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claims | 1. A nuclear island comprising:a nuclear reactor including a nuclear reactor core comprising fissile material disposed in a reactor pressure vessel;an emergency core cooling (ECC) system connected to the reactor pressure vessel through an ECC pressure vessel feedthrough;a reactor coolant inventory and purification system (RCIPS) connected to the reactor pressure vessel to inject coolant into the reactor pressure vessel through a make-up line connected to a make-up line pressure vessel feedthrough and to extract coolant from the reactor pressure vessel through a let-down line connected to a let-down line pressure vessel feedthrough; andan integral isolation valve (IIV) system including:a passive IIV comprising a check valve built into a forged flange that is connected directly to the ECC pressure vessel feedthrough and a corresponding ECC system line,a passive IIV comprising a check valve built into a forged flange that is connected directly to the make-up line pressure vessel feedthrough and the make-up line, andan active IIV comprising an active valve built into a forged flange that is connected directly to the let-down line pressure vessel feedthrough and the let-down line,wherein the passive and active IIVs each have an outer diameter along its entire length that is greater than an outer diameter of the corresponding ECC system line, make-up line, and let-down line to which it is connected. 2. The nuclear island of claim 1 further comprising:a residual heat removal (RHR) system having an RHR inlet line connected to the reactor pressure vessel through an RHR pressure vessel inlet feedthrough and an RHR outlet line connected to the reactor pressure vessel through an RHR pressure vessel outlet feedthrough;wherein the IIV system further includes:a passive IIV comprising a check valve built into a forged flange that is connected directly to the RHR pressure vessel outlet feedthrough and the RHR outline line, andan active IIV comprising an active valve built into a forged flange that is connected directly to the RHR pressure vessel inlet feedthrough and the RHR inlet line,wherein the passive and active IIVs each have an outer diameter along its entire length that is greater than an outer diameter of the corresponding RHR outlet line and RHR inlet line, respectively, to which it is connected. 3. The nuclear island of claim 2 wherein the RHR system comprises an air- or water-cooled RHR heat exchanger. 4. The nuclear island of claim 2 further comprising:a pressure sensor disposed in the reactor pressure vessel and configured to sense reactor coolant pressure; anda reactor control system configured to close the active IIV connected directly to the RHR pressure vessel inlet feedthrough in response to the reactor coolant pressure sensed by the pressure sensor exceeding a threshold pressure. 5. The nuclear island of claim 1 further comprising:a level sensor disposed in the reactor pressure vessel and configured to sense reactor coolant level;a reactor control system configured to close the active IIV connected directly to the letdown line pressure vessel feedthrough in response to the reactor coolant level sensed by the pressure sensor falling below a threshold reactor coolant level. 6. A nuclear island comprising:a nuclear reactor including a nuclear reactor core comprising fissile material disposed in a reactor pressure vessel that has a plurality of pressure vessel penetrations that exclusively carry flow into the reactor pressure vessel and at least one pressure vessel penetration that carries flow out of the reactor pressure vessel; andan integral isolation valve (IIV) system including:a plurality of passive IIVs each comprising a check valve built into a forged flange and not including an actuator, andone or more active IIVs each comprising an active valve built into a forged flange and including an actuator,wherein:each pressure vessel penetration that exclusively carries flow into the reactor pressure vessel is protected by a passive IIV whose forged flange is directly connected to the corresponding pressure vessel penetration and a corresponding line, andeach pressure vessel penetration that carries flow out of the reactor pressure vessel is protected by an active IIV whose forged flange is directly connected to the corresponding pressure vessel penetration and a corresponding line,wherein the passive and active IIVs each have an outer diameter along its entire length that is greater than an outer diameter of the corresponding line to which it is connected. 7. The nuclear island of claim 6 wherein each pressure vessel penetration that carries flow out of the reactor pressure vessel exclusively carries flow out of the reactor pressure vessel. 8. The nuclear island of claim 6 wherein each active IIV further includes a manual backup for the actuator. 9. The nuclear island of claim 6 further comprising:an emergency core cooling (ECC) system connected to the reactor pressure vessel through a pressure vessel penetration that exclusively carries flow into the reactor pressure vessel and is protected by a passive IIV whose forged flange is directly connected to the pressure vessel penetration and a corresponding ECC system line,wherein the passive IIV has an outer diameter that is greater along its entire length than an outer diameter of the corresponding ECC system line to which it is connected. 10. The nuclear island of claim 6 further comprising:a reactor coolant inventory and purification system (RCIPS) connected to the reactor pressure vessel to inject coolant into the reactor pressure vessel through a vessel penetration that exclusively carries flow into the reactor pressure vessel and is protected by a passive IIV whose forged flange is directly connected to the pressure vessel penetration and a corresponding RCIPS line,wherein the passive IIV has an outer diameter that is great along its entire length than an outer diameter of the corresponding RCIPS line to which it is connected. 11. The nuclear island of claim 10 wherein the RCIPS is further connected to the reactor pressure vessel to extract coolant from the reactor pressure vessel through a pressure vessel penetration that exclusively carries flow out of the reactor pressure vessel and is protected by an active IIV whose forged flange is directly connected to the pressure vessel penetration and a corresponding RCIPS line,wherein the active IIV has an outer diameter that is greater along its entire length than an outer diameter of the corresponding RCIPS line to which it is connected. 12. The nuclear island of claim 11 further comprising:a residual heat removal (RHR) system having:an inlet line connected to the reactor pressure vessel through a pressure vessel penetration that exclusively carries flow out of the reactor pressure vessel and is protected by an active IIV whose forged flange is directly connected to the pressure vessel penetration and the inlet line, andan outlet line connected to the reactor pressure vessel through a pressure vessel penetration that exclusively carries flow into the reactor pressure vessel and is protected by a passive IIV whose forged flange is directly connected to the pressure vessel penetration and the outlet line,wherein the passive and active IIVs each have an outer diameter that is greater along its entire length than an outer diameter of the corresponding outlet line and inlet line, respectively, to which it is connected. 13. The nuclear island of claim 6 further comprising:a residual heat removal (RHR) system having:an inlet line connected to the reactor pressure vessel through a pressure vessel penetration that exclusively carries flow out of the reactor pressure vessel and is protected by an active IIV whose forged flange is directly connected to the pressure vessel penetration the inlet line, andan outlet line connected to the reactor pressure vessel through a pressure vessel penetration that exclusively carries flow into the nuclear reactor pressure vessel and is protected by a passive IIV whose forged flange is directly connected to the pressure vessel penetration and the outlet line,wherein the active and passive IIVs each have an outer diameter that is greater along its entire length than an outer diameter of the corresponding inlet line and outlet line to which it is connected. 14. The nuclear island of claim 6 wherein the check valve of each passive IIV admits flow at above a threshold pressure into the reactor pressure vessel, while blocking flow otherwise. 15. The nuclear island of claim 6 wherein the active valve of each active IIV is a normally closed valve. 16. The nuclear island of claim 6 wherein the active valve of at least one active IIV is configured to be closed by reactor coolant pressure upon loss of actuator power. 17. A nuclear island comprising:a nuclear reactor including a nuclear reactor core comprising fissile material disposed in a reactor pressure vessel;a plurality of auxiliary systems in fluid communication with the reactor pressure vessel via pressure vessel penetrations wherein each said pressure vessel penetration is either a fluid inlet pressure vessel penetration carrying fluid into the reactor pressure vessel or a fluid outlet pressure vessel penetration carrying fluid out of the reactor pressure vessel; andan integral isolation valve (IIV) system including:at least one passive IIV comprising a check valve built into a forged flange and not including an actuator, andat least one active IIV comprising an active valve built into a forged flange and including an actuator,wherein:each fluid inlet pressure vessel penetration is protected by a passive IIV whose forged flange is directly connected to the pressure vessel penetration and a corresponding line, andeach fluid outlet pressure vessel penetration is protected by an active IIV whose forged flange is directly connected to the pressure vessel penetration and a corresponding line,wherein the passive and active IIVs each have an outer diameter that is greater along its entire length than an outer diameter of the corresponding line to which it is connected. 18. The nuclear island of claim 17 wherein the plurality of auxiliary systems includes an emergency core cooling (ECC) system and a reactor coolant inventory and purification system (RCIPS). 19. The nuclear island of claim 18 wherein the plurality of auxiliary systems further includes a residual heat removal (RHR) system. |
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051606968 | abstract | Apparatus for nuclear transmutation and power production using an intense accelerator-generated thermal neutron flux. High thermal neutron fluxes generated from the action of a high power proton accelerator on a spallation target allows the efficient burn-up of higher actinide nuclear waste by a two-step process. Additionally, rapid burn-up of fission product waste for nuclides having small thermal neutron cross sections, and the practicality of small material inventories while achieving significant throughput derive from employment of such high fluxes. Several nuclear technology problems are addressed including 1. nuclear energy production without a waste stream requiring storage on a geological timescale, 2. the burn-up of defense and commercial nuclear waste, and 3. the production of defense nuclear material. The apparatus includes an accelerator, a target for neutron production surrounded by a blanket region for transmutation, a turbine for electric power production, and a chemical processing facility. In all applications, the accelerator power may be generated internally from fission and the waste produced thereby is transmuted internally so that waste management might not be required beyond the human lifespan. |
abstract | A method for storing nuclear fuel in a container (10) including a concrete body and a fuel receiver embedded in the concrete body, comprises the steps of: providing formwork (62) for the concrete body and supporting the fuel receiver within the formwork; placing the formwork in an immersed position in a pool (54) containing a body of water; placing concrete in the immersed formwork (62); and removing the formwork with the concrete body cast therein from the pool (52). A system for manufacturing a storage container (10) for use in the method comprises: a water pool (52) of a depth at least equal to the height of the storage container (10) to be manufactured; facilities for assembling concrete formwork (62) for the concrete body (12) of the storage container (10); facilities for moving the formwork and the fuel receiver to the water pool (52); facilities for placing concrete in the formwork (62) with the formwork immersed in water in the water pool (52); and facilities for removing the formwork (62) and the concrete body (12) therein from the water pool (52). |
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claims | 1. An apparatus for containing radioactive materials having a residual heat load comprising:a body having an inner surface that forms a cavity for receiving radioactive materials, the cavity having an open top end and a dosed bottom end, the cavity having a horizontal cross-sectional profile having a perimeter formed by the inner surface of the body;a basket formed of a plurality of intersecting plates positioned in the cavity, the basket comprising a plurality of cells, the basket having an outer surface;a plurality of spacers having a top surface, a bottom surface, an outer surface, and an inner surface forming a central passageway, the spacers arranged in a stacked assembly within the cavity the basket extending through the central passageways of the spacers of the stacked assembly; andwherein an entirety of the outer surface of the basket is in conformal surface contact with the inner surfaces of the spacers. 2. The apparatus of claim 1 further comprising:the plurality of spacers having a substantially constant horizontal cross-section from the top surface to the bottom surface, the spacers arranged in the stacked assembly within the cavity so that the top and bottom surfaces of adjacent spacers are in surface contact with one another so as to form a spacer-to-spacer interface; andmeans for maintaining the spacers of the stacked assembly in alignment. 3. The apparatus of claim 1 wherein one or more of the spacers are keyed to maintain alignment of the stacked assembly. 4. The apparatus of claim 1 wherein an entirety of the outer surfaces of the spacers is in conformal surface contact with the inner surface of the body. 5. The apparatus of claim 1 further comprising:a lid enclosing the open top end of the cavity,each of the spacers comprising a passageway extending from the top surface to the bottom surface;wherein when arranged in the stacked assembly, the passageways of the spacers form a downcomer extending from a top plenum of the cavity to a bottom plenum of the cavity; andwherein the downcomer has a substantially constant horizontal cross-sectional area from the top plenum to the bottom plenum. 6. The apparatus of claim 1 wherein the stacked assembly comprises a top end and a bottom end, the stacked assembly having a substantially constant horizontal cross-section from the top end to the bottom end. 7. An apparatus for containing radioactive materials having a residual heat load comprising:a body comprising an inner surface that forms a cavity for receiving radioactive materials, the cavity having an open top end and a closed bottom end;a basket positioned in the cavity and comprising a plurality of cells, the basket having a horizontal cross-sectional profile having an external perimeter formed by an outer surface of the basket;a plurality of ring structures having a top surface, a bottom surface, an outer surface, and an inner surface forming a central passageway, the ring structures arranged in a stacked assembly within the cavity so that the top and bottom surfaces of adjacent ring structures in the stacked assembly are in direct surface contact with one another, each of the ring structures having a horizontal cross-sectional profile having an internal perimeter formed by the inner surface of the ring structure and an external perimeter formed by the outer surface of the ring structure; andwherein the basket extends through the central passageways of the ring structures so that an entirety of the external perimeter of the basket is in conformal surface contact with the internal perimeters of the ring structures. 8. The apparatus of claim 7 wherein each of the ring structures is constructed of a material having a first coefficient of thermal expansion and the body is constructed of a material having a second coefficient of thermal expansion, the first coefficient of thermal expansion being greater that the second coefficient of thermal expansion. 9. The apparatus of claim 8 wherein each of the ring structures comprise a plurality of passageways extending from the top surface to the bottom surface, and the passageways of the ring structures form a plurality of downcomers extending from a top plenum of the cavity to a bottom plenum of the cavity when arranged in the stacked assembly, each of the plurality of downcomers having a substantially constant horizontal cross-section from the top plenum to the bottom plenum. 10. The apparatus of claim 8 further comprising:a first plurality of cutouts provided at a top of the basket so that the cells are in spatial communication with each other, thereby forming a top plenum;a second plurality of cutouts provided at a bottom of the basket so that the cells are in spatial communication with each other, thereby forming a bottom plenum; andeach of the ring structures comprising a plurality of passageways extending from the top surface to the bottom surface, the passageways of the ring structures forming a plurality of downcomers extending from the top plenum to the bottom plenum when arranged in the stacked assembly; andwherein each of the plurality of downcomers has a substantially constant horizontal cross-section from the top plenum to the bottom plenum. 11. The apparatus of claim 8 wherein the first coefficient of thermal expansion is at least 20 percent times greater than the second coefficient of thermal expansion. 12. The apparatus of claim 7 further comprising:the cavity having a horizontal cross-sectional profile having a perimeter formed by the inner surface of the body; andan entirety of the external perimeters of the ring structures is in conformal surface contact with the perimeter of the body. 13. The apparatus of claim 7 wherein each of the ring structures is a unitary structure. 14. An apparatus for containing radioactive materials having a residual heat load comprising:a body comprising an inner surface that forms a cavity for receiving radioactive materials, the cavity having an open top end and a closed bottom end;a basket comprising a plurality of cells positioned in the cavity, the basket having an outer surface;a plurality of ring structures having a top surface, a bottom surface, an outer surface, and an inner surface forming a central passageway, the ring structures arranged in a stacked assembly within the cavity; andthe basket extending through the central passageways of the ring structures so that an entirety of the outer surface of the basket is in conformal surface contact with the inner surface of the ring structures. 15. The apparatus of claim 14 wherein each of the ring structures is a unitary structure. 16. The apparatus of claim 14 further comprising:the ring structures having a substantially constant horizontal cross-section from the top surface to the bottom surface, the ring structures arranged in the stacked assembly within the cavity so that the top and bottom surfaces of adjacent ring structures in the stacked assembly are in direct surface contact with one another and the stacked assembly has a substantially constant horizontal cross-section along its entire height; andmeans for maintaining the ring structures of the stacked assembly in alignment. 17. The apparatus of claim 14 wherein an entirety of the outer surfaces of the ring structures is in conformal surface contact with the inner surface of the body. 18. The apparatus of claim 14 further comprising:the cavity having, a horizontal cross-sectional profile having a perimeter formed by the inner surface of the body;each of the ring structures having an external perimeter formed by the outer surface of the ring structure; andwherein the perimeter of the cavity is circular, and wherein the external perimeter of the basket is of a shape that is not circular. 19. An apparatus for containing radioactive materials having a residual heat load comprising:a body comprising an inner surface that forms a cavity for receiving radioactive materials, the cavity having an open top end and a closed bottom end;a basket comprising a plurality of cells positioned in the cavity, the basket having an outer surface;a plurality of ring structures having a top surface, a bottom surface, an outer surface, and an inner surface forming a central passageway, the ring structures arranged in a stacked assembly within the cavity; andthe basket extending, through the central passageways of the ring structures so that a first clearance exists between the inner surfaces of the ring structures in the stacked assembly and the outer surface of the basket at ambient temperature, the first clearance circumferentially surrounding the basket; andwherein upon radioactive materials having, a residual heat load being positioned in the basket, the residual heat load from the radioactive materials causes the basket to expand so that an entirety of the outer surface of the basket is in conformal surface contact with the inner surfaces of the ring structures and the first clearance is eliminated. 20. The apparatus of claim 19 further comprising:a second clearance between the inner surface of the body and the outer surfaces of the ring structures in the stacked assembly at ambient temperature, the second clearance circumferentially surrounding the stacked assembly; andwherein upon positioning, the radioactive materials having a residual heat load in the basket, the residual heat load of the radioactive materials causes the ring structures to expand so that an entirety of the outer surfaces of the ring structures is in conformal surface contact with the inner surface of the body and the second clearance is eliminated. 21. The apparatus of claim 20 wherein the plurality of ring structures are constructed of a material having a first coefficient of thermal expansion and the inner surface of the body is constructed of a material having a second coefficient of thermal expansion, the first coefficient of thermal expansion being greater that the second coefficient of thermal expansion. |
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047770112 | description | Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is seen a fuel assembly 1 which is disposed in a water tank 3, together with a manipulator 2. Several fuel rods 4 form a fuel rod bundle which is disposed between two end plates 5, 6, leaving a spacing 7 between the end plates 5, 6 and the end of a fuel rod. Tie rods 8 fix the end plates 5, 6 in position relative to each other. Non-illustrated spacers provide for the guidance of the fuel rods and for the maintenance of the required gaps between the fuel rods. The manipulator 2 as well as the fuel assembly are supported on a base 10 of the water tank 3. The manipulator has a carrier 11 in which two probes 12 are held. As is evident, in particular from FIGS. 3 and 4, the probes are formed of a resiliently constructed flat material which has a cross-section of approximately 20.times.1 mm. The probes are mutually parallel and the wide sides thereof extend roughly parallel to the base 10 of the water tank 3. The probes 12 project from the carrier 11 at least as far as the width of a fuel assembly when considered in cross-section. Ultrasonic test heads 13, 14 are disposed at the free ends of the probes, on the lateral faces which are directed towards each other. The ultrasonic test heads are situated opposite each other and are spaced apart from each other by approximately 10 mm. One ultrasonic test head 13 is constructed as a transmitter and the other ultrasonic test head 14 is constructed as a receiver. Since the the test heads are disposed in water, the transit time of the transmitted pulse over a water path to the receiving test head 14 is a quantity which depends on the spacing of the test heads with respect to each other. The display screen portion illustrated in FIG. 5 shows the transit time 15 of the transmitted pulse 16 after traversing the water path between the ultrasonic test heads 13 and 14. The end of the transit time is clearly shown by a peak or amplitude 17. If it is intended to check whether or not the fuel assembly has grown in its overall length during insertion or residence in the nuclear reactor, the actual fuel assembly length is determined as follows. After an appropriate travelling movement of the manipulator 2 in the direction of arrows 18 and 19, the wide side of a probe 12 facing away from the ultrasonic test head 14 slowly approaches and is brought into contact with an end surface 20 of the end plate 5. The phantom illustration of the pair of probes 12 indicates the path in the direction of the arrow 18 past the height of the end surface 20 and the subsequent movement in the direction of the arrow 19. It is only then that the opposite movement takes place until contact is made with the end surface 20 of the end plate 5. The representation of the guide and drive elements of the manipulator 2 have been dispensed with for reasons of improved clarity. Due to the resilient construction of the probes 12, after contact has been made, a spacing dimension will be established between the two probes which is smaller than the spacing in a probe that is not brought into contact. The spacing dimension which is established can be seen on the display screen, as calibrated in millimeters in FIG. 5. The end of a reduced transit time 9 of the transmitted pulse 16 after traversing the smaller spacing between the probes 12 is shown by a peak or amplitude 21. The actual length dimension of the fuel assembly is made up of a constant dimension "k" between the base 10 of the water tank 3 and the wide side of the probe 12 with the manipulator 2 disposed on the base 10, plus the path of travel of the manipulator until contact is made by the wide side of the probe with the end surface 20 of the end plate 5, minus the difference between the transit times 9 and 15 which, according to the display screen portion of FIG. 5, is 1 millimeter. In FIG. 6 the upper end region of a fuel assembly 1 according to FIG. 1 is shown on a larger scale. During the operation of a reactor plant, the individual fuel rods 4 undergo a varying change in length. In a repetitive check taking place at regular spacings, the spacing between the fuel rod with the greatest elongation and the end plate 5 can be determined. For this purpose, the probe pair is inserted into the gap between the end plate 5 and the fuel rod ends in the direction of the arrow 19 while maintaining as small a spacing as possible from the lower surface of the end plate 5. After the probes have been inserted over the entire fuel assembly width, they are moved in the direction of the end plate 5. The process of approach can be expediently followed using the display screen. A reduction of the transit time of the ultrasonic signals between the transmitting and receiving test heads 13, 14 indicates contact with the end plate 5. The path of travel of the probes 12 in the direction of the fuel rod ends at the point of contact with the fuel rod having the largest elongation and is equal to the spacing dimension. The reduction in the transit time of the acoustic pulses between the ultrasonic test heads in this embodiment serves as a visible indication of the process of contact between the side of the probe facing away from the ultrasonic test head 13, 14 and the end plate 5 or a fuel rod end. FIG. 7 illustrates the application of the method for checking a deviation in alignment of an end plate 5, 6. In this case, a feeler or sensor which is constructed in the form of a roller 22, is associated with the side of the probe facing away from the ultrasonic test haad 14. FIG. 7 shows a view of the end plate 5 in the direction of the arrow VII in FIG. 1, with probes rotated through 90 degrees with respect to the device shown in FIG. 1. By bringing the roller 22 into contact at a position 23 and then at a position 24, the transit time of the transmitted pulse from the transmitting test head 13 to the receiving test head 14 is used to determine which position is nearer to a construction or alignment line 25. The difference between transit times 26 and 27 can be read off directly in millimeters on a display screen in FIG. 8. If necessary, the profile of the deviation from the construction line 25 between the positions 23 and 24 can also be documented by continuously moving the probe carrier 11 in the direction of the arrow 19. The foregoing is a description corresponding in substance to German Application No. P 35 42 204.1-33, dated Nov. 29, 1985, the International priority of which is being claimed for the instant application, and which is hereby made part of this application. Any material discrepancies between the foregoing specification and the aforementioned corresponding German application are to be resolved in favor of the latter. |
summary | ||
description | Extreme ultra violet (EUV) lithographic masks are used during the manufacturing process of semiconductor wafers and other modern electrical components. Defects of EUV lithographic masks are duplicated on multiple electrical components and thus are very costly. In order to protect EUV lithographic masks these masks are usually covered (or placed below) by pellicles. EUV lithographic masks should be inspected in order to detect defects. The detection typically includes scanning the EUV lithographic masks with low energy electrons and detecting these low energy electrons. It has been found that some modern pellicles prevent (or at least dramatically reduce) the passage of low energy electrons through the pellicles and thus prevent inspection of EUV lithographic masks that are protected by pellicles. There is a growing need to inspect EUV lithographic masks that are protected by pellicles. According to an aspect of the invention, there are provided a method and a system for inspecting EUV lithographic masks that are protected by pellicles. According to an embodiment of the invention, the method comprises: directing by electron optics, primary electrons towards a pellicle that is positioned between the electron optics and the lithography mask; wherein the primary electrons exhibit an energy level that allows the primary electrons to pass through the pellicle and to impinge on the lithographic mask; detecting, by at least one detector, detected emitted electrons and generating detection signals; wherein detected emitted electrons are generated as a result of an impingement of the primary electrons on the lithographic mask; and processing, by a processor, the detection signals to provide information about the lithography mask. According to another embodiment of the invention, there is provided a system for evaluating lithography mask, the system comprises: electron optics for directing primary electrons towards a pellicle that is positioned between the electron optics and the lithography mask; wherein the primary electrons exhibit an energy level that allows the primary electrons to pass through the pellicle and to impinge on the lithographic mask; at least one detector for detecting detected emitted electrons and for generating detection signals; wherein detected emitted electrons are generated as a result of an impingement of the primary electrons on the lithographic mask; and a processor for processing the detection signals to provide information about the lithography mask. According to another embodiment, there is provided a method for evaluating a lithographic mask, the method comprises: receiving detection signals; wherein the detection signals are generated by at least one detector that detects detected emitted electrons; wherein the detected emitted electrons are generated as a result of directing by electron optics, primary electrons towards a pellicle that is positioned between the electron optics and the lithography mask; wherein the primary electrons exhibit an energy level that allows the primary electrons to pass through the pellicle and to impinge on the lithographic mask; and processing, by a processor, the detection signals to provide information about the lithography mask. According to yet another embodiment of the invention, there is provided a system for evaluating lithography mask, the system comprises: an interface for receiving detection signals; wherein the detection signals are generated by at least one detector that detects detected emitted electrons; wherein the detected emitted electrons are generated as a result of directing by electron optics, primary electrons towards a pellicle that is positioned between the electron optics and the lithography mask; wherein the primary electrons exhibit an energy level that allows the primary electrons to pass through the pellicle and to impinge on the lithographic mask; and a processor for processing the detection signals to provide information about the lithography mask. According to an embodiment of the invention, there is provided a non-transitory computer readable medium that stores instructions for: receiving detection signals; wherein the detection signals are generated by at least one detector that detects detected emitted electrons; wherein the detected emitted electrons are generated as a result of directing by electron optics, primary electrons towards a pellicle that is positioned between the electron optics and a lithography mask; wherein the primary electrons exhibit an energy level that allows the primary electrons to pass through the pellicle and to impinge on the lithographic mask; and processing the detection signals to provide information about the lithography mask. According to various embodiments of the invention: the detected emitted electrons can be backscattered electrons that are emitted from the lithographic mask; the detected emitted electrons may exclude secondary electrons emitted from the pellicle due to an interaction of the primary electrons with the pellicle; the detected emitted electrons may exclude secondary electrons emitted from the pellicle due to an interaction of the backscattered electrons with the pellicle; the detected emitted electrons may exclude secondary electrons emitted from the pellicle due to (a) an interaction of the primary electrons with the pellicle and due to (b) an interaction of the backscattered electrons with the pellicle; the detected emitted electrons may be secondary electrons that are emitted from the pellicle due to an interaction of the backscattered electrons with the pellicle, wherein the backscattered electrons are emitted from the lithographic mask; the detected emitted electrons may exclude electrons emitted from the pellicle due to an interaction of the primary electrons with the pellicle; the detected emitted electrons exclude the backscattered electrons; the detected emitted electrons may exclude secondary electrons emitted from the pellicle due to an interaction of the primary electrons with the pellicle and masking the backscattered electrons. It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention. Because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits known to those skilled in the art, details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention. Any reference in the specification to a method should be applied mutatis mutandis to a system capable of executing the method and should be applied mutatis mutandis to a non-transitory computer readable medium that stores instructions that once executed by a computer result in the execution of the method. Any reference in the specification to a system should be applied mutatis mutandis to a method that may be executed by the system and should be applied mutatis mutandis to a non-transitory computer readable medium that stores instructions that may be executed by the system. Any reference in the specification to a non-transitory computer readable medium should be applied mutatis mutandis to a system capable of executing the instructions stored in the non-transitory computer readable medium and should be applied mutatis mutandis to method that may be executed by a computer that reads the instructions stored in the non-transitory computer readable medium. According to an embodiment of the invention there is provided a method and system for evaluating a lithography mask such as an extreme ultra violet (EUV) lithography mask and especially a 16 nanometer EUV lithography mask. FIG. 1 illustrates a system 41 for evaluating a lithography mask 10 according to an embodiment of the invention. FIG. 2 illustrates a system 42 for evaluating a lithography mask 10 according to an embodiment of the invention. FIG. 3 illustrates a system 43 for evaluating a lithography mask 10 according to an embodiment of the invention. FIG. 4 illustrates a system 44 for evaluating a lithography mask 10 according to an embodiment of the invention. The lithography mask 10 can be an extreme ultra violet EUV lithographic mask and may be a 16 nanometer EUV lithographic mask. It is illustrated in FIGS. 1-4 as including an upper layer 11, multiple intermediate layers 12-15 and a substrate (bulk) 16. System 41 includes: a. Electron optics 60 for directing primary electrons 21 towards a pellicle 30 that is positioned between the electron optics 60 and the lithography mask 10. The primary electrons 21 can be generated by electron beam source 61 of the electron optics 60. The primary electrons exhibit an energy level that allows the primary electrons to pass through the pellicle 30 and to impinge on the lithographic mask 10. Electron optics 60 may include any element that can affect the trajectory of the primary electrons 21 as well as their landing energy, or any other characteristics. The electron optics 60 may include one or more lenses, one or more apertures, one or more filters, one or more beam shaping elements, one or more beam splitters, one or more collimators, one or more deflectors, one or more accelerating elements, one or more de-accelerating elements, and may include an electron source. The electron optics 60 may include one or more detectors such as detector 70. b. Detector 70 for detecting detected emitted electrons and for generating detection signals indicative of the detected emitted electrons. The detected emitted electrons are generated as a result of an impingement of the primary electrons on the lithographic mask. c. Interface 100 for receiving the detection signals. The interface 100 may be a communication port, a memory module and the like. d. Processor 80 for processing the detection signals to provide information about the lithography mask. The system can include more than a single detector. The detector 70 can have multiple separate segments—each arranged to generate detection signals reflecting the detected emitted electrons it detected. In FIG. 1 the detector 70 is shown as being of an annular shape and four segments 71-74 that surround central aperture 76. It is noted that the number, shape and size of detector can differ from those illustrated in FIG. 1. For example, in FIG. 2 the detector 70 has a single annular segment 77 that surrounds aperture 76. The primary electrons 21 form a primary beam that passes through the pellicle 30 and this passage causes the pellicle 30 to emit a first group of emitted secondary electrons SE1 24. The primary electrons 21 impinge onto the lithographic mask 10 and result in an emission of backscattered electrons BSE 22. The backscattered electrons 22 may pass through the pellicle 30 and may case the pellicle 30 to emit a second group of secondary electrons SE2 26. It is noted that the impingement of the primary electrons 21 onto the lithographic mask 10 results in an emission of secondary electrons (not shown) that do not manage to pass through the pellicle 30 and be detected. The term “emitted electrons” may refer to the combination of backscattered electrons 22, the first group of electrons SE1 24 and the second group of electrons SE2 26. The term “detected emitted electrons” may refer to the part of the emitted electrons that are detected by the at least one detector 70. For example, even if the system 41 may be designed to detect emitted electrons of a certain type (BSE, SE1 and/or SE2) it may occur that only some of these certain type of electrons are detected. Furthermore, according to various embodiments of the invention one or more types of electrons (outs of SE1, SE2 and BSE) may be masked. This can be implemented by various known masking methods including spatial filters and energy filters. System 42 of FIG. 2 differs from system 41 of FIG. 1 by including energy filters 90 between detector 70 and the pellicle 30. These energy filters 90 can be set for masking secondary electrons such as those that belong to the second group of secondary electrons SE2 26. System 43 of FIG. 3 differs from system 41 of FIG. 1 by having a beam splitter 110 that is positioned above the pellicle 30 and directs emitted electrons towards detector 70. This arrangement can allow the plane of the pellicle 30 to be imaged onto the detector 70 although non-imaging detection can be applied by system 43. It is noted that the electron optics 60 may include the beam splitter 110. The electron optics 60 can include lenses or any other electro-static components (60′) positioned between the beam splitter 110 and detector 70 to focus the SE electron 24 and 26 on detector 70. System 44 of FIG. 4 differs from system 41 of FIG. 1 by including a beam splitter energy 110 positioned above the pellicle 30 and an energy filter 90 that can be set for masking secondary electrons such as those that belong to the first group of secondary electrons SE1 24. It is noted that the masking can be achieved by the position and shape of the detector 70. The detector 70 can be positioned and shaped in locations in which it is expected to detect emitted electrons of one type and not emitted electrons of another type. For example, the detector 70 can have (see FIG. 1) a central aperture that allows passage of the primary electrons 21 and also allows a passage of emitted electrons of the first group of secondary electrons SE1 24 to pass through without being detected. FIG. 5 illustrates method 200 according to an embodiment of the invention. Method 200 may start by stage 210 of directing by electron optics, primary electrons towards a pellicle that is positioned between the electron optics and the lithography mask. The primary electrons exhibit an energy level that allows the primary electrons to pass through the pellicle and to impinge on the lithographic mask. Stage 210 may be followed by stage 220 of detecting, by at least one detector, detected emitted electrons and generating detection signals. The detected emitted electrons are generated as a result of an impingement of the primary electrons on the lithographic mask. Stage 220 may include detecting detected emitted electrons that are backscattered electrons that are emitted from the lithographic mask (detecting BSE). Stage 220 may also include at least one out of (a) masking secondary electrons emitted from the pellicle due to an interaction of the primary electrons with the pellicle (masking SE1), and (b) masking secondary electrons emitted from the pellicle due to an interaction of the backscattered electrons with the pellicle (masking SE2). The detecting of the detected electrons can be performed by a detector that includes multiple backscattered electron detection elements. Stage 220 may include detecting detected emitted electrons that are secondary electrons that are emitted from the pellicle due to an interaction of the backscattered electrons with the pellicle (detecting SE2), wherein the backscattered electrons are emitted from the lithographic mask. Stage 220 may also include at least one out of: (a) masking secondary electrons emitted from the pellicle due to an interaction of the primary electrons with the pellicle (masking SE1), and (b) masking the backscattered electrons (masking BSE). Stage 220 may include imaging the plane of the pellicle on the at least one detector. Stage 220 may be followed by stage 230 of processing, by a processor, the detection signals to provide information about the lithography mask. The information can be indicative of the state of the lithographic mask, defects of the lithographic mask, shape of the lithographic mask and the like. The processing can include any known method or process for extracting information from detection signals. FIG. 6 illustrates method 300 according to an embodiment of the invention. Method 300 may start by stage 310 of receiving detection signals. The detection signals are generated by at least one detector that detects detected emitted electrons. The detected emitted electrons are generated as a result of directing by electron optics, primary electrons towards a pellicle that is positioned between the electron optics and the lithography mask. The primary electrons exhibit an energy level that allows the primary electrons to pass through the pellicle and to impinge on the lithographic mask. The detected emitted electrons may include backscattered electrons that are emitted from the lithographic mask (detecting BSE). The detected emitted electrons may exclude at least one out of (a) secondary electrons emitted from the pellicle due to an interaction of the primary electrons with the pellicle (masking SE1), and (b) secondary electrons emitted from the pellicle due to an interaction of the backscattered electrons with the pellicle (masking SE2). The detected emitted electrons may include secondary electrons that are emitted from the pellicle due to an interaction of the backscattered electrons with the pellicle (detecting SE2), wherein the backscattered electrons are emitted from the lithographic mask. The detected emitted electrons may exclude at least one out of (a) secondary electrons emitted from the pellicle due to an interaction of the primary electrons with the pellicle (masking SE1), and (b) backscattered electrons (masking BSE). Stage 310 may be followed by stage 320 of processing, by a processor, the detection signals to provide information about the lithography mask. The information can be indicative of the state of the lithographic mask, defects of the lithographic mask, shape of the lithographic mask and the like. The invention may also be implemented in a computer program for running on a computer system, at least including code portions for performing steps of a method according to the invention when run on a programmable apparatus, such as a computer system or enabling a programmable apparatus to perform functions of a device or system according to the invention. The computer program may cause the storage system to allocate disk drives to disk drive groups. A computer program is a list of instructions such as a particular application program and/or an operating system. The computer program may for instance include one or more of: a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system. The computer program may be stored internally on a non-transitory computer readable medium. All or some of the computer program may be provided on computer readable media permanently, removably or remotely coupled to an information processing system. The computer readable media may include, for example and without limitation, any number of the following: magnetic storage media including disk and tape storage media; optical storage media such as compact disk media (e.g., CD-ROM, CD-R, etc.) and digital video disk storage media; nonvolatile memory storage media including semiconductor-based memory units such as FLASH memory, EEPROM, EPROM, ROM; ferromagnetic digital memories; MRAM; volatile storage media including registers, buffers or caches, main memory, RAM, etc. A computer process typically includes an executing (running) program or portion of a program, current program values and state information, and the resources used by the operating system to manage the execution of the process. An operating system (OS) is the software that manages the sharing of the resources of a computer and provides programmers with an interface used to access those resources. An operating system processes system data and user input, and responds by allocating and managing tasks and internal system resources as a service to users and programs of the system. The computer system may for instance include at least one processing unit, associated memory and a number of input/output (I/O) devices. When executing the computer program, the computer system processes information according to the computer program and produces resultant output information via I/O devices. In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims. Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. Each signal described herein may be designed as positive or negative logic. In the case of a negative logic signal, the signal is active low where the logically true state corresponds to a logic level zero. In the case of a positive logic signal, the signal is active high where the logically true state corresponds to a logic level one. Note that any of the signals described herein may be designed as either negative or positive logic signals. Therefore, in alternate embodiments, those signals described as positive logic signals may be implemented as negative logic signals, and those signals described as negative logic signals may be implemented as positive logic signals. Any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality may be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected,” or “operably coupled,” to each other to achieve the desired functionality. Furthermore, those skilled in the art will recognize that boundaries between the above described operations merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments. Also for example, in one embodiment, the illustrated examples may be implemented as circuitry located on a single integrated circuit or within a same device. Alternatively, the examples may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner. Also for example, the examples, or portions thereof, may implemented as soft or code representations of physical circuitry or of logical representations convertible into physical circuitry, such as in a hardware description language of any appropriate type. Also, the invention is not limited to physical devices or units implemented in non-programmable hardware but can also be applied in programmable devices or units able to perform the desired device functions by operating in accordance with suitable program code, such as mainframes, minicomputers, servers, workstations, personal computers, notepads, personal digital assistants, electronic games, automotive and other embedded systems, cell phones and various other wireless devices, commonly denoted in this application as ‘computer systems’. However, other modifications, variations and alternatives are also possible. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps then those listed in a claim. Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. |
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046648739 | summary | FIELD OF THE INVENTION The invention relates to remotely-controlled manipulator carrier systems for use in large-area cells affected by radioactivity wherein industrial processes are conducted. The cells are part of a nuclear facility for reprocessing irradiated nuclear fuel materials. The cell is defined by an enclosure and a remotely-controlled bridge crane is arranged beneath the ceiling of the enclosure. The bridge crane includes a vertical, rotatable guide column mounted on a supporting member arranged above the process components in the cell. A support is movably mounted on the guide column for vertical movement and an extendible arm is arranged on this support. The extendible arm is adapted to receive a manipulator or other remotely-controlled device to perform manual-like maintenance operations on the process components and equipment arranged in the large-area cell. BACKGROUND OF THE INVENTION Facilities for the reprocessing of irradiated nuclear fuel materials are equipped with so-called hot cells for holding the components needed for conducting the industrial processes associated therewith. In these radiation-shielded cells, the process components are arranged in scaffold-like structures or racks as they are sometimes referred to. The maintenance work within the hot cell affected by radioactive radiation should be conducted preferably without the necessity of operating personnel entering the hot cell. Solutions have been therefore sought to conduct the maintenance work by means of remotely-controlled equipment which can be movable within the hot cell. For this purpose, it is desirable that the racks holding the components used in the industrial processes be arranged in mutually adjacent rows longitudinally along the walls of the hot cell. In this way, a center passageway is formed along which the remotely-controlled equipment for the maintenance work can be moved and for exchanging the individual process components or exchanging fully-loaded racks. For the maintenance work and the exchange of individual process components or of entire racks, a combination of remotely-controlled machines for performing manual-like operations are utilized. The overhead bridge crane passes over both rows of racks and the center passageway. After all connections and conduits are disconnected, the racks or heavy individual components are lifted from their anchor locations with the aid of the overhead bridge crane. The bridge crane then moves the racks or individual components horizontally into the transport passageway. The control room for the remotely-controlled bridge crane is located outside of the hot cell. In addition to the overhead bridge crane, a manipulator carrier apparatus is provided which can act in the horizontal direction from the central transport passageway to engage the process components and to position operating devices, maintenance device and tools. This manipulator carrier apparatus opens up the possibility for utilizing electrical servo and power manipulators as well as providing the capability for the future use of robots and programmed apparatus. Manual-like operations are performed on small components with the available manipulators and lifting devices in dependence upon the configuration of the carrier apparatus. The manipulator carrier apparatus includes a vertical guide column arranged on a second overhead bridge crane movable below the first overhead bridge crane in the direction of the longitudinal axis of the hot cell; or, if desired, the column can be mounted on a half bridge movable below the overhead bridge crane. The rotatable guide column includes an extendable arm mounted on a support which can be moved vertically up and down. The extendable arm is adapted to receive the tools or equipment for performing manual-like operations. A division of work is achieved with the arrangement of the first overhead bridge crane and the manipulator carrier apparatus. The remotely-controlled first overhead bridge crane is used primarily for holding and transporting pipe connections, components and individual racks. If necessary, the first overhead bridge crane can take over the lowering and holding of tools as well as separating and welding equipment. The manipulator carrier apparatus which is movable along the central passageway serves to guide and hold impact wrenches or other special tools which are needed to effect disassembly work. Further, the manipulator carrier apparatus and second overhead bridge crane can be adapted for accommodating video equipment or other helpful ancilliary devices. SUMMARY OF THE INVENTION It is an object of the invention to provide an arrangement to improve the utilization of the remotely-controlled equipment so that both the first overhead bridge crane and the manipulator carrier apparatus can be brought to work at the same location simultaneously. The system of the invention performs remotely-controlled, manual-like operations in a large-area hot cell of a facility for reprocessing irradiated fuel materials. The hot cell is an enclosure with process equipment disposed along at least one longitudinally extending wall thereof whereby a canyon-like passageway is defined which extends in the direction of the longitudinal axis of the enclosure. The system of the invention includes a first overhead bridge crane having a trolley movable thereon in a direction transverse to said longitudinal axis. The trolley includes hoist means for lowering and raising a device for engaging and moving a component of the process equipment in a first vertical plane transverse to the axis. A second overhead bridge crane is disposed beneath the first overhead bridge crane. Elevated track means guides the first and second bridge cranes in the enclosure in respective horizontal planes and in the direction of the longitudinal axis. The second overhead bridge crane includes an elongated supporting member arranged transversely to the longitudinal axis of the enclosure above the passageway and engages the track means for movement therealong. A manipulator assembly has a mast that extends downwardly into the canyon-like passageway from the supporting member. The supporting member and the mast conjointly define a second vertical plane transverse to the longitudinal axis of the enclosure. The manipulator assembly further includes manipulator means for performing manual-like operations on the process equipment and is mounted asymmetrically on the mast so as to be on one side thereof and in a third vertical plane transverse to the longitudinal axis of the enclosure. Means are provided for moving the first overhead bridge crane along the track means to bring the first vertical plane into coincidence with the third vertical plane so as to permit movement of the engaging device in the third vertical plane clear of the elongated supporting member whereby both the manipulator means and the engaging device can be brought simultaneously to a predetermined work location at the processing equipment. During maintenance work involving both the first overhead bridge crane for lifting and holding a component of the process equipment and the manipulator carrier apparatus made up of the above-mentioned manipulator assembly and second overhead bridge crane, the execution of manual-like movements by the manipulator assembly and the lifting movement of a component of the process equipment by the first overhead bridge crane at the same work location do not interfere with each other. The guide column or mast and the hoist cable of the first overhead bridge crane are displaced with respect to each other and are in two different vertical planes of the hot cell. |
040000389 | summary | BACKGROUND OF THE INVENTION The invention relates to a nuclear power station having a reactor arranged in natural rock or bedrock, with machine groups formed with the use of turbines, compressors, generators or other apparatus, with gas conduits and with a safety chamber for delimiting the machine groups relatively to the atmosphere. A nuclear power station of this kind is known from the periodical "Nuclear News," May 1971, Pages 36 to 39. Above a reactor buried in rock there is arranged a safety chamber constructed in an underground pit, in which the machine groups and gas conduits required for converting the thermal energy from the reactor into electrical energy are placed and which delimits the machine groups and gas conduits relatively to the atmosphere. The periodical also draws attention to the possibility of constructing the safety chamber as a cave in the rock. This known nuclear power station does not in fact disturb the appearance of the landscape and may be sufficiently protected from the environment and also from damaging the environment but the individual machine groups are set up in a single large safety chamber. The outlay involved as regards work makes it necessary to limit the construction of the machine groups. It is felt necessary because of lack of space to make various compromises at the expense of the optimum design or layout for the machine groups. SUMMARY OF THE INVENTION The present invention has as its object to provide a nuclear power station arrangement wherein the builder can design the machine groups with greater freedom of choice. This object is achieved in a nuclear power station of the type initially described according to the invention in that the individual machine groups and the gas conduits are arranged in rooms which are hollowed out in the rock, for supporting and receiving these, that at least one tunnel hollowed out in the rock leads from the atmosphere to each machine group, and that the safety chamber is formed by a tunnel chamber which is situated adjacent the machine group and is secludable from the atmosphere. It is advantageous if each tunnel has a cross-section which is sufficient to allow the transport of each machine group to which it leads. In this way a nuclear power station is obtained which is integrated in natural rock. The rock takes over the task of receiving and supporting the machine groups. The large safety chamber is substantially dispensed with. It is simply necessary to hollow out the appropriate rooms from the rock and the tunnels. This means a considerable saving in costs. The safety chamber has also been reduced to small tunnel chambers or compartments which adjoin the individual machine groups and can be closed off relative to the atmosphere. When designing the machine groups the designer is given much greater freedom of choice. The machine groups are also shielded from one another in a more satisfactory manner. The nuclear power station is completely safe as regards to influences from the environment. It also meets the requirements regarding safety in its possible effects on the environment in a more satisfactory manner. |
