minimizes a difference between dimensions of portions of a visually observable exposure field formed by adjacent ones of leaves of radiation shielding blocks arising from a difference between relative positions of the adjacent leaves. It is a further object of the present invention to provide a radiation exposure field limiting apparatus wherein each leaf of radiation shielding blocks is simplified in sectional shape and can be produced at a reduced cost. In order to attain the objects, according to the present invention, there is provided a radiation exposure field limiting apparatus for limiting an exposure field to be formed by radiations generated from a radiation generating equipment which includes a particle accelerator and a point radiation source, which comprises a plurality of radiation shielding members in the form of plates for defining an exposure field of radiations, the radiation shielding members being mounted for individual movement relative to each other with respect to a plane of a center axis of radiations generated from the point radiation source to define an exposure field of radiations having an approximated profile to that of an object to be irradiated by such radiations, a face of each of the radiation shielding members at which the radiation shielding member contacts with an adjacent one of the radiation shielding members making part of a face of a circular cone having the apex at an imaginary radiation source which is imaginarily disposed at a point spaced by a significant distance from the point radiation source. With the radiation exposure field limiting apparatus, since the apex of the face of the circular cone part of which is made by a face of each radiation shielding member is disposed at a position different from the position of the point radiation source, radiations which may otherwise pass through gaps between the radiation shielding members are eliminated, and an intended radiation exposure field can be formed clearly. Further, the difference between visually observable exposure field portions which may arise from a difference between relative positions of adjacent radiation shielding members can be restricted within an allowable range, and since the radiation shielding members are simplified in construction, they can be produced readily. Consequently, radiations can be irradiated with a high degree of accuracy and an equipment in which the radiation exposure field limiting apparatus is incorporated can be produced at a reduced cost. The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjuction with the accompanying drawings. |
description | This application is a continuation of and claims priority to U.S. patent application Ser. No. 11/831,744 which was filed Jul. 31, 2007, entitled ION IMPLANTER HAVING COMBINED HYBRID AND DOUBLE MECHANICAL SCAN ARCHITECTURE, the entirety of which is hereby incorporated by reference as if fully set forth herein. The present invention relates generally to ion implantation systems and methods, and more specifically to an ion implantation system and method for implanting ions in a plurality of operating ranges. Ion implanters are conventionally utilized to place a specified quantity of dopants or impurities within semiconductor workpieces or wafers. In a typical ion implantation system, a dopant material is ionized, therein generating a beam of ions. The ion beam is directed at a surface of the semiconductor wafer to implant ions into the wafer, wherein the ions penetrate the surface of the wafer and form regions of desired conductivity therein. For example, ion implantation has particular use in the fabrication of transistors in semiconductor workpieces. A typical ion implanter comprises an ion source for generating the ion beam, a beamline assembly having a mass analysis apparatus for directing and/or filtering (e.g., mass resolving) ions within the beam, and a target chamber containing one or more wafers or workpieces to be treated. Various types of ion implanters allow respectively varied dosages and energies of ions to be implanted, based on the desired characteristics to be achieved within the workpiece. For example, high-current ion implanters are typically used for high dose implants, and wherein medium-current to low-current ion implanters are utilized for lower dose applications. An energy of the ions can further vary, wherein the energy generally determines the depth to which the ions are implanted within the workpiece, e.g. to control junction depths in semiconductor devices. As device geometries continue to shrink, shallow junction contact regions translate into requirements for higher ion beam currents at lower and lower energies. Additionally, requirements for precise dopant placement have resulted in ever-more demanding requirements for minimizing beam angle variation, both within the beam, and across the substrate surface. For example, in certain applications, high current implants at energies down to 300 electron Volts are desirable, while concurrently minimizing energy contamination, maintaining tight control of angle variation within the ion beam as well as across the workpiece, and also while providing high workpiece processing throughput. At present, the preferred architecture to achieve high currents at low energies while minimizing angle variation is a dual-mechanical scan architecture, wherein the workpiece is mechanically scanned in two directions (e.g., a “fast” scan direction and a generally perpendicular “slow” scan direction) relative to a stationary spot ion beam. However, the relatively modest “fast” scan frequency utilizing this conventional architecture is limited by maximum accelerations that the mechanical systems can tolerate, and generally ranges between 1-3 Hz, thus limiting the maximum throughput of workpieces through the ion implanter. Ribbon beam systems, on the other hand, utilize ion beam optics for steering and shaping a ribbon-shaped ion beam, and are capable of achieving reasonably high currents at low energies. However, uniform current densities in conventional ribbon beam systems may be difficult to achieve, often at the expense of loss of angle accuracy. Hybrid scan technologies have also been provided utilizing electrostatic or magnetic “fast” scans of pencil or spot ion beams and mechanical “slow” scans of the workpiece, however, these conventional hybrid implanters further suffer beam transport problems resulting from the relatively higher space-charge density in a pencil beam and longer beam line length, especially at energies below 5 keV. Conventionally, high dose implants and lower dose implants require the utilization of separate dose-specific ion implanters, wherein each implanter is designed for the respective higher or lower dose ion implantation architecture. Such a requirement for multiple ion implanters thus increases equipment cost to the semiconductor product manufacturer, as well as increasing the cost of ownership of the particular implanters. Thus, it can be appreciated that an improved beamline architecture is desirable for providing both a high dose implant and a lower dose implant utilizing a common ion implantation system. The present invention overcomes the limitations of the prior art by providing a system, apparatus, and method that combines high current capabilities and angle control of a two-dimensional mechanical scan or “spot” ion beam implanter for high dose implants with the productivity of a hybrid scanned implanter for mid and lower dose implants. Accordingly, the following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. The present invention is directed generally toward a system and method for implanting ions in a plurality of operating ranges. In accordance with the invention, an ion implantation system is provided, wherein the ion implantation system comprises an ion source configured to generate a beam of ions having a generally elliptical cross-section, therein defining a spot or pencil ion beam. The ion implantation system further comprises a mass analyzer configured to mass resolve the beam of ions, and a beam scanning system positioned downstream of the mass analyzer. In accordance with the invention, the ion implantation system is configured to selectively operate in a first mode and a second mode, based on a desired dosage of ions and/or ion beam current or energy to be implanted into a workpiece. The first mode, for example, is associated with a first operating range, such as for low dose ion implantation. In the first mode, the beam scanning system is configured to scan the beam of ions along a single beam scan plane, thus defining a scanned ion beam. According to one exemplary aspect of the invention, a parallelizer is positioned downstream of the beam scanning system, wherein the parallelizer is configured to selectively bend the scanned ion beam into a substantially S-shape when the ion implantation system is operated in the first mode. As such, contaminants associated with the scanned ion beam are generally filtered out while concurrently parallelizing the scanned ion beam into a ribbon-shaped beam comprising a plurality of parallel beamlets, wherein the plurality of parallel beam lets have a substantially equal length. Accordingly, the plurality of parallel beamlets of the scanned ion beam can be uniformly implanted into a workpiece positioned on a workpiece scanning system residing downstream of the parallelizer. When the ion implantation system is operating in the first mode, the workpiece scanning system is configured to selectively translate the workpiece in one dimension through the scanned ion beam, therein implanting ions at the desired dosage and/or current or energy within the first operating range. Accordingly, in the first mode, the ion implantation system can be operated in a “mechanically-limited” throughput manner, wherein medium to low-dose implants can be achieved at a substantially high workpiece throughput, and wherein an upper limit of the workpiece throughput is mainly governed by mechanical capabilities (e.g., speed) of the workpiece scanning system to translate workpieces through the scanned ion beam. In accordance with the present invention, the beam scanning system is further configured to pass the beam of ions un-scanned when the ion implantation system is operated in the second mode, therein defining an un-scanned spot ion beam. The second mode, for example, is associated with a second operating range, such as a high current, or high dosage implantation. In one example, the un-scanned spot ion beam is further bent into the S-shape via the parallelizer, and the workpiece scanning system is configured to selectively translate the workpiece in two dimensions through the un-scanned spot ion beam, therein implanting ions at the desired current, energy, and/or dosage within the second operating range. Accordingly, with the ion implantation system operating in the second mode, the ion implantation system can be operated in an “implant-limited” throughput manner, wherein high-dose implants can be achieved, and wherein an upper limit of the current and/or dosage of the implants is mainly governed by the capabilities of the ion source and the ion beam transport system. To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. The present invention is directed generally toward an ion implantation system and method for implanting ions in a workpiece, wherein a plurality of differing modes of operation of the implantation system can be implemented for a plurality of operating ranges. Accordingly, the present invention will now be described with reference to the drawings, wherein like reference numerals may be used to refer to like elements throughout. It should be understood that the description of these aspects are merely illustrative and that they should not be interpreted in a limiting sense. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident to one skilled in the art, however, that the present invention may be practiced without these specific details. Referring now to the figures, FIG. 1 illustrates an exemplary ion implantation system 100 according to one aspect of the present invention, wherein the ion implantation system can be controlled to implant ions at various energies, currents, and/or dosages, as will be described herein. The ion implantation system 100 (also called an ion implanter) comprises a terminal 102, a beamline assembly 104, and an end station 106, wherein the terminal comprises an ion source 108 powered by a high voltage power supply 110. The ion source 108 is thus operable to produce an ion beam 112, and to direct the ion beam to the beamline assembly 104. The ion source 108, for example, generates charged ions that are extracted and formed into the ion beam 112, wherein the ion beam is directed along a nominal beam path 113 within the beamline assembly 104 and toward the end station 106. It should be noted that the ion beam 112 of the present invention has a relatively narrow profile (e.g., a generally circular cross-section when viewed from along the nominal beam path 113), and is hereinafter alternatively referred to as a “pencil” or “spot” ion beam. In order to generate the ions, a gas of a dopant material (not shown) to be ionized is located within a generation chamber 114 of the ion source 108. The dopant gas, for example, can be fed into the chamber 114 from a gas source (not shown). In another example, it will be appreciated that any number of other suitable mechanisms (not shown) can be implemented or utilized to excite free electrons within the ion generation chamber 114, such as RF or microwave excitation sources, electron beam injection sources, electromagnetic sources and/or a cathode operable to create an arc discharge within the chamber. Accordingly, the excited electrons collide with the dopant gas molecules, and ions are thereby generated. In general, positive ions are generated, however, the present invention contemplates the generation of negative ions, as well, and all such ion generating systems are contemplated as falling within the scope of the present invention. The ions are controllably extracted through an aperture or slit 116 in the chamber 114 via an ion extraction assembly 118, wherein the extraction assembly comprises a plurality of extraction and/or suppression electrodes 120A and 120B. The extraction assembly 118, for example, can comprise a separate extraction power supply (not shown) to bias the extraction and/or suppression electrodes 120A and 120B in order to accelerate the ions from the generation chamber 114. It will be appreciated that since the ion beam 112 comprises like charged particles, the ion beam may have a tendency to blow up or expand radially outwardly as the like charged particles repel one another. It will be further appreciated that beam blow up can be exacerbated in low energy, high current (known in the art as high perveance) ion beams, wherein many like-charged particles are moving in the same direction relatively slowly, such that an abundance of repulsive forces among the particles exists with little particle momentum to maintain the particles moving in the direction of the nominal beam path 113. Accordingly, in accordance with one example, the extraction assembly 118 is configured such that the ion beam 112 is generally extracted at an energy sufficiently high enough such that the spot ion beam does not blow up (i.e. so that the particles have sufficient momentum to overcome repulsive forces that can lead to the ion beam blowing up). In another example, in order to promote beam containment, it can be advantageous to transfer the ion beam 112 at a relatively high energy throughout the system, wherein the energy of the ion beam may be optionally reduced just prior to impacting a workpiece 122 located within the end station 106, as will be described infra. It should be noted that it can also be advantageous to generate and transport molecular or cluster ions which can be transported at a relatively high energy while being implanted with a lower equivalent energy, since the energy of the molecule or cluster is divided amongst the dopant atoms of the molecule. In accordance with another aspect of the invention, the beamline assembly 104 comprises a beamguide 124, a mass analyzer 126, and a beam scanning system 128. The beamline assembly 104, for example, may further comprise a parallelizer 130. The mass analyzer 126, for example, is generally formed at about a ninety degree angle and comprises one or more magnets (not shown), wherein the one or more magnets generally establish a dipole magnetic field within the mass analyzer. As the ion beam 112 enters the mass analyzer 126, it is correspondingly bent via the magnetic field such that ions of an inappropriate charge-to-mass ratio are generally rejected. More particularly, ions having too great or too small a charge-to-mass ratio are deflected into side walls 132 of the mass analyzer 126. In this manner, the mass analyzer 126 primarily allows only those ions in the ion beam 112 which have the desired charge-to-mass ratio to pass therethrough, wherein the ion beam 112 exits the mass analyzer through a resolving aperture 134. It will be appreciated that ion beam collisions with other particles (not shown) in the system 100 can degrade beam integrity, thus, one or more pumps (not shown) may be further included to evacuate, at least, the beamguide 124. The present invention contemplates a plurality of ranges of ion dosages that can be implanted via the ion implantation system 100. For example, the ion implantation system 100 can be configured to implant ions in a first operating range (e.g., an ion dosage ranging between approximately 5×1010 and 5×1014 ions/cm2) and a second operating range (e.g., an ion dosage ranging between approximately 5×1014 and 1×1017 ions/cm2). The first and second operating ranges, for example, may not necessarily be defined by dosage alone, but may be further defined by a combination of ion beam current and energy as well as, or in place of, the dosage. Accordingly, the first and second operating ranges maybe associated with ion dosage, ion beam current, and/or ion beam energy. In accordance with one example, a good approximation of operating range can be attained via the desired ion dosage, as will be further described infra. Conventionally, ion implantation in both the first operating range and second operating range would require two separate ion implantation systems, with each ion implantation system being configured to only implant in a respective one of the first or second ranges of dosage, current, and/or energy. The present invention advantageously utilizes a common architecture in the ion implantation system 100 to accommodate the plurality of operating ranges, wherein a control of the ion implantation system generally determines the operating range, as will be discussed hereafter. The exemplary beam scanning system 128 illustrated in FIG. 1, for example, comprises a scanning element 136 and a focusing and/or steering element (not shown), wherein power supply 140 is operably coupled to the scanning element 136 (and the focusing and steering element—not shown). The focusing and steering element (not shown), for example, may be configured to receive the mass analyzed spot ion beam 112 and to selectably focus and steer the ion beam to a scan vertex 148 of the scanning element 136. In accordance with one aspect of the invention, the ion implantation system 100 is configured to selectively operate in a first mode (e.g., associated with the first operating range) and a second mode (e.g., associated with the second operating range). In the first mode, for example, a voltage waveform can be selectively applied to the scanner plates 144A and 144B of the beam scanning system 128 via the power supply 140, wherein the applied voltage waveform is operable to electrostatically scan the spot ion beam 112 back and forth over time, thus “spreading out” the ion beam along a single beam scan plane 149 (e.g., along the X-axis, as illustrated in FIG. 1A) and defining a scanned ion beam 150, wherein the scanned ion beam can be seen as an elongate “ribbon” beam when time-averaged over a cycle of the applied voltage waveform. The scanned ion beam 150, for example, can be viewed as comprising a plurality of beamlets 151, wherein each beamlet is comprised of the spot ion beam 112 at a respective point in time over the cycle of the applied voltage waveform. The scanned ion beam 150 thus has a width 152 associated therewith when measured along the beam scan plane 149, wherein the width is greater than the cross-sectional dimension of the spot ion beam 112. The width 152 of the scanned ion beam 150, for example, can be as wide or wider than a width (not illustrated) of the workpiece 122. It should be noted that the width 152 of the scanned ion beam 150 may be altered or further focused via the parallelizer 130 downstream of the beam scanning system 128. It will be further appreciated that the scan vertex 148 can be defined as the point in the nominal beam path 113 from which each beamlet 151 appears to originate after having been scanned by the scanning element 136. In accordance with the present invention, the ion implantation system is further configured to selectively pass the spot ion beam 112 through the beam scanning system 128 generally un-scanned in the second mode, wherein the spot ion beam generally only follows the nominal beam path 113, as illustrated in FIG. 1B. Accordingly, in the second mode of operation of the ion implantation system 100, no voltage is applied to the scanner plates 144A and 144B via the power supply 140, thus letting the spot ion beam 112 travel through the beam scanning system generally unimpeded or unaltered, while benefiting from the beam transport enhancement provided by focusing properties of the parallelizer 130. Focusing elements such as dipole magnets and the like can be designed with focusing properties in both dimensions transverse to the propagation direction of the ion beam 112, wherein this focusing can counteract the expansion of the beam size, thus providing good transmission of the ion beam through restrictions in the beam line, such as vacuum enclosures, apertures etc. In accordance with another example, the ion beam 112 (e.g., the scanned beam 150 in the case of the first mode of operation illustrated in FIG. 1A or the un-scanned spot ion beam in the case of the second mode of illustrated in FIG. 1B) is passed through the parallelizer 130. The parallelizer 130, for example, comprises two dipole magnets (dipoles) 153A and 153B that are substantially trapezoidal in shape and are oriented to mirror one another. The dipoles 153A and 153B are thus configured to cause the ion beam 112 (e.g., the scanned ion beam 150 or the un-scanned spot ion beam) to bend into a substantially S-shape. Stated another way, the dipoles 153A and 153B have equal angles and opposite bend directions, wherein the dipoles are operable make the divergent beamlets 151 of the scanned ion beam 150 originating from the scan vertex 148, for example, generally parallel. The two symmetric dipoles 153A and 153B are further described in U.S. patent application Ser. No. 11/540,064, filed Sep. 29, 2006, the contents of which is hereby incorporated herein by reference in its entirety. The use of the two symmetric dipoles 153A and 153B permits, in general, isotropic, or spatially uniform properties across the scanned ion beam 150, both in terms of path length of the beamlets 151, as well as first and higher order focusing properties. Furthermore, similar to the operation of the mass analyzer 126, the S-bend serves to decontaminate the spot ion beam 112 and scanned ion beam 150, wherein the trajectories (not shown) of neutral particles and/or other contaminants (e.g., environmental particles that enter the beam downstream of the mass analyzer 120) are minimally affected by the dipoles 153A and 153B. As such, some number of these neutrals which do not get bent (such as from an injector), or get bent very little, thus do not impact the workpiece 122. The parallelizer 130, for example, causes the beamlets 151 of the scanned ion beam 150 to become parallel, such that implantation parameters (e.g., implant angle) are made generally uniform across the workpiece 122. Turning to FIG. 2, it can be seen that each of the dipoles 153A and 153B cause the beamlets 151 to bend through an angle θ 154 relative to a direction 156 parallel to the original trajectory (e.g., the nominal beam path 113) of the ion beam 112, thus giving the beam its substantially S-shape. In one example, θ 154 is between about 30 degrees and about 40 degrees, but can be any angle greater than about 20 degrees. In any event, because the two dipoles 153A and 153B mirror one another, the respective beamlets 151 are of a substantially equal length 158, as illustrated in FIG. 1A. Alternatively, this can also be stated as each of the beamlets 151 having a constant path length. The symmetry properties of the dipoles thus facilitate uniform implantation parameters (e.g., implant angle). The length 158 of the beamlets 151 is kept relatively short by using relatively small bend angles in the dipoles 153A and 153B. This is advantageous at least because it maintains an overall footprint of the implantation system 100 relatively compact. Additionally, the dipoles 153A and 153B are generally separated by a gap 160, as the illustrated example of FIG. 2. The gap 160, for example, generally provides an equal drift length for the respective beamlets 151, and may separate the dipoles 153A and 153B by a distance of two times the pole gap of the dipoles (e.g., between about 100 and about 250 millimeters). Each of the dipoles 153A and 153B may further comprise a plurality of cusping magnets (not shown), in order to help contain and/or otherwise control the ion beam 112 of FIGS. 1A and 1B passing therethrough. The cusping magnets, for example, operate as described in U.S. Pat. No. 6,414,329 to Benveniste et al., the entirety of which is hereby incorporated herein by reference. It will be appreciated that the cusping magnets generally induce a static magnetic field close to the beamline enclosure to confine electrons generated by self-neutralization or any other means so that motion of such electrons in a direction perpendicular to the magnetic field of the cusps is thereby inhibited. More particularly, the cusping magnets act to confine electrons so that it is difficult for them to move along the magnetic field and reach pole pieces or the walls of the enclosure. In this manner, any contribution of the electrons to further self-neutralization is thereby enhanced. It should be noted that various orientations, sizes, spacings and/or numbers of the cusping magnets about the dipoles 153A and 153B are possible and are contemplated as falling with the scope of the disclosure herein. Referring again to FIG. 1A, one or more deceleration stages (not shown) may be further positioned downstream of the parallelizer 130. Up to this point in the system 100, the ion beam 112 is generally transported at a relatively high energy level, such as to mitigate the propensity for beam blow up. For example, the propensity of beam blow up can be particularly high where beam density is elevated, such as at the resolving aperture 134. Similar to the ion extraction assembly 118, scanning element 136 and the focusing and steering element (not shown), the deceleration stage comprises one or more electrodes (not shown) coupled to a power supply (not shown), wherein the one or more electrodes of the deceleration stage are operable to selectively decelerate the ion beam 112 (e.g., the scanned ion beam 150). For example, deceleration of the ion scanned ion beam 150 is particularly beneficial in the first mode of operation of the ion implantation system 100, wherein the ion beam 112 can travel at a substantially high energy prior to being scanned by the beam scanning system, and the energy of the scanned ion beam 150 can be lowered prior to impacting the workpiece 122 for the implantation of ions in the first operating range. In accordance with one example, the deceleration stage (not shown) is operable to selectively further filter neutrals and other ions of non-desired energies out of the ion beam 112 (e.g., the scanned ion beam 150), and ion species of the desired energy will continue to follow the desired path of the ion beam and can be selectively decelerated via the deceleration stage. In accordance with another exemplary aspect of the invention, in the second mode of operation of the ion implantation system 100, no voltage may be applied to the electrodes of the deceleration stage (not shown), therein generally permitting the spot ion beam 112 to pass through the deceleration stage generally unaffected. It should be noted that in the present example, two electrodes 120A and 120B, and 144A and 144B are respectively illustrated in the ion extraction assembly 118 and scanning element 136. It should be further noted, however, that extraction assembly 118, scanning element 136, focusing and steering element (not shown) and deceleration stage (not shown) may comprise any suitable number of electrodes arranged and biased to accelerate and/or decelerate ions, as well as to focus, bend, deflect, converge, diverge, scan, parallelize and/or decontaminate the ion beam 112, such as provided in U.S. Pat. No. 6,777,696 to Rathmell et al., the entirety of which is hereby incorporated herein by reference. Additionally, the focusing and steering element may comprise electrostatic deflection plates (e.g., one or more pairs thereof, as well as an Einzel lens, quadrupoles and/or other focusing elements to selectively focus the ion beam 112. The end station 106 illustrated in FIGS. 1A and 1B, for example, comprises a “serial” type end station, wherein a single workpiece 122 is translated through the path of the ion beam 112 via a workpiece scanning system 167 for ion implantation thereto. Alternatively, the end station 106 may comprise a “batch” type end station, wherein a plurality of workpieces (not shown) may be placed on a spinning disk (not shown) and passed through the ion beam 112. In a preferred embodiment, the workpiece scanning system 167 is configured to support the single workpiece 122 and to mechanically scan the single workpiece in one or more dimensions or directions generally orthogonal to the ion beam path 113 through the ion beam 112. The workpiece scanning system 167, for example, may comprise the two-dimensional scanning system described in U.S. Pat. No. 7,135,691 to Berrian et al., the contents of which are hereby incorporated by reference herein in its entirety. Alternatively, any workpiece scanning system 167 capable of translating the workpiece 122 through the path of the ion beam 112 in one or more directions generally orthogonal to the ion beam path 113 is contemplated as falling within the scope of the present invention. For example, in the first mode of operation associated with the first operating range, the workpiece scanning system 167 may mechanically translate the workpiece 122 in a first direction (e.g., Y or slow scan direction) while the ion beam 112 is scanned via the scanning element 136 (thus defining the scanned ion beam 150) in a second direction (e.g., X or fast scan direction) to implant ions over the entire workpiece. Accordingly, in the first mode, the ion implantation system 100 can be operated in a “mechanically-limited” throughput manner, as will be further discussed infra, wherein medium to low-dose implants can be achieved at a substantially high workpiece throughput, and wherein the upper limit of the workpiece throughput is mainly governed by mechanical capabilities of the workpiece scanning system 167 to translate workpieces 122 through the scanned ion beam 150. In accordance with another exemplary aspect of the invention, the parallelizer 130 can be omitted, and the workpiece scanning system 167 can further rotate the workpiece 122 generally about the Y axis when the ion implantation system 100 is operated in the first mode, as described in U.S. Pat. No. 6,992,310 to Ferrara et al., the contents of which are hereby incorporated by reference herein. By rotating the workpiece 122 about the Y-axis, a substantially constant implantation angle can be achieved on the workpiece. In the first mode of operation associated with the first operating range, for example, the ion beam is scanned relative to the workpiece, as illustrated in FIGS. 3A and 3B. FIG. 3A, for example, the workpiece is shown traveling in the Y-direction (illustrated as arrow 168), while the ion beam 112 is scanned in the X-direction (illustrated as arrow 169). Accordingly, as illustrated in FIG. 3B, the ion beam 112 traces a trajectory 170 that forms stripes 171 across the workpiece 122 as the ion beam is scanned relative to the traveling workpiece. A width 172 of each stripe 171 (shown in FIG. 3A), for example, is associated with the size of the ion beam 112. Accordingly, the ion beam 112 is scanned across the workpiece 122 and then off the workpiece in order to maintain correct ion dosages, wherein one or more beam monitoring devices 173 positioned beyond the circumference of the workpiece generally permit real-time monitoring of the ion beam at extents 174 of the beam travel. Thus, as shown in FIG. 3B, a total area 175 swept by the ion beam 112 is larger than an area 176 of the workpiece 122, wherein the ion beam spends a considerable amount of time while being off the workpiece (e.g., when the ion beam does not impact the workpiece). The ratio of time on the workpiece 122 to time off the workpiece generally defines the ion beam utilization, U. Alternatively, the ion beam utilization U can be defined, at a constant ion beam current, I, as the desired quantity of dopant implanted into the workpiece to the quantity of dopant delivered by the implantation system during the implantation process. With a desired dosage D of dopant (per square unit) to be implanted into a given workpiece, the implant time timplant for the workpiece in an ion implanter can be expressed as:timplant=D×A/q/I/U (1)where A is the total area of the workpiece to be implanted by the ion beam, and q is the ion charge. It will be appreciated that, depending on the width of the ion beam, the utilization can range from highs approaching approximately 80% to less than 20% in the first mode of operation. Furthermore, with continuous operation of the ion implanter, such as when implanting ions into multiple workpieces in a serial manner, the total throughput, or total processing time ttotal per workpiece, of the ion implanter can be expressed as:ttotal=timplant+thandling (2)where timplant is the time associated with implanting ions into each workpiece as described above, and thandling is the handling time associated with transferring the workpiece into and out of the ion implanter. If, for example, the implant time timplant for a given workpiece is short, (e.g. significantly shorter than the handling time thandling), then the throughput of the implanter is mainly governed by the handling time, and the ion implanter is operated in a “mechanically limited” throughput manner or mode. In the second mode of operation associated with the second operating range, in accordance with yet another example, the workpiece scanning system 167 is configured to mechanically translate the workpiece 122 in both the first direction (Y or slow scan direction) and the second direction (X or fast scan direction) while the ion beam 122 remains an un-scanned spot ion beam for higher dosage, higher current, and/or higher energy implants, as illustrated in FIG. 1B. Accordingly, in the second mode, the ion implantation system 100 can be operated in an “implant-limited” throughput manner, as will be described infra, wherein high-dose implants can be achieved, and wherein an upper limit of the dosage, current, and/or energy of the implants is mainly governed by the capabilities of the terminal 102, beamguide 124, and mass analyzer 126. Throughput of workpieces 122 in the second mode is thus generally limited by the beam current performance of the beamline. In second mode of operation, as illustrated in FIGS. 4A and 4B, for example, the workpiece 122 is swept through the stationary ion beam 112 in both the X-direction (illustrated as arrow 177) and Y-direction (illustrated as arrow 178) where the utilization U is again defined by the time that the ion beam spends on the workpiece versus the total implant time. A real-time current monitoring device (not shown) is located, for example, behind the workpiece, such that the scan widths are given by beam size. It should be noted that while arrows 177 and 178 are illustrated as linear translations, curvilinear translations are also contemplated as failing within the scope of the present invention, such as a pendulum-type translation of the workpiece 122 in the X-direction. Speeds of two-dimensional mechanical scans of the workpiece 122 are generally slower than scan speeds of electrically and magnetically scanned systems. Accordingly, it is advantageous to mechanically scan the workpiece 122 when the implant time is long, (e.g. when the desired implant dose is high). In such a case, the times during which the ion beam 112 is off the workpiece can be made relatively short (e.g., as illustrated in FIG. 4B) by maintaining high accelerations in a workpiece turn-around area 179 (e.g., the time associated with the reversal of direction of workpiece travel), thus achieving higher utilizations than can be achieved in the first mode, (e.g., approaching 100% utilization), wherein a relative area 180 of beam/workpiece travel is minimized. Thus, in the second mode of operation, the workpiece 122 throughput is primarily defined by the implant time timplant, which is typically much larger than the workpiece handling time thandling. As seen in equation (1), higher utilizations lead to shorter implant times, thus leading to improved throughput and productivity for the ion implanter. According to yet another example, as illustrated in FIGS. 1A and 1B, a dosimetry system 182 is included in the end station 106 near the workpiece 122 for calibration measurements prior to implantation operations. In one example, during calibration, the ion beam 112 passes through dosimetry system 182, wherein the dosimetry system comprises one or more profilers 184 operable to translate along a profiler path 186, thereby measuring one or more characteristics of the ion beam (e.g., operable to measure a profile of the scanned ion beam and/or the spot ion beam 112). The profiler 184, for example, may comprise a current density sensor, such as a Faraday cup, operable to measure a current density of the ion beam 112. The measured current density, for example, may be utilized for a control of the ion implantation system via a system controller 188 operably coupled thereto. The system controller 188, for example, may comprise a computer, microprocessor, or other control system, wherein the controller is operable to control one or more of the terminal 102, mass analyzer 126, beam scanning system 128, focusing and steering element (not shown), scanning element 136, parallelizer 130, deceleration stage (not shown), and the workpiece scanning system 167. In one example, the system controller 188 is configured to receive measurement values from the dosimetry system 182 and to control the implantation of ions into the workpiece 122 based on the received measurement values. The system controller 188, for example, is operable to control the formation of the ion beam 112 via a control of the ion generation chamber 114 and extraction assembly 118. The system controller 188 is further operable to control the scanning element 136 via the power supply 140, wherein the ion beam 112 is selectively scanned, based on the desired operating range (e.g., the first mode or the second mode of operation of the ion implantation system 100). For example, based on the desired implantation dosage, current, and/or energy, the controller 188 is configured operate the ion implantation system 100 in the first mode, wherein the ion beam 112 is scanned by the beam scanning system 128 in the X direction, and wherein the controller controls the workpiece scanning system 167 to translate the workpiece 122 in the Y direction, therein implanting the workpiece with ions of the scanned ion beam 150. In the second mode, the controller 188 is configured to pass the ion beam 112 through the beam scanning system 128 generally unaltered or un-scanned, wherein the controller is further configured to control the workpiece scanning system 167 such that the workpiece 122 is translated in both the X direction and the Y direction through the generally stationary spot ion beam traveling along the ion beam path 113. Accordingly, the ion implantation system 100 can be adjusted via the system controller 188 in order to facilitate desired ion implantation based upon a desired dosage, current, and/or energy of ion implantation, as well as based on the one or more measured characteristics provided by the dosimetry system 182. In accordance with one example, the ion beam 112 can initially be established according to predetermined beam tuning parameters (e.g., the predetermined beam tuning parameters may be stored/loaded into the system controller 188). Then, based upon feedback from the dosimetry system 182, the parallelizer 130 can be adjusted to alter the degree of S-bend to alter an implantation angle, for example. Likewise, the energy level of the scanned ion beam 150, for example, can be adapted to adjust junction depths by controlling a bias applied to the electrodes 120A and 120B in the ion extraction assembly 118 and electrodes in the deceleration stage (not shown). In another example, the strength and orientation of magnetic field(s) generated in the mass analyzer 126 can be further controlled, such as by regulating the amount of electrical current running through field windings associated therewith, therein altering the charge-to-mass ratio of the ion beam 112. The angle of implantation can be further controlled by adjusting the voltage applied to the steering element (not shown). In accordance with another aspect of the present invention, FIG. 5 illustrates an exemplary method 200 for implanting ions of various operating ranges into a workpiece. It should be noted that while exemplary methods are illustrated and described herein as a series of acts or events, it will be appreciated that the present invention is not limited by the illustrated ordering of such acts or events, as some steps may occur in different orders and/or concurrently with other steps apart from that shown and described herein, in accordance with the invention. In addition, not all illustrated steps may be required to implement a methodology in accordance with the present invention. Moreover, it will be appreciated that the methods may be implemented in association with the systems illustrated and described herein as well as in association with other systems not illustrated. The method 200 begins at act 202, wherein an ion implantation system, such as the ion implantation system 100 of FIGS. 1A and 1B, is provided, wherein the ion implantation system is configured to implant ions of a spot ion beam into a workpiece in a plurality of operating ranges. For example, the ion implantation system is configured to operate in a first mode and a second mode, wherein the spot ion beam is scanned by a beam scanning mechanism in the first mode, and wherein the spot ion beam remains un-scanned in the second mode. In act 204 of FIG. 5, a set of desired criteria, such as a desired dosage, current, and/or energy of ions to be implanted into the workpiece, is provided. For example, a desired dosage of implantation is provided in act 204, wherein the desired dosage ranges from a low dosage implant (e.g., approximately 5×1010 and 5×1014 ions/cm2) to a high dosage implant (e.g., approximately 5×1014 and 1×1017 ions/cm2). The set of desired criteria, for example, may further or alternatively comprise one or more of ion beam utilization, throughput considerations, productivity considerations, or other criteria related to the implant. In act 206, the spot ion beam is formed and mass analyzed, wherein the spot ion beam has a generally circular cross section, and wherein the ion beam has an energy and current associated with the desired criteria provided in act 204. In act 208, one or more properties of the spot ion beam are quantified. For example, the current and/or size of the spot ion beam is measured or determined in act 208, such as via the dosimetry system 168 of FIGS. 1A and 1B. In act 210 of FIG. 5, an implant time (e.g., a time needed to implant ions into the entire workpiece) is determined, wherein the determination of the implant time is based on the quantified one or more properties of the spot ion beam. For example, the in order to implant the entire workpiece at the desired dosage provided in act 204, an implant time is calculated based on the quantified current and/or size of the spot ion beam. In act 212, the implant time is then compared to a scan time associated with a mechanically limited scan time. The mechanically limited scan time, for example, is a minimal time associated with operating the ion implantation system 100 in the second mode, and wherein the workpiece scanning system 167 of FIGS. 1A and 1B translates the workpiece 122 in the X direction and the Y direction at respective maximum velocities or limits. In the comparison of act 212 of FIG. 5, if the implant time determined in act 210 is generally less than the mechanically limited scan time (a relatively short implant time), and, for example, sufficient ion beam current is available, then the ion implantation system is operated in the first mode in act 214, wherein the ion beam is scanned via the beam scanning system 128, as illustrated in FIG. 1A. If the implant time determined in act 210 of FIG. 5 is generally greater than the mechanically limited scan time (a relatively long implant time), then the ion implantation system is operated in the second mode in act 216, wherein the ion beam remains un-scanned as illustrated in FIG. 1B, wherein the workpiece scanning system 167 translates the workpiece 122 in both the X direction and the Y direction. Accordingly, regardless of whether the ion implantation system 100 is operated in the first mode or the second mode, an implantation of ions into the workpiece 122 is performed in act 218, wherein the implantation meets the set of desired criteria (e.g., desired dosage). Thus, the present invention provides an architecture and method for implanting ions in a plurality of operating ranges while utilizing a common implantation system. As such, desired dosages, currents, and/or energies of ion implantations, as well as utilization and productivity efficiencies can be achieved using the common implantation system, regardless of the operating range, thus reducing equipment costs associated with dose-specific systems, and also increasing the utilization of the current ion implantation system. Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiments of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several embodiments, such feature may be combined with one or more other features of the other embodiments as may be desired and advantageous for any given or particular application. |
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047330929 | claims | 1. An internal radiation attenuation system for a radioactive environment having a substantially enclosed radioactive workspace having an internal configuration with at least one entrance portal, said workspace internal configuration having an upper wall or ceiling, side walls and a bottom wall, comprising: means for forming a frame for supporting a plurality of radiation attenuation means to substantially conform to at least a portion of the inside of said workspace internal configuration, said frame means including a plurality of interlocking segments, including means for assembling said segments into said frame means, said interlocking segments form a skeleton spaced around the periphery of said internal workspace, interlocked with one another, said frame means further including means for supporting said radiation attenuation means around at least a portion of the periphery of said internal workspace, said frame means skeleton forming a free standing support structure standing and supported primarily by said bottom wall without vertical support from said ceiling. 2. The system as defined in claim 1 wherein said support means include means for supporting said radiation attenuation means adjacent the top of said internal workspace. 3. The system as defined in claim 2 wherein said radiation attenuation means include means for movably covering the top of said internal workspace. 4. The system as defined in claim 3 wherein said movable covering means include a plurality of radiation attenuation panels and means for moving said panels in a fan shaped path covering a substantially semicircular top portion. 5. The system as defined in claim 4 wherein said support means include means for supporting said panels in at least two separate fan shaped paths. 6. The system as defined in claim 1 wherein said support means include means for supporting said radiation attenuation means adjacent the sides of said internal workspace. 7. The system as defined in claim 6 wherein said radiation attenuation means include means for movably covering the sides of said internal workspace. 8. The system as defined in claim 1 wherein said interlocking segments form a skeleton spaced around the periphery of said internal workspace, interlocked with one another. 9. The system as defined in claim 8 wherein said interlocking segments include a plurality of rib portions which interlock forming the sides, and bottom of said skeleton. 10. The system as defined in claim 9 wherein said interlocking segments include a plurality of band portions which interlock around the top periphery of said internal workspace forming the top of said skeleton. |
abstract | An insulated solution injector may include an outer tube and an inner tube arranged within the outer tube. The outer tube and the inner tube may define an annular space therebetween, and the inner tube may define a solution space within. The annular space may be configured so as to insulate the solution within the solution space. As a result, the solution may be kept to a temperature below its decomposition temperature prior to injection. Accordingly, the decomposition of the solution and the resulting deposition of its constituents within the solution space may be reduced or prevented, thereby decreasing or precluding the occurrence of a blockage. |
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041949480 | description | DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, each of the figures illustrates a locking device according to the present invention for supporting and locking a fuel assembly in place upon a support plate or grid structure with the weight of the fuel assembly and other forces acting downwardly upon the fuel assembly serving to retain it in place. In addition, the locking device may be released in order to facilitate the removal or replacement of each fuel assembly. Since retention forces are developed within the locking device due to the weight of the fuel assembly, the retention forces increase in proportion with the weight of the fuel assembly and other forces acting downwardly thereupon. Accordingly, no preload forces need be applied to the locking mechanism to counteract the natural and operational separating forces between the fuel assembly and the supporting grid plate. Generally, each of the figures illustrates a fuel assembly and fragmentary portion of a supporting grid in a reactor core in a nuclear reactor, preferably a gas cooled rector, which is not otherwise illustrated. In any event, it will be apparent from the following description that the grid or support plate comprises a number of openings for supporting a plurality of similar fuel assemblies to permit passage of coolant gas therepast. Referring now to FIG. 1, the locking device of the present invention primarily comprises a sleeve generally indicated at 10 which is positioned in an annular space formed between a bore 12 in the grid or support plate 14 and the tubular surface 16 of a fuel assembly 18. The manner in which the locking device serves to support the fuel assembly within the bore 12 of the support plate in response to the weight of the fuel assembly is described below. For purposes of the present invention, it is sufficient to understand that the fuel assembly is of generally conventional construction and is formed with inlet and outlet passages (not shown) for permitting coolant gas to flow through the fuel assembly as it passes along the bore 12 of the support plate. The support and locking sleeve 10 includes a solid tubular portion generally indicated at 20 which is arranged within a central portion of the bore 12. The sleeve also includes upper and lower portions indicated respectively at 22 and 24. The upper sleeve portion 22 comprises a plurality of axially extending members or fingers 26 which are circumferentially arranged between the bore 12 and fuel assembly surface 16. The upper end of each of the fingers 26 is formed with a wedge shaped structure 28 which forms an annular contact surface 30 facing radially inwardly and upwardly for engagement with an annular tapered surface 32 formed at the upper end of the fuel assembly. The wedged shaped structure 28 also forms a contact surface 34 opposite the surface 32 to engage the bore 12 of the support plate in a manner described in greater detail below. The lower sleeve portion 24 similarly comprises a plurality of downwardly extending elements or fingers 36 which are also circumferentially arranged between the support plate bore 12 and the surface 16 of the fuel assembly. Each of the lower fingers 36 forms a wedge shaped structure 38 at its lower end. Each wedge shaped structure 38 forms an annular contact surface 40 facing radially outwardly and downwardly for engagement with an annular tapered surface 42 at the lower end of the bore 12 in the support plate. The wedge shaped structure 38 also forms a contact surface 44 opposite the surface 42 for engagement with a surface portion of the fuel assembly in a manner described in greater detail below. The locking and support sleeve 10 functions together with the tapered surfaces 32 and 42 on the fuel assembly and support plate bore respectively to support the fuel assembly within the bore. In operation, as may best be seen on the left side of FIG. 1, the weight of the fuel assembly 18 is transmitted to the sleeve 10 through interaction of the tapered surfaces 30 and 32 and the upper fingers 26. The slope of the surfaces 30 and 32 forces the wedge shaped portions of the fingers outwardly so that the surfaces 34 are urged into engagement with the bore 12 of the support plate to develop a taper socket configuration producing relatively tight radial engagement at the upper end of the fuel assembly. Downward thrust of the sleeve developed by the weight of the fuel assembly in the manner described above is transmitted through the lower fingers 36 and the engaged, tapered surfaces 40 and 42 to the lower portion of the grid plate. Here again, the tapered slope of the surfaces 40 and 42 forces the wedge shaped portions 38 of the lower fingers inwardly so that the opposite contact surfaces 44 are urged into engagement with the surface 16 of the fuel assembly. Proper operation of the locking sleeve 10 is dependent upon the angular configuration of the surfaces 30, 32 and 40, 42 to produce an optimum coefficient of friction between those engaging surfaces and also between the opposed surfaces 34 and 44 with the bore 12 and fuel assembly surface respectively. The angle of the tapered surfaces 30 and 32 relative to the diameter of the bore 12 is selected so that frictional engagement is developed between the surfaces 34 and the bore 12 while the weight of the fuel assembly is substantially transmitted through the sleeve 10 to the tapered surfaces 40 at the lower end of the bore 12. The angle of the tapered surfaces 40 and 42 relative to the axis of the bore 12 is selected to produce radial interaction between the surfaces 44 and the fuel assembly surface while developing a wedging force so that the weight of the fuel assembly is transferred through the annular surface 42 into the support plate. As may be seen in FIG. 1, the tapered surfaces 30, 32 and 40, 42 are generally parallel and each form an angle of approximately 30.degree. relative to the axis of the bore 12. The sleeve 10 is further designed to facilitate installation and/or removal and replacement when necessary of each fuel assembly 18. In the righthand portion of FIG. 1, similar components are indicated by primed numerals. In that portion of the figure, the fuel assembly 18' and sleeve 10' are illustrated in a raised position suitable for either insertion or removal of the fuel assembly from the bore 12'. The fuel assembly 18' is adapted to raise the sleeve 10' through interaction of a lug 46' with each of the lower fingers 36'. As may be seen in the right hand portion of FIG. 1, the lug 46' engages the lower end of a finger 36' in order to raise the sleeve 10' to the position shown. Each of the lower fingers 36 is formed with a pre-stressed condition tending to urge the fingers radially outwardly. Thus, when there is no interaction between the tapered surfaces 40 and 42, the lower fingers tend to move outwardly against the bore 12 in order to facilitate removal or installation of the fuel assembly through the bottom of the support plate in a manner described in greater detail below. The upper fingers 26 are also pre-stressed and tend to move radially inwardly into engagement with the fuel assembly 18. In order to remove or install a fuel assembly, a special tool (not shown) may be used to spread all of the upper fingers 26 outwardly for example into the position illustrated for the one finger 26' at the right of FIG. 1. With the upper fingers in that position, the fuel assembly 18' may be lowered out of engagement with the bore 12' and sleeve 10'. However, with the upper fingers normally acting inwardly against the fuel assembly, its downward movement results in return of the support and locking sleeve to the configuration illustrated on the left side of FIG. 1. Accordingly, either removal or installation of the fuel assembly is possible when the upper fingers of the locking sleeve are retained in the position illustrated by the one finger 26' at the right of FIG. 1. Upward movement of the fuel assembly 18 is limited by engagement of a shoulder 48 with the bottom surface 50 of the support plate. For example, on the right side of FIG. 1, the shoulder is illustrated at 48' in abutting engagement with the bottom surface 50'. In order to even more closely limit undesirable upward movement of the fuel assembly, a mechanical restraint and flow restricting seal assembly 52 is arranged at the upper end of the bore 12. The assembly 52 includes an annular support structure 54 and a plurality of hinged restraint levers 56 which are urged outwardly by springs 58 for engagement with an annular recess 60 at the upper end of the support plate bore 12. Thus, the fuel assembly is normally locked in place within the bore by interaction of the levers 56 with the recess 60. In order to remove or install a fuel assembly however, the levers 56 may be urged inwardly to permit upward movement of the fuel assembly and locking sleeve toward the position illustrated at the right of FIG. 1. The assembly 52 also forms a sealing surface 62 which engages a similarly tapered sealing surface 64 at the upper end of the support plate bore 12 to prevent passage of coolant gas. An additional annular seal assembly 66 resiliently engages the upper end of the fuel assembly and functions to limit the passage of coolant gases between the fuel assembly and the bore 12. The seal assembly 66 internal bore size is adjustable for example in order to reduce coolant inlet flow to the fuel assembly to compensate for diminishing thermal output due to normal fuel consumption. The lower end of the fuel assembly is formed with a plurality of keys, one of which is indicated at 76. The keys 76 align with the key ways 78 formed by the support plate bore below the tapered surface 42 to receive the keys 76 and assure proper angular alignment of the fuel assembly within the bore 12. FIG. 2 illustrates an additional embodiment of the support and locking mechanism of the present invention. The assembly of FIG. 2 includes generally the same components as described above in connection with FIG. 1. Accordingly, similar numerals preceded by the additional digit "1" are employed to indicate corresponding components in FIG. 2. FIG. 2 does not include a mechanical restraint and flow restricting seal assembly as indicated at 52 in FIG. 1. However, similar restraint means could be employed with the FIG. 2 embodiment if desired. The embodiment of FIG. 2 particularly contemplates the use of the fuel assemblies of a conventional type including a venting arrangement for alleviating excessive pressure build-up within the fuel assembly. Such a venting arrangement is described for example in U.S. Pat. No. 3,743,576 issued July 3, 1973 and assigned to the assignee of the present invention. The fuel assembly 118 of FIG. 2 includes a vent outlet at 168. The central portion 120 of the sleeve and an adjacent portion of the support plate structure form passages 170 and 172 in communication with the vent outlet 168 for receiving gaseous fission products vented from the fuel assembly. Thus, the gaseous fission products may be safely removed from the fuel assembly without contaminating the coolant gas. In order to prevent contamination, three sets of seal rings 176, 178 and 180 are arranged for interaction between the central sleeve portion 120 and the bore 112 and between the central sleeve portion and the fuel assembly. The seal rings 176 and 178 are respectively arranged below and above both the vent outlet 168 and passage 170. An additional passage 182', illustrated on the right side of FIG. 2, is formed by the sleeve between the seals 178 and 180 for communication with another passage 184 in the support plate structure, at least when the fuel assembly is in an operative position as illustrated at the left of FIG. 2. Within the arrangement of fuel rings and venting passages described, the uppermost seal 180 acts as a primary flow bypass seal. The region between the uppermost seal 180 and the adjacent seal 178 is vented to the low pressure region beneath the support plate, at least when the fuel assembly is seated as illustrated on the left of FIG. 2, to prevent the higher inlet pressure from entering the fission traps and fuel rods (within the fuel assembly) which are at the lower pressure of the outlet region beneath the support plate. At the same time the passage 172 functions to receive gaseous fission products from the fuel assembly in the manner described above. In addition, an annular recess 186 is formed at the lower end of the bore 112 to receive the pre-stressed lower fingers 136 as the fuel assembly and locking sleeve are shifted upwardly. Here again, the rightward portion of FIG. 2 illustrates the fuel assembly and sleeve shifted upwardly into position indicated by primed numerals. In that position, one of the lower fingers 136' is illustrated within the recess 186'. Otherwise, it is believed apparent that the locking assembly illustrated in FIG. 2 operates in generally the same manner described above for the embodiment of FIG. 1. Lugs 188 are arranged on the fuel assembly for normally interacting with the lower fingers to prevent undesirable dropping of the fuel assembly. However, when the lower fingers are retained in the recesses 186', the lugs 188' do not engage the fingers and the fuel assembly may be inserted into or removed from the support plate bore. Accordingly, it may be seen that the two embodiments of the invention as described above provide support and locking means for retaining individual fuel assemblies within bores of a grid or support plate. Various modifications of the invention in addition to those illustrated and described above will be apparent to those skilled in the art from the preceding description and accompanying drawings. The scope of the present invention is therefore defined only by the following appended claims. |