052672859 | claims | 1. Apparatus for suppressing formation of vortices in coolant fluid of a nuclear reactor having a main core support, a generally hemispherical lower plenum below said main core support and reactor core coolant inlets, said apparatus comprising: a generally planar plate suspended below the main core support in the lower plenum and disposed generally parallel to said main core support; a plurality of support columns connecting said plate to said main core support; and a plurality of openings through said plate maintaining uniform coolant flow and pressure across reactor core coolant inlets of the reactor wherein said plate includes a generally circular inner ring portion having at least one opening through a central portion thereof, a plurality of relatively spaced spokes extending generally radially outwardly from said inner ring, and an outer ring portion connected to outer ends of said spokes and separated from said central portion by said spokes, at least some of said openings being located between said inner ring and said outer ring and separated from one another by said spokes. said support columns are connected to said outer ring portion. said support columns are angularly spaced circumferentially around said outer ring portion. said secondary core support columns are secured to said inner ring of said plate to brace said secondary core support columns against movement. said plate is positioned about 2 to 4 feet below said main core support. said plate is positioned in said lower plenum at a height equal to about 55 to 75 percent of the radius of curvature said lower plenum. a generally planar plate suspended in the lower plenum below and disposed generally parallel to the main core support of the reactor, said plate having a generally planar inner ring portion with an opening in a central portion thereof, four spaced spokes extending generally radially outwardly from said inner ring, an outer ring portion connected to outer ends of said spokes, and four relatively spaced openings defined between said inner ring and said ring portion and separated from one another by said spokes to maintain uniform coolant flow and pressure across the reactor core coolant inlets of the reactor; a plurality of support columns angularly spaced around said outer ring portion and connecting said plate to the main core support; and secondary core support means disposed in the lower plenum and having a plurality of secondary core support columns depending from said main core support, said secondary core support columns being secured to said inner ring of said plate to brace said secondary core support columns against undesired movement resulting from the circulation of the coolant fluid in the, lower plenum. said plate is positioned about 2 to 4 feet below the main core support. said plate is positioned in said lower plenum at a height equal to about 55 to 75 percent of the radius of curvature of said lower plenum. providing a generally planar vortex-suppressing plate having a generally circular inner ring portion having an opening in a central portion thereof, four relatively spaced spokes extending generally radially outwardly from said inner ring, on outer ring portion connected to outer ends of said spokes, and four relatively spaced openings defined between said outer ring and said inner ring and separated from one another by said spokes a plurality of openings therein; suspending said plate in a lower plenum of said reactor beneath and generally parallel to the main core support; and circulating the reactor coolant fluid in said lower plenum past said plate for distribution to the reactor core coolant inlets. suspending said plate about 2 to 4 feet below said main core support. positioning said plate in said lower plenum at a height equal to about 55 to 75 percent of the radius of curvature of said lower plenum. 2. The apparatus of claim 1, wherein 3. The apparatus of claim 2, wherein 4. The apparatus of claim 3, for the nuclear reactor further having secondary core support columns depending from said main core support, and wherein 5. The apparatus of claim 4, wherein 6. The apparatus of claim 4, wherein 7. Apparatus for suppressing vortices in coolant fluid circulating in a generally hemispherical lower plenum of a reactor vessel of a nuclear reactor, said nuclear reactor having a main core support disposed within said reactor vessel above said lower plenum and a plurality of reactor core coolant inlets through said main core support said apparatus comprising: 8. The apparatus of claim 7, wherein 9. The apparatus of claim 8, wherein 10. A method of suppressing formation of vortices in reactor coolant fluid in a reactor vessel of a nuclear reactor, said reactor having a main core support, a generally hemispherical lower plenum and a plurality of reactor core coolant inlets disposed within said reactor vessel, comprising the steps of: 11. The method of claim 10, further including: 12. The method of claim 10, further comprising: |
description | This application claims priority to French Patent Application No. 1060121, filed Dec. 6, 2010, the entire contents of which are incorporated by reference herein. The invention relates to a method for processing a diffuse radiation spectrum through a material exposed to incident radiation, in order to extract the primary diffuse radiation spectrum. It relates also to an associated processing device and a computer program for this processing method. The application domain of the invention extends in the first place to the spectrometry of diffuse X rays or gamma rays, in particular the analysis of materials, but it comprises also other diffusion spectrometries. This type of spectrometry can be used in the study of materials, for instance the detection of explosives. The diffusion spectrometry of X rays is based on exposing a material to incident X rays with energy equal to a few tens to a few hundreds keV. When they encounter the material on which they are projected, the X photons induce different types of interaction with the material: fluorescence or internal conversion (photoelectric effect during which the photon transfers all its energy to the material which returns it afterwards), inelastic diffusion, (or Compton effect which includes a change in the direction of the photon and a reduction of its energy), creation of positon-electron pairs (uniquely for X rays with very high energy not considered in the present invention), or Rayleigh diffusion (or elastic diffusion, a minority of the considered energies). The invention applies by preference to the processing of signals produced by X ray tubes delivering photons with energy between zero and 300 keV. This kind of photons diffuses in material. Certain characteristics of the studied materials (absorption coefficient μ (E), density, ratio Z Abetween the atomic number Z and the atomic mass number A, and the chemical composition in particular) can in principle be determined on the basis of theoretical knowledge and by obtaining the primary diffuse spectrum of the material exposed to X rays, in other words, the diffusion spectrum which would be obtained in a situation whereby each photon is interacting only once with the material. Patent application FR 0955011, filed on Jul. 20, 2009, shows for instance how certain characteristics of the material can be determined by analyzing the energy spectrum of a diffused radiation through this material, when the latter is submitted to irradiation by X rays. The X ray diffusion spectra comprise an important component of diffused photons that have interacted several times with the material. This component is called the multiple diffused spectrum. Certain information, in particular the density of the material, can be obtained based on the total diffused spectrum because the attenuating character of the material affects in similar manner the two components, primary and multiple, of the diffusion spectrum. To obtain a better estimate of this density, and other physical or chemical information, it is observed that the use of the total diffusion spectrum leads to imperfect results. PCT Application Publication No. WO2007/007247 discloses the use of a system where the studied object is placed between the radiation source and a matrix of pixellated detectors. The detector is calibrated on the basis of characteristic lines of the source, and the multiple diffusion is evaluated for each of these characteristic lines before being extrapolated to the whole spectrum. The invention is placed in another context, because it applies in the first place to an analysis system with a strongly collimated radiation source, and a detector placed in the same half-space as the source opposite the surface of the studied material, which is also strongly collimated. The goal of the invention is to remedy, in the context of this analytic arrangement, the problem mentioned above, by proposing a correction method for the total diffuse radiation spectrum in order to extract from it the primary diffuse radiation spectrum, in other words a radiation that diffused only once in the analyzed material before being detected. For this purpose, a method is proposed for extracting a primary diffuse radiation spectrum from a diffuse radiation spectrum, according to a diffusion angle, coming through a material exposed to incident radiation through a surface, the method comprises the application of a spectral response function corresponding to the multiple diffuse radiation spectrum when a photon of given energy is detected belonging to the primary diffusion spectrum, said spectral response function is organized in the form of a matrix (M), called correlation matrix, of which each value aij corresponds to the number of detected photons, with energy Ej, constituting the multiple diffuse radiation, when a photon with energy Ej is detected, of the primary diffuse radiation. Thanks to this method, the primary diffuse spectrum is easily extracted and access is gained to extensive physical-chemical information. Furthermore, this matrix calculation mode offers the advantage of efficiency, specifically in terms of programming. According to one aspect of the invention, the spectral response function provides a discretized spectrum over a finite number of energy bands. According to one advantageous aspect of the invention, the method comprises an iterative process in which each step comprises an estimation of the multiple diffusion spectrum after a preceding estimation of the primary diffuse spectrum, and a new estimation of the primary diffusion spectrum, by subtracting said estimated multiple diffusion spectrum from the detected diffusion spectrum, said iterative process is continued until a convergence criterion is satisfied related to the successive estimated primary diffuse spectra, or, as a variant, until a predetermined number of iterations is executed. According to one implementation mode, the spectral response function is the result of interpolating a plurality of spectral response spectra with monochromatic exposure of the different materials for the given depth, taking into account only the density of the material. According to another implementation mode, the spectral response function is the result of interpolating a plurality of spectral response functions of the material, for different depths. It is also proposed that the spectral response function is obtained beforehand by experimental acquisition of a response of the material for the given depth. According to one aspect of the invention, the method includes also a preliminary step consisting of estimating a first spectrum of estimated primary diffusion, by multiplying a measured total diffusion spectrum with a multiplication coefficient dependent on the inspection depth. The invention consists also of a device for extracting a primary diffuse radiation spectrum from a diffusion spectrum of a radiation diffused through a material to be analyzed, the device comprises an incident source of radiation suitable for irradiating a surface of the material to be analyzed, a detector suitable for detecting the radiation diffused through said material according to at least one preselected diffusion angle, and means for applying a spectral response function corresponding to the multiple diffused radiation spectrum when a photon is detected with given energy belonging to the primary diffusion spectrum, said spectral response function is organized in the form of a matrix (M), called correlation matrix, of which each value aij corresponds with a number of detected photons, with energy Ei, constituting the multiple diffuse radiation, when a photon is detected, with energy Ej, of the primary diffuse radiation. Advantageously and according to the invention, the radiation source and detector are both strongly collimated. On the other hand, in a variant combination, the detector is by preference placed in the same half-space as the radiation source opposite the irradiated surface of the material to be analyzed. The invention extends also to a spectrometry analysis device comprising such device for extracting the first diffuse radiation spectrum. The invention is also proposing a computer program comprising a sequence of instructions suitable, when executed by a microprocessor, for executing a method according to the invention. FIG. 1 shows the equipment arrangement employed in a method according to the invention. A source 100 emits a radiation X 150 which is collimated by a collimator 110. The radiation 150 forms a beam with very small opening angle, typically a few degrees, around the central axis of the source collimator. The source 100 is mono energetic, or poly energetic, and emits photons in continuous manner over a whole energy range. The beam is projected on a material volume 200 through a surface 210 of the material. The photons X interact with the material according to various physical phenomena mentioned in the introduction. Certain photons interact very close to the surface, while others penetrate deep into the material. A detector 300 equipped with a collimator 310 observes the surface 210 of the material 200 and measures the energy of the photons which it is able to receive thanks to its collimation angle. The positioning relative to source 100 and detector 300 and their placement opposite material 200 and their respective collimation angles define an analysis depth in the material 200, which is such that the photons measured by the detector 300 originate from a volume of material 200 called inspection volume. This volume is determined by the intersection between the solid angles according to which the source and the detector see the material volume 200. The respective collimation axes of source and detector intersect in the analyzed material. The analysis depth, or inspection depth, corresponds to the distance between the material surface and this intersection. The detector detects the photons emitted by the source and diffused according to an angle θ′=π−θ in the inspection volume. When θ′=0, there is no inelastic diffusion: the photon follows a rectilinear trajectory. Since source 100 and detector 300 are installed on the same side of surface 210, we are speaking of rear diffusion spectrum or retrodiffusion spectrum. FIG. 2 shows the diffusion spectrum measured for a given material 200. The x-axis represents the energy of the photons which extends typically from 20 to 100 keV, and the y axis represents the intensity of the detected radiation for a given energy in arbitrary unit. The total diffusion spectrum 1000 is decomposed in a primary diffusion spectrum 1100 and a multiple diffusion spectrum 1200 as mentioned in the introduction. FIG. 1 shows the trajectory of a primary diffusion photon 410. By definition, its trajectory changes only once: it is diffused only once before reaching the detector. The diffusion takes place according to the diffusion angle θ′, and occurs in the inspection volume. Just like the previously mentioned photon, these photons form the primary diffusion radiation according to angle θ′, constituted of photons diffused only once, according to this diffusion angle θ′, in the inspection volume. The trajectory is also represented of a multiple diffusion photon which was the object of various interactions with the material 200 and which changes trajectory at each of these interactions. It undergoes multiple diffusions before reaching the detector. We speak then of multiple diffuse radiation, because the radiation is constituted of photons having diffused more than once before reaching the detector. To be noted that also other photons come from the material volume 200, like photons 410 and 420, at other angles and are not measured by the detector 300. Therefore, they do not appear on the spectra of FIG. 2. A portion of the primary diffuse radiation and the multiple diffuse radiation is detected by the detector. The detector is not capable of distinguishing between these two components. The detector measures the energy of the radiation spectrum, of the total diffuse radiation, noted DT. The latter is composed of a component corresponding to the primary diffusion spectrum, noted DP, previously defined and a component corresponding to the multiple diffusion spectrum, noted DM, previously defined. The total diffusion spectrum is the sum of the primary diffusion spectrum and the multiple diffusion spectrum.DT=DP+DM FIG. 3 shows the spectral response of the diffuse radiation in reaction to monochromatic exposure of material 200 in the arrangement of FIG. 1. The material used for the example is Delrin (registered trademark) and the monochromatic excitation 2000 is performed at 150 keV. The inspection volume is at a depth of 4 cm, the angle between the incident beam and the collimation axis of the detector is 120°. As expected, it is observed that the spectral response 2100 is composed uniquely of energies smaller than the monochromatic excitation energy 2000. In an analysis system as shown in FIG. 1, in other words using retrodiffusion, measurement and collimation both of the source and the detector, the primary diffuse spectrum in response to a monochromatic excitation is linked in objective manner to the activation energy, and is in essence monochromatic. It is indicated by reference 2200 in FIG. 3. Outside the energy measured in close proximity of an energy value corresponding to the whole primary diffusion spectrum 2200, which in FIG. 3 is 104.14 keV, the whole diffusion spectrum 2100 is constituted of the multiple diffuse component DM. This spectrum DM is represented in discretized form on a limited number n of energy detection channels, by column vector ( a 1 a j … a n ) ,where aj is the intensity of the multiple diffuse radiation at energy Ej. The vector depends naturally of the material and the analysis depth. The material can be irradiated according to a large number of monochromatic energies Eirradiation-i, each energy Eirradiation-i being such that the primary diffuse radiation, according to angle θ′, detected by the detector has energy Ei. The totality of the responses of the material to monochromatic excitations, discretized for a specific number of energy detection channels is represented in the form of a correlation matrix. M = ( a 11 a 1 j … a 1 p a i 1 a ij … … … … … … a n 1 … … a np ) In this matrix, each value aij corresponds to the proportion of detected photons constituting the multiple diffuse radiation, with energy Ei when a photon is detected, with energy Ej, of the primary diffuse radiation. In this manner, each sum of coefficients aij according to a column j of this matrix corresponds to the number of detected multiple diffused photons Nmultiples-j when a primary diffuse photon with energy Ej is detected. N multiples - j = ∑ i a i , j Each column Cj of matrix M represents the energy spectrum of the multiple diffuse radiation detected when a primary diffuse photon with energy Ej is detected. This function is called spectral response function and corresponds to the detection of a primary diffuse photon with energy Ej. The method according to the invention is using the correlation matrix M to extract in iterative manner the component of the multiple diffusions in the experimentally measured diffusion spectrum. It involves an iterative process, allowing for an estimation of the primary diffusion spectrum DP. With each iteration k, an estimate is made of the primary diffusion spectrum, noted DPk. This estimate is made by using the previously defined correlation matrix, according to the following algorithm:DMk=M*DPk-1 (1)DPk=DT−DMk (2)where: DMk: estimate of the multiple diffusion spectrum during iteration k; DPk-1: estimate of the primary diffuse spectrum established at iteration k−1 when k≠1. Exceptionally, when k=1, DPk-1 is the initial estimate of the primary diffusion spectrum; DT: total diffusion spectrum, in other words diffusion spectrum measured by the detector. The iterations continue either according to a predetermined number of iterations, or until a convergence criterion is reached, as described below. The iterative process comprises an initial estimate of the primary diffuse spectrum (initialization stage) estimation of the multiple diffusion spectrum DMk corresponding to the primary diffusion spectrum of the preceding iteration DPk-1 or, during the primary iteration, of the initial estimation of the primary diffuse spectrum (DP0). This estimation is done by means of the correlation matrix M. a new estimation of the primary diffusion spectrum DPk, performed by subtracting the estimated multiple diffuse spectrum DMk from the measured diffusion spectrum DT, a new iteration step during which the multiple diffuse spectrum is evaluated on the basis of the estimation of the primary diffuse spectrum. According to the method of fixed point inversion, the two components are separated in the physically measured spectrum. The initial primary diffuse spectrum, indicated by DPk=0 is initially estimated on the simple basis of the measured total diffusion spectrum DT, multiplied by a global attenuation factor, indicated by α, between 0.3 and 1 in function of the inspection depth, for materials with density between 0.7 and 2.2. For an inspection depth of 2 cm, a value of 0.6 is used, and for an inspection depth of 4 cm, the value is 0.45. These values can be adjusted with each start up. In this way,DPk=0=αDT Knowing the total diffuse spectrum DT (this is the spectrum measured by the detector), and the initial estimation value of the primary diffuse spectrum DPk=0, the previously described steps (1) and (2) are completed, in iterative manner, until the convergence criterion is achieved or until the number of iterations reaches the predetermined number. A possible convergence criterion is the distance between the successive estimates of the two primary diffusion spectra, DPk-1 and DPk, this distance can be expressed as follows DP k - DP k - 1 = ∑ E ( DP k ( E ) - DP k - 1 ( E ) ) 2 ≤ ɛ ( 4 ) In this expression, the term E designates the energy and the term ε designates the convergence criterion. In each iteration the primary diffuse spectrum is represented in the form of a vector . ( dp 1 dp 2 … dp p ) k ,and the multiple diffuse spectrum is represented also in the form of a vector ( d m 1 d m 2 … d m n ) k .The latter is deduced through matrix calculation by the operation ( d m 1 d m 2 … d m n ) k = ( a 11 a 12 … a 1 p a 2 1 a 22 … … … … … … a n 1 … … a np ) · ( dp 1 dp 2 … dp p ) k - 1 . The energy discretization step and the dimensions of the previously described vectors are by preference identical. Also, the number of discretization channels n of the multiple diffusion spectrum is, by preference, selected equal to the number of channels p of the primary diffusion spectrum, In this case the matrix M is square. The iterative process is illustrated in FIGS. 4 and 5. FIG. 4 represents the multiplication of an intensity dpi of the estimated primary diffuse spectrum 3000 with energy i=40 keV with a column aij, where j varies from 1 to n, represented by the reference 3100 in the lower part of the figure, the result being a fraction 3200 of the estimated multiple diffuse spectrum 3300, visible on the right side of FIG. 4 and expressed in the form of a vector ( d m 1 d m 2 … d m n ) ,which is added to the spectrum fractions calculated for energy values smaller than i to obtain an intermediate total 3300. FIG. 5 shows the multiplication of another intensity dpi of the primary diffuse spectrum with energy i with a column aij, referenced 3110, the result is a second fraction of the estimated multiple diffuse spectrum 3210, the different fractions are then added to obtain the estimated multiple diffuse spectrum 3300. An important element of the invention is the use of the correlation matrix M, allowing for the estimation of the multiple diffusion spectrum DMk corresponding to a given primary diffusion spectrum DPk. This matrix must be established for a specific position of the source and the detector, relative to the analyzed object. Furthermore, the matrix depends of the following parameters: the angle θ between the collimation axes of detector and source, which conditions the diffusion angle θ′(θ=π−θ) the nature of the material constituting the analyzed object the depth of the inspection volume, which is determined by the intersection of the collimation axes of the detector and the source. Correlation matrix M can be constructed experimentally, by successive use of different sources of mono energetic X rays or gamma rays with energy Eirradiation-j, as radioactive sources with known radiation, or a synchrotron radiation, which in addition has the advantage of having great energy. The total diffusion spectrum is then measured, such as the spectrum shown in FIG. 6, and referenced 4000. The method proceeds then in three steps: First, the primary diffusion peak is determined. Starting from the total diffusion spectrum 4100, the peak 4200 is isolated corresponding to the detected primary diffused radiation. This peak extends on both sides of a known energy, Ej corresponding to the diffusion energy Eirradiation-j of the monochromatic radiation emitted by the source according to the diffusion angle θ′. This peak is finer the more the source and the detector are collimated. By preference, the half-height width of this peak corresponds to an energetic band smaller than a few keV. This peak 4200 corresponds with the primary diffusion radiation component in the detected total diffusion spectrum. During a second step, the multiple diffuse spectrum is determined. By eliminating the primary diffusion component 4200 from the total diffusion spectrum 4100, the multiple diffusion spectrum 4300 is obtained. To this end, the “under the peak” bottom is determined, in other words the continuous curve, for instance a linear curve, joining the two extremities of the 4300 spectrum from both sides of the basis of the peak 4200. Algorithms for determining the signal under a peak can be used, which are currently employed in the domain of X ray or gamma ray spectrometry. Finally, during a third step, the normalization takes place. The multiple diffusion spectrum 4300 is divided by the quantity of photons corresponding to the primary diffusion peak 4200. The multiple diffusion spectrum normalized in this manner corresponds then to the multiple diffusion spectrum obtained when a photon of primary diffuse radiation is detected. This spectrum corresponds also to a column Cj of the correlation matrix M, of which the index j corresponds with the energy Ej of the observed primary diffusion peak, the latter is determined by the energy Eirradiation-j of the radiation source and the diffusion angle θ′. The three described steps can be performed by simulation instead of experimentally, for instance by using a Monte Carlo type code for particle transport known to a person skilled in the art, and in particular MCNP, GEANT or PENELOPE. Furthermore, the correlation matrix can be constituted by combining simulation and experimentation. Renewing this operation by successively using, or simulating, different mono energetic sources, it is possible to obtain several columns of the matrix, corresponding to as many primary diffusion energies Ej, the latter are calculated after knowing the diffusion angle θ′ and the emission energy Eirradiation-j of the source employed. These different columns can be interpolated among them, for instance in linear manner, in order to construct a complete matrix, the energy discretization step between each row and each column corresponding respectively to the energy discretization step of the vector representing a multiple diffusion spectrum and to the energy discretization step of the vector representing the primary diffusion spectrum. Such a matrix is created for different measurement configurations, by varying the following parameters: the nature of the sample material position, in the sample material, of the intersection of the collimation axes of detector and source, this position corresponds to the inspection depth. In this way, for a given diffusion angle, a plurality of correlation matrices M (nature, depth) is obtained. For instance, when the diffusion angle θ′ is 120°, corresponding to an angle separating the collimation axes θ of 60°, the multiple diffusion spectrum can be simulated corresponding to a mono energetic source with energy varying between 10 and 150 keV, per 10 keV step. Fifteen simulations are then necessary. This corresponds to primary diffusion photons with energy between 9.7 keV (when the energy of the source is equal to 10 keV) and 104.14 keV (when the energy of the source is equal to 150 keV). The matrix is then completed by interpolation. These matrices are constructed for different sample materials, which can be polyethylene, water, Plexiglass (registered trademark), nylon (registered trademark), Delrin, cellulose, Kynar (registered trademark), ammonium nitrate or Teflon (registered trademark). For each material, a correlation matrix is determined for each inspection depth, the latter varying for instance between 0 and 7 cm, in steps of 0.5 cm. For the same material, matrices can also be interpolated according to the depth. FIG. 7 shows an implementation example of the iterative method for estimating the primary diffusion spectrum, previously described, using plexiglass material and a depth of 2 cm. Excellent convergence of this estimation is observed. The convergence is estimated here by calculating a normalized distance between two successive estimated primary diffuse spectra and comparison between this distance and a threshold value ε according to an equation of the type DP k - DP k - 1 = ∑ E ( DP k ( E ) - DP k - 1 ( E ) ) 2 ≤ ɛ The value is reached after 5 iterations. FIG. 8 shows also the convergence of the estimated primary diffuse spectrum for plexiglass studied at 4 cm depth. We observe good convergence regardless of the importance of the multiple diffusion relative to the primary diffusion. Materials to be explored which are not listed in the pre-established data base are the object of an interpolation, for determining their correlation matrix, starting from the matrices constituting this base, according to their density. The object of the interpolation is each coefficient of the correlation matrix M, and can be a linear interpolation. The results obtained with this approximation are excellent, because in the analytic system of FIG. 1, it is determined experimentally that the spectral distribution of the multiple diffusion for an energy of incident radiation does not depend on the material, which allows for limiting the study of the materials to their density, which determines the attenuation over the whole spectral range, for the preparation of the correlation matrix. FIG. 9 shows the spectral distribution of the multiple diffusion for a primary diffusion at 104.14 keV, which does not depend on the material and FIG. 10 shows the attenuation function for the same energy value of primary diffusion. Interpolations can also be made for the values of the incident energy and the depth, in order to effectively adapt the used matrix M to the precise values of the studied situations. The spectrum processing method described in this application can be applied to an unknown material, but of known or estimated density. An already constructed matrix is selected, corresponding to a material with a density close to the density of the unknown material. The extraction of the primary diffusion spectrum allows for more precise determination of the nature of this material, for instance by following the method described in patent application FR 09 55011, filed on Jul. 20, 2009, previously mentioned. It is then possible to repeat an extraction of the primary diffusion spectrum by means of a matrix taking into account the precise nature of the material, as it was previously determined. FIG. 11 shows the excellent convergence observed when determining the primary diffusion spectrum in this implementation mode, in the case a volume of plexiglass is studied at 3 cm depth. Convergence is reached in less than 10 iterations, with an oscillation phenomenon around the final spectrum. FIG. 12 shows the results of the decomposition in primary diffusion and multiple diffusion of a total diffusion spectrum obtained with Delrin studied at 3 cm depth. The results are compared with simulated data of the total diffusion spectrum, the primary diffusion spectrum, and the multiple diffusion spectrum. The invention is not limited to the described implementation mode but extends to all variants within the reach of a person skilled in the art. |
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051788217 | description | DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 of the drawing, a water cooled nuclear fission reactor plant 10 includes a reactor pressure vessel 12, containing a core of fissionable fuel material 14, such as enriched uranium oxide pellets contained within sealed metal tubes grouped into conveniently sized bundles. During reactor power generation operations, the neutron incited fission reaction of the fuel material is controlled by neutron-absorbent control rods or blades positioned into appropriate amounts and patterns of control rod withdrawal to produce prodigious amounts of thermal energy. The core of fuel assemblies is positioned centrally in a lower region of the reactor pressure vessel 12 spaced inward therefrom and substantially submerged in coolant water 16 which circulates through the fuel core 14 to carry away heat and form steam for work, such as driving a turbine for generating electrical power. Control rods 18 containing a neutron absorbing material such boron, are reciprocally moveable into and out from the fuel core 14 to control or govern the rate of the neutron incited fission reaction of the fuel, or to terminate the reaction. This in turn regulates the quantities of heat produced by the fuel core 14 for generating steam to perform work. Typically such nuclear reactor plants 10 are provided with an auxiliary cooling water system, including a supplementary cooling water reservoir containing a standby supply of supplementary water coolant. Cooling water auxiliary systems heretofore were commonly activated and driven with a propellant gas under pressure, such as nitrogen, as a means for propelling supplementary water coolant from a reservoir tank into the reactor vessel 12. A propellant gas source for a supplementary coolant reservoir normally consisted of high pressure gas supply tank connected with the reservoir through fluid conduit. Such means are susceptible to leakage and in turn malfunction. The reactor pressure vessel 12 of the plant 10 contains a fuel core shroud 20 encircling the inward spaced fuel core 14 as shown in FIGS. 1 and 2. The fuel core shroud 20 extends a distance both above and below the surrounded fuel core 14 to form a fuel core lower plenum area 22 comprising the region within the shroud beneath the fuel core and a fuel core upper plenum area 24 comprising the region within the shroud above the fuel core. The fuel core shroud 20 is typically an open ended cylinder in configuration, and is of smaller diameter than the pressure vessel 12 being positioned inward away from the side wall of the reactor pressure vessel 12 to provide an annular area 26 between the outside wall of the fuel core shroud 20 and the inside wall of the reactor pressure vessel 12. The annular area 26 forms a downward flow path for the circulation of coolant water 16 through its cooling cycle comprising condensed coolant feedwater 16 supplied by the vessel inlet 28 along with reactor vessel recirculating liquid water coolant 16 flowing downward through the annular area 26. The coolant water 16 then continues down and around the lower edge of the fuel core shroud 20 and upon reversing flow direction, the coolant water 16 passes upward within the fuel core shroud 20. Thus the circulating coolant water 16 flows in sequence up through the core lower plenum area 22, the fuel core 14 and the core upper plenum area 24. On passing through the heat producing fuel core 14 the coolant water 16 absorbed heat therefrom converting a portion of the coolant water to steam. The steam admixed with the remaining liquid water coolant on passing through the core upper plenum area 24, and beyond, are substantially separated by apt means whereupon the steam exits from the pressure vessel 12 to perform work. The remaining liquid water coolant 16 again reverses its flow path above the top of the fuel core shroud 20 and passes down within the annular area 26 along with some returned feedwater usually originating as condensed steam and purified make up water usually originating from power station condensate water storage tanks or evaporators to endless repeat this cooling cycle. This circulating coolant water system maintains the heat producing core of fissionable fuel submerged within the coolant water flowing thereover as a means of governing the reactor's temperature and in turn the reactor vessel pressure through continuous heat transfers from the fuel core to the circulating coolant water and the evaporation of a portion thereof into steam. However, in the unlikely event of a significant breach of a coolant water containing component of the reactor, such as a main pipe, with a resultant reactor depressurization and with a resultant substantial loss of coolant water from about the heat producing fuel core uncovering same, and/or interrupting the circulation of the coolant water, the fuel core and associated components within the reactor pressure vessel soon overheat. In accordance with this invention a standby supply of coolant water for more completely submerging the heat producing core of fissionable fuel of the reactor is provided for any instances involving inadvertent loss of coolant water. Coolant water of the standby supply in this invention is retained within the reactor pressure vessel, and the activation and the impelling or driving means are inherently passive responding to any overheating within the reactor pressure vessel. Referring to the drawings, at least one elongated chamber 32 is provided within the annular area 26. Elongated chamber(s) 32 is provided with an open lower end, and a closed upper end having a small orifice bleed vent 34. Small orifice bleed vent 34 in the closed upper end of chamber 32 is preferably vented through a small diameter tube such as a capillary tube 36 passing through the shroud 20 into the core upper plenum area 24. With the foregoing means and arrangement of this invention, coolant water 16 flowing through the reactor pressure vessel 12 during normal operating conditions fills the elongated chamber 32 through its open lower end replacing the initial gaseous contents as the gaseous contents are vented out therefrom over a prolonged period of time through bleed vent 34. Thereafter, upon the occurrence of a loss of coolant water 16 due to an accident or the like, the inevitable resultant depressurization of the hot reactor coolant will everywhere throughout the body of liquid comprising coolant water 16 produce steam formation in the form of small bubbles in a process well known as steam flashing. The flashing formation of steam within the coolant water contents of the water filled elongated chamber 32 will similarly occur because of the comparably hot conditions of this water. This inherently produced steam rising to the bleed vented closed top and rapidly expanding within the chamber 32 as the reactor depressurization transient proceeds will drive the remaining coolant water contents of the chamber 32 out through the open lower end into the pressure vessel 12 supplementing the leaking volume of coolant water 16 for submerging and cooling the heat producing fuel core. Thus, supplementary coolant water is supplied from a source within the reactor pressure vessel 12 and is applied by inherently passive means. Referring to FIGS. 1 and 3 of the drawings, a preferred embodiment of this invention comprises a plurality of spaced apart elongated chambers 32 arranged in an encircling configuration around and within the annular area 26 to provide for an enhanced supply of supplementary coolant water. The elongated chambers 32 desirably are of tubular-like construction of suitable length and diameter to reside in annular area 26 without effectively obstructing coolant water flow therethrough. An alternative or additional measure of this invention of either replacing or modifying the bleed vent 34/capillary tube 36 means and/or the passive steam driving phenomenon, comprises providing a duct 38 communicating from the vent 34 in the closed top of chamber(s) 32 to a vent and source of auxiliary pressure external of the reactor pressure vessel 12. Namely, the duct 38 extends out from the reactor pressure vessel 12 and is in communication with a valve for venting gases from the chamber(s) 32, and/or with a source of fluid pressure 40 to either provide the driving force to expel supplementary coolant water from chamber(s) 32, or augment inherently produced steam in chamber(s) 32 in expelling the supplementary coolant water out into the reactor vessel. Another embodiment of this invention is illustrated in FIGS. 2 and 4 of the drawings. In this version of the invention a supplementary coolant water chamber 42 of annular configuration is provided. Chamber 42 induces an open lower end and closed upper end having at least one small orifice bleed vent 44 communicating with a capillary tube 46 for bleeding gases from the chamber. Alternatively annular chamber 42 can be provided with a duct 38 extending outside of the reactor pressure vessel 12 for venting and/or connecting to source of auxiliary gas pressure. Annular chamber 42 extending around annular area 26 can be formed by providing an annular skirt 48 having a top wall section 50 extending generally inward from the inside of the reactor pressure vessel 12 into a limited portion of the annular area 26, with a side wall section 52 extending therefrom downward within annular area 26, generally concentric to the reactor pressure vessel 12 and the fuel core shroud 20. Thus, the outer portion of annular area 26 is closed off except at its bottom to provide a supplemental coolant water chamber 42 while the inner portion of annular area 26 remains open and available for the normal circulation of coolant water. |
054266866 | claims | 1. An x-ray source for producing masked x-ray illumination over a spatially-extended area of a semiconductor workpiece, the source comprising: a laser beam generating means for producing a laser light beam having a cross-sectional area that is commensurate in size with the spatially-extended area of the semiconductor workpiece; a spatially-extended photoelectron emitter means, intercepting the laser light beam over a light intercept area substantially as large as the laser light beam cross-sectional area, for producing electrons by the photoelectric effect over an electron production area substantially as large as the light intercept area; a high voltage means for generating an electric field for accelerating the produced electrons as an electron beam wavefront over an area substantially as large as the electron production area; a spatially extended metal foil, positioned to intercept the electron beam wavefront over substantially its entire area, for producing x-rays that are spatially extended over substantially the entire electron intercept area in response thereto; and an x-ray opaque mask, positioned to intercept the spatially-extended x-rays over substantially the entire area thereof, for masking the x-rays in order to produce masked x-rays over a spatially extended area; wherein because the produced x-rays are masked over substantially the entire area thereof, because the x-rays are produced over substantially the entire electron intercept area, because the electron intercept area is substantially the entire area of the electron beam wavefront, because the area of the electron beam wavefront is substantially as large as the area of light intercept, because the area of light intercept is substantially as large as the laser light beam cross-sectional area, and because the laser light beam cross-sectional area is commensurate in size with the spatially-extended area of the semiconductor workpiece, the masked x-rays are produced over an area that is also commensurate in size with the spatially-extended area of the semiconductor workpiece. a laser means for producing pulses of laser light that constitute a temporally intermittent laser beam. a high voltage switching means selectively operable to energize the high voltage means for a selected period of time for producing said wavefront of electrons during said period of time. a means for producing said laser beam as pulses in synchronization with the energizing of the high voltage means. an electrical switch selectively operable to energize the high voltage means in response to and in synchronization with said laser beam pulses. a photocathode; a source of a high voltage potential between the anode and the cathode. a substantially planar photocathode; illuminating with a laser light beam having a cross-sectional area that is commensurate in size with the spatially-extended area of the semiconductor workpiece a commensurately spatially-extended area of a photoelectron emitter in order to produce electrons by the photoelectric effect over the spatially-extended photoelectron emitter area; generating a high voltage electric field in order to accelerate the produced electrons as a wavefront of electrons, the wavefront occupying a spatially-extended area commensurate in size with the spatially-extended photoelectron emitter area from whence the electrons arose; intercepting the spatially-extended wavefront of electrons with a commensurately spatially-extended area of metal in order to produce x-ray radiation over the spatially-extended area of intercept; and masking the produced x-ray radiation with a x-ray radiation-opaque mask occupying a spatially extended area commensurate in size with the size of the metal in order to produce masked x-rays over a spatially extended area; wherein the cross-sectional area of the laser light beam, the photoemitter area, the area of the wavefront of electrons, the area of intercept and the x-ray radiation-opaque mask are all commensurately spatially extended, and are commensurate in size with the spatially-extended area of the semiconductor workpiece. masking the produced x-ray radiation with a mask occupying a spatially extended area and positioned against the spatially extended metal foil; and receiving the masked x-ray radiation in a photoresist sensitive thereto. illuminating with the laser light the spatially extended area of a spatially-extended photocathode consisting essentially of a semiconductor in combination with a metal. a spatially-extended photocathode consisting essentially of a semiconductor in combination with a metal. a substrate; a layer upon the semiconductor substrate. illuminating with laser light a spatially extended substantially planar area of a spatially extended photoelectron emitter in order to produce electrons by the photoelectric effect over the spatially-extended substantially-planar area; generating a high voltage electric field in order to accelerate the produced electrons as a wavefront of electrons, the wavefront occupying a spatially extended planar area; and intercepting the spatially extended wavefront of electrons with a spatially extended substantially planar metal foil in order to produce x-ray radiation over the spatially-extended substantially-planar area of intercept; masking the produced x-ray radiation with a substantially planar mask occupying a spatially extended area and positioned against the spatially-extended substantially-planar metal foil; and receiving the masked x-ray radiation in a photoresist that is sensitive thereto. 2. The x-ray source according to claim 1 wherein the laser beam generating means comprises: 3. The x-ray source according to claim 1 comprising: 4. The x-ray source according to claim 3 wherein the laser beam generating means comprises: 5. The x-ray source according to claim 4 wherein the high voltage switching means comprises: 6. The x-ray source according to claim 1 wherein the spatially extended photoelectron emitter means comprises: 7. The x-ray source according to claim 1 wherein the photoelectron emitter means consists essentially of pure metal having a low work function. 8. The x-ray source according to claim 7 wherein the pure metal having a low work function consists essentially of a metal from the group of Ta, Sm, and Ni. 9. The x-ray source according to claim 1 wherein the metal foil consists essentially of aluminum. 10. The x-ray source according to claim 1 wherein the spatially extended photoelectron emitter means comprises: 11. A method of producing masked x-ray illumination over a spatially-extended area of a semiconductor workpiece., the method comprising: 12. The method of producing x-rays according to claim 11 particularly adapted for lithography, the method further comprising: 13. The method of producing masked x-ray illumination over a spatially extended area according to claim 11 wherein the illuminating comprises: 14. The method of producing masked x-ray illumination over a spatially extended area according to claim 13 wherein the illuminating of the spatially-extended photocathode consisting essentially of a semiconductor in combination with a metal serves to illuminate a semiconductor selected from the group consisting essentially of cesium and cesium antimonide and oxides of cesium and cesium antimonide. 15. The method of producing masked x-ray illumination over a spatially extended area according to claim 13 wherein the illuminating of the spatially-extended photocathode consisting essentially of a semiconductor in combination with a metal serves to illuminate a metal selected from the group consisting of tantalum, copper, silver, aluminum and gold, and oxides of tantalum, copper, silver, and aluminum, and halides of tantalum, copper, silver, and aluminum. 16. The method of producing masked x-ray illumination over a spatially extended area according to claim 13 wherein the illuminating is of the spatially-extended photocathode consisting essentially of the metal deposited on the surface of the semiconductor. 17. The method of producing masked x-ray illumination over a spatially extended area according to claim 13 wherein the illuminating is of the spatially-extended photocathode consisting essentially of the metal substantially homogeneously mixed in bulk with the semiconductor. 18. The x-ray source according to claim 1 wherein the spatially-extended photoelectron emitter means comprises: 19. The x-ray source according to claim 18 wherein the spatially-extended photocathode's semiconductor is selected from the group consisting essentially of cesium and cesium antimonide and oxides of cesium and cesium antimonide. 20. The x-ray source according to claim 18 wherein the spatially-extended photocathode's metal is selected from the group consisting of tantalum, copper, silver, aluminum and gold, and oxides of tantalum, copper, silver, and aluminum, and halides of tantalum, copper, silver, and aluminum. 21. The x-ray source according to claim 18 wherein the spatially-extended photocathode spatially-extended photocathode consists essentially of the metal deposited on the surface of the semiconductor. 22. The x-ray source according to claim 18 wherein the spatially-extended photocathode consists essentially of the metal substantially homogeneously mixed in bulk with the semiconductor. 23. The x-ray source according to claim 18 wherein the spatially-extended photocathode's semiconductor comprises: 24. The x-ray source according to claim 23 wherein the spatially-extended photocathode's metal layer is sputtered on the photocathode's semiconductor substrate. 25. The x-racy source according to claim 23 wherein the spatially-extended photocathode's metal layer is annealed to the surface of the photocathode's semiconductor substrate. 26. A method of x-ray lithography comprising: |
052232069 | description | DETAILED DESCRIPTION OF THE INVENTION In accordance with this invention, the two main components of a composite constructed nuclear fuel container, comprising a tubular container component, or tube stock unit for forming same, and a barrier liner or lining stock unit for forming same, each of some what different metal compositions such as a zirconium alloy and a zirconium metal are separately heat treated at optimum conditions for producing specifically desired properties in each prior to the assembly and uniting into a single composite unit. Thus, the tubular container component, or tube stock unit therefor, is separately heat treated. For example, a unit composed of the commercially available zirconium alloy marketed as zircaloy-2 (U.S. Pat. No. 2,772,964, and No. 4,164,420) can be heated up to about 970.degree. C. uniformly throughout to convert the alpha phase microcrystalline structure of this alloy composition to a substantially alpha plus beta phase structure, and then rapidly cooled to preserve the heat induced structural phase. Additionally the barrier liner components, or lining stock unit therefor, is also separately heat treated. For example, a unit composed of a relatively pure zirconium metal (U.S. Pat. No. 4,372,817) can be heated up to about 900.degree. C. uniformly throughout to convert the alpha phase microcrystalline structure of this metal composition to a substantially beta phase structure, and then rapidly cooled to preserve the heat induced structural phase. Following their separate optimized heat treatments, the tubular container component and the barrier liner component, or the stock units therefor, are assembled together and combined by inserting the hollow barrier liner into and through the length of the tubular container. The combined units are then metallurgically bonded by conventional means, such as explosive bonding, into a single integrated composite of a lined tubular container for nuclear fuel. Typically the separately heat treated components for composite constructed nuclear fuel containers, comprise large diameter container tube stock and liner stock which are assembled into a composite are then reduced together in circumference in at least one and preferably a series of reduction steps repeated until the desired diameter is attained for a fuel container. In accordance with the practice of the art, the composite unit is annealed following each reduction to relieve stresses imposed by the compression of the reductions. Annealing temperatures should be below the previously imposed microstructure modifying heat treating temperatures to preclude reversing the modified microstructure, for example not above about 600.degree. C. |
abstract | There is a structure element, in particular for radiation shielding constructions, having at least one floor plate and at least one wall section and/or at least one ceiling section. The structure element is characterized in that the at least one wall section and/or the at least one ceiling section comprise/comprises at least two shell elements made from metal, plastic and/or wood and a layer which lies in between and is made from radiation shielding materials. In addition, a construction, in particular a radiation shielding construction, is proposed having at least one floor plate and/or ceiling plate which delimits a storey and a structure element described above. |
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claims | 1. A method for the production of a radioisotope or for the treatment of nuclear waste, the method comprising steps of:providing a solution of heavy water and target material in a shielded irradiation vessel, wherein the target material includes fissile material;generating an electron beam;directing the electron beam onto an x-ray converter to generate bremsstrahlung photons; andintroducing the bremsstrahlung photons into the solution, wherein the bremsstrahlung photons have energy sufficient to generate photoneutrons by interacting with nuclei of deuterons present in the heavy water and the photoneutrons to cause fission of the fissile material. 2. The method of claim 1, further comprising a step of producing yttrium-90 via neutron capture of yttrium-89 in the solution. 3. The method of claim 1, wherein the electron beam has an energy within the range of about 5 to 30 MeV. 4. The method of claim 3, wherein the electron beam has an energy within the range of about 5 to about 15 MeV. 5. The method of claim 1, wherein the x-ray converter has an atomic number of at least 26. 6. The method of claim 5, wherein the x-ray converter has an atomic number of at least 71. 7. The method of claim 1, wherein the solution includes a sub-critical amount of fissile material. 8. The method of claim 7, wherein the solution includes neutron capture material as additional target material. 9. The method of claim 7, wherein the fissile material comprises uranium-235. 10. The method of claim 9, further comprising a step of recovering molybdenum-99 as a fission product of the uranium-235. 11. The method of claim 7, wherein the fissile material comprises uranium-233. 12. The method of claim 7, wherein the fissile material comprises plutonium-239. 13. The method of claim 1, further comprising recovering the radioisotope from the solution. 14. The method of claim 13, wherein said step of recovering comprises filtering. 15. The method of claim 13, wherein said step of recovering comprises interacting the solution with sorbent. 16. The method of claim 15, further comprising rinsing the sorbent. 17. The method of claim 13, further comprising recycling the solution. 18. The method of claim 17, wherein said step of recycling comprises treating the solution with chemicals, adding heavy water, and adding target material. 19. A method for the production of a radioisotope or for the treatment of nuclear waste, the method comprising steps of:providing a solution of heavy water and target material in a shielded irradiation vessel, wherein the target material includes fissile material and fissionable material;generating an electron beam;directing the electron beam onto an x-ray converter to generate bremsstrahlung photons; andintroducing the bremsstrahlung photons into the solution, wherein the bremsstrahlung photons have energy sufficient to generate photoneutrons by interacting with nuclei of deuterons present in the heavy water and the photoneutrons to cause fission of the fissile material. 20. A device for production of a radioisotope or for the treatment of nuclear waste, the device comprising:an electron beam generator configured to generate an electron beam having an energy in the range of about 5 MeV to 30 MeV;an x-ray converter configured to receive an electron beam from said electron beam generator;a shielded irradiation vessel configured to receive bremsstrahlung photons from said x-ray converter and containing a solution of heavy water and fissile material. |
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050193210 | description | DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates an embodiment of a module 1 of a fusion generating device in accordance with the principles of the invention. A fusion power core 2 is shown housed within two clam-shaped regions 4a and 4b of a blanket 4. The blanket 4 absorbs radiation emanating from the fusion power core as a result of the fusion reaction. It is the function of the blanket 4 to absorb such radiated energy which appears mostly as neutrons generated in the fusion reaction. These neutrons could be used to generate fission in fission plates incorporated as neutron multipliers in the blanket assembly or simply for the production of heat by neutron slowing and neutron capture reactions. Such heat energy is extracted by means of a coolant passing through conduits 8 which are shown diagrammatically as penetrating the blanket region 4a. The conduit 8 may in fact be a plurality of cavities or conduits passing through both regions 4a and 4b of blanket 4 and may be of the multiple artery type so as to cover a large region of the blanket to absorb maximum amount of heat energy. The fluid conduit 8 passes to heat exchange means and pump means indicated at 10. The blanket material may, for example, be composed of graphite, fluoride salts, beryllium or other materials as well known in the art. The coolant material may be water or oil or any other suitable fluid serving a cooling/heat extracting function Heat exchange means 10 may be connected to thermal/electrical or thermal/mechanical power generating equipment. Also shown in FIG. 1 is a heat exchange means and pump means 12 associated with a conduit 14 which passes through the blanket 4 and into the fusion power core 2. The coolant flowing through conduit 14 serves to cool the field coils utilized to provide the magnetic confinement within the fusion power core 2. Only one such conduit 14 is illustrated although it is understood that a plurality of conduits may be provided (and a single or an associated plurality of heat exchange means and pump means as required) for cooling various sections of the magnetic field coils. The coolant stream may provide heat energy to heat exchange means 12 for utilization in thermal/electrical conversion equipment in order to produce electrical power therefrom and/or thermal/mechanical equipment for generation of mechanical (shaft) energy. The coolant/thermal extraction system provided by conduits 14 and heat exchange means 12 may be separate and independent from the coolant/thermal extraction system employed for the blanket 4 or alternately the two systems may utilize common components. The temperatures within the coils of the fusion core must be kept below the melting or structurally limiting temperatures of the coil materials (copper or aluminum coils, for example). The heat developed within the blanket 4, however, has no such restriction and the coolant within the blanket may thus be heated to considerably higher temperatures than the coolant passing through the fusion power core (conduits 14). The thermal/electrical conversion equipment, for example, associated with the higher temperature coolant will thus be able to operate at higher thermal/electrical conversion efficiencies than possible for the lower temperature coolant. For a fusion power core of the toroidal type, coolant is typically provided in the toroidal field coils but may also be provided for other field coils if desired (ohmic heating, vertical field or auxiliary heating coils). Additionally, coolant means similar to that shown by conduits 14 and heat exchange and pump means 12 may be provided for other regions of the fusion power core, such as a region between the toroidal shell and the toroidal coil as more fully set forth below. An alternate or additional means for cooling and obtaining thermal energy from the fusion power core 2 and blanket 4 is provided by heat exchange means and pump means 15 together with conduits 16. In this embodiment, the fluid inflow to module 1 passes between the blanket regions 4a and 4b and is heated by the fusion power core 2 which effectively serves to preheat the coolant which is subsequently heated to higher temperatures by energy from the blanket region 4. In this manner, a single coolant may be utilized with a single thermal/electrical conversion unit. Blanket 4 may also contain a tritium breeding section 17 which may contain for example lithium utilized to capture neutrons for the breeding of tritium for subsequent use in the D,T fusion reaction. Heat exchange and pump means 18 together with conduits 20 may be utilized to cool the lithium breeding section 16, or alternately, a molten fluoride salt of lithium (or lithium plus beryllium, for example) may be used to provide for tritium breeding as well as self-cooling. Appropriate tritium extraction apparatus 22 is connected to the conduits 20 to extract the tritium for subsequent utilization. An electrical control means 24 is utilized to provide the current to drive the various field coils within the fusion power core via a plurality of power conductors 26. Thus, in the case of a toroidal or tokamak-type device, conductors 26 serve to provide the necessary current for the toroidal field as well as for the ohmic heating transformer, auxiliary heating coils, vertical coils and the like. The fusion power core 2 is provided with a containment region 28 for housing the plasma. In the embodiment in which the toroidal-type fusion power core is utilized, the containment region 28 is simply the toroidal shell or vacuum cavity containing the plasma gas. Means are provided for evacuating the containment region 28 such as by utilizing a vacuum pump 30. Gas feeding means 32 are also shown for supplying the fusible fuel or gas to the containment region 28. The gas feeding means 32 may comprise for example a supply of D,T gas and remotely operable valve means for controlling flow of gas into the containment region 28. Each fusion power core 2 also may be provided with diagnostic ports 33 for measuring plasma position, density and temperature as is well known in the art. As stated below, the fusion power core 2 may be of the tokamak type and include the required toroidal magnetic field coils and ohmic heating coils. However, it is envisioned that other fusion power cores may be utilized wherein other types of magnetic confinement are obtained, e.g., stellarator confinement principles, for example. The description herein is presented in terms of specific embodiment of the tokamak-type fusion reactor and specifically utilizing a D, T fusion reaction process However, it is clear that other fusion reaction processes, for example, the D,D or D,He.sup.3 may be utilized separately, or simultaneously with D,T. A prime consideration of the present invention is the fact that the fusion power core 2 is removable from the blanket 4 and, in fact, is disposable, or recyclable. The high temperatures and high fields attained in the fusion power core result in an extremely high radiation flux significantly higher than the first wall loading heretofore assumed acceptable for practical large scale fusion reactor designs. As a result of such a high radiation flux on the first wall of the fusion power core, the fusion power core may deteriorate over a relatively short time. In this circumstance, the present invention allows for and provides a means for replacing the entire fusion power core. Depending upon specific operating parameters replacement could be required at time intervals on the order of weeks to months. However, the relatively small size of the fusion power core 2 will allow economical means of removal and subsequent disposal and/or reprocessing/recycling thereof and replacement by a new fusion power core utilizing the same blanket 4. Consequently, the blanket regions 4a and 4b are made separable, and the fusion power core 2 may be removed therefrom. For tokamak-type fusion power cores, it is possible to reprocess the fusion power core 2 such that the copper and other materials within the core may be utilized again. As an exemplary conventional frame of reference, assuming a D,T reaction, the fusion power core may have a radius on the order of 1 meter and height of approximately 1 meter. Each blanket region may typically be on the order of 1 meter thick. In practice the exact thickness and shape of the blanket is somewhat arbitrary and may be de signed to provide adequate thickness for capture of neutrons generated in the fusion power core. Additionally, the first wall of the blanket shell may be made of high Z or other materials which allow n,2n reactions to enhance blanket neutron yield thus assuring a simple T-breeding design. As shown in FIG. 2A, a plurality of modules 1.sub.1 . . . 1.sub.n, each having a corresponding blanket 4.sub.1 . . . 4.sub.n and cores 2.sub.1 . . . 2.sub.n may be arranged together to form a power generating system wherein corresponding coolant conduits 8'.sub.1 . . . 8'.sub.n are separately connected to one or more heat exchange and pump means (not shown). An alternate arrangement is shown in FIG. 2B wherein a plurality of modules 1'.sub.1, 1'.sub.2 . . . 1'.sub.n is shown with series connected coolant conduits 8".sub.1, 8".sub.2 . . . 8".sub.n. In any such series arrangement, a system bypass means 9 may be provided so that upon replacement of any individual fusion power core, the remaining assembly of modules 1' may be left operational. In FIGS. 2A and 2B, the arrows labeled 8'.sub.1, 8'.sub.2 etc. and 8".sub.1, 8".sub.2 etc. are used to represent both the blanket coolant/thermal extraction system and corresponding fusion power core coolant/thermal extraction system whether they be separate or integral systems as taught in FIG. 1. Obviously, in FIG. 2B, the fusion power core (blanket) coolant/thermal extraction system could be connected in series with a separate plurality of blanket (fusion power core) coolant/thermal extraction system for the modules. It is advantageous in these configurations to closely pack the modules 1 together so that neutrons escaping one module may be trapped in an adjacent module thereby increasing overall efficiency. FIG. 2C shows yet another embodiment of the invention wherein a plurality of fusion power cores are surrounded by a single blanket 34. FIG. 3 illustrates an electrical power generating system comprising a fusion reaction room containing an array of modules 1" such as those illustrated in FIG. 2A. Each module in the array is connected to an electrical supply, gas feeding and vacuum unit in accordance with FIG. 1 to supply both the electrical power to each individual fusion power core and the necessary gas feeding and vacuum pumping means. Also interconnected to each of the modules 1" are heat exchange means and conduits which are connected in accordance with elements 8, 10, 12 and 14 of FIG. 1 to extract heat from the blanket units as well as to provide cooling means and heat extraction means for the fusion power cores. A low temperature heat exchange means 42a forms part of the fusion power core coolant/thermal extraction system and is connected to conduit means feeding each fusion power core. For simplicity of illustration, only one such connecting line is shown. A low temperature condenser 44a is connected to the low temperature heat exchange and pump means 42a and to one state of turbine 46. A high temperature heat exchange and pump means 42b forms part of the blanket coolant/thermal extraction system for the modules 1" and is connected to conduit means for feeding each blanket. Again, for simplicity of illustration, only one such conduit means is illustrated. The high temperature heat exchange and pump means 42b is connected to a high temperature condenser 44b and to a second stage of turbine 46. The turbine 46 drives a generator 48 which supplies electrical energy to an electrical gridwork which may in turn be fed by a plurality of units similar to those shown in FIG. 3. Alternatively, instead of or in addition to the electrical conversion one may utilize the turbine 46 to provide mechanical energy such as shaft rotational energy. A remotely operable means is also provided for removing any given fusion power core from its corresponding blanket so that the fusion power core may be handled, moved, disposed of, or reprocessed to recycle valuable metals, dispose of radioactive contaminants, and/or to remanufacture and refabricate an additional (replacement) fusion power core. The remotely operable means may comprise remote handling means 51 and a recycle and disposal means 52. Remote handling means 51 may comprise an overhead crane and means for connecting and disconnecting the various conduits and cables feeding the fusion power core 2. A control room 54 is also shown for providing a monitor and control means 56 and to provide office space for personnel. Monitor and control means 56 monitors and controls the operation of the entire power generating plant and, in particular, monitors and controls each of the various elements in FIG. 1 shown associated with module 1. Additionally, plasma position, temperature and density may be monitored via diagnostic ports (33 of FIG. 1) in each module 1". An enlarged top view of a single module 1 is illustrated in FIG. 4. The fusion power core 2 is shown in cross section. The blanket is shown to be composed of two regions 4a and 4b which surround the fusion power core 2. The blanket regions 4a and 4b are also shown in cross section but may not necessarily be taken along the same horizontal plane with respect to each other. The blanket region 4a is shown permeated with an artery array of conduits 8 which serve to remove thermal energy generated by neutrons emanating from the fusion power core 2 and absorbed in the surrounding blanket 4. Although not specifically illustrated in FIG. 4, the blanket region 4b may similarly contain an array of conduits for carrying a cooling/thermal energy extraction fluid. The blanket may be comprised of a fluid material instead of the more commonly utilized solid blanket material. If desired, the fluid material may be circulated to serve both as a neutron absorbing medium and as its own coolant/thermal extraction means, i.e., the fluid may be fed via conduits to heat exchange means. The fusion power core 2 is illustrated in the preferred embodiment as comprising a tokamak-type reactor wherein plasma is contained in cavity region 101 of a toroidal shell 100 which may, for example, be composed of aluminum stainless steel, niobium, molybdenum or the like. The shell may be in the range of approximately one to a few millimeters thick, and may be coated internally with beryllium, carbides, graphite or aluminum oxide for protection. The shell may likewise be coated with an aluminum oxide or other insulating layer on the outside thereof for insulation of the shell from the surrounding conductors. A series of current carrying conductors or disk coils 102 are disposed around the toroidal shell 100 for establishing the toroidal magnetic field. A plurality of spiral grooves 103 may be provided in the disk coil 102 for passage of a cooling fluid therethrough. The grooves 103 communicate with peripheral channels 103a in the disk coils 102. The coolant fluid passing adjacent the disk coils 102 may be connected to heat exchange means as shown in FIG. 1 to remove thermal energy therefrom for utilizing same for the generation of electric power. Between the disk coil 102 and the shell 100 there may be disposed a cooling channel 104 for passage of the cooling fluid around and along the length of the shell 100. The cooling channel 104 is thus in fluid communication with the spiral grooves 103 and peripheral channels 103a. Supporting the shell 100 in the cooling channel 104 are a plurality of supports 105 which may take the form of small button-like elements or rib members surrounding the toroidal shell. The cooling channel 104 around the shell 100 (first wall) is utilized to maintain the shell at controlled temperatures. The channel may typically be on the order of one to a few millimeters wide. If necessary for stress and strength considerations, surrounding the disk coils 102 may be a support means 106 which holds the coils 102 in tension against an outer rib 108 and top and bottom support members 110. The support means 106 thus supports the disk coils 102 and shell 100 from the strong forces produced by the generated magnetic fields. Support means 106 may be fabricated, for example, from steel and may be an integral toroidal unit or a plurality of supports, one for each disk coil 102. If the support means is integral over two or more disk coils, then insulation means are provided between the disk coils 102 and support means 106 to prevent shorting out of the disk coils. The support member 110 as well as the outer rib 108 may be made of aluminum or other material and are typically insulated from the support means 106 by insulation means 112 (made, for example, of aluminum oxide). Support members 110 are held together by means of a central load carrying member 114 (made of ceramic, for example) as well as by sealed joints 116 at the periphery of the support means 106. The fusion power core 2 is provided with ohmic heating coils 120 which may take the form of an air core or saturated iron core transformer. All of the coils illustrated in FIG. 4 are utilized for ohmic heating. Additional auxiliary heating and vertical field coils may also be provided as more clearly illustrated in reference to FIG. 5 discussed below. Various coolant conduits are provided in the module 1 of FIG. 4 such as fluid conduits 124, 125, 126 and 127. Fluid conduits 124 and 125 are inflow and outflow conduits respectively which are associated with shell 100 and disk coil 102. The fluid is passed into the fusion power core 2 and circulates in grooves 103 and channels 103a of the disk coils 102 and within the cooling channel 104 adjacent and exterior to the shell 100. Fluid conduits 126 and 127 are inflow and outflow conduits respectively and associated with the ohmic heating coils (as well as vertical and auxiliary heating coils if desired). Thus, conduits 124, 125, 126 and 127 form part of the fusion power core coolant/thermal extraction system as disclosed in reference to FIGS. 1 and 3. In order to facilitate removal of the fusion power core 2 from the blanket 4 for replacement of the fusion power core, the conduits 124-127 are passed through coupling means 128 before interconnecting to the fusion power core 2. Coupling means 128 permits easy separation of the fluid conduit sections contained within the fusion power core from the external conduits leading to the heat exchange and pump means. Consequently, when the fusion power core is separated from the blanket 4, it is only necessary to disconnect the sections of the fluid conduit at the coupling means 128. Functionally similar coupling means 128' are provided for electrical connections 129 to ohmic heating (OH) coils 120 of the fusion power core 2. The fusible gas, for example, an equal mixture of deuterium and tritium is fed into the cavity region 101 of shell 100 via a fuel inlet conduit 134. Valve means (32 of FIG. 1) are connected to the fuel conduit 134 to regulate the flow of fusible fuel into the plasma cavity region 101. An extraction fuel conduit 136 is connected to pump means (30 of FIG. 1) and is provided to extract the plasma during the gas purge cycle of operation. Both conduits 134 and 136 may be provided with small nozzle means to couple to the cavity region 101. Coupling means 128 may also be provided for the conduits 134 and 136 as shown. FIG. 4 also illustrates in region 4b of the blanket 4 special fluid passages 130 for cooling regions 132 containing lithium used for breeding tritium. The tritium may later be used in the fusion power core for the D,T fusion reaction. Region 132 may contain, for example, canned lithium alloys. A neutron monitor 133 is shown positioned between the fusion power core 2 and blanket 4 to provide a means for measuring the reaction rates within the plasma. The fusion reaction rate may, of course, be indicative of the plasma temperature or density. The plasma temperatures may be determined in a conventional manner as, for example, by utilizing laser interferometer techniques via the diagnostic port 33 (FIG. 1). The overall size of the fusion power core 2 in FIG. 4 is quite small in comparison with conventional tokamak designs. In particular, the fusion power core 2 may have a major radius of approximately 50 centimeters and a minor radius of approximately 20 centimeters. The radial thickness of the disk coils 102 is approximately 10 centimeters and each coil may extend a few centimeters in thickness. One particular coil is illustrated in FIGS. 5, 5A and 5B and employs cooling grooves 103' in the form of radial grooves which may alternately be used instead of the spiral grooves shown in FIGS. 4 and 6. One portion of the disk coil is bent outwardly for alignment with the adjacent disk coil around the toroidal shell 100. The disk coils 102 are arranged around the plasma shell 100 and are placed adjacent to each other to form a complete coil producing the toroidal field. It is contemplated that 176 such disk coils may be utilized either series connected or connected in modular groups such that there are 8 separate coils per each coil group with a total of 22 coil groups. In such an arrangement each coil group would comprise one complete turn and would be electrically connected to the next coil group to form a series current path through the entire plurality of coils. FIG. 6 illustrates an enlarged sectional view of part of the fusion power core as shown in FIG. 4. Fluid conduits 124 and 126 and fuel conduit 134 have already been discussed in relation to FIG. 4. Various field coils are shown in FIG. 6 in addition to the OH coils 120. For example, field coils 142 may be used to provide a vertical field (VF) for positioning the plasma, and coils 144 may be used, if desired, as auxiliary heating coils. Auxiliary heating of the plasma may, of course, be provided by other means such as ripple currents on the VF coils 142, microwave techniques etc. FIG. 7 illustrates a cross-sectional view of the fusion power core showing the disk coils 102 and support means 106 as taken along line 7--7 of FIG. 6. Ohmic heating coils and conduits are now shown for simplicity of illustration. FIG. 7 shows the disk coils 102 with an integral support means 106'. Support means 106' is shown broken away so that the disk coils 102 may be more clearly seen. Each disk coil 102 is wedged-shaped and separated by an insulation means 152 which may take the form of a thin ceramic disk. The insulation means 152 may alternately be provided by an insulating coating on the disk coils 102. FIG. 7A illustrates the disk coils 102 with separate support means 106, one such support means associated with each disk coil 102. FIG. 8 is a side plan view of a module 1 wherein the fusion power core 2 is being removed from the blanket regions 4a and 4b by an overhead crane 160 forming part of the remote handling means 51. The blanket regions 4a and 4b are carried on support means such as a remotely operable trolley 168 for separating the blanket regions to allow removal of the fusion power core 2. For ease of illustration, various fluid conduits, gas and vacuum feed lines and electrical connection lines are not shown. The crane 160 lifts the fusion power core 2 from a support means 170 and moves it to the recycle and disposal means 52 (FIG. 3) for processing. A new or recycled fusion power core 2 is then placed on the support means 170 via the overhead crane 160 and the fluid conduit, gas feed and vacuum lines as well as electrical connections are connected via the remote handling means 51 to the new fusion power core 2. The fusion power core may be driven to ignition and thence to power producing levels in cycles utilizing gas staging techniques as described in copending application Ser. No. 755,794 filed Dec. 30, 1976 and/or utilizing e-beam techniques as described, for example in U.S. Pat. No. 3,831,101 incorporated herein by reference. In practice, each fusion power core 2 of FIG. 3 is cycled through an initial start-up stage, ignition stage and burn stage so that the residual gas in the plasma cavity 101 may be pumped out and a new gas mixture introduced at the beginning of stage 1. Power is switched into each fusion power core 2 in a sequential manner by means of the electrical supply units 40 and monitor and control means 56 of FIG. 3. For example, assume that there are 20 fusible power core units operating at a "burn time" of 25 seconds with a 30-second total cycle time. The control means for the power system activates unit 1 associated with the first fusion power core. Approximately 1.5 seconds later (30/20) power is supplied to unit 2 while continuing power to unit 1. Three seconds later, unit 3 is switched on while continuing power to units 1 and 2, etc., until all units are being driven at the 30-second cycle time. In this manner, an average power output may be supplied by the generator 48. It is expected of course, that not all of the fusion power cores will need replacement at the same time. The replacement of any given fusion power core is thus made as required, but because of the small size and simplicity of the replacement procedure such replacement takes a relatively short time and does not require shutdown of other fusion power cores. Consequently, such replacement will not appreciably affect the overall power output of the generating plant. While the invention has been described in reference to the preferred embodiments set forth above, it is evident that modifications and improvements may be made by one of ordinary skill in the art, and it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically set forth herein. |
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summary | ||
042283809 | summary | BACKGROUND OF THE INVENTION As stated in U.S. Pat. No. 4,068,147 to Daniel R. Wells, there is a need for new techniques that will permit the construction of power generating stations that are relatively small when compared to conventional thermonuclear reactor designs. It is desirable that full size power generating thermonuclear reactors be built in sizes approximately 1000 times smaller than those possible with currently proposed designs. This would enable the utilization of these power plants, for example, in spacecraft intended for deep space missions. Such compact nuclear power plants could also be utilized for marine propulsion. Also, there is a need for a system suitable for facilitating substantially direct conversion of thermonuclear energy to electrical energy without the necessity of employing complex thermal cycle machinery. For various reasons, the currently available systems are not adequate to satisfactorily meet the above needs. Although heating plasma structures entirely be conieal theta pinch compression fields provides a basic solution, previous systems relying essentially on this technique have failed because it involves heating a plasma ring that is moving with respect to the theta pinch coils. This results in a very low coefficient of coupling and a very inefficient heating process. Furthermore, the rate of rise of the compression field must be very fast, thereby requiring the use of expensive and complex equipment. Therefore, there currently exists a need for more efficient and inexpensive means for compressing and heating plasma in thermonuclear devices. SUMMARY OF THE INVENTION Accordingly, a main object of the present invention is to provide an improved system for generating energy by a thermonuclear process which overcomes the deficiencies and disadvantages of the previously employed systems. A further object of the invention is to provide an improved method and apparatus for generating electrical energy by thermonuclear processes which utilizes an improved useful geometrical arrangement of the components of reactors of the TRISOPS type. A still further object of the invention is to provide an improved nuclear reactor of the theta pinch coil type in which the compression coils act through a collapsing Lithium, FLIBE, or other suitable molten metal liner and wherein the liner can be collapsed onto the plasma vortex rings at the center of the reactor chamber after said rings have collided. A still further object of the invention is to provide an improved thermonuclear reactor of the TRISOPS type employing compression coils acting through a collapsing molten metal liner, the liner having flexible stabilizing supporting means, including means to bias it toward the inside wall surface of the associated vacuum chamber. A still further object of the invention is to provide an improved thermonuclear reactor of the TRISOPS type employing compression coils acting through a collapsible molten metal liner, wherein part or all of the molten metal liner may comprise Uranium 238, Thorium 232, or similar thermonuclear fuel material, to provide for neutron capture in fertile material in order to produce fissile fuel, such as Plutonium 239 or Uranium 233, for use in fission reactors, and wherein the liner may or may not produce heat energy for use in generating electrical energy. The present invention comprises an improved method and means for compressing plasma vortex structures by means of a stabilized collapsing molten metal liner. The molten metal liner is "spun" substantially onto the inside surface of a suitable vacuum chamber which is preferably placed in an upright or vertical position so that the molten metal, which is swirled into the liner by a suitable set of nozzles, deposits at the bottom of the vessel where it is pumped out of the chamber, processed, and returned to the top of the chamber. This swirling molten metal liner is stabilized by a flexible metal mesh constructed of stainless steel or other suitable refractory metal. This mesh is normally held near the inside surface of the vacuum chamber by a "scissor-like" articulated cage structure which allows collapse of the liner toward the center line of the chamber, and after the collapse immediately returns the mesh to the wall of the chamber. After the molten liquid metal liner has been swirled onto the collapsible supporting structure adjacent the inside surface of the vacuum chamber, a set of vortex structures or matrices are fired into the chamber from each end through openings in the mesh. These vortex structures move to the center of the vacuum chamber and stop. At this instant a suitable timing mechanism activates a set of compression coils surrounding the chamber. The rapidly rising magnetic field generated by these coils induces large electrical currents in the swirling liner. The electromagnetic Lorentz forces drive the liner toward the axis of the chamber, thus trapping any magnetic fields inside the chamber and increasing the intensity of these fields as the liner collapses. These increasing fields in turn compress and heat the stable vortex structures to thermonuclear temperatures. The entire apparatus operates in a surrounding steady-state magnetic field which allows formation of the vortex structures by the theta pinch guns or ejectors and acts as a "seed" field inside the collapsing liner, which rapidly increases as the liner approaches the center line of the apparatus. Once the compression coils have accelerated the heavy liner toward the central axis, the inertia of the liner forces the hot metal against the compressed seed field, increasing the seed field and heating the plasma structures. The metal mesh is tightly woven so that there can be little or no flow of molten metal through it. The articulated scissor cage mechanism is designed so that it holds the mesh against the inner wall surface of the vacuum chamber until the collapse begins. During collapse, this mechanism closes the ends of the mesh so that no plasma can escape. Spring loading then returns the mesh to the wall and opens the ends of the mesh so that a new set of vortex rings can enter the chamber through the ends. |
claims | 1. A method of liquid waste processing for a nuclear power plant (NPP) with boron control, wherein a waste comprises sodium and potassium salts, the method including:a) introduction of calcium nitrate into a borate solution to provide a resulting composition, precipitation of calcium borate, and separation of the calcium borate from a mother liquor of the separated calcium borate;b) obtaining solutions of boric acid and sodium and potassium hydroxides; andc) electrodialysis with the use of an electrodialysis device with cation-exchange and anion-exchange membranes;wherein in step a) the borate solution, as sodium and potassium salts, comprises nitrates and sulphates of both sodium and potassium, wherein the introduction of calcium nitrate into the borate solution causes a co-precipitation of the calcium borate and calcium sulphate;wherein in step b) the obtaining the solution of boric acid is achieved by treating the co-precipitated calcium borate and calcium sulphate with a solution of nitric acid and separating the calcium sulfate precipitate from a solution of calcium borate, which is followed by treating the solution of calcium borate with nitric acid to cause formation of a precipitate of boric acid and a solution of calcium nitrate, and separating and drying the precipitate of boric acid; andwherein step c) comprises directly subjecting the mother liquor to electrodialysis to obtain solutions of nitric acid and sodium and potassium hydroxides. 2. The method according to claim 1, wherein the calcium nitrate is introduced into the borate solution at a pH of 9.3-11.0. 3. The method according to claim 1, wherein the co-precipitated calcium borate and calcium sulphate are treated with the nitric acid solution until a pH of 5-7 is provided. 4. The method according to claim 1, wherein the calcium borate solution is treated with the nitric acid at a temperature of 10-20° C. until a pH of 1-3 is provided. 5. The method according to claim 1, wherein, after separation of the precipitate of boric acid, the solution of calcium nitrate is added to the borate solution. 6. The method according to claim 1, wherein the electrodialysis of the mother liquor is conducted in a three-chamber electrodialysis device at the ratio of the mother liquor volume Vml in the middle chamber of the electrodialysis device to the volumes of the anolyte Va and the catholyte Vc in the anodic and cathodic chambers, respectively, equal to Vml:Va=1:0.5-1.0 and Vml:Vc=1:0.4-0.6, thereby obtaining the solution of nitric acid in the anodic chamber, and the solution of sodium and potassium hydroxides in the cathodic chamber. 7. The method according to claim 1, wherein the electrodialysis is executed at a current value of 1-3 A and a voltage of 4-10 V. 8. The method according to claim 1, wherein the boric acid precipitate is washed with a nitrate solution of pH 2-3, comprising 30-35 g/l of boric acid. 9. The method according to claim 1, wherein the boric acid precipitate is dried at a temperature not exceeding 60° C. |
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description | Next, with reference to the accompanying drawings, an embodiment of the present invention will be described. FIG. 3 shows a mask pattern and an exposed resist pattern. The mask pattern and the exposed resist pattern are disposed to the strutted mask 11 as shown in FIGS. 1A and 1B. In a stripe connection boundary area (having a width of around 10 to 20 xcexcm), desired mask patterns 1 and 2 are formed with an EB (Electron Beam) mask A-3 and an EB mask B-5. In addition, on the EB mask A-3, a xe2x96xa1-shaped connection inspection box mark 4 (having a width of around 5 to 10 xcexcm) is formed. Likewise, on the EB mask B-5, a square-shaped connection inspection box mark 6 (having a width of around 5 to 10 xcexcm) is formed. In this case, a plurality of xe2x96xa1-shaped connection inspection box marks 4 and a plurality of square-shaped connection inspection box mark 6 are disposed at predetermined positions in the stripe connection boundary area. In this case, the stripe connection area is a boundary of connections of stripes of which a real chip is scanned and exposed for each stripe area having a width of 250 xcexcm. As shown in FIG. 3, a box-in-box pattern 6, 4 is a pattern of which a square pattern is overlapped with a xe2x96xa1-shaped pattern. When a connection error is measured, a light beam is scanned on the box-in-box pattern 6, 4. In other words, the box-in-box 6, 4 is a pattern composed of a square pattern and a hollowed square that surrounds the square pattern. Instead of the box-in-box pattern 6, 4, a slide caliper pattern may be used. The slide caliper pattern is a vernier pattern. As with a size measuring slide caliper, the slide caliper pattern has a shape of which a part of a xe2x96xa1 is cut. By connecting two patterns having different pitches, an error is measured. In other words, the slide caliper pattern is composed of a main scale pattern and a sub scale pattern. Corresponding to the relation of the positions of the main slide pattern and the sub slide pattern, a relative error of the positions of the scales (in this case, a connection error) is measured. The stripe boundary areas are overlapped and exposed with an electron beam. As with the resist pattern shown in FIG. 3, the connection of the exposed stripes is inspected. By measuring the amount of the connection error and extracting the shift component and rotation component, the obtained data and extracted data are fed back to the main exposing process. In the main exposing process, with the box-in-box pattern 6, 4, the connection of patterns can be inspected. Next, with reference to FIG. 4, the operation of the embodiment of the present invention will be described. FIG. 4 is a flow chart showing steps of the electron beam lithographing method according to the embodiment of the present invention. In FIG. 4, at step S1 as an EB mask forming step), a connection inspection box mark or a slide caliper pattern is placed in the vicinity of a stripe connection boundary area of a desired pattern. Thus, an EB mask is obtained. At step 2 as a pilot lithographing step, with the EB mask, a pilot lithographing process is performed. The slide caliper pattern is a pattern of which two sets of line patterns having different pitches are exposed and with the difference the amount of the deviation (error) is measured. At step S3 as a measuring step, with the connection inspection box mark or slide caliber pattern, the connection error between exposed stripes is measured. At step 4 as a main exposing step, the connection error is fed back and then the main exposing process is performed. Alternatively, the connection error may be measured several times at step 3. Whenever the connection error is measured, the measured result is fed back to the main exposing process. At step S5 as an appearance inspecting step, in addition to inspections for the size and overlap accuracy, with the connection inspection box mark or slide caliper pattern, the connection of patterns is inspected. In this case, since the box mark can be measured with a measuring unit, the connections of the patterns can be quantitatively inspected in a short time. In the case of the slide caliper pattern, although it should be manually measured, it can be quantitatively measured with an optical microscope. In this case, since it is not necessary to use a line width measuring SEM that is a vacuum unit, the TAT of the adjustment time becomes shorter than the conventional method. According to the electron beam lithographing method of the embodiment of the present invention comprising the steps of (a) forming a plurality of accuracy evaluation patterns and a desired pattern in a stripe connection boundary area so as to form an electron beam mask, (b) lithographing patterns with the electron beam mask, (c) measuring connection errors of the exposed stripes with the accuracy evaluation patterns, (d) exposing the patterns with the electron beam, and (e) inspecting the connections of the pattern that has been exposed. Thus, the following effect can be obtained. In the electron beam lithographing method according to the present invention, a plurality of accuracy evaluation patterns (box marks and slide caliper patterns) and a desired pattern are formed at a stripe boundary (exposed area) of a full transfer type or large area transfer type electron beam mask. With the electron beam mask, the stripe connected portions are overlapped and exposed. Thus, by quantitatively measuring accuracy evaluation patterns such as box marks or slide caliper patterns using an optical microscope or an automatic measuring unit, the connection adjustment time of exposed strips can be shortened and the connection accuracy can be improved. Although the present invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the present invention. |
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039393534 | abstract | An apparatus for releasably and adjustably mounting a specimen in an electron microscope. A specimen-holding unit is releasably and fluidtightly mounted in an evacuatable chamber and abutting the lens of the electron microscope in the mounted position. The unit is held in place by the pressure differential between the chamber and the atmosphere when the chamber is evacuated. The specimen-holding unit comprises a rotatable specimen-holding shaft disposed eccentrically within a rotatable support member. The specimen is variably positioned under the electron beam by the rotation of the member and the rotation of the shaft with respect to the member. |
050664514 | abstract | A control rod cluster assembly of a nuclear reactor is repeatedly repositioned in a guide tube assembly above a fuel assembly in the reactor core in single steps at separate times during a single fuel cycle. For instance, the single-step repositioning can be repeated once every month or every four months in a twelve month fuel cycle. |
047284820 | abstract | An inservice inspection of a surface area or welds of a nuclear reactor pressure vessel is carried out, without the need to remove the core barrel and lower internals therefrom, by providing access to the annular chamber between the core barrel and the pressure vessel wall through an aperture in the upper flange of the core barrel. An inspection means, such as an ultrasonic testing device is inserted into the annular chamber through the access and positioned proximate the weld to be inspected and inspection of the weld effected by the device. |
051981858 | abstract | Method and apparatus for improving coolant flow in a nuclear reactor during accident as well as nominal conditions. The reactor has a plurality of fuel elements in sleeves and a plenum above the fuel and through which the sleeves penetrate. Holes are provided in the sleeve so that coolant from the plenum can enter the sleeve and cool the fuel. The number and size of the holes are varied from sleeve to sleeve with the number and size of holes being greater for sleeves toward the center of the core and less for sleeves toward the periphery of the core. Preferably the holes are all the same diameter and arranged in rows and columns, the rows starting from the bottom of every sleeve and fewer rows in peripheral sleeves and more rows in the central sleeves. |
054460751 | description | DETAILED DESCRIPTION OF THE INVENTION Any of numerous malleable materials may provide the base for the colored exercise putties according to the invention, such as clays. However, siloxanes and siloxane reaction products are preferred because they will not dry out as water-based compositions will have a tendency to do. Particularly preferred exercise putties according to the present invention include (a) a chain-extended polysiloxane reaction product, (b) optionally a second, normal polysiloxane gum, (c) an internal lubricant such as a monounsaturated fatty acid, (d) any of a number of filler materials, and (e) in the instance of any of the originally colored masses of exercise putty described below, a small amount of pigment. The chain-extended polysiloxane reaction product is formed by reacting a polydiorganosiloxane with a reactant containing oxygen and either boron or tin. Such reactants can be any of several boron and oxygen containing reactants, such as trimethyl boroxane, pyroboric acid, boric anhydride, ethyl borate, esters of boric acid, etc. Where boron is selected as the chain-extending atom, trimethyl boroxine is a preferred reactant. The reactant may also be a tin- and oxygen-containing compound such as dibutyldiacetoxytin. This reactant is reacted with a polydiorganosiloxane that preferably is a hydroxyl end-stopped polydimethylsiloxane fluid having a viscosity of 50,000-100,000 centistokes, a weight average molecular weight of 88,000-103,000, and an average number of siloxyl units per molecule in the range of 1,200-1,400. The reactant attacks the hydroxyl groups on the ends of the polysiloxane chain to yield chain extension through the boron or tin groups. Where boron is used as the chain-extending atom, and because boron is trifunctional, the boron atom will link three polysiloxane chain ends together about fifty to one hundred percent of the time. In a particularly preferred composition, approximately 100 parts by weight of the above hydroxyl end stopped polydimethylsiloxane are reacted with approximately 3 parts by weight of trimethyl boroxine. The reaction is carried out at approximately 200.degree. F. to produce a borosiloxane reaction product. The composition preferably further includes a normal polysiloxane gum having a viscosity on the order of 1,000,000 centistokes and a Williams' plasticity between 120 and 140 mm, inclusive. This second polysiloxane may be any common polydiorganosiloxane gum. While a particularly preferred second polysiloxane is polydimethylsiloxane, the percentage of side group substitutions is largely irrelevant, as the end composition is not to be cured and little or no siloxyl crosslinking will occur. Thus, a methyl vinyl polysiloxane can as easily be used. This second polysiloxane may be trimethyl end-blocked, dimethyl vinyl end-blocked, or end blocked with other groups known in the art. The second polysiloxane is added as a plasticizer to prevent the composition from becoming tacky after extensive kneading, and may be present in the composition in the range of 10 to 50 parts by weight inclusive relative to 100 parts by weight of the chain-extended polysiloxane reaction product. Further, exercise putty which is based on nothing except borosiloxane is self-leveling and has a tendency to pool. This material cannot be left for long on carpeting or macroscopically porous surfaces as it will infiltrate the cracks and holes. Adding the second polysiloxane has the additional effect of providing some body or resistance to this self-leveling effect, such that the resulting mass will be more shape-retaining or clay-like and less fluid-like. A third constituent of the composition is an internal lubricant such as 9-octadecenoic acid, sold commercially under the trademark PAMOLYN 125 oleic acid by Hercules Incorporated of Wilmington, Del. Other monounsaturated fatty acids such as those of C.sub.17 -C.sub.18 carbon chain length can be used. The monounsaturated fatty acid is added to affect the flow properties of the two blended polymers described above and may be present in the end composition at 0.2-2.0 parts by weight per 100 parts of the chain-extended polysiloxane reaction product. A fourth principal constituent is a filler material such as a siliceous or calcareous material. Particular fillers useful for the invention include fumed silica, precipitated silica, celite, ground quartz and others commonly known in the industry. The filler materials selected should not be so highly colored as to affect the desired color of the putty mass in question; on the other hand, certain highly colored fillers such as iron oxide and titanium dioxide may be intentionally used in place of the pigments disclosed below to impart particular colors to the mass. The filler material can be present in the composition from 5 to 45 parts by weight relative to 100 parts of the reaction product. In certain applications, it may be further desired to heat the exercise putty mass prior to giving it to the patient. Certain filler materials will heat up when subjected to microwave radiation. Filler materials which are compounds and complexes containing bound water, such as hydrated silicas, have this characteristic. Representative of such compounds and complexes is precipitated silica, which is also particularly preferred because of its reinforcing capabilities. Precipitated silica has hydrated onto its surface a layer of water molecules. The water molecules themselves have OH bonds which absorb microwave energy; the silica particles heat upon exposure to this energy. Hydrated silicates and other compounds containing bound water are preferred over other water-containing mixtures because bound-water particulate materials will heat up each time after successive exposures to microwave energy. Precipitated silica acquires its boundary layers of water by the process of its manufacture. Precipitated silica is also a preferred filler constituent in that it does not rub off on the hands as carbon black, metals and the various metal oxides have a tendency to do. This is because precipitated silica is wetted by the silicone gum and therefore is retained within the composition. Further, precipitated silica is preferred because it does not mask pigments. Other lubricants may be added to the composition in addition to, and not in place of, the monounsaturated fatty acid. One of these additional additives is petrolatum, which has the particular effect of imparting an anti-sticking property to the composition. Petrolatum may be present in the composition in an amount in the range of 0 to 30 parts by weight relative to 100 parts by weight of the boro- or stannosiloxane reaction product. Glycerine may also be added in the range of 0 to 1 part by weight order to impart a shiny surface to the product. In order to form a particularly preferred composition, 100 parts by weight of a hydroxyl end-stopped polydimethylsiloxane having a viscosity of 70,000 centistokes is reacted with 3 parts by weight of trimethoxyboroxine at 200.degree. F. until a "snow" of polymerized borosiloxane reaction product results. 100 parts of this reaction product is combined with 30 parts polydimethylsiloxane gum have a Williams' plasticity of 130 mm, 1 part 9-octadecenyl acid, and 20 parts precipitated silica. For each of the colored masses initially provided according to the invention (but not the large colorless mass 14 in the embodiment illustrated in FIG. 1), a pigment is also made a portion of the composition. The color, concentration and chemical identity of these pigments is influenced by the order in which the colored masses of which they are constituents are to be combined with the main (initially colorless) mass. The first or earlier colored masses should have pigments which are weak either because they are in a low concentration or because they have a low pigmenting power, such that, when they are combined with the main mass, these pigments will not hide the further addition of stronger pigments. Pigments suitable for use with the invention should be FDA approved, of relatively low cost, and colorfast to ultraviolet light. Preferably, they are not soluble in silicone oil, because if they are they will have a tendency to come out of the putty onto the patient' hands. The pigment concentrations according to the invention are arbitrary but should be low in the first of a series of colored masses and, assuming similar coloring ability among the pigments, increase in later colored masses. Pigment concentrations in each of the colored masses may, for example, range from 1.times.10.sup.-5 parts by weight per 100 parts of the borosiloxane reaction product to 1.times.10.sup.-3 parts therein. Pigments found particularly useful for the invention are noted below with parts by weight relative to 100 parts of the borosiloxane reaction product in a preferred series of compositions: 1. Sky blue: 0.0008 parts titanium dioxide and 0.0004 parts ultramarine blue. PA1 2. Yellow: 0.000018 parts chrome yellow. PA1 3. Red: 0.0001 part C1 red 30 aluminum lake pigment. PA1 4. White: 0.0008 parts titanium dioxide. The smaller masses may be borosiloxane putty, polydiorganosiloxane putty, or mixtures thereof, with the amount of pigments added thereto being about the same regardless of which of the bases is selected. The present invention is preferably provided in a kit, the parts of which are illustrated in FIG. 1. In FIG. 1, a therapeutic kit 10 according to the invention includes a container 12 of a large mass 14 of borosiloxane putty, and a package 16 containing a plurality of pockets containing respective small, colored masses 18-32 of borosiloxane or siloxane putty. The large mass may, for example, be about 50 grams and each of the small masses 18-32 can be approximately 1 gram. The large mass 14 is preferably a colorless borosiloxane-based composition as described above; in its natural, uncolored state, the mass 14 will have a whitish, translucent appearance. Each of the smaller masses 18-32, however, are highly colored, and at least some of the colored masses 18-32 should have colors which are different from the remaining ones of masses 18-32. For example, mass 18 may be sky blue, mass 20 may be yellow, mass 22 may be red, mass 24 may be yellow again, mass 26 may be green, mass 28 may be a different shade of red, mass 30 may be white, and mass 32 may be dark blue. The sequence and hues of colors within the colored masses 18-32 are largely arbitrary. The choice and concentration of the colors should be chosen accordingly to two guiding principles: first, the beginning colored masses, such as mass 18, should be less highly colored, and the ending colored masses such as mass 32 should be more highly colored. Second, the chosen colors should be sufficiently different from each other such that streaks and the like will be quickly apparent to the physical therapist. At a minimum, the colors should alternate; for example, masses 18, 22, 26 and 30 may be colored blue and masses 20, 24, 28 and 32 white. Other preferably bright colors may be added to the ones chosen for masses 18-32 according to design choice. The kneading or manipulative process according to the invention is shown in FIGS. 2a-2c. In FIG. 2a, the main colorless mass 14 is shown being kneaded or manipulated by a patient's hand 34. At this stage, the physical therapist or physician has just given the patient a small colored mass 18, which is still intact and visually distinct within the combined mass. If on his or her next meeting with the patient, the physical therapist or physician discovered that the exercise putty was in this condition, the therapist or physician would know that not very much exercise or manipulation had been performed by the patient. In FIG. 2b, an intermediate condition is shown, wherein streaks 36 of color are beginning to spread themselves throughout the combined putty mass 38. This streaking process will start with relatively large streaks having greatly contrasting color characteristics; as the kneading or manipulation process continues, the streaks will be finer and will have less contrast as compared with the background color of the combined mass 38. In FIG. 2c, the combined mass 38 has taken on a uniform color which results from the combination of the background color of the beginning mass (in this case, the beginning mass 14) and colored mass. In this first iteration, the combined putty 38 would take on a uniform blue color where mass 18 starts out as blue. In this condition, the physical therapist would know that the putty had been manipulated or kneaded to a great extent. The patient may periodically be given further colors from the package 16 and instructed to repeat the process. For example, the blue putty which is the result of a manipulation process in a first period (such as a first day or week) is taken; the patient is instructed to add packet 20 to it, which for example may be 1.5 grams of yellow. Yellow streaks 36 (FIG. 2b) will appear, but on continued flexing the streaks will disappear and a uniform blue/green color will be obtained as is shown in FIG. 2c. Then, on day (or week) three, the blue/green combined mass will be combined by the patient with a third highly colored putty 22. The putty mass 22 may, for example, be 1 gram of red putty. The complete kneading and manipulation of this putty with the combined mass will produce a combined mass which is of a uniform light brown color. This process may be repeated indefinitely with one or more packages 16 of the putty until the combined mass is of a dark uniform color whose color can no longer be easily changed by the addition of further pigment. In an alternative embodiment, only two colors are used. With just two colors, the physical therapist supplies blue putty to the patient on day (or week) one, white putty to the patient on day (or week) two, blue putty on day (or week) three, et cetera. During each period, streaks in incompletely worked putty may be observed, either white streaks in blue or blue streaks in the white/blue blends. This process would continue until the putty is too dark to show streaks. Once the putty has become so highly pigmented that color streaks are hard to see, it can be discarded and the therapy terminated or further exercise putty substituted in its place. In an alternative embodiment, the different colors of the colored putty masses may denote increasing stiffness. The amount of the colored putty would be made more nearly equal to the mass of the colorless putty 14 in this embodiment, and the pigment concentration would be commensurately reduced. In this method, the patient would combine a colored putty of relatively low strength or stiffness with the colorless putty in a first period. In a next period, a portion of the first combined mass may be kneaded and manipulated by the patient with a second colored mass which is stiffer than the first. In summary, a novel exercise putty system and method have been disclosed in which a patient's progress may be easily observed by a physical therapist or physician by the combination of different colored exercise putties. While the detailed description has illustrated and described preferred embodiments, the present invention is not limited thereto but only by the scope and spirit of the appended claims. |
047599111 | summary | BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is related to nuclear fuel elements and in particular to gas cooled nuclear fuel elements. 2. General Background Gas-cooled particle bed reactors (PBR's) being considered for multi megawatt space power and propulsion applications in strategic defense initiative studies are cooled by a gas flowing radially inward through the annular particle bed. These reactors use fuel elements formed from particle fuel wherein the fuel particle bed is confined in the annular space between an outer porous cylinder and an inner porous cylinder. The coolant gas flows out of an axial channel defined by the inner porous cylinder. A variety of fuel elements and fuel particles are known in the art. U.S. Pat. No. 3,992,258 entitled "Coated Nuclear Fuel Particles And Process For Making The Same" discloses coated nuclear fuel particles of low density to accommodate fission gases generated during the use of the fuel particles. U.S. Pat. No. 3,928,132 entitled "Annular Fuel Element For High-Temperature Reactors" discloses a compacted fuel element of annular shape enclosed in a graphite casing constituted by an inner and outer tube. The outer tube is larger than the inner tube and also has a greater coefficient of shrinkage. U.S. Pat. No. 3,361,638 entitled "Pyrolytic Graphite And Nuclear Fuel Particles Coated Therewith" disclosed a nuclear fuel particle having a central core of fissile or fertile material surrounded by a fission-product retentive layer of true pyrolytic graphite. U.S. Pat. No. 3,311,540 entitled "Integral Boiling and Superheating Nuclear Reactor And Pressure Tube Assembly Therefor" discloses a direct cycle integral vapor generating and superheating reactor having, within each pressure tube, a plurality of concentric annular fuel elements clad in metal such as stainless steel. The coolant passes alternately downwardly and upwardly among the fuel elements from the outer flow passage to the inner flow passage. U.S. Pat. No. 3,222,773 entitled "Process For Assembling Concentrically Spaced Nuclear Fuel Elements" discloses a process of assembly for arranging cladded tubular and cylindrical nuclear fuel members within each other. U.S. Pat. No. 3,345,733 entitled "Nuclear Reactor Fuel Elements" discloses a method of constructing a nuclear fuel element of a plurality of part annular plates supported at their longitudinal edges by radial support members to define a series of spaced coaxial tubes. U.S. Pat. No. 2,985,576 entitled "Fuel Element For Nuclear Reactor", U.S. Pat. No. 3,138,534 entitled "Fuel Arrangement For A Neutronic Test Reactor", U.S. Pat. No. 3,165,448 entitled "Nuclear Reactor Core And Fuel Assembly", U.S. Pat. No. 3,422,523 entitled "Process For Fabricating Nuclear Reactor Fuel Elements", and U.S. Pat. No. 3,753,854 entitled "Production Of A Fuel Carbide With A Jacket Of Fuel Nitride, Sulfide, or Phosphide" are representative of the art. Calculations indicate that for uniformly loaded fuel elements the full potential power density of the peripheral fuel cannot be realized because of heat transfer and fuel temperature constraints in the hot region at the inner boundary of the fuel element. SUMMARY OF THE INVENTION The present invention solves the above problems in a straightforward manner. What is provided is a fuel element formed from a plurality of nested rigid porous cylinders in coaxial alignment and suitably sized to allow placement of each cylinder within the cylinder of the next largest size. Varying quantities of fissionable isotopes are distributed on each cylinder to achieve greater optimal power density than that achieved in current particle bed reactors. In view of the above, it is an object of the invention to provide a nuclear fuel element that facilitates zone loading of the nuclear fuel isotopes to enhance power density. It is another object of the invention to provide a nuclear fuel element that accommodates longitudinal thermal expansion. |
abstract | A diagnostic system to monitor digital rod position indication (DRPI) signals of a DRPI system of a nuclear power plant, including a digital diagnostic unit connected between a DRPI display cabinet and a DRPI data cabinet of the DRPI system to monitor digital rod position signals of the DRPI data cabinet. The digital rod position signals include digital rod address signals and digital rod position data signals such that the digital diagnostic unit detects signal level variation and signal timing variation of the digital rod address signals and the digital rod position data signals to determine rod position errors of the DRPI system. |
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abstract | A method of forming tungsten tetraboride, by combining tungsten and boron in a molar ratio of from about 1:6 to about 1:12, respectively, and firing the combined tungsten and boron in the hexagonal boron nitride crucible at a temperature of from about 1600 C to about 2000 C, to form tungsten tetraboride. |