abstract | The present invention provides a plasma ion beam system that includes multiple gas sources and that can be used for performing multiple operations using different ion species to create or alter submicron features of a work piece. The system preferably uses an inductively coupled, magnetically enhanced ion beam source, suitable in conjunction with probe-forming optics sources to produce ion beams of a wide variety of ions without substantial kinetic energy oscillations induced by the source, thereby permitting formation of a high resolution beam. |
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claims | 1. An irradiation apparatus for radiating a radiation beam transported from a particle accelerator onto a location to be irradiated that is positioned on an irradiation table, said apparatus comprising:a beam interruption part for interrupting said radiation beam;a position control part for controlling a position of said irradiation table such that said radiation beam is radiated onto an entire surface of a target in a plurality of irradiation zones including an overlapping zone formed by a plurality of radiations of said radiation beam; anda multileaf collimator control part for controlling said radiation beam to provide a slope to a dose distribution in said overlapping zone of said irradiation zones, respectively, such that the dose distribution is made flat over the entire surface of said target including said overlapping zone by the plurality of radiations of said radiation beam. 2. The irradiation apparatus of claim 1, wherein said slope is approximated by a straight line. 3. The irradiation apparatus of claim 1, wherein said slope has different gradients and is approximated by two or more straight lines connected with each other. 4. The irradiation apparatus of claim 1, wherein said slope changes in a stepwise manner. 5. The irradiation apparatus of claim 1, wherein said slope is approximated by a curved line. 6. The irradiation apparatus of claim 1, wherein said multileaf collimator control part includes a multileaf collimator provided with a plurality of pairs of opposed leaves, and said multileaf collimator control part decreases a dose irradiated to said overlapping zone in a direction from a boundary between said overlapping zone and a non-overlapping zone toward another irradiation zone by moving at least one of said opposed leaves in each pair. 7. The irradiation apparatus of claim 6, wherein said multileaf collimator is operated by remote control. 8. The irradiation apparatus of claim 6, wherein a direction in which said leaves are driven to move is parallel to the direction in which said dose decreases. 9. The irradiation apparatus of claim 6, wherein a direction in which said leaves are driven to move is perpendicular to the direction in which said dose decreases. 10. The irradiation apparatus of claim 1, wherein said target encloses an area to which said radiation beam is not radiated. 11. The irradiation apparatus of claim 1, further comprising a compensating filter commonly usable with at least two of said irradiation zones, wherein when irradiation is changed from one of said irradiation zones to another a filter driving mechanism drives said compensating filter to move to a position suitable for irradiation. 12. The irradiation apparatus of claim 11, further comprising a filter position verification mechanism for verifying the position of said compensating filter. 13. The irradiation apparatus of claim 1, wherein said irradiation apparatus is incorporated in a radiotherapy system. 14. An irradiation method for radiating a charged particle beam transported from a particle accelerator onto a location to be irradiated that is positioned on an irradiation table, said method comprising:dividing a location to be irradiated into a first irradiation zone and a second irradiation zone, the first irradiation zone and the second irradiation zone partially overlapping creating an overlapping zone;radiating a radiation beam to said first irradiation zone such that a distribution of a dose radiated to said overlapping zone has a slope that decreases from a boundary between said overlapping zone and a non-overlapping zone of said first irradiation zone toward said second irradiation zone; andradiating a radiation beam to said second irradiation zone such that a distribution of a dose radiated to said overlapping zone has a slope that decreases from a boundary between said overlapping zone and a non-overlapping zone of said second irradiation zone toward said first irradiation zone, with a dose distribution in a target being made flat. 15. An irradiation method for radiating a charged particle beam transported from a particle accelerator onto a location to be irradiated that is positioned on an irradiation table, said method comprising:dividing a location to be irradiated into first, second, and third irradiation zones that partially overlap, each of the first, second, and third irradiation zones having non-overlapping zones and overlapping zones adjoining each other;radiating a radiation beam to said first irradiation zone such that a distribution of a dose radiated to two overlapping zones in said first irradiation zone overlapping with at least one of said second irradiation zone and said third irradiation zone has a slope that decreases from boundaries between said overlapping zones and a non-overlapping zone in said first irradiation zone toward at least one of said second and third irradiation zones;radiating the radiation beam to said second irradiation zone in such a manner that a distribution of a dose radiated to two overlapping zones in said second irradiation zone overlapping with at least one of said first and third irradiation zones has a slope that decreases from boundaries between said overlapping zones and a non-overlapping zone in said second irradiation zone toward at least one of said first and third irradiation zones, with a total dose distribution in said overlapping zone of said second irradiation zone that overlaps with said first irradiation zone alone being made flat; andradiating the radiation beam to said third irradiation zone in such a manner that a distribution of a dose radiated to two overlapping zones of said third irradiation zone overlapping with at least one of said first and second irradiation zones has a slope that decreases from boundaries between said overlapping zones and a non-overlapping zone in said third irradiation zone toward at least one of said first and second irradiation zones, with a total dose distribution in said overlapping zones of said third irradiation zone that overlap with one of said first and second irradiation zones being made flat. |
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abstract | Safety valves accurately control closure and opening of fluid passage through the valve. Valves include a barrier that blocks the fluid until removal only by a high-energy projectile. Following removal and opening, the barrier or the projectile can flow through the valve, which remains open. Bullets, pneumatic pistons, shot, coilgun pellets and any other forceful projectile may impact and remove the barrier. The projectile is actuated with any type of chemical reaction, firing pin, spring release, accelerating circuit, ignition circuit. Catchers in the valve envelop or otherwise retain the projectile or barrier pieces and enter the fluid flow of the opened valve without blocking it. Disruptable barriers include strong but breakable glass plates, thin steel sheets, a rotatable door and other barriers that can withstand potentially over 10,000 psi of fluid pressure while closing the valve. Valves can use circuits to both monitor valve open/closed status and initiate firing the projectile. |
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045004889 | claims | 1. An encapsulated nuclear fuel unit having very low thickness-to-width and thickness-to-length ratios, comprising a piece of nuclear fuel sized approximately between 1 and 2 inches in width, 1 and 4 inches in length, and only 1/64 and 1/8 of an inch in thickness; and a rigid housing virtually enclosing and encapsulating the fuel piece, said housing having an open-ended body section defining an interior opening just slightly larger than the cross section of the fuel piece thereby being operable to receive the fuel piece and being elongated lengthwise to extend beyond the fuel piece, the body section being formed as two C-shaped channel pieces each having an intermediate wall and two short legs angled substantially normal thereto and welded together along opposite butted edges extended along the narrow sides of the body section and being of an impervious material between 0.002 and 0.01 of an inch thick, and said housing having rigid but porous end caps secured across and closing the open ends of the body section outwardly of the fuel piece. 2. An encapsulated nuclear fuel unit according to claim 1, wherein the porous material of the end cap has pores of approximately 3-10 microns size. 3. A method of encapsulating a nuclear fuel piece having a thickness of between 1/64 and 1/8 of an inch, a width between 1 and 2 inches and a length between 1 and 4 inches, comprising the steps of forming an open-ended housing by fixturing two C-shaped channel pieces, each of stainless steel of a thickness between 0.002 and 0.01 of an inch, snuggly over internal chill block means sized so that the free ends of the channel legs abut and welding the butted ends of the legs together, closing one of the open ends of the housing by welding a porous end plug thereto, loading the fuel piece into the other open end of the housing, and closing this other open end by welding a second porous end cap thereto thereby defining an encapsulated fuel unit of very low thickness-to-width and thickness-to-length ratios. 4. The method of encapsulating a nuclear fuel piece according to claim 3, further including the step of welding the channel pieces together with an electron beam in a sealed chamber having an inert gas atmosphere or a high vacuum. 5. The method of encapsulating a nuclear fuel piece according to claim 4, further including the step of fixturing the channel pieces between interior and exterior chill blocks that abut flush against the faces of the channel pieces. 6. The method of encapsulating a nuclear fuel piece according to claim 5, further including the step of using two wedges each having an inclined face angled with respect to an outer face as the internal chill block means and of sliding the inclined faces on one another in one direction to expand the outer wedge faces snug against the channel legs, and of keying the internal chill block wedges together as thus expanded. 7. The method of encapsulating a nuclear fuel piece according to claim 6, further including the step of using internal chill block wedges formed of machined and hardened tool steel. 8. The method of forming the encapsulated nuclear fuel unit according to claim 7, further including the step of forming the end caps of a stainless steel having pores of approximately 5 micron size. |
description | The present application hereby claims priority under 35 U.S.C. §119 on German patent application number DE 10 2008 032 137.0 filed Jul. 8, 2008, the entire contents of which are hereby incorporated herein by reference. At least one embodiment of the invention generally relates to a scattered radiation collimator for radiological radiation. It is well-known that scattered radiation impairs the image quality, particularly in the case of imaging tomography equipment such as, for example, X-ray computed tomography devices. It is for this reason that such tomography devices generally comprise radiation detectors which have so-called scattered radiation collimators arranged upstream of them in order to reduce the scattered radiation. Known scattered radiation collimators comprise, for example, absorber elements which are arranged next to one another in a collimation direction and are aligned in one direction with respect to their longitudinal extent, with absorber surfaces of the absorber elements running substantially perpendicular to the respective collimation direction. This makes it possible to suppress scattered radiation occurring in the collimation direction, which scattered radiation is caused, for example, by the radiation being scattered on an object to be examined. For example, DE 103 61 510 A1 discloses a collimator for a computed tomography scanner, which has a collimator lower part and a collimator upper part as holders for collimator plates. The collimator lower part comprises groove-like recesses on the end face for collimator plates in addition to lug receptacles on the bottom side. The collimator upper part is screwed to the collimator lower part in order to form the collimator. US 2006/0233298 A1 describes a collimator with an upper and a lower annular-segment-like holding element. The holding elements have grooves for collimator sheets. The annular-segment-like holding elements are connected to each other by means of side parts. The absorber elements of known scattered radiation collimators are generally comparatively thin and delicate. As a result of this, the absorber elements as such have low mechanical stability and are therefore not very dimensionally stable. Particularly in the case of acting forces, such as, for example, centrifugal forces acting on the absorber elements and transverse forces acting across the connection axis X-ray tube/X-ray detector during the operation of an X-ray computed tomography device, and other mechanical effects, the absorber elements can be deformed to such an extent that artifacts occur as a result of this. By way of example, this can be the case if the—or individual—absorber elements are deformed or displaced to the extent that detector elements of the radiation detector are shadowed. In order to avoid deformations and temporary displacements or erroneous positioning, it is common practice, for example, to adhesively bond the absorber elements using suitably designed holding lugs which extend into the intermediate spaces between the absorber elements. Aside from the comparatively high production complexity, this additionally poses the problem of irreversible erroneous positioning of the absorber elements possibly being caused by adhesive bond contraction during the curing of an adhesive bond used for the adhesive bonding. DE 10 2005 028 411 A1 describes a collimator for a beam detector which has a number of collimator sheets arranged next to one another and between which respectively at least one support element is arranged for stiffening the collimator, which support element is composed of an X-ray transparent material and supports the collimator sheets from the side. In at least one embodiment of the invention, at least one of the disadvantages according to the prior art is reduced or even removed. In particular, a scattered radiation collimator is intended to be provided, in which the absorber elements, using simple design measures, can be held in a dimensionally stable manner. From the same considerations, a radiation detector and a radiation detection device are also intended to be specified. A first aspect of at least one embodiment of the invention relates to a scattered radiation collimator for radiological radiation. The scattered radiation collimator comprises a multiplicity of absorber elements connected one behind the other in a collimation direction and at least two plate-like holding elements which are arranged substantially parallel with respect to one another. The holding elements can, for example, comprise a paired base and cover plate. The base and cover plates can be arranged on transverse and/or longitudinal edges lying opposite one another. In this case, a distance between the base and cover plate can basically correspond to the longitudinal or transverse extent of the absorber elements. It is also possible for the distance between the base and cover plate to be smaller than the longitudinal or transverse extent of the absorber elements. The holding elements have absorber element holders, for example in the form of slits or the like, to hold the absorber elements. By way of example, the absorber elements can be held on the transverse or longitudinal edges by way of the absorber element holders. According to at least one embodiment of the invention, the plate-like holding elements are connected to each other by cross beams, the cross beams running along the end face with respect to the longitudinal extent and/or with respect to the transverse extent of the absorber elements. As a result of the concept of the cross beams according to at least one embodiment of the invention, the holding elements can be made stiffer relative to each other in a simple but nevertheless effective manner, so that a relative displacement, deformation, or erroneous positioning of the holding elements caused by mechanical influences, such as, for example, forces acting as a result of the rotation and the like, are avoided as far as possible. As a result, a set position of the absorber elements fixed by the holding elements can substantially be maintained even when forces act, so that artifacts caused by deformation and the like of the absorber elements can be avoided. Apart from that, adhesively bonding the absorber elements using holding and supporting elements engaging between the absorber elements can be dispensed with as a result of the stabilizing effect of the cross beams. To this extent, the initially mentioned problem of erroneous positioning as a result of adhesive bond contraction can easily be circumvented. By way of example, the absorber element holders can be slits, recesses, depressions, in particular grooves or channels, and/or projections, etc. Such absorber element holders can be produced comparatively easily, particularly in the case of plate-like holding elements. By way of example, they make it possible to hold the absorber elements on the edge on transverse and/or longitudinal sides in a predetermined set alignment. Possible set alignments of the absorber elements, which can, for example, be collimator sheets, are: a parallel alignment of the collimator sheets or a confocal alignment of the collimator sheets. So that particularly secure holding of the absorber elements by the absorber element holders can be ensured, it is possible for the absorber elements to have notches on the edge side and/or protruding lugs which engage into the absorber element holders. Lugs or notches of the type mentioned above can be provided on one or more sides of the absorber elements. With regard to a particularly high mechanical stability of the scattered radiation collimator with respect to external effects, it is advantageous if in each case at least one cross beam is arranged or attached to at least two end faces lying opposite one another. A particularly high stability can be achieved if two crossing cross beams are attached to at least one of the end faces lying opposite one another. In the case of two crossing cross beams, the crossing cross beams can form an integrally-formed cross brace in order to simplify production and mounting. In order to ensure that the scattered radiation collimators can be adjoined from all sides, the cross beams on the end faces can at least partially be lowered into the absorber elements. To this end, the absorber elements can have recesses corresponding to the profile of the cross beams on the end face. The cross beams can be lowered into a channel-like incision formed by the recesses. It is advantageous if the recesses are formed such that mechanical contact between the absorber elements and the cross beams is avoided. This makes it possible to avoid erroneous positioning of the absorber elements possibly caused by mechanical contact. In the case of tightly fitting recesses, such erroneous positioning can be caused, for example, as a result of production tolerances of the recesses and cross beams. It is also possible for different thermal expansion coefficients of the cross beams and absorber elements to lead to stresses and hence possibly lead to deformations and erroneous positioning of the absorber elements. In order to keep an impairment of measurement results caused by the scattered radiation collimator with cross beams as low as possible, the holding elements and the cross beams can be suitably arranged and designed. In this case, it is advantageous if an attenuation of the radiation in the radiation transit direction, caused by a combination of holding elements and cross beams, in the region of the cross beams, is approximately equal to an attenuation of the radiation in the radiation transit direction, caused by the holding elements only, in a cross-beam-free region. Here, the term “radiation transit direction” is intended to be understood to mean that direction in which the radiation is intended to pass through when used in the intended manner. It is understood that the absorber elements also effect attenuation in the radiation transit direction as a result of their thickness. However, this should not change any aspect of the present definition of the radiation transit direction. Under closer scrutiny, the radiation transit direction is fixed by that direction in which the radiation is intended to pass through between the absorber elements without hindrance. In the case of confocally aligned absorber elements, the radiation transit direction is a local variable which depends on the confocal alignment of the absorber elements. To the extent that the demands with respect to precision make it necessary and the production complexity is justified, the cross beams can, in those regions in which said beams are lowered in the recesses, be designed such that their degree of absorption substantially corresponds to that of the absorber elements. A particularly stable embodiment which can be produced easily can be achieved by the cross beams running substantially diagonally on the end face. In this case, substantially diagonally is intended to mean that the cross beams run from one corner of a holding element, transversely across the end face, and to a corner of another holding element. The phrasing “substantially diagonally” is also intended to include the case where the clear distance between two holding elements is smaller than the transverse or longitudinal extent of the absorber elements. In order to attach the cross beams to the holding elements, substantially arbitrary attachment device(s) or attachment methods can be considered which depend, inter alia, on the material and geometry of the holding elements and cross beams. In particular, the cross beams and the holding elements can be connected to one another by means of bolts or pins and corresponding bores, by means of screws and/or by means of an adhesive connection. A second aspect of at least one embodiment of the invention relates to a radiation detector comprising a detection unit for detecting radiological radiation and a scattered radiation collimator according to the first aspect of the invention arranged upstream of the detection unit. A third aspect of at least one embodiment of the invention relates to a radiation detection device, in particular an X-ray computed tomography device, comprising a radiation detector according the second aspect of at least one embodiment of the invention. Advantages and advantageous effects of the second and third aspect of at least one embodiment of the invention result directly from the advantages and advantageous effects of the first aspect of at least one embodiment of the invention. Various example embodiments will now be described more fully with reference to the accompanying drawings in which only some example embodiments are shown. Specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. The present invention, however, may be embodied in many alternate forms and should not be construed as limited to only the example embodiments set forth herein. Accordingly, while example embodiments of the invention are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments of the present invention to the particular forms disclosed. On the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of the invention. Like numbers refer to like elements throughout the description of the figures. It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments of the present invention. As used herein, the term “and/or,” includes any and all combinations of one or more of the associated listed items. It will be understood that when an element is referred to as being “connected,” or “coupled,” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected,” or “directly coupled,” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between,” versus “directly between,” “adjacent,” versus “directly adjacent,” etc.). The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the terms “and/or” and “at least one of” include any and all combinations of one or more of the associated listed items. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper”, and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, term such as “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein are interpreted accordingly. Although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, it should be understood that these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are used only to distinguish one element, component, region, layer, or section from another region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the present invention. In the figures, equivalent or functionally equivalent elements are always designated by the same reference symbol. The illustrations in the figures are schematic and not to scale, and the scale can vary between figures. Without loss of generality, the invention will be described below on the basis of X-ray computed tomography. FIG. 1 schematically shows an X-ray computed tomography device 1, comprising a patient support table 2 for supporting a patient 3 to be examined. The X-ray computed tomography device 1 furthermore comprises a gantry 4 with a tube/detector system rotatably mounted about a system axis 5 in the azimuthal direction φ. The tube/detector system in turn comprises an X-ray tube 6 and an X-ray detector 7 arranged opposite thereto. During operation of the X-ray computed tomography device 1, X-ray radiation 8 is emitted by the X-ray tube 6 in the direction of the X-ray detector 7 and is detected by means of the X-ray detector 7. The X-ray detector 7 has a number of radiation detector modules 9 to detect the X-ray radiation 8. Scattered radiation 10 is generated when the X-ray radiation 8 passes through the body of the patient 3 and during the interaction processes occurring thereby. The scattered radiation 10 leads to a reduced image quality in the tomographic illustrations or images generated from the recorded data of the X-ray computed tomography device 1. The occurrence of scattered radiation 10 is illustrated schematically for the azimuthal direction φ in FIG. 2. Analogous results hold for the occurrence of scattered radiation in the direction of the system axis 5; this is not described in any more detail. Reference is made to the fact that the following explanations also hold for scattered radiation collimators whose collimation direction corresponds to the direction of the system axis. The radiation detector modules 9 generally comprise one or more, e.g. modular, scattered radiation collimators with a multiplicity of absorber elements 11 to suppress the azimuthal scattered radiation 10. In accordance with the confocal beam geometry in the present case, the absorber elements 11 are aligned confocally with a focus 12 of the X-ray tube 6. The absorber elements 11 are arranged one behind the other in the azimuthal direction φ, which in the present example corresponds to the collimation direction. In the present case, a radiation transit direction 13 corresponds to the radial direction with respect the focus 12. Inasmuch as there is an intension of providing further absorber elements for suppressing scattered radiation in the direction of the system axis 5, said elements are arranged one behind the other in the direction of the system axis 5 and are preferably arranged confocally with respect to the focus 12. The absorber elements 11 are generally delicate, fine small plates or sheets with a comparatively small thickness. As a result of this, the absorber elements 11 do not have a particularly high mechanical stability. However, relatively large acceleration forces act on the absorber elements 11 in the case of a circular or helical scan of the patient 3, in which the tube/detector system is rotated about the system axis 5. This makes it possible for the absorber elements 11 to be deformed and displaced temporarily. This leads to erroneous positioning of the absorber elements 11 which in turn can lead to artifacts in the images. Forces which act across the focus 12/radiation detector module 9 connection axis can in particular cause displacements and erroneous positioning of the absorber elements 11. Such forces are, in a simplified manner, referred to as transverse forces in the following text. As already mentioned initially, it is common practice to insert holding lugs into the intermediate spaces between the absorber elements and to adhesively bond said lugs to the absorber elements by means of an adhesive bond to avoid displacements and deformations caused by transverse forces. However, contraction processes of the adhesive bond inevitably lead to the absorber elements being positioned erroneously or being deformed. In particular, such production-dependent, artifact-inducing erroneous positioning and deformations can be avoided using the solution according to the invention, as will be explained in more detail in the following text. FIG. 3 shows the radiation detector module 9 illustrated schematically in FIG. 1 in more detail. The radiation detector module 9 has two detection units 15 mounted on a support 14 in the direction of the system axis. In general, an arbitrary number of detection units 15 can be arranged on a correspondingly designed support or on other attachment apparatuses. In this respect, the illustrated refinement with two detection units 15 mounted on the support 14 should not be seen as limiting. In the present case, the support 14 has through-holes 16 for attaching a plurality of supports 14 on a holding frame of the X-ray detector 7 not shown in any more detail, for example by way of screws. FIG. 1 shows that a plurality of radiation detector modules 9 or supports 14 are mounted one behind the other in the azimuthal direction φ on the holding frame. In the embodiment of FIG. 3, each detection unit 15 respectively has a scattered radiation collimator 17 connected upstream thereof, the scattered radiation collimators 17 being arranged next to one another like tiles in accordance with the detection units 15. However, it is also possible that, deviating from the illustration of FIG. 3, only one scattered radiation collimator is provided which spans both detection units 15. The scattered radiation collimators 17 can be adhesively bonded to the detection units 15 or can be attached in any other suitable manner. FIG. 4 shows one of the scattered radiation collimators 17 in detail. As mentioned previously, the scattered radiation collimator 17 comprises a multiplicity of absorber elements 11 connected one behind the other in the collimation direction, i.e. the azimuthal direction 9. The scattered radiation collimator 17 comprises two plate-like holding elements 18 which are arranged substantially parallel with respect to one another in order to hold the absorber elements 11. For the purposes of simplification, in the following text, one of the holding elements 18 is referred to as the base plate 18A and the other is referred to as the cover plate 18B. In the present example embodiment, the holding elements 18 are arranged on longitudinal edges 19 of the absorber elements 11. However, additionally or optionally, it is also possible for one or more holding elements to be arranged on transverse edges 20 of the absorber elements 11. Each holding element 18 comprises absorber element holders 21 for holding the absorber elements 11, as can be seen in more detail in FIG. 5, which shows an individual holding element 18. In the present example, the absorber element holders 21 are designed as slits, into which lugs engage which are formed on the longitudinal edges 19 of the absorber elements 11 and are not illustrated in any more detail. The absorber element holders 21 hold the absorber elements 11 according to a respectively desired set position and bearing. As an alternative thereto, or additionally, it is also possible for the absorber element holders 21 to be designed in the form of recesses and/or depressions, in particular grooves or channels. In this case, the channels or depressions can for example be designed such that the absorber elements 11 can be inserted on the edge side with longitudinal 29 or transverse 20 edges and can be held therewith. Differently designed absorber element holders are feasible, such as, for example, projections in the form of pins, rails, or the like, arranged in pairs. FIG. 4 shows that the holding elements 18, that is to say the base plate 18A and the cover plate 18B, are connected to one another by means of cross beams 22. The cross beams 22 run on the end face, that is to say on the end face of the scattered radiation collimator 17 spanned by the transverse edges 20, with respect to the longitudinal extent of the absorber elements 11. Only one end face is visible in the perspective side view of FIG. 4. However, in an analogous manner, cross beams 22 run on the end face which is not visible and faces away in the illustration. The perspective frontal view of FIG. 6 shows this. The cross beams 22 attached according to the invention can, in a simple design and production-technical manner, counteract displacement, deformation, and erroneous positioning of the absorber elements 11 caused in particular by transverse forces. The concept of the cross beams 22 according to an embodiment of the invention accordingly makes reliable and stable positioning of the absorber elements 11 possible. The holding elements 18 and cross beams 22 can for example be produced relatively simply, and therefore economically, and moreover with a very high precision, by way of an injection molding method. In the present example, respectively two crossing cross beams 22 are arranged on the end faces lying opposite to one another. The cross beams 22 run diagonally on the end faces. The diagonal profile of the cross beams 22 is particularly advantageous from the point of view of stability. Furthermore, such a profile makes comparatively uncomplicated attachment possibilities of the cross beams 22 on the holding elements 18 possible; this will be explained further below in more detail. It is feasible, and convenient within the scope of embodiments of the invention, if only one cross beam 22 is present on one or all end faces. In any case, for reasons of stability, at least two cross beams 22 should be present in an opposing, crossing arrangement. Furthermore, it is within the scope of embodiments of the invention if in each case non-crossing cross beams are present on one or on both end faces. Such cross beams could, for example, run substantially parallel to one another and connect the holding elements 18 to each other. In order to avoid displacements, deformations, and erroneous positioning when transverse forces are acting, further arrangements and designs of cross beams 22 are feasible. For example, it would also be feasible for holding elements 18 to be attached to the end faces and the cross beams 22 to run transversely over the longitudinal edges 19. Returning to the refinement shown with reference to the figures, two crossing cross beams 22 are arranged on each end face. In this case, the cross beams 22 on an end face can be designed as bracing elements which are independent of one another. Alternatively, it is also possible for the cross beams 22 to be combined as one unit on a respective end face. This is shown in FIG. 7, where the cross beams 22 are connected to each other and in this manner form an integrally-formed cross brace 23. Such cross braces 23 can reduce the mounting complexity of the cross beams 22 on the holding elements 18. So that the scattered radiation collimators 17 can be arranged adjacently to one another on the end faces whilst avoiding comparatively large gaps, it is particularly advantageous if the cross beams 22 are at least in part lowered into the absorber elements 11 on the end face. To this end, the absorber elements 11 can have recesses 24 corresponding to the—in this case diagonal—profile of the cross beams 22. This can be seen in FIG. 8 which shows the scattered radiation collimator 17 without cross beams 22. The recesses 24 are designed such that the cross beams 22 can at least in part be lowered into said recesses. The depth of the recesses 24, that is to say the degree to which the cross beams 22 can be lowered into the recesses 24, can be selected according to the desired or permissible projection of the cross beams 22 beyond the transverse edges 20 of the absorber elements 11. The recesses 24 are preferably designed such that mechanical contact between the absorber elements 11 and the cross beams is avoided. By way of example, the recesses 24 can be designed to be so large that, taking into account the production tolerances of recesses 24 and cross beams 22, a gap remains between the cross beams 22 and recesses 24, even after the cross beams 22 have been mounted. Otherwise, it could be possible for erroneous positioning of the absorber elements 11, caused by mechanical contact, to occur for example during the mounting of the cross beams 22, that is to say when the cross beams 22 are inserted into the recesses 24. The shape of the recesses 24 can basically be selected freely under the proviso that the cross beams 2 can be lowered to the desired extent, preferably whilst avoiding mechanical contact with the absorber elements 11. By way of example, it can be possible to select shapes which can be produced particularly easily and cost-effectively, such as, for example, rectangular or circular shapes. The holding elements 18 and the cross beams 22 cause an—albeit comparatively small—absorption of the X-ray radiation 8 in the radiation transit direction 13. So that the scattered radiation collimator 17 has an absorption profile which is as even as possible in the radiation transit direction 13, it is advantageous if the holding elements 18 and the cross beams 22 are arranged and designed such that an attenuation of the X-ray radiation 8 in the radiation transit direction 13, caused by the holding elements 18 and the cross beams 22, in the region of the cross beams 22, is approximately equal to an attenuation of the X-ray radiation 8 in the radiation transit direction 13, caused by the holding elements 18 only, in a cross-beam-free region. A corresponding refinement in the present example embodiment is clear from a combined view of FIGS. 5, 7 and 8. FIGS. 5 and 8 show that the holding elements 18 respectively have an offset 25 on the respective end face in that region in which the cross beams 22 come to rest. The offset 25 is selected such that the holding elements 18 and the cross beams 22 do not overlap when viewed in the radiation transit direction 13. So that a uniform absorption profile can be ensured, the cross beams 22 of the respective end faces are designed such that their absorption—when viewed in the radiation transit direction 13—locally substantially equals the local absorption of the two holding elements 18 outside of the region of the cross beams 22. In order to achieve a substantially equal absorption, it is possible to select cross beams 22 with a correspondingly dimensioned cross section and/or with a suitable material composition. With respect to a uniform absorption profile of the scattered radiation collimator 17, attachment elements for attaching the cross beams 22 on the holding elements 18 should also, if provided, be designed appropriately. In the present refinement, in particular taking account of FIGS. 7 and 8, the cross beams 22 have protruding pins 26 or bolts which are inserted or pressed into corresponding bores 27 of the holding elements 18 in order to mount the cross beams 22 on the holding elements 18. As long as at least the pins 26 are produced from the same material as the holding elements 18, it is also possible to avoid a discontinuous change of the absorption property in the region of the attachment elements. It is within the scope of an embodiment of the invention for the bores 27 and the pins 26 to be interchanged, or for the cross beams 22 and the holding elements 18 to respectively have bores 27 into which a pin is inserted in order to mount the cross beams 22. Within the scope of an embodiment of the invention, it is also possible for other or additional attachment possibilities to be used. For example, an adhesive connection between the cross beams 22 and the holding elements 18 is possible. The concept according to an embodiment of the invention allows the provision of a scattered radiation collimator 17 which makes particularly dimensionally stable holding of the absorber elements 11 possible, in particular when transverse forces are acting. Furthermore, the scattered radiation collimator 17 according to the invention can be produced in a particularly simple and therefore cost-effective manner. Overall, particularly on the basis of the exemplary embodiments explained with reference to the figures, it is clear that the object on which an embodiment of the invention is based is achieved. The patent claims filed with the application are formulation proposals without prejudice for obtaining more extensive patent protection. The applicant reserves the right to claim even further combinations of features previously disclosed only in the description and/or drawings. The example embodiment or each example embodiment should not be understood as a restriction of the invention. Rather, numerous variations and modifications are possible in the context of the present disclosure, in particular those variants and combinations which can be inferred by the person skilled in the art with regard to achieving the object for example by combination or modification of individual features or elements or method steps that are described in connection with the general or specific part of the description and are contained in the claims and/or the drawings, and, by way of combinable features, lead to a new subject matter or to new method steps or sequences of method steps, including insofar as they concern production, testing and operating methods. References back that are used in dependent claims indicate the further embodiment of the subject matter of the main claim by way of the features of the respective dependent claim; they should not be understood as dispensing with obtaining independent protection of the subject matter for the combinations of features in the referred-back dependent claims. Furthermore, with regard to interpreting the claims, where a feature is concretized in more specific detail in a subordinate claim, it should be assumed that such a restriction is not present in the respective preceding claims. Since the subject matter of the dependent claims in relation to the prior art on the priority date may form separate and independent inventions, the applicant reserves the right to make them the subject matter of independent claims or divisional declarations. They may furthermore also contain independent inventions which have a configuration that is independent of the subject matters of the preceding dependent claims. Further, elements and/or features of different example embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. Still further, any one of the above-described and other example features of the present invention may be embodied in the form of an apparatus, method, system, computer program, computer readable medium and computer program product. For example, of the aforementioned methods may be embodied in the form of a system or device, including, but not limited to, any of the structure for performing the methodology illustrated in the drawings. Even further, any of the aforementioned methods may be embodied in the form of a program. The program may be stored on a computer readable medium and is adapted to perform any one of the aforementioned methods when run on a computer device (a device including a processor). Thus, the storage medium or computer readable medium, is adapted to store information and is adapted to interact with a data processing facility or computer device to execute the program of any of the above mentioned embodiments and/or to perform the method of any of the above mentioned embodiments. The computer readable medium or storage medium may be a built-in medium installed inside a computer device main body or a removable medium arranged so that it can be separated from the computer device main body. Examples of the built-in medium include, but are not limited to, rewriteable non-volatile memories, such as ROMs and flash memories, and hard disks. Examples of the removable medium include, but are not limited to, optical storage media such as CD-ROMs and DVDs; magneto-optical storage media, such as MOs; magnetism storage media, including but not limited to floppy disks (trademark), cassette tapes, and removable hard disks; media with a built-in rewriteable non-volatile memory, including but not limited to memory cards; and media with a built-in ROM, including but not limited to ROM cassettes; etc. Furthermore, various information regarding stored images, for example, property information, may be stored in any other form, or it may be provided in other ways. Example embodiments being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. |
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description | This application claims the benefit under 35 USC 119 of the following prior filed provisional applications: 60/393,116, filed Jul. 3, 2002; and 60/400,047, filed Aug. 2, 2002; and is a continuation of nonprovisional application: Ser. No. 10/431,573 filed May 8, 2003 which has been allowed but has not yet issued. This invention relates to a vibratory transducer which is particularly suited for use in a viscometer, a viscometer-densimeter, or a viscometer-mass flowmeter. To determine the viscosity of a liquid flowing in a pipe, use is frequently made of meters which, using a vibratory transducer, comprising a flow tube communicating with the pipe, and control and evaluation electronics connected thereto, induce shear or friction forces in the fluid and derive therefrom a measurement signal representing the viscosity. U.S. Pat. No. 4,524,610, U.S. Pat. No. 5,253,533, U.S. Pat. No. 6,006,609, or EP-A 1 158 289, for example, disclose in-line viscometers, i.e., viscometers connectable into a fluid-conducting pipe, with a vibratory transducer which responds to the viscosity of the fluid flowing in the pipe and comprises: a single straight flow tube for conducting the fluid which vibrates in operation and communicates with the pipe via an inlet tube section and an outlet tube section; an excitation assembly which in operation excites at least part of the flow tube into torsional vibrations about an axis of vibration aligned with the flow tube; and a sensor arrangement for locally sensing vibrations of the flow tube. As is well known, straight flow tubes, when excited into torsional vibrations about an axis aligned with the flow tube, cause shear forces to be produced in the fluid flowing through the tube, whereby vibrational energy is removed from the torsional vibrations and dissipated to the fluid. This results in the torsional vibrations of the flow tube being damped, so that additional excitation energy must be supplied to the flow tube to maintain those vibrations. The applied excitation energy can be measured in a suitable manner, and the viscosity of the fluid can be derived therefrom. In operation, the flow tubes of such transducers, which are used in in-line viscometers, for example, are generally excited at an instantaneous resonance frequency of a fundamental torsional mode, particularly with the vibration amplitude maintained at a constant value. It is also common practice to excite the flow tubes for viscosity measurements, simultaneously or alternately with the torsional mode, into flexural vibrations, preferably at a resonance frequency of a fundamental flexural mode, see also the above referred to U.S. Pat. No. 4,524,610. Since this flexural resonance frequency is also dependent on the instantaneous density of the fluid in particular, such meters can also be used to measure the density of fluids flowing in pipes. Compared with the use of bent flow tubes for viscosity measurements, the use of straight flow tubes vibrating in the manner described above, as is well known, has the advantage that shear forces are induced in the fluid over virtually the entire length of the flow tube, particularly with a great depth of penetration in the radial direction, so that very high sensitivity of the transducer to the viscosity to be measured can be achieved. Another advantage of straight flow tubes is that they can be drained residue-free with a high degree of reliability in virtually any position of installation, particularly after a cleaning operation performed in-line. Furthermore, such flow tubes are much easier and, consequently, less expensive to manufacture than, for example, an omega-shaped or helically bent flow tube. An essential disadvantage of the prior art transducers lies in the fact that in operation, torsional vibrations can be transmitted from the transducer via the flow tube and any transducer case that may be present to the connected pipe. This, in turn, may result in a zero shift and, thus, in measurement inaccuracies. Furthermore, the loss of vibrational energy to the transducer's environment may result in a substantial deterioration of efficiency and possibly also in a degradation of the signal-to-noise ratio in the measurement signal. It is therefore an object of the invention to provide a vibratory transducer which is particularly suited for a viscometer and which in operation, even if it uses only a single, particularly straight, flow tube, is dynamically well balanced over a wide fluid density range and nevertheless has comparatively little mass. To attain the object, the invention provides a vibratory transducer for a fluid flowing in a pipe, the transducer comprising a flow tube for conducting the fluid which in operation vibrates at a predeterminable frequency, an excitation assembly acting on the flow tube for vibrating the flow tube, a sensor arrangement for sensing vibrations of the flow tube, and a torsional vibration absorber fixed to the flow tube. The flow tube communicates with the pipe via an inlet tube section, ending in an inlet end of the flow tube, and via an outlet tube section, ending in an outlet end of the flow tube. Primarily in order to produce shear forces in the fluid, in operation, the flow tube performs, at least intermittently, torsional vibrations about a longitudinal flow-tube axis at an instantaneous torsional frequency. To reduce or avoid the dissipation of vibrational energy from the transducer to the connected pipe, in operation, the torsional vibration absorber is a least partially vibrated out of phase with the vibrating flow tube. In a first embodiment of the invention, the vibrating torsional vibration absorber is driven only by the vibrating flow tube. In a second embodiment of the invention, the torsional vibration absorber is fixed to the flow tube on the inlet and outlet sides. In a third embodiment of the invention, the torsional vibration absorber has a torsional natural frequency greater than 0.8 times the vibration frequency of the flow tube. In a fourth embodiment of the invention, the torsional vibration absorber has a torsional natural frequency less than 1.2 times the vibration frequency of the flow tube. In a fifth embodiment of the invention, the torsional vibration absorber is formed by an inlet-side absorber subunit and an outlet-side absorber subunit. In a sixth embodiment of the invention, the transducer comprises a transducer case coupled to the flow tube on the inlet and outlet sides. In a seventh embodiment of the invention, the transducer comprises an antivibrator fixed to the flow tube at the inlet and outlet ends, particularly an antivibrator coaxial with the flow tube. In an eighth embodiment of the invention, additional masses are provided on the flow tube. A basic idea of the invention is to dynamically balance torques developed by the torsionally vibrating flow tube with equal reactive torques generated by means of the torsional vibration absorber, which, for example, may be driven only by the flow tube. One advantage of the invention lies in the fact that the transducer, despite possible operational variations in the density and/or viscosity of the fluid, is balanced in a simple and robust manner such that internal torques can be largely kept away from the connected pipe. Another advantage is that as a result of this constructionally very simple vibration isolation, the transducer according to the invention can be made very compact and very light. While the invention is susceptible to various modifications and alternative forms, exemplary embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the the particular forms diclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the intended claims. FIG. 1 shows a meter designed to be inserted in a pipe (not shown) for measuring the viscosity of a fluid flowing in the pipe. In addition, the meter may also designed to measure the mass flow rate and/or the density of the fluid. It comprises a vibratory transducer through which the fluid to be measured flows in operation. FIGS. 2 to 6 show schematically embodiments and developments of such vibratory transducers. To conduct the fluid, the transducer comprises an essentially straight flow tube 10, particularly a single tube, which in operation performs, at least intermittently, torsional vibrations about its longitudinal axis and is thus repeatedly elastically deformed. To permit flow of fluid through flow tube 10, the latter is connected to a fluid-conducting pipe (not shown) via an inlet tube section 11 and an outlet tube section 12. Advantageously, flow tube 10, inlet tube section 11, and outlet tube section 12, which are aligned with each other and with an imaginary longitudinal axis L, are integrally formed, so that a single tubular semifinished product, for example, can be used for their manufacture; if necessary, however, flow tube 10 and tube sections 11, 12 may also be made from separate semifinished products that are subsequently joined together, for instance welded together. For flow tube 10, virtually any of the materials commonly used for such transducers, e.g., steel, titanium, zirconium, etc., may be used. If the transducer is to be nonpermanently connected with the pipe, a first flange 13 and a second flange 14 may formed on inlet tube section 11 and outlet tube section 12, respectively; if necessary, however, inlet and outlet tube sections 11, 12 may also be connected with the pipe directly, for instance by welding or brazing. Furthermore, as shown schematically in FIG. 1, an external support system 100, here shown in the form of a transducer case receiving or enclosing the flow tube 10, is fixed to inlet and outlet tube sections 11, 12, see FIGS. 1 and 3. To produce friction forces in the fluid that correspond to the viscosity of the fluid, in operation, flow tube 10 is at least intermittently excited into torsional vibrations, particularly in the range of a torsional natural resonance frequency, such that it is twisted about its longitudinal axis L essentially according to a torsional natural vibration mode shape, cf., for instance, U.S. Pat. No. 4,524,610, U.S. Pat. No. 5,253,533, U.S. Pat. No. 6,006,609, or EP-A 1 158 289. The flow tube 10 is excited at a torsional frequency fexcT corresponding as exactly as possible to a natural resonance frequency of that fundamental torsional eigenmode in which flow tube 10 is twisted essentially unidirectionally over its entire length. In the case of a flow tube 10 of special steel with a nominal diameter of 20 mm, a wall thickness of about 1.2 mm, and a length of about 350 mm and with attachments (see below), a natural resonance frequency of this fundamental torsional eigenmode may be of the order of about 1500 to 2000 Hz, for example. In an embodiment of the invention, during operation of the transducer, flow tube 10, in addition to being excited into torsional vibrations, is excited, particularly simultaneously therewith, into flexural vibrations in such a way as to be deflected essentially according to a natural first flexural vibration mode shape. To this end, flow tube 10 is excited at a flexural vibration frequency corresponding as exactly as possible to a lowest natural flexural resonance frequency of flow tube 10, so that the vibrating, but empty flow tube 10, as shown schematically in FIGS. 7a and 7b, is deflected essentially symmetrically with respect to a central axis perpendicular to the longitudinal axis and has a single antinode. In the case of a flow tube 10 of special steel with a nominal diameter of 20 mm, a wall thickness of about 1.2 mm, and a length of about 350 mm as well as with the usual attachments, for example, this lowest flexural resonance frequency may be of the order of about 850 to 900 Hz. When a fluid flows through the pipe, so that the mass flow rate m is nonzero, Coriolis forces are induced in the fluid by flow tube 10 vibrating in a flexural mode. The Coriolis forces react on flow tube 10, thus causing an additional deformation (not shown) of flow tube 10 according to a natural second flexural vibration mode shape, which is coplanar with the first flexural vibration mode shape. The instantaneous shape of the deformation of flow tube 10, particularly in regard to its amplitudes, is also dependent on the instantaneous flow rate m. The second flexural vibration mode shape, the so-called Coriolis mode, may be, for instance, an asymmetric flexural vibration mode shape with two or four antinodes, as is usual with such transducers. To generate mechanical vibrations of flow tubes 10, the transducer further comprises an excitation assembly 40, particularly an electrodynamic exciter. Excitation assembly 40 serves to convert electric excitation energy Eexc supplied from control electronics (not shown), for instance with a regulated current and/or a regulated voltage, into an excitation moment Mexc which acts on flow tube 10, for instance in a pulsed manner or harmonically, and elastically deforms the tube in the manner described above, and, if flow tube 10 is additionally excited into flexural vibrations, into a laterally acting excitation force. The excitation moment Mexc may be bidirectional as shown schematically in FIG. 4 or 6, or unidirectional, and be adjusted in amplitude, for instance by means of a current-and/or voltage-regulator circuit, and in frequency, for instance by means of a phase-locked loop, in the manner familiar to those skilled in the art. From the electric excitation energy Eexc necessary to maintain the torsional vibrations and the contingently additionally excited flexural vibrations of flow tube 10, the viscosity of the fluid can be derived in the manner familiar to those skilled in the art, cf. in particular U.S. Pat. No. 4,524,610, U.S. Pat. No. 5,253,533, U.S. Pat. No. 6,006,609, or EP-A 1 158 289. The excitation assembly 40 may be, for example, a simple solenoid with a cylindrical excitation coil which is attached to the transducer case 100 and, in operation, is traversed by a suitable excitation current, and with a permanent magnetic armature which is fixed to the outside flow tube 10, particularly at the midpoint thereof, and rides, at least in part, in the excitation coil. Excitation assembly 40 can also be implemented with one or more electromagnets as shown in U.S. Pat. No. 4,524,610, for example. To detect vibrations of flow tube 10, a sensor system as is commonly used for such transducers may be employed which senses the motions of flow tube 10, particularly on the inlet and outlet sides thereof, by means of at least a first sensor 51, but contingently also by means of a second sensor 52, and converts them into corresponding sensor signals S1, S2. Sensors 51, 52 may be, for example, electrodynamic velocity sensors as shown schematically in FIG. 1, which perform relative vibration measurements, or electrodynamic displacement sensors or acceleration sensors. Instead of electrodynamic sensor systems, sensor systems using resistive or piezoelectric strain gages or optoelectronic sensor systems may be used to detect the vibrations of flow tube 10. As mentioned above, on the one hand, the torsional vibrations are damped by a desired energy loss to the fluid, which is sensed, particularly for the purpose of measuring viscosity. On the other hand, however, vibrational energy may also be removed from the vibrating flow tube 10 if components mechanically coupled to the flow tube, such as transducer case 100 or the connected pipe, are also excited into vibration. While the energy loss to case 100, even though undesired, could still be calibrated, at least the energy loss to the transducer's environment, particularly to the pipe, occurs in a