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062228978 | summary | FIELD OF THE INVENTION This invention relates generally to nuclear reactors and, more particularly, to methods and apparatus for inspecting core spray and jet pump riser inlet piping. BACKGROUND OF THE INVENTION A reactor pressure vessel of a boiling water reactor typically has numerous piping systems. Such piping systems are utilized, for example to transport water throughout the reactor pressure vessel. For example, core spray piping delivers water to a reactor core. Over the life of the reactor, the piping is often inspected to verify integrity. For example, the piping welds must be periodically inspected for Inter Granular Stress Corrosion Cracking (IGSCC). Based upon such inspections, the piping may require either repair or replacement. Problems arise when attempting to inspect small or tight radius elbows on small diameter core spray piping and jet pump riser inlet piping systems, using volumetric ultrasonic techniques. Known pipe inspecting apparatus may not be able to sufficiently cover pipe elbow weld areas for volumetric ultrasonic examinations because of an inability to maintain ultrasonic transducer contact with the surface to be examined. It would be desirable to provide an easy to use pipe inspection apparatus and methods for inspecting nuclear reactor piping. Preferably, the apparatus and methods would utilize a motion that more readily conforms to the contours of the pipe elbows. It would also be desirable to facilitate maintaining ultrasonic transducers of the inspection apparatus in sufficient contact with the surface of the piping being inspected to allow the ultrasonic transducers to produce accurate scan readings. SUMMARY OF THE INVENTION These and other objects may be attained by apparatus for inspecting piping and welds of pipe elbows in a reactor pressure vessel of a boiling water reactor which, in one embodiment, includes a scan head having a pair of spaced apart ultrasonic transducer probes and a motor that moves the scan head axially along the pipe elbow. The scan head allows the transducer probes to remain substantially in contact with the pipe elbow while the scan head traverses the pipe elbow. The ultrasonic transducer probes can detect flaws in the piping and the welds of the pipe elbows and are contoured to conform to the piping. The scan head also includes a scan platform, a connector, and a pair of transducer arms each having a first end and a second end. Each transducer arm first end is connected to one of the transducer probes. The connector attaches the transducer arms to the scan platform and permits the transducer probes to orbit freely about the connector producing a gimbals type movement. The freedom of movement of the transducer probes allows the probes to remain in contact with the pipe elbow. The scan platform includes an arcuate cutout having a size and shape to accommodate the piping. The connector slides along the arcuate cutout. As the connector slides along the cutout, the transducer probes and the transducer arms are caused to rotate, at least partially, about a circumference of the piping. The inspecting apparatus further includes a pivot arm having a first end and a second end connected to the scanner platform. A pivot pin is connected to the pivot arm first end. The motor pivots the pivot arm second end about the pivot pin. The pivoting of the pivot arm second end causes the transducer probes to pivot substantially about the pivot pin. A method of inspecting piping and welds of a pipe elbow using the above described scan apparatus includes positioning the scan head and the ultrasonic transducer probes such that the probes are substantially in contact with the pipe elbow surface. The motor is then used to pivot the transducer probes substantially about the pivot pin which allows the transducer probes to travel axially along the pipe elbow while the probes remain in substantial contact with the pipe elbow surface. The scan head, during this pivoting movement, inspects the piping and the welds in the pipe elbow to detect flaws. The scan apparatus moves with a wrist scan motion to enhance the ability of the probes to remain in contact with the pipe elbow surface during the inspection and to enable ultrasonic transducer signals to better penetrate the piping and welds of the pipe elbow. The piping and the weld material of the pipe elbow are inspected as the transducer probes move substantially perpendicular to the weld. In operation, the scan head moves axially along the pipe elbow in a first direction from a first axial point to a second axial point. The connector then slides along the cutout, incrementally rotating, in a raster type manner, the probes. The scan head then moves axially along the pipe elbow in a second direction from the second axial point to the first axial point. Again, the connector slides along the cutout, incrementally rotating, in a raster type manner, the probes. Each incremental rotation of the probes moves the probes about a partial circumference of the piping. The axial movement of the probes and the incremental rotation at the ends of the axial stroke are repeated until the probes have inspected the entire surface of the pipe elbow. A method of positioning ultrasonic transducer probes to examine piping and welds of a pipe elbow begins by locating the above described scan apparatus at the pipe elbow. Since the above described scan apparatus is for use in a reactor pressure vessel of a boiling water reactor, the scan apparatus can be deployed in water to a depth of more than about 60 feet. The scan head is then moved to allow at least a portion of the piping to enter the scan platform cutout. After the transducer probes are positioned substantially in contact with the pipe elbow, the scan head is moved axially along the pipe elbow while the transducer probes are maintained in substantial contact with the pipe elbow. The axial movement of the pipe elbow begins at a first axial point and moves in a first direction to a second axial point. The connector is then moved incrementally along the arcuate cutout which causes the transducer probes and the transducer arms to rotate partially about the circumference of the piping. The scan head is then moved axially along the pipe elbow in a second direction from the second axial point to the first axial point. Again, the connector is moved incrementally along the arcuate cutout which causes the transducer probes and the transducer arms to rotate partially about the circumference of the piping. The axial movement of the probes and the incremental rotation at the ends of the axial stroke are repeated until the probes have investigated the entire surface of the pipe elbow. The above described scan apparatus allows the ultrasonic transducers to remain in contact with the outer surface of the pipe elbow until the entire inspection has been completed. The wrist scan motion enhances the transducers abilities to contour to the piping surface and enable the transducer signals to better penetrate the surface. The wrist scan motion also enhances the signal dynamics of the transducers enabling the operator to discern between geometric reflections and cracks. |
043280714 | description | In FIG. 1, there is shown the main vessel of an integrated-type fast nuclear reactor 2 suspended from a shield roof 4, said roof being in turn supported by a massive concrete structure 6. Within the interior of the main reactor vessel 2 are placed the primary vessel 8 and the diagrid 10 which supports the reactor core 12 and serves to supply this latter with liquid metal coolant. Since reference is made to a reactor of the integrated type in the example under consideration, there is also shown a primary heat exchanger 14 suspended from the shield roof 4 and a primary pump 16 which is also suspended from said roof. Provision is made within the top shield roof 4 for the small rotating shield plug 18 which is rotatable about the axis X--X' and is in turn mounted within the large rotating shield plug 20. Said large plug is capable of rotating about its axis Y--Y' which coincides with the axis of the main reactor vessel 2. It is thus apparent that any point located in fixed relation to the small rotating shield plug 18 is endowed with planetary motion with respect to the stationary shield roof 4 by a combination of movements of rotation of the small shield plug 18 and of the large shield plug 20. Various devices 22 such as the handling grab and a number of other installations have also been shown diagrammatically on the small rotating shield plug 18. There are also shown the top containment casing 24 which surrounds the space above the shield roof 4. The whole of the foregoing description can relate to any fast reactor of the integrated type such as the French Superphenix reactor, for example. In this general figure, there is also shown very diagrammatically the device 30 for guiding tubes and cables which are intended to penetrate through the small rotating shield plug to the reactor core lid according to a first embodiment. Said mechanism 30 comprises a hanger 32 constituted by a vertical portion of column 32a and by a horizontal portion or arm 32b, said hanger 32 being fixed on the periphery of the small rotating shield plug. The hanger 32 is constructed in such a manner that the extremity of the horizontal arm 32b of the hanger 32 is disposed on the axis X--X' of rotation of the small shield plug 18. From the extremity of the horizontal portion 32b of the hanger 32, there extends upwards a vertical central portion forming a bundle-type assembly 34 which is so arranged that its vertical axis coincides with the axis of rotation X--X' of the small shield plug. The device 30 further comprises the following components disposed substantially in the same horizontal plane: a first lever-arm 36 pivotally coupled to the upper extremity of the bundle-type assembly 34 and a second lever-arm 38 pivotally mounted on a pin 40 having a vertical axis with respect to the first arm 36 and with respect to a second vertical pivot-pin 42 which is rigidly fixed to the containment casing 24 of the nuclear reactor. The length of the vertical portion 32a of the hanger 32 is such that a height H is left free above the small rotating shield plug, thus making it possible to position a number of different devices 22. The cables and tubes which are intended to pass through the small rotating shield plug are fixed respectively on the two guiding arms 36 and 38, extend vertically downwards so as to constitute the bundle-type assembly 34 and are guided by the two elements of the hanger 32 to the different points of the small rotating shield plug 18 at which said cables or tubes are intended to penetrate through this latter. Before proceeding to a more detailed description of the guiding device according to the first embodiment, the advantage and the general operation of the device may already be understood by referring more particularly to FIGS. 2 and 3. During the combined movement of rotation of the small shield plug 18 and of the large shield plug 20, the two lever-arms 36 and 38 are caused to pivot about pins 46 and 42 in such a manner that the vertical axis of the bundle-type assembly 34 remains in coincident relation with the axis of rotation of the small shield plug. It is understood in particular that, by virtue of the presence of these two lever-arms, irrespective of the position of the small rotating shield plug with respect to the large shield plug and more specifically irrespectively of the position of the point at which the hanger 32 is fixed on the small shield plug, the extremity of the lever-arm 36 can be maintained in the line of extension of the axis X--X' of the small shield plug by pivotal displacement of the lever-arms 36 and 38 with relative angles of pivotal motion which are of small amplitude irrespective of the movements of rotation of the small and large shield plugs with respect to a reference position. It can be understood that this limitation of angular displacements is a particularly important feature of the invention since cables or tubes which have a certain degree of stiffness must pass around the different articulations. As will be explained hereinafter, this makes it possible in particular to limit the dead length of cables and tubes while also limiting the value of stress induced in the tubes or cables under the action of the twisting effort produced by rotational displacement of the small and large shield plugs. It is readily apparent that one of the problems to be solved is that of twisting of the cables or tubes in the vertical guiding portion corresponding to the bundle-type assembly 34. In more precise terms, the problem is to control the degree of twist in order to prevent introduction of excessive localized stresses in said tubes or cables since the repetitive character of such stresses would be liable to result in damage. The construction of this portion of the device will now be described in greater detail in order to provide a clearer idea of its originality. For the purpose of guiding in the vertical direction in the bundle-type assembly 34, the different cables or tubes 50 are fixed on annular guide-plates such as the plate 52 and these latter are uniformly spaced between the horizontal portion 32b of the hanger 32 and the arm 36. In fact, a distinction must be drawn between a bottom guide-plate 52a which is rigidly fixed to the arm 32b of the hanger 32, a top guide-plate 52b which is associated with the arm 36 and intermediate guide-plates 52c. At least in the portion corresponding to the bundle-type assembly 34, the cables or tubes 50 are fixed on the guide-plates 52 in uniformly spaced relation and thus constitute the equivalent of a squirrel cage. As will become apparent hereinafter, the different guide-plates are positioned relative to each other by means of three cables secured to the bottom ring 52a and to the top ring 52b. It is further apparent that twisting of the cables 50 is produced at the time of rotation of the upper arms 36 and 38. This twisting movement in one direction or in the other is clearly accompanied by a reduction in the vertical height H' between the bottom guide-plate 52a which is rigidly fixed to the arm 32b and the top guide-plate 52b with respect to an intermediate value. As will be explained in detail below, the guide-plate 52b is consequently not directly secured to the arm 36 but is coupled to this latter through the intermediary of a vertical jack which permits displacements of the top guide-plates 52b in order to absorb the reductions or increases in said height H' with respect to an intermediate value. In FIG. 4, there is shown in greater detail one example of construction of an intermediate guide-plate 52c. This latter is designed in the form of a ring, the external face of which is provided with semicylindrical recesses 60. The cables such as the cable 50 are placed at the level of the guide-plate within a circular sleeve 62. Said sleeve is in turn placed within a recess 60 and clamped in position by means of an individual clamping member such as the member 64. Said clamping member 64 is fixed in the guide-plate 52 by means of four screws such as the screw 66. It is apparent that the imbricated shape 68 of said clamping members makes it possible to reduce the space between two consecutive cables 50 while fixing the clamping member 64 in position on each side of the sleeve 62. As can readily be understood, each clamping member has a semi-cylindrical bore 69 which cooperates with the bore 60 of the intermediate guide-plate 52c. It is further apparent that exactly the same nethod is adopted for fixing the cables or tubes 50 in the guide-plates 52a and 52b. Furthermore and as mentioned earlier, the different intermediate guide-plates 52c are maintained in position by means of supporting cables 70. By way of example, provision is made for three supporting cables. As will be explained hereinafter, said cables are fixed on the top guide-plate 52a and on the bottom guide-plate 52b and are, of course, also fixed within the intermediate guide-plates 52c. In FIG. 4, there is also shown a preferred mode of clamping of the cables 70. This clamping operation is carried out by placing two semi-cylindrical half-shells 74a and 74b within a bore 72 formed in the guide-plate 52. Clamping of the cable within the guide-plate 52 is carried out, for example, by means of two cone-point set-screws 76 which are intended to clamp one of the half-shells in position (namely the half-shell 74a, for example) with respect to the upper half-shell. It is apparent that the cables 70 are then secured to each intermediate guide-plate. It is important to lay emphasis on the double function performed by the supporting cables 70. The first function is to maintain equality of spacing between two consecutive guide-plates during relative movement of the arm 36 with respect to the hanger 32 in the vertical direction. The other function is to ensure uniform distribution between the different intermediate guide-plates 52c of the angle through which the top guide-plate 52b rotates with respect to the bottom guide-plate 52a which is rigidly fixed to the hanger. It may be stated in more precise terms that, by virtue of the supporting cables 70, if the top guide-plate 52b rotates through an angle A with respect to the bottom guide-plate 52a, the angle between two consecutive guide-plates will be substantially equal to A/(n+1) if there are n intermediate guide-plates 52c. It should be added that the second function could be performed in a different way. For example, provision could be made in the case of each guide-plate for two stops which are rigidly fixed to the guide-plate located immediately beneath and limit the movement of rotation of the guide-plate considered to a maximum angle, taking into account the maximum angle which can appear between the bottom guide-plate 52a and the top guide-plate 52b. In more general terms, the guide-plates are connected to each other in such a manner as to ensure that the total angle of rotation between the top guide-plate and the bottom guide-plate is distributed between the different guide-plates in proportion to the distance between the plates in the vertical direction and that they retain a common axis of symmetry of revolution. In FIG. 5, there is shown the articulation between the bundle unit 34 and the arm 36, and more precisely the mode of compensation for variations in length resulting from twisting of the cables or tubes 50. The top guide-plate 52b is not rigidly fixed to the arm 36 but coupled to this latter by means of the operating rod of a jack designated by the general reference 80. The jack body is designated by the reference 80a, the jack piston is designated by the reference 80b and the operating rod of the jack is designated by the reference 80c. Said operating rod is rigidly fixed to a yoke 82 which is guided in vertical translational motion as a result of cooperation of guides 84 and rollers 86 attached to the yoke 82 which is rigidly fixed to a transmission rod 88. Said rod is fixed on the one hand on the top guide-plate 52b and on the other hand on the yoke 82 by means of a system 90 forming a universal-joint assembly. It is thus apparent that the top guide-plate 52b is in fact rotationally coupled to the arm 36 but capable of displacement in a vertical direction with respect to the arm 36. More specifically, the jack 80 is controlled in such a manner as to exert a constant force. Thus, under the action of rotation of the top guide-plate 52b with respect to the bottom guide-plate 52a, displacement of the guide-plate 52b takes place progressively only during the course of twisting of all the cables 50. Moreover, FIG. 5 shows diagrammatically the upper extremity 70a of the suspension cable 70 associated with a nut system 92 which makes it possible to carry out the adjustment. FIG. 3 is a top view showing diagrammatically the complete device for guiding cables or tubes. There are again shown the horizontal arm 32b of the hanger 32, the arm 36 which is pivotally mounted with respect to said hanger, the arm 38 which is pivoted about the pin 40 with respect to the arm 36, the arm 38 being in turn pivoted about the pin 42 with respect to a support bracket 100 which is rigidly fixed to the dome 24. The movements of the arms 36 and 38 in order to follow the movements of the small and large rotating shield plugs are produced by two actuating devices designated by the reference 102 for the movement of the arm 36 with respect to the arm 38 about the pivot-pin 40 and by the reference 104 for the movement of the arm 38 about the pivot-pin 42 with respect to the fixed support bracket 100. In the embodiment shown in FIG. 3, each actuating means 102 or 104 consists of a hydraulic jack 106 which is rigidly fixed to the arm 36, for example, and the operating rod 108 of which is adapted to carry a toothed rack 110. Said toothed rack is disposed in meshing engagement with a toothed pinion 112 which is rigidly fixed to the arm 38 and the shaft of which is coaxial with the pivot-pin 40. It can readily be understood that, by initiating operation of the jack 106, the arm 36 is displaced in pivotal motion with respect to the arm 38. Furthermore, the device comprises a mechanical detector as designated for example by the reference 114 for detecting any movement of the arm 36 with respect to the arm 38. A signal which is representative of said movement is transmitted by said detector to the control unit. Exactly the same principle of operation applies to the actuating device 104. In this embodiment, it is apparent that the control unit includes follow-up control devices for causing the actuating devices 102 and 104 to displace the extremity of the arm 36 in such a manner as to ensure that said extremity remains vertically above the center of the small rotating shield plug. It will naturally be understood that other actuating means could be employed. This result can readily be obtained in accordance with well-known control techniques involving the use of detectors 114, for example, and achieving follow-up control between said mechanical means 104 and 102 and the mechanical means for producing displacements of the small and large rotating shield plugs. It is therefore unnecessary to describe this follow-up control system in greater detail. In the foregoing description, it has been considered that the hanger 32 was rigidly fixed on the small rotating shield plug or in other words that there did not exist any possibility of pivotal displacement of the vertical portion 32a with respect to the small rotating shield plug. However, for certain handling operations in the nuclear reactor, it may prove advantageous to provide maximum clearance within the entire space which extends above the small rotating shield plug. It is for this reason that, in accordance with an improved alternative embodiment, provision can be made for a possibility of pivotal displacement of the hanger about its vertical axis Z--Z'. This form of construction will now be described in greater detail with reference to FIG. 6. In this embodiment, the horizontal arm 32'b is capable of rotating in a horizontal plane about the vertical column 32'a. It is apparent that, under these conditions, the same problem arises in regard to twisting of the cables or tubes 50 which are placed along this portion of the guiding device. This problem is solved substantially in the same manner as in the case of the bundle portion 34. It should nevertheless be borne in mind that rotational displacement of the hanger takes place at much less frequent intervals than the bundle portion 34 and that the applied forces are also of much lower value. The vertical column 32'a is provided with a base 32c which is rigidly fixed to the small rotating shield plug 18. The arm 32'b of the hanger is pivotally mounted on the upper extremity of the vertical column 32'a in known manner. Guiding of the cables or tubes 50 is carried out by means of annular plates which are similar to the plates 52. There is thus shown a bottom annular guide-plate 52'a which is rigidly fixed to the base 32c, a top annular guide-plate 52'b which is rigidly fixed to the arm 32'b and one or a number of intermediate annular plates 52'c, said intermediate annular plate or plates being capable of moving freely in translational motion and in rotational motion with respect to the vertical column 32'a. The cables or tubes 50 are fixed at uniform intervals on the periphery of the guide-plates so as to form the equivalent of a squirrel cage. As in the case of the bundle portion 34, the intermediate annular plate or plates are connected to the top plate 52'b by means of supporting cables 70'. Said cables 70' are fixed on the top guide-plate 52'b by means of resilient devices 125 which perform much the same function as the jack 80 while taking into account that the movements take place at a much lower frequency. Moreover, the vertical column 32'a passes through the horizontal arm 32'b. An actuating device 120 makes it possible to cause rotational displacement of the arm 32'b about the vertical column 32'a. Said actuating device can consist of a jack 122, the operating 122a of which is rigidly fixed to a toothed rack 122b in cooperating relation with a pinion 124 which is rigidly fixed to the upper extremity of the vertical column 32'a. The actuating device is associated with a sensor for detecting rotational motion of the arm 32'b (not shown). It is readily apparent that, when inititiating a movement of rotation of the vertical column 32'a, it is also necessary to initiate the corresponding movements of rotation of the arms 36 and 38. This function is performed by the general control unit of the device. It would not constitute any departure from the invention if the vertical column 32'a were rotatable with respect to the base 32c which is rigidly fixed to the small rotating shield plug 18, in which case the arm 32'b would be rigidly fixed to the vertical column 32'a. It can readily be understood in this case that the actuating device 120 is rigidly fixed to the small rotating shield plug and transmits its movement to the foot of the vertical column 32'a. It should be added that, taking into account the weight of the assembly constituted by the two guiding arms 36 and 38 and by the cables and tubes 50 which are secured to said arms, it may be preferable to support the arms 36 and 38. Provision can accordingly be made for a cable which is attached at its lower end to the pivot-pin 40 of the arm 38 and is secured at its upper end to a slide-block, said slide-block being capable of moving within a circular guide rail which is rigidly fixed to the dome 24. It can be mentioned by way of example that, by virtue of the two supporting arms 36 and 38, the maximum movements of rotation of the large shield plug are followed with maximum angles of pivotal displacement at the level of the articulations or pivot-pins 40 and 42, these angles being equal respectively to 45.degree. and 62.degree.. It is apparent that the reduced value of these angles considerably simplifies the problem of the flexibility loops or "slack" which the cables and tubes must possess at the level of said articulations. It is understood that, by means of the device according to the invention, the space which extends above the small and large rotating shield plugs can be freed to the maximum extent in order to facilitate positioning of the different mechanisms on the shield plugs. Furthermore, the guiding means make it possible to limit the degrees of curvature or of twist applied to the cables or tubes during the different movements, thus considerably reducing the stresses applied to said cables or tubes and increasing the useful life of these latter. The device further permits accessibility of the cables, with the result that these latter are conveniently interchangeable. In the foregoing description, the bottom guide-plate 52a is rigidly fixed to the hanger 32 in order to provide maximum clearance for the small rotating shield plug. However, in some cases or in other applications of the device, it would be possible to dispense with the hanger. The bottom guide-plate 52a would then be directly fixed on the small shield plug 18 at its center of rotation. The system would then be simplified in the case of the first alternative embodiment. In the second alternative embodiment of the invention which will now be described, the hanger is in fact dispensed with. The bundle-type assembly described earlier is replaced in this case by a "rigid axis" assembly which provides a direct connection between the center of rotation of the small rotating shield plug and the free end of the horizontal articulated arm. Moreover, the general problem to be solved is again the same since it remains necessary to avoid twisting of the cables or ducts. The central portion of the rigid vertical mast 150 is shown in FIGS. 7a and 7b and the upper end of said mast is shown in FIG. 9. It is recalled that the lower end of the mast is rigidly fixed at the center of the small rotating shield plug 18 and that the axis of the mast coincides with the axis of rotation of the plug. FIGS. 7a and 7b show one form of construction of the intermediate guide-plates 52'c. The external periphery of the guide-plate 52' is identical with the external periphery of the intermediate guide-plates 52c and therefore does not call for any further description. Similarly, horizontality of the guide-plates is achieved by means of cables 70 as in the previous embodiments. The difference lies in the cooperation between the intermediate guide-plate and the mast 150; this cooperation permits equal distribution of the total rotation between the intermediate guide-plates. The guide-plate 52'c is pierced by a central bore 152. Within the interior of said bore 152 and around the mast 150, provision is made for a ring 154 in which is formed a central bore 156. The ring 154 is coupled for translational motion with the guide-plate 52'c on the one hand by means of its flat portion 154' which penetrates into the recessed portion 158 of the guide-plate 52'c and on the other hand by means of circlips 160. However, the ring 154 is capable of rotating with respect to the guide-plate. The flat portion 154' of the ring is provided at its external periphery with at least one recess 162 corresponding to an angle .alpha. at the center. Said recess 162 is adapted to cooperate with a stud 164 which is rigidly fixed to the guide-plate. Provision could naturally be made for a plurality of recesses 162 and for a plurality of studs 164. The internal face of the ring 154 corresponding to the bore 156 is also provided with a recess 166 corresponding to an angle at the center equal to .beta.. This second recess 166 is adapted to cooperate with a key 168 fixed on the mast by any known means. It will readily be apparent that provision can be made for a plurality of recesses 166 and for a plurality of keys 168 in order to accommodate applied stresses. It is therefore apparent that two assemblies are provided for limiting the angle of rotation of the guide-plate 52c with respect to the mast 150, the effects of which are added. The total possible range of angular displacement has the value .alpha.+.beta.. In FIG. 8, there is shown a simplified form of construction of the guide-plate 52"c. Provision is made simply for a key 168' which is rigidly fixed to the mast 150 and for a recess 170 formed in the internal face of the guide-plate 152"c. There is thus obtained only one possibility of rotation equal to .beta.. The top guide-plate 52'b is shown in FIG. 9. As already explained with reference to FIG. 5, the guide-plate 52'b must be capable of displacement in the vertical direction under the action of a jack and is rotationally coupled to the extremity of the horizontal arm 36. The upper extremity 150a of the mast 150 is rotatably mounted in the extremity of the arm 36, for example by means of two ball-type or roller-type thrust bearings. The extremity 150a is fitted internally with the body 174a of a pneumatic jack 174, the operating rod 174b of which is rigidly fixed to a horizontal guide-plate 176 by means of the ball thrust bearing 175. The pipe 174c for supplying the jack 174 is also shown. The guide-plate 176 is connected to the top guide-plate 52'c by means of rods such as the rod 178. Said rods 178 traverse the arm 36 through bores 180. It is therefore apparent that the top guide-plate 52'b is rotationally coupled to the arm 36 but that the top guide-plate 52b is capable of vertical displacement along the mast 150 under the action of the jack 174. Under the action of a movement of rotation of the mast 150, the ducts or pipes 50 assume a helical configuration and the jack moves downwards. The tensile load does not change since the pneumatic circuit comprises either a large volume of gas or an expansion and overflow system. As already mentioned, the intermediate guide-plates 52'c or 52"c serve to ensure equal distribution of the total angle of rotation between the bottom guide-plate which is rigidly fixed to the shield plug and the top guide-plate 52'b which is rotationally coupled to the arm. The intermediate guide-plate 52"c of FIG. 8 permits a maximum rotation .beta.=100.degree. if it is desired to provide three keys 168' and three recesses 170. By means of guide-plates of the type designated by the reference 52"c, it is therefore possible to obtain all values of angular displacement up to .+-.50.degree.. The system constituted by the recess 162 and the stud 164 of FIG. 7a permits a maximum angle of rotation .alpha.=340.degree.. It is in fact necessary to take into account the diameter of the stud 164 and the length of the heel or projecting portion to be left on the periphery of the ring. If the guide-plate is intended to have an angular displacement within the range of .+-.50.degree. to .+-.170.degree., it is clearly unnecessary to make use of the angle .beta.. The recess 166 will therefore have a width equal to the width of the key 168. The angle .alpha. is zero. If the guide-plate is intended to have an angular displacement within the range of .+-.170.degree. to .+-.220.degree., the guide-plate 52'c is put to use by employing the two rotation-limiting systems. By way of example, if five intermediate guide-plates are employed and the total angle of displacement has a value of .+-.150.degree., the following intermediate guide-plates can be employed successively: In the case of the two intermediate guide-plates located nearest the bottom guide-plate, plates of the type designated by the reference 52"c will be employed with respective values in respect of .beta.=0; in the case of .alpha., the respective values will be .+-.75.degree., .+-.100.degree. and .+-.125.degree.. There is then obtained an accurate distribution of the helix of the ducts 50 of the bundle over the entire length of the vertical mast 150, especially under the most exacting conditions which correspond to maximum design values of angular displacement which cannot be exceeded. Contraction of a bundle unit calls for adapted ducts and, as in any system which works in torsion, the ducts 50 will be judiciously arranged so as to ensure that the center of torsion of the bundle unit corresponds as closely as possible to the principal axis of the guide-plates and of the mast. It should nevertheless be added that the horizontal arms 36 and 38 could be replaced by other means for supporting the cables and ducts between the fixed point and the upper extremity of the main bundle portion 34. It would also be possible to employ conventional flexible connections such as festoons or garlands as employed in known manner for linear displacements of traveling bridge cranes, whether these latter are combined with sliding linear arms or not. Furthermore, the device according to the invention and in particular the structure of its main bundle portion is applicable to fields other than nuclear reactors equipped with eccentric rotating shield plugs. The fuel assembly storage pools associated with fast reactors can be mentioned by way of example. Ducts arranged in a bundle can also be used to supply fuel assembly storage magazines of the rotary drum or carousel type. Broadly speaking, the invention is applicable whenever a bundle of cables, pipes or ducts having a certain degree of stiffness is intended to pass through a rotating component. |
abstract | According to an aspect, a vapor powered apparatus for generating electric power includes a liquid chamber that contains a working fluid and a first heat exchanger that transfers heat from fluid coming from a heat source to working fluid coming from the liquid chamber, where the transferred heat vaporizes at least a portion of the working fluid to provide a working pressure of the vaporized working fluid. The apparatus includes a pressure motor to convert the working pressure of the vaporized working fluid into mechanical motion for a power generator. The apparatus includes a vapor chamber to capture the vaporized working fluid and a second heat exchanger to use working fluid from the liquid chamber to condense the captured vaporized working fluid. An exchanger fluid system provides the working fluid to the second heat exchanger from a bottom portion of a pool of working liquid in the liquid chamber. |
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description | 1. Field This invention pertains in general to radiation filters/reflectors and, in one particular embodiment, more specifically to a variable neutron reflector for a nuclear reactor core. 2. Related Art In a nuclear reactor for power generation, such as a pressurized water reactor, heat is generated by fission of a nuclear fuel such as enriched uranium, and transferred to a coolant flowing through a reactor core. The core contains elongated nuclear fuel rods mounted in proximity to one another in a fuel assembly structure, through and over which the coolant flows. The fuel rods are spaced from one another in co-extensive parallel arrays. Some of the neutrons and other atomic particles released during nuclear decay of the fuel atoms in a given fuel rod pass through the spaces between fuel rods and impinge on fissile material in adjacent fuel rods, contributing to the nuclear reaction and to the heat generated by the core. Movable control rods are dispersed throughout the nuclear core to enable control of the overall rate of the fission reaction, by absorbing a portion of the neutrons, which otherwise would contribute to the fission reaction. The control rods generally comprise elongated rods of neutron absorbing material and fit into longitudinal openings or guide thimbles in the fuel assemblies running parallel to and between the fuel rods. Inserting a control rod further into the core causes more neutrons to be absorbed without contributing to fission in an adjacent fuel rod; and retracting the control rods reduces the extent of neutron absorption and increases the rate of the nuclear reaction and the power output of the core. FIG. 1 shows a simplified conventional nuclear reactor primary system, including a generally cylindrical pressure vessel 10 having a closure head 12 enclosing a nuclear core 14 that supports the fuel rods containing the fissile material. A liquid coolant, such as water or borated water, is pumped into the vessel 10 by pump 16 through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18 typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown) such as a steam driven turbine generator. The reactor coolant is then returned to the pump 16 completing the primary loop. Typically, a plurality of the above-described loops are connected to a single reactor vessel 10 by reactor coolant piping 20. Commercial power plants employing this design are typically on the order of 1,100 megawatts or more. More recently, Westinghouse Electric Company LLC has proposed a small modular reactor in the 200 megawatt class. The small modular reactor is an integral pressurized water reactor with all primary loop components located inside the reactor vessel. The reactor vessel is surrounded by a compact high pressure containment. Due to both limited space within the containment and the low cost requirement for integral pressurized light water reactors, the overall number of auxiliary systems needs to be minimized without compromising safety or functionality. For that reason, it is desirable to maintain most of the components in fluid communication with the primary loop of the reactor system within the compact, high pressure containment. Typical control rod drive mechanism used in existing and proposed small modular reactors require moving parts to be positioned in locations where mechanical and electro-mechanical failure of the mechanism represents a serious operating concern. It is very difficult or impractical to repair failures associated with these control mechanisms and associated position indication sensors while the reactor is operating. Conventionally, some reactors have employed dense materials to surround a reactor core with the dense materials having a high potential to produce large angle scattering collisions with escaping neutrons, and a low absorption potential, to minimize the neutron leakage from the reactor. This type of material is said to “reflect” escaping neutrons back into the reactor where they can contribute to additional fission reactions. The result is that less reactivity or fissionable material is needed to create a critical reactor configuration. Given a critical reactor configuration with no reflector, the addition of a reflector allows the fission reaction rate in the core to increase, thus producing a higher power level. It is an object of this invention to replace the functionality of some or all of the control rod drive mechanisms with moving parts with a system with no moving parts. It is a further object of this invention to provide such a system with no moving parts that is wholly contained outside the core. These and other objects are achieved employing a device in which an albedo of the device can be varied, for controlling the transmission of radiation from a source on a first side of the device to a second location on a second side of the device. The device comprises a fluid reservoir containing a magneto-rheological fluid in an interior of the fluid reservoir. A magnetic field generator is positioned on the outside of the reservoir for establishing a magnetic field across the magneto-rheological fluid. A control system controls the magnetic field generator to vary the magnetic field across the magneto-rheological fluid to change the albedo of the device in accordance with a demand signal. In one preferred embodiment, the device is a nuclear reflector supported around at least a portion of a circumference of a core of the nuclear reactor with the nuclear reflector having at least a first state and a second state wherein the strength of the magnetic field is varied by the control system to change the nuclear reflector between the first state and second state to change the albedo of the device. Preferably, the magnetic field is established along an axial direction of the core of the nuclear reactor and desirably, the magnetic field is a plurality of discrete magnetic fields spaced around a circumference of the core of the nuclear reactor. Desirably, the plurality of discrete magnetic fields are respectively established by a plurality of independently operated electromagnets. In one embodiment, the device includes a monitoring system connected to the control system, for monitoring the radiation on the second side of the device and controlling the magnetic field to control a level of the radiation on the first side of the device through a program directing the demand signal. Preferably, the demand signal is a power demand signal of the reactor. In still another embodiment, the demand signal controls a level of the nuclear reactions within the core of the nuclear reactor by changing the magnetic field. In one such embodiment, the change in the magnetic field is the primary mechanism for controlling the nuclear reactions within the core during normal operation of the reactor. Preferably, if power is lost to the magnetic field generator, the magneto-rheological fluid transitions to a state that shuts down the reactor. FIGS. 2 and 3 illustrate a small modular reactor design which can benefit from the control system of this invention for controlling the rate of the nuclear reactions within the core 14. FIG. 2 shows a perspective view of the reactor containment of a modular reactor design to which this invention can be applied. The reactor containment illustrated in FIG. 2 is partially cut away, to show the reactor pressure vessel 10 and its integral, internal components. FIG. 3 is an enlarged view of the reactor pressure vessel shown in FIG. 2. Like reference characters are used among the several figures to identify corresponding components. In an integral pressurized water reactor such as illustrated in FIGS. 2 and 3, substantially all of the components typically associated with the primary side of a nuclear steam supply system are contained in a single reactor pressure vessel 10 that is typically housed within a high pressure containment vessel 32 capable of withstanding pressure of approximately 2500 psig, along with portions of the safety systems associated with the primary side of the nuclear steam supply system. The primary components housed within the pressure vessel 10 include the primary side of the steam generator, reactor coolant pumps 28, the pressurizer 22 and the reactor itself. The steam generator system 18 of a commercial reactor, in this integral reactor design, is separated into two components, a heat exchanger 26 which is located in the reactor vessel 10 above the reactor upper internals 30 and a steam drum (not shown in the drawings) which is maintained external to the containment 32. The steam generator heat exchanger 26 includes within the pressure vessel 10/12, which is rated for primary design pressure and is shared with the reactor core 14 and other conventional reactor internal components, two tube sheets 54 and 56, hot leg piping 24 (also referred to as the hot leg riser), heat transfer tubes 58, which extend between the lower tube sheet 54 and the upper tube sheet 56, tube supports 60, secondary flow baffles 36 for directing the flow of the secondary fluid medium among the heat transfer tubes 58 and secondary side flow nozzles 44 and 50. The heat exchanger 26 within the pressure vessel head assembly 12 is thus sealed within the containment 32. The external-to-containment steam drum is comprised of a pressure vessel, rated for secondary design pressure. The external-to-containment steam drum includes centrifugal-type and chevron-type moisture separation equipment, a feed water distribution device and flow nozzles for dry steam, feed water, recirculating liquid and wet steam, much as is found in a conventional steam generator design 18. The flow of a primary reactor coolant through the heat exchanger 26 in the head 12 of the vessel 10 is shown by the arrows in the upper portion of FIG. 3. As shown, heated reactor coolant exiting the reactor core 14 travels up and through the hot leg riser 24, through the center of the upper tube sheet 56 where it enters a hot leg manifold 74 where the heated coolant makes a 180 degree turn and enters the heat transfer tubes 58 which extend through the upper tube sheet 56. The reactor coolant then travels down through the heat transfer tubes 58 that extend through the lower tube sheet 54 transferring its heat to a mixture of recirculated liquid and feed water that is entering the heat exchanger through the sub-cooled recirculation input nozzle 50 from the external steam drum, in a counterflow relationship. The sub-cooled recirculating liquid and feed water that enters the heat exchanger 26 through the sub-cooled recirculation input nozzle 50 is directed down to the bottom of the heat exchanger by the secondary flow baffles 36 and up and around heat exchange tubes 58 and turns just below the upper tube sheet 56 into an outlet channel 76 where the moisture-laden steam is funneled to the wet steam outlet nozzle 44. The wet saturated steam is then conveyed to the external steam drum where it is transported through moisture separators which separate the steam from the moisture. The separated moisture forms the recirculated liquid which is combined with feed water and conveyed back to the sub-cooled recirculation input nozzle 50, to repeat the cycle. Control of the fission process in the core of these types of conventional reactors is largely provided by the control rods. Typical control rod drive mechanisms used in many conventional reactors require moving parts to be positioned in locations where mechanical and electro-mechanical failure of the mechanisms represents a serious operating concern. It is very difficult or impractical to repair failures associated with these control mechanism and associated position indication sensors while the reactor is operating. There is a need to replace or greatly supplement the functions required to be performed by the control rods that does not require moving parts inside the reactor vessel or reactor core. This invention provides a device and associated application methodology for adjusting the core reactivity balance without use of any moving mechanical equipment and associated position sensors. The core reactivity balance of a nuclear reactor may be significantly affected by changing the number of neutrons that leak out of the reactor core and do not return to contribute to the amount of fission occurring inside the reactor. This fact is well understood by those skilled in the art of nuclear engineering. A static hardware device, such as that described in U.S. Pat. No. 8,615,465, is often installed around nuclear reactors specifically to reduce the number of neutrons that leak out of the reactor core before they contribute to the fission rate inside the reactor. These devices are given the general term “reflector” since they act as if neutrons exiting the core region are reflected back inside the core. Use of a suitable reflector allows the amount of power generated by a given amount of uranium inside the reactor core over a fixed time interval to be increased. Another way to describe this effect is that the reflector increases the core reactivity. Consequently, if one changes the properties of the reflector, the core reactivity is affected. Changes in core reactivity will change the core power level. Hence, being able to control changes in the neutron reflection properties of the neutron reflector will control the core power level. FIG. 4 is a graphical illustration of the relative neutron distribution with and without the use of a neutron reflector. The neutron reflector effectiveness is often indicated by the term “albedo.” The albedo of the reflector is defined to be the ratio of the number of neutrons entering the reflector to the number returned to the core region. If all the neutrons entering the reflector region return to the core, the albedo is 1.0. This situation will provide a maximum positive contribution to core reactivity. Water has an albedo of approximately 0.8. The density and material composition of the reflector determine the albedo. Steel produces a higher albedo than water. Being able to effectively change the density and material composition of the reflector during core power operation will therefore provide the ability to control the core power without the use of control rods or chemical shim changes. A magneto-rheological fluid has effective density and composition properties that change when a magnetic field is applied across the fluid. The typical magneto-rheological fluid comprise very small ferro-magnetic spheres 40 suspended in some type of viscous oil 38 as shown in FIG. 5. The magnetic particles which are typically micrometer of nanometer scale spheres or ellipsoids, are suspended within the carrier oil 38, and are distributed randomly and in suspension under normal circumstances. When a magnetic field is applied, however, the microscopic particles (usually in the 0.1-10 micrometer range) align themselves along the lines of magnetic flux as illustrated in FIG. 6, where the arrows indicate the direction of the magnetic flux. The object of this invention is to control the density of the ferro-magnetic particles in the fluid and the impact the metal density in the fluid has on the reflector properties. When the magnetic field is applied, the ferro-magnetic particles line up along the lines of magnetic field strength between the north and south poles of the magnets. This alignment creates gaps of fluid that preferentially either absorb or forward diffused neutrons preventing them from returning to the reactor core. This effectively reduces the neutron scattering cross section of the reflector which reduces the albedo. The preferred embodiment of this device controls the radial neutron leakage from one or more of the sides of a small reactor type. An example of this type of reactor is the integral modular reactor described above or a typical TRIGA-style pool reactor. This reflector can operate effectively inside the reactor vessel as shown in FIG. 2 by reference character 42 where it is placed around the core between the reactor vessel and the core barrel, or it can work by being positioned outside the reactor vessel as shown in FIG. 3. Preferably, each face of the reactor core will have its own device with the capability for independent control. The device comprises a series of independently controlled electromagnets and a non-ferromagnetic fluid tank containing the magneto-rheological fluid. The magneto-rheological fluid comprises very small ferromagnetic particles (e.g., —Fe, FE-W—Ni alloys) in a suspension with a viscous liquid with a viscosity low enough to allow rapid movement of the magnetic particles when under the influence of the forces produced by the magnetic fields. FIGS. 7 and 8 schematically illustrate electromagnets numbered 1 through 5 that cause the distribution of magnetic particles in the magneto-rheological fluid to preferentially locate in the areas of the highest magnetic field strength. This action collects the metals that compose the suitable neutron reflector into a small effective area in the magneto-rheological fluid. Since the fluid particles in this configuration do not serve as good neutron reflector materials, neutron leakage out of the reactor will increase significantly adjacent to the areas of highest magnetic particle concentration, and decrease in the areas where the more suitable reflector material concentration is forced to increase. The magneto-rheological fluid is supported within the panel 46 which is positioned between the north and south poles of the electromagnets 1-5 that are individually controlled and electrically powered from a source 48. Each of the magnets 1-5 is separately controlled to respectively create discrete, independent magnetic fields. Using the configuration illustrated in FIG. 7 and applying currents to electro magnets 1 and 5 will draw the magnetic particles away from the center of the panel 46 containing the magneto-rheological fluid, increasing the leakage from a larger area about the center of the panel. This action will increase neutron leakage from the majority of the most significant areas of the reactor and cause reactor power to decrease. In order to increase reactor power, the current supplied to electro magnets 1 and 5 is eliminated, and current is supplied to electro magnet 3. Electro magnets 2 and 4 are used to re-establish a uniform magnetic particle and liquid neutron reflective material distribution once the desired power level has been established. The same principle can be used to reshape the radial power distribution. In a similar way the configuration shown in FIG. 8 can be used to reshape the axial power distribution. The control of the current supplied to each of the electromagnets is determined primarily from a reactor power demand signal. There is a feedback to the current supply controller 62 based on signals from prompt responding neutron detectors 64 (e.g., —Pt, Co detectors) located immediately behind the fluid tank 46 of the reflector 42. The controller 62 receives information on reactor power level and rate of reactor power level change from the neutron detector 64. The current controller evaluates the difference between the current reactor power level and the demanded power level and the rate at which the current reactor power level is changing to determine which electromagnets need to be activated to achieve the desired power level change and then stabilize the reactor at the targeted power level. Reference character 52 figuratively illustrates the information supplied from the reactor control system to the current controller 62. Pre-established correlations between current supply, magnetic field strength, and magnetic particle density (reflector liquid displacement) and reactor power change rate (reactivity change) are included in the controller to enable the current supplied to each electromagnet to require the minimum amount of correction from the controlled feedback loop. In the event that electromagnet power is lost, the heavy magnetic particles will settle to the bottom of the fluid panel causing the reactor to shut down. While the preferred embodiment was described in an application to nuclear fission rate control, this concept's application is well beyond the nuclear power environment. While the primary application of this concept is the control of the transmission of neutron radiation, the principle can be used to control the transmission of any form of electromagnetic or particulate radiation. As an example, the equilibrium magnetic particulate distribution may result in a fluid that is completely opaque. Applying current to electromagnets 1 and 5 could cause the center portion of the panel to become translucent or transparent. This application could be used to electrically control visible light transmission through a window. Accordingly, the term “albedo” as used herein shall be understood to include a measure of the transmissivity of any form of electromagnetic or particulate radiation. While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof. |
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063273225 | abstract | A transfer device for moving a poison rod assembly between fuel cells in a nuclear fuel storage facility coupleable to an overhead crane. The poison rod assembly has a plurality of poison rods disposed in rows. The device includes an elongated outer member, an inner member, and a gripper assembly supported by the inner member. The inner member is slidably disposed within the elongated outer member. The overhead crane is coupled to the inner member for sliding said inner member between an upper position and a lower position. An interlock assembly selectively couples the inner member and elongated outer member. Thus, moving the crane, which is coupled to the inner member either moves the inner member relative to the elongated outer member when the interlock is not engaged, or, moves both the inner member and the elongated outer member when the interlock is engaged. |
039379710 | abstract | A focused shield for use in connection with a radiation therapy machine or the like is constructed using a method and apparatus that results in a shield having a bevel-walled aperture. The aperture is cut into a focused shield blank and has a predetermined configuration corresponding to a selected area of a patient to be exposed to a field of radiation with the angularity of the aperture sidewalls relative to the radiation rays being such that any rays of a radiation beam in alignment with the aperture enter the same and do not strike the sidewalls thereof but rather pass through the aperture in an unimpeded manner. The method and apparatus employed in making the focused shield incorporates the use of a focused shield blank having a convexly rounded bottom side and a blank holding fixture, adapted for use with a band saw or the like, having a bowl-shaped concavity for receiving the blank and positioning the same on a surface that has a radius of curvatuve corresponding to the curvature of the blank bottom wall during the cutting of the aperture into the blank. |
description | This application is a continuation of, claims priority to, and claims the benefit of U.S. application Ser. No. 14/027,423, filed Sep. 16, 2013, which claims the benefit and priority of Provisional Application No. 61/718,215, filed Oct. 25, 2012. This application likewise claims the benefit of and priority to Provisional Application No. 61/718,215, filed Oct. 25, 2012. The contents of both applications are hereby incorporated by reference in their entireties. This invention relates to a composition and process for processing radioactive waste materials to render them suitable for shipment and/or storage. Radioactive waste materials, especially those resulting from the processing of uranium and plutonium, are particularly dangerous to transport to sites for final disposition, such as long-term storage or further processing. Such waste encompasses a wide range of material, and may include piping, building materials, machinery and equipment, furniture, weapons casings and the like. Radioactive waste, especially from the processing of uranium and plutonium, is usually buried for its final disposition. The current state of technology includes the steps of filling all of the interstitial spaces in the radioactive material with cement, and then micro-encapsulating the material with more cement. There are several shortcomings to this method. First, the resultant encapsulating material is very heavy. Cement has a typical density about 120 lbs/ft3, so it would not be unusual to have a large piece of contaminated equipment weigh in excess of 100,000 lbs. This necessitates the use of expensive, heavy equipment to move these structures. Second, the pouring of cement in situ over the encapsulated material (i.e. in the landfill) is an extraordinarily inefficient use of space. A large amount of cement is spilled over the sides of the material due to the inexact nature of pouring cement. This causes much more landfill space to be used than would be the case with a more focused process. Third, cement is well known to crack when exposed to tensile stress, temperature extremes, or when non-optimal water/cement ratios are used. When cracking in these monolithic structures occurs, there is a greater risk that radioactive waste will migrate from the structure into an uncontrolled environment. The use of polyurethanes for the purpose of encapsulation of radioactive materials is known in the prior art. The known prior art describes the use of one of several types of cement/mortar, sand, filler, or other additives to the polyurethane to either create a high density monolithic block, or as an aid for radiation attenuation. The novelty of the present invention resides in the lack of solid fillers or cement/mortar, as well as the optional inclusion of an elastomeric coating to encapsulate and protect the radioactive material from possible damage in transport. UK Patent No. GB2047946 to Pordes et al. discloses the encapsulation of radioactive waste material, particularly wet ion exchange resin, by dispersing the waste in an aqueous emulsion of an organic polyol, a polyisocyanate and an hydraulic cement, and allowing the emulsion to react and form a monolithic block. U.S. Pat. No. 7,250,119 to Sayala discloses the use of naturally occurring minerals in synergistic combination with formulated modified cement grout matrix, polymer modified asphaltene and maltene grout matrix, and polymer modified polyurethane foam grout matrix to provide a neutron and gamma radiation shielding product. U.S. Pat. No. 4,100,860 to Gablin et al. discloses a shipping container overpack for transportation of radioactive materials, and includes a leakproof receptacle for containing and protecting the material against accidental release. The receptacle has spaced inner and outer shells into which polyurethane foam is poured to create a stress skin structure. U.S. Pat. No. 4,486,512 to Tozawa et al. discloses a waste sealing container constructed by depositing a foundation of zinc over a steel base, then coating an organic synthetic resin paint containing a metal phosphate over the foundation coating, and thereafter coating an acryl resin, epoxy resin, and/or polyurethane paint. The above-described processes and resulting structures retain many of the disadvantages of the prior art, and thus a more cost-effective, efficient and safe means of processing radioactive waste for shipping and storage is needed. Therefore, it is an object of the invention to provide encapsulation materials and methods for application in the field of radioactive materials that do not require a cementitious material or grout as a constituent part of the material. It is another object of the invention to provide a mechanism for safe transport of radioactive materials with far less weight (approximately 1/20th the weight of cement) and occupying far less space in its burial site. It is another object of the invention to provide encapsulation materials and methods for application in the field of radioactive material that provides superior tensile strength and elongation that will resist cracking for long periods of time, unlike cementitious materials, which are subject to deterioration over time. The present invention includes the use of a foaming plastic, optionally covered with an elastomeric coating, for the purpose of encapsulating radioactive material that may or may not have been coated with a primer to render it attenuated and properly encased for safe transport while mitigating the risk of radioactive materials escaping. These and other objects of the invention are achieved by providing process for encapsulating a radioactive object to render the object suitable for shipment and/or storage, and including the steps of preparing a plastic material, causing the plastic material to react with a foaming agent, generating a foaming plastic, encapsulating the radioactive object in the foaming plastic, and allowing the foaming plastic to solidify around the radioactive object to form an impervious coating. According to one aspect of the invention, the step of encapsulating the radioactive object includes the steps of filling a void in the object with the foaming plastic and encasing the object in an outer layer of foaming plastic. According to another aspect of the invention, the step of encapsulating the radioactive object includes the step of placing the object in a bag before encasing the object in an outer layer of foaming plastic. According to another aspect of the invention, the step of encapsulating the radioactive object includes the step of applying an outer layer of an elastomeric coating to the object. According to another aspect of the invention, a process for encapsulating a radioactive object to render the object suitable for shipment and/or storage is provided, and includes the steps of preparing a plastic material, causing the plastic material to react with a foaming agent, generating a foaming plastic, placing a radioactive object in a container, encapsulating the container in the foaming plastic, and allowing the foaming plastic to solidify around the container to form an impervious coating. According to another aspect of the invention, the method includes the steps of evacuating displaced air from the container as the container is encapsulated and transferring the air to another treatment location. According to another aspect of the invention, a method of encapsulating a radioactive object to render the object suitable for shipment and/or storage includes the steps of preparing a plastic material, causing the plastic material to react with a foaming agent, generating a foaming plastic, and encapsulating the object in the foaming plastic. The step of encapsulating the object in the foaming plastic includes the steps selected from the group consisting of placing a radioactive object in a container, encapsulating the container in the foaming plastic, and allowing the foaming plastic to solidify around the container to form an impervious coating; and encapsulating the radioactive object in the foaming plastic, allowing the foaming plastic to solidify around the radioactive object to form an impervious coating. According to another aspect of the invention, the step of encapsulating the radioactive object includes the steps of filling a void in the object with the foaming plastic and encasing the object in an outer layer of foaming plastic. According to another aspect of the invention, various formulations are disclosed having various physical characteristics suitable for encapsulating objects in a foaming plastic in preparation for shipment and storage. Referring now specifically to the drawings, FIG. 1 is a flow diagram showing by way of example an iteration of the method steps that may be used to carry out the method according to one preferred embodiment of the invention. First, candidate objects are examined to determine the appropriateness for treating with foaming plastic in downstream steps. Some objects may be incinerated or processed by different methods. Those objects, such as described above, selected for processing are prepared based on the type and physical characteristics of the object. For example, objects such as piping may first be cleaned and loose material, particularly in the interior of the pipe, either removed or primed onto the surface. The selection and preparation steps will determine the particular process to be used in the next steps. As shown in FIG. 1, large objects, such as machinery, barrels, and the like may be placed in a container, and then encapsulated by filling the container with foaming plastic. Other materials, such a piping, may be first injected with foam, then the exterior encapsulated with foaming plastic. The foaming plastic expands into interstitial cracks, fractures and surface irregularities. This effectively fixes the radioactive material in place in or on the object and protects it from later contact or removal. Whether or not the object is encased with an outer layer of foam plastic, the object may then optionally be placed in a bag to further protect against eventual leakage. Once completely encapsulated according to the selected method steps, the object is ready to be shipped to a burial site for burial. Referring now to FIG. 2, a typical object that may be radioactively contaminated, a length of pipe 10, is processed by priming or otherwise stabilizing the interior surface, then forming holes 12 in the pipe 10. The method is advantageous when dealing with long lengths of pipe, hose or other elongate object where, due to the length of the object, it may be impractical to inject foaming plastic into the object through or adjacent one end. Plastic is foamed in a foam generator 14 and conveyed through a hose 16 to the holes 12, and foam “F” is injected into the holes 12 successively from one end of the pipe 10 to the other. A temporary or permanent cap 20 may be placed over the ends of the pipe 10 as shown to prevent foam from exiting the pipe 10 through its ends. After injection of the foam in complete, the holes 12 are plugged or capped. FIGS. 3 and 4 illustrate that once the pipe 10 has been filled with foam “F” as shown in FIG. 2, the exterior of the pipe 10 may optionally be coated with a layer 22 of foam “F”. Referring to FIG. 5, an object, for example, a length of I-beam 30 is first sealed in a heavy plastic bag 32. Then, foam “F” is used to completely encapsulate the bagged I-beam 30. Optionally, an elastomeric coating 34 may be placed over the foam “F”. The elastomeric coating 34 will provide greater resistance to tensile and tear stress, damage during transport, and cracking. Referring now to FIGS. 6 and 7, a method for encapsulating large, bulky objects is explained. By way of example, barrels 40, which may themselves be contaminated and/or containing radioactively-contaminated waste, liquid or solid, are placed on pallets 42 and fastened in a suitable manner, as by straps 44. One or more pallets 42 and barrels 40 are then placed in a container 46, for example, as shown in FIG. 7, and then the entire container 46 is filled with foam “F” by injecting it from the foam generator 14 through hose 16. In some instances it will be necessary to provide an outlet 48 to permit contaminated air displaced by the introduction of the foam “F” to be removed to another location 50 for treatment. After the container 46 is filled, it is shipped to a suitable location for burial. More generally, a foaming plastic such as the foam “F” can be used to encapsulate primed or unprimed radioactive waste, thus containing and immobilizing the waste, making it safe to transport to a landfill. The foaming plastic can be poured, sprayed, or otherwise dispensed in and around the contaminant, allowing the foam to rise and fill the interstitial spaces. The foam can also be dispensed over already encapsulated objects that may or may not be primed to render it completely macro-encapsulated and attenuated for further transport. The foam can be injected into pipes, ductwork, or other contaminated spaces where it will fill the voids and immobilize any radioactive materials. The methods of forming a foam generally include providing a blowing agent composition of the present disclosure, adding (directly or indirectly) the blowing agent composition to a foamable composition, and reacting the foamable composition under the conditions effective to form a foam or cellular structure. Any of the methods well known in the art, such as those described in “Polyurethanes Chemistry and Technology,” Volumes I and II, Saunders and Frisch, 1962, John Wiley and Sons, New York, N.Y., which is incorporated herein by reference, may be used or adapted for use in accordance with the foam embodiments. Polyisocyanate-based foams are prepared, e.g., by reacting at least one organic polyisocyanate with at least one active hydrogen-containing compound in the presence of the blowing agent composition described in this application. An isocyanate reactive composition can be prepared by blending at least one active hydrogen-containing compound with the blowing agent composition. According to preferred embodiments of the invention, the blend contains at least 1 and up to 50, preferably up to 25 weight percent of the blowing agent composition, based on the total weight of active hydrogen-containing compound and blowing agent composition. Active hydrogen-containing compounds include those materials having two or more groups which contain an active hydrogen atom which reacts with an isocyanate. Preferred among such compounds are materials having at least two hydroxyl, primary or secondary amine, carboxylic acid, or thiol groups per molecule. Polyols, i.e., compounds having at least two hydroxyl groups per molecule, are especially preferred due to their desirable reactivity with polyisocyanates. Additional examples of suitable active hydrogen containing compounds can be found in U.S. Pat. No. 6,590,005. For example, suitable polyester polyols include those prepared by reacting a carboxylic acid and/or a derivative thereof or a polycarboxylic anhydride with a polyhydric alcohol. The polycarboxylic acids may be any of the known aliphatic, cycloaliphatic, aromatic, and/or heterocyclic polycarboxylic acids and may be substituted, (e.g., with halogen atoms) and/or unsaturated. Examples of suitable polycarboxylic acids and anhydrides include oxalic acid, malonic acid, glutaric acid, pimelic acid, succinic acid, adipic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, trimellitic acid anhydride, pyromellitic dianhydride, phthalic acid anhydride, tetrahydrophthalic acid anhydride, hexahydrophthalic acid anhydride, endomethylene tetrahydrophthalic acid anhydride, glutaric acid anhydride acid, maleic acid, maleic acid anhydride, fumaric acid, and dimeric and trimeric fatty acids, such as those of oleic acid which may be in admixture with monomeric fatty acids. Simple esters of polycarboxylic acids may also be used such as terephthalic acid dimethylester, terephthalic acid bisglycol and extracts thereof. The polyhydric alcohols suitable for the preparation of polyester polyols may be aliphatic, cycloaliphatic, aromatic, and/or heterocyclic. The polyhydric alcohols optionally may include substituents which are inert in the reaction, for example, chlorine and bromine substituents, and/or may be unsaturated. Suitable amino alcohols, such as monoethanolamine, diethanolamine or the like may also be used. Examples of suitable polyhydric alcohols include ethylene glycol, propylene glycol, polyoxyalkylene glycols (such as diethylene glycol, polyethylene glycol, dipropylene glycol and polypropylene glycol), glycerol and trimethylolpropane. Suitable additional isocyanate-reactive materials include polyether polyols, polyester polyols, polyhydroxy-terminated acetal resins, hydroxyl-terminated amines and polyamines, and the like. These additional isocyanate-reactive materials include hydrogen terminated polythioethers, polyamides, polyester amides, polycarbonates, polyacetals, polyolefins, polysiloxanes, and polymer polyols. Other polyols include alkylene oxide derivatives of Mannich condensates, and aminoalkylpiperazine-initiated polyethers as described in U.S. Pat. No. 4,704,410 and U.S. Pat. No. 4,704,411. The low hydroxyl number, high equivalent weight alkylene oxide adducts of carbohydrate initiators such as sucrose and sorbitol may also be used. In the process of making a polyisocyanate-based foam, the polyol(s), polyisocyanate and other components are contacted, thoroughly mixed and permitted to expand and cure into a cellular polymer. The particular mixing apparatus is not critical, and various types of mixing head and spray apparatus may be used. It is often suitable, but not necessary, to preblend certain of the raw materials prior to reacting the polyisocyanate and active hydrogen-containing components. For example, it is often useful to blend the polyol(s), blowing agent, surfactant(s), catalyst(s) and other components except for polyisocyanates, and then contact this mixture with the polyisocyanate. Alternatively, all the components may be introduced individually to the mixing zone where the polyisocyanate and polyol(s) are contacted. It is also possible to pre-react all or a portion of the polyol(s) with the polyisocyanate to form a prepolymer. The invention is further described according to the several examples set out below: A rigid polyurethane foam with the following composition and physical properties was produced by dispensing through high pressure impingement mix equipment. INGREDIENT%Polyol blend34.78Crosslinkers1.45Water0.48Fire retardant3.60Viscosity suppressant1.09Surfactants0.72Catalysts0.14Blowing agent6.04Polymeric Isocyanate51.70TOTAL100.00 Free Rise Core Density: 2.4 lbs/ft3 Molded Core Density: 2.8 lbs/ft3 Compressive Strength: 37 lbs/in2 UL Bulletin 94: Passes HBF Mil-PRF-26514G Meets Type 1, Class 1 Mil-PRF-83671B Meets Class 1, Category 1 The foam was dispensed into pipes ranging in diameter from 2 inches to 8 inches. The foam completely filled the pipe, rendering the radioactive material encapsulated. The piping could then be safely cut into sections without the risk of releasing radioactive materials, and safely transported to a designated site for burial. A rigid polyurethane foam with the following composition and physical properties was produced by dispensing through high pressure impingement mix equipment: INGREDIENT%Polyol blend34.45Crosslinkers3.83Water0.05Fire retardant3.83Viscosity suppressant1.41Surfactants0.72Catalysts0.12Blowing agent3.44Polymeric Isocyanate52.15TOTAL100.00 Free Rise Core Density: 6.3 lbs/ft3 Compressive Strength: 135 lbs/in2 UL Bulletin 94: Passes HBF The foam was pumped into large cylindrical spaces up to 40 inches diameter and 40 inches high for encapsulation of uranium converters. It allowed the converters, which comprise hundreds of tubes for uranium enrichment, to then be safely moved in their entirety to a designated site for burial. There was no need to cut the converters and potentially risk leaking radioactive material. A rigid polyurethane foam with the following composition and physical properties was produced by dispensing through high pressure impingement mix equipment: INGREDIENT%Polyol blend39.38Crosslinkers1.65Water0.12Viscosity suppressant3.07Surfactants0.47Catalysts0.12Blowing agent2.36Polymeric Isocyanate52.83TOTAL100.00 Free Rise Core Density: 6.0 lbs/ft3 Compressive Strength: 160 lbs/in2 The foam is used to encapsulate and immobilize large volume spaces. This can be a dumpster-like container, piping, ductwork, or any large volume space with or without interstitial spaces to fill. A rigid polyurethane foam with the following composition and physical properties was produced by dispensing through high pressure impingement mix equipment: INGREDIENT%Polyol blend33.50Crosslinkers4.78Water0.10Fire retardant4.31Viscosity suppressant0.57Surfactants0.38Catalysts1.82Blowing agent2.39Polymeric Isocyanate52.15TOTAL100.00 Free Rise Core Density: 6.5 lbs/ft3 Compressive Strength: 150 lbs/in2 The foam is sprayed onto equipment or encapsulating bags to smooth out the surface, and attenuate the radioactive material. A polyurea elastomeric coating with the following composition and physical properties was produced by dispensing through high pressure impingement mix equipment to form an outer coating: INGREDIENT%Polyetheramine blend 42.31Amine Crosslinker4.81Moisture Scavenger0.96Isocyanate Prepolymer51.92TOTAL100.00 Tensile Strength: 3000 lbs/in2 Tear Strength: 436 lbs/in Elongation: 364% Shore Hardness: 70 Shore D The elastomeric material is sprayed over equipment or encapsulating bags or foaming plastic encapsulants to create a durable outer coating that is resistant to puncture, tensile stress, and damage during transport to its final disposition. A composition and process for encapsulating radioactive wastes to render them suitable for shipment according to the invention have been described with reference to specific embodiments and examples. Various details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description of the preferred embodiments of the invention and best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims. |
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description | This application claims the benefit of Austrian Patent Application Serial Nos. A 755/2004, filed 30 Apr. 2004 and A 1492/2004, filed 6 Sep. 2004 The invention relates to improvements of a multi-beam pattern definition device for use in a particle-beam processing apparatus. More in detail, the invention relates to a pattern definition device for use in a particle-beam processing apparatus, which device is adapted to be irradiated with a beam of electrically charged particles and allow passage of the beam only through a plurality of apertures which are of identical shape and define the shape of beamlets permeating them; the apertures are arranged within a pattern definition field according to a specific arrangement for defining the pattern on a target; with said apertures are associated corresponding blanking openings located such that each of the beamlets traverses that blanking opening which corresponds to the aperture defining the beamlet respectively, and each blanking opening is provided with a deflection means that can be controlled by a blanking signal between two deflection states, namely, a first state (‘switched on’) when the deflection means has assumed a state in which particles radiated through the opening are allowed to travel along a desired path, and a second state (‘switched off’) when the deflection means is deflecting particles radiated through the opening off said path. In other words, the particle beam is generated by an illumination system, and it illuminates a pattern definition (PD) means having an array of apertures which define a beam pattern to be projected on a target surface. The passage of each beam through an aperture can be controlled so as to allow (‘switch on’) or effectively deactivate (‘switch off’) the passage of particles of the beam through the respective apertures. The beam permeating the aperture array (or more exactly, through the switched-on apertures of the array) forms a patterned particle beam bearing a pattern information as represented by the spatial arrangement of the apertures. The patterned beam is then projected by means of a particle-optical projection system onto the target (for instance, a semiconductor substrate) where an image of the apertures is thus formed to modify the target at the irradiated portions. One important application of processing apparatus of this kind is in the field of nano-scale patterning, by direct ion beam material modification or ion beam induced etching and/or deposition, used for the fabrication or functionalization of nano-scale devices, particularly having sub-100 nm feature sizes. Another important application of processing apparatus of this kind is in the field of particle-beam lithography used in semiconductor technology, as a lithography apparatus, wherein, in order to define a desired pattern on a substrate surface, the wafer is covered with a layer of a radiation-sensitive photoresist, a desired structure is imaged onto the photoresist by means of a lithography apparatus which is then patterned by partial removal according to the pattern defined by the previous exposure step and then used as a mask for further structuring processes such as etching. In the context of this disclosure, however, rather than an indirect process such as a photoresist structuring, the invention is primarily discussed in the context of a direct structuring process which employs an ion beam material modification where the particle beam itself causes modification of the substrate to be structured. The potential use of an addressable aperture plate for direct pattern transfer by charged particle beam projection has been investigated since more than a decade. One early discussion is given by B. Lischke et al., Microelectronic Engineering 9, 1989, pp. 199-203. Later, in 1997, I. L. Berry et al., in J. Vac. Sci. Technol. B, 15 (6), 1997, pp. 2382-2386, presented a writing strategy based on a blanking aperture array and an ion projection system. Arai et al., U.S. Pat. No. 5,369,282, discuss an electron-beam exposure system using a so-called blanking aperture array (BAA) which plays the role of the PD means. The BAA carries a number of rows of apertures, and the images of the apertures are scanned over the surface of the substrate in a controlled continuous motion whose direction is perpendicular to the aperture rows. The rows are aligned with respect to each other in an interlacing manner so that the apertures form staggered lines as seen along the scanning direction. Thus, the staggered lines sweep continuous lines on the substrate surface without leaving gaps between them as they move relative to the substrate, thus covering the total area to be exposed on the substrate. In the U.S. Pat. No. 5,369,282, the apertures of every second row align and the pitch between neighboring apertures in a row is twice the width of an aperture; in general, an alignment of rows is possible based on any number n, the pitch then being n times the width of an aperture. Yasuda et al., U.S. Pat. No. 5,359,202 and U.S. Pat. No. 5,260,579 use a similar BAA for exposing a substrate with a pattern, but the need to fed control lines to each of the blanking apertures makes the internal structuring very complex and prone to unwanted disturbing effects such as cross-talking and transit time delays. The above-mentioned article of I. L. Berry et al. describes a PD device comprising a “programmable aperture array” with an array of 3000×3000 apertures of 5 μm side length with an n=4 alignment of rows and staggered lines. The aperture array contains additional logic circuitry, thus implementing an electronic mask scanning system in which the pattern information is passed by means of shift registers from one aperture to the next within a row. The article proposes to use a 200× demagnification ion-optical system for imaging the apertures of the BAA onto the substrate. Starting from Berry's concept, E. Platzgummer et al., in the U.S. Pat. No. 6,768,125 (=GB 2 389 454 A), present a multi-beam direct write concept, dubbed PML2 (short for “Projection Mask-Less Lithography #2”), employing a PD device comprising a number of plates stacked on top of the other, among them an aperture array means (aperture plate) and a blanking means (blanking plate). These separate plates are mounted together at defined distances, for instance in a casing. The aperture array means has a plurality of apertures of identical shape defining the shape of beamlets permeating said apertures, wherein the apertures are arranged within a PD field composed of a plurality of staggered lines of apertures, wherein the apertures are spaced apart within said lines by a first integer multiple of the width of an aperture and are offset between neighboring lines by a fraction of said integer multiple width. The blanking means has a plurality of blanking openings arranged in an arrangement corresponding to the apertures of the aperture array means, in particular having corresponding staggered lines of blanking openings. The teaching of the U.S. Pat. No. 6,768,125 with regard to the architecture and operation of the PD device, and in particular the architecture of its blanking plate, are hereby included as part of this disclosure. According to the PML2 concept, the image formed by the beam is moved continuously along a straight path over each die field; additional scanning of the beam in a direction perpendicular to the scanning direction is not necessary (except, where needed, to compensate for lateral travel motion errors of the scanning stage). Furthermore, gray scales can be generated by subsequent exposures of apertures located in line, so that a shift register approach can be effectively applied to create gray scale patterns (of a predetermined bit size, e.g. 5 or more bits) while only the substrate is moved. The PML2 concept involves the use of a large amount of memory on the aperture plate, located next to the apertures either in between or in the vicinity. A main distinctive feature of the PML2 over prior art is a row offset between blocks of regular apertures on the aperture plate, intended for the placement of shift register electronic circuits on the blanking plate. The key for the realization of an addressable mask is the so-called Micro-Electro and micro-Mechanical System (MEMS) technology, which allows the monolithic fabrication of hundred thousands up to millions of apertures together with the electronics needed to control the beam switching and data management. Since the minimum feature size of industrial MEMS devices is about 100 to 200 times larger than the typical critical dimension in lithography (for example the size of apertures and micro-deflectors), a powerful large field high resolution optical projection system is obligatory for exploitation of the advanced MEMS fabrication technologies in the field fast writing applications, such as for example mask-less lithography. The main advantage of the multi-beam approach inherent to the PLM2 is the vast enhancement of the writing speed compared to single beam writers, due to the fact that the charged particle beam comprises a plurality of sub-beams, dynamically structured by an aperture plate including switchable blanker devices. The improved productivity (over other prior art, such as Arai et al. and Yasuda et al.) arises mainly from the following features: the possible number of sub-beams directed parallel to the substrate and the possible density of apertures per area is significantly increased, resulting in relaxed requirements for the particle source; single beam blanking is achieved by a continuous data stream and a simplified data line architecture, where only one aperture row (=number of lines×one aperture) is to be fed into the PD field per clock cycle, the signal traveling by shift registers over the PD field; the influence of space charge is reduced since the beam current is distributed over a large cross section as a consequence of using a broad beam; a high degree of redundancy is produced using a large number of fractional exposures (apertures in line) to cumulate the desired exposure dose, which permits a gray scale generation during single pass scanning. However, with the PLM2 layout, like with other prior art, the following main problems arise: using the physical address grid available by the BAA of prior art, in particular Arai et al. and Berry et al., only an insufficient number of gray scale levels is achieved in single pass exposures in order to fulfill lithographic requirements (1 nm address grid for 45 nm node), as the size of the PD field is limited in large field projection systems. The consequences are a poor process latitude, untolerable values for the line edge roughness, which is related to the physical address grid, and insufficient pattern placement accuracy, or in case of a multi pass strategy, reduced throughput and unwanted alignment errors. the need for a distortion-free imaging of a large pattern field as a consequence to the requirement to bring thousands of apertures in overlay during the scanning process (to exploit the high redundancy); and dealing with an unavoidable current dependent (=pattern dependent) image distortion and de-focusing, limiting the usable current and requiring pattern homogenization, which will involve a time consuming data pre-processing. In view of the above, it is the task of the present invention to find a way to overcome the deficiencies of prior art and allow the use of an addressable mask for applications such as mask-less lithography or nano-scale beam patterning applications. A central aim is to enhance the resolution that can be obtained with a PD device, realizing a finer address grid and reducing the line edge roughness despite an unchanged or even reduced number of apertures compared to prior art. Furthermore, it shall allow the irradiation of pixels at the target according to a gray scale, i.e. with exposure levels interpolated between a minimal (‘black’) and maximal (‘white’) exposure dose. This task is solved by a PD device as described in the beginning wherein the positions of the apertures taken with respect to a direction perpendicular to a scanning direction and/or parallel to it are offset to each other by not only multiple integers of the effective width of an aperture taken along said direction, but also by multiple integers of an integer fraction of said effective width—the latter kind of offsets are referred to as ‘fractional offsets’ in the context of the invention. In this context, ‘scanning direction’ denotes the direction along which the image of the apertures formed by the charged-particle beam on a target surface is moved over the target surface during an exposure process. The basic idea of the invention is to realize the positions of the aperture not only along an imaginary basic grid which results when copies of the base shape of the apertures are assembled to a contiguous covering of the plane area, but to multiply the possible positions by interpolating additional imaginary grids. (The base shape is usually a square or rectangle oriented along the scanning direction, but it could be another shape as well, in particular a regular polygon such as a hexagon). These grids mesh into each other, which is why they are referred to as “interlocking grids” in the context of the invention. It should be appreciated that not all points of these grids are occupied by an aperture; in fact, only a small fraction is, since the apertures need sufficient ‘solid’ space between each other (cf. the discussion below with FIG. 2). On account of the invention a finer resolution is available on the target surface even though the individual spots formed by each image of an individual aperture are not decreased in size. Furthermore, the invention enables a downscaling of the lithographic node, for example from 45 mn to 32 nm lines and spaces resolution, without simultaneously downscaling the critical dimensions in the PD device. Hence, the invention helps to circumvent the feature size limitation of state-of-the-art MEMS technology. All together, the invention allows to significantly reduce the diameter of the optical beam and the required PD device in PML2 with the purpose to relax optical performance requirements, such as for example the absence of distortion, and to improve productivity by using several columns in parallel. In a preferred embodiment of the invention, the device comprises an aperture array means for forming a number of beamlets and a blanking means for controlling the passage of selected beamlets. Said aperture array means then has a plurality of apertures of identical shape defining the shape of beamlets permeating said apertures which are arranged within a pattern definition field as described above; and said blanking means has a plurality of blanking openings arranged in an arrangement corresponding to the apertures of the aperture array means. Preferably, the fractional offsets are integer multiples of ½N times the effective width of an aperture, where N is a positive integer, preferably greater than 1. In a preferred way to arrange the apertures according to the invention the different grids are located on different regions on the aperture field, so as to simplify the overall layout. Accordingly, the pattern definition field may be segmented into several domains, each domain being composed of a plurality of staggered lines of apertures running along the scanning direction, wherein the apertures are spaced apart within said lines by an integer multiple of the effective width of an aperture and are offset between neighboring lines by a fraction of said integer multiple width, wherein along the direction perpendicular to the scanning direction, the apertures of a domain are offset to each other by multiple integers of the effective width of an aperture, whereas the offsets of apertures of different domains are fractional offsets. In this case, in order to facilitate calculation of the irradiation patterns, the domains may have arrangements of apertures which are corresponding as regards the relative position of aperture within each domain. In particular, the domains may have the same number of apertures. Suitable numbers of domains with different fractional offsets are 22N-1 or 2N (with the integer N as above), in the latter case the fractional offsets preferably run along a diagonal of an aperture's base shape. In a preferred embodiment of the invention the group blanking signals are fed to the PD field partly at a side running parallel to the orientation of lines, partly at a side running perpendicular. This further reduces the density of lines at the feeding sites. In a suitable realization of the aperture arrangement, the shape of the apertures is substantially equivalent to a two-dimensional geometrical base shape of a contiguous covering of the plane, such as a square or a regular hexagon. The advantage of a contiguous covering arrangement is that the influence of the optical imaging blur on the dose distribution on the wafer (=aerial image), in particular on the gray levels needed for a specific feature, is as small as possible, so that an intrinsic radial variation of the optical imaging blur inside the projected image of the PD device can be tolerated. If the blur is in the range of the spot size, the same advantage can be achieved by choosing the area of the apertures substantially equivalent to a two-dimensional geometrical base shape of a contiguous covering of the plane, whereas the shape may differ from that of a contiguous covering, in particular by modifying the edges of a polygonal base shape such as rounding the edges or beveling (canting). The total shape should be enlarged, if necessary, so the total area of the modified shape is kept, and the area of the shape of the apertures is the same as that of the original polygonal base shape. This means, for example, that instead of perfectly square-shaped apertures it is possible to use corner-rounded square shaped apertures with equal area. The latter allows higher tolerances in the fabrication process. In general, a hexagonal arrangement has the advantage that it has the highest possible density of pixels per area combined with the highest degree of symmetry, both improving the achievable line placement precision and line edge roughness at limited number of apertures in the PD means (=limited size of the PD field). In order to have a reservoir of ‘extra’ openings which can be accessed when needed, in particular if one or more other blanking openings are found to be defect, additional blanking openings may be provided which are activated or deactivated for operation. The activation/deactivation can, for instance, be done by a structuring step such as irradiation by focused ion, electron or laser beam. Thus, blanking opening may be provided for which the line feeding the respective blanking signal to said blanking opening(s) comprises a component which is accessible on a surface of the device by a structural modification and which is adapted to change its transmissivity for the group blanking signal between an electrically connecting state and a blocking state upon treatment by said structural modification. In particular, the component may be realized as a conductor segment adapted to be modified, possibly irreversibly, between an electrical well-conducting and a non-conducting state. In a further aspect of the invention, in order to realize an efficient provision of the blanking signals to the individual blanking openings, the blanking signals are derived from feeding lines, each feeding line serving blanking signals for a number of blanking openings, which are propagated through a series of shift registers into a sequence of intermediate buffer means, one buffer means for each blanking opening, and the data contained in the buffer means are activated by a common trigger signal. In this case, the blanking signal for each blanking opening may be a multi-bit signal, said signal coding a duration of how long within one exposure time the respective aperture is switched on—in terms of a fraction between zero (switched off) and one (fully switched on). Preferably, the blanking is done by a small change to the angle of the beamlet which is deflected only so far that it does not reach the target or any of the devices at the target position. To this end, the deflection means may be adapted to deflect, in the switched off state, the particles to an absorbing surface of the exposure apparatus mounted after the PD device as seen in the direction of the particle beam. Pattern Definition System The preferred embodiments of the invention discussed in the following are based on the pattern definition (PD) system disclosed in the U.S. Pat. No. 6,768,125, used in a ion-beam processing apparatus. In the following, the technical background of the PD system, as far as relevant to the invention, is first discussed with reference to FIGS. 1 to 5 (which correspond, with modifications where appropriate, those of the U.S. Pat. No. 6,768,125), then embodiments of the invention in the PD system are discussed. It should be appreciated that the invention is not restricted to the following embodiments, which merely represent some of the possible implementations of the invention. An overview of a processing apparatus employing the preferred embodiment of the invention is shown in FIG. 1. In the following, only those details are given as needed to disclose the invention; for the sake of clarity, the components are not shown to size in FIG. 1. The main components of the apparatus 100 are—corresponding to the direction of the lithography beam lb, pb which in this example runs vertically downward in FIG. 1—an illumination system 101, a PD system 102, a projecting system 103, and a target station 104 with the substrate 41. The whole apparatus 100 is contained in a vacuum housing 105 held at high vacuum to ensure an unimpeded propagation of the beam lb, pb along the optical axis cx of the apparatus. The particle-optical systems 101, 103 are realized using electrostatic or electromagnetic lenses. The illumination system comprises, for instance, an electron gun 11, an extraction system 12 as well as a condenser lens system 13. It should, however, be noted that in place of electrons, in general, other electrically charged particles can be used as well. Apart from electrons these can be, for instance, hydrogen ions, heavier ions, charged molecules or clusters, the choice of projectile depending on the desired beam-substrate interaction. The extraction system 12 accelerates the particles to a defined energy of typically several keV, e.g. 10 keV. By means of a condenser lens system 13, the particles emitted from the source 11 are formed into a wide, substantially telecentric particle beam serving as lithography beam lb. The lithography beam lb then irradiates a PD device 20 which, together with the devices needed to keep its position, form the PD system 102. The PD device 20 is held at a specific position in the path of the lithography beam lb, which thus irradiates a plurality of apertures 21 (see FIG. 2). Some of the apertures are “switched on” or “open” so as to be transparent to the incident beam in the sense that they allow the portion of the beam (beamlet) that is transmitted through it to reach the target; the other apertures are “switched off” or “closed”, i.e. the corresponding beamlets cannot reach the target, and thus these apertures are effectively non-transparent (opaque) to the beam. The pattern of switched-on apertures is chosen according to the pattern to be exposed on the substrate, as these apertures are the only portions of the PD device transparent to the beam lb, which is thus formed into a patterned beam pb emerging from the apertures (in FIG. 1, below the device 20). The temperature distribution over the PD device 20 is kept stable by appropriate heating or cooling elements, with optional means 28, 29 for irradiation cooling in addition to thermally conductive cooling. The pattern as represented by the patterned beam pb is then projected by means of an electro-magneto-optical or purely electro-optical projection system 103 onto the substrate 41 where it forms an image of the switched-on mask apertures 21. The projection system 103 implements a demagnification of, for instance, 200× with two crossovers c1, c2. The substrate 41 is, for instance, a silicon wafer covered with a photo-resist layer. The wafer 41 is held and positioned by a wafer stage 40 of the target station 104. The apparatus 100 may further comprise an alignment system 60, which allows to stabilize the position of the image of the mask apertures (image field mf, FIG. 3) on the substrate with respect to the particle-optical system by means of reference beams which are formed in the PD system by reference marks 26 at the side of the PD field pf (FIG. 2); the principles of an alignment system are described in the U.S. Pat. No. 4,967,088. For instance, correction of image position and distortion can be done by means of a multipole electrode 315, 325; additionally, a magnetic coil 62 can be used to generate a rotation of the pattern in the substrate plane. In the embodiment of the invention shown in FIG. 1, the projection system 103 is composed of two consecutive electro-magneto-optical projector stages 31, 32. The lenses used to realize the projectors 31, 32 are shown in FIG. 1 in symbolic form only, as technical realizations of particle imaging systems are well known in the prior art, such as, for instance, the U.S. Pat. No. 4,985,634 (=EP 0 344 646) of the applicant (assignee). The first projector stage 31 images the plane of the apertures of the device 20 into an intermediate plane e1 which in turn is imaged onto the substrate surface by means of the second projector stage 32. Both stages 31, 32 employ a demagnifying imaging through crossovers c1, c2. The demagnification factor for both stages is chosen such that an overall demagnification of several hundred results, e.g. 200×. A demagnification of this order is in particular suitable with a lithography setup, in order to alleviate problems of miniaturization in the PD device. A stop plate 204 is provided at, for instance, the position of a crossover c1, in order to block out beam components which are deflected off the regular beam path. In both projector stages the respective lens system is well compensated with respect to chromatic and geometric aberrations; furthermore, a residual chromatic aberration of the first stage 31 can be compensated by suitable fine correction of the electrode potentials in the second stage 32. As a means to shift the image laterally as a whole, i.e. along a direction perpendicular to the optical axis cx, deflection means 315, 325 are provided in one or both of the projector stages. The deflection means can be realized as, for instance, a multipole electrode system which is either positioned near to the crossover, as shown in FIG. 1 with the first stage deflection means 315, or after the final lens of the respective projector, as is the case with the second stage deflection means 325 in FIG. 1. In this apparatus, a multipole electrode is used as deflection means both for shifting the image in relation to the stage motion and for correction of the imaging system in conjunction with the alignment system. These deflection means 315, 325 are not to be confused with any additional deflection array means that may be present within the PD device which are primarily intended for correction of individual beamlets (Austrian patent application A 1711/2003 of the applicant/assignee). FIG. 2 shows a plan view of the (basic) arrangement of apertures in the PD device 20. A plurality of square-shaped apertures 21 is provided which are arranged within a PD field pf in, basically, a regular array in which the apertures 21 are aligned along adjacent lines pl, wherein in each of the lines pl the same number of apertures is present. Seen along the direction perpendicular to the lines pl, the apertures form a sequence of rows r1, r2, r3; in the embodiment shown, the rows r1-r3 are not adjacent but spaced apart. The apertures are arranged in the PD field pf according to a skewed regular arrangement described in detail further below. The arrangement of the apertures according to the invention is a variation of a basic grid an example of which is depicted in FIG. 2. According to the basic arrangement, the apertures of every n-th row align (in FIG. 2, n=3) as the pitch pn between neighboring rows is n times the width w of an aperture, i.e., pn=n×w. The offset pm between neighboring rows is m times the width of an aperture, pm=m×w (the specific arrangement of FIG. 2 has m=4), and within a line pl, the offset of apertures is n·pm=n×m×w (in FIG. 2, n×m=12). Thus, the apertures cover only 1/(n×m) of the area of the field pf and, at a time, only one out of n×m image elements can be exposed as shown in FIG. 3; the other elements are exposed in subsequent steps by means of moving the substrate along the “scanning direction” sd relative to the image of the apertures. For details about spatial arrangement and circuitry to control the apertures, the reader is referred to the U.S. Pat. No. 6,768,125. FIG. 3 illustrates the image field mf produced on the substrate; for the sake of clarity it is assumed that all apertures are switched on in this figure. The width fw of the image field is the width L of the PD field pf reduced by the demagnification factor of the projection system. The image field is composed of a plurality of image elements mx (also referred to as pixels). For a given position of the image field on the substrate, each of the apertures 21 of the aperture array corresponds to an image element mx, but as the apertures only cover a fraction of the PD field area, only a corresponding fraction of the number of image elements (shown hatched in FIG. 3) can be exposed at a time. In order to expose also the other image elements, the substrate is moved under the beam so as to shift the image field on the substrate. It will be clear that any alternative way may be used for effecting a relative motion of the image over the substrate; for instance, in one suitable way for nano-scale beam patterning with a stable substrate position, the beam is deflected over the substrate. FIG. 3a illustrates the exposure of pixels in subsequent positions of the motion of the substrate through the possible positions for the case n×m=3×4=12; the pixels are accordingly referenced with letters a to l (the pixels shown hatched are position a). The whole image field mf is moved over the surface of the photoresist-covered wafer serving as substrate 41 so as to cover the total area of the substrate surface. The scanning direction sd may also reversed when one sequence of die fields is finished and imaging of the next sequence begins (boustrophedonal motion as in FIG. 4 of U.S. Pat. No. 6,768,125). FIGS. 4 and 5 show the architecture of the PD system 102, namely, in FIG. 4 a top view and in FIG. 5 a longitudinal-sectional view. FIG. 6 shows a detail of FIG. 5, illustrating the configuration of the set of plates constituting the PD system 102 of the present embodiment along two apertures. The PD system 102 comprises a number of plates 22 mounted in a stacked configuration, realizing a composite device whose components serve respective functions. Each of the plates 22 is realized as a semiconductor (in particular silicon) wafer in which the structures were formed by micro-structuring techniques known in the art. The lithography beam traverses the plates through an array of apertures in the PD field pf (FIG. 5). Each aperture corresponds to a set of openings 210, 220, 230 which are defined in the plates 22 (FIG. 6). The thickness of each of the plates 22 is about 500 μm to 50 μm in the area of the apertures; their mutual distance is in the order of 10 μm to 1 mm. It should be noted that in FIGS. 5 and 6, the dimensions in the longitudinal axis (z-axis parallel to the optical axis of the apparatus) are enlarged and not to scale. The blanking of the beamlets is controlled by means of a blanking means realized as a blanking plate 202 which comprises an array of openings 220 (“blanking openings”), each corresponding to an aperture. Each blanking opening 220 comprises a set of beam blanking electrodes 221 as well as the circuitry 222 for controlling the electrodes 221a,221b, which are accommodated, for instance, on the lower surface layer of the blanking plate 202. The blanking electrodes 221, serving as aperture deflection plates as described below, are formed around the blanking openings by perpendicular growth employing state-of-the-art techniques. In order to provide a better shielding of the blanking openings against cross-talking and other unwanted effects, one of the electrodes 221a may be formed so as to have a substantial height over the blanking plate 202. Preferably, this electrode 221a is connected with a uniform potential (e.g., ground potential) for all apertures, while to the other electrode 221b the controlling potential for switching between the ‘on’ and ‘off’ states is applied. Further details about the layout of the blanking plate 202 and its circuitry 222 can be found in the U.S. Pat. No. 6,768,125. The PD system 102 further comprises an aperture array means which serves to define the shape of the beamlets laterally and which is here realized as an aperture array plate 203 (in the following in short ‘aperture plate’) with an array of openings having a width w3, positioned after the cover and blanking plates 201, 202. More details about the layout of the aperture plate 203 can be found in the U.S. Pat. No. 6,768,125. Preferably, the sequence of the functional plates, the cover plate, the blanking plate and the aperture plate, is chosen in a way that plate-to-plate alignment as well as heating, charging and contamination effects inside and between the plates of the PD system can be controlled easily. The arrangement shown in FIG. 6, in which the cover plate is positioned on top, the blanking plate in the middle and the aperture plate at the bottom as seen from the direction of the incoming beam, is just one possible arrangement. Other possibilities might be an arrangement of a PD device 102′ shown in FIG. 18 where the aperture plate is combined with the function of the cover plate to a “beam forming plate” 204 which is positioned above the blanking plate 203, or the beam forming plate is bonded directly on the blanking plate. In this case, the beamlet bm is defined upon passing the aperture 230′ with the appropriate width w3. FIG. 18a shows the option of another variant of the PD device 102″ with a two plate system where the blanking electrodes are placed in the space between the aperture plates. The beam deflection angles for blanking are generally very small, typically 0.5 to 5 thousands of a radian, so that the distance between the deflected beam and the side wall of the apertures is sufficient. One important advantage of variant 102′ is that it enables a PD device consisting of only two plates, where both the upper and the lower plate can be used as part of a “diverging lens” (cf. U.S. Pat. No. 5,801,388 and U.S. Pat. No. 6,326,632 of the applicant/assignee). This advantage is made possible by placing the blanking electrodes between the plates, so that the electrodes of high aspect ratio do not lead to a deformation of the electric field gradient outside the interstitial space between the two plates which is applied to realize the diverging lens. Further, an important advantage of variant 102′ is offered by choosing a small distance between the plates to efficiently reduce cross-talk related limitations of the PD device (e.g. 10 μm between the upper side of the longer electrodes used as shielding electrodes and the beam forming plate above). A small distance can be achieved for example by stacking the two aperture plates using appropriate spacing elements. In front of the aperture and blanking plates 202,203, as seen in the direction of the lithography beam, a cover means realized as a cover plate 201 is provided in order to protect the other plates from irradiation damage. The cover plate 201 takes up the majority of the impingent lithography beam lb; the particles can only pass through the openings 210, formed in an array corresponding to that of the blanking plate, which openings make up only a small fraction of the total area of the blanking field bf. More details about the layout of the cover plate 201 can be found in the U.S. Pat. No. 6,768,125. It is the aperture 230 of width w3 (rather than the initial opening in the cover plate 201) which defines the lateral shape of the beamlet emerging from the system 102 (corresponding to the width w of an aperture in FIG. 2). Therefore, strictly speaking the term ‘apertures’ should be reserved to the openings of defined shape and width w (FIG. 2) as defined by the beamlet-defining apertures 230, in contrast to ‘opening’ which is used as generic term; however, the term ‘aperture’ is also used to denote the set of corresponding openings 210,230,220 through which one beamlet bm propagates, as in FIG. 6. The width w2 of the blanking opening 220 is greater than the width w1 of the opening 210 in the cover plate 201, so the beamlet bm defined by the latter opening will pass through the former opening without affecting the controlling circuitry 222 on the blanking plate 202. For instance, the width w2 can be 7 μm (as compared to the defining width of the aperture of w=5 μm). The beamlet bm transgresses the subsequent openings of the plates 22 along the path p1 and is then imaged in the imaging system (FIG. 1), provided the blanking electrodes 221a,221b are not energized; this corresponds to the “switched-on” state of the aperture (with respect to the switching state, no distinction is made between the blanking opening, the respectively associated aperture or beamlet defined by that aperture). A “switched-off” aperture is realized by energizing the electrodes 221a,221b applying a transverse voltage. In this state, the blanking electrodes 221a,221b deflect the beamlet bm off the path p1 to a deviating path p0 so the beamlet will be absorbed, for instance at the stop plate 204 (FIG. 1) positioned at some place after the PD device. It should be appreciated that the beamlet bm will be deflected in the switched-off state by an angle which will be rather small, and the beamlet may still pass through the aperture 230 as shown in FIG. 6; however, the deflection caused by this angle is sufficient to bring about a lateral deviation of the angel at a later position that it is easy to block off the (“switched-off”) beamlet. Referring again to FIGS. 4 and 5, the plates 22 are held by chucks 23 which are positioned with respect to each other by means of actuators 24,25 realized as piezoactuators or nanopositioning elements of known type. The vertical actuators 25 may also be left off in order to save space; then the positioning between the plates is defined by the height of the chucks 23 themselves which then are simply stacked on each other. One of the chucks, in FIG. 5 for instance the chuck of the last plate, may be formed as a cup 233 so as to facilitate lateral positioning of the other chucks. Preferably, the plates 22 and chucks 23 are produced from the same material, e.g. silicon, or materials having the same thermal expansion behavior in the operating temperature range. The chucks also provide for the electric supply of the blanking plate 202; for the sake of clarity, the electric lines are not shown in the figures. In the plates 22 openings 26 are provided for the definition of reference beams rb. The shape of the reference beams rb is defined, for instance, in the opening formed in the aperture plate 203, whereas the corresponding openings in the other plates are wide enough so as to let pass the radiation for the reference beams rb. The reference beams rb and the patterned beam pb are then imaged towards the substrate plane; in contrast to the patterned beam pb, however, the reference beams rb do not reach the substrate 41 but are measured in the alignment system 60 as already mentioned above. The chucks 23 further have alignment openings 236 which serve as alignment markers for relative positioning of the chucks 23 and the plates 22 they hold. Interlocking Grids According to the state-of-the-art layout discussed above, a regular grid is formed by spots with a certain spotsize and the spot distance is equal to this spotsize in both directions (FIGS. 2 and 3). The positions of apertures according to such a regular imaginary grid are referred to as basic grid in the following. According to the invention, the apertures are arranged along “interlocking grids” in order to create additional exposure spots on the substrate which are interpolated between the positions of spots of the basic grid. Some apertures are shifted (offset) with respect to the basic grid in horizontal and vertical direction by a fraction of the effective aperture width, in particular ½N (N is a positive integer, preferably N≧2) of the diameter for square or rectangular grids. In the following, the invention is described in the context of an arrangement of apertures realized on a basic grid of n=5 and m=6 (i.e., n×m=30). The minimum feature size to be illuminated on the wafer is 45 nm (referred as 45 nm node with 45 nm resolved lines and spaces), and the spot size is 25 nm (pixel width x in FIG. 3, equaling geometric image size of one aperture). The image field width fw is 24 μm; in order to produce this image field in connection with a 200× demagnification projection system (see above), the square-shaped PD field has a width L=4.8 mm, corresponding to a number of lines pl=L/w=960, and 960 bit streams are to be addressed by the incoming data stream. In the direction across, there are fw/(n·x)=L/(n·w)=192 apertures in each of the rows r1-r3. As already pointed out, the blanking plate has an array of blanking openings, hereinafter called the blanking field bf, which reflects the arrangement of apertures in the aperture field in direct correspondence. One suitable embodiment of a blanking plate 702 according to the invention is shown in the plan view of FIG. 7 (the other plates of a PD device, such as the cover and aperture plates, would have to be dismounted to reveal this view). In the center of the blanking plate 702, the blanking field bf can be found. Like with the arrangement of apertures, there are 960 lines having 192 blanking openings each in the blanking field bf. Since this arrangement of openings cannot be represented to scale, the field bf is represented with a cross-hatching only in FIG. 7. At the periphery of the blanking plate 702, the data stream of the pattern to be produced is fed in via a number of pad connections 713. Since due to necessary size of the pad connections their number is limited, the pattern signal is multiplexed to a smaller data width at a higher data rate, with the data width corresponding to the number of pad connections. Converter means 721, 722, 723, 724, which are preferable positioned surrounding the field bf, decode the data to the control signals which are applied to the blanking openings in the field bf. These signals are applied using a number of feeding lines 711 (only a part of the feeding lines are shown in the figure for sake of clarity; actually, there are 960 feeding lines 711 plus several control signal lines, as explained below). FIG. 8 shows the upper left edge of the aperture field pf. In the embodiment shown, the first n=5 rows (as seen from the left) are aligned so as to cover the whole width of the aperture field (running vertically in FIG. 8), and thus form a first set Da1. The next set Da2 of n=5 rows are offset by one half of the diagonal of the square of an aperture with regard to the basic grid defined by the first set. The third set Da3 of rows is offset by one half of the aperture width along the Y direction from the basic-grid position; the fourth set Da4 by one half of the aperture width along the X direction. The next four sets Da5, Da6, Da7, Da8 are offset by multiples of a quarter of aperture width along the X and Y directions as denoted in FIG. 8. Each of the sets Da1-Da8 thus forms a “domain” of apertures representing one of eight interlocking grids. Following the eighth set Da8, another set Db1 aligning with the first set Da1 is realized, starting a new sequence of domains. FIG. 9 shows the way the aperture field pf is segmented into such domains D. The sequence of domains Da1 through Da8 is repeated three times, resulting in a total of thirty-two domains Da1-Da8, Db1-Db8, Dc1-Dc8, Dd1-Dd8. Domains denoted with identical digits represent the same grid, while the grids of domains with different digits are offset to each other, they are interlocking with respect to each other. FIG. 10 shows a detail of the aperture arrangement in the first two domains Da1, Da2, and FIG. 11 a like detail of a border region between the second and third domains Da2, Da3. The arrangement of apertures which would be present if the arrangement of domain Da1 were continued into the consecutive domains Da2, Da3 is shown as hatched squares. The detail of FIG. 10a shows that the apertures of the domain Da2 are offset by one half of the aperture size along the diagonal, i.e., by {½, ½} of the basic grid. The detail of FIG. 11a shows the offset of the apertures corresponding to {½, 0} with respect to the basic grid. FIG. 12 shows a graphical survey of the fractional offsets of the domains Da1-Da8 using a sequence of details like that of FIGS. 10a and 11a. In the embodiment underlying FIGS. 8-12, the number of domains with different grids is eight, realizing multiple interlocking grids with fractional offsets that are multiples of ¼ of the aperture width. In general, the number of domains and the fractional offsets can be chosen freely as suitable for the individual application; preferably there are 22N-1 domains realizing fractional offsets equal to multiples of the binary inverses, ½N (with the sum of the X and Y offsets equal to an even multiple of ½N). Interlocking arrays allow spatial pixel interpolations, and hence an improved flexibility with respect to image placement and line edge roughness. Moreover, the required number of gray levels (see below) for a pre-defined address grid may be significantly reduced. Interlocking grids can be used to interpolate an existing pixel representation, for example, to reduce the required number of gray levels—which may be important when down-scaling of the PD field is desired—or to improve resolution by using a smaller address grid which is important to define intricate structures such as the structures of transistor gates. The necessary data pre-calculation, such as for example the calculation of a linear interpolation interlocking grid for a given gray pixel data, can be done during operation of the structuring process or off-line beforehand. It is recalled that the need for a gray scaling in direct-write lithography comes from the situation that semiconductor industry design rules imply smaller and smaller address grids in order to match the specifications with respect to image placement and line edge roughness. For example, using a geometric spot size of 25 nm with about 22.5 nm blur and an address grid with 25 nm pitch, simulations show that 46 gray levels are sufficient to place the features with the required precision of 1 nm on the wafer (meaning a line-position error below 0.5 nm and a line-width error below 0.5 nm at all grid positions). Additionally, to meet lithography specifications the total line width variation has to be less than about 10% of the critical dimension (=nominal minimum line width), which means that the single side line edge roughness has to be less than 5% of line width. It is remarked that the line edge roughness is a measure for the linearity of lines, expressed by the deviation of a straight line of the level curve of the total dose distribution at the exposed wafer (aerial image) after cutting off at the threshold dose. The main advantages of gray pixels arranged in multiple interlocking grids are: 1) It is possible to place a line with 45 nm and 32 nm line-width with a required precision of 1 nm (virtual grid) on the wafer (calculations were made with 25 nm spots, 22.5 nm blur, 14% background and 16 gray levels). 