practically nonreproducible or even unpredeterminable manner. To suppress such a loss of torsional vibration energy to the environment, the transducer comprises a torsional vibration absorber 60, which is fixed to flow tube 10 on the inlet and outlet sides. According to the invention, torsional vibration absorber 60 serves to absorb at least part of the torsional vibration energy lost by the single flow tube 10 being twisted about its longitudinal axis L, thus keeping this energy away from the transducer's environment, particularly from the pipe connected to the transducer. To this end, at least one of the torsional resonance frequencies of the torsional vibration absorber is tuned as precisely as possible to the torsional frequency fexcT, at which the flow tube 10 is predominant-ly vibrated in operation. As a result, at least portions of the torsional vibration absorber 60 perform torsional vibrations which are out of phase with, particularly opposite in phase to, torsional vibrations of flow tube 10. In addition, the torsional vibration absorber tuned in this way is fixed to flow tube 10 in such a manner that even with the absorber caused to vibrate, particularly in phase opposition to flow tube 10, the inlet tube section and the outlet tube section are substantially free of torsional stress. The use of such a torsional vibration absorber is predicated particularly on recognition that the flow tube 10, vibrated in the above-described manner, has at least one torsional resonance frequency which, in contrast to its flexural resonance frequencies, for example, is correlated with the density or viscosity of the fluid only to a very small degree, and which can thus be maintained substantially constant in operation. Accordingly, at least one of the torsional resonance frequencies of such a torsional vibration absorber can be tuned comparatively precisely to the torsional resonance frequency of the flow tube to be expected in operation. At least for the above-described case where excitation assembly 40 is connected with flow tube 10 and transducer case 100, the vibrating torsional vibration absorber is driven indirectly, namely virtually exclusively by the vibrating flow tube 10. As shown in FIG. 3 or 6, in yet another embodiment of the invention, torsional vibration absorber 60 comprises a first rotator 61A of predeterminable moment of inertia, coupled to flow tube 10 via a first torsion-spring body 61B of predeterminable torsional rigidity, and a second rotator 62A of predeterminable moment of inertia, coupled to flow tube 10 via a second torsion-spring body 62B of predeterminable torsional rigidity. Torsion-spring bodies 61A, 61B may be formed from thick-walled, short metal rings of suitable mass, for example, while for torsion-spring bodies 61B, 62B, short, comparatively thin-walled metal tube lengths may be used whose length, wall thickness, and cross section are so chosen that the required torsional rigidity is achieved. For the case shown here, where the two rotators 61A, 62A, which are disposed symmetrically with respect to the midpoint of the flow tube 10, are not rigidly connected with one another, torsional vibration absorber 60 is formed by an inlet-side first absorber subunit 61 and an outlet-side second absorber subunit 62. If necessary, the two rotators may additionally be coupled directly, namely rigidly or flexibly. Consequently, rotators 61A, 62A may also be formed by a single tube enclosing the flow tube 10 and fixed to the latter by means of the two torsion springs 61B, 62B in the manner described above. For the manufacture of the two absorber subunits 61, 62, virtually the same materials as those suitable for flow tube 10 may be used, i.e., special steel, for example. In still another embodiment of the invention, the two absorber subunits 61, 62, as shown schematically in FIGS. 7a and 7b, are formed in the manner of a cantilever and are so disposed in the transducer that a centroid M61 of the inlet-side absorber subunit and a centroid M62 of the outlet-side absorber subunit are spaced from flow tube 10, particularly in line with the flow tube. In this manner, mass moments of inertia applied at the respective fixing points, namely at an inlet end 11# and an outlet end 12#, eccentrically, i.e., not at the associated centroids M61 and M62, can be developed by means of the two absorber subunits 61, 62. This has particularly the advantage that for the case where flow tube 10, as mentioned above, is vibrated in a flexural mode, laterally acting inertial forces can be at least partially balanced, cf. in particular applicant's international Patent Application PCT/EP02/02157, which was not published prior to the filing data of the present application. According to a further development of the invention, in order to further minimize disturbing effects on flow tube 10, torsional vibration absorber 60 comprises an antivibrator 20 extending essentially parallel to flow tube 10. Conversely, the loss of torsional vibration energy to the connected pipe is further reduced by means of antivibrator 20. Antivibrator 20 may be in the form of a tube, as shown schematically in FIGS. 2 and 3, and be so connected to flow tube 10 at the inlet end 11# and outlet end 12# as to be essentially coaxial with flow tube 10, as shown in FIG. 3. Materials suitable for antivibrator 20 are virtually the same as those that can be used for flow tube 10, i.e., special steel, titanium, etc. In this development of the invention, excitation assembly 40, as shown in FIG. 2, is advantageously so designed and so positioned in the transducer as to act on flow tube 10 and antivibrator 20 simultaneously, particularly differentially. To this end, in the embodiment shown in FIG. 4, excitation assembly 40 has at least a first excitation coil 41a, which in operation is at least intermittently traversed by the excitation current or a partial excitation current, and which is fixed to a lever 41c connected with flow tube 10 and acts differentially on flow tube 10 and antivibrator 20 via this lever 41c and an armature 41b fixed to the outside of antivibrator 20. One advantage of this arrangement is that the cross section of antivibrator 20, and hence the cross section of transducer case 100, is kept small while excitation coil 41a is easily accessible, particularly during assembly. Another advantage of this design of excitation assembly 40 is thant any cup cores 41d used, which are not negligibly heavy particularly with nominal diameter above 80 mm, can also be fixed to antivibrator 20 and thus have virtually no effect on the resonance frequencies of flow tube 10. At this point it should be noted, however, that, if necessary, it is also possible to fix excitation coil 41a to antivibrator 20, and armature 41b to flow tube 10. Correspondingly, sensor arrangement 50 may be so designed and so positioned in the transducer that the vibrations of flow tube 10 and antivibrator 20 are sensed differentially. In the embodiment shown in FIG. 5, sensor arrangement 50 comprises a sensor coil 51a which is fixed to flow tube 10 outside all principal axes of inertia of sensor arrangement 50. Sensor coil 51a is positioned as closely as possible to an armature 51b fixed to antivibrator 20 and is so magnetically coupled to the latter that a variable measurement voltage influenced by rotational and/or lateral relative motions between flow tube 10 and antivibrator 20 is induced in the sensor coil. With the sensor coil 51a positioned in this way, both the above-mentioned torsional vibrations and the optionally excited flexural vibrations can be sensed simultaneously in an advantageous manner. If necessary, however, it is also possible to fix sensor coil 51a to antivibrator 20, and armature 51b, which is coupled to sensor coil 51a, to flow tube 10. For the above-described case where in operation, flow tube 10 is additionally excited into flexural vibrations, antivibrator 20 may further serve to dynamically balance the transducer for a specified fluid density value, for example a value most frequently expected during operation of the transducer or a critical value, to the point that any transverse forces produced in the vibrating flow tube 10 are at least intermittently completely balanced, so that flow tube 10 will practically not leave its static rest position, cf. FIGS. 7a and 7b. Accordingly, in operation, antivibrator 20, as shown schematically in FIG. 7b, is also excited into flexural vibrations, which are essentially coplanar with the flexural vibrations of flow tube 10. In another embodiment of the invention, a lowest torsional resonance frequency of torsional vibration absorber 60 is not greater than 1.2 times the torsional resonance frequency of flow tube 10. In a further embodiment of the invention, a lowest torsional resonance frequency of the torsional vibration absorber is not less than 0.8 times the torsional resonance frequency of flow tube 10. In yet another embodiment of the invention, antivibrator 20 has a lowest torsional resonance frequency f20 which is different from the respective torsional resonance frequencies f61, f62 of absorber subunits 61, 62. The torsional resonance frequency f20 of antivibrator 20 may chosen to be essentially equal to the torsional frequency fexcT at which the flow tube 10 is excited in operation. This results in flow tube 10 and antivibrator 20 vibrating torsionally out of phase with each other, namely essentially in phase opposition. At least for this case, antivibrator 20 advantageously has a torsional rigidity or torsional elasticity similar or equal to that of flow tube 10. However, it has also proved to be advantageous if the respective torsional resonance frequencies f61, f62 of the two absorber subunits 61, 62 are chosen to be essentially equal to the torsional frequency fexcT. For that case, the torsional resonance frequency f20 of antivibrator 20 will advantageously be chosen to lie below or above the expected torsional vibration fexcT. If necessary, antivibrator 20 may also be of multipart construction, as shown, for example, in U.S. Pat. No. 5,969,265, EP-A 317 340, or WO-A 00/14485, or be implemented with two separate antivibrators fixed to flow tube 10 at the inlet and outlet ends, see FIG. 6. Particularly for this case, in which antivibrator 20, serving as an internal support system, is formed by an inlet-side and an outlet-side antivibrator subunit, the external support system 100, too, may be of two-part construction, consisting of an inlet-side and an outlet-side subsystem, cf. FIG. 6. According to a further development of the invention, as shown schematically in FIG. 3, counterbalance bodies 101, 102 are provided which, fixed to flow tube 10, permit precise setting of its torsional resonance frequencies and, thus, improved matching to the signal processing circuitry. The counterbalance bodies 101, 102 may be, for instance, metal rings slipped over flow tube 10 or metal plates fixed to the flow tube. According to yet another development of the invention, as shown schematically in FIG. 3, grooves 201, 202 are provided in antivibrator 20 which make it possible in a simple manner to precisely set the antivibrator's torsional resonance frequencies, particularly to lower the torsional resonance frequencies by lowering the torsional rigidity of the antivibrator. While the grooves 201, 202 in FIG. 2 or 3 are shown essentially evenly distributed in the direction of the longitudinal axis L, they may also be unevenly distributed in the direction of this axis if necessary. As is readily apparent from the above explanations, the transducer according to the invention is characterized by a multitude of possible settings which enable those skilled in the art, particularly even after specification of external and internal mounting dimensions, to achieve high-quality balancing of torsional forces produced in flow tube 10 and in antivibrator 20, and hence to minimize the loss of torsional vibration energy to the environment of the transducer. While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description is to be considered as exemplary not restrictive in character, it being understood that only exemplary embodiments have been shown and described and that all changes and modifications that come within the spirit and scope of the invention as described herein are desired to protected. |
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040615340 | abstract | A nuclear reactor cooled by a freezable liquid has a vessel for containing said liquid and comprising a structure shaped as a container, and cooling means in the region of the surface of said structure for effecting freezing of said liquid coolant at and for a finite distance from said surface for providing a layer of frozen coolant on and supported by said surface for containing said liquid coolant. In a specific example, where the reactor is sodium-cooled, the said structure is a metal-lined concrete vault, cooling is effected by closed cooling loops containing NaK, the loops extending over the lined surface of the concrete vault with outward and reverse pipe runs of each loop separated by thermal insulation, and air is flowed through cooling pipes embedded in the concrete behind the metal lining. |
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abstract | Method of fabricating a fuel rod, comprising providing an effective amount of a metal oxide in the fuel rod to generate steam and mitigate the tendency for secondary hydriding. Fuel rods fabricated according to the method of the invention are also provided. |
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claims | 1. An apparatus for inspecting overlapping figures comprising:a chip overlap inspection unit configured to input a data file on each chip of a plurality of chips arranged in a writing pattern, and inspect an existence of an overlap between each region of a plurality of chips, based on arrangement data on each region of the plurality of chips, wherein the plurality of chips are different chips to be written;a setting unit configured to set, with respect to the plurality of chips, a plurality of hierarchies and a plurality of cell regions of each of the plurality of hierarchies;an extraction unit configured to extract, with respect to a plurality of chips where the overlap occurs, a cell region where the overlap is located, from a higher hierarchy level to a lower hierarchy level in order, wherein the plurality of chips where the overlap occurs are different chips to be written, the cell region extracted is included in one of the plurality of chips where the overlap occurs and another cell region extracted is included in the other one of the plurality of chips where the overlap occurs;a figure overlap judging unit configured to judge an existence of an overlap between a figure in the cell region extracted and a figure in the other cell region extracted; andan output unit configured to output data on a plurality of figures overlapping. 2. An apparatus for inspecting overlapping figures comprising:a chip overlap inspection unit configured to input a file of data on each chip of a plurality of chips arranged in a writing pattern, and inspect an existence of an overlap between each region of a plurality of chips, based on arrangement data on each region of the plurality of chips, wherein the plurality of chips are different chips to be written;a setting unit configured to set, with respect to the plurality of chips, a plurality of cell regions for each hierarchy of the data on the each chip;an extraction unit configured to extract, with respect to a plurality of chips where the overlap occurs, a cell region where the overlap is located, from a higher hierarchy level to a lower hierarchy level in order, wherein the plurality of chips where the overlap occurs are different chips to be written, the cell region extracted is included in one of the plurality of chips where the overlap occurs and another cell region extracted is included in the other one of the plurality of chips where the overlap occurs;a figure overlap judging unit configured to judge an existence of an overlap between a figure in the cell region extracted and a figure in the other cell region extracted; andan output unit configured to output data on a plurality of figures overlapping. 3. An apparatus for inspecting overlapping figures comprising:a chip overlap inspection unit configured to input a data file on each chip of a plurality of chips which are arranged in a writing pattern and in which a plurality of hierarchies and a plurality of cell regions for each of the plurality of hierarchies are defined in such a manner that the plurality of cell regions become smaller in order according to the plurality of hierarchies, and to inspect an existence of an overlap between each region of a plurality of chips, based on arrangement data on each region of the plurality of chips, wherein the plurality of chips are different chips to be written;an extraction unit configured to extract, with respect to a plurality of chips where the overlap occurs, a cell region where the overlap is located, from a higher hierarchy level to a lower hierarchy level in order, wherein the plurality of chips where the overlap occurs are different chips to be written, the cell region extracted is included in one of the plurality of chips where the overlap occurs and the another cell region extracted is included in the other one of the plurality of chips where the overlap occurs;a figure overlap judging unit configured to judge an existence of an overlap between a figure in the cell region extracted and a figure in the other cell region extracted; andan output unit configured to output data on a plurality of figures overlapping. 4. A charged particle beam writing apparatus comprising:a chip overlap inspection unit configured to input a data file on each chip of a plurality of chips arranged in a writing pattern, and inspect an existence of an overlap between each region of a plurality of chips, based on arrangement data on each region of the plurality of chips, wherein the plurality of chips are different chips to be written;a setting unit configured to set, with respect to the plurality of chips, a plurality of hierarchies and a plurality of cell regions of each of the plurality of hierarchies;an extraction unit configured to extract, with respect to a plurality of chips where the overlap occurs, a cell region where the overlap is located, from a higher hierarchy level to a lower hierarchy level in order, wherein the plurality of chips where the overlap occurs are different chips to be written, the cell region extracted is included in one of the plurality of chips where the overlap occurs and another cell region extracted is included in the other one of the plurality of chips where the overlap occurs;a figure overlap judging unit configured to judge an existence of an overlap between a figure in the cell region extracted and a figure in the other cell region extracted;an output unit configured to output data on a plurality of figures overlapping; anda writing unit configured to write a writing pattern in which a plurality of chips having no overlap in a figure hierarchy are arranged, onto a target workpiece by using a charged particle beam. 5. A charged particle beam writing apparatus comprising:a chip overlap inspection unit configured to input a file of data on each chip of a plurality of chips arranged in a writing pattern, and inspect an existence of an overlap between each region of a plurality of chip regions, based on arrangement data on each region of the plurality of chips, wherein the plurality of chips are different chips to be written;a setting unit configured to set, with respect to the plurality of chips, a plurality of cell regions for each hierarchy of the data on the each chip;an extraction unit configured to extract, with respect to a plurality of chips where the overlap occurs, a cell region where the overlap is located, from a higher hierarchy level to a lower hierarchy level in order, wherein the plurality of chips where the overlap occurs are different chips to be written, each other, the cell region extracted is included in one of the plurality of chips where the overlap occurs and another cell region extracted is included in the other one of the plurality of chips where the overlap occurs;a figure overlap judging unit configured to judge an existence of an overlap between a figure in the cell region extracted and a figure in the other cell region extracted;an output unit configured to output data on a plurality of figures overlapping; anda writing unit configured to write a writing pattern in which a plurality of chips having no overlap in a figure hierarchy are arranged, onto a target workpiece by using a charged particle beam. 6. A charged particle beam writing apparatus comprising:a judging unit configured to input a data file on each chip of a plurality of chips which are arranged in a writing pattern and in which a plurality of hierarchies and a plurality of cell regions for each of the plurality of hierarchies are defined in such a manner that the plurality of cell regions become smaller in order according to the plurality of hierarchies, and to judge an existence of an overlap between each region of a plurality of chip regions, based on arrangement data on each region of the plurality of chips, wherein the plurality of chips are different chips to be written;an extraction unit configured to extract, with respect to a plurality of chips where the overlap occurs, a cell region where the overlap is located, from a higher hierarchy level to a lower hierarchy level in order, wherein the plurality of chips where the overlap occurs are different chips to be written, the cell region extracted is included in one of the plurality of chips where the overlap occurs and another cell region extracted is included in the other one of the plurality of chips where the overlap occurs;a figure overlap judging unit configured to judge an existence of an overlap between a figure in the cell region extracted and a figure in the other cell region extracted;an output unit configured to output data on a plurality of figures overlapping; anda writing unit configured to write a writing pattern in which a plurality of chips having no overlap in a figure hierarchy are arranged, onto a target workpiece by using a charged particle beam. 7. A method for inspecting overlapping figures comprising:inputting a data file on each chip of a plurality of chips arranged in a writing pattern, and inspecting an existence of an overlap between each region of a plurality of chips, based on arrangement data on each region of the plurality of chips, wherein the plurality of chips are different chips to be written;setting, with respect to the plurality of chips, a plurality of hierarchies and a plurality of cell regions of each of the plurality of hierarchies;extracting, with respect to a plurality of chips where the overlap occurs, a cell region where the overlap is located, from a higher hierarchy level to a lower hierarchy level in order; wherein the plurality of chips where the overlap occurs are different chips to be written, the cell region extracted is included in one of the plurality of chips where the overlap occurs and another cell region extracted is included in the other one of the plurality of chips where the overlap occurs;judging, using hardware or a programmed computer, an existence of an overlap between a figure in the cell region extracted and a figure in the other cell region extracted; andoutputting data on a plurality of figures overlapping. 8. A method for inspecting overlapping figures comprising:inputting a file of data on each chip of a plurality of chips arranged in a writing pattern, and inspecting an existence of an overlap between each region of a plurality of chips, based on arrangement data on each region of the plurality of chips, wherein the plurality of chips are different chips to be written;setting, with respect to the plurality of chips, a plurality of cell regions for each hierarchy of the data on the each chip;extracting, with respect to a plurality of chips where the overlap occurs, a cell region where the overlap is located, from a higher hierarchy level to a lower hierarchy level in order, wherein the plurality of chips where the overlap occurs are different chips to be written, the cell region extracted is included in one of the plurality of chips where the overlap occurs and another cell region extracted is included in the other one of the plurality of chips where the overlap occurs;judging, using hardware or a programmed computer, an existence of an overlap between a figure in the cell region extracted and a figure in the other cell region extracted; andoutputting data on a plurality of figures overlapping. 9. A method for inspecting overlapping figures comprising:inputting a data file on each chip of a plurality of chips which are arranged in a writing pattern and in which a plurality of hierarchies and a plurality of cell regions for each of the plurality of hierarchies are defined in such a manner that the plurality of cell regions become smaller in order according to the plurality of hierarchies, and judging, using hardware or a programmed computer, an existence of an overlap between each region of a plurality of chip regions, based on arrangement data on each region of the plurality of chips, wherein the plurality of chips are different chips to be written;extracting, with respect to a plurality of chips where the overlap occurs, a cell region where the overlap is located, from a higher hierarchy level to a lower hierarchy level in order, wherein the plurality of chips where the overlap occurs are different chips to be written, the cell region extracted is included in one of the plurality of chips where the overlap occurs and another cell region extracted is included in the other one of the plurality of chips where the overlap occurs;judging an existence of an overlap between a figure in the cell region extracted and a figure in the other cell region extracted; andoutputting data on a plurality of figures overlapping. |
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043022910 | summary | BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to nuclear power generating plant structures, and more particularly, to a totally submersible platform and containment system for a nuclear power generating plant. 2. Description of the Prior Art Large nuclear power generating plants are currently sited on land near a water supply necessary for use in evaporative cooling of their associated steam condensers. Competitive land and water uses are making it extremely difficult to procure new land sites for nuclear generating plants and their transmission systems. The unique features of each land site also make it difficult to introduce standardization and "mass" production methods into nuclear power plant design so as to benefit from assembly line production techniques. Particular difficulties with land-sited nuclear power generating plants are: 1. engineering and social conflicts due to the safety and aesthetic characteristics of nuclear power generating plants; PA1 2. the environmental impact of nuclear power generating plants on the water resources in their locale due to their need to reject heat; and PA1 3. the need for isolation of nuclear power generating plants from potential ground motions resulting from seismic events in the vicinity of the plant location. One possible solution to these difficulties can be effected by placing the plant underground. Several recent studies of this type of siting have indicated that the cost penalties involved do not warrant the potential benefits. The studies also suggest a requirement for necessary geological siting criteria which may not exist in many locations. Also, underground siting does not provide in itself for seismic isolation. Another possible solution to land siting of nuclear power generating plants, particularly for situations where electrical energy use centers are located near the ocean, is the employment of offshore surface mooring of a floating nuclear power plant, such as is incidentially shown in PERRY, et al. (U.S. Pat. No. 3,794,849). However, obvious major problems arise in such a design from meteorological, surface, and artificial causes. An attractive alternative to either land siting or offshore surface mooring of a nuclear power plant is the employment of an underwater mooring site. Such a location would protect the plant from adverse meteorological and surface conditions, as well as provide greater isolation from potential ground motions due to seismic activity. Further, undersea siting of nuclear power plants would minimize the danger of theft or sabotage of nuclear materials, since special equipment is required in order to gain access to the submerged plant. Further still, such siting offers several engineering advantages over alternative methods. For example, all nuclear reactors of the water-cooled steam generator type require vast amounts of water to cool their associated steam condensers. For higher efficiency in generating electrical power, this cooling water should have as low a temperature as possible. Further, discharged cooling water should be remixed with its source as quickly and efficiently as possible in order to reduce any impact on the environment due to thermal pollution. Undersea siting of a nuclear power plant simply and easily meets the above criteria: the supply of cooling water is virtually unlimited, the water temperature at a depth of a few hundred feet is significantly lower than the surface temperature, and efficient remixing of effluent cooling water is readily achieved by convection currents. A further engineering advantage of undersea siting over alternate methods is the presence thereby of a virtually unlimited heat sink for the reactor in the event of a loss-of-coolant accident, thus preventing dangerous overheating of the reactor core. SCHANZ, (U.S. Pat. No. 3,115,450) discloses a nuclear reactor situated within a small spherical containment pressure vessel for siting on land, but this embodiment, unlike the present invention, is not believed to be designed to withstand the very large external pressures present at even a few hundred feet beneath a body of water. BRAY, (U.S. Pat. No. 3,118,818) also shows a submersible nuclear thermoelectric power plant for use in low power (approximately 50 kilowatts) situations, but this design is not believed to be feasibly expandible to the size (approximately 3400 megawatts) necessary to provide sufficient power to offset the cost of construction of such a plant. It should be appreciated that while spherical pressure vessels for nuclear reactors are described in the prior art, as in HAFTKE, (U.S. Pat. No. 3,087,883), and also that underwater pressure spheres are well known, as in MOUTON, JR., (U.S. Pat. No. 4,004,429), the prior art does not show the combination of elements of the present invention, as more fully discussed herein. In the present invention, the spheres used must not only be able to withstand the large external pressure of the surrounding body of water, but also the internal steam pressure that might occur in the event of a loss-of-coolant nuclear accident. A loss-of-coolant nuclear accident may occur when a failure happens somewhere in one or more of the coolant loops of a nuclear reactor. In such a situation, the water normally in and around the reactor is vaporized by the heat generated by the reactor, thereby increasing the internal pressure of the enclosing containment sphere tremendously. It should be further appreciated that although submersible sea platforms of various types are shown in the art, as in GIBSON, et al. (U.S. Pat. No. 3,486,343) and British Patent No. 963,083, the present invention shows a configuration of spherical pressure vessels housing a nuclear steam power generating plant mounted on a platform for total submergence which is unique. The structure of the present invention is particularly rigid, strong, and stable, and the preferred embodiment provides for direct access from any one pressure sphere to the adjacent spheres. It is therefore an object of this invention to provide a safe and convenient means to house a nuclear power generating plant beneath a body of water. A further objective is to isolate a nuclear power generating plant from potential ground motion resulting from seismic events in the vicinity. Another object is to increase the thermal efficiency of such a plant by submerging the plant at depths having a significantly colder temperature than does the surface. Still a further object of this invention is to provide for more efficient dissipation of the heat rejected from the plant's condensers, due to convection mixing of the effluent waters with the surrounding body of water. Yet another object is to reduce the demands on limited water supplies which a land-sited plant requires. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objectives and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which the presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limit of the invention. SUMMARY OF THE INVENTION The invention in its preferred embodiment comprises a triangular platform structure formed of three tubular platform members, or legs. To the midpoint of each leg is connected a tubular truss member which converges to the vertical axis of the platform, the truss member there being attached to a perpendicular axial tubular member, or kingpost. Each leg is strengthened by circumferential stiffening rings, and is provided with bulkhead-equipped compartments, thus permitting access to sections of a leg. Other portions of each leg are used as independent flotation, ballast, or auxiliary storage tanks. The three legs are joined together at the apexes of the platform triangle by ferroconcrete fittings, or elbows, which house jacking gear and pile support sleeves if a pile foundation is used to anchor the platform as hereinbelow described. Placed upon the three quadrilateral areas formed by the trussed triangular platform are three large interconnected spherical pressure vessels, each having an internal diameter of about 150 feet. The nested inner shell of each sphere is of 2 inch thick welded carbon steel, with strengthening ribs and stiffening rings providing additional structural support. The surrounding outer spheres are of 6.5 feet thick ferroconcrete. Each sphere is equipped with a top polar cap piercing the sphere and fitted with a transfer trunk, pressurized hatch, and a mating flange for connection to underwater service vessels. The spheres are constructed to withstand water pressure to a depth of 1500 feet with a safety factor of 2.5 at the maximum permitted depth. A first one of the pressure spheres houses a pressurized water nuclear reactor and its associated nuclear steam system, a nuclear refueling system, and a containment system. The containment system is comprised of a set of sub-systems designed to control and encompass the nuclear reactor and its by-products in the event of a nuclear mishap. This first sphere is specially reinforced and designed to withstand and contain the contents of the coolant system of a nuclear reactor if a loss of coolant accident occurs. A second of the spheres houses a steam turbine-generator unit, a reactor feed water purity control system, and reprocessing facilities for chemical reactor control additives. The steam turbine-generator unit derives its motivating power from the nuclear steam system housed in the first sphere. Associated with this second sphere are twin steam condenser units housed in the two platform legs upon which the second sphere rests. A third sphere contains the control and operating system for the entire plant, personnel living facilities, life support systems, emergency storage batteries, power transformers, workshops and auxilliary support systems necessary for normal and emergency plant operations. The life support system includes sub-systems for oxygen generation, carbon-dioxide elimination, atmosphere contaminant removal, waste disposal, food and water supplies, and heating, cooling, and ventilation. The three pressure spheres housing the various components of the nuclear power generating plant are interconnected with each other by means of bulk-headed pressure locks. The spheres also have bulk-headed pressure access hatches to the two platform legs upon which each rests. Also attached to the triangular platform are three secondary pressure vessels, cylindrical in shape. Affixed between the reactor sphere and the generating sphere is a double compartmented tank, the top half containing concentrated boric acid solution used for chemical reactor control, the bottom half holding emergency core cooling water. Another tank containing high purity make-up feed water for the reactor is situated between the reactor sphere and the support sphere. The remaining secondary pressure vessel is a fully sealable personnel airlock/decompression chamber affixed between the support sphere and the generating sphere. Used primarily for transferring personnel and supplies, this chamber can also serve as an emergency escape unit in the event of a major catastrophe. The entire platform is attached to the bottom of a body of water either by foundation piles or a cable mooring system in combination with dynamic positioning and stabilization of the platform itself. By the use of the structure of the present invention, problems with prior art designs are substantially overcome. |
claims | 1. An apparatus comprising:a pressurized water reactor (PWR) including a pressure vessel and a nuclear reactor core comprising fissile material disposed inside the pressure vessel at the bottom of the pressure vessel; anda secondary core containment structure including:a containment basket comprising zirconia insulation containing the bottom of the pressure vessel, the containment basket spaced apart from the bottom of the pressure vessel by a clearance gap, the containment basket having an open top located at an elevation above the top of the nuclear reactor core, anda radiological containment structure containing the PWR and the secondary core containment structure, the radiological containment structure having a sump in which the bottom of the pressure vessel is disposed; andconduits disposed between the containment basket and the bottom of the pressure vessel and having inlets disposed both above the top of the containment basket and in the sump and outlets inside the containment basket, so that water disposed in the sump flows into the conduits via the inlets and discharges into the containment basket via the outlets. 2. The apparatus of claim 1 wherein the clearance gap between the containment basket and the bottom of the pressure vessel is no larger than one meter. 3. The apparatus of claim 1 wherein the containment basket of the secondary core containment structure further includes an inner steel liner and an outer steel support structure with the zirconia insulation disposed between the inner steel liner and the outer steel support structure. 4. The apparatus of claim 3 wherein the outer steel support structure includes bottom supports via which the secondary core containment structure is bottom-supported on a floor of the sump of the radiological containment structure. 5. The apparatus of claim 4 further comprising:zirconia insulation boards disposed on the floor of the sump of the radiological containment structure. 6. The apparatus of claim 1 wherein the secondary core containment structure further includes a cylindrical collar comprising refractory fiber insulation, the cylindrical collar extending upward from the open top of the containment basket and spaced apart from the pressure vessel by a clearance gap. 7. The apparatus of claim 1 wherein the secondary core containment structure is effective to support the weight of a corium mass comprising the nuclear reactor core in the containment basket of the secondary core containment structure. 8. An apparatus comprising:a nuclear reactor including a pressure vessel and a nuclear reactor core comprising fissile material disposed inside the pressure vessel at the bottom of the pressure vessel; anda secondary core containment structure including a containment basket comprising insulation with a maximum stable temperature of at least 2200K cladded by steel;a radiological containment structure having a sump containing the nuclear reactor and the secondary core containment structure; andconduits disposed between the containment basket and the bottom of the pressure vessel and having inlets disposed both above the top of the containment basket and in the sump and outlets inside the containment basket, so that water disposed in the sump flows into the conduits via the inlets and discharges into the containment basket via the outlets,wherein the bottom of the pressure vessel and the nuclear reactor core are disposed inside the containment basket with the containment basket spaced apart from the bottom of the pressure vessel by a clearance gap, and the containment structure has an open top located at an elevation above a top of the nuclear reactor core. 9. The apparatus of claim 8 wherein the containment basket of the secondary core containment structure comprises zirconia insulation cladded by steel. 10. The apparatus of claim 9 wherein the clearance gap between the containment basket and the bottom of the pressure vessel is no larger than one meter. 11. The apparatus of claim 9 wherein the containment basket includes bottom supports via which the secondary core containment structure is bottom-supported on a floor of the sump of the radiological containment structure. 12. The apparatus of claim 9 further comprising:zirconia insulation disposed on the floor of the sump of the radiological containment structure beneath the nuclear reactor and the secondary core containment structure. 13. The apparatus of claim 9 wherein the secondary core containment structure further includes a cylindrical collar comprising refractory fiber insulation, the cylindrical collar extending upward from a lip of the containment basket. 14. The apparatus of claim 9 wherein the secondary core containment structure is effective to support the weight of a corium mass comprising the nuclear reactor core in the containment basket of the secondary core containment structure. 15. The apparatus of claim 9 wherein the secondary core containment structure further comprises pipes arranged to inject water into the clearance gap between the containment basket and the bottom of the pressure vessel. |
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050162675 | description | DETAILED DESCRIPTION OF THE INVENTION By way of example of the first aspect of the invention, the simple case of a parabolically curved micro-channel plate with parallel faces will now be considered, with reference to FIG. 1A. The example is confined to the case of x-rays. For mathematical convenience, certain simplifying assumptions shall be applied to this example, viz that: (i) the reflectivity of the channel walls is perfect (that is 100%) for x-rays incident on the walls at grazing angles up to the critical angle .gamma..sub.c for total external reflection; PA1 (ii) the thickness of the walls is negligible relative to the diameter of the channels; PA1 (iii) the focusing properties can be considered in one dimension at a time; PA1 (iii) the x-rays emanate isotropically from a point source, at least over the small solid angular ranges relevant to the effective angular apertures of the device; PA1 (v) the micro-channel plate consists of substantially parallel straight-walled channels perpendicular to the two parallel end faces of the plate; and PA1 (vi) at most single reflection occurs in the channels. PA1 1. They are more compact (e.g. 1 or 2 mm thick) than, say, single-bore glass x-ray guide tubes (e.g. 20 cm long) and can focus with much shorter focal lengths so that they may be incorporated with minimal modification of existing instruments and the air path can be shorter leading to lower absorption losses in the air; PA1 2. They are rigid with no moving parts in the device itself and are stable in an x-ray beam; PA1 3. They are quite efficient; PA1 4. They may be readily produced economically by mechanically bending of conventional micro-channel or micro-filament plates or can be moulded thermally to a wide variety of shapes in order to produce desired focusing properties in two or three dimensions; PA1 5. They also act as short wavelength filter, hence reducing harmonic contamination when used in conjunction with x-ray monochromators. PA1 6. Can produce focusing and collimation in 2-dimensions with a large effective angular aperture. PA1 7. Capable of producing very short focal lengths. For example, conventional plate glass mirrors have a minimum focal length of the order of 1 m, whereas the device of the invention can achieve a focal length of the order of 1 cm. PA1 8. Can allow for fine tuning of device in situ to optimize focusing properties. PA1 9. Can automatically provide collimation out of the focusing plane due to their action of fine Soller slits. PA1 10. Can be used to produce quasi-parallel beams from extended sources. Assuming ray optics, the x-ray focusing properties of a flat (i.e. uncurved) two-dimensional, lens device according to the first aspect of the invention are illustrated in FIG. 1A. It will be better appreciated from what follows that this and the other diagrams are not to scale and exaggerate the size of the channels for purposes of illustration. Micro-capillary plate 10 has multiple tubular channels 12 which are elongate and open-ended. A divergent beam 14 from source S is focused as convergent beam 16 by plate 10. The reflection efficiency E at a point y above the origin O is here defined as: ##EQU1## where .DELTA..phi..sup.ter and .DELTA..phi..sup.channel are respectively the angular apertures for total external reflection and for intercepting the cross-section of the channel at height y above the optic axis. Integrated reflectivity refers to the integral of expression (1) over the full effective angular aperture of the focusing collimator and is an angle in radian measure. For illustrative purposes, and as noted in part at assumption (vi) above, the effective angular aperture of the device may be considered to be limited by the minimum of the angle at which double reflection in the channel begins to become possible and the angle at which total external reflection at the channel wall no longer becomes possible. In practice the aperture will usually be limited by the value of .gamma..sub.c rather than by the single reflection condition. For a given value of .gamma..sub.c (i.e. choice of channel-wall material), the optimum efficiency of the focusing device within the single reflection condition is given by choosing ##EQU2## Calculations have been made for parameter values typical of the sorts of values which may be achieved for the devices in practice and which would be suitable (but not necessarily optimum) for achieving focusing. For example, the selected .gamma..sub.c value refers to quartz glass while the d/t value is typical of commonly available micro-channel plates. It has been found that integrated reflectivities of the order of 1 mrad in one-dimension are in principle possible with these parameter values (and 5 mrad if t/d were optimized in the manner described in (2) above). Integrated reflectivities of this order correspond to a flux increase of order 13 for Gelll Bragg reflection and CuK.alpha. radiation, if collimation is achieved to better than 15 seconds of arc. If a focusing distance l.sub.F is desired for a source distance on the other side of the plate of l.sub.S, then the channel at height y above the x axis, that is the central optical axis of the diverging x-ray beam emanating from source S, should be tilted by the angle w(y) given by: ##EQU3## where .rho. is the radius of curvature of the plate 10 required to produce w(y). The general flat plate, parallel channel case is geometrically explained in FIG. 1A and 1B. The general focusing condition is shown in FIG. 2: here, the inclination of the channel side walls progressively change from channel to channel with increasing distance from the optical axis. The result is an enhanced focusing effect. A special case of equation (3) occurs when l.sub.f equals infinity and corresponds to the production of a quasi-parallel x-ray beam from a point source. The geometry for this case is illustrated in FIG. 3. In FIG. 3, the side walls of each channel are curved end-to-end by virtue of the bending of the micro-capillary plate about the z axis: this is demonstrated by the parallelism of the emerging beam segments reflected by each channel side wall from a divergent beam segment received from source S. By way of example, with reference to FIG. 3, where l.sub.S is 100 mm, the channel width and length are respectively 0.025 mm and 1.0 mm, and the critical angle .gamma..sub.c is 5 mrad, the bending displacement at y=10 mm from the axis of the x-ray beam passing through the plate is 0.25 mm. A bending of a micro-channel plate to this extent clearly involves no severe mechanical problems in practice. Alternatively, the curving of the micro-capillary plate may be carried out by slump forming on heating the plate above the appropriate glass softening temperature. The channels may be tapered, shaped or may be of non-circular cross-section, e.g. hexagonal, to produce special or improved focusing effects, and to reduce off-axis aberrations. The aforedescribed exemplification assumed that the thickness of the walls in the micro-capillary plate matrix is negligible relative to the diameter of the channels. In reality, a capillary to matrix cross-section ratio of about 50% is typical and this simply results in a reduced transmission intensity. However, by careful design of the micro-capillary plate, a capillary to matrix cross-section ratio as high as 90% is presently possible. As mentioned, the principle of increasing inclination of the side walls of the channels, as shown in two dimensions in FIG. 3, may be readily extended to three dimensions by curving a micro-filament plate so that its outer and inner surfaces in which the channels open are of part paraboloidal formation. By varying the curve in the two dimensions, different effects can be produced in the respective dimensions, e.g. collimation in one plane and focusing to substantially a spot in the other. It will be understood that even in two dimension, a physical embodiment of the first aspect of the invention is possible in the form of a stack of thin x-ray mirror plates, and would have practical applications. FIG. 4 shows such an embodiment of lens device 10 ''' according to the first aspect of the invention. Multiple metal sheets 11 are fixed by suitable spacers (not shown) at uniform intervals in a stack. The sheets 11 are highly polished and reflective to x-rays, and the device is effective to focus a divergent x-ray beam from a source S substantially to a focus F. The sheets may be of variable increasing inclination and be curved under tension, as with the previously described embodiment. It will be seen that the cavities between the stack form multiple open-ended channels 12''' arranged across the optical path. In a particular embodiment, an aperture may be formed in the lens device (in any of the above forms) to allow unimpeded propagation of a direct portion of the incident beam consistent with the collimation requirements of the instrument. This aperture may then be bordered by an x-ray lens device in accordance with the invention to gather additional x-ray flux outside the aperture. In general, the front and back faces of eg, plate 10 may be shaped to optimise performance according to desired parameters. In an instrumental application, an x-ray lens device according to the first aspect of the invention may be provided in conjunction with an x-ray source tube, for example in place of the existing pin hole or rectangular slit aperture which is the effective source of x-rays from the tube. A collimating and focusing device according to the first aspect of the invention provides a very practical and cost effective means for increasing the x-ray intensity and flux in a wide variety of x-ray scattering instruments such as x-ray powder diffractometers, four circle diffractometers, small-angle scattering systems and protein crystallography stations. It should also be of value in the construction of x-ray microprobes, microscopes and telescopes. This will be especially so where conventional systems use very primitive x-ray optics, such as narrow slits or pin hole collimation. Micro-channel and micro-filament plates are very well suited to mechanical and plastic deformation as a means to achieving the desired focusing or collimating properties, in contrast to the case of single crystal diffraction systems which are much more difficult to bend with a high risk of damage. A closely similar application of such device also pertains to the case of collimating and focusing of neutrons. The advantages of x-ray lens devices according to the first aspect of the invention include: Table 1 is a summary of properties of some exemplary devices according to the first aspect of the invention, including an indication of a practical set of values for hypothetical but highly practical case. __________________________________________________________________________ SUMMARY OF PROPERTIES OF FOCUSING COLLIMATORS FOR A POINT SOURCE AND PARALLEL CHANNELS WITH WALLS OF NEGLIGIBLE THICKNESS FOCUSING TO A FOCUSING TO A POINT QUASI-PARALLEL BEAM __________________________________________________________________________ 1. maximum value of .phi. such that .gamma..sub.c (5 .times. 10.sup.-3) 2.gamma..sub.c (10 .times. 10.sup.-3) total external reflection can still occur in channel (.phi..sup.ter) 2. maximum value of .phi. such that at most only one reflection ##STR1## (0.025) ##STR2## (0.05) can occur in channel (.phi..sup.apert) 3. effective anngular semi- aperture of collimator (.phi..sup.apert) ##STR3## (5 .times. 10.sup.-3) ##STR4## (10 .times. 10.sup.-3) 4. semi-aperture of collimator on y-scale (y.sup.apert) ##STR5## (0.5 mm) ##STR6## (1.0 mm) 5. Reflection efficiency at y when aperture is .gamma..sub.c ##STR7## (0.4 y) ##STR8## (0.2 y) 6. mean efficiency averaged in 1-dimension out to effective ##STR9## (0.1) ##STR10## (0.1) aperture limit of system for .gamma..sub.c limited case. 7. intergrated reflectivity of focusing collimator when ##STR11## (1 .times. 10.sup.-3) ##STR12## (2 .times. 10.sup.-3) system is .gamma..sub.c limited (note factor of 2 to cover .+-. y contributions). 8. bending locus for MCP in order to achieve focusing x = 0 ##STR13## (-0.0025 y.sup.2) 9. bending requirements for z = 0 z = 0 sagittal focusing with 1.sub.F.sup.sag = 1.sub.s 10. integrated reflectivity if t/d value is optimized to ##STR14## (5 .times. 10.sup.-3) 2 .times. .gamma..sub.c (10 .times. 10.sup.-3) match .gamma..sub.c (i.e. d/t = .gamma..sub.c) distance to focus from 0 1.sub.s (100 mm) .omega. (.infin.) error in focusing along x - axis: (i) spatial spread 2t (2 mm) . . . (ii) angular divergence ##STR15## (10 .times. 10.sup.-3) ##STR16## (0.05 .times. 10.sup.-3) __________________________________________________________________________ N.B. Values in parenthesis relate to values of relevant quantities when the following representative values of the key quantities are chosen: ##STR17## - Turning now to the second aspect of the invention, the condensing-collimating channel-cut monochromator illustrated in FIG. 5 and 6 is a single perfect or nearly perfect-crystal of silicon, germanium or other suitable material. The crystal has been cut to form the converging channel 22 with opposed perpendicular lateral faces 24, 26. These faces are cut at respective angles, known as asymmetry angles (see FIG. 15), of .alpha..sub.l =0, .alpha..sub.2 =10.degree. to the Bragg lll planes 17 of the crystal. In operation, the at least partially collimated incident x-ray beam 28 is multiply reflected and emerges as a relatively spatially condensed and angularly collimated pencil 30. Monochromator 20 is usually formed in silicon or germanium because of their ready availability in near perfect-crystal form and the reflections typically chosen are the lll reflections because they have the largest structure factor and so the largest wave-length band-pass or angular acceptance and hence lead to the highest integrated (with respect to angle of divergence at exit face) reflectivity from the monochromator. However, other reflections may be chosen and these may confer advantages in special cases. The channel-cut crystal monochromator of FIGS. 5 and 6 has been made in accordance with certain specified tolerances, viz that for CuK.alpha..sub.l radiation (1.54051 Angstrom), the emergent x-ray beam will have a FWHM angular divergence less than 1 minute of arc, a wavelength band-pass of the order of 2.5 by 10.sup.-4, and a spatial condensation factor of about 6. By the latter is meant that, in the plane of diffraction, the ratio of the width of the incident beam to emergent beam is about 6. An example spatial condensation of the beam is shown in FIG. 7, in which image A shows the beam incident to the monochromator and image B (on the same scale as image A) shows the emergent beam. FIG. 8 is a contour plot of the spatial condensation factor, as just defined, for various values of the asymmetry angle, .alpha..sub.1, at the first lateral face of the channel, plotted against values of the asymmetry angle, .alpha..sub.2, at the second face. It will be seen that the spatial condensation factor increases with increasing .alpha..sub.1 and that, for a given .alpha..sub.1 value, increasing values of .alpha..sub.2 further enhance the condensing effect. However, these observations must be considered together with the effects of varying asymmetry angles on bandwidth, angular collimation and integrated reflectivity. For example, FIG. 9 is a contour plot of the full width of the reflectivity curve (that is the reflectivity versus the angle of divergence of the existing beam) taken as twice the standard deviation of the reflectivity distribution. FIG. 10 is a contour plot of integrated reflectivity (i.e. reflectivity integrated with respect to angle of divergence at the exit face of monochromator) versus the asymmetry angle .alpha..sub.2 for various values of .alpha..sub.1. It will be noted that for a given value of .alpha..sub.1, the integrated reflectivity tends to increase with increase in .alpha..sub.2. It seems from these curves that a good net result for silicon lll planes and CuK.alpha. radiation is obtained for .alpha..sub.1 =0 and .alpha..sub.2 =+10.degree.. A significant improvement in spatial condensation is obtained with this difference relative to no difference (FIG. 8) and integrated reflectivity is still quite high (FIG. 10), while angular collimation remains within acceptable limits and certainly below the aforementioned criterion of 1 minute of arc. For general choices of asymmetry angles for multiple reflections in a channel, the net reflectivity curve must be calculated as the product for each face treated according to the dynamical theory of x-ray diffraction. FIG. 11 shows the individual and integrated reflection curves for the ideal case (graph A), at which, as mentioned, .alpha..sub.l =0 and .alpha..sub.2 =10.degree., and for two less satisfactory arrangements (graph B: .alpha..sub.1 =9.degree., .alpha..sub.2 5.degree. and graph C: .alpha..sub.1 =3.degree.,.alpha..sub.2 =10.degree.). The former reduces the final intensity and the latter gives too sharp a peak in the net curve. The reflectivity peak for a single reflection from a perfect-crystal falls off quite slowly with angle (as can be seen in FIG. 11), with the result that long tails may occur in the primary beam coming off the monochromator and swamp the small-angle scattering intensity from the sample. Bonse and Hart showed that the undesirable tails in the beam coming rom a perfect-crystal could be reduced in intensity by man orders of magnitude, with negligible reduction in peak intensity, by using multiple reflections in a parallel-face channel-cut monochromator. For parallel faces in a channel, the reflectivity curve for a series of m identical pairs of reflections in a channel is just the m.sup.th power of the reflectivity curve for one pair. This relationship is not so for general choices of asymmetery angles for multiple reflections in a channel but the overall effect remains: the net reflectivity is the product of the individual reflectivities for the individual faces. The embodiment of FIGS. 5 and 6 uses a small number of such reflections-and the reduction of the tails can be seen in FIG. 11. The tails may be reduced even further by careful design involving increasing the numbers of faces. This may involve splitting up one or both faces of the channel. FIG. 12 diagrammatically depicts one such design viewed in plan with values for .alpha..sub.1 =0.degree., .alpha..sub.2 =10.degree., .alpha..sub.3 =-10.degree. and .alpha..sub.4 =10.degree. respectively for the four successive reflections in the monochromator. The reflectivity curves for the faces and for the device as a whole are depicted in FIG. 13. This embodiment has high reflectivity in the central range of Bragg reflection but in addition has the desirable property that the Bragg tails fall off as approximately the eights power of the angular devation from the Bragg condition. It should be noted that, the net spatial condensation factor for a monochromator with reflectance at m faces is the product of the spatial condensations at the individual faces. In the case where beams possessing a high-degree of plane polarization are required, this may be achieved by choosing reflections having 2.theta..sub.B (i.e. twice the Bragg angle) close to 90.degree. for the given wavelength. For example, for CuK.alpha., the 333 or 511 reflections of silicon or germanium are suitable. Although the discussion above of channel-cut monochromators in accordance with the second aspect of the invention has been in terms of parallel-beam optics, improvements in integrated reflectivity of such devices is clearly possible if the faces of the monochromator are suitably bent or if surface modification is carried out, for example, by ion implantation, liquid phase epitaxy or molecular-beam epitaxy. Since reflectivity of a perfect crystal depends on atomic number, one approach would be to grow an epitaxial layer or implant and anneal a heavier atom material at or near the surface of a perfect crystal of, e.g. silicon. Similarly, production of a lattice parameter gradient perpendicular to the diffracting planes, for example by the sort of means mentioned above, leads to an increase in the width of the reflectivity curves in a manner very similar to that of crystal bending. Variation of lattice parameters parallel to the diffracting planes can also lead to a one or two dimensional focusing effect similar to that achievable by bending. Improvements in transmitted power of the monochromator system of the second aspect of the invention may be achieved by use of a pre-collimator such as a bent crystal monochromator with lattice parameter gradient or x-ray mirror, or a lens means according to the first aspect of the invention. The ideal incident beam for the monochromator is collimated at least to some extent and the device of the first aspect of the invention is ideal for such pre-collimation. The monochromator of itself accepts a maximum angle or divergence in the incident beam of approximately 15"; the angular acceptance from the source can be increased from 15" to 11/2.degree. by use of the lens device of the first aspect of present invention between the source and the monochromator as shown in FIG. 16. In more advanced versions of the present types of monochromators, the degree of overlap of the two reflectivity curves, and hence the angular divergence of the beam coming from the monochromator, could be varied extrinsically by making a flexure cut in the monochromator and by using a piezo-electric or electro-magnetic transducer to vary the angle between the sets of Bragg planes corresponding to each face. An arrangement adaptable to this varability is shown in FIG. 14. Such an extension of the invention makes possible the development of compact multi-stage beam-condensing monochromators of ultimate beam condensing power, estimated to be of the order of 1 micron or less, and typically limited by the depth of penetration of the x-ray beam into the crystal face. The monochromator of the invention is of particular value in small-angle x-ray scattering and x-ray powder diffraction systems in that the incident beam on the sample is condensed to a width consistent with the detector pixels of position-sensitive detectors. The monochromator would also be valuable in x-ray microprobes for x-ray fluoresence analysis, scanning x-ray probes and for medical diagnostic and clinical purposes, in scanning x-ray lithography and as analyser crystals in powder diffractometers and fluorescence spectrometers. The described arrangement has been advanced merely by way of explanation and many modifications may be made thereto without departing from the spirit and scope of the invention which includes every novel feature and combination of novel features herein disclosed. |