2) The process latitude is nearly independent from the background dose. 3) The process latitude is increased (if a 45 nm line is written with 25 nm spots, 22.5 nm blur, 14% background and 16 gray levels, the process latitude is in the range of 15% and for a 32 nm line—with the same parameters—about 10%). 4) The process latitude is nearly independent from the line position. 5) The process latitude is not decreased in the corners. 6) The total line-width variation is less than about 0.7% of the critical dimension (=nominal minimum line-width) for a 45 nm line and lower than 1.86% for a 32 nm line. The main advantage of the multiple interlocking grids is an increased process latitude with a certain spotsize and blur. Simulations made by the applicant (assignee) proved that the dose latitude is nearly linear scaling with the spotsize, therefore all spotsizes can be interpolated linearly between 50-77%. Gray Scales The term gray scale refers to the possibility of irradiation of the pixels at the target at exposure levels whose values are not only a minimal (‘black’) or a maximal (‘white’) exposure dose, but also intermediate levels. A composite approach for addressing gray values is employed whose principles were outlined in the U.S. Pat. No. 6,768,125 and are here adapted for the multiple interlocking grids, namely (i) gray scaling by superposition, namely by irradiation of the same pixel through a plurality of apertures that are operated at different ‘transparency’, i.e. deflection states, and (ii) gray scaling by fractional irradiation, namely by switching on apertures during a fraction of the individual irradiation cycles only. As already mentioned every eighth domain has the same grids, so the apertures of these domains—for instance, domains Da1, Db1, Dc1 and Dd1—cover the same pixels on the target substrate in the course of scanning motion. This can be used to produce a gray scale by superposition, in this case a scale between 0 to 4, depending on how many of the respectively corresponding apertures in the four domains are switched on. A finer gray scale is achieved by implementation of fractional irradiation during an irradiation cycle. For this, each blanking opening is controlled with regard to its deflection state by a blanking signal comprising more than one bit. For instance, the blanking signal may comprise three bits which allows 23=8 values of gray shadings for a single irradiation cycle. For example, if the three bits form a binary signal 101 (equivalent to a decimal number 5), the corresponding aperture is switched on for ⅝ of the irradiation cycle. In general, the blanking signal will comprise p components, p>1, allowing for up to 2p gray shadings assuming a binary signaling. Thus, each irradiation cycle may be split into a sequence of up to 2p split cycles. The scanning motion may be halted during a whole irradiation cycle, or only for each of the split cycles. In the latter case, the optical system (in particular the deflection means 315, 325; see FIG. 1) may be used to shift the image laterally on the target so as to adjust with the factual motion of the target 41 on the target station 104. In this context it should be mentioned that in a variant, the X offsets of the domains may be all chosen as 0; in this case where fractional X offsets are needed, they may be introduced by appropriate shifts of the image by means of the optical system in the mentioned manner. The schematic diagram of FIG. 13 shows the processing of the blanking signals in the blanking plate. The blanking signals are fed in through p-bit lines 131 each of which serves one line pl of blanking openings (called ‘Line of Apertures’ in FIG. 13), respectively, by providing a sequence of bits. In the example discussed here with 32 blanking openings per line and p=3, a 48-bit sequence is fed in to each blanking signal line 131; only one line of blanking openings is shown in FIG. 13 for clarity. The data are propagated through shift registers 132 (triggered by a common shift-clock signal 13c) and distributed to temporary registers 133 serving as buffers, one p-bit buffer for each blanking opening. Before commencement of an irradiation cycle, the data stored in the p-bit buffers is taken to the blanking circuitry of the blanking openings 134 upon a uniform activation trigger signal 13t. The data of the blanking signal lines 131 are fed to the chip in which the blanking field bf is realized in form of a small number of data streams (of 64 Mbps for instance) which is demultiplexed to the blanking signal lines 131 by demultiplexers 139 located on the same chip (since the data stream has a much higher clock rate, the demultiplexers are controlled by a clock signal 13d different from the clock signal 13c). For instance, each data stream 130 is demultiplexed to 32 blanking signals 131. Gray Groups It should be mentioned that in a variant of the invention (not shown here), handling of the data transfer—in particular if the number of overlaying apertures (blanking openings) is greater than four—could be facilitated by using a gray group strategy with the coding of the gray scales generated by superposition of apertures. To generate a distinct number of exposures, in total adding up to a given intensity, a binary color increment can be used instead of a linear color increment, where for example a 4-bit pixel is build up by 4 gray channels, comprising each N*2n apertures, with n corresponding to the gray color channel 1 to 4 (or higher), and N the number of the apertures of the lowest (non-zero) gray level. Then, any gray level spanned by the 4-bit range can be expressed by a simple binary number. For example, to create gray values between “0” and “15” a linear independent basis consisting of ‘gray groups’ comprising 1, 2, 4 and 8 blanking openings respectively may be used. For instance, gray level “13” is generated by a binary signal (1011), which means, that columns 1, 3 and 4 are active and contribute fractional doses of 1/16, 4/16 and 8/16 to the cumulated exposure dose, whereas column 2 does not contribute at all. The crucial and important advantage of the binary channel approach is that by subdividing the 16 levels into only 4 ‘gray group’ channels, data transfer issues become much simpler as compared to the straightforward strategy used in the U.S. Pat. No. 6,768,125. Choice of Multiple Interlocking Grids If each pixel is written with 8 gray levels, every image spot requires information equivalent to 3×8=24 bits (3 bits gray information times the number of domains overlaying at each spot). To reduce the data transfer there are two ways: 1) use less gray levels, and 2) use less grids number.If the gray level numbers are reduced, it is hard to achieve the required virtual grid of 1 nm. As becomes already clear from the embodiment shown in FIGS. 8 to 12, the domains need not realize all possible combinations of multiples of the base fractional offset. For instance, a grid with offset {0, ¼} is not realized in that embodiment, as are the other grids where the sum of the X- and Y-offsets is an odd multiple of ¼. Thus only every second of the possible grids is actually realized. In fact it was found that the numbers of grids can be further reduced, for example from 8 to 4, namely the 4 grids with X=Y: {0, 0}, {¼, ¼}, {½, ½}, {¾, ¾}. This reduces the amount of data to be fed into the PD device to a half. Calculations showed that all results of the multiple interlocking grids with 8 domains as in FIG. 8 are reproduced with this reduced set as well. A disadvantage of this grid arrangement is a non-homogenous distribution of the spots. Example of Sputter Process FIGS. 14 to 17 illustrate the progress of a trench-sputter process on a semiconductor substrate used as target. The aim of the sputter process is formation of a long dove-tail trench of 600 nm width and 200 nm depth; the trench is assumed to be straight and uniform along its length. FIG. 14 shows the process after exposition through the apertures of the first sequence Da1-Da8 of domains, FIG. 15 after the second sequence Db1-Db8, FIG. 16 after the third sequence Dc1-Dc8, and FIG. 17 depicts the result of the complete sputtering process, which is the cumulated result of exposition to all four domains. The substrate is a silicon wafer, which is irradiated with argon ions at an energy of 30 keV with a current density of 0.75 mA/cm2 of each spot on the substrate; the sputter time is 3.65 s for each of the four sequences. In each of FIGS. 14 to 17, the upper diagram shows the ion current density ICD, given in mA/cm2, across the width of the trench (horizontal coordinate). The dotted lines represent the contributions of each spot which is exposed on the substrate and produced by one aperture in the corresponding line along the X coordinate (parallel to the scanning direction). The sum of these individual contributions is the full line, the total ion current density ICD. All lengths are given in nm. The lower diagram shows the actual profile SPP of the sputtered substrate after irradiation of the respective sequence of domains. These results were obtained by a numerical computer simulation of the sputter process done by the applicant (assignee). (Note that the left side of the trench is at a lateral coordinate of 200 nm.) The dotted line ISP represents an ideal dove-tail shape given for comparison; it represents an ideal result which, however, cannot be reached by any real sputter process. As can be seen from these drawings, in particular FIG. 17, the invention allows to produce a sputtered surface profile of high quality and smoothness (i.e., the deviation of the profile SPP to the ideal dove-tail shape is lower than 2% of the sputtered pattern). These drawings also illustrate that by virtue of this method, it becomes possible to remove layer by layer off the target substrate in successive steps. With prior-art methods this was not possible since the individual spots were to narrow, resulting in an assemblage of isolated spots on the target rather than a smooth irradiation profile. The multiple interlocking grids according to the invention in the field of lithography e-beam direct write applications result in the following particular advantages: For high voltage electron beam lithography, which is suitable to achieve highest resolutions in lithography, in particular when 100 keV electrons are used, an unwanted dose contribution (proximity effect) is caused by electron backscattering from the wafer (i.e., the target). 100 keV backscattered electrons are typically spread out substantially to dimensions of about 30 μm before reaching the resist covered surface. Forward scattering, for comparison, which is dominant at low energies, causes only a minor contribution. To correct for the proximity effect caused by 100 keV electrons, the problem can be turned into one of a smooth background dose correction and is practically independent of the particular feature shape. For example, backscattering of 100 keV electrons in silicon gives rise to an extra dose of about 14% of the deposited dose (over 30 μm averaged) in the resist if maximum 50% pattern density is assumed. For very low pattern densities, for example in case of isolated lines with no patterns in the neighborhood, backscattering plays no significant role for the total dose. The effective strategy for proximity correction made possible by the invention is an adjustment of the edge slope and cut-off position of the total dose distribution (=deposited dose plus extra dose due to backscattering) by using interlocking-grid gray level spots with adequate gray levels, yielding the as-designed edge positions at optimized edge slope over the entire address grid. The described method of interlocking grids allows a very fine address grid (line position, line width) where all features can be realized with practically the same process latitude. In the case of zero background but also in presence of 14% background it is possible to adjust the edge slope of the deposited energy profile and the cut-off position, allowing process latitudes of sufficient quantity over the entire grid independent of the local pattern density. For common high-contrast resist materials the resolution and process latitude can be derived from the aerial image (spatial distribution of deposited dose). As common for chemically amplified resists, the latent image (i.e. the spatial distribution of exposed and unexposed resist) is proportional to the exponential of the negative intensity of the aerial image. The resulting gamma-like behavior leads to the common (and good) approximation that the feature width and position after the lithographical process can be derived by “cutting off” the aerial image at a certain dose level, usually one half of the maximum dose level. In the examples shown in the following, the dose distribution is cut off at 50% of the average dose level, giving the reader an impression on the achievable width and position of lines after development, using the new possibilities offered by the invention. Extra Apertures; Defect Correction In order to take into account for possible defects or other problems in the context of gray scaling, it is possible to include spare blanking openings which can be enabled or disabled depending on the number of defects per line. To provide this option, more apertures than necessary could be structured for some or all channels, or, what seems more useful, an additional “extra aperture” group could be added in each line, so a specific number of apertures per line can be “activated” when needed, for example physically by focused ion beam modification of the corresponding connection lines, or by software controlled switching using implemented logics. Details can be found in the applicant's AT 755/2003 and patent applications derived from it. Variant Apertures It is worthwhile to note that there are other ways to realize interlocking grids. One example is shown in FIGS. 19 and 20, namely, an arrangement of hexagonal apertures (or having a shape close to a hexagon, for example circular) in the PD field (FIG. 19) for addressing a triangulated target grid (FIG. 20). The apertures (or blanking openings) are arranged in three domains hm1, hm2, hm3. Within each domain, which may be sub-partitioned in to gray groups according to the above considerations, the apertures are positioned with respect to each other according to an hexagonal array (which may be elongated in the direction of scanning), where neighboring lines are spaced apart by 3/2 of the side length a of an hexagon, and a suitable value for the offset ho of apertures in neighboring lines is ho=(k+½)·a·√3, with k≧1 being an integer number. In general, any offset is possible if an appropriate signal phase transformation is provided for the offset lines. The distance between the apertures of different domains, d12 and d23, may be chosen as equal to the offset ho, or as (k′+½)·a·√3, or in general, any offset if an appropriate signal phase transformation is applied to the respective first aperture of the groups belonging to the respective domain. The distances d12, d23 will usually be equal to each other, but could also take different values. With these values, the hexagonal apertures of FIG. 19 are appropriate for generating a regular distribution according to a triangulated target grid hg shown in FIG. 20. In this figure, the dots represent the physical access grid on the target (centers of exposure dots), and the solid lines show the envelope of the geometric spots of the apertures of all three domains hm1 . . . hm3. |
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059303204 | abstract | Support assemblies for allowing RPV radial expansion while simultaneously limiting horizontal, vertical, and azimuthal movement of the RPV within a nuclear reactor are described. In one embodiment, the support assembly includes a support block and a guide block. The support block includes a first portion and a second portion, and the first portion is rigidly coupled to the RPV adjacent the first portion. The guide block is rigidly coupled to a reactor pressure vessel support structure and includes a channel sized to receive the second portion of the support block. The second portion of the support block is positioned in the guide block channel to movably couple the guide block to the support block. |
claims | 1. A method for maintaining liquid lithium on a surface area of internal walls of a reactor chamber, the method comprising:installing at least one layer of at least one porous open-cell tile on the surface area of the internal walls of the reactor chamber,wherein a portion of the at least one tile facing an interior of the reactor chamber is divided into a plurality of channels; andapplying an electric charge to the liquid lithium;flowing the liquid lithium into an interior network of open cells of the at least one tile;circulating the liquid lithium through the interior network of the at least one tile via at least one of the open cells to allow for the liquid lithium to seep from the interior network of the open cells to the channels on an external surface of the at least one tile that faces the interior of the reactor chamber; andoutputting the circulated liquid lithium from the at least one tile. 2. The method of claim 1, wherein the interior network of the at least one tile and the channels of the at least one tile are manufactured from a high-temperature resistant, porous open-cell material. 3. The method of claim 2, wherein the high-temperature resistant, porous open-cell material is a ceramic foam, andwherein the channels are hydraulically and electrically separated from one another. 4. The method of claim 1, wherein the at least one tile is manufactured from a high-temperature resistant, porous open-cell material. 5. The method of claim 1, wherein the method further comprises installing at least one magnetic coil between the at least one tile and the surface area of the internal walls of the reactor chamber. 6. The method of claim 1, wherein at least one voltage source is used to provide the electric charge. 7. The method of claim 1, wherein the at least one tile has a constant porosity. 8. The method of claim 1, wherein the at least one tile includes an input plenum,wherein the liquid lithium is inputted into the at least one tile via the input plenum. 9. The method of claim 8, wherein the input plenum is a hollow piece of metal. 10. The method of claim 1, wherein the at least one tile includes an output plenum,wherein the liquid lithium is outputted from the at least one tile via the output plenum. 11. The method of claim 10, wherein the output plenum is a hollow piece of metal. 12. The method of claim 1, wherein a flow rate of the circulation of the liquid lithium within the interior network of the at least one tile is varied over time. 13. A system for maintaining liquid lithium on a surface area of internal walls of a reactor chamber, the system comprising:at least one porous open-cell tile, wherein a portion of the at least one tile facing an interior of the reactor chamber is divided into a plurality of channels; andthe reactor chamber, wherein at least one layer of the at least one tile is installed on the surface area of the internal walls of the reactor chamber,wherein the at least one tile allows for electrically charged liquid lithium to be flowed into an interior network of open cells of the at least one tile,wherein the at least one tile further allows for the liquid lithium to be circulated throughout the interior network of the at least one tile via at least one of the open cells to allow for the liquid lithium to seep from the interior network of the open cells to the channels on an external surface of the at least one tile that faces the interior of the reactor chamber, andwherein the at least one tile further allows for the circulated liquid lithium to be outputted from the at least one tile. 14. A tile for maintaining liquid lithium on a surface area of internal walls of a reactor chamber, the tile comprising:a high-temperature resistant, porous open-cell material;a plurality of channels; andan interior network of open cells in an interior of the tile for circulating electrically charged liquid lithium within the interior network of the tile via at least one of the open cells to allow for the liquid lithium to seep from the interior network of the open cells to the channels on an external surface of the at least one tile that faces an interior of the reactor chamber. |
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description | Conventional managed information environments, such as a Storage Area Network (SAN), typically employ an interconnection of storage arrays operable for storing large quantities of data, in which the storage arrays are responsive to a management application such as an SNMP (Simple Network Management Protocol) based application. The SAN includes a plurality of host computers coupled to users for storage and retrieval of the data in the storage array devices. The SAN, therefore, supports an enterprise such as a corporation or business entity with conventional information storage and retrieval services via the SAN. The SAN management application allows administration activities, such as monitoring and maintenance of the storage arrays, for ensuring maximum throughput and efficiency of the data to and from users via the SAN. A conventional SAN, therefore, employs a plurality of hosts, each connected to one or more storage arrays. The storage arrays each include a plurality of individual storage units, also known as disk drives or spindles, operating as an integrated storage medium. In a configured SAN, each of the storage arrays may have different numbers, types, and arrangements of the storage units, and are often from multiple vendors. Further, the storage within a storage array may be partitioned or designated according to data redundancy or protection schemes, such as shadowing, journaling, and RAID arrangements, and may also be partitioned for usage by certain subsets of users. Accordingly, tracking consumption of available storage and identifying areas of excessive or sparse consumption becomes a formidable task. In a storage area network, multiple storage arrays having varying amounts of storage and often from different vendors provide a large aggregate body of storage capacity to support a user community. In such a storage array network, a system manager or operator is often charged with the responsibility for maintaining the storage arrays in an efficient manner, i.e. ensuring adequate storage to users or user groups and avoiding excessive or disproportionate consumption of storage by hosts using the storage, the maintenance of which is also known as so-called “SAN hygiene.” It would therefore be beneficial to provide allocation reports as an indication of storage consumption in a manageable and readable form which indicates sparse or excessive consumption of storage in a pinpointed manner and which avoids extraneous or excessive information which can tend to cloud or dilute salient data. Configurations of the invention are based, in part, on the observation that it is difficult for a SAN manager to identify all related and derived fields pertaining to storage allocation among multiple heterogeneous storage arrays. Conventional reports tend to focus on only a particular type or vendor of storage array, requiring interpretation and synthesis of multiple report formats. Other conventional reports may not include related or derived fields in a single report, requiring offline computations and/or analysis to arrive at the desired allocation report results. Therefore, configurations of the invention discussed below substantially overcome many of the drawbacks with conventional storage allocation reports by presenting foundation variables, device specific parameters and computed, derived fields for different types of storage arrays, without burdening the allocation report with extraneous parameters, through the use of a layout indicative of the information included on the report. Accordingly, configurations of the invention define a layout indicative of foundation variables, device attributes and derived fields requested in such a storage allocation report. The layout may be predetermined from a menu pulldown, or may be customized from among available allocation parameters and report formats. Foundation variables are generally hardcoded items such as the vendor, type, and name of a particular device (storage array). Device attributes are items specific to a device which may be modified by reconfiguring, such as redundancy partitioning and emulation. Derived parameters are items deterministic from other items, which are computed according to a particular device usage metric, or allocation metric. The layout therefore indicates the requested allocation parameters for a report, indicative of the foundation variable, device attributes, and derived fields, and also indicates the device usage metrics for computing the derived fields from the foundation variables and device attributes. In this manner, a SAN operator may request an allocation report indicative of only the information sought, and need not correlate multiple reports or manually synthesize report output for determining derived fields. In further detail, the method of reporting device allocation in the storage area network includes generating and storing allocation data indicative of allocation parameters of manageable entities in the storage area network, and receiving a layout indicative of a user specified subset of the allocation parameters for display in an output report, in which each of the parameters in the subset corresponds to output fields operable for display. A management application identifies, from the output fields, derived fields specified in the layout, in which the derived fields are determinable from other allocation parameters, and determines usage metrics operable to compute the derived fields from the allocation parameters. The management application then computes the identified derived fields from the determined usage metrics using the allocation parameters for display to a user as an allocation report on a display console. In the exemplary configuration discussed further below, the allocation parameters are values concerning storage arrays in the SAN and include foundation variables, device attributes, and derived fields, in which the foundation variables are predetermined, the device attributes correspond to a configuration of a particular storage array device, and the derived fields are operable for computation from other allocation parameters. As indicated above, each of the storage arrays includes a plurality of storage array devices, or disk drives. The management application is operable to compute the derived fields from predetermined usage metrics, in which the usage metrics are indicative of allocation parameters and operations for computing the derived fields. The usage metrics are further operable to indicate predetermined allocation parameters, such that the management application may retrieve device attributes from at least one storage array device, and compute the derived fields from operations specified by the usage metrics. Further, the allocation parameters presented as the output fields are indicative of a progression of consumed storage, including available storage, reserved storage, configured storage, allocated storage, and used storage. In the exemplary configuration, computing the derived fields for the allocation report includes identifying corresponding allocation parameters from storage arrays of each of a plurality of vendors, and coalescing the corresponding parameters for generating the output fields. As a typical SAN includes storage arrays from multiple vendors, coalescing typically involves identifying different vendor specific allocation parameters employed for computing a derived field. Further, the layout may be indicative of a filter selection, in which the filter selection specifies allocation parameters for inclusion as output fields. Such a filter selection may incorporate a filtering criteria including at least one of LUN masking, HBA ports, masking and vendors. The invention as disclosed above is described as implemented on a computer having a processor, memory, and interface operable for performing the steps and methods as disclosed herein. Other embodiments of the invention include a computerized device such as a computer system, central processing unit, microprocessor, controller, electronic circuit, application-specific integrated circuit, or other hardware device configured to process all of the method operations disclosed herein as embodiments of the invention. In such embodiments, the computerized device includes an interface (e.g., for receiving data or more segments of code of a program), a memory (e.g., any type of computer readable medium), a processor and an interconnection mechanism connecting the interface, the processor and the memory. In such embodiments, the memory system is encoded with an application having components that, when performed on the processor, produces a process or processes that causes the computerized device to perform any and/or all of the method embodiments, steps and operations explained herein as embodiments of the invention to allow execution of instructions in a computer program such as a Java, HTML, XML, C, or C++ application. In other words, a computer, processor or other electronic device that is programmed to operate embodiments of the invention as explained herein is itself considered an embodiment of the invention. Storage area networks employ allocation reports to identify configurations of storage among the storage arrays in the network. In a large storage area network (SAN), it may be difficult for a SAN manager to identify all related and derived fields pertaining to storage allocation among multiple heterogeneous storage arrays. Conventional reports tend to focus on only a particular type or vendor of storage array, requiring interpretation and synthesis of multiple report formats. Other conventional reports may not include related or derived fields in a single report, requiring offline computations and/or analysis in order to arrive at the desired allocation report results. Device allocation reports generated by a SAN management application substantially overcome these shortcomings by presenting foundation variables, device specific parameters, and computed, derived fields for different types of storage arrays, without burdening the allocation report with extraneous parameters through the use of a user selectable layout indicative of the information included on the report. The SAN management application discussed further below defines a layout indicative of foundation variables, device attributes, and derived fields, requested in an allocation report. Foundation variables are generally hardcoded items such as the vendor, type, and name of a particular storage array. Device attributes are items specific to a device which may be modified by reconfiguring, such as redundancy partitioning and emulation (shadowing, RAID, etc.). Derived parameters are items deterministic from other allocation parameters (i.e. foundation variables and device attributes), which are computed according to a particular device usage, or allocation, metric. The layout therefore indicates the requested allocation parameters for a report, indicative of the foundation variable, device attributes, and derived fields, and also indicates the device usage metrics for computing the derived fields from the foundation variables and device attributes. In this manner, a SAN operator may request an allocation report indicative of only the information sought, and need not correlate multiple reports or manually synthesize report output for determining derived fields. Further, the allocation report may be employed to complement or verify data in other reports, as many reports may be derived from the allocation data gathered and stored by the mechanism discussed herein. FIG. 1 is a context diagram of an exemplary managed information environment 100 including a storage area network 110 and suitable for use with configurations of the invention. Referring to FIG. 1, the storage area network (SAN) 110 is an interconnection of storage arrays 102-1 . . . 102-3 (102 generally), hosts 104-1 . . . 104-2 (104 generally), agents 106-1 . . . 106-3 (106 generally), and other manageable entities, collectively 108-N (108 generally), including connectivity devices, databases, and other nodes (not specifically shown). Each of the storage arrays 102 includes one or more storage array devices 114-N (e.g. disk drives). The SAN 110 connects to a server 120 executing a management application 122 operable to monitor and manipulate the manageable entities 108-N in the SAN. The management application 122 is responsive to a console 130 for receiving report and layout selections 132 via user requests and for generating allocation reports 134 for presentation to a user or operator on the report display 136. A repository 140, such as a managed object database, connects to the server 120 and stores layouts 142, allocation data 144, and usage metrics 146. The individual layouts 142-1 . . . 142-N are user selectable reporting formats indicative of allocation parameters and other report content of the allocation reports 134. The allocation data 144, as indicated above, includes raw data gathered from the storage arrays 102 by the agents 106 on a periodic bases for SAN management support. The usage metrics 146 define interrelations between the various allocation parameters and identify deterministic relations between various allocation parameters, such as allocation parameters computable from other allocation parameters for presentation in the allocation report 134. FIG. 2 is a flowchart of generation of a device allocation report 134 concerning the environment in FIG. 1. Referring to FIGS. 1 and 2, the method of reporting device allocation in the storage area network 110 as disclosed herein includes generating allocation data 144 indicative of allocation parameters of manageable entities 108, such as storage arrays 102, in the storage area network, as shown at step 200. The agents 106, in communication with the storage arrays 102, gather the allocation data 144, typically as part of routine SAN 110 maintenance. In the exemplary arrangement, the allocation data 144 may be an aggregated file of markup data operable for further processing by a variety of processes in the management application 122. For example, the allocation data 144 may be a file in a markup language, such as an XML file, which is further parseable according to XML syntax such as via a SAX parser, as is known to those of skill in the art. The management application 122 receives a layout 142-N indication from the console 130 GUI, in which the layout 142 is indicative of a user or operator specified subset of allocation parameters for display in the output allocation report 134, as depicted at step 201. In such a layout, each of the allocation parameters in the subset corresponds to output fields operable for display, and are therefore indicative of the content of the allocation report 134 for the report display 136. As indicated above, the layout may be predetermined from a typical windows menu pulldown, or may be customized from among available allocation parameters and report formats, or other suitable GUI mechanism, such as radio buttons, file selection, or free-form text entry. The management application 122 identifies, from the output fields indicated in the selected layout 142-N, derived fields specified in the layout 142-N, in which the derived fields are those which are determinable from other allocation parameters, as depicted at step 202. The management application 122 then determines the applicable usage metrics 146, discussed further below, operable to compute the indicated derived fields from the predetermined allocation parameters from the allocation data 144, as shown at step 203. The derived fields are indicative of storage quantums and/or totals which are deterministic from other allocation parameters, either the foundation variables or the device attributes, which generally are obtainable from the gathered allocation data 144. The usage metrics 146 define the operations for computing the derived fields from the underlying foundation variables and device attributes. The management application 122 then employs the usage metrics 146 to compute the identified derived fields from the determined usage metrics using the allocation parameters (e.g. foundation variables and device attributes), as depicted at step 204. FIG. 3 is a data flow diagram of the device allocation report generation using the flowchart of FIG. 2. Referring to FIGS. 1 and 3, the agents 106 periodically gather the allocation data 144 as part of ongoing SAN maintenance and hygiene activity. A metadata parser 150 in the management application 122 receives a device selection 148 via the user requested layout 142-N. The metadata parser 150 parses the allocation data 144 and retrieves a set of foundation variables 152 for each of the selected devices 148. The layout 142 is further indicative of fields 149, corresponding to allocation parameters selected for r output on the resulting allocation report 134. As indicated above, the allocation parameters displayed on the allocation report 134 include foundation variables 152, device attributes 156 and derived fields 160, discussed further below. A compute layout process 154 identifies the device attributes 156 corresponding to the selected allocation parameters for the selected storage array devices 114. The compute layout process 154 may retrieve the device attributes from the foundation variables 152 or from the allocation data 144. The device attributes 156 and foundation variables 152 are then employed by the usage metrics 146 corresponding to the requested layout 142. A compute device metrics 158 process is responsive to the usage metrics 146 and computes derived fields 160 from the device attributes 156 and the foundation variables 152. The foundation variables 152, device attributes 156, and derived fields 160 indicated by the requested layout 142 are then employed as allocation parameters for the allocation report 134. FIG. 4 is an exemplary disk array employed for computing derived fields for the device allocation report 134 in FIG. 3. Referring to FIGS. 3 and 4, the exemplary disk array 102 has a plurality of storage array devices including devices 114-11 and 114-12. Each storage array device 114-11 and 114-12 represents a physical disk drive, or spindle, in a storage array 102. Raw disk capacity refers to the size of the unformatted physical disk, or 100 Gb for each of array devices 114-11 and 114-12 in the example shown. Accordingly, the two array devices (disks) 114-11 and 114-12 are actually two physical array disks configured as array device 102, having a raw device capacity of 100 Gb. Raw device capacity refers to the capacity of a device plus the capacity required for protection capacity, i.e. mirroring, shadowing and/or parity RAID, defined for the device. Usable capacity is that portion of raw device capacity that is reserved for host use and for internal array operations. Accordingly, usable capacity is the raw device capacity minus the capacity required for the protection level capacity defined for the device. In the example shown, each of the array devices has a raw disk capacity of 100 GB, of which 20 Gb is allocated 170 and 80 Gb remains available 172. Array device 114-11 therefore has 20 Gb usable capacity, shown by oval 180, and employs 20 Gb of protection capacity, shown by oval 182. The raw device capacity is therefore 20 Gb usable capacity on device 114-11+20 Gb protection capacity on device 114-12. Accordingly, it follows that usable capacity is raw capacity minus the protection capacity, or 40 Gb raw −20 Gb protection capacity=20 Gb usable device capacity. Accordingly, the derived parameters raw device capacity 185 and usable device capacity 187 are given by the following usage metrics: 20 Gb usable + 20 Gb protection =40 Gb raw device capacity18540 Gb raw − 20 Gb protection =20 Gb usable device capacity187Alternate arrangements employ other usage metrics for aggregating and distinguishing the various subdivisions and partitions of storage apportioned among the disk drives 114-N in the storage arrays 105. FIGS. 5-7 are a flowchart of device allocation reporting according to FIG. 3 in greater detail. Referring to FIGS. 1, 3 and 5-7, the management application 122 begins reporting device allocation in the storage area network 110 by generating allocation data indicative of allocation parameters of manageable entities in the storage area network, as depicted at step 300. The agents 106-N, responsive to periodic polling and/or event driven maintenance activity, gather allocation data 144 indicative of report variables for the storage array devices 114, as shown at step 301, for storage in the repository 140. The console 130 presents a GUI or other display to a user for presenting a selection of available layouts 142, or report formats. The GUI presents the selection via a pulldown menu or other console 130 selection, and the management application 122 receives the selected layout 142, as shown at step 302, in which the layout 142 is indicative of a user specified subset of allocation parameters for display in the output allocation report 134, as depicted at step 302. Through the indicated layout 142, the user selects a particular allocation report 134 including particular allocation parameters. Each of the allocation parameters in the subset, therefore, corresponds to output fields operable for display in the allocation report 134. The different selectable layout 142-1.142-N allow specification of various allocation parameters. In the exemplary configuration, the allocation parameters include foundation variables 152, device attributes 156, and derived fields 160, in which the foundation variables 152 are predetermined, the device attributes 156 correspond to a configuration of a particular storage array device, and the derived fields 160 are operable for computation from other allocation parameters, as disclosed at step 303. In further detail, the management application 122 receives a user selection 132 of one of the available layouts 142-N indicative of a set of storage array devices 114 and output fields for inclusion the output report 134, in which the output fields include one or more of the foundation variables 152, device attributes 156, and derived fields 160, as depicted at step 304. Further, in particular configurations, the layout 142 is indicative of a filter selection operable to specify allocation parameters for inclusion as output fields, as shown at step 305. The filter selection may include particular target values or ranges of values, for example. In the exemplary SAN environment 100, the filtering criteria may includes parameters such as LUN masking, HBA ports, masking, and vendors. Criteria such as LUN masking are operable to logically group storage arrays, as is known in the art. The layout selection 142 specified by the user, therefore, indicates the desired allocation report 134, and specifies the format and fields therein. Further, configurations allow the user to select from predefined layouts 142, or to define custom layouts. The layout 142 is also indicative of the derived fields 160 for the report. Accordingly, the management application 122 identifies, from the layout, foundation variables 152 corresponding to the storage array devices 114 selected for inclusion in the output report, as shown at step 307. The foundation variables 152, generally, are static labels or specifications associated with a storage array, such as the physical number of disk drives and total capacity. The management application 122 further identifies, from the layout 142, device attributes corresponding to the selected output fields for the selected storage array devices 114, in which the device attributes are specific to a particular storage device, as shown at step 308. The device attributes 156, therefore, are generally configurable fields which are specific to the storage array device 114, but which are modifiable through configuration management, such as partitioning for protection and redundancy. The management application 122 identifies, from the desired output fields, derived fields 160 specified in the layout 142, in which the derived fields 160 are determinable from other allocation parameters, i.e. from the foundation variables 152, device attributes 156, and also from other derived fields 160, as shown at step 309. The derived fields 160, therefore, are computable from other allocation parameters, such as by summation of storage discussed above with respect to FIG. 4. Accordingly, the management application 122 determines the applicable usage metrics 146 operable to compute the derived fields 160 from the predetermined allocation parameters, as shown at step 310. The usage metrics 146 are stored in the repository 140 as procedures, and are further operable to indicate predetermined allocation parameters and mathematical computations and operations for computing the derived parameters, as depicted at step 311. Having identified the usage metrics 146, the management application 122 retrieves the device attributes from the storage array 102 and/or storage array devices 114 implicated by the applicable usage metrics 146, as shown at step 312. The management application then computes the derived fields 160 from operations specified by the usage metrics 146, as shown at step 313. Computing further includes coalescing the allocation parameters by identifying different vendor specific allocation parameters employed for computing a particular derived field 160, as depicted at step 314. In the SAN 110, storage arrays 102 of multiple different vendors may be present. Accordingly, computing the derived fields 160 includes identifying corresponding allocation parameters from devices of each of a plurality of vendors, as shown at step 315, and coalescing the identified corresponding parameters for generating the output fields. Therefore, the usage metrics 146 coalesce the allocation parameters by identifying the vendor specific parameters contributing to or included in the computation of the derived parameters 160. The usage metrics 146 are indicative of vendor and/or device specific allocation parameters and operations for computing the derived fields, as shown at step 316. Typically, each storage array 102 has an application programming interface (API) specific to that vendor or device. The management application 122 includes or inherits the API to communicate with the storage array 102. Accordingly, the management application 122 identifies the corresponding parameters in the API of each vendor, and employs the corresponding parameters in computing the derived fields 160 to compute the derived parameter correctly despite dissimilar APIs and labels of API obtained fields. The management application 122 formats the foundation variables 152, device attributes 156, and derived fields 160 corresponding to the selected layout 142 for output, as depicted at step 317. The reported output fields reflecting the allocation parameters, therefore, are indicative of a progression of available storage, reserved storage, configured storage, allocated storage, and used storage, depending on the usage metric employed and the derived parameters sought, based on the layout 142, as disclosed at step 318. FIGS. 8-10 are exemplary screen displays of device allocation reports 134 illustrating the foundation variables 152, device attributes 156, and derived fields 160. Referring to FIGS. 4, and 8-10, an allocation report 134 representing the layout 142 for accessible used devices 154-1 is shown. In the exemplary report, the foundation variables include the array 190-1, array type 190-2, array device name 190-3 and host/HBA 190-12. The device attributes include the array device type 190-4, the array allocation type 190-9, the emulation 190-10 and Number of ports 190-11. The derived fields include, as indicated above, the raw device capacity 185 and usable device capacity 187, and also allocation 190-5, LUN masking 190-6, mapping 190-7, volume group use 190-8, host accessible 190-13, host allocated 190-14, host accessible description 190-15 and host allocated description 190-16. The derived fields 160 concerning allocation and accessibility are computable according to predetermined metrics, or rules. FIG. 11 illustrates allocation rules 192 based on particular device attributes 156. Referring to FIG. 12, the host 104-11 connects to storage arrays 102-11 . . . 102-15, each illustration a particular set of conditions defining the allocation rules 192 and associated derived fields 160. Alternate configurations may employ alternative allocation rules for defining consumed storage, such as a set of rules depicted form a flowchart or state specification defining related fields. In further detail, in the exemplary configuration encompassing a storage area network, the following allocation parameters may be specified using the available layouts (note that various vendor specific names may be trademarks of their respective companies). Further, the derived fields Host Accessible Description and Host Allocated Description further include criteria for identifying these fields from the underlying device attributes. The device allocation report, in the exemplar configuration discussed herein, lists all array devices and provides details about each device including the capacity of the device (raw and usable), whether the device is allocated, and whether it is accessible to hosts. With respect to raw and usable capacity, the term raw is used in the both in the context of physical disk capacity and device capacity. Raw disk capacity refers to the size of the unformatted physical disk. For example, Raw—Total (GB) is the sum of the capacities of the unformatted physical disks in the array. Raw device capacity refers to the capacity of the device plus the capacity required for the protection level (mirrors or parity RAID) defined for the device. In general, usable capacity is that portion of raw device capacity that is reserved for host use and for internal array operations. Usable capacity is raw device capacity minus the capacity required for the protection level (mirrors or parity RAID) defined for the device. Descriptions of the allocation parameters included in the reports follow: Array Allocated?: Indicates if the device is allocated for use by the array. The following types of devices are considered array allocated: Symmetrix LUN masked primary logical devices and non-established replica devices mapped to access-controlled front-end ports Primary logical devices and non-established replica devices mapped to non access-controlled front-end ports Local replicas (split or synchronized) associated with array allocated primary devices (includes multihop to n levels) Remote replicas (split or synchronized) associated with array allocated local replicas or array allocated primary devices external to this array (includes multihop to n levels) LUN masked VDEVs mapped to access-controlled front-end ports VDEVs mapped to non access-controlled front-end ports Devices included in API device groups System resources CLARiiON Access Logix enabled—LUNs bound to Storage Groups that also contain an HBA plus system resources Access Logix disabled—all configured LUNs plus system resources HDS/HP XP LUN masked devices mapped to security-enabled ports Devices mapped to non security-enabled ports System resources IBM ESS LUN masked primary logical devices mapped to access-controlled front-end ports Primary logical devices mapped to non access-controlled front-end ports HP StorageWorks EMA—LUN masked primary logical devices mapped to access-controlled front-end ports Array Allocation Type: Describes how the array device has been allocated within the array. If the array device is not allocated, this column is blank. If the host is known this also describes how the device is considered allocated to a host. Symmetrix Valid values are Primary, Local Replica, Remote Replica, and System Resource: Primary—Primary devices Local Replica—BCV devices, VDEVs, and non-established replicas Remote Replica—R2 devices System Resource—VCMDB devices, Gatekeeper devices, SFS devices, RAD devices, COVD devices, saved pool devices, DRVs and their mirrors, and spare disks CLARiiON—Valid values are Primary and System Resource. HDS/HP XP—Valid values are Primary and System Resource. IBM ESS—Valid value is Primary. HP StorageWorks EMA—Valid value is Primary. Array Device Capacity—Raw (GB): Total capacity of this logical device (non-meta, meta head, or meta member) including any defined protection capacity Array Device Capacity—Usable (GB): Capacity on this logical device (non-meta, meta head, or meta member) available for use by the array. Does not include any defined protection capacity. Array Device Type: Array device type Not reported for arrays discovered by the Storage Agent for SMI. Array Type: Type of array For arrays discovered by the Storage Agent for SMI, the array type is reported based on the information available from the SMI provider implemented on the array. Consistency Group Name: Name of the consistency group to which the device belongs. Applies to Symmetrix devices only. A consistency group is a user-defined group of Symmetrix SRDF devices used to maintain the integrity of remote replication of the devices. Emulation: Emulation mode of the device. Applies to Symmetrix and HDS/HP XP arrays only. Device emulation modes for Symmetrix devices include FBA and CKD. For HDS/HP XP arrays, the logical devices (LDEVs) associated with a RAID group may have an emulation mode (for example, OPEN-L). Host Accessible?: Indicates if the array device is accessible to the host Host Accessible Description: Describes why or why not the array device is accessible to the host LUN Masked, Not Connected Primary logical device or non-established replica that is LUN masked and mapped to an access-controlled front-end port that is not physically connected Mapped to port, Not Connected Specific to Symmetrix arrays. Primary logical device or non-established replica mapped to front-end port of iSCSI type Non Lun Masked, Not Connected Specific to Access Logix disabled CLARiiON. Primary logical device mapped to front-end port, but not physically connected to host Lun Masked, Visible to Host Primary logical device or non-established replica that is LUN masked and mapped to access-controlled front-end port and is physically connected through direct attach or switch that is not yet discovered by FCC agent Lun Masked, Zoned and Connected to Host Primary logical device or non-established replica that is LUN masked and mapped to access-controlled front-end port and is physically connected through discovered switch Visible to Host through non access controlled port Primary logical device or non-established replica that is physically connected through direct attach or switch that is not yet discovered by FCC agent Zoned and Connected to Host through non access controlled port Primary logical device or non-established replica mapped to non access controlled front-end port and is physically connected through a discovered switch Host Device Found Logical device for which host has created a host device. Only seen if host agent is running. Host Allocated?: Indicates if the device is allocated to the host. The following are considered host allocated: Symmetrix LUN masked primary logical devices and non-established replica devices mapped to access-controlled front-end ports Primary logical devices and non-established replica devices that mapped to non access-controlled front-end ports which are physically connected to hosts Local replicas (split or synchronized) associated with array allocated primary devices (includes multihop to n levels) Remote replicas (split or synchronized) associated with array allocated local replicas or array allocated primary devices (includes multihop to n levels) LUN masked VDEVs mapped to access-controlled front-end ports VDEVs mapped to non access-controlled front-end ports CLARiiON Access Logix enabled—LUNs bound to Storage Groups that also contain an HBA Access Logix disabled—all configured LUNs on a CLARiiON that is physically connected to a host HDS/HP XP LUN masked devices masked to security-enabled ports Devices mapped to non security-enabled ports IBM ESS LUN masked primary logical devices mapped to access-controlled front-end ports Primary logical devices mapped to non access-controlled front-end ports which are physically connected to hosts HP StorageWorks EMA—LUN masked primary logical devices mapped to access-controlled front-end ports Host Allocated Description: Describes why or why not the array device is allocated to the host Accessible Primary Device • LUN masked primary logical devices and non-established replicas mapped to access-controlled front-end ports which are physically connected to hosts Primary logical devices mapped to non access-controlled front-end ports which are physically connected to hosts Accessible replica with no Source device • Non-established replica devices that are LUN masked and mapped to access-controlled front-end ports that are physically connected to hosts Non-established replica devices mapped to non access-controlled front-end ports that are physically connected to hosts Accessible replica with unaccessed source device • Replica devices that are LUN masked and mapped to access-controlled front-end ports that are physically connected to hosts but the source of the replica is not accessible Replica devices that are mapped to non access-controlled front-end ports that are physically connected to hosts but the source of the replica is not accessible Accessible Virtual Device Applicable to Symmetrix arrays only for these conditions: VDEVs LUN masked and mapped to access-controlled front-end ports that are physically connected to hosts VDEVs mapped to non access-controlled front-end ports that are physically connected to hosts Device mapped to iSCSI port Primary logical devices and non-established replica devices mapped to front-end ports of iSCSI type LUN Masked but not accessible device Primary logical devices and non-established replicas that are LUN masked and mapped to access-controlled front-end ports, but are not physically connected Member of API Device Group API device group is array allocated, but device not host allocated Replica of an Allocated Primary BCV device of an array allocated primary device Replica of an Allocated Replica R2 device of an array allocated replica device Replica of a primary allocated to same host BCV device and primary device are allocated to the same host. This typically occurs when the primary device and the BCV are accessing the same host. Replica of a replica allocated to same host BCV and R2 device are allocated to the same host. This typically occurs when the BCV device and the R2 device are accessing the same host. LUN Masked?: Indicates if the array device is LUN masked. A device is considered LUN masked if: Symmetrix, HP StorageWorks EMA, IBM ESS—the device is LUN masked to an HBA CLARiiON—the device is part of a CLARiiON Storage Group that also contains at least one HBA HDS/HP XP the device is LUN masked to an HBA or Host Group the LUN Group that contains the device is LUN masked to an HBA or Host Group the device and the HBA are in a common Storage Domain Mapped?: Indicates if the array device is mapped to a front-end port. A device is considered mapped if: Symmetrix, HDS/HP XP, HP StorageWorks EMA, IBM ESS—the device is mapped to a front-end port CLARiiON Access Logix enabled—the device is part of a Storage Group Access Logix disabled—all devices are considered mapped Meta Device Capacity—Raw (GB): Total capacity on this logical device (non-meta, meta head, or meta member) that can be made available to hosts including any defined protection capacity. For non-meta devices and meta heads, it is the same as Meta Device Capacity—Usable plus any defined protection capacity. Always 0 (zero) for a meta member. Meta Device Capacity—Usable (GB): Capacity on this logical device (non-meta, meta head, or meta member) that can be made available to hosts. Does not include any defined protection capacity. For a non-meta device, it is the same as Array Device Capacity—Usable. For a meta head, it is the sum of the meta head and its associated meta members. Always 0 (zero) for a meta member. Used By File System?: Indicates if the array device is used by a host file system Used by Volume Group?: Indicates if the array device is used by a volume group User-defined Device Capacity—Raw (GB): For CLARiiON metaLUNs only, user-defined portion of the Meta Device Capacity—Usable. For all other array device types, same as Meta Device Capacity—Usable. User-defined Device Capacity—Usable (GB): For CLARiiON metaLUNs only, user-defined portion of Meta Device Capacity—Raw. For all other array device types, same as Meta Device Capacity—Raw. As indicated above, the exemplary configuration also provides filtering of the output based on particular allocation parameters. In particular, a filter tab or other suitable GUI icon is operable to define and employ a filter as follows: Use the Filter tab to filter the data that appears in your report. You can specify up to 10 filters. The following filter options are available. Column Select the types of data (columns) by which you want the report filtered. The list boxes contains all possible columns in the type of report. However, selecting a column for filtering only means that the report will be filtered by the data in that column. For the column to actually appear in the report, you must select that column on the Table tab. For example, you might create a filter so that you only see data for hosts that have more than one CPU, but not want the number of CPUs column to appear in the report. Operator Select an operator for the data type selected in Column. The type of operators that are valid depends on the type of data. Filtering data is case-sensitive. If you are filtering on a column that contains alphabetic characters and you want to include certain values regardless of their case, choose one of the ignore case operators. Value Enter the value or values that you want the operator to act upon. If the chosen operator allows more than one value or there are any restrictions on the type of value you can enter (for example, numeric only), the value field will contain a message to that effect. In particular configurations, the data type for switch port numbers may be changed to allow alphanumeric (instead of numeric only) values. The SAN simulation framework and mechanism disclosed herein may encompass a variety of alternate deployment environments. In a particular configuration, as indicated above, the exemplary SAN management application discussed may be the EMC Control Center (ECC) application, marketed commercially by EMC corporation of Hopkinton, Mass., assignee of the present application. Those skilled in the art should readily appreciate that the programs and methods for reporting device allocation in a storage area network as defined herein are deliverable to a processing device in many forms, including but not limited to a) information permanently stored on non-writeable storage media such as ROM devices, b) information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media, or c) information conveyed to a computer through communication media, for example using baseband signaling or broadband signaling techniques, as in an electronic network such as the Internet or telephone modem lines. The operations and methods may be implemented in a software executable object or as a set of instructions embedded in a carrier wave. Alternatively, the operations and methods disclosed herein may be embodied in whole or in part using hardware components, such as Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), state machines, controllers or other hardware components or devices, or a combination of hardware, software, and firmware components. While the system and method for reporting device allocation in a storage area network has been particularly shown and described with references to 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. Accordingly, the present invention is not intended to be limited except by the following claims. |