047918012 | abstract | Reversible fuel assembly grid tab repair tool having a tab bending member for remotely reforming fuel assembly grid tabs, wherein the tool has means for fine alignment of the tab bending member with the grid tab to be reformed. The grid tabs may be formed on the top and bottom edges of a grid strap which wraps a plurality of fuel rods disposed in the fuel assembly. The repair tool comprises a generally L-shaped frame having a vertical first leg and a horizontal second leg. Extending from the terminal end of the second leg is an alignment blade capable of being interposed between two fuel rods adjacent each side of the grid tab to be reformed. Attached to the top surface of the second leg at the terminus thereof is an anvil block having an anvil surface for bending the grid tab thereagainst when the grid tab is interposed between the anvil surface and the tab bending member. Extending from the anvil surface are a plurality of register pins capable of abutting the top and bottom edges of the grid strap for fine alignment of the tab bending member with the grid tab. A lever member pivotally connected to the repair tool is capable of translating the grid tab bending member towards the anvil surface for bending the grid tab against the anvil surface when the grid tab is interposed between the tab bending member and the anvil surface. |
claims | 1. A basket assembly for receiving a plurality of fuel assemblies and configured to accommodate an irregular fuel assembly, comprising:a basket having a grid comprising a neutron absorbing material and defining spacing between fuel assembly compartments, the grid defining a first compartment for receiving a first fuel assembly and a second compartment for receiving a second fuel assembly, wherein the cross-sectional area of the second compartment is larger than the cross-sectional area of the first compartment, the basket assembly configured to receive in the first compartment a first fuel assembly, the first fuel assembly being a regular fuel assembly, and the basket assembly configured to receive in the second compartment the second fuel assembly, the second fuel assembly being the irregular fuel assembly that requires a relatively larger cross-sectional dimension than a cross-sectional dimension of the regular fuel assembly, wherein the irregular fuel assembly includes at least one irregular fuel rod. 2. The basket assembly of claim 1, wherein the irregular fuel rod is selected from the group consisting of a bowed fuel rod, a twisted fuel rod, a deformed fuel rod, a damaged fuel rod, bottled fuel debris, and any combinations thereof. 3. The basket assembly of claim 1, wherein the cross-sectional area of the second compartment is larger than the first compartment by a multiple of the dimensions of the first compartment. 4. The basket assembly of claim 1, wherein the second compartment has the same cross-sectional shape as the first compartment. 5. The basket assembly of claim 1, wherein the grid defines a plurality of first compartments. 6. The basket assembly of claim 1, wherein the grid defines a plurality of second compartments. 7. The basket assembly of claim 1, wherein the cross-sectional area of the second compartment is equal to an array of cross-sectional areas of the first compartments. 8. The basket assembly of claim 1, wherein the cross-sectional shape of the first compartment is square. 9. The basket assembly of claim 1, wherein the cross-sectional shape of the second compartment is square. 10. The basket assembly of claim 1, wherein the first and second fuel assemblies are spent fuel assemblies. 11. The basket assembly of claim 1, further comprising a third compartment for receiving a third fuel assembly, wherein the cross-sectional area of the third compartment is larger than the cross-sectional areas of the first and second compartments. 12. The basket assembly of claim 11, wherein the cross-sectional area of the second compartment is larger than the first compartment by a multiple of the dimensions of the first compartment. 13. The basket assembly of claim 11, wherein the third compartment has the same cross-sectional shape as the first and second compartments. 14. The basket assembly of claim 11, wherein the cross-sectional shape of the third compartment is square. 15. The basket assembly of claim 11, wherein the grid defines a plurality of third compartments. 16. A basket assembly for receiving a plurality of fuel assemblies and configured to accommodate an irregular fuel assembly, comprising:a basket having a grid comprising a neutron absorbing material and defining spacing between fuel assembly compartments, the grid defining at least a first compartment configured for receiving a first fuel assembly, wherein the first fuel assembly is a regular fuel assembly, and a second compartment configured for receiving a second fuel assembly comprising a plurality of fuel rods including at least one irregular fuel rod, wherein the second fuel assembly is the irregular fuel assembly that requires a relatively larger cross-sectional dimension than a cross-sectional dimension of the regular fuel assembly, and wherein the cross-sectional area of the second compartment is larger than the cross-sectional area of the first compartment by a multiplication factor. 17. A basket assembly including a plurality of fuel assemblies and configured to accommodate an irregular fuel assembly, comprising:a basket having a grid comprising a neutron absorbing material and defining spacing between fuel assembly compartments, the grid defining at least a first compartment and a second compartment, wherein the cross-sectional area of the second compartment is larger than the cross-sectional area of the first compartment; anda regular fuel assembly disposed in the first compartment and the irregular fuel assembly comprising a plurality of fuel rods including at least one irregular fuel rod disposed in the second compartment,wherein the irregular fuel assembly requires a relatively larger cross-sectional dimension than a cross-sectional dimension of the regular fuel assembly. |
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052805085 | description | DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a fuel bundle B is shown, removed from a reactor. The fuel bundle has been placed in a holding pool 12, and had its lifting handle, channel and upper tie plate removed. As these items are conventional, they are not shown herein. Typically, the bundle is held vertically upright in a fuel handling machine (also conventional and not shown). For the convenience of the reader, the machine is likewise omitted, and the bundle is shown in the upright position. The worker manipulates handles H. Handles H effect the latching and unlatching of grasping mechanism G. Grasping mechanism G attached to the part length rod A. This grasping of the part length rod A must be made in a matrix of upstanding full length rods F. Referring to FIGS. 2 and 3, the dimensions of this problem can be understood. Specifically, full length rods F and an interior water rod W are of the order of 160" in length. The part length rod A is approximately 100" in length. Viewing the schematic of FIG. 2, it can be seen that spacer S1 and spacer S2 overlie the top of part length rod A. Part length rod A is at spacer S3, finally braced in the full upright position. It becomes apparent that access to a tip T at the end of part length rod A must occur through the cell matrix of spacers S1 and S2, overlying tip T. Referring to FIG. 3, an enlarged perspective of the upstanding fuel bundle is illustrated. A pole P with a grasping tool G at the bottom portion thereof is shown penetrating the matrix of the upstanding fuel bundle, including the lower tie plate L, the full length fuel rods F, a water rod W, to the top of the part length rods A. Part length rods A at spacer S3 as can be seen in order for pole P and grasping tool G to reach tip T of the part length rod A, passage through the cell matrix of spacers S1 and S2 has to occur. Referring to FIG. 4A, a part length rod A is shown. The part length rod includes a threaded lower tip 20, and attached sealed tube 22, an end tip T. Threaded tip 20 seals tube 22 at one end. Tip T seals the tube at the opposite end. A group of fuel pellets 30 compressed by spring 32 complete the construction of the part length fuel rod. Viewing the rod A as set forth in FIG. 4A, two important things are worthy of note: First, in the preferred embodiment of this invention, part length rod A is typically threadedly attached to lower tie plate L. Typically, lower tie plate L defines female threads. The lower threaded tip 20 defines corresponding male threads. To absolutely assure against vertical motion of the part length rod A, the rod is rotated with threaded engagement to lower tie plate L (see FIG. 3). Secondly, the part length rod includes a tip T. It is this tip T illustrated in more detail in FIG. 4B which constitutes the invention hereof. Referring to tip T, this tip includes a conventional sealing portion 40, the function of which is known in the prior art and here will be briefly described. Typically, portion 40 seals the end of the tube 22. It has an aperture 42 through which the fuel may be pressurized by gases chosen to suppress fusion gas production during the reactive lifetime of the fuel rod. As this is well known in the prior art, it will not be further discussed herein. Portion 44 of the tip T constitutes the novel portion of the tip. Consequently, it will now be described in more detail. First, tip T includes a cylindrical mass 46. This cylindrical mass has three distinctive features. First, it is provided at the upper end with a rounded gathering surface 47. As will hereinafter be more fully understood, gathering surface 47 enables a tool in a substantially blind condition to be gathered to the tip T for the removal and replacement of the rod. Secondly, the tip includes a longitudinal keyway 50. Keyway 50 is displaced from the longitudinal axis 49 of the cylindrical mass 46. It functions to provide a surface on tip T through which substantial torque can be applied. Thus, by manipulating the tip T at keyway 50, threads 20 (see FIG. 4A) can be screwed and unscrewed for insertion and removal of the part length rod A. Finally, a female cylindrical segment 54 has been removed from the cylindrical mass 46. Preferably, this removed section 54 is at the bottom end of keyway 50, at some distance removed from rounded tip 47. As will hereinafter made more apparent, female cylindrical segment 54 enables the tip T to be positively grasped to enable lifting parallel to axis 49 of the entire part length rod A. Having set forth the construction of the tip, three discrete tools will now be described: First, and with reference to FIGS. 6, 7A and 7B, a tool including a spring key and male cylindrical segment will be discussed. Thereafter, and with reference to FIGS. 8, 9A and 9B, a tool which grasps only the periphery of the tip T for both lifting and rotation will be set forth. The reader will understand that this tool will not be utilized where either large lifting forces or large turning or torque forces are required on the partial length rod A. Finally, and with respect to FIGS. 10A and 10B, a socket for applying a large turning force on the part length rod A will be set forth. Referring to FIG. 6, the pole P utilized with this invention is set forth. Pole P includes an upper cable attachment clevis 60, having a crossbore 62. This clevis 60 at crossbore 62 is conventionally attached to an overhead cable mechanism 14. Utilizing the pendulous weight of pole P and grasping tool G, verticality of the entire pole P arrangement is assured. It is necessary that pole P have relatively moving parts. These relatively moving parts are utilized for latching and unlatching of the grasping mechanism G. This being the case, and at the top of the pole, there is a knurled knob 64 and a threaded inner rod 66. Knurled knob 64 bears against a knurled handle 68. Handle 68 connects to an outer portion of the pole P. Looking at the end of the pole adjacent grasping mechanism G, it can be seen that there is required an outer portion of the pole 70, and an inner portion of the pole 72. Further, it will be understood that to operate the grasping mechanism G, relative reciprocation of the members 70, 72 must occur. Accordingly, knurled handle 68 is attached to outer portion 70. Threaded shaft 66 is attached to inner portion 62. By manipulation of handle 68, relative to shaft 66, corresponding movement of outer portion 70 relative to inner portion 72 occurs along the full length of the pole. Accordingly, the grasping mechanism G may be manipulated. Thirdly, and finally, the reader must realize that the pole P is of considerable length. Not only must the pole penetrate 60" into the radioactive environment of the upwardly exposed fuel bundle, but the pole must pass through a sufficient amount of water so that the maintenance personnel M manipulating the pole P are shielded from the ambient radiation. It thus will be understood that the sheer removal of the maintenance personnel M from the point of manipulation of the part length rod A constitutes one of the difficulties encountered in this invention. Because of this overall length, it is required to provide for breaking the pole P into two discrete sections. Accordingly, opposed flanges 80 on outer section 70 and a corresponding joining device on inner pole section 72 (not shown) are utilized so that pole P can be shipped in two separate and discrete sections. Having set forth the operation of the pole and emphasizing that the outer section 70 reciprocates with respect to the inner section 72, attention may now be devoted to a first embodiment of the grasping tool shown in FIGS. 7A and 7B. Affixed to shaft 72 there is included a cylinder 90. Cylinder 90 defines a concentric bore 92. Bore 92 is exposed outwardly to and towards tip T, and receives tip T concentrically thereof. Cylinder 92 is provided with an end which mates with taper 47 at the end of tip T. Cylinder 90 is slotted with a longitudinal keyway 94. It is the function of keyway 94 to receive a spring loaded tang 96. Spring loaded tang 96 and its attachments can be simply summarized. Tang 96 includes a second thickened end 97 which end 97 forms the weld point to cylinder 90. Tang 96 extends from weld point 97 to a key 98. It is the function of the key 98 to fit into keyway 50. As will hereinafter be described, registry of key 98 to keyway 50 provides a tactile signal to maintenance personnel M manipulating pole P so that rotational registration of gripping mechanism G can be determined at a distance of more than 20011 under conditions where observation of the engagement of the gripping mechanism G to the tip T simply cannot effected. Finally, and here shown at the distal end of tang 96, there is provided a male cylindrical segment 99. Male cylindrical segment 99 is complementary to female cylindrical segment 54. That is to say, once key 98 registers to keyway 50, and cylinder 90 at bore 92 is fully advanced onto tip T, male cylindrical segment 99 fits into female cylindrical segment 54, to completely fill in the cylindrical profile of the tip T. It can be seen with respect to FIG. 7A that such engagement is about to occur. By referring to FIG. 7B, locking of the grasping mechanism G can now be set forth. Referring to grasping mechanism G, it can be seen that outer pole segment 70 is connected to a sleeve section 102. Sleeve section 102 is open, so as to fit over tip T, when tip T has male cylindrical segment 99 occupying female cylindrical segment 54, key 98 occupying keyway 50, and tang 96 fully received within its slot 94. Having disclosed these constructions, the operation of sleeve 102 to lock to tip T of part length rod A can now be easily understood. With simultaneous reference to FIG. 1A, pole P is manipulated with respect to bundle B at a position overlying the part length rod P. Since the pole P and the grasping mechanism G do not have a dimension exceeding that dimension of an individual cell within spacers S1, S2, lowering of the grasping mechanism G at the end of pole P through spacers S1, S2 at the corresponding cell position easily occurs. When contact is made with tip T, pole P is rotated. At the same time, key 98 is deflected upwardly by gathering surface 47 on tip T. Rotation of pole P will occur until key 98 registers to keyway 50. At this point, the keyway 50 and key 98 will cause the relative rotation of pole P with respect to part length rod A to suddenly cease. This cessation of rotation will be felt by the maintenance worker M, some distance from fuel bundle B as he stands on catwalk 16. Once this rotational registration has occurred, grasping mechanism G will be advanced onto tip T. Such advancing will occur until a full position of penetration is reached. At this juncture, female segment 54 on tip T will be occupied by male segment 99. This will be accompanied by an additional tactile indication, which will include the end of the limit of travel of pole P and the grasping mechanism G down onto the top of part length rod A at tip T. Thereafter, sleeve 102 will be advanced. It will advance to a position overlying tang 96 at key 98 and male cylindrical segment 99. Firm locking of the grasping mechanism G and pole P to the part length rod A will occur. Typically, the grasping mechanism G will be maintained firmly fixed at tip T during its inspection. Such inspection can include the conventional removal of flocculants and other debris from the exterior of the part length rod A, with visual photographic and other non-destructive examinations occurring to the part length fuel rod A. Presuming that either the part length fuel rod A will be returned to the bundle B or alternately be replaced in bundle B, releasing of the part length rod A must be understood. It can be seen that male cylindrical segment 99 is provided with a complementary climbing surface 122. Presuming that sleeve 102 is withdrawn by corresponding withdrawal of outer section 70 of pole P, upward vertical movement of pole P will no longer result in lifting of partial length fuel rod A. Instead, male cylindrical segment 99 will climb free of female cylindrical segment 94, through the coaction of the climbing surface 122 with the edge of the female cylindrical segment. This will cause corresponding lifting of tang 96, enabling complete withdrawal of the grasping mechanism G. With respect to the apparatus set forth in FIGS. 6, 7A and 7B, the reader will understand that a universal type tool has been disclosed. The disclosed tool provides positive locks to tip T at the end of partial length rod A. These positive locks include both vertical lifting and application of torque to tip T. Referring to FIGS. 8, 9A and 9B, a second type of pole P, here denominated P', and gripping mechanism G, here denominated G', is set forth. Referring to FIG. 8, pole P includes clevis 62 overlying two counter-rotating handles 60, 162. Handle 162 is held stationary. Handle 60 is rotated. Upon such rotation, outer member 70 moves longitudinally of pole P with respect to inner member 72. As will hereinafter be explained with more detail, it effects engagement and disengagement of grasping mechanism G'. As before, an opposed flange 80 on outer section 70 and a mechanism (not shown) on inner mechanism 72 enables breaking of the pole P for convenient shipping and/or storage. Construction of the particular gripping mechanism G' is easy to understand. Inner member 72 has fastened thereto a cylinder 190. Cylinder 190 defines a female bore 192, for receiving the cylindrical portion 46 of tip T at the end of part length rod A. Typically, cylinder 190 is slit. It is slit at three respective cylinder slits 194, at approximate 120.degree. intervals, about an axis 196 of the cylinder. Additionally, the cylinder side wall is tapered. It tapers from a narrow dimension at the upper end 198 of the cylinder to a thickened dimension 199, at the lower end of the cylinder. Stopping here and ignoring all other constructions, the insertion and removal of cylinder 190 over tip T can be understood. As gripping tool G' comes down onto and over tip T, the respective segments of cylinder 190 will move away from cylinder 46. When it moves away from cylinder 46, capture of the tip T will occur. The reader can further see that the taper having a thin portion 198 at the upper end of bore 192 and a thickened portion at the lower end 199 of bore 192, will have an advantage in gripping the cylindrical side walls 46 of tip T. Specifically, the cylinder from top to bottom will fit flush with respect to the top to bottom cylindrical side walls of cylinder 46 of tip T. There remains to be understood how cylinder 190 may be firmly locked to the exterior of tip T. A second and reciprocating tip 202 is provided. Tip 202 has a thick portion 204 at the upper end, and a thinned portion 206 at the lower end. Sleeve 202, unlike cylinder 190, does not have slits. Accordingly, and once it is advanced over cylinder 190, firm capture of cylinder 190 will occur. Assuming that cylinder 190 is over the cylindrical portion 196 of tip T, and sleeve 202 is advanced over cylinder 190, firm engagement of the exterior of tip T will occur. At this point, the reader can note two differences from the mechanism set forth in FIGS. 9A and 9B, with respect to the mechanisms of FIGS. 7A and 7B. First, and presuming that pole P is utilized to apply torqued partial length rod A at tip T, no positive lock with respect to any keyway will occur. Accordingly, at higher degrees of torque, slippage may be expected. In the ordinary case, and presuming that threads 20 of partial length rod A are not stuck with respect to the lower tie plate, removal of the partial length rod A may occur. However, if sticking occurs, use of another tool may well be desired. Secondly, no locking of the grip mechanism G' occurs with respect to the female cylindrical cavity 54. Accordingly, if large lifting forces are required, again tool substitution may be utilized. It will be appreciated that the engagement of the tool mechanism G' shown on FIGS. 9A and 9B is relatively easy. This being the case, and assuming normal attachment of part length rod A in a fuel bundle B, use of this tool will be preferred. It may be desired to apply just torque to tip T. This being the case, the tool of FIGS. 10A and 10B may well be utilized. Referring to FIGS. 10A and 10B, a solid tool sleeve 300 is illustrated. Tool sleeve 300 has been slotted at a weld preparation area 302 for the receipt of a key 304. Key 304 is affixed as by welding to slot 302. Sleeve 300 is conventionally attached to a pole P; relatively reciprocating parts are not required. In operation, sleeve 300 fits over and receives tip T. Key 304 is registered to keyway 50 in tip T. Rotation under high torque of tip T and attached part length rod A can occur. Such a tool can be used as desired for partial length rod removal. It is to be noted that the grasping tool arrangements here shown are capable of being conveniently manipulated with respect to the end of a partial length rod A. It will further be understood that absolute verticality of the tip T of the end of the partial length rod A is never required. The respective gathering surfaces and conformance of the various grasping mechanisms G and G' enable the practical operating parameters of a reactor to be accommodated. |
048809888 | summary | Reference to related to patent, assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference: U.S. Ser. No. 828,830, filed Feb. 12, 1986, RUDZKI 4,747,645, May 31, 1988 Reference to related disclosure: German Pat. No. 20 24 288 The present invention relates to a testing apparatus to test samples for resistance to weathering and light irradiation, and more particularly to such an apparatus which includes a radiation source which emits light in the infrared (IR), visible, and ultraviolet (UV) spectrum of radiation, and in which the sample is exposed to the radiation from the light source. Background Weathering and light testing apparatus in which radiation is directed to samples are known, see for example U.S. Pat. No. 4,747,645, based on application Ser. No. 828,830, filed Feb. 12, 1986, Rudzki. A suitable radiation source is a xenon lamp. Such lamps, which either are tubular or have a small, essentially point source radiation element, emit radiation in the visible, UV and IR range. It has previously been proposed to filter IR radiation emitted from the source by placing between the source an the sample a filter which is transparent to the visible and the UV portion of the emitted radiation, but selectively reflects IR radiation. The filter is spaced from the radiation source and, for example, is cylindrical, surrounding the elongated or point-type radiation source. A flat surface can be located within the filter to absorb the radiation. German Pat. No. 20 24 288 HENSIEK describes a test apparatus to test samples for resistance to light and weather influences. Such apparatus are used to test various materials with respect to aging, by accelerating the impinging radiation, so that a time compressed test can be carried out. To obtain good correlation between natural weathering and radiation effects on the sample, it is necessary to match the spectral energy distribution of the light closely to that of natural sunlight. It is customary to utilize xenon radiators in apparatus of this type. Xenon radiators have a high proportion of radiation in the IR range, and specifically between about 700 to 1000 nm. To obtain a radiation spectrum which matches daylight as closely as possible, it is necessary to substantially reduce IR radiation. This also reduces heating of the sample. The structure, as proposed, has an external filter in form of a cylindrical jacket, the interior space of which is separated into three regions by mirror surfaces. Each one of the separated regions is associated with an elongated radiation source, extending in axial direction with respect to the filter cylinder. Besides the desired UV and visible light spectrum radiation, a substantial proportion of IR radiation is emitted by the light source. The mirror surfaces are transparent for IR radiation, but reflect UV radiation at the surface. The cylindrical jacket is transparent for UV and visible radiation, reflecting, however, IR radiation into the interior of the cylinder. The IR radiation passing through the mirror surfaces is absorbed within the structure. Heat, which results, is removed from the absorption space. The Invention It is an object to simplify a structure which provides, essentially only UV and visible spectral radiation, which is efficient, and in spite of simplicity of construction permits absorption of a substantial portion of IR radiation emitted from the radiation source without, however, reducing UV and visible radiation emitted to the outside of the apparatus. Briefly, the radiation source which may be a lamp having essentially point or small area light emitting characteristics, or may be an elongated tubular lamp, is located within a cylindrical filter defining a central axis, which filter is transparent to UV and visible radiation but reflects IR radiation internally. The radiation source is located eccentrically with respect to the cylinder axis of the filter cylinder, and positioned in a plane which includes the axis of the cylinder and the radiation source, the plane also retaining a radiation absorbing element defining a plane surface. The radiation absorbing element extends from an inside wall of the cylindrical filter up to the radiation source; another portion of the radiation absorbing element may extend from the light source towards the opposite side of the inside wall of the cylinder, in line with the major portion of the radiation absorbing element extending part-diametrically across the cylindrical filter. The arrangement has the advantage of utmost simplicity while being highly efficient. The radiation source is in the plane of at least one IR radiation absorbing surface. The radiation source, thus, is not in the axis of the hollow cylindrical filter so that the entire IR radiation which is emitted from the radiation source can be reflected by the cylindrical filter internally of the cylindrical structure and to the IR radiation absorbing surface. The heat generated, thereby, can be conducted away from the IR radiation absorbing surface. On the other hand, UV radiation and visible light radiation can pass through the essentially cylindrical filter without any impediment towards the outside, and, for example, against the test sample. The construction is very simple and inexpensive, since no expensive filter and mirror surfaces are needed. The region between the cylindrical jacket and the radiation source, namely that region plane which is opposite and coplanar to the zone of the radiation absorbing surface which extends towards the radiation source, is practically free from radiation with respect to reflected IR radiation from the inner surface of the cylinder of the filter. A further surface can be located within the cylindrical filter of the apparatus which is located in the plane of the main radiation absorbing surface and extends from the source, in the back of the main surface of the inner wall of the cylindrical filter. This arrangement provides for subdivision of the interior space of the filter into two halves of essentially the same size. Preferably, the radiation source is so located within the inside of the filter that it is spaces from the inner surface of the cylinder by a distance which corresponds to approximately one-quarter of the interior diameter of the filter. The axis of the radiation source should be offset from the axis of the cylindrical filter by preferably at least one half the outer diameter of the bulb of the radiation source; in other words, no portion of the radiation' source should intersect the axis of the cylinder, or be tangent to the axis of the cylinder. Dimensioning the apparatus in such a manner obtains reflection of all of the IR radiation from the inner wall of the filter on to the radiation absorbing surface. In accordance with a preferred feature of the invention, the radiation source is an elongated tubular element, having an axis parallel to the axis of tubular filter. This provides for uniform intensity of radiation over the entire longitudinal extent of the tubular filter. It is also possible, however, to use an essentially punctiform radiation source; if so, it is preferably located essentially in the center of the filter, that is, at half the axial length of the cylinder. The filter itself is a cylinder which is coated at the inside or at the outside with a material which, preferably, is a dialectric, and is characterized by good reflectivity within the IR spectral range while having high transparency for radiation in the UV and visible range. |
description | This application claims the benefit of Chinese Patent Application No. 200910132511.8 filed Mar. 31, 2009, which is hereby incorporated by reference in its entirety. Embodiments described herein relate to a filter and an X-ray imaging apparatus, and more particularly to a filter that adjusts energy spectra of X-ray, and an X-ray apparatus provided with the filter. A X-ray imaging apparatus irradiates X-ray to a subject by adjusting energy spectrum of the X-ray with a filter. The filter is provided in a collimator box attached to an X-ray tube. In order to obtain desired spectrum, the filter can be used by switching multiple filter plates which are attached to a rotating disc, as disclosed in the Japanese Patent Application No. HEI11-76219. In this prior art, the energy spectrum can be adjusted in a wide range according to the actual needs, but in the construction in which the filter plates are switched by the rotating disc, a four-step adjustment is about all this construction can provide. Therefore, widening the adjustment range causes a rough step, while providing more steps narrows the adjustment range. When a multi-step adjustment is made possible in the rotating disc system anyway, the rotating disc to which a great number of filters are attached is increased in size, thus unrealistic. In addition, the U.S. Pat. No. 7,260,183 reveals a filter which is provided with a cam having a curve to adjust laminated filter plates, in order to realize a filter that can make a fine spectrum adjustment in a wide rage and can be miniaturized. However, there are some disadvantages in the technical solution of this prior art: 1. when there are more than two cams in a set of cams, the curve becomes very complex and thus the cam will be in an execrable stressed state, even the angle of the force driving the cam is approximate to the angle of friction. When the cam is rotated to a certain direction or to a certain state, it will be in a more execrable stressed state; 2. since one driving system will drive at most two filter plates in practice, more driving systems are needed to drive two or more filter plates, and thus the disadvantages of high cost, large size and complex structure are caused. An embodiment of the present invention provides a filter, which is provided with less driving systems to realize a fine spectrum adjustment in a wide range, and which has advantages of simple structure, low cost and high reliability. A X-ray imaging apparatus fitted with said filter is also provided. Moreover, embodiments of the present invention provide a filter, comprising: at least two filter plates for adjusting X-ray energy spectrum; a pair of rails for fixedly supporting the filter plates, said filter plates being provided in a laminated structure on the guiders of the pair of rails; two cams which are respectively provided with a groove curve on a surface thereof; and a driving wheel for driving the cams contacted with it, wherein the filter further comprises at least two link levers, each link lever being connected to one of the filter plates at one end and being mounted to an axis at the other end in the manner that the link lever is rotatable about the axis, and each link lever being provided with a pin which is in cooperation with the groove curve of the corresponding cam so that the link lever proceeds with a reciprocating movement according to rotation of the corresponding cam to move the filter connected to it into or out of the X-ray passing space; the cams are respectively located at different sides of the driving wheel to move the filter plates into the X-ray passing space from different directions. The number of the filter plates is two; the corresponding cams are respectively located at the left and right sides of the driving wheel, each cam being provided with a groove curve at only one surface thereof; and the filter has two link levers, each link lever being connected to one of the filter plates and being in cooperation with the groove curve of one of the cams. The filter further comprises a third filter plate and a third link lever, the third link lever being connected to the third filter plate at one end thereof and being provided with a pin; on the other surface of one of the cams is provided with a groove curve which is in cooperation with the pin on the third link lever. The groove curve on each cam is designed according to the following method: according to N, the number of the filter thickness states that can be switched, calculate the minimum rotation angle of each cam in each thickness switch by 360°/N; set R1, the corresponding radius of the cam groove curve as the minimum one when the filter plate is in the O region retreating from the X-ray passing space; set R2, the corresponding radius of the cam groove curve as the maximum one when the filter plate is in the W region advancing into the X-ray passing space; determine whether the driving wheel is rotated clockwise or anticlockwise; set the O region and W region of the filter plates according to the space size and structure of the collimator box; determine whether the cams are located at the left side or the right side of the filter; and create and design the cam groove curves according to the table below: Switch Order of the Filter Plates1234NCam PositionFilter Plate 1O region or W. . .. . .. . .. . .which side ofregion?the filter?Filter Plate 2O region or W. . .. . .. . .. . .which side ofregion?the filter?Filter Plate . . .O region or W. . .. . .. . .. . .which side ofregion?the filter?Operating State ofIs any filter. . .. . .. . .. . .the Filterworking? wherein the O region represents the non X-ray passing space; and the W region represents the X-ray passing space. Assuming the filter plate 1 and the filter plate 2 have thickness of h1 and h2 respectively, then the filter composed of the filter plates has four switches in thickness, i.e. N=4, and the four switches are 0, h1, h2, and h1+h2, wherein 0 indicates that there is no filter in the X-ray passing space; h1 indicates that there is a filter with thickness of h1 in the X-ray passing space; h2 indicates that there is a filter with thickness of h2 in the X-ray passing space; h1+h2 indicates that there is a filter with thickness of h1+h2 in the X-ray passing space; and corresponding to each thickness switch, the cams will be rotated by an angle of 90° calculated by 360°/4; assuming the driving wheel is rotated anticlockwise and the two cams are rotated clockwise, then the table below for the two groove curves can be designed: Switch Order of the Filter Plates1234Cam PositionFilter Plate 1OWOWright sideFilter Plate 2OOWWleft sideOperating State of the0h1h2h1 + h2Filter The filter plates are made of copper or tin; h1 and h2 are respectively 0.1 mm and 0.2 mm. The filter plate 1 and the filter plate 2 have thickness of h1, h2 respectively; with an additional filter plate 3 with thickness of hn, the filter composed of the filter plates has four thickness switches plus a switch of thickness hn, i.e. N=5, and the switches are 0, h1, h2, h1+h2, and hn; and corresponding to each thickness switch, the cams will be rotated by an angle of 72° calculated by 360°/5; assuming the driving wheel is rotated anticlockwise and the cams located respectively at left side and right side of the driving wheel are rotated clockwise, then a table below for three groove curves can be designed, wherein groove curves for respectively controlling the filter plate 1 with thickness of h1 and the filter plate 3 with thickness of hn are respectively provided on the front surface and the back surface of the cam located at the right side: Switch Order of the Filter Plates12345Cam PositionFilter Plate 1OWOWOright sideFilter Plate 3OOOOWright sideFilter plate 2OOWWOleft sideOperating State of the0h1h2h1 + h2hnFilter The filter plate 1 and the filter plate 2 are made of copper or tin; h1 and h2 are respectively 0.1 mm and 0.2 mm; and the filter plate 3 is made of tin or copper. The filter plate 1 and the filter plate 2 have thickness of h1 and h2 respectively; with an additional filter plate 3 with thickness of hn, the filter composed of the filter plates has four thickness switches plus one more thickness switch of hn+h2+h1, i.e. N=5, and the switches are 0, h1, h2, h1+h2, and hn+h1+h2; and corresponding to each thickness switch, the cams will be rotated by 72° calculated by 360°/5; assuming the driving wheel is rotated anticlockwise and the cams located respectively at the left and right sides of the driving wheel are rotated clockwise, then the table below for three groove curves can be designed, wherein groove curves for respectively controlling the filter plate 1 with thickness of h1 and the filter plate 3 with thickness of hn are respectively provided on the front surface and the back surface of the cam located at the right side: Switch Order of the Filter Plates12345Cam PositionFilter Plate 1OWOWWright sideFilter Plate 3OOOOWright sideFilter plate 2OOWWWleft sideOperating state of0h1h2h1 + h2hn + h1 + h2the Filter The filter plate 1 and the filter plate 2 are made of copper or tin; h1 and h2 are respectively 0.1 mm and 0.2 mm; and the filter plate 3 is made of tin or copper. Another embodiment of the present invention provides an X-ray imaging apparatus for imaging a subject by X-ray via a filter, wherein said filter comprises: at least two filter plates for adjusting X-ray energy spectrum; a pair of rails for fixedly supporting the filter plates, said filter plates being provided in a laminated structure on the guiders of the pair of rails; two cams which are respectively provided with a groove curve on a surface thereof; and a driving wheel for driving the cams contacted with it, wherein the filter further comprises at least two link levers, each link lever being connected to one of the filter plates at one end and being mounted to an axis at the other end in the manner that the link lever is rotatable about the axis, and each link lever being provided with a pin which is in cooperation with the groove curve of the corresponding cam so that the link lever proceeds with a reciprocating movement according to rotation of the corresponding cam to move the filter connected to it into or out of the X-ray passing space; the cams are respectively located at different sides of the driving wheel to move the filter plates into the X-ray passing space from different directions. The filter has two filter plates; the corresponding cams are respectively located at the left and right sides of the driving wheel, each cam being provided with a groove curve at only one surface thereof; and the filter has two link levers, each link lever being connected to one of the filter plates and being in cooperation with the groove curve of one of the cams. The filter further comprises a third filter plate and a third link lever, the third link lever being connected to the third filter plate at one end thereof and being provided with a pin; on the other surface of one of the cams is provided with a groove curve which is in cooperation with the pin on the third link lever. The groove curve on each cam is designed according to the following method: according to N, the number of the filter thickness states that can be switched, calculate the minimum rotation angle of each cam in each thickness switch by 360°/N; set R1, the corresponding radius of the cam groove curve as the minimum one when the filter plate is in the O region retreating from the X-ray passing space; set R2, the corresponding radius of the cam groove curve as the maximum one when the filter plate is in the W region advancing into the X-ray passing space; determine whether the driving wheel is rotated clockwise or anticlockwise; set the O region and W region of the filter plates according to the space size and structure of the collimator box; determine whether the cams are located at the left side or the right side of the filter; and create and design the cam groove curves according to the table below: Switch Order of the Filter Plates1234NCam PositionFilter Plate 1O region or W. . .. . .. . .. . .which side ofregion?the filter?Filter Plate 2O region or W. . .. . .. . .. . .which side ofregion?the filter?Filter Plate . . .O region or W. . .. . .. . .. . .which side ofregion?the filter?Operating State ofIs any filter. . .. . .. . .. . .the Filterworking?wherein the O region represents the non X-ray passing space; and the W region represents the X-ray passing space. Assuming the filter plate 1 and the filter plate 2 have thickness of h1 and h2 respectively, then the filter composed of the filter plates has four switches in thickness, i.e. N=4, and the four switches are 0, h1, h2, and h1+h2, wherein 0 indicates that there is no filter in the X-ray passing space; h1 indicates that there is a filter with thickness of h1 in the X-ray passing space; h2 indicates that there is a filter with thickness of h2 in the X-ray passing space; h1+h2 indicates that there is a filter with thickness of h1+h2 in the X-ray passing space; and corresponding to each thickness switch, the cams will be rotated by an angle of 90° calculated by 360°/4; assuming the driving wheel is rotated anticlockwise and the two cams are rotated clockwise, then the table below for the two groove curves can be designed: Switch Order of the Filter Plates1234Cam PositionFilter Plate 1OWOWright sideFilter Plate 2OOWWleft sideOperating State of the0h1h2h1 + h2Filter The filter plates are made of copper or tin; h1 and h2 are respectively 0.1 mm and 0.2 mm. The filter plate 1 and the filter plate 2 have thickness of h1, h2 respectively; with an additional filter plate 3 with thickness of hn, the filter composed of the filter plates has four thickness switches plus a switch of thickness hn, i.e. N=5, and the switches are 0, h1, h2, h1+h2, and hn; and corresponding to each thickness switch, the cams will be rotated by an angle of 72° calculated by 360°/5; assuming the driving wheel is rotated anticlockwise and the cams located respectively at left side and right side of the driving wheel are rotated clockwise, then a table below for three groove curves can be designed, wherein groove curves for respectively controlling the filter plate 1 with thickness of h1 and the filter plate 3 with thickness of hn are respectively provided on the front surface and the back surface of the cam located at the right side: Switch Order of the Filter Plates12345Cam PositionFilter Plate 1OWOWOright sideFilter Plate 3OOOOWright sideFilter plate 2OOWWOleft sideOperating State of the0h1h2h1 + h2hnFilter The filter plate 1 and the filter plate 2 are made of copper or tin; h1 and h2 are respectively 0.1 mm and 0.2 mm; and the filter plate 3 is made of tin or copper. The filter plate 1 and the filter plate 2 have thickness of h1 and h2 respectively; with an additional filter plate 3 with thickness of hn, the filter composed of the filter plates has four thickness switches plus one more thickness switch of hn+h2+h1, i.e. N=5, and the switches are 0, h1, h2, h1+h2, and hn+h1+h2; and corresponding to each thickness switch, the cams will be rotated by 72° calculated by 360°/5; assuming the driving wheel is rotated anticlockwise and the cams located respectively at the left and right sides of the driving wheel is rotated clockwise, then the table below for three groove curves can be designed, wherein groove curves for respectively controlling the filter plate 1 with thickness of h1 and the filter plate 3 with thickness of hn are respectively provided on the front surface and the back surface of the cam located at the right side: Switch Order of the Filter Plates12345Cam PositionFilter Plate 1OWOWWright sideFilter Plate 3OOOOWright sideFilter plate 2OOWWWleft sideOperating state of0h1h2h1 + h2hn + h1 + h2the Filter The filter plate 1 and the filter plate 2 are made of copper or tin; h1 and h2 are respectively 0.1 mm and 0.2 mm; and the filter plate 3 is made of tin or copper. In the present invention, a single driving wheel drives multiple cams that have different curves. The cams are respectively located at different sides of the driving wheel, so that the cams driven by the driving wheel move the filter plates connected thereto into FOV of the X-ray respectively from different sides (FOV is the abbreviation of Field of View). In the present invention, the number of driving systems and motors are reduced, and a driving system can satisfy the case of four cam curves. Therefore, the present invention reduces the components number and complexity in structure, enhances reliability and cuts the cost. In the meantime, the structure according to the present invention increases the margin between the filter plates, and thus decreases the requirement on filter plate position repeatability accuracy, because the size of the filter plates can be optimized and increased, and the filter plates can be withdrawn sufficiently. In addition, by the cam design method according to the present invention, the cam curve becomes more simple and the strained condition is improved. Embodiments according to the present invention will be described in detail in the following text with reference to the Drawings. It should be apprehended that the present invention shall not be limited to these specific embodiments. FIG. 1 shows the structure of a X-ray imaging apparatus according to the present invention. The X-ray imaging apparatus is an example of the present invention, and comprises an X-ray irradiating device 10, X-ray detecting device 20 and an operator controller 30. The X-ray irradiating device 10 and the X-ray detecting device oppose to each other via a subject 40. The X-ray irradiating device 10 has an X-ray tube 12 and a collimator box 14. A filter 16 and a collimator 18 are accommodated in the collimator box 14. The filter 16 is one example of the present invention. X-ray emitted from the X-ray tube 12 whose energy spectra are adjusted by the filter 16 is irradiated to the subject 40 through an opening of the collimator 18. The filter 16 can make the energy spectra variable. The collimator 18 has the opening that is variable. The X-ray passing through the subject 40 is detected by the X-ray detecting device 20 to be inputted to the operator controller 30. The operator controller 30 reconstructs the radioscopic image of the subject based upon an inputted signal. The reconstructed radioscopic image is displayed on a display 32 of the operator controller 30. The operator controller 30 further controls the X-ray irradiating device 10. The control of the X-ray irradiating device 10 by the operator controller 30 includes the control of the filter 16 and the control of the collimator 18. It should be noted that the filter 16 and the collimator 18 can manually be adjusted according to actual need. FIG. 2 is a view showing the structure of a filter 16 according to the present invention. As shown in FIG. 2, filter plates 161, 162 are provided in a laminated structure on a pair of rails 172, 174. The rails 172, 174 are parallel with each other, each rail having parallel rails in equal amount with the filter plates. The two ends of each filter plate are respectively inserted into the corresponding guider of the pair of rails 172, 174, so that the filter plates are formed into a laminated structure, and each filter plate is movable parallelly between the pair of rails 172, 174. The filter plates 161, 162 are respectively connected to one ends of link levers 261, 262, while the other ends of the link levers 261, 262 are mounted to axes 272, 274 to be rotatable about the axes 272, 274. Cams 461, 462 for driving the link levers 261, 262 are provided with groove curves, while the link levers 261, 262 are provided with pins to cooperate with the groove curves on the cams 461, 462. The two cams 461, 462 are located respectively at different sides of the driving wheel 800, which drives the cams 461, 462 simultaneously to rotate and thus to move the filter plates 161, 162 into and out of the FOV region of the X-ray from different directions. As shown in FIG. 3, the link levers 261, 262 driven by the cams 461, 462 make the filter plates 161, 162 proceed with a reciprocating movement respectively along the rails 171, 172. The filter plate 161 and/or the filter plate 162 are in the advancing state into the X-ray passing space when they are present in the W region, while they are in the retreating state when they are respectively present at the left and right O region. In the present invention, the structure is simplified, since the filter plates 161, 162 are moved in/out through the reciprocating movement of the link levers 261, 262 driven by the rotation of the cams 461, 462, while the rotation of the cams are driven by a single driving wheel 800. As stated above, through rotation, the cams 461, 462 have the function of switching positions of the filter plates in a binary mode between the advancing position where the filter plates move into the X-ray passing space and the retreating position where the filter plates move out of the X-ray passing space. As shown in FIG. 3, the groove curve of each cam according to the present invention is designed following the method below: 1. According to N, the number of the filter thickness states that can be switched, calculate the rotation angle of each cam in each thickness switch by 360°/N; 2. Set the O region to correspond to R1, the minimum radius of the cam groove curve; 3. Set the W region to correspond to R2, the maximum radius of the cam groove curve; 4. Determine the rotation direction of the driving wheel 800, e.g. the clockwise direction or the anticlockwise direction; 5. Set the O region and W region of the filter plate according to the space size and structure of the collimator box; 6. Determine which side of the filter the cam is positioned, e.g. the left side or the right side; 7. Create a table for designing the groove curve of the cam and a graph of the groove curve. The method for designing the groove curve will be described through the specific examples below. Assuming there are two filter plates 161, 162 made of copper which have thickness of 0.2 mm and 0.1 mm respectively, then the filter composed of the same will have four different thickness switches, i.e. N=4 (N=4, e.g. 0.0 mm, 0.1 mm, 0.2 mm, 0.3 mm; wherein 0.0 mm indicates that no filter is present in the X-ray passing space; 0.1 mm indicates that a filter with thickness of 0.1 mm is present in the X-ray passing space; 0.2 mm indicates that a filter with thickness of 0.2 mm is present in the X-ray passing space; 0.3 mm indicates that a filter with thickness of 0.3 mm is present in the X-ray passing space), and corresponding to each thickness, the cam shall be rotated by an angle of 90° calculated by 360°/4; assuming the driving wheel 800 is rotated anticlockwise, then the two cams 461, 462 are rotated clockwise. Two groove curves 471, 472 are designed on the cams 461, 462 as shown in FIG. 4A according to Table 1. TABLE 1Switch Order of the Filter Plates1234Cam PositionFilter plate, 0.1OWOWright sidemmFilter plate, 0.2OOWWleft sidemmOperating state0.0 mm0.1 mm0.2 mm0.3 mmof the Filter FIG. 4C shows the filter mounted according to Table 1 and FIGS. 4A and 4B, with a operating state as shown in FIGS. 4D-4G. It can be seen that from the 0.0 mm state to the 0.3 mm state, each cam is rotated by 90° with each operating state switch; between the 0.0 mm state and the 0.1 mm state, or between the 0.0 mm state and the 0.3 mm state, the minimum cam rotation angle is 90°; between the 0.1 mm state and the 0.3 mm state, or between the 0.3 mm state and the 0.1 mm state, the minimum cam rotation angle is 180°; between the 0.0 mm state and the 0.2 mm state or between the 0.1 mm state and the 0.3 mm state, the minimum cam rotation angle is 180°. For example, there are two filter plates 161, 162 made of copper which have the thickness of 0.2 mm and 0.1 mm respectively; and a filter plate 163 which has the thickness of hn (as shown in FIGS. 5D and 5E). Assuming the filter composed of the above filter plates has 4 copper filter thickness switches and 1 tin filter thickness switch, i.e. N=5, (N=5, e.g. 0.0 mm, 0.1 mm, 0.2 mm, 0.3 mm, hn), then corresponding to each thickness switch, the cam shall be rotated by an angle of 72° calculated by 360°/5; assuming the driving wheel 800 is rotated anticlockwise, then the cams 461, 462 at the left side and the right side of the driving wheel 800 respectively will be rotated clockwise. Three groove curves 571, 572, 573 as shown in FIGS. 5A-5C are designed according to Table 2, wherein on the front surface and the back surface of the cam 462 are respectively provided with groove curves 572, 573 for respectively controlling the copper filter plate 162 with the thickness of 0.1 mm and the tin filter plate. A third link lever 263 is further included, with one of its end connected to the tin filter plate. A pin (with no reference number) is provided on the third link lever 263 to cooperate with the groove curve 573 on the back surface of the cam 462. TABLE 2Switch Order of the Filter Plates12345Cam PositionFilter Plate, 0.1OWOWOright sidemmFilter Plate, hnOOOOWright sideFilter Plate, 0.2OOWWOleft sidemmOperating state0.00.10.20.3hnof the Filtermmmmmmmm FIGS. 5D and 5E are two schematic drawings showing the front side and the back side of the filter mounted by two cams 461, 462 according to Table 2 and FIGS. 5A-5C. The operating states of the filter is shown in FIGS. 5F-5J. It can be seen that from the 0.0 mm state to the 0.3 mm state and then to the hn state, each cam is rotated by 72° along with each operating state switch, wherein between the 0.0 mm state and the hn state or between the hn state and the 0.0 mm state, the minimum cam rotation angle is 72°; between the 0.1 mm state and the 0.2 mm state and the hn state, the minimum cam rotation angle is 144°. If the required thickness switch states include four copper filter thickness switches and one tin filter thickness plus 0.3 mm, i.e. N=5, (N=5, 0.0 mm, 0.1 mm, 0.2 mm, 0.3 mm, hn+0.3 mm), then three groove curves 671, 672, 673 as shown in FIGS. 6A-6C are designed according to Table 3, wherein groove curves 672, 673 are provided on the front surface and the back surface of the cam 462 for respectively controlling the copper filter plate 62 with thickness of 0.1 mm and the tin filter plate 163. TABLE 3Switch Order of the Filter Plates12345Cam PositionFilter Plate, 0.1OWOWWRight sidemmFilter Plate, hnOOOOWRight sideFilter Plate, 0.2OOWWWLeft sidemmOperating state0.00.10.20.3hn +of the Filtermmmmmmmm0.3 mm FIGS. 6D and 6E are schematic drawings showing the front side and the back side of the filter mounted by the two cams 461, 462 designed according to Table 3 and FIGS. 6A-6C. The operating states of the filter are shown in FIGS. 6F-6J. It can be seen that from the 0.0 mm state to the 0.3 mm state and then to the hn+0.3 mm state, each cam will be rotated by 72° along with each operating state switch. Of course, all the filter plates as described in the present invention can be made of copper or tin or other materials which are suitable for filtering. In FIGS. 4D-4G, 5F-5J, and 6F-6J, 0.0CF, 0.1CF, 0.2CF and 0.3CF refer to copper filter, 0.0 mm; copper filter, 0.1 mm; and copper filter, 0.2 mm; and copper filter, 0.3 mm respectively; TinF refers to tin filter. |