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055925227 | claims | 1. A control rod for a nuclear reactor, comprising: a cruciform control rod body having four elongated, substantially vertically extending wings arranged generally 90.degree. relative to one another, each wing including a plurality of elongated, generally vertically extending discrete structural members spaced from one another and discrete horizontally extending structural members adjacent opposite ends of said vertically extending members defining a plurality of vertically extending, side-by-side compartments; a sheath disposed along opposite sides of said vertically and horizontally extending members, said structural members being welded to one another and said sheaths being welded to said structural members whereby said members and said sheath seal said compartments externally of said wing; neutron-absorbing material disposed in each said sealed compartment; and a passage through a vertically extending member in part defining adjacent compartments enabling passage of gas under pressure generated by reaction of at least certain of the neutron-absorbing material with neutrons from one of said adjacent compartments through said passage into another of said adjacent compartments. 2. A control rod according to claim 1 wherein said neutron-absorbing material comprises boron carbide disposed in a plurality of capsules, said plurality of capsules being disposed in said compartments. 3. A control rod according to claim 2 wherein said capsules are confined within said compartments by said members and said sheaths and without structural interconnection therewith. 4. A control rod according to claim 2 wherein said neutron-absorbing material comprises hafnium. 5. A control rod according to claim 1 including a central tie rod comprised of a plurality of planar plates each secured along opposite edges to innermost vertical members of a pair of wings lying in a common plane, the plates being superposed one over the other. 6. A control rod according to claim 5 wherein said tie rod plates have a slot at one end for receiving a portion of a vertically adjacent tie rod plate whereby the tie rod plates interconnect with one another and are alternately secured to right angularly related planar extending wings. 7. A control rod for a nuclear reactor comprising a control rod body having a plurality of elongated, laterally spaced, generally vertically extending parallel discrete structural members in part defining a plurality of side-by-side compartments each containing neutron-absorbing material, discrete structural members generally right angularly related and joined to said parallel vertically extending members at opposite ends of said compartments to close said compartments in the plane thereof, a sheath on opposite sides of said discrete structural members closing the sides of said compartments, said members being welded to one another and said sheaths being welded to said members to form sealed compartments, and an opening in at least one of said members in part defining adjacent compartments enabling passage of gas under pressure generated by reaction of the neutron-absorbing material with neutrons from one of said adjacent compartments through said opening to another of said adjacent compartments to maintain said adjacent compartments under substantially equal pressure. 8. A control rod according to claim 7 wherein said neutron-absorbing material comprises boron carbide disposed in a plurality of capsules, said plurality of capsules being disposed in said compartments. 9. A control rod according to claim 8 wherein said capsules are confined within said compartments by said members and said sheath and without structural interconnection therewith. 10. A control rod according to claim 8 wherein said neutron-absorbing material comprises hafnium. 11. A control rod according to claim 7 including a central tie rod comprised of a plurality of planar plates each secured along opposite edges to innermost structural members of said plurality thereof lying in a common plane, the plates being superposed one over the other. 12. A control rod according to claim 11 wherein said tie rod plates have a slot at one end for receiving a portion of a vertically adjacent tie rod plate whereby the tie rod plates interconnect with one another and are alternately secured to right angularly related planar extending wings. |
048246077 | abstract | A process for denitrating aqueous, nitric acid and salt containing waste solutions, comprises mixing the waste solution at room temperature with ethyl alcohol, and heating the mixture to at least 75.degree. C. |
049833510 | claims | 1. In a nuclear reactor pressure vessel having an outer enclosure defined by a generally cylindrical sidewall with a generally vertical central axis and upper and lower edges, and top and bottom heads secured in sealed relationship to the upper and lower edges, respectively, of the cylindrical sidewall, and the vessel enclosing therein a core including a plurality of elongated fuel element assemblies mounted in parallel axial relationship between vertically spaced lower and upper core support plates supported at respective, lower and intermediate, vertically spaced positions of the cylindrical sidewall of the vessel and an upper internals support plate supported within and adjacent the upper end of the cylindrical sidewall of the vessel, an instrumentation system comprising: plural instrumentation thimbles, each of elongated and generally cylindrical sidewall configuration and defining an elongated interior passageway of a predetermined, fixed diameter, and having an open, upper end, a closed, lower end and having flow holes in the sidewall adjacent the lower end, said thimbles being disposed in generally parallel axial relationship centrally within respective fuel element assemblies and thus at predetermined positions intermediate the upper and lower core support plates; plural bores in the upper core support plate in alignment with the respective, plural instrumentation thimbles; plural head penetrations and associated, plural head penetration columns, each said head penetration being of generally elongated, cylindrical configuration and extending in parallel axial relationship through the top head of the vessel and in sealed relationship therewith and having a first, lower end within the interior of the vessel top head and a second, upper end exterior of the vessel top head; each of said plural head penetration columns being of generally elongated, cylindrical configuration and correspondingly having a cylindrical sidewall and a first, lower end and a second, upper end; means for supporting the first, lower ends of said head penetration columns on the upper internals support plate, said head penetration columns extending in parallel axial relationship through said respective head penetrations and said second, upper ends thereof being disposed above the second, upper ends of the respective head penetrations; means for mechanically securing each said head penetration column to its associated head penetration and for sealing the head penetration column to the second, upper end of said associated, head penetration; plural detector guide tubes, each defining an interior passageway, as aforesaid, extending from the upper end of an associated head penetration column and passing therewith through the respectively associated head penetration and into the interior of said upper head and through the sidewall of the penetration column to respective, said predetermined positions and correspondingly aligned with respective said instrumentation thimbles; said upper internals support plate further comprising a plurality of bores positioned in alignment with said predetermined positions and respectively associated instrumentation thimbles; plural elongated, generally cylindrical column tubes defining respective interior passageways therethrough, as aforesaid, each extending through a respective bore in said upper internal support plate and having a first upper end at said predetermined position and a second lower end disposed adjacent the open, upper end of an associated instrumentation thimble; plural first means connecting corresponding said detector guide tubes to said first upper ends of said respective column tubes; and plural second means connecting said second, lower ends of said column tubes to said respective, open and upper ends of said instrumentation thimbles. each said second means comprises a ball and cone joint defined by the open and upper end of the associated instrumentation thimble and the second, lower end of the associated, aligned column tube. plural resilient biasing means respectively associated with said column tubes and secured to the upper internals support plate, for producing an axially downward resilient force on the respective column tube for maintaining the associated ball and cone connection in sealed engagement. plural upper internals support columns of generally elongated cylindrical configuration, each extending between and secured at the first, lower and second, upper ends thereof to said upper core plate and said upper internals support plate, respectively, and receiving a respective one of said plural column tubes coaxially therethrough; said plural upper internals support columns being in axial aligned relationship with respective said plural instrumentation thimbles; the second, upper end of each said upper internals support column being received through a corresponding, axially aligned bore in the upper internals support plate and defining an interior, cylindrical spring chamber therein; each said column tube having a collar affixed thereon and received within the associated spring chamber; and a spring mounted within the spring chamber and urged against the collar on the corresponding column tube for resiliently urging the column tube axially downwardly to maintain a sealed connection of the associated ball and cone joint. the lower end of each said spring chamber defines an abutment surface relative to said collar on said column tube; and all of said upper internal support columns and associated said column tubes being removable by upward axial movement of said upper support plate. plural guide tube jumper bundles associated with respective said head penetration columns, each jumper bundle defining therein a plurality of elongated interior passageways, each as aforesaid, respectively corresponding to the plural detector guide tubes of the respectively associated head penetration column; and plural flanged disconnect joints respectively associated with said plural jumper bundles and with the associated detector guide tubes of respective, said plural head penetration columns, each said flanged joint comprising: 2. An instrumentation system as recited in claim 1, wherein: 3. A system as recited in claim 2, wherein there is further provided: 4. A system as recited in claim 3, further comprising: 5. An instrumentation system as recited in claim 4, wherein: 6. An instrumentation system as recited in claim 34, further comprising: |
059784296 | description | DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a cross section 1 through a reactor core, in which square fuel elements 2 stand closely adjacent to one another in the manner of a checkerboard. With the exception of the edge regions, four such fuel elements each form a square, in which a respective measuring lance designated by (1), (2) . . . (28) is arranged only at one corner. A measuring lance of this type is therefore generally arranged at the common corner of four mutually abutting squares made up of four fuel elements each. For the evaluation of the signals of these measuring lances, once more with the exception of edge regions, four such squares are combined into a "region" (for example the adjacent, differently hatched regions 2', 2"), which thus register virtually the entire region of the core in the manner of a checkerboard. Since each measuring lance (reference numeral 3 in FIG. 2) comprises four sensors 4a, 4b, 4c and 4d arranged one above another in a sleeve pipe 5, four sensor signals are supplied to the entire device for monitoring the core via the corresponding measuring lines 6 of the measuring lances (1) . . . (28). Each of these measuring lines 6 thus carries the sensor signals assigned to one region. In FIG. 1, the individual measuring lances, respectively assigned to a region, are designated by one of the letters A, B, C and D, these letters specifying the assignment of the corresponding measuring lances and their sensor signals to a system of a total of p monitoring systems (here p=4). These monitoring systems operate redundantly and in each case supply their own monitoring signal and, if appropriate, alarm signal. These signals are only processed into an endstage monitoring signal or endstage output alarm signal in a system selection. The corresponding monitor monitoring the core therefore contains only systems which are respectively independent of one another (no sensor signal is processed in more than one system), and the regions monitored by the systems do not overlap. In the event of a failure of a measuring lance, although an entire region of 16 fuel elements is no longer monitored, only one of the redundantly operating systems is influenced thereby, while the other systems are not affected by the failure. Sensors of adjacent region are in this case also always assigned to different systems. These principles are also maintained in the case of other configurations of the core (for example larger cores) and of the measuring lances (for example 34 instead of 28 measuring lances). In the present case, the 28 measuring lances are distributed among systems having seven regions each (in general an arbitrary region will be designated by m and the total number of regions of a system p by M.sub.p. In the example, therefore, M.sub.p =7 applies to all the systems). In this assignment all the regions respectively register an identical number (namely four) of sensor signals, whereas in the general case for the individual regions, the number of sensor signals can also be different. This can primarily be provided if the above-mentioned "linear" assignment, in which each sensor is assigned to a maximum of one single system, is not performed. Each sensor signal is initially subjected to a plausibility control by means of a selection stage 8 in its region channel, firstly those sensor signals being separated out which lie outside the proper operating range of the sensors, as is also provided in the above-mentioned patent to Watford et al. From the remaining output signals from properly operating sensors, however, differing from this prior art, only a minimum number (here: two) is selected, to be specific generally the signals of the lowest sensors which are available. In general, the signals from sensors which are arranged linearly one above another specifically differ only little and, in particular, they show the same time profiles, virtually without a phase shift, which can be traced back to the local power pulsation in this region. The sensors are therefore in principle able to substitute for one another. Taking into account the lowest sensors (4a and 4b in FIG. 2), however, offers a slight advantage, since in the critical region of high power and low coolant throughput, the flux in the lower regions of the fuel elements executes more pronounced oscillations than in the upper regions. In other words, the corresponding extreme values (amplitudes) of the oscillation can be registered more distinctly. In particular, an analog filter 9' for the sensor signals can be connected upstream of the selection stage 8 which receives the sensor signals, whereby the sensor signals are subjected to a "2 out of 4" selection on processing in the component 10, which may simultaneously undertake a conversion of the analog input signals into digital output signals, so that instead of the analog filter 9' connected upstream, a digital filter 9 can also be connected downstream (in signal flow direction). In addition, in the case of this filter 9 a summation of the two output signals of the evaluation stage 8 is also performed, in order to obtain an instantaneous value for the flux into the appropriate region which is averaged over the model variation of the individual sensors. This corresponds to the summation of the sensor signals in the individual "cells" of the above-mentioned patent to Watford et al. However in the case of the prior art, the corresponding "cell signal" is formed from sensor signals which are also used in the monitoring of other regions and in other systems. Finally, in a standardization unit, a current measured value A(t)-A* is formed at the output of the filter 9 from the current signal A(t). The measured value can be standardized, for example, to the average signal level A* of this region. As described in the prior art, the average level can be formed by an integrator 10 in that the signal A(t) measured over a relatively long integration time period is integrated. This standardization supplies an alternatingly positive and negative measured value, so that the oscillation amplitudes lie symmetrically about a zero point and can be registered easily. However, digital signal processing makes it possible, also without great outlay, to register the amplitude of a half-period in each case, even in the case of otherwise standardized or unstandardized signals S. In that case, it may then be advantageous for the threshold values to be predefined as absolute values instead of relative values. Finally, the further processing of the signal S is suppressed as long as it lies under a threshold value A.sub.o for the normal signal noise, and therefore a determination of extreme values ("Peaks" or "amplitudes"), which could be assigned to an oscillation, is not possible (threshold value element 11). With reference to FIG. 3, the region channel of a system p contains an evaluation stage 12, in which firstly, in a first computing stage, the point in time T.sub.n is recorded at which an initially increasing signal value S, which lies above the noise limit A.sub.o, has risen to an extreme value A.sub.n and drops once more (positive peak). As an alternative--or preferably in addition--a negative peak is also registered as the peak A.sub.n and its point in time T.sub.n, that is to say an extreme value which lies beyond the noise limit A.sub.o which is formed by an initially falling and then rising (negative) value of the signal S. This extreme value registration 13 is followed by a further plausibility check 14 which, for example, is constructed similarly to the description in the Watford et al. patent, and checks whether the time interval DT.sub.n, which can be registered in the extreme value monitoring 13, between the currently registered point in time T.sub.n and the previously registered point in time T.sub.n-1 can correspond to an oscillation within the critical frequency band between 0.3 and 0.7 Hz. A further evaluation element 15 additionally checks whether the registered time interval DT.sub.n virtually coincides with the last-registered time interval DT.sub.n-1. If this is not the case, then the registered peaks are not the amplitudes of an oscillation which is virtually undamped and could increase to hazardous extreme values; the further evaluation of the last-determined peak A.sub.n is then suppressed. If, on the other hand, these are values which can be assigned to the amplitude of an oscillating variable, then by means of a corresponding confirmation signal a subsequent computing element 16 is activated, which determines from the last-determined peaks their "rate of increase" ##EQU1## If, therefore, the respective signal value S can be described mathematically by a variable S(t).multidot.cos .OMEGA.T, then this rate of increase corresponds to the differential ##EQU2## In the case of evaluating positive and negative extreme values, for example, it indicates the growth of the extreme value in each case following a half-period DT=T.sub.n -T.sub.n-1 of the oscillation. In the monitoring unit 17 ("checking"), a monitoring element 18 now forms a signal, in accordance with predetermined monitoring criteria which are described in more detail below, which signal indicates, for example as the binary signal in the state "0", that there is no hazardous oscillation corresponding to any of the monitoring criteria, whereas the state "1" of the corresponding monitoring signal sets off an alarm (item 19). This alarm signal, together with other information which, for example, identifies the region in which the monitoring criteria has responded, can be output to an display unit and/or stored in a memory for the purpose of documentation of the process. This construction of the region channel m is advantageously provided in each region channel, as is indicated at the top left in FIG. 4 in the field "system 1" for each region of the total number M1 of regions of the system p=1, and in the right field "system P" for all the regions (total number M.sub.p) of the system p=P. The linear alarm region signals (for example also entered into the element 19) represent a M.sub.p -multiple binary signal, corresponding to the number M.sub.p of the region channels, from which a N.sub.mp -multiple binary signal is formed in a region selection stage 20, in order to indicate that a bit corresponding to an alarm has been set at least in a number N.sub.mp of the regions of this system. In FIG. 4, the corresponding alarm region signals are combined once into a visual indication 21, where N.sub.m =1 is selected. This means that the visual alarm 21 is triggered as soon as the bit corresponding to the alarm is set in at least one region channel. Each system therefore contains a selection element in which N.sub.mp =1 is predetermined, i.e. a "1 out of 7" selection 22 (for example an OR element in digital evaluation) is executed, and the visual indication 21 is set, whereas if a second selection element 23 N.sub.mp =2 is set, a "2 out of 7" selection takes place. To be precise, an appropriate alarm bit in the region signal is only set if the monitoring criterion is satisfied respectively in at least two regions of the system, in order to rule out a false alarm as a result of processing errors. A system selection is now made in an output stage 24 which sets an alarm output signal if at least a minimum number N.sub.p from the total number P of the systems contains a set alarm signal. In this case, this system selection comprises a "1 out of 4" circuit 25 which is set to N.sub.p =1 and outputs an alarm signal (item 26) which is visually indicated in a display 27 and indicates that a critical oscillation has been discovered in one of the systems. A "2 out of 4" selection 28, set to N.sub.p =2, sets an alarm (item 27') which on the one hand can likewise be indicated in the display 27 and on the other hand acts on the reactor control 29 and there triggers a stabilization strategy which is stored in a memory 29' as an appropriate program. In general, in each system the processing elements of the region channel which are illustrated in FIGS. 2 and 3 can be implemented by means of a central computer with its own power supply, a central processing unit, an input module for 32 analog input signals and an appropriate output module for 32 digital signals, the computer being utilized to about 50% given an operating frequency of 32 MHz with the parallel processing of the 28 sensor signals, which are contained in the 32-bit input of the computer. An advantageous sampling rate for the input signals is 50 Hz or more, but at least 20 Hz should be ensured. The usual processing elements for the sensor signals offer sufficient space for the processor units of the systems. The output signals of these system processors can be connected to a commercially available microcomputer, in which the received region signals are processed and stored. This processor also contains the programs which are necessary to make the system selection and, in accordance with predefined strategies, to supply the signals which are necessary in the reactor control system for carrying out the respective stabilization measures. Optical fibers can advantageously be used as connecting lines. The stabilization measures are explained with reference to FIG. 5, which does not take into account a scale corresponding to the actual relationships. A course of the relative region measured value S is assumed and from its values which lie above the noise limit A.sub.o the rate of increase DA is determined if the oscillation exceeds a threshold value or limit value A.sub.lim. Here, the extreme case is assumed where, after a predefined maximum value A.sub.max of the amplitudes had been exceeded, a total SCRAM is initiated, a number N' (N'=2 here) oscillation periods being needed until it is sufficiently effective, whereas only a number N (for the purpose of illustration, N=3 is assumed here; in realistic conditions, N is very much larger) of oscillation periods has elapsed until the amplitudes of the relative measured value S pass through the region between A.sub.lim and A.sub.max. A curve 30 in FIG. 6 corresponds to the extreme case represented in FIG. 5 by the slope 30 (half envelope), further curves 34, 33, 32 and 31 being specified in FIG. 6 whose rate of increase DA is respectively lower by the factor 1/2, 1/3, 1/4 and 1/5. It can be seen from FIG. 6 that in the case of these rates of increase a SCRAM, which would be triggered when the threshold value A.sub.max were exceeded, is not yet necessary; instead the time or number N' of oscillation periods DT which is or are necessary for the effectiveness of the SCRAM permits the reactor to continue operating at power for a certain number N of periods. The number N can be seen from the point of intersection of the curves 32, 33 . . . with the curve F(A.sub.4) . For oscillations whose amplitudes grow still more weakly than the curve 32 when the threshold value A.sub.lim is exceeded, it can be assumed that such weakly increasing transient transitions inherently decay, so that it is provisionally not necessary to intervene in the reactor operation for a number N of operating periods, this number resulting from the point of intersection of the corresponding curves with the limit curve 35. An upper threshold value A4 in this case ensures that it is still possible, even in the case of an unchangingly growing amplitude, for a SCRAM to be initiated, the number N'=2 of oscillation periods still being available for its effectiveness. In FIG. 5, the relationship which is given by the curve F(A.sub.4) between the rates of increase DA and the periods N which are still available before the initiation of a SCRAM, following the exceeding of the threshold value A.sub.max are reproduced as a corresponding limit curve F(DA). A curve of this type--taking into account a sufficient safety margin--can be determined from model calculations for the behavior of the reactor under transient conditions and also from the comparison of such model calculations with actually observed reactor states and, for example, can be stored as a characteristic curve in a memory. It is then sufficient, when the threshold value A.sub.lim is exceeded, to make use of the respectively detected rate of increase in order to take the appropriate value N (that is to say the values N.sub.1, N.sub.2, N.sub.3, N.sub.4 for the curves 31, 32, 33, 34). When the amplitude value of A.sub.max is exceeded, a counter can be set to the appropriate value N, and counted down with each confirmation signal (FIG. 3). The reactor operation then does not need to be interrupted by a total SCRAM, provided the counter reading has not been counted down to zero. Here too, the total SCRAM only needs to be initiated when the amplitude threshold value A.sub.4 has been reached. As a rule, however, the oscillation has already inherently been damped within this time and decays once more, which can in particular be ensured by an alarm signal being set when the threshold value A.sub.max is exceeded, in this alarm stage that alarm signal prevents only changes in the operating state being undertaken in the control system which could lead to an increase in power and hence to a further transient excitation of the oscillation. In this case, therefore, only if the threshold value A.sub.max is exceeded is a stabilization strategy simply followed which corresponds to a low-ranking alarm stage and does not require any interruption in the reactor operation, in particular no SCRAM, as long as a highest-ranking alarm stage with a total SCRAM is not present as a result of exceeding the curve given in FIG. 7 and/or exceeding the threshold value A.sub.4. However, it is also possible to dispense with a characteristic curve which, for each value DA, determines the corresponding time which is still available before a SCRAM (number of periods N), and instead to monitor the rate of increase DA by means of appropriate threshold value detectors for the exceeding of specific threshold values, as specified in FIG. 7 by DA.sub.1, DA.sub.2, DA.sub.3 and DA.sub.4. If, therefore, there is for example a rate of increase which lies below the threshold value DA.sub.1, it is then possible to wait for a corresponding number of periods N, in which no safety measures at all are yet necessary, that is to say no stabilization strategy with a special intervention in the reactor control is necessary. In the region between the threshold values DA.sub.1 and DA.sub.2 (alarm stage I), provision can, for example, be made for the reactor operation to be allowed to continue for a number N.sub.2 of oscillation periods, in which case it may be advantageous to prevent the reactor from being raised to increased power. In the alarm stage II, the duration for this reactor operation can be limited to a number N.sub.3 of reactor periods. It is also possible, in order to improve the damping, to provide for some of the control rods to be moved slowly into the reactor, which corresponds to a reduction in the reactor power, as is provided operationally when a lower power is demanded of the reactor. The threshold values DA.sub.3 and DA4 for the rate of increase determine an alarm stage III, in which the reactor can still run further for a number N.sub.4 of periods. It is also possible in this case to move some of the absorber rods in rapidly, which is referred to as a "partial SCRAM". Only when the threshold value DA.sub.4 is exceeded does a total SCRAM appear necessary in a highest-ranking alarm stage. A further variant of the invention is explained with reference to FIGS. 8 to 10. Rates of increase are shown in FIG. 8, in this case for the amplitudes of the relative region signal S, which correspond to the curves 32 and 33 of FIG. 6. These amplitudes are determined at the point in time at which they exceed the threshold value A.sub.lim. It is assumed that the alarm stage II has been detected by the monitoring stage and a stabilization strategy has been initiated in which the reactor power is to be stabilized by moving the control rods in slowly. In the envelope 33, the amplitudes which occur under these conditions are indicated by continuous lines. The stabilization strategy corresponding to alarm stage II--if it were maintained at amplitude values which lie above a threshold value indicated by A.sub.3 and illustrated by peaks which are drawn with dashed lines--would lead to a total SCRAM having to be initiated with the threshold value A.sub.4. However, a total SCRAM of this type should be avoided. Therefore, when the threshold value A.sub.3 is reached, a transition is made from the stabilization strategy discussed in conjunction with the alarm stage II in FIG. 7 (slow insertion of absorber elements) to a higher alarm stage with a higher-ranking stabilization strategy, namely the above-mentioned partial SCRAM. As a result, the oscillation is now damped more heavily, with the result that the amplitudes no longer increase and the threshold value A.sub.4 is not reached and, in turn, the total SCRAM is not initiated. The curve 32 shows that the threshold value A.sub.3 can also be set higher in this case, given a lower rate of increase, than in the case of a higher rate of increase. In this embodiment, therefore, it is not the rate of increase which is monitored for the exceeding of threshold values. Instead, the instantaneous detected rate of increase is used to predefine a threshold value for the amplitude values themselves. The dependency of the threshold value on the rate of increase can by contrast in turn be determined in accordance with a calibration curve, similar to FIG. 7, or the threshold value A.sub.3 can also be changed in discrete steps by means of an appropriate division of the region available for the rate of increase into individual alarm stages. This embodiment provides the advantage that changes in the decay rate of the oscillation, which occur during reactor operation even after the threshold value A.sub.lim has been exceeded, are taken into particular account. This is shown by the curve 40 in FIG. 8, in which it is initially assumed that the oscillation grows so weakly as it exceeds the threshold value A.sub.lim that an intervention in the reactor control system is not necessary. However, it is assumed that the operating personnel have performed an increase in the power at time t.sub.b via the operational reactor control system. As a result, the transiently excited oscillation is considerably amplified. This leads to the situation where the amplitude A, whose rise was initially low when it crossed over A.sub.lim and which has not triggered any alarm, now assume the value of curve 33, so that the amplitude A is now monitored with regard to its exceeding the threshold value A.sub.3. Also, the stabilization strategy ("partial SCRAM") which is proper in the alarm stage III is initiated. This results in the increasing oscillation being damped more heavily, so that even in this case the threshold value A.sub.4 is in practice no longer reached. The result, of course, is that a total SCRAM has been averted. In a similar manner to the threshold value A.sub.3 for the amplitude A, which leads to the initiation of the partial SCRAM, it is of course also possible for an appropriate threshold value A.sub.1 and A.sub.2 to be introduced for the lower-ranking stabilization strategies (blocking of an increase in power, alarm stage I; or slow introduction of additional absorber elements, alarm stage II). This is shown in FIG. 9 using an oscillation whose envelope increases at a relatively low rate. In this case the extreme values (amplitudes) of the oscillation lie on an envelope curve 41 and are monitored for exceeding the threshold values A.sub.1, A.sub.2, A.sub.3 which, in accordance with the high number N of oscillation periods which are available in the case of this rate of increase, lie relatively close to the threshold values A.sub.th and A.sub.max. When the threshold value A.sub.1 is exceeded, the first alarm stage is set, whose stabilization strategy provides only for the blocking of an increase in power. As a result, although the rate of increase is lowered, the oscillation is not yet sufficiently damped. When the threshold value A.sub.2 is exceeded, however, the power of the reactor is lowered and the oscillation is damped in such a way that further growth to the threshold values A.sub.3, A.sub.max, A.sub.th already no longer occurs. The curve 42 shown in FIG. 10 is based on a relatively high rate of increase DA, for which reason the threshold values A.sub.1, A.sub.2 and A.sub.3 are set lower in this case--depending on the detected rate of increase DA--than in FIG. 9. Hence, the alarm stage II (threshold value A.sub.2) is already reached relatively early, and the "partial SCRAM" provided in the alarm stage III as a result of exceeding the threshold value A.sub.3 is also performed earlier. This leads to the desired damping of the oscillation and prevents the threshold value A.sub.max from being exceeded. By this means, total shutdown is prevented even in this unfavorable case. The resetting of the respective alarm stages can be performed, for example, when the amplitude once more falls below the threshold value A.sub.lim. FIG. 11 illustrates an embodiment for the monitoring in the command channels of the system 1, the region selection stage for the alarm signals, which are set in the region signal by means of this monitoring, and the monitoring device in the corresponding system channel. In this case, the region monitoring in the first region of the channel 1 is illustrated in the fields in each case designated by "region 1", the region signal assigned to this first region and corresponding to the threshold value A.sub.1 being fed to a threshold value detector which sets a logic alarm signal "1" if the region signal S exceeds the threshold value A.sub.1. The threshold value is taken from a memory 52 for a characteristic curve. In accordance with the stored characteristic curve, this threshold value A.sub.1 corresponds to the value DA of the current rate of increase determined in the region channel 1 (component 16, FIG. 3). Using the logic output signal of the threshold value detector 51, on the one hand an indicator and/or memory unit 53 can be driven, which now forms an alarm region signal AA1, which is assigned to the first alarm stage, for the monitoring signal AA1, which is assigned to the first alarm stage and to the first region channel. In a similar way, the relative region signal S is performed in an evaluation unit 54 (not shown in more detail) with respect to the threshold value A2, and in a monitoring stage containing the threshold value detector 55, the characteristic curve memory 56 and the indicating and/or memory unit 57, with respect to the threshold value A.sub.3 and the alarm stage III. What is not shown is that the signal S can be monitored for the exceeding of a fixedly predefined threshold value A.sub.4 by means of a further threshold value detector. The corresponding elements are present in each region signal of the system and are also indicated for the last region "region M.sub.p " in the right-hand part of FIG. 11, using the reference symbols 51', 52' . . . 57'. The monitoring signals which are formed by the threshold value detectors 51, 51' in the individual region channels (i.e., a 7-bit signal in the case of M.sub.p =7) can be summed in a summing element 60. This signal thus indicates in how many region channels the corresponding threshold value detector 51 has set an alarm signal of stage I. If this number is greater than or equal to a predefined number N.sub.mp, then an appropriate interrogation unit 61 sets a corresponding alarm system signal. In this case, the interrogation unit 61 performs this interrogation twice, the minimum number N.sub.mp being set to 1 for a first alarm system signal AA1'. This signal AA1' can then be used to indicate, via a corresponding system selection (in the simplest case a summing element--not shown--for all the signals AA1' from all the redundantly operating system channels), whether and how many systems are generating the alarm stage I. In addition, in this system monitoring 61, N.sub.mp =2 is also set. A corresponding signal AA1" is output if at least two regions report the alarm of stage I. This signal can be used in the system selection to form an alarm output signal from all the alarm signals which are generated in the redundantly operating systems. The alarm output signal can intervene in the control system of the reactor and there block an operational increase in the reactor power. In the simplest case it is sufficient if the system selection forms only a "1 out of 4" selection, i.e., it combines the appropriate signals AA1" of the four systems by means of an "OR" element. However, in order to reliably avoid unnecessary disturbances to the reactor operation, which could be produced by faulty processing in one of the systems, a minimum number N.sub.p for the system signals, in which the alarm stage I is set, is advantageously predefined for the intervention in the reactor operation. This can be carried out in a simple way in that the logic signals AA1" of the systems are added and produce the intervention in the reactor control system only if the sum is greater than or equal to 2. In a similar way, the monitoring signals assigned to the alarm stages II and III of the individual regions of the system can be processed via the summing elements 62, 64 into corresponding signals which, in the interrogation units 65, 66 for generating the alarm system signals assigned to these stages, supply AA3' and AA3". The [lacuna] from the alarm signals AA2" of the four system signals are further processed (not illustrated) in the same way as was described with reference to the signals AA1", and form an alarm output signal assigned to this alarm stage II, which signal intervenes in the reactor operation in such a way that not only is an increase in the reactor power blocked but the reactor power is even reduced in accordance with the programs which are provided for normal reactor operation. In the same way as was described with regard to the signals AA1" of the first alarm stage, the alarm signals AA3" assigned to the alarm stage III are also further processed and form an alarm output signal assigned to this alarm stage III. The signal triggers a "partial SCRAM" in accordance with the stabilization strategy assigned to this alarm stage. Finally, it should be noted that the alarm signals formed by means of the fixedly set threshold value A.sub.4 are further processed in the same way, so that, in an emergency, a total SCRAM is triggered corresponding to the highest alarm stage. The invention therefore ensures on the one hand that the unstable state of the reactor is monitored with a sufficient redundancy in order to be able to make a reliable statement about the unstable state, given failure of individual sensors, measuring lances or computing elements; on the other hand the invention allows a minimum in intervention in the reactor operation in order to damp the instability. In this case, a total SCRAM is virtually ruled out in accordance with all experience and estimations, so that the fourth alarm stage--a total SCRAM--can be viewed as completely superfluous. The constructional elements provided for monitoring the threshold value A.sub.max and the transmission elements for an alarm signal assigned to this highest alarm stage are therefore described only as an option which may also be dispensed with. |
abstract | The present disclosure significantly reduces the waiting time from inserting a specimen holder into an electron microscope until high quality data acquisition is possible. Characterizing the present disclosure, it is a specimen holder partly made of low thermal expansion material. The low thermal expansion material can be any of group 4, 5 or 6 in the periodic table of the elements. |
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abstract | In a method of reducing corrosion of a material constituting a nuclear reactor structure, an electrochemical corrosion potential is controlled by injecting a solution or a suspension containing a substance generating an excitation current by an action of at least one of radiation, light, and heat existing in a nuclear reactor, or a metal or a metallic compound forming the substance generating the excitation current under the condition in the nuclear reactor to allow the substance generating the excitation current to adhere to the surface of the nuclear reactor structural material, and by injecting hydrogen in cooling water of the nuclear reactor while controlling the hydrogen concentration in a feed water. |
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claims | 1. A multileaf collimator comprising:a first leaf block group including a plurality of leaf blocks arranged in a direction;a second leaf block group including a plurality of leaf blocks arranged in the same direction as the first leaf block group, the leaf blocks of the second leaf block group being opposed to the leaf blocks of the first leaf block group in a direction orthogonal to the direction in which the leaf blocks of the first leaf block group are arranged;a plurality of drive mechanisms provided on the respective leaf blocks of the first and second leaf block groups, the drive mechanisms moving the leaf blocks of the first or second leaf block group in an oncoming direction in which the leaf blocks of the first or second leaf block group come close to the leaf blocks of the second or first leaf block or in a departing direction in which the leaf blocks of the first or second leaf block group depart from the leaf blocks of the second or first leaf block group;a plurality of magnetic layers which are provided on the respective leaf blocks of the first and second leaf block groups so as to be positioned on faces of the leaf blocks along a moving direction of the leaf blocks, each of the magnetic layers having a first magnetized part which is magnetized in a north pole and a second magnetized part which is magnetized in a south pole;a plurality of magnetic sensors which are provided on the respective leaf blocks of the first and second leaf block groups, the magnetic sensors being stationary in a noncontact state with respect to the respective leaf blocks, the magnetic sensors varying output signals when the respective leaf blocks are moved in the oncoming direction or the departing direction; anda control device which controls the drive mechanisms according to the output signals delivered by the respective magnetic sensors so that a space defined between the leaf blocks of the first and second leaf blocks is adjusted into a target configuration. 2. The multileaf collimator according to claim 1, wherein the first and second magnetized parts magnetized in the north and south poles respectively are disposed alternately in a direction of movement of the leaf blocks. 3. The multileaf collimator according to claim 1, wherein each leaf block of the first and second leaf block groups has a face extending in a direction of movement of the leaf blocks, the face having concave and convex portions formed alternately, and each magnetic layer is disposed on the face of each leaf block so as to cover the concave and convex portions. 4. A control device for a multileaf collimator which includes a first leaf block group including a plurality of leaf blocks arranged in a direction, a second leaf block group including a plurality of leaf blocks arranged in a same direction as the first leaf block group, the leaf blocks of the second leaf block group being opposed to the leaf blocks of the first leaf block group in a direction orthogonal to the direction in which the leaf blocks of the first leaf block group are arranged, a plurality of drive mechanisms provided on the respective leaf blocks of the first and second leaf block groups, the drive mechanisms moving the leaf blocks of the first or second leaf block group in an oncoming direction in which the leaf blocks of the first or second leaf block group come close to the leaf blocks of the second or first leaf block or in a departing direction in which the leaf blocks of the first or second leaf block group depart from the leaf blocks of the second or first leaf block group, a plurality of magnetic layers which are provided on the respective leaf blocks of the first and second leaf block groups so as to be positioned on faces of the leaf blocks along a moving direction of the leaf blocks, each of the magnetic layers having a first magnetized part which is magnetized in a north pole and a second magnetized part which is magnetized in a south pole, a plurality of magnetic sensors which are provided on the respective leaf blocks of the first and second leaf block groups, the magnetic sensors being stationary in a noncontact state with respect to the respective leaf blocks, the magnetic sensors varying output signals when the respective leaf blocks are moved in the oncoming direction or the departing direction, and a control device which controls the drive mechanisms according to the output signals delivered by the respective magnetic sensors so that a space defined between the leaf blocks of the first and second leaf blocks is adjusted into a target configuration, the control device comprising:a movement starting unit which carries out a movement starting process in which movement of the leaf blocks in the departing direction is processed with a predetermined origin position serving as a starting point when an operation of the drive mechanism is started, the movement starting unit carrying out the movement starting process for everyone of the leaf blocks of the first and second leaf block groups;a movement amount detection unit which carries out a movement amount detection process in which an amount of movement of each leaf block in the departing direction is detected on the basis of the origin position based on a variation in an output signal delivered by the magnetic sensor, the movement amount detection unit carrying out the movement amount detecting process for every one of the leaf blocks of the first and second leaf block groups;a determination unit which carries out a determining process which determines whether a result of detection by the movement amount detection unit has reached a target movement amount, the determination unit carrying out the determining process for every one of the leaf blocks of the first and second leaf block groups; anda movement stopping unit which carries out a movement stopping process in which the movement stopping unit stops an operation of the drive mechanism when the determination unit has determined that a result of detection by the movement amount detection unit has reached the target movement amount, the movement stopping unit carrying out the movement stopping process for every one of the leaf blocks of the first and second leaf block groups. 5. A method of controlling a multileaf collimator which includes a first leaf block group including a plurality of leaf blocks arranged in a direction, a second leaf block group including a plurality of leaf blocks arranged in the same direction as the first leaf block group, the leaf blocks of the second leaf block group being opposed to the leaf blocks of the first leaf block group in a direction orthogonal to the direction in which the leaf blocks of the first leaf block group are arranged, a plurality of drive mechanisms provided on the respective leaf blocks of the first and second leaf block groups, the drive mechanisms moving the leaf blocks of the first or second leaf block group in an oncoming direction in which the leaf blocks of the first or second leaf block group come close to the leaf blocks of the second or first leaf block or in a departing direction in which the leaf blocks of the first or second leaf block group depart from the leaf blocks of the second or first leaf block group, a plurality of magnetic layers which are provided on the respective leaf blocks of the first and second leaf block groups so as to be positioned on faces of the leaf blocks along a moving direction of the leaf blocks, each of the magnetic layers having a first magnetized part which is magnetized in a north pole and a second magnetized part which is magnetized in a south pole, a plurality of magnetic sensors which are provided on the respective leaf blocks of the first and second leaf block groups, the magnetic sensors being stationary in a noncontact state with respect to the respective leaf blocks, the magnetic sensors varying output signals when the respective leaf blocks are moved in the oncoming direction or the departing direction, and a control device which controls the drive mechanisms according to the output signals delivered by the respective magnetic sensors so that a space defined between the leaf blocks of the first and second leaf blocks is adjusted into a target configuration, the method comprising:starting movement of the leaf blocks in the departing direction with a predetermined origin position serving as a starting point when an operation of the drive mechanism is started, the movement starting step being carried out for every one of the leaf blocks of the first and second leaf block groups;detecting an amount of movement of each leaf block in the departing direction on the basis of the origin position based on a variation in an output signal delivered by the magnetic sensor, the movement amount detecting step being carried out for every one of the leaf blocks of the first and second leaf block groups;determining whether a result of detection by the movement amount detection unit has reached a target movement amount, the determining step being carried out for every one of the leaf blocks of the first and second leaf block groups; andstopping an operation of the drive mechanism when the determination unit has determined that a result of detection by the movement amount detection unit has reached the target movement amount, the movement stopping step being carried out for every one of the leaf blocks of the first and second leaf block groups. 6. A radiation treatment machine comprising:a treatment table on which a patient is put;a radiation generator which applies medical treatment radiation to an affected part of the patient on the treatment table; anda multileaf collimator which adjusts the radiation applied to the patient according to a shape of the affected part, the multileaf collimator comprising:a first leaf block group including a plurality of leaf blocks arranged in a direction;a second leaf block group including a plurality of leaf blocks arranged in the same direction as the first leaf block group, the leaf blocks of the second leaf block group being opposed to the leaf blocks of the first leaf block group in a direction orthogonal to the direction in which the leaf blocks of the first leaf block group are arranged;a plurality of drive mechanisms provided on the respective leaf blocks of the first and second leaf block groups, the drive mechanisms moving the leaf blocks of the first or second leaf block group in an oncoming direction in which the leaf blocks of the first or second leaf block group come close to the leaf blocks of the second or first leaf block or in a departing direction in which the leaf blocks of the first or second leaf block group depart from the leaf blocks of the second or first leaf block group;a plurality of magnetic layers which are provided on the respective leaf blocks of the first and second leaf block groups so as to be positioned on faces of the leaf blocks along a moving direction of the leaf blocks, each of the magnetic layers having a first magnetized part which is magnetized in a north pole and a second magnetized part which is magnetized in a south pole;a plurality of magnetic sensors which are provided on the respective leaf blocks of the first and second leaf block groups, the magnetic sensors being stationary in a noncontact state with respect to the respective leaf blocks, the magnetic sensors varying output signals when the respective leaf blocks are moved in the oncoming direction or the departing direction; anda control device which controls the drive mechanisms according to the output signals delivered by the respective magnetic sensors so that a space defined between the leaf blocks of the first and second leaf blocks is adjusted into a target configuration. 7. The radiation treatment machine according to claim 6, wherein the first and second magnetized parts magnetized in the north and south poles respectively are disposed alternately in a direction of movement of the leaf blocks. 8. The radiation treatment machine according to claim 6, wherein each leaf block of the first and second leaf block groups has a face extending in a direction of movement of the leaf blocks, the face having concave and convex portions formed alternately, and each magnetic layer is disposed on the face of each leaf block so as to cover the concave and convex portions. |