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062953346 | description | DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic diagram showing an X-ray exposure system assembled with an SR light transmission system according to an embodiment of the invention. The outline of the X-ray exposure system has been described already, and so the description thereof is not duplicated herein. In the SR light transmission system of this embodiment, the structures of the mirror 15, swinging mechanism 16 and window flange 37 are different from those of conventional systems. These different structures will be later detailed. First, a change in a substantial focal length when the mirror 15 swings will be discussed. FIG. 2 is a schematic diagram showing a relation between the optical axis of SR light and the reflection plane of the mirror 15. When the mirror is set at a reference position, SR light SR.sub.1 radiated from a light source P.sub.s is incident upon a reference reflection point P.sub.r1 on the reflection plane of the mirror. Light SR.sub.r reflected at the reference reflection point P.sub.r1 is applied to an exposure plane S of a semiconductor substrate. A swing axis P.sub.o is on a line extending from the reflection plane RF.sub.1 of the mirror at the reference position. L.sub.1 is defined as a distance between the light source P.sub.s and the reference reflection point P.sub.r1, L.sub.2 is defined as a distance between the reference reflection point P.sub.r1 and the exposure plane S. R is defined as a distance between the reference reflection point P.sub.r1 and the swing axis P.sub.o, and .theta. is defined as an angle between the mirror reflection plane and the optical axis of the incidence light SR.sub.1. It is assumed that the mirror reflection plane is a curved plane symmetrical with a rotation center axis in the incidence plane. Consider now that the mirror swings by an angle from the reference position. Representing the reflection point at this time as P.sub.r2, a distance x between the reference reflection point P.sub.r1 and the reflection point P.sub.r2 is given by: EQU x=R.times.sin(.DELTA..theta.)/sin(.theta..sub.o -.DELTA..theta.) EQU .apprxeq.R.times.sin(.DELTA..theta.)/(sin(.theta..sub. o)-sin(.DELTA..theta.)) (1) The conditions that the SR light SR.sub.1 radiated from the light source P.sub.s and reflected by the mirror becomes parallel light fluxes, are given by: EQU f=L.sub.1 +x (2) where f is a focal length of the mirror. A fundamental equation of the mirror satisfies: EQU f=r/(2 sin(.theta..sub.o -.DELTA..theta.)) (3) where r is a radius of curvature of the reflection plane at the reflection point P.sub.r2. By erasing .DELTA..theta. from the equations (1) to (3), r is expressed as a function of x as: EQU r=((L.sub.1 +x)/(R+x)).multidot.2R sin(.theta..sub.o) EQU .apprxeq.(1-(1-L.sub.1 /R)/(1-x/R)).multidot.2R sin(.theta..sub.0) (4) If x/R<<1, the equation (4) is rewritten as: EQU r.apprxeq.(1-(1-L.sub.1 /R).multidot.(1-x/R)).multidot.2R sin(.theta..sub.o) (5) Namely, the radius r of curvature can be approximated by a linear function of x. This indicates that the mirror reflection plane has nearly a conical surface. If the mirror reflection plane has a conical surface having the radius r of curvature given by the equation (5), the shape of light reflected from the mirror at any swing angle and vertically projected upon the horizontal plane is always that of parallel light fluxes. If R=L.sub.1 is substituted into the equation (5), then it is represented by: EQU r.apprxeq.2L.sub.1 sin(.theta..sub.o) (6) Therefore, the radius r of curvature is constant at any value x. This means that the mirror reflection plane has a cylindrical surface. Therefore, if the swing radius R is equal to the distance L.sub.1 between the reflection reference point P.sub.r1 and the light source P.sub.s and the mirror reflection plane is made to have a cylindrical surface having the radius of curvature given by the equation (6), then it is possible to make the shape of light reflected from the mirror and vertically projected upon the horizontal plane have that of parallel light fluxes. Next, the results of simulation made to verify the effects of the mirror 15 having a cylindrical surface will be described with reference to FIGS. 3A to 3C. FIGS. 3A to 3C show the shapes of SR light exposure areas on the exposure plane. The abscissa corresponds to the horizontal direction, and the ordinate corresponds to the direction along which the optical axis of reflected SR light extends. Only one side of the shape of a radiation area is shown in FIGS. 3A to 3C, the whole shape being symmetrical with right and left. The spread angle of SR light radiated from the light source was set to +/-10 mrad in the horizontal direction, to +/-0.5 mrad in the vertical direction, and an angle .theta..sub.0 between the incidence light optical axis and the reflection plane at the mirror reference position was set to 1.8.degree.. The distances L.sub.1 and L.sub.2 shown in FIG. 2 were set to 3 m and 6 m, respectively. In the case of FIG. 3A, the swing radius R was set to 0, i.e., the mirror was swung around the reference reflection point P.sub.r1. Shapes a.sub.1 to a.sub.5 show the radiation areas of SR light when the angle e shown in FIG. 2 was varied. The shape a.sub.1 corresponds to the radiation area at the angle .theta. of 1.61.degree., and the shape a.sub.5 corresponds to the radiation area at the angle .theta. of 1.94.degree.. The shapes a.sub.2 to a.sub.4 correspond to the angles between 1.61.degree. and 1.94.degree.. As the angle .theta. becomes small, the radiation area moves upward and the expansion in the horizontal direction becomes large. This means that the focal length f given by the equation (2) becomes long and the convergence force of the cylindrical mirror weakens. In the case of FIG. 3B, the swing radius R was set to 3 m, i.e., R=L.sub.1. A shape b.sub.1 corresponds to the radiation area at the angle .theta. of 1.67.degree., and the shape b.sub.5 corresponds to the radiation area at the angle .theta. of 1.89.degree.. The shapes b.sub.2 to b.sub.4 correspond to the angles between 1.67.degree. and 1.89.degree.. The case of FIG. 3B satisfies the conditions of the equation (6) so that reflected SR light becomes parallel light fluxes. Therefore, a variation of the sizes of radiation areas when the angle .theta. is varied, is small. In the case of FIG. 3C, the swing radius R was set to 10 m. A shape c.sub.1 corresponds to the radiation area at the angle .theta. of 1.72.degree., and the shape C.sub.5 corresponds to the radiation area at the angle .theta. of 1.89.degree.. The shapes C.sub.2 to C.sub.4 correspond to the angles between 1.72.degree. and 1.89.degree.. In the case of FIG. 3C, as the angle .theta. is made small, the lateral width of the radiation area becomes narrow. This is because the influence of an elongated focal length by the smaller angle .theta. is less than the influence of a shortened distance (L.sub.2 -x) between the reflection point P.sub.r2 and the exposure plane S shown in FIG. 2. The shapes of radiation areas at the swing radius R=L.sub.1 are not uniform, the reason of which is that light shifted from the optical axis was not compensated in fundamental calculations of simulation. Comparison between FIGS. 3A and 3C shows that the change in the area and shape of a radiation area is smaller in FIG. 3C than in FIG. 3A. Namely, in order to make the energy density on the exposure plane uniform, it is preferable to set the swing radius R larger than the distance L.sub.1 between the light source P.sub.s and the reference reflection point P.sub.r1. Next, examples of the structure of the swinging mechanism shown in FIG. 1 will be described with reference to FIGS. 4A to 4C. FIG. 4A shows a first example of the swinging mechanism. One end of a lever 21 is pivotally supported by a support shaft 20 whose relative position to the mirror box 12 is fixed. A drive point of a drive mechanism 22 mounted on the mirror box 12 couples to the level 21 near at its other end. The mirror 15 is supported in the mirror box by two links 23A and 23B. Portions of the links 23A and 23B extend out of the mirror box 12 and fixed to the lever 21 near at its other end portions. The regions via which the links 23A and 23B pass through the wall of the mirror box 12 are maintained to be hermetically sealed with bellows to maintain vacuum. A relative position of the mirror 15 to the support shaft 20 is set so that a straight line extending from the reflection plane of the mirror 15 toward the incoming opening 13 intersects with the support shaft 20. As the driving mechanism 22 moves up and down the drive point, the lever 21 swings around the support shaft 20 so that the mirror 15 coupled thereto also swings around the support shaft 20. FIG. 4B shows a second example of the swinging mechanism. In the case of FIG. 4A, the mirror 15 is fixed to the swinging lever 21 near at its end portions. In contrast, in the case of FIG. 4B, the mirror 15 is coupled to a lever 25 via a motion length changing mechanism 26. The lever 25 is pivotally supported by a support member 27 fixed to the mirror box 12, and its one end is driven up and down by a driving mechanism 28. As the level 25 swings, the mirror 15 swings similar to the case of FIG. 4A, with the help of the motion length changing mechanism 26. Use of the motion length changing mechanism 26 therefore reduces the size of the swinging mechanism. FIG. 4C shows a third example of the swinging mechanism. The mirror 15 is supported via links 29A and 29B by electrical driving mechanisms 24A and 24B which drive the links 29A and 29B up and down. By controlling the phases of up/down motions of the links 29A and 29B and adjusting the amplitudes of the up/down motions properly, the mirror 15 swings similar to the case of FIG. 4A. Next, the structure of the vacuum duct 30 shown in FIG. 1 near at its output port will be described with reference to FIGS. 5A and 5B. FIG. 5A is a cross sectional view showing the structure of the vacuum duct 30 near at its output port, and FIG. 5B is a side view as seen along an arrow A shown in FIG. 5A. The vacuum duct 30 is fixed near at its end portion to a base 60 by a support column 32. A flange 33 is formed at the end of the vacuum duct 30 to which one flange 35 of a metal bellows 34 is mounted. Another flange 36 of the bellows 34 is mounted on the window flange 37. As shown in FIG. 5B, the window flange 37 is formed with an opening having a shape defined by curves of upward convex obtained by curving virtual horizontal lines. A beryllium thin film 31 of about 20 .mu.m thickness is welded or soldered to cover the opening. A protective hood 38 extending outward is mounted on the circumference of the opening. This protective hood 38 prevents the beryllium thin film 31 from being broken. Slide bearings 40 are mounted on right and left extensions of the window flange 37. The slide bearings 40 are fitted around guide shafts 41 so that each bearing 40 slides in a direction guided by the corresponding guide shaft 41. The guide shaft 41 is fixed via opposite bosses 42, support plates 43 and the like, to the vacuum duct 30. The guide direction of the guide shaft is parallel to a cross line between a plane vertical to the center axis of the vacuum duct 30 and a vertical plane including the center axis. The window flange 37 is coupled to a drive rod 45 of a linear driving unit 46, via an L-shaped metal jig 44 mounted on a lower extension of the window flange 37. The linear driving unit 46 rotates a motor 47 to reciprocally move the drive rod 45 along the guide direction of the guide shaft 41. In response to a reciprocal motion of the drive rod 45, the window flanges 37 moves reciprocally. The window flange 37 is reciprocally moved synchronously with a swing motion of the optical axis of reflected SR light travelling in the vacuum duct 30. SR light passes through the beryllium thin film 31 and is applied to the exposure plane of the semiconductor substrate 51 shown in FIG. 1. As shown in FIGS. 3A to 3C, the cross section of reflected SR light has a shape defined by curves of upward convex. Since the beryllium thin film 31 is made to have a shape corresponding to the cross section of SR light, the window area can be made small. Accordingly, ac compared a simple circle shape of the window, the beryllium thin film can be made thick and its breakage can be avoided. Although the window shape shown in FIG. 5B is defined by upwardly convex curves, it may be defined by downwardly convex curves if the reflection plane of the mirror 15 shown in FIG. 1 is directed upward and SR light is reflected upward. In the above, the uniformed energy density, when the swing axis of the mirror 15 shown in FIG. 1 is set on the light source side, has been described. Setting the swing axis on the light source side has the effect of not only uniforming the energy density but also improving a runout. Next, the effects of improving a runout will be described. While a broad area on the exposure plane is exposed with reflected SR light whose optical axis is swung, if the optical axis of SR light incident upon one point on the exposure area is vertical to the exposure plane, then the optical axis of SR light incident upon another point remote from the one point is oblique to the exposure plane. Since the exposure mask and exposure plane are disposed with a certain gap therebetween, the oblique optical axis makes a mask pattern not correctly transferred to the exposure plane. This phenomenon is called a runout. In order to alleviate the effects of a runout, it is preferable to set the swing center of reflected SR light as far from the exposure plane as possible. If the swing axis of the mirror 15 shown in FIG. 1 is disposed on the light source side from the exposure plane, it can be understood from approximate calculations that a substantial distance between the swing center of reflected SR light and the exposure plane becomes roughly equal to a sum of the swing radius R and the distance L.sub.2 between the reference reflection point P.sub.r1 and the exposure plane S. Therefore, the swing center of reflected SR light is made remote from the exposure plane and so the effects of a runout can be alleviated. Converging SR light in the horizontal plane has been described with reference to FIGS. 3A to 3C. Next, converging SR light in the vertical plane will be described. If the mirror 15 shown in FIG. 1 is given some radius of curvature around an axis vertical to the incidence plane of SR light, the SR light can be converged in the vertical plane. Such a mirror can be obtained, for example, by curving a cylindrical mirror along a plane including the center axis of the mirror. By representing the radius of curvature of the mirror 15 by r.sub.t and an angle .theta. between the incidence light optical axis and the reflection plane by (.theta..sub.0 -.DELTA..theta.), a focal length f.sub.t with respect to the incidence plane is given by: EQU f.sub.t =(r.sub.t /2)sin(.theta..sub.0 -.DELTA..theta.) (7) The conditions that reflected SR light becomes parallel light fluxes satisfy: EQU f.sub.t =L.sub.1 +x (8) By erasing .DELTA..theta. from the equations (1), (7) and (8), the radius r.sub.t of curvature is expressed as a function of x by: EQU r.sub.t =(2/sin(.theta..sub.o))(x.sup.2 /R+(1+L.sub.1 /R)x+Li (9) By selecting the radius r.sub.t of curvature to satisfy the equation (9), reflected SR light vertically projected upon the incidence plane becomes parallel light fluxes. Assuming that in FIG. 1 the distance L.sub.1 is 3 m, the distance L.sub.2 is 6 m, and the angle .theta..sub.o is 1.8.degree., the radius r.sub.t of curvature is 191 m at the reference position of the mirror 15, i.e., at x=0. FIG. 6A shows the shapes of radiation areas on the exposure plane obtained by using a troidal mirror having a radius r of curvature of 188 m in a plane vertical to the incidence plane and a radius r.sub.t of curvature of 191 m in the incidence plane. The abscissa corresponds to the horizontal direction, and the ordinate corresponds to the swing direction of reflected SR light. Shapes d.sub.1, d.sub.2 and d.sub.3 correspond to the radiation areas at the angles .theta. shown in FIG. 2 of 1.63.degree., 1.8.degree. and 1.92.degree., respectively. As compared with the shapes shown in FIGS. 3A to 3C, it is seen that the width in the vertical direction is compressed. As above, if the width of the radiation area in the vertical direction is compressed, the width of the window 31 of the beryllium thin film shown in FIG. 5B can be made narrower. Therefore, the strength of the beryllium thin film can be increased more and the safety of the system can be improved. FIG. 6B shows the shapes of radiation areas obtained by using a toroidal mirror having a radius r.sub.t of curvature 127 mm in the incidence plane. Shapes e.sub.1, e.sub.2 and e.sub.3 correspond to the radiation areas at the angles e shown in FIG. 2 of 1.60.degree., 1.8.degree. and 1.94.degree., respectively. With the radius r.sub.t of curvature of 127 mm, reflected SR light is focused on the exposure plane as viewed along the direction perpendicular to the incidence plane. It is seen that the width in the vertical direction is compressed more than the shapes shown in FIG. 6A. By selecting the radius r.sub.t of curvature to such a value allowing reflected SR light to be focussed on the exposure plane, the area of the window 31 of the beryllium thin film can be reduced. However, as the width of the radiation area in the vertical direction is made narrower, the energy density increases in inverse proportion with it. As the energy density becomes too high, a local temperature rise may occur, thereby increasing a danger during malfunction or other detective operations. Optimum exposure conditions can be set by changing the radius r.sub.t of curvature and the energy density. As described above, the radius of curvature of the reflection plane in the incidence plane is very large. As a means for forming a mirror with such a large radius of curvature, a method is known in which mirror raw material is placed on a mount having an inversely curved surface and worked to have a proper curvature to thereafter dismount the mirror from the mount. In this case, it is necessary to form a mount having the inversely curved surface. Another method is to give a bending moment to a mirror to form a curved surface. FIG. 7 is a schematic cross sectional view of a mirror mount capable of giving a bending moment to a mirror 15. The mirror 15 is placed in tight contact with one side of a mirror mount plate 17. The other side of the mirror mount plate 17 is provided with arms 18 extending vertically from the mirror mount plate, at opposite end portions of the other side. An adjusting screw 19 is inserted into the end portion of each arm 18, and coupled to an expansion rod 19A. The distance between each arm 18 and the end of the expansion rod 19 is adjusted with the adjusting screw to apply a bending moment to the mirror mount plate 17 and curve the mirror 15. With this method, since a uniform bending moment is generated over the whole length of the mirror mount plate 17, the radius of curvature becomes constant if the mirror mount plate 17 has the same cross sectional area over the whole length thereof. Consider the x-y coordinate system where the direction of a bending moment in the mount plane of the mirror mount plate 17 is the x-axis and the normal direction is the y-direction. The shape of the mount plane of the mirror mount plate 17 is given by a differential equation: EQU d.sup.2 y/dx.sup.2 =M/E.sub.z (10) where M is a bending moment, E is a Young's modulus of the material, and I.sub.z is a cross sectional secondary moment. Linear equation approximation for the equation (9) produces: EQU r.sub.t =(2/sin(.theta..sub.o)).multidot.((1+L.sub.1 /R)x+L.sub.1 i) (11) Substituting into the equation (11) EQU (2/sin(.theta..sub.o)).multidot.(1+L.sub.1 /R)=a EQU (2/sin(.theta..sub.o))=b (12) yields: EQU r.sub.t =ax+b (13) The cross sectional shape in the incidence plane of the reflection plane of the mirror 16 is given as a solution of the following differential equation: EQU d.sup.2 y/dx.sup.2 =1/r.sub.t =1/(ax+b) (14) Comparison between the equations (10) and (14) introduces: EQU I.sub.z =(M/E).multidot.(ax+b) (15) If the elastic secondary moment I.sub.z of the mirror mount plate 17 satisfies the equation (14), a desired radius of curvature is obtained. If the thickness of the mirror mount plate 17 is constant, the elastic secondary moment I.sub.d is proportional to the width of the mirror mount plate 17. Namely, a desired radius of curvature can be obtained by changing the width of the mirror mount plate 17 in correspondence with (ax+b) in the x-axis direction. Another embodiment of the invention will be described next. FIG. 8A is a plan view of the outgoing vacuum duct shown in FIG. 1 near at its output port, FIG. 8B is a cross sectional view taken along one-dot chain line B2--B2 shown in FIG. 8A, and FIG. 8C is a side view as seen along an arrow C2 shown in FIG. 8A. A flange 30a is mounted on the output port of the vacuum duct 30. A window flange 37 having a square opening is coupled to the flange 30a. A coupling region between the flange 30a and window flange 37 is sealed with an O-ring. A window frame 39 is welded to the output port surface of the window flange 37 to surround the opening. The output port surface of the window frame 39 is worked to have a cylindrical shape. The center axis of this cylindrical surface is perpendicular to the center axis of the vacuum duct 30 (optical center axis of SR light travelling in the duct) and parallel to the vertical plane. A beryllium thin film 31 is welded or soldered to the cylindrical surface of the window frame 39. The beryllium thin film 31 is worked to have a cylindrical shape in correspondence with the cylindrical surface of the window frame 39. A plane beryllium thin film may be deformed along the cylindrical surface of the window frame 39. The vacuum of the vacuum duct 30 is retained by the beryllium thin film 31. Next, the function of the cylindrical beryllium thin film 31 will be described with reference to FIGS. 9A and 9B. FIG. 9A shows a shape of an SR radiation area on the exposure plane of the semiconductor substrate shown in FIG. 1. Consider an x-y coordinate system having as the x-axis the horizontal direction on the exposure plane and as the y-axis the axis perpendicular to both the optical center axis of SR light and the x-axis. It is assumed that an SR light radiation area 60 (corresponding to a beam cross section of SR light) is defined by a circular line having a radius R. The length of the radiation area 60 cut along a straight line parallel to the y-axis is represented by a function of a coordinate value x. Therefore, a radiation time of a point in the exposure plane when the radiation area 60 is moved at a constant speed along the y-axis direction becomes a function of the coordinate value x. The radiation time of a point at the coordinate value x is proportional to (1-(x/R).sup.2).sup.-1/2. If the photon density of SR light before it becomes incident upon the beryllium thin film is constant, the exposure amount P.sub.x of the point on the exposure plane assuming that the beryllium thin film does not exist, is given by: P.sub.x =P.sub.o (1-(x/R).sup.2).sup.-1/2 (16) where P.sub.o is an exposure amount of a point on a straight line (y-axis) at x=0. FIG. 9B shows the cross section of the beryllium thin film 31 and the optical axis of SR light, respectively in the plane perpendicular to a generator of the cylindrical surface of the beryllium thin film 31 shown in FIG. 8A. By representing a film thickness of the beryllium thin film 31 by T and a radius of curvature of the cylindrical surface by r, an SR light optical path length t.sub.x of the beryllium thin film 31 at the coordinate value x is given by: EQU t.sub.x =T(1-(x/r).sup.2).sup.2).sup.-1/2 (17) If an SR light absorption coefficient of the beryllium thin film is .mu., the exposure amount Q.sub.x on the exposure surface when SR light transmits through the beryllium thin film is given by: EQU Q.sub.x =P.sub.x.multidot.exp(-.mu.t.sub.x) (18) From the equations (16) to (18), the following equation is established: EQU Q.sub.x =P.sub.o (1-(x/R).sup.2).sup.-1/2.multidot.exp(-.mu.T(1-(x/r).sup.2).sup.-1/2 (19) Since EQU Q.sub.x =P.sub.o.multidot.exp(-.mu.T) (20), the following equation is obtained from the equations (19) and (20): EQU Q.sub.x /Q.sub.o =(1-(x/R).sup.2).sup.-1/2.multidot.exp((-.mu.T)((1-(x/r).sup.2).sup.-1/2 -1)) =(1-(x/R).sup.2).sup.-1/2.multidot.exp((-.mu.T)((1-(x/kR).sup.2).sup.-1/2 -1)) (21) where EQU r=kR (22) FIGS. 10A to 10C are graphs showing Q.sub.x /Q.sub.o as a function of x/R by using the equation (21). FIGS. 10A to 10C correspond to the values k of 0.8, 1.0 and 1.2, respectively. In the graphs, one-dot chain line, dotted line, broken line and solid line correspond to the value Q.sub.o /P.sub.o, i.e., exp(-.mu.T), of 0.2, 0.4, 0.6 and 0.8, respectively. If Q.sub.x /Q.sub.o =1, the exposure amounts on the exposure plane are uniform. In all the cases shown in FIGS. 10A to 10C, in the area with a small x/R, Q.sub.x /Q.sub.o is near "1", and as the value x/R becomes large, it goes apart from "1". SR light contains different wavelengths in a predetermined range and the transmittivity of a beryllium thin film depends upon the wavelength. Q.sub.o /P.sub.o in average is predicted in the range from 0.4 to 0.6. If Q.sub.o /P.sub.o is in a range from 0.4 to 0.6, Q.sub.x /Q.sub.o goes lower right at k=0.8, is nearly flat at k=1.0, and goes upper right at K=1.2. It is therefore preferable to set the value k in a range from 0.8 to 1.2, and more preferable to set it near to 1. In the above studies, it has been assumed that while the SR exposure area is moved on the exposure plane, the shape of the SR exposure area does not change and is defined by circular lines. Even if these assumptions cannot be established strictly, a variation of exposure amounts on the exposure plane can be made small by adjusting the value k. If the shape of an exposure area is defined by curved lines other than circular lines, the beryllium thin film is curved to have a suitable shape matching the shape of the exposure area. In this case, it can be expected that a variation of exposure amounts can be suppressed. In the example shown in FIG. 8A, the beryllium thin film 31 is curved convexly toward the inside of the vacuum duct 30. Similar effects as above can be expected even if the beryllium thin film 31 is curved convexly toward the outside of the vacuum duct 30. The above structures may be applied not only to the case where the mirror is swung to swing SR light, but also to the case where the semiconductor substrate is moved while the mirror is fixed. The fundamental teachings of the above embodiment are applicable not only to an X-ray exposure system, but also to other systems where SR light is intended to be attenuated in the plane perpendicular to the optical axis, with some intensity attenuation distribution. If an intensity attenuation in one direction is desired, the thin film is curved along the cylindrical surface formed by a generator perpendicular to the optical axis. Various intensity distributions are possible by changing the curved shape. The shape of an SR light beam in a plane perpendicular to the optical axis is generally short in a first direction and long in a second direction perpendicular to the first direction. A thin film made of material capable of attenuating SR light may be disposed in the optical path of SR light and, in this case, the thin film is curved in a plane parallel to both the optical axis direction of SR light and the second direction. With such a thin film, SR light can be attenuated by an amount different at different points in the second direction. In the example shown in FIG. 9A, the cross section of SR light is defined by circular lines having a radius R of curvature. In an actual case, the radiation area 60 of SR light changes its radius R while being moving in the y-axis direction. Although the radiation area 60 is considered as being approximately formed along circular lines, it is considered strictly as being formed along curved lines having no inflection point. It is also considered that the SR light intensity (the number of photons per unit area) in the radiation area 60 is not constant but follows a function of a coordinate value x. Another embodiment capable of making exposure amounts constant by incorporating the above considerations, will be described. FIG. 11A shows the outline of an X-ray exposure system according to another embodiment. The fundamental structure of the X-ray exposure system shown in FIG. 5 is similar to that shown in FIG. 1. Each constituent element of the X-ray exposure system shown in FIG. 5 is represented by using identical reference numerals to those used in FIG. 1. Only different structures from that of the X-ray exposure system shown in FIG. 1 will be described hereinafter. The outgoing vacuum duct 30 is coupled via a vacuum bellows 17 to the outgoing opening 14 of the mirror box 12, and supported on a base via a vacuum duct driving mechanism 18. The vacuum duct driving mechanism 18 swings the outgoing vacuum duct 30 so that a relative position of the outgoing vacuum duct 30 and the optical center axis of SR light travelling therein will not change while the optical center axis is swung up and down by a swinging motion of the reflection mirror 15. A beryllium thin film 31 is adhered to the window flange 37. As shown in FIG. 11B, the window flange 37 is formed with a circular window 37a. The shape of the window 37a has an integrity with that of an SR beam cross section travelling in the outgoing vacuum duct 30. The window 37a is covered with the beryllium thin film 31 whose details will be later given. Since the relative position of the outgoing vacuum duct 30 and the optical center axis of SR light travelling therein will not change, the SR light always passes through the window 37a and is output to the outside of the outgoing vacuum duct 30 even if the SR light center axis is swung up and down. An area of the beryllium thin film 31 shown in FIG. 11B is smaller than that shown in FIG. 8C. Therefore, it can easily endure a pressure difference between the outside and inside of the outgoing vacuum duct 30. Furthermore, as will be described in the following, the structure shown in FIG. 11B is advantageous in that a temperature rise of the beryllium thin film 31 is suppressed. The beryllium thin film 31 plays a roll of a filter for absorbing longer wavelength components of SR light. The energy of SR light absorbed in the beryllium thin film 31 is about 50% of the total energy. Absorption of SR light generates heat in the beryllium thin film 31. This heat is dissipated through thermal conduction to the window flange 37, through radiation from the beryllium thin film and through convection of external air in contact with the beryllium thin film 31. If the temperature of the beryllium thin film 31 rises to 250.degree. C. or higher, the strength of the film lowers considerably. In order to endure a pressure difference between the inside and outside of the outgoing vacuum duct 30, it is preferable to maintain the temperature of the beryllium thin film 30 not higher than 250.degree. C. In the range of about this temperature, thermal conduction to the window flange 37 becomes dominant for thermal dissipation. As shown in FIG. 11B, since the area of the window 37a is small and elongated, the thermal conduction efficiency can be improved so that a temperature rise of the beryllium thin film 31 can be suppressed. Next, uniformity of in-plane exposure amounts on the surface of a semiconductor substrate 51 will be discussed. FIG. 12A shows the intensity distribution of SR light applied to the surface of a semiconductor substrate 51. A rectangle ABCD shows a half area of the radiation area, and a straight line AD is a center line of the radiation area. A center of a line segment AD is represented by G and a center of a line segment BC is represented by H. Consider an x-y-z coordinate system on the exposure plane, where the x-axis corresponds to the straight line GH, the y-axis corresponds to a straight line DA, and the z-axis corresponds to a direction normal to the exposure plane. SR light having a beam cross section elongated in the x-axis direction is scanned along the y-axis direction on the exposure plane from a point D toward a point A to thereby expose the rectangle ABCD area. Assuming that the x-axis direction indicates the intensity of SR light, the intensity distributions of SR light are represented by curved surfaces P.sub.1, P.sub.2 and P.sub.3 respectively at the start of scanning, at the half position of scanning the exposure area, and at the end of scanning. A solid shape defined by each curved surface can be represented by a wall standing in parallel to the x-z plane and slightly curved around a straight line parallel to the z-axis direction. The curvature and height of each wall are determined in accordance with the shape of the reflection mirror 15 shown in FIG. 11, a geometrical layout of SR light and the reflection mirror 15, a thickness of the beryllium thin film 31 and the like. A swing angular velocity of the reflection mirror 15 is assumed to be constant. FIG. 12B shows a distribution of exposure amounts in the rectangle ABCD when SR light is scanned once on the exposure plane. A height of each point on a curved surface A' B' H' C' D' G'from the x-y plane indicates the exposure amount at that point. For example, the exposure amount at a point A is represented by a length of a line segment AA'. SR light radiated from the electron circular orbit 3 shown in FIG. 11A spreads greatly in the horizontal direction and slightly in the vertical direction. A contact point of the optical center axis of SR light incident upon the reflection mirror 15 with the electron circular orbit 3 may be considered as a light source of SR light. Using this light source as an origin, an angle of the optical center axis of SR light relative to the horizontal direction is represented by .theta..sub.x and an angle relative to the vertical direction is represented by .theta..sub.Y. A swing angle of the reflection mirror relative to its reference position is represented by .phi.. For example, .phi. is 0 when the optical center axis of SR light reaches the center point G of the exposure area. As the swing angle .phi. changes, an incidence angle of SR light changes and the curved wall representative of the SR light intensity shown in FIG. 12A moves in the y-axis direction. When the swing angle .theta. is fixed, e.g., when SR light is applied to the area corresponding to the curved surface P.sub.1, the angle .theta..sub.x corresponds to a direction of a curved line of a top edge of the curved line P.sub.1 vertically projected upon the x-y plane, whereas the angle .theta..sub.y corresponds to a direction perpendicular to the projected curved line. These angles .theta..sub.x and .theta..sub.y are defined in the same manner also for the curved surfaces P.sub.2 and P.sub.3. As the swing angle .theta. and angles .theta..sub.x and .theta..sub.y are determined, the coordinates (x, y) on the exposure plane and the SR light intensity P at this point can be definitely determined. The values x, y and P can therefore be given by: EQU x=F.sub.x (.theta..sub.x,.theta..sub.y,.phi.) EQU y=F.sub.y (.theta..sub.x,.theta..sub.y,.phi.) EQU P=F.sub.p (.theta..sub.x,.theta..sub.y,.phi.) (23) Generally, a line intersecting each of the curved surfaces P.sub.1 to P.sub.3 shown in FIG. 12A and indicating the SR light intensity distribution on the exposure plane at a certain time can be approximated by a Gaussian distribution curve. In order to facilitate the analysis to follow, the curved wall defined by each of the curved surfaces P.sub.1 to P.sub.3 is assumed as a wall having a rectangular cross section with a uniform thickness at any .theta..sub.x. and it is assumed that the area of the rectangular cross section is indicated by an integrated value of Gaussian distribution at the cross section position .theta..sub.x. If a thickness of the wall is W and a height thereof is H(.theta..sub.x, .phi.), the following equation stands: EQU W.multidot.H(.theta..sub.x,.phi.)=.intg.F.sub.p (.theta..sub.x,.theta..sub.y,.phi.)d.theta..sub.y (24) FIG. 13 is a graph corresponding to FIG. 12A with the above-described assumptions. The walls defined by the curved surfaces P.sub.1 to P.sub.3 shown in FIG. 12A are replaced by walls P.sub.1 to P.sub.3 having a uniform thickness W and a rectangular cross section. In FIG. 13, a center line (line corresponding to .theta..sub.y =0) of the wall showing the intensity distribution of SR right is used as a representative of the radiation area of SR light at a certain time. In this case, the intensity of SR light at that time is represented by a function of only x and .phi.. The intensity P of SR light is therefore given by: EQU P=F.sub.1 (x,.phi.) (25) An intensity distribution of SR light in the y-axis direction is represented by a function of a swing angle .phi.. FIG. 14 is a graph showing an example of a distribution of the SR light intensity in the y-axis direction, as normalized by the intensity at the point G. Even if the SR light intensity in the y-axis direction is not constant, this intensity in the y-axis direction can be made uniform by changing the swing angular velocity of the reflection mirror 15 in accordance with a change in the swing angle .phi.. This method will be later described. FIG. 15 shows an example of the SR light intensity distribution in the x-axis direction, as normalized by the SR light intensity on the center line AGD of the exposure area. Three curves P.sub.1 to P.sub.3 shown in FIG. 15 correspond SR light beams applied to the positions of the walls P.sub.1 to P.sub.3 shown in FIG. 13. FIG. 15 shows a convergence characteristics of the reflection mirror. A convergence coefficient K, is defined as: EQU K.sub.1 (x,.phi.)=F.sub.1 (x,.phi.)/F.sub.1 (0,.phi.) (26) Referring to FIG. 16, consider the case where SR light is applied to an area having a width W defined along a circular line having a radius R and scanned in the y-axis direction to expose the whole exposure area. A width of a beam in the y-axis direction exposing a point having the coordinate value x in the exposure area is given by: EQU W/cos(.alpha.) (27) Since it is assumed that the width is constant, the exposure amount at the position on a line having a center angle a takes a value of the exposure amount at the position on the center line multiplied by: EQU 1/cos(.alpha.) (28) By representing the center line (line corresponding to .theta..sub.y, 0 shown in FIG. 13) of the beam cross section by a curve y=g(x, .phi.), the following equation stands: EQU tan(.alpha.)=dy/dx=(.differential./.differential.x).multidot.g(x,.phi.) (29) By using the exposure amount on the center line (x=0) of the exposure area as a reference, the exposure amount K.sub.2 (x, .phi.) at the coordinate value x (hereinafter K.sub.2 is called an inclination coefficient) is given by: EQU K.sub.2 (x,.phi.)=1/cos(.alpha.)=(1+((.differential./ .differential.x).multidot.g(x,.phi.)).sup.2).sup.1/2 (30) The rightmost term in the equation (30) is applicable not only to the case where the center line of the beam cross section is defined along a curved line, but also to the general case where it is defined along a smooth curved line without any inflection point. FIG. 17 is a graph showing an example of the equation (30). Three curves P.sub.1 to P.sub.3 shown in FIG. 17 correspond SR light beams applied to the positions of the walls P.sub.1 to P.sub.3 shown in FIG. 13. FIG. 18 shows an example of the cross section of an SR light beam to be applied to the exposure plane. Now consider that the radiation position of SR light moves from P.sub.1 to P.sub.2. As the radiation position moves along the y-axis direction, the curvature of the beam cross section changes. Therefore, the motion amount L.sub.o of a radiation position on the center line (x=0) of the radiation area is different from the motion amount L.sub.x at the coordinate value x. This means that the motion speed of the radiation point changes depending upon the coordinate value x. This variation of the motion speed is required to be considered when the exposure amount on the exposure plane is calculated. A minute change dyo of the radiation position motion amount on the center line (x=0) of the exposure area, as the swing angle (changes finely by do, is given from the equation (23) by: EQU dy.sub.o =((.differential./.differential..phi.).multidot.F.sub.y (0,0,.phi.).multidot.d.phi.) (31) A minute change dy.sub.x at the coordinate value x is given by: EQU dy.sub.x =((d/d.phi.).multidot.F.sub.y (.theta..sub.x,0,.phi.).multidot.d.phi.) (32) A deformation coefficient K.sub.3 (.theta..sub.x, .phi.) is defined by: EQU K.sub.3 (x,.phi.)=dy.sub.x /dy.sub.o (33) From the equations (31) to (33), the following equation is derived: K.sub.3 (x,.phi.)=(.differential./.differential..phi.).multidot.F.sub.y (.theta..sub.x,0,.phi.)/(d/d.phi.).multidot.F(0,0,.phi.) (34) FIG. 19 is a graph showing an example of the deformation coefficient given by the equation (34). Three curves P.sub.1 to P.sub.3 shown in FIG. 19 correspond SR light beams applied to the positions of the walls P.sub.1 to P.sub.3 shown in FIG. 13. From the above studies, it can be understood that the distribution characteristics of exposure amounts in the x-axis direction on the exposure plane are represented by a product of the convergence coefficient K.sub.1 illustratively shown in FIG. 15, inclination coefficient K.sub.2 illustratively shown in FIG. 17, and deformation coefficient K.sub.3 illustratively shown in FIG. 19. This product is defined as a distribution coefficient K.sub.d (x, .phi.) which is: EQU K.sub.d (x,.phi.)=K.sub.1 (x,.phi.).multidot.K.sub.2 (x,.phi.).multidot.K.sub.3 (x,.phi.) (35) FIG. 20 is a graph showing an example of the distribution coefficient kd(x, 0). Three curves P.sub.1 to P.sub.3 shown in FIG. 19 correspond SR light beams applied to the positions of the walls P.sub.1 to P.sub.3 shown in FIG. 13. If a .phi. dependency of the distribution coefficient K.sub.d (x, .phi.) is small, i.e., if the three curves P.sub.1 to P.sub.3 are not separated greatly, it can be expected that the exposure amount can be made nearly uniform by adjusting the attenuation amount of SR light in the beryllium thin film shown in FIG. 11A in the x-axis direction. A representative one of various distribution coefficients K.sub.d shown in FIG. 20 obtained when the swing angle .phi.is changed, is represented by F.sub.d (X). For example, K.sub.d (X, 0) when the optical center axis of SR light is positioned at the middle point (point G) in FIG. 12A on the center line (x=0) of the exposure area, is represented by F.sub.d (x). EQU F.sub.d (x)=K.sub.d (X,0) (36) The number N of photons passed through the beryllium thin film is given by: EQU N=N.sub.o.multidot.exp(-.mu.t) (37) wherein N is the number of photons of SR light-before it enters the beryllium thin film, .mu. is a line absorption coefficient which changes depending upon the photon energy (wavelength), and t is an optical path length of SR light in the beryllium thin film. The energy spectrum of SR light changes with a layout of the optical system. Therefore, in calculating the intensity of SR light transmitted through the beryllium thin film, it is necessary to integrate the equation (37) over the whole range of energy spectra. It is assumed herein from a more macro viewpoint that the intensity P of SR light transmitted through the beryllium thin film is represented by the form like the equation (37), namely by: EQU P=P.sub.o.multidot.exp(-.mu..sub.o t) (38) where P.sub.o is the intensity of SR light before it transmits through the beryllium thin film, and .mu..sub.o is a line absorption coefficient (constant). A thickness of the beryllium thin film is represented by T and an incidence angle of SR light upon the beryllium thin film is represented by .theta.. The angle .theta. is an angle between the optical axis of SR light and a normal to the beryllium thin film at the incidence point. In this case, a transmission thickness t of SR light at the incidence point is given by: EQU t=T/cos(.theta.) (39) In order to make the distribution coefficient K.sub.d realize the attenuation effects of the beryllium thin film, the following equation is to be satisfied: EQU (P.sub.o.multidot.exp(-.mu..sub.o T))/(P.sub.o.multidot.exp(-.mu..sub.o T/cos(.theta.)))=F.sub.d (X) (40) By arranging this equation (4), the following equation is obtained: EQU 1/cos(.theta.)=1/(.mu..sub.o T).multidot.ln(F.sub.d (x)) (41) By using the beryllium thin film having a shape satisfying the equation (41), irregularity of the exposure amount on the exposure plane in the x-axis direction (horizontal direction) can be relaxed. FIG. 21 shows an example of the beryllium thin film having a shape satisfying the equation (41). SR light 61 having a curved beam cross section transmits through a beryllium thin film 31 and irradiates upon the exposure surface of a semiconductor substrate 51. SR light 61 is in plane symmetry with the vertical plane including the optical center axis of SR light. Consider a virtual plane 62 parallel to the optical center axis of SR light 62 and to the horizontal line vertical to the optical center axis. A u-v rectangular coordinate system is defined on the virtual plane 62, the u-axis direction being the horizontal direction. The beryllium thin film 31 is disposed along a cylindrical surface with the normal direction to the virtual plane 62 as its generator. If the shape of an image 31a of the beryllium thin film 31 vertically projected upon the virtual plane 62 is given by: EQU V=F.sub.Be (u) (42) then the following equation stands: EQU tan(.theta.)=dF.sub.Be (u)/du (43) Since the u-axis on the virtual plane 62 corresponds to the x-axis on the exposure plane, the variable u in the equation (43) can be replaced by the variable x in the equation (41). From the equations (41) and (43), it stands: EQU dF.sub.Be (x)/dx=((1+1/(.mu..sub.o T).multidot.ln(F.sub.d (x))).sup.2 -1).sup.1/2 (44) By integrating the equation (44), a proper shape of the beryllium thin film 31 can be obtained. In the above calculations, the equation (38) is used as an approximate equation representative of the intensity of SR light after it transmits through the beryllium thin film. If the following general equation (45) is adopted: EQU P=H(t) (45), the proper shape of the beryllium thin film is represented by: EQU dF.sub.Be (x)/dx=(((1/T.multidot.H.sup.-1.multidot.(H(T)/F.sub.d (x))).sup.2)-1).sup.1/2 (46) where H.sup.-1 is an inverse function of H. As described with FIG. 21, if the beryllium thin film 31 is formed having the shape satisfying the equation (44), irregularity of the exposure amount on the exposure plane in the x-axis direction can be relaxed. Next, a method of relaxing irregularity of the exposure amount in the y-axis direction will be described. Irregularity of the intensity of SR light such as shown in FIG. 14 is present in the y-axis direction on the exposure plane. The normalized SR light intensity distribution shown in FIG. 14 is defined as an intensity coefficient K.sub.4 (.phi.) which is given by: EQU K.sub.4 (.phi.)=F.sup.1 (0,.phi.)/F.sub.1 (0,0) (47) Irregularity of the exposure amount on the exposure plane in the y-axis direction is formed by the irregularity of the SR light intensity shown in FIG. 14 as well as by a variation of the motion speed of an area applied with SR light in the y-axis direction. The variation characteristics of the motion speed in the y-axis direction are normalized by the motion speed when the optical center axis of SR light passes the middle point G of the exposure area, and is represented by a scan coefficient K.sub.5 (.phi.) which is given by: EQU K.sub.5 (.phi.)=(d/d.phi.)(F.sub.y (0,0,0))/(d/d.phi.)(F.sub.y (0,0,.phi.)) (48) Irregularity of the exposure amount in the y-axis direction can be evaluated from a product of the intensity coefficient K.sub.4 (.phi.) and scan coefficient K.sub.5 (.phi.). This product is defined as a speed coefficient Ks(.phi.) which is: EQU K.sub.s.phi.)=K.sub.4 (.phi.).multidot.K.sub.5 (.phi.) (49) Irregularity of the exposure amount on the exposure plane in the y-axis direction can therefore be relaxed by changing the swing angular velocity of the reflection mirror in accordance with the swing angle .phi., i.e., the incidence angle of SR light, so as to compensate for the speed coefficient K.sub.5 (.phi.). In the course of above calculations, the SR light radiation area is covered by using the center line of the curved walls P.sub.1 to P.sub.3 shown in FIG. 7 and the effects of the thickness of each wall are not considered. The equation (3) is an approximate equation. In the course of above studies, the theory has been developed by using the SR light intensity on the center line (x 0) in the exposure area as a reference. From these reasons, even if the reflection mirror 15 is swung so as to compensate for the speed coefficient K.sub.s (.phi.), the effects of uniforming the exposure amount may be small at the peripheral area of the exposure area remote from the center line. In order to enhance the effects of uniforming the exposure amount over the whole exposure area, the SR light intensity at the position apart from the center line may be used as a reference. Specifically, the shape of the beryllium thin film is first determined from the equation (44), and the swing angular velocity of the reflection mirror 15 is determined from the equation (49). An evaluation experiment of the exposure amount distribution is made by using the beryllium thin film having the determined shape and the determined swing angular velocity of the reflection mirror 15. In accordance with this evaluation results, similar evaluation experiments or simulations through calculations are repeated by changing little by little the shape of the beryllium thin film and the swing angular speed of the reflection mirror 15. With this method, a variation of exposure amounts is expected to be reduced further. In the latter of the embodiments described above, the shape of the beryllium thin film has been studied in order to uniform the SR light intensity distribution in the x-axis direction shown in FIG. 13. The shape of the beryllium thin film may be optimized in order to uniform the SR light intensity distribution in the y-axis direction. With reference to FIGS. 22A and 22B, description will be given for a method of optimizing the shape of the beryllium thin film in order to uniform the SR light intensity distribution in the y-axis direction. FIG. 22A is a graph corresponding to the graph shown in FIG. 13. In this graph, the walls P.sub.1 to P.sub.3 shown in FIG. 13 are moved in parallel in the y-axis direction to make the positions of the walls P.sub.1 to P.sub.3 on the y-axis coincide with each other as shown in FIG. 22A. In this graph, the x'-z' coordinate system with the upper edges of the walls P.sub.1 to P.sub.3 being projected upon the x-z plane corresponds to the graph shown in FIG. 20. The y"-z" coordinate system shows the upper edges of the walls P.sub.1 to P.sub.3 projected upon the y-z plane. Also in the y"-z" coordinate system, the SR light intensity is irregular in the y"-axis direction. This irregularity can be relaxed by using an optimum shape of the beryllium thin film which is determined by exchanging the variables x and y used in the above-described studies of uniforming the intensity distribution in the x-axis direction. FIG. 22B is a diagram corresponding to FIG. 21. In FIG. 21, the generator of the cylindrical surface along the beryllium thin film 31 is perpendicular to the virtual plane 62, whereas in FIG. 22B, the beryllium thin film is disposed along the cylindrical surface having a horizontal generator. Also with the shape of the beryllium thin film 31 shown in FIG. 22B, similar effects described with FIG. 21 can be obtained, i.e., the SR light intensity distribution can be made nearly uniform. In the case of FIGS. 20 and 21, an optimum shape of the beryllium thin film is determined in accordance with the images of the walls P.sub.1 to P.sub.3 shown in FIG. 13 vertically projected on the x-z plane, whereas in the case of FIGS. 22A and 22B, an optimum shape of the beryllium thin film is determined in accordance with the images of the walls P.sub.1 to P.sub.3 vertically projected on the y-z plane. An optimum shape of the beryllium thin film may be determined from other projections. With reference to FIGS. 23A and 23B, a method of determining an optimum shape of the beryllium thin film from a different projection will be described. FIG. 23A illustrates a projection and corresponds to FIG. 22A. Supposing a rotary axis z' at the origin R.sub.o of the y-axis and an R'-axis in the x-y plane intersecting the z'-axis, the upper edges of the walls P.sub.1 to P.sub.3 are rotatively projected around the rotary axis z' upon the z'-R' plane. The shape of the beryllium thin film is optimized by reducing the R'dependency of the rotatively projected images P.sub.1 ' to P.sub.3 '. The position R.sub.1 on the R'-axis is represented by using the coordinate values (x, y) in the x-y plane as: EQU R.sub.1 =(x.sup.2 +(R.sub.o-y).sup.2).sup.1)1/2 (50) In order to reduce a variation of SR light intensities, the shape of the beryllium thin film is determined by reducing a variation of SR light intensities in accordance with R.sub.1. FIG. 23B shows an example of the shape of the beryllium thin film 31 wherein the optical path length of SR light in the beryllium thin film 31 is changed with R.sub.1. A rotary axis 63 is supposed which is in the vertical plane including the optical center axis of SR light and parallel to, and spaced apart by a distance Ro from, the optical center axis. The beryllium thin film is disposed along a rotary surface 64 having the rotary axis 63 as its rotation center. The shape of the rotary surface 64 is determined in such a manner that the R' dependency of the rotatively projected images P.sub.1 ' to P.sub.3 ' in the z'-R' plane shown in FIG. 23A is reduced. Also with the shape of the beryllium thin film 31 shown in FIG. 23B, similar effects described with FIG. 21 can be obtained, i.e., the SR light intensity distribution on the exposure plane can be made nearly uniform. If R.sub.o is set to .infin. the shape similar to that shown in FIG. 22B is obtained. Three optimum shapes of the beryllium thin film have been described with reference to FIGS. 21, 22B and 23B. In any of these shapes, the attenuation amount of SR light in the beryllium thin film changes in the in-plane of the film, so that the exposure amount distribution on the exposure plane in the horizontal direction while the reflection mirror 15 is swung, becomes more uniform than the attenuation amount of SR light in the beryllium thin film is not considered. In the case of FIG. 21, the beryllium thin film has a uniform thickness, and the shape thereof is determined so that a cross line between the beryllium thin film and a plane including both the optical center axis of SR light and a horizontal line perpendicular to the optical center axis, is curved. In the case of FIG. 22, the shape of the beryllium thin film is determined so that a cross line between the beryllium thin film and a vertical plane including the optical center axis of SR light, is curved. It is not simple to determine which is optimum among the shapes shown in FIGS. 21, 22B and 23B, as the shape of the beryllium thin film. In order to uniform the SR light intensity over the whole exposure plane, it is sufficient if a variation of distribution coefficients is reduced. The distribution coefficients for the shape shown in FIG. 21 are shown in FIG. 20. The distribution coefficients for the shape shown in FIG. 22B are shown on the y"-z" plane of FIG. 22A. The distribution coefficients for the shape shown in FIG. 23B are shown on the R'-z' plane of FIG. 23A. FIG. 24 is a graph showing the calculation results of dispersion of distribution coefficients which are calculated by obtaining a curve representative of an average of distribution coefficients by using the least square method and by using the average. The abscissa represents Ro shown in FIG. 23A, and the ordinate represents a dispersion of distribution coefficients in a relative value, the value "1" corresponding to R.sub.o =.infin.. A distance between the light source and the incidence point of the reflection mirror 15 was set to 3 m, a distance between the reflection mirror 15 and the exposure plane was set to 2 m, the reflection mirror 15 had a main radius of 110 mm and a subsidiary radius of 286 m, and an incidence angle was set to 88.8.degree.. It is seen that as R.sub.o is changed, the dispersion of distribution coefficients becomes large. R.sub.o =.infin. or -.infin. corresponds to the case of FIG. 22B. As R.sub.o takes near the radius of curvature of the beam cross section of SR light, dispersion changes to divergence. The beryllium thin film has preferably the shape which reduces dispersion of distribution coefficients. However, if the degree of curvature of the beryllium thin film becomes large, the exposure amount distribution is easily affected by the SR light beam width (corresponding to the thickness W of each wall P.sub.1 to P.sub.3 shown in FIG. 13). With a saucer type curved surface of the beryllium thin film such as shown in FIG. 23B, a variation of thicknesses or the like is likely to occur during drawing work of forming the film. Whether which shape among those shown in FIGS. 21, 22B and 23B is adopted is preferably determined synthetically by considering dispersion, work ease, and the like. In the above embodiments, a beryllium thin film having a uniform thickness is curved or another type of a beryllium thin film is used to provide an SR light attenuation distribution. A thickness distribution of a beryllium thin film may be used to optimize a transmission optical path length of SR light. FIG. 25 is a partially broken perspective view showing an example of a beryllium thin film having a thickness distribution. A thin portion 32 corresponding to the SR light beam cross section is formed in the beryllium thin film in the area through which SR light transmits. The thickness of the thin portion 32 is set so that a change in the distribution coefficients shown in FIG. 20 is compensated. The beryllium thin film can be formed by a generally precise cutting work. Discharge work, electrolytic polishing or the like may also be used. Since the contact surface to the window flange 37 is plane, vacuum sealing can be performed with ease by using an O-ring or the like. The thin portion 32 shown in FIG. 25 is made stepwise so as to grasp the shape thereof easily from this drawing. The surface of the thin portion 32 may be worked to have a smooth three-dimensional curved surface. In the embodiment shown in FIGS. 11A to 25, the outgoing vacuum duct 30 is swung so as not to change the relative position between the optical center axis of SR light and the beryllium thin film 31. In this case, the optical center axis of SR light always passes a particular point of the beryllium thin film. FIG. 26A shows the transmission area of SR light in the in-plane of the beryllium thin film 31 without a change in the relative position between the optical center axis of SR light and the beryllium thin film 31. Three curves P.sub.1 to P.sub.3 correspond SR light beams applied to the positions of the walls P.sub.1 to P.sub.3 shown in FIG. 12A. If the amplitude of an up/down swing of the optical center axis of SR light is made different from the amplitude of a motion of the beryllium thin film, a transmission position of SR light in the in-plane of the beryllium thin film changes. FIG. 26B shows changes in the transmission positions of SR light. Three curves P.sub.1 to P.sub.3 correspond to SR light beams applied to the positions of the walls P.sub.1 to P.sub.3 shown in FIG. 12A. If the beryllium thin film is provided with a thickness distribution corresponding to the positions of the curves P.sub.1 to P.sub.3, a change in the speed coefficient K.sub.s given by the equation (49) can be compensated. FIG. 27 is a partially broken perspective view showing an example of a beryllium thin film capable of compensating a change in the speed coefficient K.sub.s. As compared to FIG. 25, the thin portion 32 of this example expands in the swing direction of SR light. The area through which SR light transmits moves in this thin portion 32 in response to the swing of the reflection mirror 15. The thin portion 32 is provided with the thickness distribution capable of compensating for a change in the speed coefficient K.sub.s as the reflection mirror 15 swings. In the embodiment shown in FIGS. 11A to 25, the swing angular velocity of the reflection mirror 15 is changed with the swing angle .phi. to compensate for a change in the speed coefficient K.sub.s. If the beryllium thin film is formed to have the shape shown in FIG. 27, a change in the speed coefficient Ks can be compensated while the swing angular velocity of the reflection mirror 15 is maintained constant. In this case, the structure of the mirror swinging mechanism 16 can be simplified. The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. It is apparent that various modifications, improvements, combinations, and the like can be made by those skilled in the art. |
claims | 1. A sealing element fastening system for at least one sealing element for a pressure vessel having at least one opening, the sealing element fastening system comprising:a sealing part being a cover for the at least one opening, said sealing part having an accommodating groove formed therein for said at least one sealing element, wherein,in an operating state of the pressure vessel, said at least one sealing element is at least partially inserted in said accommodating groove;said sealing part having indentations formed therein, said indentations each having a respective sealing element fastening device disposed therein; and said indentations each being sealed by a respective fill element in the operating state of the pressure vessel, said fill element occupying a majority of a space within said indentation, and said fill element being in sealing engagement against a surface of the pressure vessel for blocking a potential flow of fluid into said indentation. 2. The system according to claim 1, wherein each said respective sealing element fastening device has a ring-shaped holding element with an integrally molded holding arm, and is fixed to said sealing part in the operating state of the pressure vessel. 3. The system according to claim 2, wherein in the operating state of the pressure vessel, said integrally molded holding arm engages into a recess on said at least one sealing element and fixes said at least one sealing element on said sealing part. 4. The system according to claim 3, wherein:said sealing part has a respective complementary counter thread at each of said indentations; andeach said respective sealing element fastening device has a bolt nut and a threaded bolt with two separate threaded portions including a first threaded portion and a second threaded portion, said first threaded portion passes through said ring-shaped holding element in the operating state of the pressure vessel and is screw-connected into said respective counter thread, said second threaded portion passes through said fill element in the operating state of the pressure vessel and accommodates said bolt nut as a fixing device for said fill element. 5. The system according to claim 4, wherein each said respective sealing element fastening device has a ring-shaped projection disposed between said first and second threaded portions and functioning as a stop member for said fill element and for said holding element. 6. The system according to claim 5, wherein said ring-shaped projection has a number of recesses formed therein functioning as a point of application for a tool. 7. The system according to claim 6, wherein said bolt nut has a number of further recesses formed therein, which in form and relative position to each other are identical to said recesses on said ring-shaped projection, said further recesses function as a point of application for the tool. 8. The system according to claim 3,wherein said fill element has a respective central opening formed therein; anda clamping bush is provided through each said holding element and is pressed into said central opening in said fill element for a rotational fixing of said holding element on said fill element. 9. The system according to claim 8, further comprising a fastening screw penetrating said fill element, said holding element and said clamping bush, in the operating state of the pressure vessel, said fastening screw being screw-connected into a complementary counter thread formed in said sealing part. 10. The system according to claim 1, wherein said sealing part is a cover. 11. A pressure vessel, comprising:at least one sealing element;a sealing element fastening system for said at least one sealing element, said sealing element fastening system including;a sealing part being a cover for an opening in a pressure vessel, said sealing part having an accommodating groove formed therein for said at least one sealing element, wherein,in an operating state of the pressure vessel, said at least one sealing element is at least partially inserted in said accommodating groove;said sealing part having indentations formed therein, said indentations each having a respective sealing element fastening device disposed therein; andsaid indentations each being sealed by a respective fill element in the operating state of the pressure vessel, said fill element occupying a majority of a space within said indentation, and said fill element being in sealing engagement against a surface of the pressure vessel for blocking a potential flow of fluid into said indentation. 12. The pressure vessel according to claim 11, wherein the pressure vessel is a reactor pressure vessel. |
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063109387 | summary | BACKGROUND OF THE INVENTION This invention relates generally to computed tomography (CT) imaging and, more particularly, to methods and apparatus for calibration of z-axis tracking loops for positioning a CT x-ray beam of a multi-slice CT imaging system. In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile. In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display. In a multi-slice system, movement of an x-ray beam penumbra over detector elements having dissimilar response functions can cause signal changes resulting in image artifacts. Opening system collimation to keep detector elements in the x-ray beam umbra can prevent artifacts but increases patient dosage. Known CT systems utilize a closed-loop z-axis tracking system to position the x-ray beam relative to a detector array. It would be desirable to provide improved methods and apparatus for calibration of such systems. In particular, it would be desirable to provide improved methods and apparatus for determining calibration parameters such as: (1) a target beam position at which to maintain the x-ray beam; (2) a transfer function to convert sensed tracking information into a beam position in millimeters; and (3) valid limits of the transfer function. BRIEF SUMMARY OF THE INVENTION There is therefore provided, in one embodiment, a method for determining tracking control parameters for positioning an x-ray beam of a computed tomography imaging system having a movable collimator positionable in steps and a detector array including a plurality of rows of detector elements. The method includes steps of obtaining detector samples at a plurality of collimator step positions while determining a position of a focal spot of the x-ray beam; determining a beam position for each detector element at each collimator step utilizing the determined focal spot positions, a nominal focal spot length, and geometric parameters of the x-ray beam, collimator, and detector array; and determining a calibration Parameter utilizing information so obtained. For example, in determining a target beam position at which to maintain the x-ray beam, the method also includes steps of determining an detector element differential error according to ratios of successive collimator step positions; and selecting a target beam position for an isocenter element in accordance with the determined element differential errors. The above described system provides improved tracking calibration for CT imaging systems utilizing z-axis tracking loops for positioning x-ray beams. |
052895122 | abstract | A nuclear propulsion reactor. A reactor vessel is provided with an annular first core and a cylindrical second core that is radially encompassed by the first core. Nuclear fuel elements in the first core provide first stage heating of propellant as they direct the propellant axially through the first core. The second core, which contains fissionable material in a highly refractory form, is in fluid communication with the first core for receiving the heated propellant. Fission reactions in the second core driven by leakage neutrons from the first core provide second stage heating of the propellant as it passes therethrough. The second core directs the coolant to a propellant nozzle for providing propulsive thrust. |
description | The present invention relates to a particle beam therapy system utilized in the medical field and R&Ds and particularly to data processing of the position and the size of a particle beam in a particle beam therapy system of a scanning type such as a spot-scanning type or a raster-scanning type. In general, a particle beam therapy system is provided with a beam generation apparatus that generates a charged particle beam, an accelerator that is connected with the beam generation apparatus and accelerates a generated charged particle beam, a beam transport system that transports a charged particle beam that is accelerated by the accelerator so as to gain predetermined energy and then emitted, and a particle beam irradiation apparatus, disposed at the downstream side of the beam transport system, for irradiating a charged particle beam onto an irradiation subject. Particle beam irradiation apparatuses are roughly divided into apparatuses utilizing a broad irradiation method in which a charged particle beam is enlarged in a dispersion manner by a scatterer, and the shape of the enlarged charged particle beam is made to coincide with the shape of an irradiation subject in order to form an irradiation field; and apparatuses utilizing a scanning irradiation method (the spot-scanning method, the raster-scanning method, and the like) in which an irradiation field is formed by performing scanning with a thin, pencil-like beam in such a way that the scanning area coincides with the shape of an irradiation subject. In the broad irradiation method, an irradiation field that coincides with the shape of a diseased site is formed by use of a collimator or a bolus. The broad irradiation method is a most universally utilized and superior irradiation method where an irradiation field that coincides with the shape of a diseased site is formed so as to prevent unnecessary irradiation onto a normal tissue. However, it is required to create a bolus for each patient or to change the shape of a collimator in accordance with a diseased site. In contrast, the scanning irradiation method is a high-flexibility irradiation method where, for example, neither collimator nor bolus is required. However, because these components for preventing irradiation onto not a diseased site but a normal tissue are not utilized, there is required a positional accuracy of beam irradiation that is the same as or higher than that of the broad irradiation method. Patent Document 1 discloses a beam position monitor, for a particle beam therapy system, that has a purpose of solving the problem that in a raster-scanning irradiation method in which when the irradiation position is changed, the charged particle beam is not stopped, the accuracy of beam position measurement is deteriorated mainly by the fact that the electric charges collected during the scanning of the charged particle beam and the electric charges collected when the scanning has been completed cannot accurately be distinguished from each other. The beam position monitor according to Patent Document 1 is provided with a collection electrode (corresponding to a sensor unit of the position monitor) for collecting collection charges produced by ionization of the charged particle beam, and a signal processing circuit that performs a beam position calculation for determining the beam position by utilizing collection charges. The signal processing circuit is provided with an I/V converter that generates a voltage signal obtained by I/V-converting the current output from the collection electrode; a digital signal generation circuit that generates a digital signal related to the collection charges when the voltage signal is inputted thereto; a timing signal transmission/reception unit that receives a signal, as a timing signal, that is generated at a time when the charged particle beam, which is scanned from a scanning-stop irradiation point (corresponding to an irradiation spot in a spot scanning irradiation method) to the next scanning-stop irradiation point, is in the non-scanning state (in which the charged particle beam is stopped at the scanning-stop irradiation point); and a beam position calculation unit that calculates a beam position by use of a digital signal related to collection charges when a digital signal, related to the collection charges, generated by the digital signal generation circuit at the timing when the timing signal transmission/reception unit receives the timing signal, is inputted thereto. [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-60523 (Paragraphs 0008 through 0011, FIG. 7) In the invention disclosed in Patent document 1, the irradiation position at a time when the scanning of a charged particle beam is stopped at a scanning-stop irradiation point set on a treatment subject and then the charged particle beam is irradiated can be obtained only once at the timing of a signal to be generated in the non-scanning state where no charged particle beam is scanned; however, because a plurality of irradiation position calculations are not implemented during irradiation at the scanning-stop irradiation point, there is posed a problem that even when the charged particle beam moves and falls out of the tolerance range, that irradiation position cannot be detected. Even in the case where a particle beam therapy system provided with the beam position monitor according to Patent Document 1 has a function of stopping the irradiation of a charged particle beam when the irradiation position is abnormal, no abnormality detection signal can be generated for an abnormality caused after the timing of data collection at a single scanning-stop irradiation point; therefore, there is posed a problem that the irradiation of the charged particle beam cannot immediately be stopped. The present invention has been implemented in order to solve the foregoing problems; the objective thereof is to obtain a beam data processing apparatus that detects the irradiation position of a charged particle beam even when in an irradiation spot, a position abnormality of the charged particle beam is caused during the irradiation of the charged particle beam and that can generate an abnormality detection signal for indicating the position abnormality of the charged particle beam. A beam data processing apparatus according to the present invention is provided with a plurality of channel data conversion units that perform AD conversion processing in which each of the plurality of analogue signals outputted from a position monitor that detects with a plurality of detection channels the passing position of a charged particle beam is converted into a digital signal; a position size processing unit that calculates a beam position, which is the passing position of the charged particle beam in the position monitor, based on voltage information items obtained through processing by the plurality of channel data conversion units; an abnormality determination processing unit that determines whether or not the beam position is within a tolerance range, based on a desired position of the charged particle beam and a position allowable value, and that generates a position abnormality signal when determining that the beam position is not within the tolerance range; and an integrated control unit that controls the plurality of channel data conversion units in such a way that while the charged particle beam is stopped at an irradiation spot, digital signal conversion processing is implemented two or more times. The channel data conversion unit has a plurality of A/D converters, a demultiplexer that distributes the respective analogue signals to the A/D converters at different timings, and a multiplexer that switches the respective digital signals processed by the A/D converters at different timings so as to output them to the position size processing unit. A beam data processing apparatus according to the present invention is provided with two or more A/D converters for each detection channel of a position monitor, and the two or more A/D converters are operated at different timings while a charged particle bean is stopped at an irradiation spot; therefore, it is made possible to detect the irradiation position of the charged particle beam even in the case where while the charged particle beam is irradiated at an irradiation spot, a position abnormality of the charged particle beam is caused and to generate an abnormality detection signal for indicating the position abnormality of the charged particle beam. Embodiment 1 FIG. 1 is a diagram representing the configuration of a beam data processing apparatus according to Embodiment 1 of the present invention. FIG. 2 is a diagram illustrating the configuration of a particle beam irradiation apparatus provided with the beam data processing apparatus according to Embodiment 1 of the present invention; FIG. 3 is a schematic configuration diagram illustrating a particle beam therapy system according to Embodiment 1 of the present invention. In FIG. 3, a particle beam therapy system 51 includes a beam generation apparatus 52, a beam transport system 59, and particle beam irradiation apparatuses 58a and 58b. The beam generation apparatus 52 includes an ion source (unillustrated), a prestage accelerator 53, and a synchrotron 54. The particle beam irradiation apparatus 58b is provided in a rotating gantry (unillustrated). The particle beam irradiation apparatus 58a is provided in a treatment room where no rotating gantry is installed. The function of the beam transport system 59 is to achieve communication between the synchrotron 54 and the particle beam irradiation apparatuses 58a and 58b. Part of the beam transport system 59 is provided in the rotating gantry (unillustrated), and that part includes a plurality of deflection electromagnets 55a, 55b, and 55c. A charged particle beam, which is a particle beam such as a proton beam generated in the ion source, is accelerated by the prestage accelerator 53 and enters the synchrotron 54, which is an accelerator. The particle beam is accelerated to gain predetermined energy. The charged particle beam launched from the synchrotron 54 is transported to the particle beam irradiation apparatuses 58a and 58b by way of the beam transport system 59. The particle beam irradiation apparatuses 58a and 58b each irradiate the charged particle beam onto an irradiation subject 15 (refer to FIG. 2). A charged particle beam 1 generated in the beam generation apparatus 52 and accelerated to gain predetermined energy is led to the particle beam irradiation apparatus 58 by way of the beam transport system 59. In FIG. 2, the particle beam irradiation apparatus 58 is provided with X-direction and Y-direction scanning electromagnets 2 and 3 that scan the charged particle beam 1 in the X direction and the Y direction, respectively, which are directions perpendicular to the charged particle beam 1; a position monitor 4; a dose monitor 5; a dose data converter 6; a beam data processing apparatus 11; a scanning electromagnet power source 7; and an irradiation management apparatus 8 that controls the particle beam irradiation apparatus 58. The irradiation management apparatus 8 is provided with an irradiation control computer 9 and an irradiation control apparatus 10. The dose data converter 6 is provided with a trigger generation unit 12, a spot counter 13, and an inter-spot counter 14. The traveling direction of the charged particle beam 1 is the Z direction. The X-direction and Y-direction scanning electromagnets 2 and 3 scan the charged particle beam 1 in the X direction and the Y direction, respectively. The position monitor 4 detects a beam passing position (gravity center position) and a beam size through which the charged particle beam 1 that has been scanned by the X-direction scanning electromagnet 2 and the Y-direction scanning electromagnet 3 passes. The dose monitor 5 detects the dose of the charged particle beam 1. The irradiation management apparatus 8 controls the irradiation position of the charged particle beam 1 on the irradiation subject 15, based on treatment plan data created by an unillustrated treatment planning apparatus; when the dose measured by the dose monitor 5 and converted into digital data by the dose data converter 6 reaches the desired dose, the charged particle beam 1 is stopped. The scanning electromagnet power source 7 changes setting currents for the X-direction scanning electromagnet 2 and the Y-direction scanning electromagnet 3, based on control inputs (commands), which are outputted from the irradiation management apparatus 8, to the X-direction scanning electromagnet 2 and the Y-direction scanning electromagnet 3. In this Description, the scanning irradiation method for the particle beam irradiation apparatus 58 will be explained assuming that it is the raster-scanning irradiation method in which when the irradiation position of the charge particle beam 1 is changed, the charged particle beam 1 is not stopped, i.e., it is a method in which as is the case with the spot scanning irradiation method, the beam irradiation position travels through spot positions one after another. The spot counter 13 measures the irradiation dose for a time during which the beam irradiation position of the charged particle beam 1 is stopped. The inter-spot counter 14 measures the irradiation dose for a time during which the beam irradiation position of the charged particle beam 1 moves. The trigger generation unit 12 generates a starting signal with which the beam data processing apparatus obtains new data of the position monitor 4 while the beam irradiation position is stopped. In FIG. 1, the beam data processing apparatus 11 includes an X data processing unit 16 that processes an X-channel signal of the position monitor 4 and a Y data processing unit 17 that processes a Y-channel signal of the position monitor 4. The X data processing unit 16 is connected with the position monitor 4 by way of an X-channel signal line 18; the Y data processing unit 17 is connected with the position monitor 4 by way of a Y-channel signal line 19. Each of the X-channel signal line 18 and the Y-channel signal line 19 includes signal lines, the number of which is the same as the number of a plurality of channel data conversion units 21. The respective configurations of the X data processing unit 16 and the Y data processing unit 17 are the same as each other. The X data processing unit 16 will be explained as an example. The X data processing unit 16 is provided with an integrated control unit 22, a position size processing unit 23, an abnormality determination processing unit 24, a data memory 25, and the plurality of channel data conversion units 21, the number of which coincides with the number of a plurality of X-channel signals of the position monitor 4. In FIG. 1, as far as the channel data conversion units 21 are concerned, only two of them are illustrated; the channel data conversion units between the two channel data conversion units 21a and 21n are omitted by providing a plurality of dots instead of them. Each of the X-channel signal line 18 and the Y-channel signal line 19 is illustrated with a single thick line in FIG. 1, for the purpose of preventing the drawing from becoming complex. In the position monitor 4, a sensor unit is provided in the form of a mesh, and a great number of detection channels (channels in the X direction and channels in the Y direction) are provided. These many channels each output a current signal, as an analogue signal. The respective analogue signals of Xch (the X channel) of the position monitor 4 are connected with the channel data conversion units 21a through 21n, in the X data processing unit 16; the respective analogue signals of Ych (the Y channel) are connected with the channel data conversion units 21a through 21n, in the Y data processing unit 17. Xch corresponds to the X direction of the particle beam irradiation apparatus 58; Ych corresponds to the Y direction of the particle beam irradiation apparatus 58. The channel data conversion unit 21a is provided with a current/voltage converter 31 that converts a current signal outputted from the position monitor 4 into a voltage signal, a demultiplexer 32 that distributes a voltage signal outputted from the current/voltage converter 31 to a plurality of A/D converters (analogue/digital converters) 33, a plurality of A/D converters 33, for example, five A/D converters 33a through 33e that perform A/D conversion processing in which the respective voltage signals distributed by the demultiplexer 32 are converted into digital signals, and a multiplexer 34 that switches and outputs the signals to which the A/D conversion processing has been applied in the plurality of A/D converters 33a through 33e. In FIG. 1, as far as the A/D converters 33 are concerned, only three of them are illustrated; the A/D converters between the two A/D converters 33b and 33e are omitted by providing a plurality of dots instead of them. The integrated control unit 22 controls a plurality of channel data conversion units 21, the position size processing unit 23, and the abnormality determination processing unit 24. The respective channel data conversion units 21 are connected with the integrated control unit 22 by way of a demultiplexer switching signal line 26, a multiplexer switching signal line 27, and an A/D-related signal line 28. Each of the demultiplexer switching signal line 26 and the multiplexer switching signal line 27 includes signal lines, the number of which is the same as the number of a plurality of channel data conversion units 21. The A/D-related signal line 28 includes signal lines, the number of which is the same as the number of the A/D converters 33. A data ID signal line 29 (ID: identification) includes a spot ID data line and a sub-ID notification signal line. The integrated control unit 22, for example, receives a 25% dose completion signal sigb1, a 50% dose completion signal sigb2, a 75% dose completion signal sigb3, and a 100% dose completion signal sigb4, which are four starting signals with which the beam data processing apparatus 11 obtains new data of the position monitor 4. Each of the 25% dose completion signal sigb1, the 50% dose completion signal sigb2, the 75% dose completion signal sigb3, and the 100% dose completion signal sigb4 is a proportion dose completion signal that indicates that the dose of the charged particle beam 1 at an irradiation spot where the charged particle beam 1 is stopped has become a predetermined proportion of the desired dose. Each of the demultiplexer switching signal line 26, the multiplexer switching signal line 27, and the A/D-related signal line 28 is illustrated with a single thick line in FIG. 1, for the purpose of preventing the drawing from becoming complex. The position size processing unit 23 is connected with the integrated control unit 22 by way of the data ID signal line 29. In order to prevent the drawing from becoming complex, the data ID signal line 29 is expressed by a single thick line in FIG. 1. Each of the signal lines illustrated with a thick line is a signal line through which two or more kinds of signals are transmitted. The position size processing unit 23 receives a voltage Vi that has been A/D-converted by the plurality of channel data conversion units 21, and calculates, based on the voltages Vi, a beam position P in such a manner as a gravity center is calculated. The position size processing unit 23 calculates a beam size S based on the voltages Vi, as if a standard deviation is calculated. The beam size S is a length corresponding to 1 σ of a one-dimensional Gaussian distribution. The position size processing unit 23 of the X data processing unit 16 calculates a beam position Px in the X direction and a beam size Sx; the position size processing unit 23 of the Y data processing unit 17 calculates a beam position Py in the Y direction and a beam size Sy. The position size processing unit 23 stores the calculated beam position P and beam size S in the data memory 25. The beam position Px and the beam size Sx are stored in the data memory 25 of the X data processing unit 16; the beam position Py and the beam size Sy are stored in the data memory 25 of the Y data processing unit 17. Based on preset data PD received from the irradiation control apparatus 10 before the charged particle beam 1 is irradiated, the abnormality determination processing unit 24 determines whether or not there exists an abnormality in the beam position P or in the beam size S, i.e., whether or not the beam position P and the beam size S are allowable. When determining that the beam position P is not allowable, the abnormality determination processing unit 24 outputs a position abnormality signal sige1 to the irradiation control apparatus 10. When determining that the beam size S is not allowable, the abnormality determination processing unit 24 outputs a size abnormality signal sige2 to the irradiation control apparatus 10. The abnormality determination processing unit 24 of the X data processing unit 16 outputs a position abnormality signal sige1x and a size abnormality signal sige2x; the abnormality determination processing unit 24 of the Y data processing unit outputs a position abnormality signal sige1y and a size abnormality signal sige2y. Each of the integrated control units 22 of the X data processing unit 16 and the Y data processing unit 17 receives a spot ID, an inter-spot travel completion signal siga, and a spot ID strobe sb1 from the irradiation control apparatus 10 and outputs an ADC processing abnormality signal sige3 to the irradiation control apparatus 10. SID in FIG. 1 denotes a spot ID. The integrated control unit 22 of the X data processing unit 16 outputs an ADC processing abnormality signal sige3x to the irradiation control apparatus 10; the integrated control unit 22 of the Y data processing unit 17 outputs an ADC processing abnormality signal sige3y to the irradiation control apparatus 10. The ADC processing abnormality signal sige3 is outputted when all of the A/D converters 33a through 33e of the channel data conversion unit 21 are processing. For example, in the case where each of the A/D converters 33a through 33e outputs an ADC processing starting signal sigd1 and no ADC processing ending signal sigd2, the ADC processing abnormality signal sige3 is outputted when the integrated control unit 22 receives trigger signals (siga, sigb1, sigb2, sigb3, and sigb4), which indicate that collection of the next data has been instructed. As the reference character for a spot ID, SID is utilized; however, in the case where a spot ID and the reference character SID are expressed in series, the spot ID is expressed as “spot identity SID” in order to prevent confusion between spot ID and reference character SID. Each of the abnormality determination processing unit 24s of the X data processing unit 16 and the Y data processing unit receives the preset data PD and a preset value strobe sb2 from the irradiation control apparatus 10 and outputs the position abnormality signal sige1 and the size abnormality signal sige2 to the irradiation control apparatus 10. After the irradiation of the charged particle beam 1 has been completed, the respective position size processing units 23 of the X data processing unit 16 and the Y data processing unit each output actual performance data data1 on the beam position P and the beam size S to the irradiation control computer 9. As described later, the actual performance data data1 includes information indicating a positional abnormality and a beam size abnormality. The position size processing unit 23 of the X data processing unit 16 outputs actual performance data data1x to the irradiation control computer 9; the position size processing unit 23 of the Y data processing unit 17 outputs actual performance data data1y to the irradiation control computer 9. Even in the case where when the charged particle beam 1 is being irradiated at an irradiation spot, a positional abnormality or a size abnormality of the charged particle beam 1 is caused, the beam data processing apparatus 11 detects the beam position P, which is the irradiation position of the charged particle beam 1, and the beam size S of the charged particle beam 1; then, the beam data processing apparatus 11 generates an abnormality detection signal indicating a positional abnormality or a size abnormality of the charged particle beam 1. The position abnormality signal sige1 is an abnormality detection signal indicating a positional abnormality of the charged particle beam 1. The position abnormality signal sige2 is an abnormality detection signal indicating a size abnormality of the charged particle beam 1. The operation of the beam data processing apparatus 11 will be explained by use of a timing chart. FIG. 4 is a timing chart for explaining the operation of the beam data processing apparatus; FIG. 5 is a timing chart for explaining the operation of the integrated control unit. The timing chart in FIG. 4 represents an example in which data on the position monitor 4 is collected five times at a single irradiation spot and each time the data is collected, the beam position P and the beam size S are calculated based on the AD-converted data. Measurements 1 through 5 each represent the respective timings of AD conversion processing and calculation of the beam position P and the beam size S. A period designated by “adc” indicates the period of AD conversion processing; a period designated by “c” indicates the period of calculation of the beam position P and the beam size S. The trigger signals for starting the five A/D conversion processing actions, are, for example, the inter-spot travel completion signal siga, the 25% dose completion signal sigb1, the 50% dose completion signal sigb2, the 75% dose completion signal sigb3, and the 100% dose completion signal sigb4, as described above. The irradiation control apparatus 10 of the irradiation management apparatus 8 transmits a scanning start command sigs1 to the respective apparatuses in the particle beam therapy system 51; then, the irradiation of the charged particle beam 1 is started. The irradiation control apparatus 10 transmits the spot identity SID, the spot ID strobe sb1, and the preset value strobe sb2 to the beam data processing apparatus 11. The integrated control unit 22 of the beam data processing apparatus 11 receives the spot identity SID at the rising timing of the spot ID strobe sb1. At the rising timing of the preset value strobe sb2, the abnormality determination processing unit 24 of the beam data processing apparatus 11 designates data corresponding to the spot identity SID, in the preset data PD, that has been preliminary received. The preset data PD includes a desired position Pd (Pdx, Pdy) and a position allowable value AP for performing abnormality determination on the beam position P (Px, Py) and a desired beam size Sd (Sdx, Sdy) and a size allowable value AS for performing abnormality determination on the beam size S (Sx, Sy). The irradiation control apparatus 10 transmits a beam-on command sigs2 to the beam generation apparatus 52. The charged particle beam 1 is led from the beam generation apparatus 52 to the particle beam irradiation apparatus 58 by way of the beam transport system 59. The irradiation control apparatus 10 transmits to the beam data processing apparatus 11 the inter-spot travel completion signal siga indicating that setting of instructions, to the X-direction scanning electromagnet 2 and the Y-direction scanning electromagnet 3, that corresponds to the initial spot identity SID (“0001” in FIG. 4) has been completed. When the initial inter-spot travel completion signal siga is transmitted, the integrated control unit 22 switches the demultiplexers 32 so that data of the position monitor 4 is inputted to the first A/D converter 33a and switches the multiplexers 34 so that processing data of the first A/D converter 33a is outputted to the position size processing unit 23. As represented in Measurement 1, when a measurement delay time T2, which is a mode transition time in the monitor, has elapsed after the rising timing of the initial inter-spot travel completion signal siga, the first A/D converter 33a starts AD conversion processing. When the dose of the charged particle beam 1 comes into the states of 25% dose completion, 50% dose completion, 75% dose completion, and 100% dose completion of the desired dose at the irradiation spot corresponding to the spot identity SID, the trigger generation unit 12 of the dose data converter 6 outputs the integrated control unit 22 the 25% dose completion signal sigb1, the 50% dose completion signal sigb2, the 75% dose completion signal sigb3, and the 100% dose completion signal sigb4, respectively. When receiving the ADC processing starting signal sigd1 (refer to FIG. 5) for the first A/D converter 33a through an after-mentioned demultiplexer procedure, the integrated control unit 22 switches the demultiplexers 32 so that data of the position monitor 4 is inputted to the second A/D converter 33b. After that, when receiving the 25% dose completion signal sigb1, the integrated control unit 22 outputs the ADC processing starting command sigc3 to the second A/D converter 33b so as to make the second A/D converter 33b start AD conversion processing. As represented in Measurement 2, when the measurement delay time T2, which is the mode transition time in the monitor, has elapsed after the rising timing of the 25% dose completion signal sigb1, the second A/D converter 33b starts AD conversion processing. Similarly, when receiving the ADC processing starting signal sigd1 for the second A/D converter 33b, the ADC processing starting signal sigd1 for the third A/D converter 33c, and the ADC processing starting signal sigd1 for the fourth A/D converter 33d, the integrated control unit 22 sequentially switches the demultiplexers 32 so that data of the position monitor 4 is inputted to the third A/D converter 33c, the fourth A/D converter 33d, and the fifth A/D converter 33e, respectively. After the demultiplexer procedure has been completed, when receiving the 50% dose completion signal sigb2, the 75% dose completion signal sigb3, and the 100% dose completion signal sigb4, the integrated control unit 22 outputs the ADC processing starting command sigc3 to the third A/D converter 33c, the fourth A/D converter 33d, and the fifth A/D converter 33e, respectively. AS represented in Measurements 3 through 5, when the measurement delay time T2, which is the mode transition time in the monitor, has elapsed after each of the rising timings of the 50% dose completion signal sigb2, the 75% dose completion signal sigb3, and the 100% dose completion signal sigb4, the third A/D converter 33c, the fourth A/D converter 33d, and the fifth A/D converter 33e, respectively, start AD conversion processing. As described above, taking a particle that arrives in a delayed manner into consideration, the start of AD conversion processing is delayed. The counters 1 and 2 in FIG. 4 represents the counting periods (dose measurement periods) of the spot counter 13 and the inter-spot counter 14, respectively. The upper side (state 1) of each waveform suggests that the counting is being performed, and the lower side (state 0) of each waveform suggests that the counting is stopped. The scanning state in FIG. 4 represents the state of scanning the charged particle beam 1. The upper side (state 1) of the scanning state suggests that the charged particle beam 1 is stopped at an irradiation spot, and the lower side (state 0) of the scanning state suggests that the charged particle beam 1 is traveling to the next irradiation spot. Even when the scanning of the charged particle beam 1 to the next irradiation spot has been started, the spot counter 13 continues the measurement until the spot delay time T1 elapses. The spot delay time T1 corresponds to a control delay before control of the X-direction scanning electromagnet 2 and the Y-direction scanning electromagnet 3 for the scanning to the next irradiation spot is started. The integrated control unit 22 controls the demultiplexer 32 and the multiplexer 34, based on the ADC processing starting signal sigd1 and the ADC processing ending signal sigd2 (refer to FIG. 5) for each of the A/D converters 33a through 33e. That is to say, the integrated control unit 22 collects beam data (data of Measurement 1) at a time when the charged particle beam 1 has started to halt at an irradiation spot and beam data (data pieces of Measurements 2 through 5) at a time when the dose has reached a predetermined value, and controls the demultiplexer 32 and the multiplexer 34 so that the beam data is transmitted to the position size processing unit 23 (a data collection procedure). Then, based on the collected beam data, the position size processing unit 23 calculates the beam position P and the beam size S (a position size calculation procedure). When the next spot identity SID (“0002” in FIG. 4) is transmitted from the irradiation control apparatus 10 to the beam data processing apparatus 11, the integrated control unit of the beam data processing apparatus 11 receives the spot identity SID at the rising timing of the spot ID strobe sb1 and repeats the foregoing data collection procedure and position size calculation procedure. After receiving the beam position P and the beam size S from the position size processing unit 23, the abnormality determination processing unit 24 determines whether or not the beam position P and the beam size S are allowable, based on the preset data PD. For example, when determining, with regard to the result of the second calculation in Measurement 1, that the beam position P is not allowable, the abnormality determination processing unit 24 outputs the position abnormality signal sige1 to the irradiation control apparatus 10. The irradiation control apparatus 10 receives the position abnormality signal sige1 and then cancels the beam-on command sigs2 after an interlock operation time T3 has elapsed. In this situation, cancellation of the beam-on command sigs2 is equivalent to issuing a command of turning off the beam. In response to the cancellation of the beam-on signal sigs2, the beam generation apparatus 52 stops the generation of the charged particle beam 1. With reference to FIG. 5, the operation of the integrated control unit 22 will be explained in detail. The integrated control unit 22 controls the demultiplexer 32, the A/D converter 33, and the multiplexer 34, by use of a demultiplexer switching command sigc1, a multiplexer switching command sigc2, the ADC processing starting command sigc3, the ADC processing starting signal sigd1, and the ADC processing ending signal sigd2. FIG. 5 represents an example in which in order to control the five A/D converters 33a through 33e, the demultiplexer switching command sigc1, the multiplexer switching command sigc2, the ADC processing starting command sigc3, a data ID signal sigc4, the ADC processing starting signal sigd1, and the ADC processing ending signal sigd2 are utilized. The respective numerals in parentheses added to signal names correspond to Measurements 1 through 5. The explanation will be made under the assumption that each of Measurements 1 through 5 is implemented by use of the A/D converters 33a through 33e. When receiving the scanning start command sigs1 outputted from the irradiation control computer 9, the integrated control unit 22 outputs the demultiplexer switching command sigc1(1), the multiplexer switching command sigc2(1), and the data ID signal sigc4(1) so as to activate the first A/D converter 33a. When receiving the demultiplexer switching command sigc1(1), the demultiplexer 32 switches the transmission paths so that a signal outputted from the current/voltage converter 31 is transmitted to the A/D converter 33a. When receiving the multiplexer switching command sigc2(1), the multiplexer 34 switches the transmission paths so that data outputted from the A/D converter 33a is transmitted to the position size processing unit 23. When receiving the data ID signal digc4(1), which is a signal for notifying a spot identity SID and a sub-ID, the position size processing unit 23 and the abnormality determination processing unit 24 store the spot identity SID and the sub-ID of data transmitted from the current/voltage converter 31. In this situation, the sub-ID also indicates what number the present measurement is. For example, in the case where the measurement is implemented five times at a single irradiation spot, the sub-ID is from 1 to 5. When receiving the inter-spot travel completion signal siga outputted from the irradiation control apparatus 10, the integrated control unit 22 outputs the ADC processing starting command sigc3(1) to the A/D converter 33a. When receiving the ADC processing starting command sigc3(1), the A/D converter 33a outputs the ADC processing starting signal sigd1(1) to the integrated control unit 22 after the measurement delay time T2 has elapsed and starts AD conversion processing. When receiving the ADC processing starting signal sigd1(1) from the A/D converter 33a, the integrated control unit 22 outputs the demultiplexer switching command sigc1(2) to the demultiplexer 32. When receiving the demultiplexer switching command sigc1(2), the demultiplexer 32 switches the transmission paths so that a signal outputted from the current/voltage converter 31 is transmitted to the A/D converter 33b. When receiving the 25% dose completion signal sigb1 outputted from the dose data converter 6, the integrated control unit 22 outputs the ADC processing starting command sigc3(2) to the A/D converter 33b. When receiving the ADC processing starting command sigc3(2), the A/D converter 33b outputs the ADC processing starting signal sigd1(2) to the integrated control unit 22 after the measurement delay time T2 has elapsed and starts AD conversion processing. With regard to the second and subsequent transmission-path switching actions by the demultiplexer 32, when receiving the ADC processing starting signal sigd1, which indicates that the A/D converter 33 corresponding to the immediately previous sub-ID has received the signal from the current/voltage converter 31 and then AD conversion processing has been started, the integrated control unit 22 outputs to the demultiplexer 32 the demultiplexer switching command sigc1 for switching the transmission paths so that the signal from the current/voltage converter 31 is transmitted to the A/D converter 33 corresponding to the next sub-ID (a demultiplexer switching procedure). Also in Measurements 3 through 5, the demultiplexer switching procedure is implemented in a similar manner as described above. In FIG. 5, when receiving the ADC processing starting signal sigd1(5), the integrated control unit 22 outputs to the demultiplexer 32 the demultiplexer switching command sigc1(1) for switching the transmission paths so that the signal from the current/voltage converter 31 is transmitted to the A/D converter 33a corresponding to the next sub-ID. Next, a multiplexer switching procedure will be explained. After the AD conversion processing has been completed, the A/D converter 33a outputs the ADC processing ending signal sigd2(1) to the integrated control unit 22. When receiving the ADC processing ending signal sigd2(1), the integrated control unit outputs the multiplexer switching command sigc2(2) to the multiplexer 34 and outputs the data ID signal sigc4(1) to the position size processing unit 23 and the abnormality determination processing unit 24. When receiving the multiplexer switching command sigc2(2), the multiplexer 34 switches the transmission paths so that data outputted from the second A/D converter 33b is transmitted to the position size processing unit 23. When receiving the data ID signal digc4(2), the position size processing unit 23 and the abnormality determination processing unit 24 store the spot identity SID and the sub-ID that are transmitted next. The multiplexer switching procedure will be summarized. When receiving the ADC processing ending signal sigd2, the integrated control unit 22 outputs the multiplexer switching command sigc2 corresponding to the next sub-ID to the multiplexer 34 and outputs the data ID signal sigc4 corresponding to the next sub-ID to the position size processing unit 23 and the abnormality determination processing unit 24. Also in Measurements 3 through 5, the demultiplexer switching procedure is implemented in a similar manner as described above. Next, the operation at the irradiation spot corresponding to the second spot identity SID (“0002” in FIG. 4) will be explained. As described above, before the pulse of the second inter-spot travel completion signal siga is outputted from the irradiation control apparatus 10, when receiving the demultiplexer switching command sigc1(1) outputted from the integrated control unit 22, the demultiplexer 32 switches the transmission paths so that a signal outputted from the current/voltage converter 31 is transmitted to the A/D converter 33a. When receiving the inter-spot travel completion signal siga outputted from the irradiation control apparatus 10, the integrated control unit 22 outputs the ADC processing starting command sigc3(1) to the A/D converter 33a. When receiving the ADC processing starting command sigc3(1), the A/D converter 33a outputs the ADC processing starting signal sigd1(1) to the integrated control unit 22 after the measurement delay time T2 has elapsed and starts AD conversion processing. The operation thereafter is the same as the operation described above. The operation of the position size processing unit 23 will be explained by use of a flowchart. FIG. 6 is a flowchart for explaining the operation of the position size processing unit. In the step ST01, the position size processing unit 23 receives the spot identity SID and the sub-ID. In the step ST02, the position size processing unit 23 receives a voltage Vi, which has been AD-converted, from the channel data conversion unit 21. In the step ST03, the respective voltages Vi are converted into predetermined proportions wi. The proportion wi corresponds to the weight utilized when the beam position P is calculated in such a manner as a gravity center is calculated. In the step ST04, the position size processing unit 23 calculates, through the equations (1) and (2) below, the beam position P (Px, Py) in such a manner as a gravity center is calculated. The position size processing unit 23 of the X data processing unit 16 calculates the beam position Px; the position size processing unit 23 of the Y data processing unit 17 calculates the beam position Py.Px=Σ(wix×Xi)/Σwix (1)Py=Σ(wiy×Yi)/Σwiy (2) where Xi is the X coordinate of “i” in the X channel of the position monitor 4, and Yi is the Y coordinate of “i” in the Y channel of the position monitor 4; wix is the proportion obtained converting the voltage Vi of “i” in the X channel, and wiy is the proportion obtained converting the voltage Vi of “i” in the Y channel. In the step ST04, after completion of the calculation of the beam position P (Px, Py), the position size processing unit transmits the calculated beam position P (Px, Py) to the abnormality determination processing unit 24. In the step ST05, the position size processing unit 23 stores the beam position P (Px, Py) for each spot identity SID and sub-ID in the data memory 25. In the step ST06, the position size processing unit 23 calculates, through the equations (3) and (4) below, the beam size S (Sx, Sy) in such a manner as a standard deviation is calculated.Sx=sqr(Σwix×(Xi−Px)2)/Σwix) (3)Sy=sqr(Σwiy×(Yi−Py)2)/Σwiy) (4) where sqr is a function for performing a root calculation, and n is the sum of calculation subjects in the X and Y channels. In the step ST06, after completion of the calculation of the beam size S (Sx, Sy), the position size processing unit 23 transmits the calculated beam size S (Sx, Sy) to the abnormality determination processing unit 24. In the step ST07, the position size processing unit 23 stores the beam size S (Sx, Sy) for each spot identity SID and the sub-ID in the data memory 25. The operation of the abnormality determination processing unit 24 will be explained by use of a flowchart. FIG. 7 is a flowchart for explaining the operation of the abnormality determination processing unit. In the step ST11, the abnormality determination processing unit 24 receives the spot identity SID and the sub-ID. In the step ST12, the abnormality determination processing unit 24 receives the beam position P (Px, Py) from the position size processing unit 23. In the step ST13, the abnormality determination processing unit 24 determines whether or not there exists a positional abnormality. In other words, the abnormality determination processing unit 24 determines whether or not the absolute value of the difference ΔP between the beam position P (Px, Py) and the desired position Pd (Pdx, Pdy) is larger than the position allowable value AP. In the case where the absolute value of the difference ΔP is larger than the position allowable value AP, the abnormality determination processing unit 24 transmits the position abnormality signal sige1 to the irradiation control apparatus 10. The abnormality determination processing unit 24 of the X data processing unit 16 transmits the position abnormality signal sige1x to the irradiation control apparatus 10; the abnormality determination processing unit 24 of the Y data processing unit 17 transmits the position abnormality signal sige1y to the irradiation control apparatus 10. In the step ST14, the abnormality determination processing unit 24 receives the beam size S (Sx, Sy) from the position size processing unit 23. In the step ST15, the abnormality determination processing unit 24 determines whether or not there exists a beam size abnormality. In other words, the abnormality determination processing unit 24 determines whether or not the absolute value of the difference ΔS between the beam size S (Sx, Sy) and the desired beam size Sd (Sdx, Sdy) is larger than the size allowable value AS. In the case where the absolute value of the difference AS is larger than the size allowable value AS, the abnormality determination processing unit 24 transmits the size abnormality signal sige2 to the irradiation control apparatus 10. The abnormality determination processing unit 24 of the X data processing unit 16 transmits the size abnormality signal sige2x to the irradiation control apparatus 10; the abnormality determination processing unit 24 of the Y data processing unit 17 transmits the size abnormality signal sige2y to the irradiation control apparatus 10. In the step ST16, in the case where it is determined that there exists a positional abnormality or a beam size abnormality, the abnormality determination processing unit 24 stores in the data memory 25 information indicating that there exists a positional abnormality or a beam size abnormality, for each spot identity SID and sub-ID. The beam data processing apparatus 11 according to Embodiment 1 can detect twice or more times the irradiation position P and the beam size S of the charged particle beam 1 while the charged particle beam 1 is stopped at an irradiation spot. Each time detecting the irradiation position P and the beam size S of the charged particle beam 1, the beam data processing apparatus 11 can determine whether or not there exists a positional abnormality or a size abnormality of the charged particle beam. Accordingly, the beam data processing apparatus 11 twice or more times detects the irradiation position P and the beam size S of the charged particle beam 1 while the charged particle beam 1 is stopped at an irradiation spot, and determines whether or not there exists a positional abnormality or a size abnormality of the charged particle beam each time the irradiation position P and the beam size S of the charged particle beam 1 are detected; therefore, even in the case where when the charged particle beam 1 is stopped at an irradiation spot and is being irradiated, a positional abnormality or a size abnormality is caused in the charged particle beam 1, it is made possible to determine that there exists the positional abnormality or the size abnormality of the charged particle beam 1. When a positional abnormality or a size abnormality is caused in the charged particle beam 1, the beam data processing apparatus 11 transmits the position abnormality signal sige1 indicating the occurrence of the positional abnormality or the size abnormality signal sige2 indicating the occurrence of the size abnormality to the irradiation control apparatus 10 of the irradiation management apparatus 8. When receiving the trigger signal (siga, sigb1, sigb2, sigb3, or sigb4), which indicates that collection of the next data has been instructed, the beam data processing apparatus 11 transmits the ADC processing abnormality signal sige3 to the irradiation control apparatus 10 when all the A/D converters 33a through 33e of the channel data conversion unit 21 are performing processing (when AD conversion processing is being performed). In the case where while the charged particle beam 1 is irradiated, a positional abnormality, a size abnormality, or an AD conversion processing abnormality of the charged particle beam 1 is caused, the irradiation control apparatus 10 receives the position abnormality signal sige1, the size abnormality signal sige2, or the ADC processing abnormality signal sige3 from the beam data processing apparatus 11, so that interlock processing, which is emergency stop processing, can be implemented. The beam data processing apparatus 11 according to Embodiment 1 is provided with two or more A/D converters 33 for each channel of the position monitor 4; therefore, the two or more A/D converters 33 can sequentially operate. Accordingly, even in the case where a single AD conversion processing takes a long time, data collection and AD conversion processing at a single irradiation spot can be implemented twice or more times. The beam data processing apparatus 11 can make two or more A/D converters 33 sequentially operate; therefore, unlike a beam data processing apparatus provided with only a single A/D converter 33 for each channel, the beam data processing apparatus 11 can start collection of data before on-going processing by the A/D converter 33 has been completed. Accordingly, it is not required to create a treatment plan restricted by the processing time of the A/D converter 33; thus, a treatment plan in which a single therapy irradiation ends in a shorter time is able to create. In other words, the therapy time can be shortened. The particle beam irradiation apparatus 58 according to Embodiment 1 is provided with the beam data processing apparatus 11; therefore, in the case where while the charged particle beam is irradiated, a positional abnormality, a size abnormality, or an AD conversion processing abnormality of the charged particle beam 1 is caused, the particle beam irradiation apparatus 58 receives the position abnormality signal sige1, the size abnormality signal sige2, or the ADC processing abnormality signal sige3 from the beam data processing apparatus 11 so that interlock processing, which is emergency stop processing, can be implemented. Accordingly, in the case where a positional abnormality, a size abnormality, or AD conversion processing abnormality is caused in the charged particle beam 1, the particle beam irradiation apparatus 58 can stop the irradiation of the charged particle beam 1 in a short time. The particle beam therapy system 51 according to Embodiment 1 is provided with the beam data processing apparatus 11; therefore, in the case where while the charged particle beam 1 is irradiated, a positional abnormality, a size abnormality, or an AD conversion processing abnormality of the charged particle beam 1 is caused, the particle beam therapy system 51 receives the position abnormality signal sige1, the size abnormality signal sige2, or the ADC processing abnormality signal sige3 from the beam data processing apparatus 11 so that interlock processing, which is emergency stop processing, can be implemented. Accordingly, in the case where a positional abnormality, a size abnormality, or AD conversion processing abnormality is caused in the charged particle beam 1, the particle beam therapy system 51 can stop the irradiation of the charged particle beam 1 in a short time. As described above, the beam data processing apparatus 11 according to Embodiment 1 is provided with a plurality of channel data conversion units 21 that perform AD conversion processing in which each of the plurality of analogue signals outputted from the position monitor 4 that detects with a plurality of detection channels the passing position of the charged particle beam 1 is converted into a digital signal; the position size processing unit 23 that calculates the beam position P, which is the passing position of the charged particle beam 1 in the position monitor 4, based on voltage information items obtained through processing by the plurality of channel data conversion units 21; the abnormality determination processing unit 24 that determines whether or not the beam position P is within a tolerance range, based on the desired beam size Pd of the charged particle beam 1 and the position allowable value AP, and that generates the position abnormality signal sige1 when determining that the beam position P is not within the tolerance range; and the integrated control unit 22 that controls the plurality of channel data conversion units 21 in such a way that while the charged particle beam 1 is stopped at an irradiation spot, digital signal conversion processing is implemented two or more times. The channel data conversion unit 21 includes a plurality of A/D converters 33, the demultiplexer 32 that distributes respective analogue signals to the A/D converters 33 at different timings, and the multiplexer 34 that switches the respective digital signals processed by the A/D converters 33 at different timings so as to output them to the position size processing unit 23. As a result, a plurality of A/D converters 33 can be sequentially operated; therefore, it is made possible to detect the irradiation position P of the charged particle beam 1 even in the case where while the charged particle beam 1 is irradiated at an irradiation spot, a position abnormality of the charged particle beam 1 is caused and to generate an abnormality detection signal for indicating the position abnormality of the charged particle beam 1. The particle beam therapy system 51 according to Embodiment 1 is provided with the beam generation apparatus 52 that generates the charged particle beam 1 and accelerates it by means of the accelerator 54, the beam transport system 59 that transports the charged particle beam 1 accelerated by the accelerator 54; and the particle beam irradiation apparatus 58 that irradiates the charged particle beam 1 transported by the beam transport system 59 onto the irradiation subject 15. The particle beam irradiation apparatus 58 has the scanning electromagnets 2 and 3 that scan the charged particle beam 1 to be irradiated onto the irradiation subject 15 and the beam data processing apparatus 11 that applies calculation processing to the state of the charged particle beam 1 that has been scanned by the scanning electromagnets 2 and 3. In the particle beam therapy system 51 according to Embodiment 1, the beam data processing apparatus 11 of the particle beam therapy system 51 is provided with a plurality of channel data conversion units 21 that perform AD conversion processing in which each of the plurality of analogue signals outputted from the position monitor 4 that detects with a plurality of detection channels the passing position of the charged particle beam 1 is converted into a digital signal; the position size processing unit 23 that calculates the beam position P, which is the passing position of the charged particle beam 1 in the position monitor 4, based on voltage information items obtained through processing by the plurality of channel data conversion units 21; the abnormality determination processing unit 24 that determines whether or not the beam position P is within a tolerance range, based on the desired beam size Pd of the charged particle beam 1 and the position allowable value AP, and that generates the position abnormality signal sige1 when determining that the beam position P is not within the tolerance range; and the integrated control unit 22 that controls the plurality of channel data conversion units 21 in such a way that while the charged particle beam 1 is stopped at an irradiation spot, digital signal conversion processing is implemented two or more times. The channel data conversion unit 21 includes a plurality of A/D converters 33, the demultiplexer 32 that distributes respective analogue signals to the A/D converters 33 at different timings, and the multiplexer 34 that switches the respective digital signals processed by the A/D converters 33 at different timings so as to output them to the position size processing unit 23. As a result, a plurality of A/D converters 33 can be sequentially operated; therefore, in the case where while the charged particle beam 1 is irradiated, a positional abnormality or a size abnormality of the charged particle beam 1 is caused, the position abnormality signal sige1 or the size abnormality signal sige2 is received from the beam data processing apparatus 11, so that interlock processing, which is emergency stop processing, can be implemented. The difference ΔP between the beam position P (Px, Py) and the desired position Pd (Pdx, Pdy), which is calculated by the abnormality determination processing unit 24, can also be transmitted, as position feedback information, to the irradiation control apparatus 10. The irradiation control apparatus 10 may control the position of the charged particle beam 1, based on the position feedback information. Through the foregoing method, a positional deviation, which is not large enough to be determined as a positional abnormality, can be corrected. In the flowchart in FIG. 6, an example has been explained in which the beam position P is calculated and then the beam size S is calculated; however, it may be allowed that two calculation units perform parallel processing of the beam position P and the beam size S. In this case, in the flowchart in FIG. 7, it may be allowed that data, out of the beam position P and the beam size S, that arrives earlier than the other is determined (position abnormality determination or size abnormality determination). 1: charged particle beam 2: X-direction scanning electromagnet 3: Y-direction scanning electromagnet 4: position monitor 5: dose monitor 11: beam data processing apparatus 15: irradiation subject 21, 21a, 21n: channel data conversion unit 22: integrated control unit 23: position size processing unit 24: abnormality determination processing unit 32: demultiplexer 33, 33a, 33b, 33c, 33d, 33e: A/D converter 34: multiplexer 51: particle beam therapy system 52: beam generation apparatus 54: synchrotron 58, 58a, 58b: particle beam irradiation apparatus 59: beam transport system P, Px, Py: beam position Pd, Pdx, Pdy: desired position AP: position allowable value S, Sx, Sy: beam size Sd, Sdx, Sdy: desired beam size AS: size allowable value siga: inter-spot travel completion signal sigb1: 25% dose completion signal sigb2: 50% dose completion signal sigb3: 75% dose completion signal sigb4: 100% dose completion signal sigc3: ADC processing starting command sigd1: ADC processing starting signal sigd2: ADC processing ending signal sige1: position abnormality signal sige2: size abnormality signal |
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042631637 | description | THE SPECIFIC DESCRIPTION OF THE INVENTION One part of this invention comprises a furnace for heating a material to form a heated product and/or an expanded or bloated product. The furnace comprises a first circular member 30 and a second circular member 32. In FIG. 2, it is seen that the first circular member 30 is in the configuration of a torus having a central opening or passageway 34. In FIG. 3, it is seen that the second circular member 32 is in the configuration of a torus having a central opening or passageway 36. In FIG. 1, it is seen that the first circular member 30 is positioned below the second circular member 32. The first circular member 30 can rotate and the second circular member 32 can be stationary. The first circular member 30 comprises a plenum chamber and also a support for fire brick 38. In FIG. 6, there is illustrated the support structure for the fire brick 38 of the first circular member 30. It is seen that there is a bottom support plate 40 in the configuration of a torus. Then, there is an outer circular wall 42. There is also an inner circular wall 44 defining the opening 34. Also, there are two in-between circular wall supports 46 near the wall 42 and 48 near the wall 34. Projecting inwardly on the upper part of the wall 42 is a support ledge 50. And, projecting inwardly of the wall 44 is a support ledge 52. Then, on the upper part of the wall 46 is a support ledge 54 and on the upper part of the wall 48 is a support ledge 56. The firebrick 38 rests on the support ledges. In FIG. 2, it is seen that there are three circular courses of brick. There is an outer circular course of brick 56, a middle course of brick 58, and an inner circular course of brick 60. In FIG. 2, it is seen that the firebrick 38, in a plan view, are in the figure of the frustum of a trapezoid. The firebrick in the outer course 76 are larger in size than the first brick in the middle course 58. The firebrick in the middle course 58 are of a larger size than the firebrick in the inner course 60. It is to be understood that a furnace may have only one course of firebrick or may have a large number of courses of firebrick. For illustrative purposes, there is illustrated in the first circular member 30 three courses of firebrick. There are four upright pedestals 60, spaced at 90.degree. with respect to each other. Each of the pedestals 60 has a supporting foot 62. Also, on the upper end of each of the pedestals 60, there is an inwardly directly shaft 64 and a roller 66 is positioned on the shaft. In FIGS. 1 and 6, it is seen that the bottom support plate 40 rests on the rollers 66 and that the first circular chamber 30 can rotate on these rollers 66. Further, on the upper part of the upright pedestals 60, there is an upwardly directed shaft 68. There is positioned on the upwardly directed shaft 68 a roller 70. In FIGS. 1, 2, and 6, it is seen that the outer circular wall 42 is positioned between the four rollers 70. The first circular member 30 can rotate between the rollers 70 and can be positioned by these rollers 70. In FIGS. 1, 2, and 6, there is illustrated a feed system for feeding an air-combustible gas mixture, such as propane or butane, to the plenum chamber of the first circular member 30. In FIG. 6, the plenum chamber is identified by reference numeral 72. There is an inlet pipe 74 connecting with a mixing chamber 76. The mixing chamber 76 has an outlet nozzle 78. There is an adapter 80 which fits over the outlet nozzle 78. The adapter connects with two arms 82 and 84. In FIG. 6, it is seen that the arm 82 connects with an opening 86 in the bottom support plate 40. Likewise, the arm 84 connects with another opening in the bottom support plate 40. The arms 82 and 84 are welded to the bottom support plate 40. There is attached to the adapter 80 a sprocket 88. Also, positioned near the mixing chamber 76 is a motor and variable drive gear box 90 having outlet shaft 92. On the outlet shaft 92, is a sprocket 94. A chain 96 connects the sprocket 88 and the sprocket 94. With the actuation of the motor and variable drive gear box 90, the sprocket 94, the chain 96, and the sprocket 88 move so as to rotate the adapter 80 and the arms 82 and 84 and the first circular member 90. With the rotation of the sprocket 88 and, correspondingly, the first circular member 30, the material placed on the firebrick also rotates. The firebrick 38 is a porous brick and allows the mixture of air and combustible gas to pass from the plenum chamber and through the interstices of the brick to the surface of the brick. From experience, I have found that the ends and sides of the firebrick 38 should be painted with a "temperature resistant" paint or a fireproof paint 100. This paint is impervious to the flow of the air-combustible gas mixture and thereby restricts the flow of the air-combustible gas mixture to passing through the brick. In placing the brick 38 on the support ledges, there is used a silicone sealant 102 to seal between the surface of the brick and the surface of the ledge. The firebrick may be one of many suitable bricks, such as K30 B and W. There may be placed on top of the firebrick, a layer 104 of aluminum oxide or silicone carbide. The porosity of the layer 104 of aluminum oxide or silicone carbide is of the same porosity as of the firebrick 38. The layer 104 is harder than the firebrick 38 so as to resist abrasion. Further, I consider that it is desirable that the firebrick 38 be as uniform as possible with respect to dimension and with respect to weight. It is possible within a narrow tolerance range to have the firebrick 38 in a course of the same general dimensions with respect to length, width, and thickness and also the same general porosity. Naturally, the firebrick 38 will vary in dimensions from one course to another course but in the same course, the firebrick should be of the same general characteristics. The second circular member 32 comprises a number of refractory brick 110 positioned above the firebrick 38 of the first circular member 30. In FIG. 1, it is seen that there is an upper circular ring 112. The refractory brick 110 are suspended from the ring 112 by means of bolts 114. In FIG. 1, it is seen that the spacing between the refractory brick 110 and the firebrick 38 remains constant. In FIG. 1, it is seen that on the outside of the refractory brick 110 and the second circular member 32 that there is a depending circular rim 116. The depending circular rim 116 assists in maintaining the material being processed between the firebrick 38 and the refractory brick 110. In FIG. 7, there is illustrated the upper circular ring 112 and bolts 114 connecting with the refractory brick 110. In FIG. 7, it is seen that the length of the bolts 114 vary so as to have some of the refractory brick 110 farther away from the ring 114 than other refractory brick. The result is that some of the refractory brick 110 are closer to the firebrick 38. The reason for this is that when the material to be processed is initially placed on the firebrick 38, it is of a, relatively, small volume. After a while, this material expands into a larger volume and in order to accommodate the larger volume, the refractory brick 110 must be positioned farther away from the firebrick 38. In FIG. 7, it is seen that the refractory brick 110 have a recess 118. There is a nut 120 screwed onto the threaded end of the bolt 114 and in said recess 118. In FIG. 8, there is illustrated a split upper ring 122. The split upper ring is in the form of a spiral having a lower end 124 and a upper end 126. A number of bolts 114 connect with the ring 122 and also connect with and support the refractory brick 110. In FIG. 8, the bolts 114 can be of, substantially, the same length even though some of the refractory brick 110 are positioned closer to the firebrick 38 than some of the other refractory brick 110. Again, the spacing of the refractory brick 110 with respect to the firebrick 38 is to accommodate the various size and volume of the material being processed. In FIG. 3, it is seen that there is a void 130 between two adjacent refractory brick 110 so as to allow material to be introduced between the second circular member 32 and the first circular member 30. This is more clearly illustrated in FIG. 4 wherein it is seen that there is a chute 132 or conveyor 132 for introducing product onto the firebrick 38. It is to be remembered that the lower circular member 30 and the firebrick 38 rotate while the upper circular member 32 and the refractory brick 110 as stationary or do not rotate. With the opening of void 130 in the upper circular member 32, the material to be processed can be introduced by means of the chute 132 so as to fall onto the firebrick 38. In FIG. 4, it is seen that the lower circular member 30 rotates in a clockwise direction. Also, in FIG. 4, it is seen that there is a doctor blade 134. The doctor blade extends to the outer edge or periphery of the lower circular member 30 so as to cause the processed material to flow toward the periphery of the member 30. There is positioned, partially, under the member 30 a sloping conveyor or chute 136. The material which has been processed is forced by the doctor blade to fall onto the chute 136 and be removed from between the firebrick 38 and the refractory brick 110. In FIG. 7, it is seen that there is an opening 130 between the adjacent refractory brick 110 attached to the circular ring 112. Likewise, in FIG. 8, it is seen that there is an opening 130 between firebrick 110 attached to the split circular ring 122. The openings 130 illustrated in FIGS. 7 and 8 make it possible to introduce the material to be processed onto the firebrick 38 of the lower circular member 30. In FIG. 11, there is illustrated the doctor blade 134 positioned above the firebrick 38 and positioned so as to drop the product 138 onto the sloping conveyor 136. In FIGS. 1 and 2, it is seen that there are spaced apart pedestals 140, on the outside of the upright pedestal 60. On the lower part of the pedestal 140, there is a foot 142. On the upper part of the pedestals 140, there is a flange or arm 144. There depends from the flange or arm 144 a support 146, such as a bolt or a rod. The bolt or rod 146 also connects with the upper circular 112 or the split ring 122 so as to support the ring and the refractory brick 110 above the firebrick 38. In FIGS. 1 and 3, there is illustrated an exhaust system for the products of combustion and, possible, some of the resulting product from the process. In FIGS. 1 and 3, it is seen that there is a hood 150 positioned above the opening 36 in the upper circular member 32. The hood 150 connects with the exhaust pipe 152. It is possible to exhaust the gases from the furnace 28 through the exhaust pipe 152 and into the atmosphere. Sometime, it may be desirable to separate entrained solids in the exhaust gases. Therefore, there is illustrated in FIG. 1, in broken line or phantom line, a cyclone 154 which connects with the exhaust pipe 152 by means of an inlet pipe 156. The cyclone 154 has a lower exhaust pipe 158. On the upper end of the cyclone 154, there is a motorvan-passageway 160 for directing the exhaust gases from the cyclone 154 and into a bag house 162. It is seen that some of the solid particles in the exhaust gases are separated in the cyclone 154 and flow out of the cyclone through the exhaust 158. Also, the gases which flow from the cyclone 154 into the bag house 162 can flow out of the bag house. The bag house will remove the small particulate solids in the exhaust gases. In FIG. 16, there is illustrated another species of a lower circular member 170. It is seen that this species comprises a bottom support plate 172 surrounded by an upwardly directed circumscribing rim 174. Further, it is seen that positioned on the lower support plate 172 are a number of flat bars 176 having bends 178. These flat bars can be positioned on edge on the plate 172. In the zigzag flat bars 178, there may be passageways 180. There is positioned on top of the zigzag bars 178, expanded metal 182. Then, there is positioned on top of the expanded metal 182, firebrick 184. The firebrick 184 may have the sides and ends painted with a high temperature fire resistant paint so as to seal the firebrick. Further, the tops of the firebrick 184 may be coated with a block of aluminum oxide or silicone carbide of substantially the same porosity as the firebrick 184. The reader is to understand that the firebrick 184 are porous and that the mixture of air and combustible gas can pass through the porous firebrick 184 and also the coating of aluminum oxide or silicone carbide so as to be able to burn on top of the firebrick 184. It is to be understood that that part of the lower circular member 170 between the lower plate 172, the outer circumscribing rim 174, the inner circumscribing rim 186, and the lower part of the firebrick 184 is a plenum chamber 188. There can be introduced into the plenum chamber 188, the air and combustible gas mixture. To introduce the air and combustible gas mixture into the plenum chamber 188, there are four arms 190 connecting with the outlet nozzle 78 of the mixing chamber 76. The four arms 190 may be square tubes. In the lower plate 172, there are a number of openings 192. Aligned with the openings 192 in the plate 172 are openings 194 in the tubes 190. The tubes 190 can be welded to the lower plate 172 so as to form a rigid structure with the openings 192 and the openings 194 aligned for introducing the air-combustible gas mixture into the plenum chamber 188. Again, it is to be realized that the lower circular member 170 rotates as does the lower circular member 30 as, previously, explained in a foregoing part of this written description. One of the advantages of the lower circular member 170 is the firebrick 184 need not be cut into a trapezoidal configuration. There is a saving in time and money by using standard fire brick 184. The firebrick 184 are supported on the expanded metal 182 which, in turn, is supported on the lower support plate 172. The lower support plate 172 is supported by the rollers 66, see FIG. 1, and the description of the rollers 66. Further, the lower circular member 170 is prevented from a sideways motion by the rollers 70 positioned on the upright pedestal 60. In FIGS. 17 and 18, there is illustrated an upper circular member 200 having a central opening 202. The upper central member 200 comprises sheet metal 204 in the configuration of a spiral, see FIG. 18. Sheet metal 204 is not continuous as there is a break to form an opening 206. There is attached to the sheet metal 206 refractory brick 208. The refractory brick 208 can be attached by means of sheet metal screws 210 and an adhesive 212. Also, there is on the periphery of the sheet metal 204, and depending therefrom, a circumscribing depending rim 214. This rim 214 assists in maintaining the product in the furnace 28 between the rotating lower circular member and the upper stationary circular member. Again, the opening 206 is to allow material to be introduced onto the rotating lower member. An advantage of the upper circular member 200 is that it is not necessary to cut the refractory brick into the configuration of a frustum of a cone, see refractory brick 110 in FIG. 3. The refractory brick 208, in the main, can be standard, commercially, available brick. The use of this standard brick results in a less expensive upper circular member 200. Also, the sheet metal 204 can be shaped into the form of the split circular spiral. The material 220 to be processed can be positioned on the chute or conveyor 132 and then placed on the firebrick of the first circular member, see FIG. 4. In FIG. 4, it is seen that this first circular member rotates in a clockwise direction and that the material is processed into semiprocessed material 222. After the material has been further processed into a product 224, the material can be removed from the lower circular member by means of doctor blade 134. The product 224 will fall onto the chute 136 for further treatment, such as packaging, or for use, such as in light weight concrete, gardens, insulation, filter aid, and the like. Some of the material which can be processed in this furnace 28 are perlite, vermiculite, volcanic ash, pumice, zeolite, clay, diatomaceous earth, carriers for radioactive materials, titanium dioxide, salt cake, and the like. The furnace 28 can achieve a temperature in the range of about 2500.degree. to 2600.degree. F. This is a sufficiently high temperature to process these materials. It is possible to introduce the material 220, viz., perlite, vermiculite, volcanic ash, pumice, and zeolite into the furnace 28. This material 220 on the firebrick will be heated and expand. For example, the density of the perlite 220 being introduced into the furnace 28 may be in the range of about 80 pounds per cubic foot while the density of the processed perlite 224 or expanded perlite 224 may be in the range of 5 pounds to 10 pounds per cubic foot. For example, the expanded perlite may have a density in the range of 3 pounds to 4 pounds per cubic foot and may range in particle size from +50 mesh to +100 mesh. Perlite in the range of 15 to 20 pounds per cubic foot may have a particle size of about 5/8 of an inch. I have found that it is not necessary to use a flux with perlite. The perlite can be expanded without the use of a flux, such as sodium carbonate, potassium carbonate, sodium oxide or potassium oxide. With the operation of the furnace 28, I have noticed that the perlite is, apparently, annealed and is stronger or tougher than expanded perlite made in a rotary furnace or a verticle furnace. The use of the expanded perlite can be for purposes of insulation, a filter aid for food products, an aggregate in light weight concrete, and a soil conditioner for horticultural purposes. Vermiculite can be treated in a manner similar to the treatment of perlite. Vermiculite is a schist and comprises a mixture of vermiculite and hornblend. The vermiculite ore can be passed through a rotary dryer and then screened to a size in the range of 1/8 inches to +40 mesh to form the material 220 to be expanded. The vermiculite 220 is introduced into the rotary furnace 222 and processed to form the processed vermiculite 224. The processed vermiculite 224 is softer than the processed perlite 124. However, the processed vermiculite can be used for purposes of insulation and as an aggregate in the formation of insulation board. It is possible to saw the insulation board, nail the insulation board, and use the insulation board in building a structure, such as a house or a shop. In addition to expanding or bloating materials, such as perlite and vermiculite, in the furnace 28, this furnace can also be used for calcining materials. For example, materials 220 which can be calcined are diatomaceous earth, clay, cement, titanium dioxide, fly ash, volcanic ash, natural zeolites, and pumice, to name a few. Many of these materials are of such a small size, even approaching the size of powder, it is not reasonable to process these materials in the furnace 28. In order to process these materials, it is necessary to add a binder to make the material somewhat sticky to form a sticky material. Then the sticky material is placed on a screen or a similar device and agitated to cause the sticky material to ball up or to agglomerate so as to form a agglomerated material. In this process of forming the agglomerated material, the particle size can be, readily, controlled. The transformation of the powder material to a larger and specific particle size by the agglomeration process allows precise control of the bed thickness of the material 220 on the fire brick of the furnace 28. To make the agglomerated product, there is employed a binder. The binder can be one of many chemicals or a combination of chemicals varying from plain water to complex chemicals depending upon the requirements of the material being treated. For example, in the calcining of diatomaceous earth for filter aid products, there is added to the diatomaceous earth a sodium flux, such as sodium carbonate. However, other sodium compounds can work as well as sodium carbonate, such as sodium hydroxide, sodium chloride, sodium silicate, and corresponding potassium compounds. One of the fluxes can be in liquid form such as sodium carbonate diluted with water. In the case of the calcining of clay, the addition of water as a binder and then the balling up or agglomeration of the clay can be achieved. The agglomerated clay particles will hold together long enough to satisfy the bed thickness requirement in the furnace 28. In certain instances, where higher heat requirements might be needed, they can be achieved with the use of air and a natural gas combustion mixture; the binder can be a fuel, either liquid or solid. For example, in the processing of cement, there may be mixed coal and cement and an oxidizing agent to form the agglomerated product. The coal is burned in the furnace 28 and the ash from the coal can become integrally mixed with the cement. If the ash from the coal is detrimental to the product 224 from the furnace 28, then liquid petroleum can be used as the binding agent in the agglomeration process. The oxidizing agent may be a potassium chlorate. In the calcining of these materials, the step of forming agglomerated products is important. By being able to have the agglomerated products within a certain range of sizes or within a certain size range, it is possible to control the thickness of the material 220 on the firebrick. This makes possible a more precise control of heat transfer to the material 220. The heat transfer to the material 220 can be quicker and easier for a controlled bed thickness as contrasted with a bed thickness which is not controlled. Also, a more uniform processed product 224 can be realized with a controlled bed thickness of the material 220. The finely divided material and the powder which have been processed to make a agglomerated product 220 can be further processed in the furnace 28 to calcine the agglomerated product. In calcining the agglomerated product 220, the product is heated to a high temperature without fusing the product 220 to make the processed product 224. In the calcining operation, the agglomerated product 220 undergoes changes, such as oxidation, and also changes, such as forming a smooth or glasslike surface on the processed product 224. An example will assist in explaining the agglomerating process and also the calcining process. A suitable subject is diatomaceous earth which is a nonmetallic mineral composed of about 80% to 90% amorphous silica. The silica is the skeletal remains of diatoms in the ocean, millions of years ago. The crude diatomaceous earth is mined by open pit methods and transported to a plant site. The mined, crude diatomaceous earth is crushed, dried, and then pulverized and foreign material separated. At this stage of the process, the crushed diatomaceous earth will pass 90% through a 325 mesh screen (44 microns). The crushed diatomaceous earth is mixed with a flux. The flux can be sodium carbonate or sodium silicate, or sodium oxide, or potassium carbonate or potassium oxide, or potassium silicate. It is advantageous to mix the flux with water to form a liquid flux. The liquid flux is mixed with the crushed diatomaceous earth and formed into the agglomerated product 220. Then, the agglomerated diatomaceous earth 220 can be introduced into the furnace 28 and heated to form the processed diatomaceous earth product 224 which has a glaze or a glassy surface on the individual agglomerated particles. In the furnace 28, the sodium in soda ash reacts with the silica of the diatomaceous earth, at a temperature of about 1850.degree. F., to form the product 224. Similarly, fine particles of clay can be mixed with water and processed to form agglomerated clay. The agglomerated clay 220 can be introduced into the furnace 28 and heated and processed to form agglomerated clay products 224, which have a smooth or glassylike surface. Similarly, fine particles of titanium dioxide, fly ash, volcanic ash, cement, natural zeolite, pumice, and the like can be mixed with a flux, such as a sodium salt like sodium carbonate or sodium silicate or a potassium salt like potassium carbonate or potassium silicate and formed into agglomerated products 220 which can be introduced into the furnace 28 to form a processed product 224 having a smooth or glassy surface. The calcined clay has a higher brightness and opacity than natural clay and therefore is valuable in the manufacture of high-gloss paper. The calcined clay has better hiding power in the high-gloss paper. A calcined diatomaceous earth, calcined in the temperature range of about 1750.degree.-1900.degree. F. can be used as a filter aid, used as a filler in paint and also used as a filler in paper. In regard to diatomaceous earth which is used as a filter aid, the calcining process can be valuable in rejuvinating the filter aid. For example, the filter aid comprising spent diatomaceous earth can be processed and mixed with a binder, such as sodium silicate to form a agglomerated product 220 in the form of a discrete unit or a ball. Then, this discrete unit or ball 200 can be introduced into the furnace 28 and heated to a temperature in the range of about 1750.degree.-1900.degree. F. to form a new filter aid comprising the glazed or glassy diatomaceous earth ball or discrete unit. A result of this is the reusing of diatomaceous earth and the elimination of the step of throwing away used diatomaceous earth which has served a purpose as a filter aid. In addition to being able to calcine volcanic ash and pumice, it is also possible to expand the volcanic ash and pumice in the furnace 28 in the same manner that perlite and vermiculite are expanded, as above disclosed. Further, zeolite can be expanded in the furnace 28 and also can be calcined in the furnace 28. In FIG. 12, there is illustrated a process for treating radioactive material so that the radioactive material can be stored. Radioactive material 230 and solid particles 232, such as diatomaceous earth, clay, cement, fly ash, volcanic ash, natural zeolites, and pumice, are mixed together at step 234 to form a mixture of glomulated product. Then, at step 236, the agglomerated product or mixture is calcined to form a solid encapsulated material. The encapsulated material comprises the radioactive material and the solid particles. The encapsulated material can be stored at step 238. Or, the encapsulated material can be mixed with a shielding material, such as lead or boraxo or polyethylene at step 240 to form a shielded encapsulated material comprising the radioactive material. In this manner, there are prepared small, discrete, solid particles comprising radioactive material and which small, solid, discrete particles can be coated with a shielding material to lessen the radiation from the small, discrete particles. The shielded encapsulated material can be stored at 242. The process of FIG. 12 makes it possible to transform the radioactive material, usually in a liquid form, into a solid and then to coat the solid with radioactive shielding material so as to make it possible to more, safely, store the radioactive material. In FIG. 13, there is illustrated a process for treating objects 250 contaminated with radioactive material. For example, objects 250 which are contaminated with radioactive material are paper, clothing, gloves, rubber, plastic and the like which are used in the area of radioactive material. In this process, the objects 250 can be frozen at step 252 to form a solid frozen object. The objects 250 may be frozen by being contacted with liquid nitrogen so as to form a brittle, solid, frozen object. Then, in step 254, the brittle, solid, frozen object can be comminuted to small pieces. The brittle, solid, frozen objects may be comminuted in a ball mill or hammer mill or appropriate apparatus. The solid, frozen objects are processed at step 256 by burning so as to leave a radioactive residue which can be collected. The radioactive residue may be trapped in stack gases by a filter. It is to be remembered that radioactive particles are discrete particles and are not gases. The radioactive particles are solid and therefore can be trapped by a filtering means. Further, the step 256 reduces the volume of the objects containing the radioactive material. Prior to steps 252, 254, and 256, the volume of the objects containing the radioactive material was quite large. With these steps, the volume of the radioactive material is reduced to a more manageable volume. The radioactive residue 258 is mixed with a solid 260. The solid 260 may be a chemical which can be calcined, for example, diatomaceous earth, clay, cement, titanium dioxide, fly ash, volcanic ash, natural zeolites, pumice, and the like. At step 262, the radioactive residue and the solid 260 are mixed to form a mixture 264. The mixture 264 may be stored at 266. The mixture 264 may be calcined in the furnace 28, see step 268, to form a calcined mixture. The calcined mixture has a smooth or glossy appearance and is a solid. The calcined mixture in step 270 can be mixed with a shielding material, such as lead, boraxo, polyethylene and the like to form a shielded calcined material. At step 272, the shielded calcined material can be stored. The shielded calcined material is a solid and the radioactive waste is stored as a solid. The shielding of the radioactive waste lessens the radiation escaping into the surrounding atmosphere from the radioactive waste. The calcined mixture from step 268 can be stored at 274. If the solid 260 be perlite or vermiculite or volcanic ash or pumice, then the mixture 264 can be processed in the furnace 28 at step 276 to form an encapsulated, radioactive residue. The radioactive residue can be stored at step 278. The encapsulated, radioactive residue is a solid and can be easily handled in the solid form. At step 280, the encapsulated, radioactive residue can be mixed with a shielding material, such as lead, borax, or polyethylene to form a shielded, encapsulated, radioactive material. The shielded, encapsulated, radioactive material can be stored at step 282. In FIG. 13, it is seen that there has been provided a process for treating an object contaminated with the radioactive material and then to store the resulting radioactive residue either in a calcined form or in an encapsulated form. In both the calcined form and the encapsulated form, the radioactive residue is a solid and can be, readily, handled. In FIG. 14, there is illustrated a process for processing salt cake 300. Salt cake comprises radioactive material and may be a solid, a liquid, and a mixture of solids and liquids. In the processing step, salt cake 300 is mixed with clay 302 to form agglomerated particles. These agglomerated particles can be classified as to size and introduced into the furnace 28. In the furnace 28, the agglomerated particles of clay and salt cake can be heated to form calcined particles 306. These calcined particles 306 are a solid and have a glassy or glossy appearance. It is to be remembered that these calcined particles 306 are discrete units of, substantially, the same size as the agglomerated particles formed by mixing the clay and salt cake. At step 308, the calcined particles can be stored. At a desirable time, the calcined particles 306 can be taken from storage 308 and processed in step 310 to form retrieved radioactive material 312. The retrieved radioactive material 312 may be used in a suitable and desirable manner. Instead of storing the calcined particles 306, it may be desirable in step 314 to coat these calcined particles with a shielding material 316. The shielding material may be lead, borax, polyethylene, to name a few suitable shielding materials. The coating of the calcined particles of the shielding material results in shielded particles 318. The shielded particles 318 are safer to store than the calcined particles 306 and are, therefore, more easily stored than the calcined particles 306. At a suitably desirable time, the shielded particles 318 can be processed at step 320 to form retrieved radioactive material 312. In FIG. 15, there is illustrated the process of mixing a particle 330 with a binder 332. As previously stated, the particle may be diatomaceous earth, clay, cement, titanium dioxide, fly ash, volcanic ash, natural zeolites, pumice, to name a few. The binder may be water, sodium carbonate, sodium silicate, sodium oxide, potassium oxide, potassium carbonate, potassium silicate, to name a few suitable binders. At step 334, the particle 330 and the binder 332 may be mixed to form a mixture and then the mixture agglomerated to a suitable particle size to form a agglomerated mixture 336. The agglomerated mixture 336 may be introduced into the furnace 28 and the agglomerated mixture calcined at step 338. As previously explained, in the calcining of the agglomerated mixture, there is formed glossy or glassy or smooth particles identified as a calcined mixture 340. The calcined mixture 340, has previously been referred to as the processed product 224 and the agglomerated mixture has previously been referred to as the agglomerated product 220. In FIG. 4, the doctor blade 134 need not touch the firebrick of the lower circular member 30 but, instead, can be an air doctor blade for blowing or moving the product 224 across the firebrick and toward the sloping conveyor 136 for removal from the vicinity of the furnace 28. One of the advantages of the furnace 28, as compared with a rotary furnace or a vertical furnace or a horizontal stationary furnace is that less air is required in the furnace 28 than with any of the other furnaces. For example, with the furnace 28, the air required is the air of combustion to burn the fuel. There is no need to heat extraneous air for removing the expanded product, such as bloated perlite or bloated vermiculite or bloated volcanic ash or bloated pumice from the furnace. In the other furnaces, air is needed for both combustion and the removal of the bloated product from the furnace. With the furnace 28, it is possible to heat the furnace to a temperature of about 2000.degree. F. in approximately 5 minutes. With the furnace 28, as compared to the above enumerated furnaces, it is not necessary to predry the material to be processed to a moisture content of less than 1%. It is possible to use perlite having a moisture content in the range of 3% to 10%. With the above enumerated furnaces, such as a vertical furnace or a horizontal furnace or a rotary furnace, it is necessary to dry the material to be processed to a moisture content less than one percent. In these furnaces, the residence time is approximately one second. In the furnace 28, the residence time can be varied to suit the material to be processed and the residence time can be varied from about five seconds to sixty seconds. In fact, the residence time can be varied over a much wider range of time than from five seconds to sixty seconds as the residence time may be two minutes or three minutes. The fuel which can be used and introduced into the plenum chamber can be a liquified petroleum gas, propane, butane, water gas, diesel in gaseous form, and the like. As previously stated, there can be admixed with the material to be treated a solid fuel such as coal or there can be used diesel as a binder in forming the agglomerated particle to be introduced into the furnace. In a rotary furnace, the fuel efficiency is approximately 10%. A highly efficient rotary furnace may have a fuel efficiency of 30%. With the furnace 28, I estimate that the fuel efficiency varies between approximately 60% to 85%. Again, a main reason for this difference in fuel efficiency is that it is not necessary to heat extraneous air in the furnace 28 while it is necessary to heat extraneous or carrier air in the rotary furnace. Another reason for the greater fuel efficiency of the furnace 28 is that it is not necessary to heat such a large mass as compared with the rotary furnace. The furnace 28 is more compact, less mass, smaller size, and therefore there is not a large mass of material to heat as compared with the rotary furnace. Also, there is less heat loss from the furnace 28 as compared with the rotary furnace. As previously stated, the furnace 28 can be used to regenerate filter aids. For example, a filter aid prepared from perlite or a filter aid prepared from diatomaceous earth can be regenerated in the furnace 28 at a temperature in the range of about 1500.degree. F. to 2000.degree. F. This results in a saving in the processing of a filter aid and also means that it is not necessary to discard used filter aids. In the furnace 28, it has been shown and described that the spacing between the reflector brick and the firebrick can be varied to accommodate the material 220 to be processed into the product 224. Initially, when the material 220 is introduced into the furnace 28, the spacing between the reflector brick and the firebrick is a small distance. This results in more radiant heat on the material 220. If the material 220 expands into an intermediate product 22, the spacing between the reflector brick and the firebrick is increased to accommodate the larger size. Then, near the end of the cycle or process, the spacing between the reflector brick and the firebrick is greatest to accommodate the expanded material. With this furnace, it has been noticed that it has been possible to expand perlite particles and make the particles extremely strong compared to expanded perlite particles from a rotary furnace or a vertical furnace or a stationary horizontal furnace. The expanded perlite particles from the furnace 28 were stronger than the expanded perlite particles from one of the three above-enumerated furnaces. Further, with the furnace 28, it is possible to expand perlite particles of comparatively large size into comparatively strong expanded perlite particles. This has not been accomplished in the perlite industry with a rotary furnace or a vertical furnace or a horizontal furnace. As previously stated, the firebricks were selected so as to be as uniform as possible. A standard for uniformity was that the bricks were to weigh, substantially, the same. Also, the bricks were painted on the ends and sides with a fireproof paint so as to seal the ends and sides. Then, the combination of air and combustible gas could be introduced into the plenum and this combination flow through the brick and onto the surface of the brick where the combination was ignited and burned. In a test, it was estimated that the fuel consumption was, with the furnace 28, in the range of 3,000,000 BTU's per ton of product, such as expanded perlite. With a rotary furnace or a vertical furnace or a stationary horizontal furnace, the fuel consumption is in the range of 4,000,000-4,500,000 BTU's per ton of expanded perlite. It is seen that there is a saving of approximately one-third to one-half of the fuel in the furnace 28 as compared with one of the other three furnaces. To assist in maintaining a long life for the firebrick, there was attached a block of porous aluminum oxide on top of the firebrick. One of the requisites for the aluminum oxide was that it would have a porosity equal to that of the firebrick. In the furnace 28, it is seen that the adapter 80 is free to rotate, with the first circulator member 30, around the outlet nozzle 78. The adapter 80 and the outlet nozzle 78 function as a swivel. The adapter 80 can rotate, completely, around the nozzle 78 along with the rotation of the first circular member 30. The gas passes through the pipe 74, mixing chamber 76, outlet nozzle 78, adapter 80, arms 82 and 84, or 190 and passes into the plenum chamber and flows through the porous brick so as to be burned on top of the firebrick. The material 220 to be processed is heated by conduction from the surface of the firebrick, by convection of gas, products of combustion, flowing from the firebrick to the material 220, and also by radiation from the reflector brick positioned above the firebrick and also above the material 220. Further, it can be considered that the material to be processed is contacted with the hot surface of the firebrick and with combustible materials juxtapositioned to said hot surface, and there is reflected heat energy radiating from said hot surface toward said material. The flowing of the mixture of air and combustible gas through the firebrick and also on top of the firebrick assists in keeping the material 220 being processed from sticking to the firebrick. The flowing gases raise or elevate the material being processed from the surface of the firebrick so as to lessen the possibility of the material sticking and adhering to the firebrick. With this method of burning the gas on the upper surface of the firebrick, it is possible to attain a temperature in the range of about 2600.degree. F. The material 220 is heated by radiation from the reflector brick and which reflector brick or which reflector may be as close as one-quarter of an inch to the firebrick. When the material 220 is, initially, placed on the firebrick, the reflectors may be as close as one-quarter of an inch to the firebrick. With the expansion of the material 220, it is necessary to position the reflections farther away from the firebrick so as to allow the expanded material or processed material 222 to be carried by the rotating firebrick to the outlet of the furnace 28. If the reflectors were not positioned farther away from the firebrick, then the reflectors would interfere with the movement of the material 222 being processed. In certain instances, the reflectors may be as much as one and one-half inches away from the firebrick to accommodate the material being processed. I consider that one of the advantages of this invention is that the retention time of the material to be processed in the furnace 28 can be accurately controlled. As previously stated, the retention time can be varied from five seconds up to two or three minutes or even longer. The ability to vary the retention time makes it possible to process material in a manner which has not been previously processed. For example, if, after the particle has been expanded, the expanded particle is retained in the furnace or heat zone, the surface of the particle tends to fuse. This causes a slight shrinkage in the particle but the strength of the particle is greatly increased. Because the retention time in the furnace can be controlled, much larger particles can be expanded with the furnace 28 than with the other furnaces, such as the vertical furnace, the rotary furnace, or the horizontal furnace. Again, remember that after the particle has been expanded and if it be retained in the furnace, an annealing process or a fusing of the surface of the particle takes place to increase the strength of the particle. This creates the possibility of making high-strength, light-weight concrete blocks. In the forming of expanded or bloated particles and also in the calcining of particles with the furnace 28 and with my method, it is possible to expand and calcine materials without the necessity of drying the materials as contrasted with the conventional processing methods in a vertical furnace or rotary furnace or horizontal furnace wherein the material to be processed must be almost bone dry. As stated, air and a gas, such as liquified natural gas or propane or butane, can be mixed and burned on top of the firebricks to realize in the temperature range of about 2600.degree. F., and, it is conceivable, that in place of air there can be used oxygen so as to form an oxygen-natural gas mixture which can be burned on top of the firebrick. The heating process can be used for calcination of properly prepared mixtures of aluminal-silicate materials and radioactive waste products, primarily, salt cake or sodium nitrate-nitrite complex containing cesium 137 and other radioactive products. With calcination, there is a fusion of the aluminal-silicates with the radioactive product thereby causing the radioactive products to be encased in a glasslike matrix. The radioactive products become a solid, nonleachable form of material suitable for short term or long term storage. Again, this is of value as in many instances, the salt cake or sodium nitrate-nitrite complex containing cesium 137 and other radioactive products may be in a liquid form or may be in a liquid-solid combination or mixture. The salt cake or sodium nitrate-nitrite complex can be mixed with a calcining agent like diatomaceous earth, clay, cement, fly ash, volcanic ash, natural zeolites, pumice, and the like, to name a few and the salt cake can be used as a binder. Then, the resulting mixture can be made into agglomerated particles of a desired size and then these agglomerated particles of the salt cake and the carrier, such as diatomaceous earth or clay can be calcined and fused so that a solid results and which solid contains the radioactive materials. A contributing factor to storing the salt cake and radioactive materials in solid form is the step of agglomerating the mixture of the radioactive materials and the diatomaceous earth or clay or the like into balls or discrete units of, substantially, the same size. With the formation of these agglomerated particles of discrete size and of substantially the same dimensions, it is possible to have a precise control of the bed thickness of the particles to be processed on the firebrick. The control of the bed thickness with the control of the retention time makes it possible to carry out the calcination step and, when desirable, the bloating or expanding step on the various particles. This, in turn, permits faster and more efficient use of the heat transfer to the particles being processed. The agglomeration process also allows for the reprocessing of waste materials into useful products. An example is perlite fines can be processed into a perlite aggregate, a mixture of spent filter aid (either diatomaceous earth or perlite) and unexpanded perlite ore can be reprocessed back into a filter aid or can be processed to make a light-weight aggregate for use in concrete or a mixture of fly ash and unexpanded perlite ore can be processed to produce a light-weight aggregate or a filter aid. Again, I consider that one of the main advantages of the furnace 28 and this method is the ability to achieve a product equal to or superior than achieved with a vertical furnace, a stationary horizontal furnace, or a rotary furnace with less fuel consumption. The fuel consumption of this furnace and method is in the range of about one-half to two-thirds of that achieved with one of those enumerated furnaces. In this regard, an article by Herbert A. Stein, "MEASURES FOR CONSERVATION OF FUEL IN THE EXPANSION OF PERLITE" states: "With the high cost of fuel today, it is more important than ever to reduce the amount of fuel used in expanding perlite. PA0 "Perlite expanding processes are often operated at very low fuel efficiency, sometimes less than 25%. Some of the reasons for this are as follows: PA0 "1. Present-day expanders are co-current, that is, the perlite and the flame enter the expansion chamber together and leave together. This means that the hot gases leave the furnace at a higher temperature than the expanded perlite. The co-current operation is in contrast to the counter-current operation used in cement kilns and boilers, for example, where the incoming feed absorbs heat coming from the hot zone. PA0 "2. Present-day expander tubes are uninsulated and made of metal, to operate at a lower temperature in order to avoid fusion and damage to the tube. As a result, more heat often passes through the furnace tube wall than is needed to expand the perlite. PA0 "3. Many perlite expanding processes do not involve recovering wasted heat by using it to preheat the ore or the combustion air. PA0 "4. Many furnaces, especially verticals, are operated with too much air flow, well in excess of what is needed for combustion. This air must be heated to the operating temperature. PA0 "5. Many furnaces are operated at a production rate which is too low for good fuel efficiency per ton, often because the auxilliary equipment (such as cyclones, air locks, and cooling and bagging facilities) is too small to handle the higher production rate. PA0 "6. Another cause of high fuel consumption due to too low a production rate is the use of an ore which is coarser than necessary for the intended end use of the expanded perlite." As contrasted with the comments of Herbert A. Stein, I consider that there is not a waste of fuel with my furnace and my method. The air introduced into my furnace is used for combustion and not for conveying the products from the furnace. Therefore, there is less heat energy required as it is not necessary to heat extraneous carrier air. Also, the product is at a higher temperature, upon leaving the furnace, than the temperature of the products of combustion. The production rates of my furnace can be varied to accommodate the material to be processed and also with my furnace, used in conjunction with the agglomeration process, it is not always necessary to use cyclones, air locks, cooling and bagging facilities and the like. I question if it be possible to use agglomerated particles in a rotary furnace or a vertical furnace or a stationary horizontal furnace. I think that with my furnace there is an expansion of materials which can be processed to make a useful product. I consider my invention to be new, useful, and unobvious. I consider my invention to be new as I do not know of another furnace or another method for processing a material to make an expanded product or a calcined product. I do not know of another furnace having upper and lower bricks and wherein a material to be processed can be placed on rotating firebricks for being heat processed and also wherein the air introduced into the furnace and the air used in my method is, essentially, the air for combustion purposes and not for carrier purposes. I consider my invention to be useful as it can be used to expand or bloat material such as vermiculite, perlite, volcanic ash, and pumice. Also, my furnace and my method can be used for calcining materials such as diatomaceous earth, clay, titanium dioxide, cement, fly ash, volcanic ash, zeolite, perlite, vermiculite, pumice, and the like. These products can be used for horticultural purposes fly ash, storing of radioactive wastes in a solid matrix, for lightweight concrete, and the like. I consider my invention to be unobvious as, again, I have not seen or heard of another furnace or method similar to my furnace and method. In preparing this patent application, a patent search was not made but information I know in regard to a rotary furnace, a vertical furnace, a stationary horizontal furnace for processing perlite and vermiculite has been disclosed. Also, there has been called to my attention three U.S. Pat. Nos. 2,659,521; 2,672,483; and, 2,572,484. |
050948016 | description | DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a typical pressurizer 10 used in a nuclear reactor coolant system. Pressurizer 10 is a vertical, cylindrical vessel with replaceable electric heaters 12 in its lower section that extend through nozzles 14 in the vessel wall 16 into the lower portion of pressurizer 10. Nozzles 14 extend through the vessel wall 16 which is approximately six inches thick and made of carbon steel or low-alloy steel. As seen in FIG. 2, a cladding 18 normally made from stainless steel, is used on the interior surface of the wall 16 for corrosion protection. For ease of illustration, only that portion of heaters 12 that extend into pressurizer 10 are shown. When it is necessary to replace one of original nozzles 14 as a result of damage or corrosion, the entire nozzle 14 is removed from pressurizer 10 and the remaining bore in vessel wall 16 is enlarged so as to accept the present invention. As seen in FIG. 2, the invention is generally referred to by the numeral 20. Heater sleeve 20 is generally comprised of an outer sleeve 22 and an inner sleeve 24. Outer sleeve 22 is sized to fit within bore 26 which has been enlarged to accept the invention and to remove any degraded material in vessel wall 16. Outer sleeve 22 is machined for a shrink fit and installed in bore 26 after the weld pad buildup 28 has been prepared. As seen in FIG. 2 the interior or upper end of outer sleeve 22 is cut to match the angle of the interior of vessel wall 16 and cladding 18 and is positioned so as to have its interior or upper end slightly below the upper edge of cladding 18. Once installed in this position outer sleeve 22 is structurally welded to the outer diameter of vessel wall 16 on weld pad buildup 28 as indicated at weld 30 using a partial penetration weld process. The interior or upper end of outer sleeve 22 is seal welded to cladding 18 as indicated at seal weld 32 by automated remote welding equipment that fits through outer sleeve 22. The partial penetration weld at weld 30 is accommodated by the lower end of outer sleeve 22 having a thickened wall. The interior wall of outer sleeve 22 is then machined to insure that there is proper clearance for inner sleeve 24 and that it will be properly aligned with support plate 34 seen in FIG. 1. Inner sleeve 24 is installed inside outer sleeve 22 with a small clearance therebetween and extends into the interior of pressurizer 10 beyond cladding 18 a minimum of one-half inch and a maximum of one and seven-eighth on the high side of cladding 18. This limits the amount of sludge that can build up between the two sleeves and around the heaters and thus reduces radiological hot spots and problems with heater replacement. Inner sleeve 24 is structurally welded to outer sleeve 22 as indicated by the numeral 36 using a partial penetration weld. If necessary, inner sleeve 24 is then machined in the weld area to insure that there is no obstruction to the installation of electric heater 12. As seen in FIG. 2, the portion of electric heater 12 which is known in the art and relative to the invention is comprised of heating element 38 encased in heater sheath 40. Heater sheath 40 transfers heat to the water in pressurizer 10 while protecting heating element 38 from damaging direct contact with the water. Heater sheath 40 is welded to inner sleeve 24 as indicated by numeral 42 to prevent leakage of water between the two and maintain pressure in pressurizer 10. Heater locking bushing 44 is welded onto the outer diameter of inner sheath 24 and is provided with threads 46 for threadably engaging a support collar not shown on the lower part of electric heater 12. In operation, when there is damage to one of nozzles 14 or corrosion has degraded the material in vessel wall 16 of pressurizer 10, electric heater 12 is removed and the damaged nozzle 14 is removed. Bore 26 is enlarged by machining to remove degraded material. Outer sleeve 22 is installed in the enlarged bore so that its upper end is adjacent the upper edge of cladding 18 (substantially flush with the interior of pressurizer 10) and it is then structurally welded to the outer surface or diameter of vessel wall 16 on weld pad buildup 28 by a partial penetration weld indicated at point 30. The upper end of outer sleeve 22 is seal welded to the inner surface of pressurizer 10 or cladding 18 at point 32 to prevent water from contacting the low alloy material of vessel wall 16. The interior diameter of outer sleeve 22 is then machined to the same center as the original bore to maintain the original heater alignment and a free path for inner sleeve 24. Inner sleeve 24 is installed inside outer sleeve 22 with a small clearance and extended into pressurizer 10 beyond the upper end of outer sleeve 22. This reduces the buildup of debris between inner sleeve 24 and heater sheath 40 and reduces potential problems with future heater removal operations. Inner sleeve 24 is welded to the lower end of outer sleeve 22 by a partial penetration weld to prevent water leakage and maintain pressure during operations. The inner diameter of inner sleeve 24 is sized to receive a heater of the same size as that originally installed in pressurizer 10. Electric heater 12, formed from heating element 38 encased in heater sheath 40 is inserted through inner sleeve 24 into pressurizer 10 and the appropriate hole in support plate 34. Heater sheath 40 is welded to the lower end of inner sleeve 24 to prevent water leakage and maintain pressure. Heater locking bushing 44 is welded to inner sleeve 24 and threadably attaches to the remainder of the heater assembly which is known in the art. Because many varying and differing embodiments may be made within the scope of the inventive concept herein taught and because many modifications may be made in the embodiment herein detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense. |
048062774 | summary | BACKGROUND OF THE INVENTION The present invention relates to a method of decontaminating solid surfaces, and particularly to a method of decontamination which is suitable for the purpose of decontaminating solid surfaces as a measure designed to reduce the potential danger of personnel being exposed to radioactive substances in nuclear installations. DESCRIPTION OF THE PRIOR ART A method of decontaminating solid surfaces in which bubbles are produced in a liquid and the impulsive forces produced when the bubbles burst are employed for separating and removing substances adhered to the solid surfaces has certain advantages in that it is also suitable for the decontamination of articles having complicated forms, produces only a small amount of secondary waste solution, and does not necessitate any use of chemicals, and it has thus recently attracted attention. This method includes ultrasonic washing methods such as the one disclosed in Japanese Unexamined Patent Publication No. 104799/1980. This method utilizes pressure vibrations in a liquid caused by ultrasonic waves whereby bubbles are repeatedly generated and allowed to burst in the liquid. This method is thus able to remove so-called soft clads such as substances that have adhered to the outer layer of a solid surface but has not been able to remove so-called hard clads such as oxide films in the depths of the object to be decontaminated. In addition, since the ultrasonic decontamination apparatus used in practice comprises an ultrasonic generator, a piezoelectric transducer, and a cleaning bath and since it utilizes a method in which an object to be decontaminated is decontaminated while being immersed in a liquid in the cleaning bath, it has been impossible to decontaminate piping or instruments in the state in which they are installed. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of decontamination which overcomes the disadvantages of the above-described conventional method of decontaminating solid surfaces by means of ultrasonic waves and which is capable of effectively removing hard clads and other substances even in the state wherein piping and instruments are fixed in position. The present invention is characterized by comprising the steps of immersing an object to be decontaminated in a liquid, producing bubbles in the liquid by blowing vapor therein, and causing the bubbles to burst on a solid surface which is brought into contact with the liquid and which constitutes the object to be decontaminated so that substances adhered to the solid surface are separated and removed therefrom by the impulsive force produced when the bubbles burst. The present invention can produce bubbles which are extremely large in comparison with those formed by an ultrasonic washing method, and the method of the invention employs vapor and thus is capable of obtaining a greater impulsive force, whereby an excellent decontamination effect can be obtained. The function and effect of the decontamination method of the present invention is described in detail below. (1) Effect of decontamination The present invention is similar to the abovedescribed ultrasonic washing method in that both involve the production of bubbles in a liquid and deal with decontamination by utilizing the impulsive force produced when the bubbles burst. In the present invention, however, it is possible to produce bubbles having a larger diameter than those of the ultrasonic washing method by directly injecting vapor in the liquid and the impulsive force produced is proportional to the third power of the initial diameter of the bubbles, whereby an impulsive force which is greater than that obtainable by the ultrasonic washing method can be achieved. Therefore, not only soft clads that are adhered to the surface in the outer layer thereof but also hard clads in the depths can be removed. (2) Soundness after decontamination Since the present invention uses no decontaminating agent or abrasive, no agent remains after decontamination and no adverse effect upon the soundness of piping or instruments is suffered. (3) Reduction of the amount of waste solution In the method of decontaminating solid surfaces in accordance with the present invention, the amount of secondary waste solution produced following the decontamination work is equal to the amount of vapor injected as the source of generation of bubbles, but the volume of the vapor is reduced to about 1/1500 when condensed and the amount of waste solution to be dealt with is thus kept to a small amount. (4) Workability The present invention uses only vapor and thus exhibits a very high level of safety in comparison with conventional methods of decontamination that use specific chemicals and high-pressure water. In addition, the invention generates no dust during the decontamination work and thus allows a working environment to be kept under sanitary conditions. (5) Applicable field Since the present invention employs the impulsive force produced when bubbles burst, it is possible to decontaminate a surface having a complicated form. In addition, since the method of decontamination utilizes only the injection of vapor, it is possible to decontaminate the inside of piping or an instrument while installed in situ. (6) Prevention of the spread of contamination In the present invention, since all the operations of decontamination are conducted in a liquid and the pressure of the vapor to be injected may be low, there is no possibility of the contamination being spread due to splashed water. |
061538094 | abstract | A polymer coating is applied to the surface of a phosphate ceramic composite to effectively immobilize soluble salt anions encapsulated within the phosphate ceramic composite. The polymer coating is made from ceramic materials, including at least one inorganic metal compound, that wet and adhere to the surface structure of the phosphate ceramic composite, thereby isolating the soluble salt anions from the environment and ensuring long-term integrity of the phosphate ceramic composite. |
047088435 | description | DESCRIPTION OF A PREFERRED EMBODIMENT In FIG. 1 is seen a tank 1 of a pressurized water nuclear reactor closed at its upper part by a hemispherical cover 2, traversed by a sealed vessel 3 within which a pawl mechanism 4 is arranged enabling the displacement of an operating rod of great length, at the lower part of which is fixed an absorber unit which can be moved inside the reactor by the device 4. Only one vessel 3 has been shown, but it will be understood that the control unit of the reactor includes a large number of devices each enabling the displacement of an absorber unit. The sealed vessels 3 are connected at their upper part to an anti-earthquake plate 6 of high strength itself held with respect to the wall 7 of the pool of the reactor by a set of anti-earthquake bracing rods 8. A set of lifting rods 9 enables the raising of the cover of the tank by a lifting unit 10 which can be coupled to the travelling bridge crane serving the reactor. A ventilating skirt 12 and ventilating devices (not shown) enable cooling air to be circulated at the level of the driving mechanism and device 4. A heat-insulating jacket 13 enables the cover 2 of the tank to be thermally insulated from the outside medium. If it is desired to lift the cover of the tank, it is necessary to dismount the anti-earthquake bracing rods 8 connecting the plate 6 to the walls of the pool as well as the ventilation ducts connecting the ventilating devices to the ducts 14 and to carry out the raising of a complex and extremely heavy unit with the lifting device 10. It is also necessary to arrange a forced ventilating system at the level of the motors 4, although the heat loss at the level of the sealed vessel over the whole portion of this sealed vessel situated above insulating jacket 13 is extremely large. On the other hand, the height of the sealed vessel 3 above the mechanisms 4 must be at the minimum equal to the height of the fuel assemblies, which increases all the more the bulk of the tank of the reactor or of its cover. It must finally be noted that any intervention or change of part in the motor or the mechanism situated in the sealed vessel necessitates the prior dismounting and removal of the anti-earthquake plate. Referring to FIG. 2, an embodiment of the control unit according to the invention is seen, of which also only a single sealed vessel has been shown. The cover 16 of the tank 15 is traversed by this sealed vessel 17 extending the tank upwards to a certain height. The driving mechanism of the control cluster in its vertical movement inside the core is constituted by a screw and nut device at least partly positioned below the cover 16 of the tank, which enables the height of the sealed vessel above the cover of this tank to be reduced. The rotary movement of the nut driving the screw in a translation movement is transmitted to this nut positioned beneath the cover of the tank from the motor 18 located at the upper part of the sealed vessel 17. The screw driven in translation is connected to an intermediate rod enabling the fastening of the absorber cluster to the mechanism. A cluster support 19 enables the guidance of the cluster when the latter is extracted from the core of the reactor over a sufficient length. For comparison, there is shown, in dashed lines, a sealed vessel 20 associated with a driving device located in the lower portion of this sealed vessel, such as that used in the devices of the prior art. It is seen that this vessel 20 occupies a much greater height above the cover of the tank than the height occupied by the vessel 17. A strong vertical structure 23 is fixed to the cover 16 of the tank and is extended upwards to a level somewhat below the level of the sealed vessels such as 17 where the driving devices 18 are located. This structure 23 includes high strength vertical elements 24 and horizontal elements 25 constituting a polygonal contour at the lower part and at the upper part of the prismatic shaped structure 23. The stiffener elements 26 provide for the rigidity of the whole of the structure 23. A horizontal plate 27 rests on the horizontal elements 25 at the upper part of the structure and includes openings 28 for the passage of the upper parts of the sealed vessel 17. The openings 28 permit passage of the vessels 17 with a certain play, so that the vessels can move freely under the effect of thermal expansion and of the deformations of the tank cover. In fact, the tank of the reactor and of the sealed vessels contain water under high pressure and at high temperature, so that deformations are possible during the operations of the reactor. An adapter part 30 provides the connection between the plate 27 and the sealed vessel 17 and enables shocks between them to be damped in the case of earthquake. The motors 18 are generally constituted by an immersed rotor located inside the vessel 17 whose axle rotates the nut of the screw-nut mechanism through a driving part and through a stator located outside the sealed vessel. The motor 18 and the corresponding portion of the vessel are placed inside a duct 32 which permits ventilation of the motor 18 by natural draft. The group of ducts such as 32 associated with each of the mechanisms arranged in a sealed vessel constitutes a dismountable unit structure 33 fixed by means of a support 34 to the upper portion of the vertical structure 23. The assembly 33 constituted by the ducts 32 is arranged a little above the horizontal plate 27 assuring the holding of the upper part of the sealed vessels 17 in case of earthquake. An enclosure 36 of insulating material permits the thermal insulation of the cover of the tank and of the part of the sealed vessel 17 located below the motor 18, with respect to the external medium. This heat-insulating envelope 36 of large size hence encloses the group of sealed vessels 17 passing through the cover 16 of the tank. The envelope 36 is closed at its upper part by a horizontal insulating plate 37 positioned on the strengthening plates 27 and including openings corresponding to each of the sealed vessels containing a mechanism. Vertical tie-rods 40 enable the raising of the cover of the tank by means of a lifting device such as the device 10 shown in FIG. 1. It is seen that the vertical strengthening structure 23 and the plate 27 enable an anti-earthquake protecting device for the control unit of the reactor to be constituted, without the use of tie-rods fixed to the walls of the pool of the reactor. In addition, the position of the motor above the anti-earthquake plate and the free access to the upper part of the sealed vessel enables maintenance operations on the motor and on the mechanism situated inside the sealed vessel to proceed without the necessity of previously dismounting the anti-earthquake plate. This is very advantageous since these operations are carried out in the presence of ionizing radiation. On the other hand, it could be necessary to provide an anti-missile protective slab of reinforced concrete positioned above the pool of the reactor for the protection of the latter. In contrast, the use of ducts 32 for the ventilation of the motors 18 obviates use of forced ventilation according to the devices of the prior art. With ducts of 2 m in height, a draft enabling an exchange coefficient between the air and the metal of the order of 10 watts per m.sup.2 and per degree C. has been achieved. This, combined with the use of a screw-nut unit which has a high reduction ratio, and which consequently enables the driving power necessary to effect the movement of the cluster to be limited, leads to an equilibrium temperature in the coils of the motor below 300.degree. C. During handling of the tank cover, it is possible to limit the height of the assembly which must be lifted by dismounting the assembly 33 holding the ventilation ducts 32 of the motors. This one-piece and light assembly can easily be separated from the remainder of the control unit. It will be seen that the principal advantages of the device according to the invention are the reduction in height above the cover of the tank of the control unit, the elimination of anti-earthquake braces fixed to the walls of the reactor, easy access to the motor and to the mechanism situated in the sealed vessel, natural cooling of the motors in the aerating ducts, efficient heat insulation of the sealed vessels enclosing the mechanisms, and faster and easier handling of the cover of the tank. The invention is not limited to the embodiment which has just been described, but also comprises all modifications thereof. Thus, it is possible to construct the strengthening structure fast to the cover of the tank in a form different from that of a framework structure as in the example which has been described. It is possible, for example, to build this structure in the form of a strong sheath, or, if it is desired to preserve a framework, to add tranverse reinforcements passing between the sealed vessels if the height of the sealed vessels cannot be reduced by the use of a screw-nut device located beneath the cover of the tank. It is also possible to construct the aeration ducts of the motors in a form different from the one-piece unit which has been described which, however, has the advantage of enabling dismounting of a group of ducts corresponding to the group of mechanisms. It is also possible to construct a heat insulation for the sealed vessels, not by using not a single insulating material for the group of vessels, but by separately insulating each of these vessels with an insulating material. Finally, the control unit according to the invention can be used not only for a pressurized water nuclear reactor with a control moving absorber clusters by means of a screw-nut mechanism, but also for any other nuclear reactor where the control is carried out by vertical movement of clusters of absorber material in the core of the reactor, and/or the displacement mechanisms for the clusters of absorbent material are positioned in sealed vessels communicating with the inside of the tank of the reactor. |
claims | 1. A system for attitude control of an underwater vehicle comprising:an underwater vehicle that includes a thruster;an assist device that is coupled to said underwater vehicle with a cable and includes cable handling equipment and a thruster;a cable attachment-to-attachment distance arithmetic operation unit that detects a distance between a cable attachment of said underwater vehicle and a cable dispenser, serving as a cable attachment, of the assist device; anda control unit that stores the relationship between the cable attachment-to-attachment distance and cable length including a predetermined amount of slack and determines the amount of cable to be wound up or let out based on said cable length associated with said detected cable attachment-to-attachment distance. 2. The system for attitude control of the underwater vehicle according to claim 1,wherein said assist device includes a depth finder of said assist device and an image acquisition unit that acquires images of an upper surface of said underwater vehicle,said underwater vehicle includes a depth finder of said underwater vehicle and a plurality of light emitting devices on the upper surface of a hull thereof, andsaid cable attachment-to-attachment distance arithmetic operation unit determines said cable attachment-to-attachment distance based on water depth data from said depth finder of said assist device and said depth finder of said underwater vehicle and said captured images, said images including images of said light emitting devices and being input from said image acquisition unit. 3. The system for attitude control of the underwater vehicle according to claim 2,wherein said assist device and said underwater vehicle include an inclinometer of said assist device and an inclinometer of said underwater vehicle, respectively,said cable attachment-to-attachment distance arithmetic operation unit further comprising:a relative depth arithmetic operation unit that determines vertical distance between said cable attachments based on a difference between water depth data from said depth finder of said assist device and water depth data from said depth finder of said underwater vehicle;a relative horizontal-distance arithmetic operation unit that determines horizontal distance between said cable attachments by obtaining a difference between a tilt angle from said inclinometer of said assist device and a tilt angle from said inclinometer of said underwater vehicle and performing rotation correction on said captured images including said light emitting devices with said differential tilt angle; anda relative distance/yaw angle arithmetic operation unit that determines said yaw angle from said cable attachment of said underwater vehicle to said cable dispenser of said assist device and said cable attachment-to-attachment distance based on said vertical distance and horizontal distance between the cable attachments. 4. The system for attitude control of the underwater vehicle according to claim 1,wherein said assist device includes a sonar transmitter having a plurality of crystal resonators arranged in a two-dimensional matrix,said underwater vehicle includes a sonar receiver that receives ultrasound from said sonar transmitter, andsaid cable attachment-to-attachment distance arithmetic operation unit determines said cable attachment-to-attachment distance based on the results of transmission and reception of the ultrasound between said sonar transmitter and said sonar receiver. 5. The system for attitude control of the underwater vehicle according to claim 1,wherein said assist device includes a laser transmitter and said underwater vehicle includes a laser receiver, andsaid cable attachment-to-attachment distance arithmetic operation unit determines said cable attachment-to-attachment distance based on the results of transmission and reception of the laser between said laser transmitter and said laser receiver. 6. The system for attitude control of the underwater vehicle according to claim 1,wherein said control unit further comprising:a cable length table lookup unit that includes a cable length table storing said relationship between said cable attachment-to-attachment distance and cable length including a predetermined amount of slack and extracts a cable length associated with said detected cable attachment-to-attachment distance from said cable length table; anda cable length storing unit that stores a current cable length obtained concurrently with operation of said cable handling equipment, wherein said control unit determines said amount of cable to be wound up or let out by comparing said extracted cable length with said current cable length. 7. The system for attitude control of the underwater vehicle according to claim 2,wherein said control unit further comprising:a cable length table lookup unit that includes a cable length table storing said relationship between said cable attachment-to-attachment distance and cable length including a predetermined amount of slack and extracts a cable length associated with said detected cable attachment-to-attachment distance from said cable length table; anda cable length storing unit that stores a current cable length obtained concurrently with operation of said cable handling equipment, wherein said control unit determines said amount of cable to be wound up or let out by comparing said extracted cable length with said current cable length. 8. The system for attitude control of the underwater vehicle according to claim 3,wherein said control unit further comprising:a cable length table lookup unit that includes a cable length table storing said relationship between said cable attachment-to-attachment distance and cable length including a predetermined amount of slack and extracts a cable length associated with said detected cable attachment-to-attachment distance from said cable length table; anda cable length storing unit that stores a current cable length obtained concurrently with operation of said cable handling equipment, wherein said control unit determines said amount of cable to be wound up or let out by comparing said extracted cable length with said current cable length. 9. The system for attitude control of the underwater vehicle according to claim 4,wherein said control unit further comprising:a cable length table lookup unit that includes a cable length table storing said relationship between said cable attachment-to-attachment distance and cable length including a predetermined amount of slack and extracts a cable length associated with said detected cable attachment-to-attachment distance from said cable length table; anda cable length storing unit that stores a current cable length obtained concurrently with operation of said cable handling equipment, wherein said control unit determines said amount of cable to be wound up or let out by comparing said extracted cable length with said current cable length. 10. The system for attitude control of the underwater vehicle according to claim 5,wherein said control unit further comprising:a cable length table lookup unit that includes a cable length table storing said relationship between said cable attachment-to-attachment distance and cable length including a predetermined amount of slack and extracts a cable length associated with said detected cable attachment-to-attachment distance from said cable length table; anda cable length storing unit that stores a current cable length obtained concurrently with operation of said cable handling equipment, wherein said control unit determines said amount of cable to be wound up or let out by comparing said extracted cable length with said current cable length. 11. A method for attitude control of an underwater vehicle, the method being performed by a system including said underwater vehicle with a thruster and an assist device coupled to said underwater vehicle with a cable and including cable handling equipment and a thruster, the method comprising:detecting a distance between a cable attachment of said underwater vehicle and a cable dispenser, serving as a cable attachment, of said assist device;referring to a cable length table storing the relationship between cable attachment-to-attachment distance and cable length including a predetermined amount of slack to extract a cable length associated with said detected cable attachment-to-attachment distance; andcontrolling operation of said cable handling equipment based on the amount of cable to be wound up or let out associated with said extracted cable length. |
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claims | 1. Transport container for nuclear fuel assemblies, of right prismatic shape, comprising: an external envelope; and an internal structure comprising an external envelope and an internal structure comprising a cradle having a tilting arrangement for mounting the cradle tiltable in the external envelope about an axis of transverse direction and a reception and holding unit resting on the cradle, defining at least one housing for receiving and holding a fuel assembly, and having a frame for supporting at least one fuel assembly comprising at least two walls supporting two lateral faces of a fuel assembly and two pivoting end walls for holding longitudinal end parts of the fuel assembly and at least one door mounted pivoting on the frame between an open position to give access to the fuel assembly housing and a closed position in which the door, the end walls and the support walls of the frame, ensure complete closure of the housing of the fuel assembly and a protection and containment of the fuel assembly, independently of the external envelope. 2. The container according to claim 1 , wherein the frame is a frame supporting two fuel assemblies having a T-shaped transverse section, a support base common to two housings of the two assemblies and a separation wall between the housings of the fuel assemblies and the internal structure of the container has two doors having a L-shaped transverse section, each door being articulated to a longitudinal edge of the support base of the frame, in a longitudinal direction of the frame, via a first edge of the door. claim 1 3. The container according to claim 2 , wherein the doors have pegs projecting towards the outside in the longitudinal direction at their longitudinal ends and in that the end walls closing the longitudinal ends of the housings of the fuel assemblies have, on their external edge, slots into which are introduced the pegs of the doors in the closed position of the doors and of the end plates. claim 2 4. The container according to claim 2 , wherein the doors have, in their closed position, a second edge pulled down against an end edge of the median separation wall of the frame, the second edges of the doors pulled down on the end edge of the median separation wall of the frame and the end edge of the median part of the frame having locking parts having openings which are aligned in the longitudinal direction of the internal part of the container, for the introduction of a locking rod into the locking parts which have aligned openings. claim 2 5. The container according to claim 1 , characterized in that the walls of the internal structure of the container around the lateral walls of the at least one housing for a fuel assembly are double walls formed by metal sheets and spacers having a central space filled with a neutron-absorbing resin, so that the spacers and the high density neutron-absorbing resin ensure the mechanical integrity of the walls. claim 1 6. The container according to claim 1 , wherein at least one of the closure walls of the end faces of the housing of a fuel assembly comprises adjustable means for holding the fuel assembly in the longitudinal direction of the container. claim 1 7. The container according to claim 1 , wherein the closure walls closing the lateral faces of the housing of the fuel assembly have means for holding the fuel assembly in transverse directions, these means consisting of pads which are movable in transverse directions of the fuel assembly, and manoeuvrable from the outside of the container, so as to come to press against faces of the spacer-grids of the fuel assembly. claim 1 8. The container according to claim 1 , characterized in that the container comprises, furthermore, between each of the longitudinal ends of the internal structure and each of the longitudinal ends of the external envelope, a shock-absorber formed by a disc of balsa covered by a stainless steel sheet. claim 1 |
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042696615 | abstract | A top nozzle for a nuclear reactor fuel assembly which includes an orifice plate designed to receive control rod guide tubes extending upwardly from the top of the fuel assembly. Nozzle pin extensions connected to the orifice plate are axially coextensive with the guide tubes and project upwardly through a top nozzle hold-down plate, and through openings in the upper core plate in the reactor. The hold-down plate is spring biased into an upper core plate engaging position and since the pin extensions are adapted to move in the core plate and against the action of the springs, the top nozzle thus becomes effective in accommodating upwardly acting hydraulic forces which tend to lift the fuel assembly, and to accommodate differential expansion of parts in the fuel assembly as compared to the core barrel in which the assembly is located. The number of pins disposed in the hold-down plate move in slots formed in each of the guide tube extensions and the uppermost position of the fuel assembly is determined when the pin is in the bottom of its slot in each of the guide tubes. |
048083690 | abstract | Two systems of low-pressure core spray appratuses have coolers, and are adapted to supply cooling water to a core spray header provided in a core-surrounding cylindrical shroud in a reactor pressure vessel. Two systems of high-pressure core flooding apparatuses and one system of high-pressure coolant injection apparatus are adapted to supply the cooling water to a region formed between the shroud and reactor pressure vessel. The elevation of the openings, which are in the reactor pressure vessel, of the high-pressure core flooding apparatuses and high-pressure coolant injection apparatus are higher than that of the core spray header. A pipe for returning the cooling water in the reactor pressure vessel to the above-mentioned coolers is connected to either the portion of the interior of the reactor pressure vesssel which is below the core or the portion of the interior of the reactor pressure vessel which is between the walls of the shroud and reactor pressure vessel. The above-mentioned pipe is provided therein with a valve which is adapted to be closed when the nuclear reactor stops being operated in an emergency in which the breakage of a pipe occurs, and to be opened when the nuclear reactor is stopped under normal conditions. |
claims | 1. A radiation therapy apparatus, comprisingan inner layer comprising:a radiation delivery device configured to emit primary radiation;a primary radiation shielding device configured to receive the primary radiation;one or more rails mounted on an inner surface of the inner layer, wherein the radiation delivery device and the primary radiation shielding device are disposed on the one or more rails; anda treatment table disposed inside the one or more rails, wherein the treatment table is configured to rotate around the vertical axis and the one or more rails are configured to rotate 360 degrees around the treatment table to deliver 4π steradians of radiation coverage to a patient positioned on the treatment table; andan outer layer comprising a secondary radiation shielding device integrated into the radiation therapy apparatus and configured to block secondary radiation, the secondary radiation shielding device comprising:a cylinder-shaped portion;a ring-shaped portion, the ring-shaped portion disposed to cover the radiation delivery device, and the primary radiation shielding device; anda housing configured to cover the secondary shielding device. 2. The apparatus of claim 1, wherein the one or more rails are disposed in a circular shape and configured to rotate around a horizontal axis. 3. The apparatus of claim 2, wherein the primary radiation shielding device is disposed opposite the radiation delivery device. 4. The apparatus of claim 1, wherein the apparatus is configured so that the radiation delivery device and the primary radiation shielding device rotate around a horizontal axis in synchrony with each other. 5. The apparatus of claim 1, wherein the housing is dome-shaped. 6. The apparatus of claim 1, wherein the housing is cylinder-shaped. 7. The apparatus of claim 1, wherein the primary radiation shielding device is configured to block at least 99.9% of radiation incident upon the primary radiation shielding device. 8. The apparatus of claim 1, wherein the secondary radiation shielding device is configured to block at least 99.9% of radiation incident upon the secondary radiation shielding device. 9. The apparatus of claim 1, wherein the cylinder shaped portion includes a closable opening through which a patient may pass in preparation for radiation therapy. 10. The apparatus of claim 9, wherein the cylinder shaped portion includes one or more doors coupled to a central portion of the cylinder shaped portion, the one or more doors covering and opening the closeable opening. 11. The apparatus of claim 1, further comprising one or more imaging sources and one or more imaging panels. 12. The apparatus of claim 11, wherein the one or more imaging sources and the one or more imaging panels are coupled to the one or more rails. 13. The apparatus of claim 11, wherein the one or more imaging sources and the one or more imaging panels are disposed inside the ring shaped portion. 14. The apparatus of claim 1, wherein the cylinder-shaped portion extends along the longitudinal center of the housing within the one or more rails. 15. The apparatus of claim 14, wherein the treatment table is movable and configured to slide inside the cylinder shaped portion. 16. The apparatus of claim 15, further comprising a gap in the cylinder shaped portion having the width of the one or more rails,the gap configured to avoid obstructing movement of the radiation delivery device and the primary radiation shielding device as the radiation delivery device and the primary radiation shielding device rotate around the horizontal axis. 17. The apparatus of claim 16, wherein the cylinder-shaped portion is configured to receive a portion of the treatment table within the gap to enable radiation emitted from the radiation delivery device to reach the patient positioned on the portion of the treatment table within the gap. 18. The apparatus of claim 16, wherein the ring shaped portion is coupled to the cylinder shaped portion at both sides of the gap. 19. The apparatus of claim 16, wherein the treatment table is configured to move between a first position over the gap and a second position away from the gap. 20. The apparatus of claim 1, wherein the steradians are solid angle units describing a direction of primary radiation applied to the patient disposed on the treatment table. |
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summary | ||
claims | 1. A chimney structure comprising:a guide structure defining an opening having a plurality of gaps; anda plurality of chimney partitions including 1 to N chimney partitions concentrically arranged between the plurality of gaps and spaced apart from each other on the guide structure, the 1 to N chimney partitions each defining a curved opening over the opening of the guide structure for providing a fluid flow path therethrough, and N being an integer greater than 1. 2. The chimney structure of claim 1, whereinthe 1 to N chimney partitions each have a tubular shape, andthe 1 to N chimney partitions have different radii. 3. The chimney structure of claim 1, wherein each one of the 1 to N chimney partitions is spaced apart from an adjacent one of the 1 to N chimney partitions by a same distance. 4. The chimney structure of claim 3, wherein a value of the same distance is configured to reduce the formation of Eddy currents if a steam mixture flows through the 1 to N chimney partitions. 5. The chimney structure of claim 1, whereinan upper surface of the guide structure defines M grooves spaced apart from each other,M is an integer greater than or equal to N,parts of the 1 to N chimney partitions are in the M grooves defined by the upper surface of the guide structure. 6. The chimney structure of claim 1, further comprising:a top plate on the 1 to N chimney partitions, whereinthe top plate defines an opening over the curved openings of the 1 to N chimney partitions. 7. The chimney structure of claim 1, further comprising:a plurality of slats between two adjacent chimney partitions among the 1 to N chimney partitions, whereinthe plurality of slats are configured to divide a space between the two adjacent chimney partitions into smaller sections. 8. The chimney structure of claim 7, whereinthe plurality of slats are attached to an inner surface of an outer chimney partition among the two adjacent chimney partitions,the plurality of slats are configured to be positioned toward the inner chimney partition among the two adjacent chimney partitions in order to divide the space between the two adjacent chimney partitions into smaller sections, andthe plurality of slats are configured to be positioned against an inner surface of the outer chimney partition among the two adjacent chimney partitions in order to avoid dividing the space between the two adjacent chimney partitions into smaller sections. 9. The chimney structure of claim 7, whereinthe plurality of slats are attached to the inner chimney partition among the two adjacent chimney partitions,the plurality of slats are configured to be positioned toward the outer chimney partition among the two adjacent chimney partitions in order to divide the space between the two adjacent chimney partitions into smaller sections, andthe plurality of slats are configured to be positioned against the inner chimney partition among the two adjacent chimney partitions in order to avoid dividing the space between the two adjacent chimney partitions into smaller sections. 10. The chimney structure of claim 1, whereinthe 1 to N chimney partitions each have a tubular shape, andeach one of the 1 to N chimney partitions is spaced part from an adjacent one of the 1 to N chimney partitions by a distance that is about equal to an inner diameter of the 1st chimney partition among the 1 to N chimney partitions. 11. The chimney structure of claim 10, whereinthe inner diameter of the 1st chimney partition is about 16 inches,a diameter of the Nth chimney partition is about 30 feet,a height of the 1 to N chimney partitions ranges from about 18 to 22 feet along an axial direction of the 1 to N chimney partitions, andN is greater than or equal to 4 and less than or equal to about 12. 12. The chimney structure of claim 1, whereinthe 1 to N chimney partitions include 2nd to (N−1)th chimney partitions between the 1st and the Nth chimney partitions in sequential order,the 1 to N chimney partitions each have a tubular shape,the 1 to N chimney partitions have different radii, anda separation distance between the 1st chimney partition and the 2nd chimney partition is different than a separation distance between two adjacent chimney partitions among the 2nd to (N−1)th chimney partitions. 13. The chimney structure of claim 1, whereinthe 1 to N chimney partitions include steel, anda thickness of 1 to N the chimney partitions ranges from about 0.25 inches to about 0.50 inches. 14. A reactor comprising:a reactor wall; andthe chimney structure of claim 1 secured to the reactor wall. 15. A chimney structure comprising:a tubular chimney housing having an input opening and an output opening sharing a central axis with the input opening and defining a fluid flow path therethrough; anda tubular partition arranged in and spaced from the tubular chimney housing to provide a central fluid flow path through the tubular partition and a first annular fluid flow path through an annular spacing defined by the tubular chimney housing and the tubular partition;wherein the chimney structure is configured to permit a fluid to flow from an input end to an output end of the tubular chimney housing through both the central fluid flow path and the first annular fluid flow path. 16. A method of manufacturing a chimney structure, comprising:concentrically arranging a plurality of chimney partitions on a guide structure, the guide structure defining an opening having a plurality of gaps,the plurality of chimney partitions including 1 to N chimney partitions concentrically arranged between the plurality of gaps and spaced apart from each other on the guide structure,the 1 to N chimney partitions each defining a curved opening over the opening of the guide structure for providing a fluid flow path therethrough,N being an integer greater than 1. 17. The chimney structure of claim 15, whereinthe tubular partition defines an opening that has a polygonal cross-section, andthe tubular chimney housing has a polygonal cross-section. 18. A chimney structure comprising:a guide structure defining an opening;a plurality of chimney partitions including 1 to N chimney partitions concentrically arranged and spaced apart from each other on the guide structure, the 1 to N chimney partitions each defining a curved opening over the opening of the guide structure, and N being an integer greater than 1; and one ofat least one rod secured to the guide structure,a plurality of divider plates extending through the 1 to N chimney partitions, the divider plates and the 1 to N chimney partitions defining a plurality of curved opening sections, based on sectionally dividing the curved openings of the 1 to N chimney partitions, anda partition structure surrounding the 1 to N chimney partitions, an inner surface of the partition structure defining a round opening, the partition structure including divider plates that divide the round opening into round opening sections, the 1 to N chimney partitions each having a tubular shape, and the 1 to N chimney partitions having different radii. 19. The chimney structure of claim 18, wherein the chimney structure includes the: at least one rod secured to the guide structure. 20. The chimney structure of claim 18, wherein the chimney structure includes the plurality of divider plates extending through the 1 to N chimney partitions, the divider plates and the 1 to N chimney partitions define the plurality of curved opening sections, based on sectionally dividing the curved openings of the 1 to N chimney partitions. 21. The chimney structure of claim 18, wherein the chimney structure includes the partition structure surrounding the 1 to N chimney partitions, the inner surface of the partition structure defines the round opening, the partition structure includes divider plates that divide the round opening into round opening sections, the 1 to N chimney partitions each have the tubular shape, and the 1 to N chimney partitions have different radii. 22. The chimney structure of claim 21, wherein the divider plates are metal sheets, the divider plates include notches corresponding to the 1 to N chimney partitions, and the 1 to N chimney partitions are in the notches of the divider plates. |
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047330899 | abstract | A radiographic intensifying screen comprising a support and at least one phosphor layer comprising a binder and a phosphor dispersed therein. The sharpness of image provided by the screen and the adhesion between the phosphor layer and the support are both remarkably improved by providing onto the surface of the support a great number of pits having a mean depth of at least 1 .mu.m, a maximum depth of more than 1 ranging to 50 .mu.m, and a mean diameter at the opening of at least 1 .mu.m. |
description | This application claims priority from Japanese Patent Application No. 2003-206629 filed Aug. 8, 2003, which is hereby incorporated by reference herein. 1. Field of the Invention The present invention relates to an X-ray multi-layer mirror used as a reflecting mirror in a projection optical system or the like and an X-ray exposure apparatus using the X-ray multi-layer mirror. 2. Related Background Art In an X-ray exposure apparatus using a soft X-ray having a wavelength of 13.5 nm or less, a multi-layer film structure is necessarily used to obtain a mirror having a high reflectance outside a total reflection incident angle range. There has been widely known a structure using a distributed Bragg reflecting material similar to a ¼-wavelength stack having an entirely constant film thickness. For example, when a mirror for an X-ray having a wavelength of 13.4 nm includes an Mo/Si alternate layer having a constant film thickness fundamental structure, which is composed of an Mo layer having a film thickness of 2.8 nm and an Si layer having a film thickness of 4.1 nm, a maximum theoretical reflectance exceeds 70% (see Japanese Patent Application Laid-open No. 2001-51106). However, in the Mo/Si alternate layer having the constant film thickness fundamental structure, a range of an X-ray incident angle at which a high reflectance is obtained is only within about 10 degrees. Therefore, with respect to a light beam incident at an incident angle outside this range, a significant reduction in reflectance is caused. According to Japanese Patent Application Laid-open No. 2001-51106, there is proposed an alternate multi-layer film having a non-uniform film thickness structure in which Ru or the like is used as a third material for the Mo/Si alternate layer and the thickness of each layer is changed by numeric repetition optimization processing, for example, the film thickness of the Mo layer is changed from about 1 nm or less to near 2 nm. However, in the Mo/Si alternate layer which is widely used for a mirror for the soft X-ray having the wavelength of 13.5 nm and made of two materials of Mo and Si, Mo and Si are easy to react with each other and form a compound of MoSix at an interface. Therefore, when a design film thickness obtained by optimization processing reduces, a design value is significantly different from a reflection characteristic after film formation in some cases. The present invention has been made in view of the problem that is not solved in the above-mentioned background art. An object of the present invention is to provide an X-ray multi-layer mirror capable of surely realizing a target reflectance in a wide incident angle range and an X-ray exposure apparatus using the X-ray multi-layer mirror. In order to attain the above-mentioned object, an X-ray multi-layer mirror according to the present invention includes an Mo/Si alternate layer with a non-uniform film thickness structure, which is produced by conducting optimization processing for widening an X-ray reflection characteristic on a constant film thickness fundamental structure of an Mo/Si alternate layer having the X-ray reflection characteristic, wherein each of all of Mo layers and Si layers for forming the non-uniform film thickness structure is designed to have a film thickness of 1.5 nm or more. In the Mo/Si alternate layer, it was found from a sectional TEM photograph or the like that an MoSix layer formed in an interface between the Mo layer and the Si layer has a thickness of about 1 nm. Therefore, even when the X-ray multi-layer mirror having the non-uniform film thickness structure in which the Mo layer has a film thickness of 1.5 nm or less is designed by the optimization processing for widening an incident angle range in the reflection characteristic, a target reflection characteristic cannot be obtained because of the MoSix layer caused in the interface by actual film formation. Therefore, the optimization processing is conducted under conditions that each of the layers has the film thickness of 1.5 nm or more, whereby the Mo/Si alternate layer having the non-uniform film thickness structure in which the influence of an interface layer is reduced is realized. Thus formed X-ray multi-layer mirror having a wide range reflection characteristic is used to be able to significantly improve an optical characteristic of an X-ray exposure apparatus. The embodiments of the present invention will be described with reference to the drawings. FIG. 1A shows the film structure of an X-ray multi-layer mirror according to an embodiment of the present invention. The X-ray multi-layer mirror includes an Mo/Si alternate layer 20 having a non-uniform film thickness structure in which Si layers 11 and Mo layers 12 each having a design film thickness of 1.5 nm or more are alternately stacked on a substrate 10. That is, in order to realize a reflection characteristic with a wider incident angle range in respect of an Mo/Si alternate layer having a constant film thickness fundamental structure in which the Si layers 11 and the Mo layers 12 are alternately stacked and the film thickness of the layers is kept constant in the stack, the Mo/Si alternate layer has the non-uniform film thickness structure determined by optimization processing so that the film thickness of each of the layers becomes 1.5 nm or more. When the Mo/Si alternate layer is formed by sputtering or the like, as shown in an enlarged form in FIG. 1B, it was found from a sectional TEM photograph or the like that an MoSix layer having a thickness of about 1 nm is generated in an interface between the Si layer 11 and the Mo layer 12. In order to avoid the influence of the MoSix layer on the reflection characteristic, the optimization processing is conducted under conditions that a minimum thickness of each of the layers is set to 1.5 nm or more. Therefore, the reflection characteristic of the Mo/Si alternate layer having the non-uniform film thickness structure after the film formation can be prevented from significantly deviating from a design value. FIG. 9 shows an angle reflection characteristic of Fundamental Example 1. In Fundamental Example 1, an Mo/Si alternate layer composed of 121 layers in total has a distributed Bragg reflecting structure (constant film thickness fundamental structure) similar to a ¼ wavelength stack having an entirely constant film thickness, and a high reflectance region to a soft X-ray having a wavelength of 13.5 nm being in an incident angle range of 0 degrees to 15 degrees. Similarly, FIG. 10 shows an angle reflection characteristic of Fundamental Example 2. In Fundamental Example 2, an Mo/Si alternate layer having a constant film thickness fundamental structure and composed of the Mo layers and the Si layers each having a constant film thickness has a high reflectance region having an incident angle range of 18 degrees to 23 degrees and composed of 121 layers in total. In order to increase a region having a reflectance of, for example, 50% or more at a design wavelength of 13.5 nm in these fundamental structures, optimization processing using a known thin film design program is conducted under conditions that a minimum film thickness of each of the layers is set to 1.5 nm. (Embodiment 1) FIG. 2 is a graph showing a film thickness distribution of the Mo/Si alternate layer in the case where Fundamental Example 1 having the angle reflection characteristic shown in FIG. 9 is subjected to the optimization processing under the above-mentioned condition. In FIG. 2, a layer No. 1 indicates a first layer on a substrate. A film structure having 121 layers in total is used. A thickness of each of the Si layer and Mo layer is set to 1.5 nm or more. FIG. 3 is a graph showing an angle reflection characteristic of the film structure in FIG. 2. As compared with the X-ray multi-layer mirror having the fundamental structure of the ¼ wavelength stack with the constant film thickness as shown in FIG. 9, an incident angle range in which the reflectance is 50% or more increases by 5 degrees to become a range of 0 degrees to 20 degrees. FIG. 4 is a schematic diagram showing a sputtering film formation apparatus used in this embodiment. The sputtering film formation apparatus includes a vacuum chamber 901, a vacuum pump 902 for vacuuming in the vacuum chamber 901, a movable mask 904 which is movable by a movable mask control device 903, a shutter 906 controlled by a shutter control device 905, a rotating mechanism 907 for holding and rotating a substrate, an Si target 908, an Mo target 909, a DC power source 910, an RF power source 911, a computer 912, and Ar/Xe gas introduction control device 913. All control systems are connected with the computer and can be integrally controlled. The Mo/Si alternate layer is formed by the following operation using the sputtering film formation apparatus. B-doped polycrystalline Si material and a metal Mo material are provided as the targets 908 and 909 each having a diameter of four inches, respectively. The targets 908 and 909 are rotated and then the materials are alternately selected to alternately form the Si layer and Mo layer on the substrate. Low expansion glass having a diameter of 500 mm and a thickness of 35 mm is used for the substrate and rotated during the film formation. The shutter 906 and the movable mask 904 for controlling a film thickness distribution on the substrate are provided between the substrate and the targets 908 and 909. With respect to a process gas introduced into a film formation atmosphere, an Ar gas is introduced at 10 sccm during the Si film formation and an Xe gas is introduced at 50 sccm during the Mo film formation. With respect to the power applied to the targets 908 and 909, high frequency (RF) power of 150 W having a wavelength of 13.56 MHz is applied during the Si film formation and DC power of 100 W is applied during the Mo film formation. A film thickness of each of the Si layer and the Mo layer is controlled with time by the computer 912. FIG. 5 is a graph showing film formation data for each of the layers, which is inputted to the computer 912. The mirror thus formed has a preferable reflection characteristic in which a reflectance of 50% is maintained at an incident angle of up to 20 degrees, as a design value. FIG. 6 shows a reflection reduced projection optical system of a reflection type reduced projection exposure apparatus (stepper) using the X-ray multi-layer mirror according to Embodiment 1. A soft X-ray having a wavelength of 13.5 nm is used for a light source. A pattern formed in a reflection type mask 1100 serving as an original plate is transferred to a resist on a wafer 1107 serving as the substrate by the reflection reduced projection optical system which is composed of first, second, third, fourth, fifth, and sixth reflection mirrors multi-layer mirror according to Fundamental Example 1 as shown in FIG. 9 is used for the reflection mirrors 1101, 1102, 1104, and 1106 in which an incident angle range thereon is 5 degrees or less. The X-ray multi-layer mirror according to Embodiment 1 is used for the reflection mirrors 1103 and 1105 in which an incident angle range thereon is 5 degrees or more. Therefore, when a pattern of 0.1 μm on a mask is used, a resist pattern having a size of 0.025 μm is obtained with high precision. (Embodiment 2) FIG. 7 is a graph showing a film thickness distribution of each of the Mo layer and the Si layer in the case where Fundamental Example 2 shown in FIG. 10 is subjected to the optimization processing under conditions that the minimum film thickness is set to 1.5 nm. In FIG. 7, a layer No. 1 indicates a first layer on a substrate. A film structure is composed of 121 layers in total. A thickness of each of the Si layer and Mo layer is set to 1.5 nm or more. FIG. 8 is a graph showing an angle reflection characteristic of the film structure in this embodiment. As compared with the angle reflection characteristic shown in FIG. 10, the range in which the reflectance is 50% or more increases by 3 degrees or more to become a range of 15 degrees to 25 degrees. The Mo/Si alternate layer was formed by the same film formation method as in Embodiment 1 and the reflection characteristic thereof was examined. As a result, the X-ray multi-layer mirror having a wide reflection characteristic as designed was obtained. In Embodiments 1 and 2, the film formation method for each of the layers is not limited to sputtering and may be another film formation method such as an evaporation. According to the above-mentioned structure of the present invention, the following effect is obtained. It is possible to produce an X-ray multi-layer mirror having a high reflectance over a wide incident angle range. Therefore, a performance of an X-ray exposure apparatus can be improved. |
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062721976 | description | Referring now to the drawings and where FIG. 1 shows a perspective view of a known fuel pin assembly 10, also called a fuel "element", for a PWR nuclear reactor core (not shown) and FIG. 2 shows a graph of average oxide layer thickness on fuel pin cladding. The assembly 10 comprises a plurality of fuel pins 12 arranged in a square array; top 14 and bottom 16 nozzles for locating the assembly in the reactor core; and, structural spacer grids 18 for aligning the pins 12 parallel to each other and to the assembly axis. The assembly shown in FIG. 1 is about 4 m in length, intervening parts 20 of the fuel pin length having been removed in the view shown. The structural spacer grids 18 have a plurality of apertures, each one accepting a fuel pin 12 or a thimble tube 22 (see FIGS. 3, 4 and 5) as appropriate and have resilient spring fingers (not able to be seen) in the apertures to grip the fuel pin to prevent any relative movement and consequent fretting damage: In operation, coolant water is pumped upwardly from the bottom nozzle 16 through the assembly 10 to exit via the top nozzle 14. Turbulence inducing vanes (not able to be seen) are provided on the grids 18 to promote mixing of the coolant and hence improve cooling of the fuel pins 12. The graph of FIG. 2 does not relate to the specific fuel assembly 10 of FIG. 1 which is merely exemplary of fuel pin assemblies. FIG. 2 shows a curve 30 showing average oxide layer thickness on the cladding against axial position on the fuel pin 12. In the curve 30, maxima 32 and minima 34 (only one of each indicated by arrows for the sake of clarity) are shown at various positions along the fuel pin length. The fuel pin assembly corresponding to the curve 30 has structural spacer grids located at positions which lie between a maxima and the preceding minima, e.g. at position 36 and other corresponding positions. The greatest average oxide layer thickness occurs at the maxima 38 and represents an oxide thickness which is effectively life-limiting for the fuel pin. The ability to reduce the oxide thickness at this position, and also possibly at the preceding maxima 40, would enable the full burn-up potential of the fuel per se to be utilised or would allow the fuel assembly to operate at a higher power level or a combination of these advantages. In the fuel pin assembly according to the present invention there is provided an additional grid 50 as shown in FIGS. 3 and 4, of which FIG. 3 shows a perspective view of part of the area of a short axial portion of a fuel pin assembly and FIG. 4 a plan view thereof. A plurality of fuel pins 12 are again shown, the fuel pin comprising an outer tubular sheath or cladding 22 which is filled with fuel material (not shown). Interspersed amongst the fuel pins 12 are thimble tubes 52 which receive control rods (not shown) to control the power output of the reactor. The grid 50 is stamped from sheet material such as Zircaloy (trade mark) and comprises a continuous framework 56 surrounding apertures 58 through which both the fuel pins 12 and thimble tubes 52 extend. However, the fuel pins 12 do not touch the framework 56 whereas the thimble tubes 52 are swaged outwardly at the location 60 where they pass through the apertures 58 so as to fixedly grip the framework 56 and thus locate the grid 50 in a desired position and orientation relative to the fuel pins 12. During the stamping operation, turbulence inducing means 61 in the form of vanes are integrally formed with the framework 56, the vanes being deflected away from the plane of the original sheet material during the forming operation to a desired configuration so as to optimise turbulence in the coolant water, indicated only schematically by the arrows 62, flowing through the fuel pin assembly 10. The grid 50 extends only up to the outer ring 64 of fuel pins since the outer ring of pins run at a lower temperature and have a consequently thinner oxide thickness layer. The grid 50 is positioned intermediate adjacent spacer grids 18 in the vicinity of the maxima 38 (and also possibly in the vicinity of maxima 40). The increased turbulence and improved coolant mixing caused by the grid 50 lowers the temperature of the fuel pins at this point and consequently also reduces the oxide thickness. The low volume or mass of the mixing grid 50 does not significantly increase the parasitic loss due to neutron capture. FIG. 5 shows a slightly modified grid 50 having turbulence inducing vanes 70 of different form. Other features remain essentially the same as in FIGS. 2 and 3. FIGS. 6 and 7 show a small part of the framework 56 of a grid 50. In this modification, some or all of the individual framework members 80 are twisted out of the plane of the original sheet from which the grid is stamped or pressed to an angle so as to promote turbulence in the coolant thereby. Vanes as described above with reference to FIGS. 3, 4 and 5 may or may not be employed depending upon the specific geometry and requirements of the fuel assembly 10 in question. |
040642046 | abstract | Nuclear fuel rods are manufactured utilizing a graphite flour-pitch matrix formulation containing an additive. The matrix formulation has a decreased viscosity at fabrication temperatures which permits manufacture of the fuel rods with lower fabrication pressures. Also, the matrix formulation does not cause the fuel rod to adhere or bond to the fuel element during heat treatment of the fuel rod in the fuel element. The nuclear fuel rods are suitable for use in high temperature gas cooled nuclear reactors. |
051961610 | abstract | A fail-safe storage rack is provided for interim storage of spent but radioactive nuclear fuel rod assemblies. The rack consists of a checkerboard array of substantially square, elongate receiving tubes fully enclosed by a double walled container, the outer wall of which is imperforate for liquid containment and the inner wall of which is provided with perforations for admitting moderator liquid flow to the elongate receiving tubes, the liquid serving to take up waste heat from the stored nuclear assemblies and dissipate same to the ambient liquid reservoir. A perforated cover sealing the rack facilitates cooling liquid entry and dissipation. |
055880363 | abstract | A radiographing operation is performed by using an X-ray CT apparatus for performing a radiographing operation to a subject lying on a bed, which comprises a bed drive means, an X-ray tube irradiating an X-ray to the subject, a detecting means for detecting an irradiated X-ray, a gantry on which said X-ray tube and said X-ray detecting means are mounted, a gantry drive means for rotationally driving the gantry, an exposure information file means into which past exposure information for the subject is stored, and a main controller for controlling the X-ray tube, the bed drive means and the gantry drive means. The radiographing condition according to which the radiographing operation is performed is set in accordance with a past exposure information stored in the exposure information file and at least one of the bed, the X-ray tube and the gantry. |
claims | 1. An X-ray collimator of a computed tomography system, wherein the X-ray collimator comprises a plurality of first plates extending in the circumferential direction of the computed tomography system and a plurality of second plates extending in the axial direction of the computed tomography system; the first plates and the second plates are inserted and engaged with each other; each of the first plates is provided with a plurality of first slits spaced apart in a circumferential direction, and each of the second plates is provided with a plurality of second slits spaced apart in an axial direction,the height of the first slit is larger than half of the entire height of the first plate or the height of the second slit is larger than half of the entire height of the second plate, andtwo adjacent first plates and two adjacent second plates define a through hole, and extensions of all of side walls of the through hole intersect at a focal spot of an X-ray source, so that X-rays can pass through the through hole in a straight line;wherein the X-ray collimator further comprises a positioning plate, the positioning plate is provided with a plurality of positioning grooves extending in the circumferential direction, and each of the positioning grooves is engaged with and positioned relative to an upper end of a corresponding first plate for further positioning;each of the positioning grooves is a run-through groove; andeach of the positioning grooves comprises two or more sub-grooves spaced apart in the circumferential direction, and a solid portion between two adjacent sub-grooves is set to have a predetermined width, and the upper end of each first plate is provided with a notch for accommodating the solid portion. 2. The X-ray collimator according to claim 1, wherein the cross joints of the first slits and the second slits are positioned by adhesive bonding or welding. 3. The X-ray collimator according to claim 1, wherein a first guard plate is provided at each of two ends of each first plate, and a second guard plate is provided at each of two ends of each second plate, and the first guard plates and the second guard plates enclose the ends of the first plates and the second plates respectively. 4. The X-ray collimator according to claim 3, wherein the thickness of the first guard plate is substantially half of the thickness of the second plate, and the thickness of the second guard plate is substantially half of the thickness of the first plate. 5. A computed tomography system, comprising an X-ray detector and an X-ray collimator for collimating X-rays radiating to the X-ray detector, wherein the X-ray collimator comprises a plurality of first plates extending in a circumferential direction of the computed tomography system and a plurality of second plates extending in an axial direction of the computed tomography system; the first plates and the second plates are inserted and engaged; each of the first plates is provided with a plurality of first slits spaced apart in the circumferential direction, and each of the second plates is provided with a plurality of second slits spaced apart in the axial direction,the height of the first slit is larger than half of the entire height of the first plate; or the height of the second slit is larger than half of the entire height of the second plate,two adjacent first plates and two adjacent second plates define a through hole, and extensions of all of side walls of the through hole intersect at a focal spot of an X-ray source, so that X-rays can pass through the through hole in a straight line;wherein the X-ray collimator further comprises a positioning plate, the positioning plate is provided with a plurality of positioning grooves extending in the circumferential direction, and each of the positioning grooves is engaged with and positioned relative to an upper end of a corresponding first plate for further positioning;each of the positioning grooves is a run-through groove; andeach of the positioning grooves comprises two or more sub-grooves spaced apart in the circumferential direction, and a solid portion between two adjacent sub-grooves is set to have a predetermined width, and the upper end of each first plate is provided with a notch for accommodating the solid portion. 6. The computed tomography system according to claim 5, wherein the X-ray collimator is mounted on the X-ray detector, and each through hole corresponds to a detecting unit of the X-ray detector; a plurality of partition grooves are provided in partition regions between pixels of the X-ray detector, and the lower end of the first plate is embedded into the partition groove. 7. The X-ray collimator according to claim 1, wherein the height of the first slit is lower than 5 mm or the height of the second slit is lower than 5 mm. 8. The computed tomography system according to claim 5, wherein the height of the first slit is lower than 5 mm or the height of the second slit is lower than 5 mm. |
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