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description | This is a Non-Provisional Application claiming the benefit of co-pending Provisional Application Ser. No. 61/340,612 filed on Mar. 18, 2010. Not Applicable Not Applicable 1. Field of the Invention The present invention relates generally to the field of plasma physics. More particularly, the invention concerns a method and apparatus for compressing plasma to a high energy state. 2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 By way of brief background, in 1942, Enrico Fermi began discussing the idea of joining light nuclei by nuclear fusion to generate a large source of energy. He suggested burning deuterium, an abundant stable-isotope of hydrogen. Today, the two primary approaches to the problem of achieving fusion power production have been Magnetic Confinement (MCF) and Laser Inertial Confinement (ICF) demonstration devices, such as the International Thermonuclear Experimental Reactor (ITER) tokamak that uses MCF or the National Ignition Facility (NIF) that uses ICF. These plasma experiments scale to very large sizes, measuring double-digit meters across. Reactors based on these approaches scale to even larger sizes because they occupy either extreme of the density conditions necessary to fulfill the Lawson criterion for simultaneously achieving an energetic plasma for sufficient duration. MCF attempts to sustain a low-density 1020 m−3 plasma for a long duration of about 2 to 4 seconds, using external magnetic fields, but suffers from plasma instabilities. ICF attempts to hold a high-density 1028 m−3 plasma for nanoseconds. Magnetized Target Fusion (MTF) mitigates the problems encountered at either extreme by sustaining a medium-density 1024 m−3 plasma for only several milliseconds, while simultaneously reducing the minimum reactor size and cost as compared to MCF or ICF. Los Alamos National Laboratory (LANL) began early research into MTF, but became hampered by the impetus to scale their experiments to use the nearby Shiva Star capacitor bank as a power source, instead of scaling by best available theory and experiment. The Shiva Star facility is located at Kirtland Air Force Base in Albuquerque, N. Mex. They did not optimize their proof-of-principle design based on physics, but rather on their power supply limitations. Another weakness in their approach was the use of a theta pinch, instead of a more efficient antenna method to form a Compact Torus (CT) plasma structure. Lastly, they adhere to a non-reusable compression method (an aluminum can crusher), for single-shot experimentation. A Canadian company improved upon this earlier implementation and attempted a smaller-scale MTF approach, one with lower input energy needs. However, this approach introduced high-atomic-number impurities (such as lead) that quench the plasma by radiation losses before ignition occurs. Controlling the timing of the acoustic-compression method of this company is also problematic. The California Institute of Technology and Lawrence Livermore National Laboratory (LLNL) focused on injecting a compact torus (CT) into a tokamak, to sustain the latter. Their prototype ‘Compact Torus Accelerator’ experiment showed that it was possible to both translate and compress a compact torus plasma structure by moving it relative to a tapered wall. However, they also experienced impurity problems (iron from steel electrodes) and did not attempt to extend their initial achievement to a curved geometry, such as a spiral. The University of Washington Plasma Physics Laboratory has long advocated cleanliness requirements to avoid plasma impurities. They also utilize newer and more efficient methods to form and accelerate compact toroids. However, the pure research of the University is not focused on advanced plasma compression for MTF and the University has not attempted to translate a CT along a curved wall made of beryllium or lithium-silicon, which are much lower-Z materials than their walls (made of silicon dioxide). Prior art compact toroid compression mechanisms, include, but are not limited to the following: a. Explosive (liner technology)—For example the Los Alamos/Shiva Star and like projects. Such mechanisms are not reusable, require high input energy requirements and necessitate large system size. b. Pneumatic (gas injection)—Such mechanisms typically exhibit pressure instabilities and are generally too slow for large plasmas. c. Hydraulic (hydro-forming wall)—For example, the Canadian ‘General Fusion’ MTF concept. Such mechanisms, which require sub-microsecond-precision timing, require highly complex control systems. Also, the liquid walls of such mechanisms add high-atomic-number contaminants to the plasma that significantly increase radiation loss rates from the plasma. d. Mechanical (piston)—For example, the Canadian ‘General Fusion’ concept. Such mechanisms, which require repetitive sub-microsecond timing, require highly complex control systems. e. Electrical (relay-piston)—For example, the Canadian ‘General Fusion’ concept. Such mechanisms, which require repetitive sub-microsecond timing require highly complex control systems. f. Magnetic (coil-current spike)—This mechanism has been tried in connection with many research programs, from the early TRISOPS (experiment at the University of Florida) to the University of Washington Plasma Physics Laboratory's latest CT devices. Such mechanisms require good timing, a large energy input, and may induce a plasma instability. The thrust of the present invention is to provide a compact toroid plasma structure compression assembly that is superior to and overcomes the problems associated with the various mechanisms described in the preceding paragraphs. More particularly, through analysis of the disadvantages of the aforementioned prior approaches, it has been possible to derive a unique set of design features that yield a novel approach with a distinct advantage. The details of these novel design features will be described further in the specification that follows. With the foregoing in mind, it is an object of the present invention to provide a compressor assembly of novel design within which a plasma can be efficiently compressed to a high energy state. More particularly, it is an object of the invention to provide a compressor assembly of the aforementioned character, which includes an elongated spiral passageway within which a compact toroid (CT) plasma structure can be efficiently compressed to a high-energy state by compressing the CT using its own momentum against the wall of the spiral passageway in a manner to induce heating by conservation of energy. Another object of the invention is to provide a compressor assembly of the character described in the preceding paragraph, which includes a burn chamber that is in communication with the spiral passageway and into which the compressed CT is introduced following its compression. Another object of the invention is to provide a burn chamber of the character described in the preceding paragraph, in which a magnetic sensor is embedded in the burn chamber for measuring the magnetic field vector versus time. Another object of the invention is to provide a compressor assembly of the character described in the preceding paragraph, in which the burn chamber comprises a toroidal ring of constant cross-section, having at least one entrance port for receiving the compressed CT and having a multiplicity of smaller exhaust ports. Another object of the invention is to provide a method for compressing a CT to a high-energy state using a compressor having an elongated spiral passageway by injecting the CT into the spiral passageway in a manner to avoid ricochet of the CT along the walls of the passageway. More particularly, in accordance with the method of the invention, ricochet is avoided by ensuring that the bulk axial kinetic energy of the CT at the point of injection is greater than the design “target” thermal energy sought to be achieved at the end of compression. Another object of the invention is to provide a method of the character described in the preceding paragraph in which thermal conduction losses and particle diffusion losses are avoided by embedding a large magnetic field within the CT during formation, prior to launching the CT into the elongated spiral passageway. A highly magnetized CT impedes both thermal conduction losses and particle diffusion losses perpendicular to the embedded magnetic field lines. Another object of the invention is to provide a method of the character described in the preceding paragraphs, in which thermal conduction losses and particle diffusion losses are avoided by applying a plasma-impurity impeding coating to the walls of the elongated spiral passageway. Examples of these coatings include low atomic number materials, such as beryllium or lithium-silicon. Another object of the invention is to provide a method of the character described in the preceding paragraphs in which, following compression of the CT to the design “target” thermal energy, the CT is introduced into a burn chamber comprising a toroidal ring of constant cross-section having at least one entrance port for the compressed CT and having a multiplicity of smaller exhaust ports. Another object of the invention is to provide a method of the character described in which, following compression of the CT to the design “target” thermal energy, the CT is introduced into a burn chamber and after the burn is complete, the compressed CT is caused to dissipate into a neutral gas, which is pumped out of the burn chamber by means of a suitable vacuum pump. The forgoing as well as other objectives of the invention will be achieved by the apparatus illustrated in the attached drawings and described in the specification which follows. Definitions As used herein, the following symbols have the following meanings: SymbolMeaninga0Bohr radiusa12½ for single reactant, otherwise 1Asplasma surface areaAwwall surface areaBmagnetic flux densitycspeed of lightDdeuteriumDeelectron particle diffusivitye0elementary chargeE0incoming ion energy for sputteringE2electron allowed energy statesEHhydrogen ground state energyEthsputtering threshold energygfbfree-bound gaunt factorgfffree-free gaunt factorhPlanck constantHhydrogenHeheliumjeelectron sheath current to wallkBoltzmann constantKLtotal transparency factorKn22nd-order Bessel functionLiion inertial lengthmeelectron massmiion massmPproduct ion massnneutron, or principal quantum no.n1, n2respective reactant densitiesneelectron densityngasneutral gas densityniion densitynPreactant ion particle densityNaion density * fractional ionizationNZreactant ion density * charge/massqabsolute sputtering yieldQPreaction product energyrradiusr0field null radiusrciion cyclotron radiusreclassical electron radiusriion radiusrwwall radiusRyRydberg energySnKRCstopping power for KrC potentialttime step durationTtritiumTeelectron temperatureTiion temperatureTptransient radial temp. profileVdion velocity distributionViion most-probable thermal speedVpreaction product ion velocityVplasma volumeWPvariable of integration for energyZaverage ion charge in plasmaZPion product chargeαfine-structure constantβethermoelectric coefficientγratio of specific heatsΔrplasma effective thicknessεreduced energy for sputteringε0electric permittivity of free spaceHproduct particles fraction that stayθvariable of integration for timeκewelectron-wall thermal conductivityκiwion-wall thermal conductivityλsputtering decrease at low energyΛeplasma parameter for electronsΛiplasma parameter for ionsμsputtering decrease fit parameterμ0magnetic permeability free spaceπgeometric piσcsbeam reaction cross-sectionσmmomentum transfer cross-sectionσ∥electric conductivity parallel B<σv>integrated reaction cross-sectionτieion-electron equilibration timeTtime that lost product particles stayφradial particle profile in timeXeelectrons to products velocity ratioXiions to products velocity ratioΨmagnetic flux radial profile in timeFusion The process by which two light nuclei combine to form a heavier one. The fusion process releases a tremendous amount of energy in the form of fast moving particles. Because atomic nuclei are positively charged—due to the protons contained therein—there is a repulsive electrostatic, or Coulomb, force between them. For two nuclei to fuse, this repulsive barrier must be overcome, which occurs when two nuclei are brought close enough together where the short-range nuclear forces become strong enough to overcome the Coulomb force and fuse the nuclei. The energy necessary for the nuclei to overcome the Coulomb barrier is provided by their thermal energies, which must be very high. For example, the fusion rate can be appreciable if the temperature is at least of the order on 10 keV—corresponding roughly to 100 million degrees Kelvin. The rate of a fusion reaction is a function of the temperature, and it is characterized by a quantity called reactivity. The reactivity of a D-T reaction, for example, has a broad peak between 30 keV and 100 keV. Field-Reversed Configuration (FRC) An example of a compact toroid plasma structure is a Field-Reversed Configuration which is formed in a cylindrical coil which produces an axial magnetic field. First, an axial bias field is applied, then the gas is pre-ionized, which “freezes in” the bias field, and finally the axial field is reversed. At the ends, reconnection of the bias field and the main field occurs, producing closed poloidal magnetic field lines. A review well known to those skilled in the art is found in “Field Reversed Configurations,” M. Tuszewski, Nuclear Fusion, Vol. 28, No. 11, (1988), pp. 2033-2092. Compact Toroid The FRC belongs to the family of compact toroids. “Compact” implies the absence of internal material structures (e.g. magnet coils) allowing plasma to extend to the geometric axis. “Toroid” implies a topology of closed donut-shaped magnetic surfaces. The FRC is differentiated from other compact toroids by the absence of an appreciable toroidal magnetic field within the plasma. Prime-Mover Subsystem As used herein, prime-mover subsystem means a system for converting fusion-generated ion and/or neutron thermal energy to electrical energy. The prime-mover subsystem may comprise a heat exchanger and may also comprise various types of selected direct-conversion subsystems of a character also well known by those skilled in the art. The Apparatus of the Invention Referring now to the drawings and particularly to FIG. 1, one form of the apparatus of the invention for compressing plasma to a high energy state is there shown and generally designated by the numeral 20. This form of the apparatus comprises a compressor 22, a vacuum pump subsystem 24 connected to the compressor by an outlet port 25 and a wall-cleaning subsystem that is operably associated with the compressor. The wall-cleaning subsystem here comprises heater blankets 26a, such as those readily commercially available from BH Thermal Corporation of Columbus, Ohio and like sources, a glow discharge cleaning (GDC) system 26b such as a system that is readily commercially available from XEI Scientific, Inc. of Redwood City, Calif. and an ion gettering pump 26c of the character readily available from commercial sources such as SAES Getters USA of Colorado Springs, Colo. Apparatus 20 also includes a plasma source subsystem 28 that here comprises stator antenna coils with pre-ionization capability, such as those commercially available from sources such as Alpha Magnetics of Hayward, Calif., a gas pulse injection valve with fire control unit 30 of the character that is available from Parker Hannifin of Pine Brook, N.J., and a ejector coil subsystem 32 that is also available from Alpha Magnetics. The pre-ionization process is preferably powered by a radiofrequency generator of the character that can be obtained from T & C Power Conversion of Rochester, N.Y. As will be discussed in greater detail in the paragraphs that follow, a prime-mover subsystem, which is generally designated in FIG. 1 by the numeral 34, must be operably associated with a compressor 22 to convert the fusion-generated ion and/or neutron thermal energy to electrical energy. Prime-mover 34 here comprises a heat exchanger of a character well understood by those skilled in the art. Attached to the heat exchanger is a steam turbine, which is, in turn, attached to an electrical generator (not separately shown in the drawings). The prime-mover subsystem can also comprise various types of selected direct-conversion subsystems of a character also well known by those skilled in the art. A highly unique feature of the apparatus of the present invention is the previously identified compressor 22, the details of construction of which are illustrated in FIGS. 2 through 4 of the drawings. In the present form of the invention, the plasma compressor 22 comprises first and second sealably interconnected portions 36 and 38 that are constructed from a material selected from the group consisting of aluminum, steel, copper, silicon, magnesium, carbon-carbon composites, nickel super alloys, tungsten, or other refractory alloys (such as molybdenum, niobium or rhenium). Preferably, portions 36 and 38 are formed using a conventional computer numerically controlled (CNC) machine, or a conventional electrical discharge machine (EDM), or by casting methods. As best seen in FIGS. 3 and 4 of the drawings, each of the portions 36 and 38 is provided with an elongate spiral passageway 40 having continuous wall 40a. Each of the spiral passageways has an inlet 40b and an outlet 40c (FIG. 3). Disposed proximate the center of the compressor 22 and in communication with the outlet of the spiral passageway is the important burn chamber 41, the construction and operation of which will presently be described. Also forming a part of the compressor 22 is an inlet port component 42 and an inner ring 44 that is operably associated with the burn chamber 41. Inlet port component 42 is in communication with the inlet of the spiral passageway 43 (FIG. 4) that is formed when portions 36 and 38 are joined together in the manner illustrated in FIG. 2 of the drawings by brazing, welding, diffusion bonding, or mechanical assembly (with bolts and seals). As illustrated in FIG. 2, spiral passageway 43 is of progressively decreasing diameter with the smallest diameter of the passageway being in communication with the burn chamber 41. Both the inlet port component and the inner ring are also preferably formed from a material selected from the group consisting of aluminum, steel, copper, silicon, magnesium, carbon-carbon composites, tungsten, or other refractory alloys. In order to avoid contamination of the plasma during the compression process, the wall of the elongated spiral passageway 40 of the compressor 22, as well as all other internal surfaces of the compressor that are exposed to the plasma, must be provided with a coating “C” preferably comprising either lithium-silicon, beryllium, or diboride ceramic, all of which are electrically conductive and low atomic-number materials (see FIGS. 3 and 4A). With respect to the lithium-silicon coating, it is to be noted that because pure lithium metal reacts with water vapor in the air, it is necessary that it be strictly maintained under vacuum between the point of manufacture of the coating powder and its application to the internal walls of the compressor. For certain applications, an electrically-conductive diboride ceramic or similar composite coating that consists of low atomic-number elements, which sputter slowly, could also advantageously be used to coat the internal walls of the compressor. The various techniques for coating the interior walls of the compressor are well known to those skilled in the art. For beryllium coatings, these techniques are fully described in a work entitled Beryllium Chemistry and Processing, Kenneth A. Walsh, Edgar E. Vidal, et al, ASM International (2009) (see particularly, Chapter 22, “Beryllium Coating Processes”, Alfred Goldberg, pp. 361-399). Once machined and properly coated, the inlet port component 42, the inner ring 44 and the inner walls of the compressor 22 that are exposed to the plasma are carefully cleaned and the various components of the compressor are joined together in the manner well understood by those skilled in the art, such as by brazing, welding, diffusion bonding, or mechanical assembly. After further cleaning and leak checks, the compressor 22 is integrated with the other subsystems of the apparatus of the invention in the manner depicted in FIG. 1 of the drawings. These subsystems include the previously described vacuum pump subsystem 24, the wall-cleaning subsystem that comprises heater blankets 26a, a glow discharge cleaning (GDC) system 26b and an ion gettering pump 26c and the plasma source subsystem 28. After these various subsystems have been interconnected with the compressor and the completed system has been thoroughly tested, the prime-mover subsystem 34 is interconnected with the compressor 22 in the manner indicated in FIG. 1 of the drawings. Prior to operating the apparatus of the invention, it is desirable to include a variety of well-known diagnostic tools around the apparatus (not shown in the drawings), such as a high-speed x-ray camera for observing shots, along with a neutron diagnostic, plus Rogowski coils for timing the ejection speed of the CT through the input port, as well as the speed of the CT in the burn chamber 41. Before considering the methods of the invention an alternate embodiment of the compressor unit will be considered. This alternate form of the compression unit is illustrated in FIGS. 6-9 of the drawings and is generally designated by the numeral 52. This embodiment is similar in many respects to the embodiment shown in FIGS. 1 through 5 and functions in a substantially identical manner. The primary difference between this latest embodiment of the invention and the previously described embodiment resides in the fact that the compressor is constructed from an electrically conductive, metallic alloy having a low atomic number, such as a beryllium alloy. More particularly, in this latest embodiment of the invention, portions 54 and 56 of the compressor unit 52 are formed from a block of beryllium alloy using a conventional computer numerically controlled (CNC) machine, or a conventional electrical discharge machine (EDM), or by casting method. As in the earlier described embodiment of the invention and as illustrated in FIGS. 7 and 8 of the drawings, each of the portions 54 and 56 is provided with an elongated spiral passageway 58 having continuous wall 58a. Each of the spiral passageways has an inlet 58b and an outlet 58c (FIG. 7). Also forming a part of the compressor 52 is an inlet port component 60, outlet port component 61 and an inner ring 62, the functions of which are substantially identical to the functions of inlet port 42 and the inner ring 44 of the previously described embodiment. Both the inlet port component and the inner ring are also preferably formed from a low atomic number, electrically conductive material, such as a beryllium alloy. Once machined, the inlet port component 60, the inner ring 62 and portions 54 and 56 are carefully cleaned and connected together in the manner well understood by those skilled in the art, such as by brazing, welding, diffusion bonding, or mechanical assembly using bolts and seals. After portions 54 and 56 are fused together the elongated spiral passageways 58 formed in each of the portions cooperate to define a spiral passageway 63 (FIG. 8). As illustrated in FIG. 8, spiral passageway 58 is of progressively decreasing diameter with the smallest diameter of the passageway being in communication with the burn chamber 65. Disposed proximate the center of the compressor 52 and in communication with the outlet of the spiral passageway 63 is the important burn chamber 65 of this latest form of the invention, the construction and operation of which is substantially identical to the previously identified burn chamber 41. Other candidate materials for use in constructing the compression structure 52 include Carbon-Carbon composites and refractory metal alloys (both higher atomic number materials than Beryllium). The use of the beryllium alloy material in constructing the compressor is somewhat less desirable than the use of the more common materials such as steel, copper, silicon, magnesium, tungsten or other refractory alloys, all of which absorb x-rays better than beryllium. Additionally, the use of these materials is considerably less hazardous and the materials combine the function of a vacuum structural wall and x-ray shielding wall into one component. It is to be understood that a variety of gasses, including but not limited to: hydrogen, deuterium, deuterium-tritium mixtures, pure tritium, helium-3, diborane and mixtures thereof can be used with the compression apparatus of the invention. In the case that the compression apparatus is used to compress a deuterium-rich gas to ignition and/or “burn” conditions, a portion of the burn ash will contain the rare gas helium-3. This is because the helium-3 generated from the reacted deuterium has a slower initial speed than other generated particles, such as tritium, and thus more easily thermalizes in the plasma. However, its nuclear fusion reaction rate is also slower than the tritium-deuterium reaction rate, such that it is not consumed as fast as the thermalized tritium. As a result of this breeding process, the ash from deuterium reactions accumulates the rare stable isotope helium-3. In order to collect the helium-3, a filtration system attached to the vacuum pumps will need to separate the isotopes in the exhaust. This apparatus is used to collect and purify the helium-3, as well as other exhaust products (such as tritium) that should not be vented to atmosphere from the pump exhaust. Additionally, hydrogen-1 (protons) and helium-4 could be obtained from the exhaust using an isotopic separating filtration system. The first step in carrying out the method of the present invention is to form a compact torus (CT) plasma structure. One type of CT is the Field Reversed Configuration (FRC). An FRC is formed in a cylindrical coil which produces an axial magnetic field. First, an axial bias field is applied, then the gas is pre-ionized, which “freezes in” the bias field, and finally the axial field is reversed. At the ends, reconnection of the bias field and the main field occurs, producing closed field lines. Following the formation of the CT, unlike the previously identified prior art methods which involve the use of compact toroid compression mechanisms, the CT, which is identified in the drawings by the numeral 68, is launched at high speed into the inlet port component 42 of the plasma compressor of the invention. As will be discussed in greater detail in the paragraphs that follow, as the CT travels through the plasma compressor it is crushed against a low atomic number material wall of the elongated spiral by means of its own inertia, inducing heating by conservation of energy. The internal thermal energy of the CT increases as its kinetic energy decreases. As the CT compresses against the walls of the spiral passageway 43, the pressure force it exerts has a vector component in the opposite direction to its forward motion (unless the walls are of constant cross-section). Therefore, it is important that the bulk axial kinetic energy of the CT at the point of ejection be greater than the design “target” thermal energy at the end of compression, to avoid a ricochet effect along the walls. The wall of the spiral passageway 43, as well as the other walls of the plasma compressor into which the CT comes in contact, absorb a portion of the heat, the degree to which is significantly reduced by embedding a large magnetic field within the CT during formation, prior to ejection. A highly magnetized CT impedes both thermal conduction losses and particle diffusion losses from its core to the walls. Once compressed to the design “target” thermal energy, the compressed CT 68a enters a comparatively short transfer conduit 70, which guides it away from the plane of symmetry of the compressor, and into the burn chamber 41. As previously discussed, the burn chamber comprises a toroidal ring of constant cross-section, with a single entrance port for the compressed CT 68a (FIGS. 3 and 7), and multiple smaller exhaust ports 72 (FIG. 5) which are in communication with the vacuum system 24. After the burn is complete, the compressed CT 68a dissipates into neutral gas, which is pumped out through the main vacuum exit port 74. Referring to FIGS. 5 and 9 of the drawings, it is to be noted that the inner ring is provided with a circular hole 78, which is adapted to receive an alignment gauge pin during assembly (not shown). After assembly, the alignment gauge pin is removed, leaving two through-holes that can be conveniently used for the insertion of diagnostic probes, such as a Rogowski coil loop. A major advantage of the method of the present invention is that neutral beams are not necessary for heating the plasma, maintaining the compact toroid plasma thermal energy, or providing stability to the plasma structure. Another advantage of the method is that collapsible walls are not needed for compressing the plasma. Additionally, in practice, the compression apparatus of the invention can be used multiple times. By way of background, in burning deuterium, which is an abundant stable-isotope of hydrogen, the reaction cycle consists of the following five equations:Primary neutron-branch 2D+2D→3He(0.8 MeV)+01n(2.4 MeV)Primary proton-branch 2D+2D→3T(1.0 MeV)+1H(3.0 MeV)Secondary helion-branch 2D+3He→4He(3.7 MeV)+1H(14.7 MeV)Secondary triton-branch 2D+3T→4He(3.5 MeV)+01n(14.0 MeV)Tertiary triton-branch 3T+3T→4He(3.8 MeV)01n(3.8 MeV)+01n(3.8 MeV) It is important to understand that in carrying out the method of the present invention, the wall of the spiral passageway, as well as any surface that the CT plasma structure comes in direct line-of-sight contact with, be clean, of low atomic number, and sputter slowly. These features will minimize losses due to impurities entering the plasma from the walls. In addition, it is beneficial for the walls to be electrically conductive, as this minimizes the loss due to synchrotron (cyclotron) radiation from the heated plasma by reflecting the emitted millimeter-wavelength light back into the plasma for re-absorption. This becomes apparent upon reviewing the fundamental equations governing the energy balance for the system. The equation for the power gained by fusion reactions is:Fusion Gain Pf=a12n1n2(συ) A.1 The loss equations for electrons, ions, and particle transfer appear respectively in FIGS. 10, 11 and 12 of the drawings with all variables being as defined in the previously set forth symbol definition table. A key observation, based on these equations, as well as prior experiment literature, is that avoiding impurity-driven losses is a crucial requirement for maintaining a hot plasma. To accomplish this, it is essential that the plasma not come into contact with high atomic number (high Z) materials, such as steel. The end-result of impurities in the plasma is that the loss rates increase by orders of magnitude. There are multiple loss paths due to high-Z contamination. The volumetric radiation power loss mechanisms that increase most significantly with Z are Bremsstrahlung, Recombination, and Excitation Line. However, the average Z also influences thermal conduction losses and even thermalization rates. Bremsstrahlung radiation is strongly affected by the average ion charge Z of the plasma, as the multi-pole non-relativistic equation A.2 (FIG. 10) indicates. In addition to this equation, it is important to calculate both the dipole and relativistic versions of the Bremsstrahlung loss rate, as well as all the quantum-mechanical “gaunt factor” corrections for each ion species, before arriving at the dominant loss rate due to Bremsstrahlung radiation. Bremsstrahlung occurs in the x-ray spectrum and leaves the plasma. However, Bremsstrahlung is dominant only at high energy levels that are commensurate with burn conditions. For this reason, and the fact that the plasma is transparent to x-rays, Bremsstrahlung is usually the primary loss mechanism considered in simulation programs. At lower energy levels, which the plasma must pass through in order to get from a neutral-gas state to burn conditions, recombination and excitation line radiation dominate the plasma's radiative loss mechanisms. This is especially the case for high-impurity content plasma. Recombination radiation, governed by equation A.3 (FIG. 10), is the loss most strongly affected by Z. As can be seen inside the integrand, recombination radiation is extremely sensitive to increases in Z. It can be orders of magnitude less than Bremsstrahlung for a pure hydrogenic plasma, but can rapidly exceed Bremsstrahlung at lower energy levels from even moderate impurity content. Thus, by controlling impurities, the recombination radiation loss mechanism can be minimized. Similarly, excitation line radiation in equation A.4 (FIG. 10) is affected by Z. Although not as apparent from this top-level equation, the calculation of Na utilizes a nonlinear function with Z as a directly dependant variable. Recombination and line radiation are often over-looked in sizing calculations, as they are assumed to be negligible as compared to Bremsstrahlung. This is the case under certain circumstances, but it is important to include their equations in case impurities enter the plasma. Overall, it is always beneficial (loss-reducing) to minimize the average Z. This is best accomplished by keeping impurities out of the plasma by utilizing clean, low-Z walls that sputter at as low a rate as possible. In a clean, but non-magnetized plasma, the dominant loss mechanism is usually thermal conduction to the walls (equations A.6 and A.8—FIGS. 10 and 11), followed by particle diffusion (equation A.15—FIG. 12). Increasing the ambient magnetic field parallel to the walls inhibits these losses, but it also gradually increases the loss from Synchrotron radiation (equation A.5—FIG. 10). From simulations, a compact torus (CT) plasma can sustain several hundred Tesla before Synchrotron radiation exceeds the Bremsstrahlung radiation loss rate. This is because the plasma is highly absorbent to the millimeter-wave spectrum emitted by Synchrotron radiation and electrically-conductive walls efficiently reflect Synchrotron radiation, as well as the fact that Synchrotron radiation is not affected by Z. Other losses included in the tables are ion Bremsstrahlung (equation A.10—FIG. 11) and ion Synchrotron (equation A.11—FIG. 11) radiation, which are comparatively minor to their electron counterparts in quasi-neutral plasmas. Neutral drag (equation A.9—FIG. 11) is also a comparatively small loss, but its inclusion enables prediction of how high a vacuum is required to sustain a moving plasma with negligible drag loss. Similarly, simulating sputtering of impurities from the wall (equation A.16—FIG. 12) and tracking magnetic dissipation (equation A.7—FIG. 10) allow estimation of how many impurities a wall will impart to a transient plasma and how long its internal magnetic field will last, respectively. The remaining effects of ion-to-electron kinetic transfer collisions (equation A.12—FIG. 11), product energy ion apportionment (equation A.13—FIG. 11), product energy ion thermalization (equation A.14—FIG. 12), and particle thermalization (equation A.17—FIG. 11) are essential to accounting for the allotment of energy and particles coming from core burn dynamics. In effect, they determine not the burn rate, but rather how to apportion the fusion energy coming from the original gain equation A.1, given the state of the plasma as instigated by an external device. Once the governing equations are accounted for, it is possible to perform an optimization of the parameters for the method of the invention. By way of example, for deuterium gas, a convenient diameter for the starting and ending CT is 137 and 19 millimeters, respectively. The initial embedded magnetic field is preferably on the order of 6±1 Tesla and the minimum initial plasma ion density is approximately 5×1015 particles per cubic centimeter. For optimum performance, the ejection speed of the CT requires a minimum of 4.8×106 meters per second and the minimum amount of time required for compression is on the order of 2 microseconds. Having now described the invention in detail in accordance with the requirements of the patent statutes, those skilled in this art will have no difficulty in making changes and modifications in the individual parts or their relative assembly in order to meet specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention, as set forth in the following claims. |
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description | This application is a U.S. National Phase application of PCT/US2004/027062, filed Aug. 19, 2004, the entirety of which is incorporated herein by reference. This application also claims the benefit of U.S. Provisional Patent Application No. 60/555,772, filed Mar. 24, 2004, and U.S. Provisional Patent Application No. 60/576,556, filed Jun. 3, 2004. This document concerns an invention relating generally to atom probes, also known as atom probe microscopes. The atom probe (also referred to as an atom probe microscope) is a device which allows specimens to be analyzed on an atomic level. A basic version of a conventional atom probe might take the following form. A specimen mount is spaced from a detector, generally a microchannel plate and delay line anode. A specimen is situated in the specimen mount, and the charge (voltage) of the specimen holder is adapted versus the charge of the detector such that atoms on the specimen's surface ionize and “evaporate” from the specimen's surface, and travel to the detector. Generally, the voltage of the specimen is pulsed so that the pulses trigger evaporation events with the timing of the pulses, thereby allowing at least a rough determination of the time of evaporation. The specimen's atoms tend to ionize in accordance with their distance from the detector (i.e., atoms closer to the detector tend to ionize first), and thus the specimen loses atoms from its tip or apex (the area closest to the detector) first, with the tip slowly eroding as evaporation continues. Measurement of the time of flight of the ionized atoms from the specimen to the detector allows determination of the mass/charge ratio of the ions (and thus the identity of the evaporated atoms). Measurement of the location at which the ions impinge on the detector allows determination of the relative locations of the ionized atoms as they existed on the specimen. Thus, over time, one may build a three-dimensional map of the identities and locations of the constituent atoms in a specimen. Owing to the number of atoms potentially contained in a specimen, and the time required to collect these atoms, specimens are often formed of a sample of a larger object. Such specimens are often formed by removing an elongated core from the object—often referred to as a “microtip”—which represents the structure of the sampled object throughout at least a portion of its depth. Such a microtip specimen is then usually aligned in the specimen holder with its axis extending toward the detector, so that the collected atoms demonstrate the depthwise structure of the sampled object. The rodlike structure of the microtip also beneficially concentrates the electric field of the charged specimen about its apex (its area closest to the detector), thereby enhancing evaporation from the apex. Ionizing (evaporating) energy need not be delivered solely by means of electric fields. For example, atom probes have been developed wherein the specimen is pulsed thermally, as well as electrically, to assist with evaporation. In some prior arrangements, a laser is situated adjacent to the specimen mount to direct laser pulses at the specimen, thereby briefly heating it to induce evaporation (see, e.g., Kellogg et al., Reference 12 in the accompanying bibliography). However, such arrangements are not common because it can be difficult and time-consuming to focus the laser beam onto a microtip specimen (more particularly, onto its apex). Further, owing to this difficulty, a laser beam of relatively wide diameter is needed, but this undesirably decreases the power density of the laser (unless laser power consumption is increased, which is also undesirable). In addition, the wide beam heats a greater area of the microtip specimen, and such heat can lead to uncertainties in mass determination because the retained heat in the specimen promotes greater variation in ion evaporation times. An alternative approach proposed by Kelly et al. (Reference 1 in the accompanying bibliography) utilizes an electron beam rather than a laser and reduces heating problems, though beam focusing and specimen heating can still pose problems. As a result, most atom probes enhance evaporation by use of other features. One such feature that may be used is a counter electrode, an electrode with a central aperture, which is situated closely spaced from the specimen between the specimen and detector (see, e.g., Miller at al., Reference 18 in the accompanying bibliography). The counter electrode is usually attractively changed with respect to the specimen so that it will enhance evaporation from the specimen, causing atoms to ionize and fly through the counter electrode's aperture toward the detector. Counter electrodes are generally used for one or more of the following purposes. First, by situating the aperture of the counter electrode about the apex of the tip, the evaporating electrical field about the apex can be greatly enhanced, thereby allowing the use of evaporating voltage pulses of lower magnitude. Owing to equipment limitations, voltage pulses of lower magnitude usually allow faster pulsing, and thus faster evaporation rates from the specimen (and faster data acquisition). In some cases, counter electrodes are used to concentrate the evaporating field about a selected microtip on a specimen bearing multiple microtips, such that ion evaporation only occurs from the single microtip. In this situation, the counter electrode is often referred to as a “local electrode” since it allows localized evaporation (see, e.g., Kelly at al., Reference 2 in the accompanying bibliography). To achieve more focused evaporation, the local electrode generally has a much smaller aperture than a conventional counter electrode, e.g, on the order of 5-50 micrometers rather than on the order of a few millimeters. Second, counter electrodes can be used to improve the mass resolution of the atom probe (i.e., to better calibrate measurements of ion times of flight between the specimen and detector). When atom probe voltages are pulsed, atoms tend to evaporate about the peaks of the pulses, leading to a small spread in departure times. Further, a late-departing ion may be in the region of the specimen as the voltage pulse on the specimen decays, and thus the ion may be influenced by the time-varying electrical field emitted by the specimen, leading to greater uncertainty in its true departure time (and thereby in the ion's time of flight, and in the determination of the ion's mass). However, if the counter electrode is situated sufficiently close to the specimen that a departing ion falls under the influence of the counter electrode's electric field before the specimen's voltage pulse significantly decays, the ion's flight will largely be decoupled from the time-varying field, thereby reducing its effect. Third, counter electrodes are sometimes used to shield the specimen from components in the flight path that might affect the electric fields near the specimen apex. As an example, if an atom probe microscope has a movable detector, the field on the specimen may be increased if the detector is moved closer, thereby enhancing the possibility of ion evaporation at unwanted times and complicating operation. However, the counter electrode, being situated between the specimen and the detector, can partially isolate the specimen from the detector and reduce the influence of the detector's field. The invention, which is defined by the claims set forth at the end of this document, is directed to atom probes and methods for their operation which allow advantages over prior atom probes. A basic understanding of some of the preferred features of the invention can be attained from a review of the following brief summary of the invention, with more details being provided elsewhere in this document. An atom probe includes a specimen mount whereupon a specimen to be analyzed may be mounted, with the specimen mount being chargeable to impart an ionization voltage on a specimen situated within the mount. A detector is spaced from the specimen mount to detect ions evaporated from the specimen. A counter electrode having an electrode aperture is situated between the specimen mount and detector, with the aperture having an aperture entry oriented along an aperture plane. The aperture plane is preferably located at or very close to the specimen apex (i.e., the specimen apex is preferably within, or close to entering, the aperture entry). As in prior atom probes, the specimen mount and detector may be charged to voltages which are nearly sufficient to ionize atoms at the specimen apex, and if desired, “overvoltage” pulses may be applied to the counter electrode to produce timed ionization events wherein ions evaporate when at least some of the pulses are applied. However, ionization is preferably primarily induced by a laser (or other energy beam source, e.g., an electron beam generator) which is spaced from the counter electrode and specimen mount on the opposite side of the aperture plane from the specimen mount, and which is oriented to emit a beam through the aperture of the counter electrode and toward the specimen mount to impinge on the specimen. A laser is a preferred energy beam source because it can be pulsed at very high frequencies, with pulses having widths on the level of picoseconds, thereby generating ionization events at the specimen with far greater mass resolution than in prior atom probes (since ion departures occur over the very narrow window of the laser pulse, allowing ion departure times to be specified with far greater precision). Where a laser is used, the laser preferably has a beam size substantially smaller than that used in prior laser atom probes such that it has a beam diameter of substantially less than 1 mm upon reaching the specimen. Most preferably, it has a beam diameter of less than or equal to 0.5 mm at the specimen. A smaller beam size (and thus a smaller spot size on the area of interest on the specimen) is useful because it heats less of the specimen, thereby better isolating ionization to the area of interest. Localized heating also promotes more rapid heat dissipation in the specimen, so that retained heat does not promote late ionization and miscalculated ion departure times. However, situating the laser such that its beam is aligned through the aperture, and using a smaller beam size than in prior atom probes, generates significant difficulties: aligning the beam through the aperture will almost inevitably situate the laser more distantly from the specimen than if the conventional arrangement is used (wherein the laser and specimen are situated on the same side of the counter electrode and its aperture plane, with the laser situated adjacent to the specimen), which makes it difficult to align the beam onto the area of interest on the specimen. This difficulty is compounded as the spot size is decreased, particularly since a smaller beam will be more subject to “drift” (i.e., gradual misalignment over time owing to vibration, laser imperfections, thermal expansion/contraction of the components between the laser and the specimen mount, etc.). Thus, even if a beam can be focused on a specimen apex or other area of interest at the outset of a data acquisition session, data may degrade over time as the beam drifts with usage of the laser. These difficulties are overcome in two ways. First, coarse alignment of the beam onto the specimen (or at least close to it) is promoted by the use of a counter electrode aperture of small size, preferably of less than about 0.1 mm (and more preferably less than about 0.05 mm). This size is roughly on the order of most microtip atom probe specimens, and since the specimen will be roughly centered in the aperture during atom probe data acquisition form the specimen, one can coarsely align the beam with the specimen by aligning the beam with the aperture. Such alignment can be detected visually, as by monitoring video or microcamera images taken adjacent the specimen mount, and/or by monitoring the output of photodetectors situated about the specimen mount (which can detect laser light projected through the counter electrode aperture). Fine alignment of the beam may then be attained by use of an automated beam alignment methodology which quickly locates the specimen apex (or other area of interest) without the need for tedious “hunting” by the atom probe's operating personnel. In this methodology, the laser (or other energy beam) is directed toward the specimen and swept in one or more dimensions over a sweep area of predefined size on (or near) the specimen; for example, it might be swept in a sinuous or zig-zagging pattern to cover some sweep area, or it might merely be swept in one dimension along a line. During the sweep, one or more parameters indicative of the interaction between the energy beam and the specimen are monitored. Exemplary parameters of this nature include the collection rate of any ions detected by the detector (with higher collection rates usually tending to be more indicative of the beam's impingement near the specimen apex, where atoms are more likely to ionize); the mass resolution of any ions detected by the detector (i.e., the degree to which detected ions may have their mass/charge ratios clearly correlated to particular atomic species, since good correlation indicates that the ion departure times—which are set by the laser pulse—are being accurately determined); the voltage applied to the specimen mount (since a beam focused on the specimen apex should produce detectable ions at lower specimen voltages than if the beam was not focused on the specimen apex); any reflected portions of the laser beam (since monitoring the image of the reflected beam may show whether the specimen apex is being illuminated); and any scattered portions of the laser beam (since the diffraction pattern of the beam may also indicate whether the specimen apex is being illuminated). The monitored parameters are compared to predefined alignment criteria, for example, whether the parameter(s) for a swept location have acceptable values, and/or whether they indicate that a location along the sweep is more likely near the specimen apex than a prior location. If a subset of the sweep is found that meets the alignment criteria—for example, if certain swept locations define some subset of the sweep area which have better ionization rates (detector collection rates), ion mass resolution, etc.—a new sweep area can be defined, one which is reduced in size to encompass at least this subset. Most preferably, the parameters are monitored to identify a single location along the sweep which has optimal parameters (those which most closely correspond to a point near the specimen apex), and this location is defined as a “home location” about which the new smaller sweep area is defined. The process is then repeated, with the beam being swept over this new smaller sweep area to locate a new subset of the sweep area (e.g., a single home location) which better meets the predefined alignment criteria, a new reduced sweep area being defined about this subset, and so forth. The process thereby iteratively shrinks and moves the sweep area about the home location, or about the collection of optimal locations, to home in on the specimen apex. Once the alignment criteria are met to some predefined level of accuracy (e.g., once the identified home locations do not significantly change between subsequent sweeps), the sweeping may be halted, and it can be assumed that the specimen apex has been located. On the other hand, if the parameters monitored during the sweep of the sweep area do not meet the alignment criteria—for example, if they are equivocal regarding the location of the specimen apex—the sweep area is increased rather than decreased before repeating the process, so as to better increase the chances that the areas on the specimen (and more particularly, near its apex) are swept. As an example, a swept area may illustrate little or no change in the monitored parameters from one location to another, indicating that the swept area of the specimen (or perhaps the counter electrode, if the beam has drifted badly) does not seem to be close to the specimen apex at all. The sweep area can then be increased to hopefully locate a home location (or other subarea/subset of the sweep area) that meets the alignment criteria, in which case the process may then iteratively shrink the sweep areas about such locations. Alternatively, if an increased sweep area does not locate a home location or other subarea that meets the alignment criteria, the sweep area can again be increased until a promising subarea is located (at which point the sweep area may be iteratively shrunk about this subarea). Once the specimen apex (or other desired location on the specimen) has been located by use of the foregoing procedure, atom probe analysis may commence: the specimen, counter electrode, and detector may each be charged to levels conducive to ionization of the specimen, and the laser (or other energy beam source) may be pulsed to add sufficient energy to the specimen that ionization occurs (with the counter electrode being simultaneously pulsed as well, if desired). Since beam drift may occur over time, the foregoing beam fine alignment process may periodically be repeated during data acquisition, perhaps after a certain number of data acquisition cycles occur, and/or after certain parameters (such as evaporation rates, mass resolution, etc.) appear to indicate that the beam is no longer centered on the desired area of the specimen. The utility of the atom probe is further enhanced if the laser (or other energy beam) contains laser light (or other energy) of at least two different wavelengths when it reaches the specimen. Since different materials can exhibit greater susceptibility to ionization at energies at different wavelengths, using a mixture of wavelengths can enhance the versatility of an atom probe by allowing it to more efficiently analyze a wide range of materials. While a mixture of wavelengths can be generated by using multiple lasers and combining their beams with the use of dichroic mirrors or other elements, a particularly preferred arrangement is to use a single laser (with emitted laser light at a single wavelength), and generate harmonic wavelengths in the same beam by use of nonlinear crystals or other harmonic-generating optics. Using a single laser avoids the need to synchronize pulse timing among different lasers, and also avoids the cost and space issues arising from use of multiple lasers. Cost and space issues are also reduced if the laser is placed outside of the vacuum chamber containing the specimen mount and detector, and if the laser delivers its beam to the specimen through a window defined in the vacuum chamber (with intermediate mirrors, lenses, or other optical elements being used along the beam if needed). This avoids the need for a laser configured for high-vacuum environments (which can enhance expense), and also avoids the need for the laser to occupy the limited available space within the vacuum chamber. Locating the laser outside the chamber (or at any other location which is distant from the specimen mount) can lead to greater problems with beam alignment on the specimen, but by use of the aforementioned alignment method, these difficulties can be at least partially overcome. Some of the aforementioned difficulties may also be at least partially alleviated if the aperture of the counter electrode is configured to receive a laser or other energy beam at the entry of the aperture, and concentrate the energy beam before it leaves the exit of the aperture so that the beam is focused on the specimen with greater intensity. This may be done, for example, by configuring the electrode aperture as a concentrating reflector having a parabolic, hyperboloid, or other configuration which internally reflects the beam energy as it travels from a larger aperture entry to a smaller aperture exit so that the incident beam is concentrated to greater intensity when emitted onto the specimen. Such an arrangement can at least partially compensate for beam misalignment by “catching” and redirecting a misaligned beam so that it is focused on the specimen. Since such an arrangement can better tolerate minor beam misalignment, it is also well suited to receive multiple laser or other energy beams and direct any misaligned beams onto the specimen. Further advantages, features, and objects of the invention will be apparent from the following detailed description of the invention in conjunction with the associated drawings. Turning initially to FIGS. 1a and 1b, an exemplary laser atom probe, depicted generally by the reference numeral 100, is illustrated in both schematic form (FIG. 1a) and in a perspective view of an actual prototype (FIG. 1b). The laser atom probe 100 includes a specimen mount 102 with a specimen 104 mounted thereon, an opposing detector 106 for receiving ions evaporated from the specimen 104, and a counter electrode 108 situated between the specimen mount 102 and the detector 106. The specimen mount 102 is movable to allow positioning of the specimen 104 within or closely spaced from the aperture 110 of the counter electrode 108, with the apex or other area of interest on the specimen 104 preferably being situated at the aperture plane 112 which defines the entry of the aperture 110. A vacuum chamber 114 surrounds all of the foregoing components. As can be seen from the references noted in the Bibliography provided later in this document, the foregoing arrangement of the specimen mount 102, detector 106, and counter electrode 108 are conventional in the field of atom probes, though a variety of configurations and operating modes are possible. As examples, the counter electrode 108 may take the form of a local electrode (as shown in U.S. Pat. No. 5,440,124, Reference 2 in the Bibliography); additional counter or “intermediate” electrodes may be present (as shown in U.S. Pat. No. 6,580,069 to Cerezo, Reference 4 in the Bibliography); or other features may be present. In one conventional mode of operation, the specimen mount 102 and detector 106 are charged to “boost” voltages which are nearly sufficient to cause ionization of the specimen 104 (generally with the specimen mount 102 being charged to about 75% of the ionization energy threshold), usually with the specimen 104 being positively charged and the detector 106 being negatively charged. The counter electrode 108 and/or the specimen mount 102 may then be pulsed with an “overvoltage,” i.e., with a charge surpassing the ionization threshold of the specimen 104, so that ions are evaporated during the pulses (and their flight times to the detector 106 can be measured from the pulses). The foregoing arrangement may be adapted to adjust the conventional atom probe for use as the laser atom probe 100 by providing a laser 116 which is oriented to project its beam through the aperture 110 of the counter electrode 108, and onto the apex or other area of interest on the specimen 104. In the exemplary laser atom probe 100 of FIGS. 1a and 1b, the vacuum chamber 114 includes a viewing tube 118 ending in a window 120, and the laser 116 is situated outside the window 120 to emit its beam 122 through the window 120 and subsequently through the counter electrode aperture 110. However, as discussed later in this document, other placements for the laser 116 are possible as well (e.g., within the chamber 114). Several operational modes are possible, with two being particularly preferred. In one mode, the specimen mount 102 may be charged to some boost voltage amounting to a significant fraction of the ionization energy threshold, and the remainder of the ionization energy may be provided by the laser 116, which is pulsed to deliver ionization energy to the specimen 104 in much the same manner as conventional overvoltage pulses. In this case, the counter electrode 108 may simply serve as an uncharged ground plane. In a second mode, the boost voltage is again applied by the specimen mount 102, and the remaining ionization energy may be shared by the laser 116 and counter electrode 108. In this case, the counter electrode 108 provides overvoltage pulses which bring the specimen 104 close to ionization, and the laser 116 then provides the energy necessary to cause ion evaporation. With this arrangement, the boost voltage can be decreased, and thus stress on the specimen 104 may be decreased, since the combined energy imparted by the laser 116 and counter electrode 108 may constitute a greater amount of the total energy needed to surpass the ionization threshold. To describe the prototypical laser atom probe 100 of FIG. 1b in greater detail, it uses an atom probe manufactured by Imago Scientific Instruments Corporation (Madison, Wis. USA) with a laser 116 having a diode-pumped Ti:Sapphire oscillator (the Verdi-V5 pump laser with a Mira Optima 900-F cavity, both from Coherent, Inc., Santa Clara, Calif., USA), which produces 8 nJ pulses at a nominal repetition rate of 76 MHz. A cavity dumper (the Pulse Switch cavity dumper from Coherent, Inc.) is used to increase the pulse energy to 60 nJ and decrease the repetition rate to 100 KHz. These components are merely exemplary, and other suitable equipment allowing the same or different outputs is available from Coherent, Inc. or from other laser equipment suppliers such as Spectra-Physics, Inc. (Mountain View, Calif., USA). The aforementioned Coherent, Inc. Pulse Switch cavity dumper includes second and third harmonic generators, which may be beneficially utilized in a manner to be discussed later in this document. The pulsed beam 122 of the laser 116 is then directed through the window 120 and the counter electrode aperture 110, and onto the apex or other area of interest on the specimen 104. The laser beam 122 is oriented at an angle of slightly less than 45 degrees relative to the ion travel axis (i.e., the center of the cone defined by the flight paths of the ions from the specimen 104, with the flight cone being designated in FIG. 1a at 124 and the ion travel axis being designated at 126). This angle is suitable to prevent the beam 122 from intersecting with the flight cone 124, while at the same time allowing the beam 122 to impinge on the specimen 104 through the counter electrode aperture 110. However, if the components of the atom probe 100 are altered (e.g., if the size of the counter electrode aperture 110 and/or the length of flight path 126 are changed), the angle of the beam 122 may vary. In the prototypical atom probe 100, the flight path 126 is nominally 6 cm (though this may be adjusted, most easily by moving the detector 106). Such a flight path 126 beneficially allows data acquisition cycles on the order of 1 MHz, thereby greatly reducing the time for specimen analysis (on the order of minutes instead of hours or days), thereby greatly enhancing the industrial applicability of the atom probe 100. Longer (or shorter) flight paths may be used with corresponding impact on data acquisition rates: as the flight path grows longer and/or as the pulse frequency grows more rapid, the start of one data collection cycle will begin overlapping with the end of the prior data collection cycle, leading to greater data interpretation and control burdens. Adjustment of the flight path 126 will also have corresponding effects on field of view and magnification (with magnification being proportional to flight path length and field of view being inversely proportional), with the 6 cm flight path providing a field of view between 1.5-2.0 steradians. The counter electrode 108 may take planar, concave, or other forms, with the prototypical laser atom probe 100 using a planar counter electrode having an aperture 110 which preferably has a diameter of less than about 0.05 mm. Larger apertures 110 are possible, though it is preferred that they have diameters of less than 0.1 mm; as will be discussed later in this document, the use of a smaller aperture greatly assists with the nontrivial task of focusing the laser 116 (which is far more distant than in prior laser atom probes) onto the specimen 104. Further, a conical or other concave counter electrode 108 having an aperture 110 of the same order or smaller might be useful if the counter electrode 108 is to serve as a local electrode, e.g., for analysis of individual adjacent microtips on the same specimen 104. The laser 116 uses a more tightly focused beam 122 than in prior laser atom probes, preferably of less than 1 mm diameter and more preferably less than 0.5 mm (as received at the specimen 104). The prototypical atom probe 100 has a spot size (beam diameter) of approximately 0.02 mm at the specimen 104. This beneficially reduces the volume of the specimen 104 that is subjected to laser heating, resulting in better heat dissipation and thus reducing the time-of-departure spread for ionized atoms (i.e., less energy is retained in the specimen 104, thereby deterring later ionization events after the pulse is delivered). Additionally, since a lower-diameter beam 122 has greater power density than a larger-diameter beam (assuming the same input power), use of a lower-diameter beam 122 allows the use of lower power (and less expensive) lasers 116. For example, if one compares two beams of equal power density, one having a diameter of 0.05 mm and the other having a diameter of 1 mm, the 0.05 mm beam only needs 1/400 the power of the 1 mm beam (since power density varies in accordance with the square of the diameter). A variety of collimators, lenses, and other optics may be used to focus the beam 122, and the focusing optics will be dependent on the laser 116 chosen for use in the atom probe 100. The prototypical atom probe 100 of FIG. 1b utilizes a coated biconvex lens (model PLCX-38.1-103.0-UV-355-532, CVI Laser LLC, Albuquerque, N. Mex., USA) mounted on a conventional positioning stage (Model 423) fitted with motorized actuators (Model LTA-HS), both provided by Newport Corp., Irvine, Calif., USA. These components, which are not shown in the drawings, are situated outside of the vacuum chamber 114 adjacent the viewport 120. The positioning stage therefore allows repositioning of the focusing lens to attain the desired beam diameter/spot size at the specimen 104. FIG. 2 provides a plot of control and feedback signals over a single data acquisition cycle. The control system for the laser atom probe 100 (a personal computer) delivers a trigger pulse PT to the laser 116. After receiving the trigger pulse PT, the cavity dumper of the laser 116 releases the next available laser pulse PL. Since an indeterminate amount of time T1 passes between the trigger pulse PT and the generation of a laser pulse PL, a fast photodiode is used to generate a pickoff pulse PP which may then be used by the control system as the nominal pulse departure time Tp (and in turn as the departure time of any ion subsequently detected at the detector 106). Rather than constantly (and unnecessarily) collecting data from the detector 106, data acquisition at the detector 106 is enabled after the passage of a small period of time T2 after the pickoff pulse PP (at Td), thereby better limiting data collection to ion arrivals. The mass-to-charge ratio of the detected ions (and thus the identities of their atoms) can then be determined by subtracting Tp from the arrival time. Data acquisition at the detector 106 is disabled at some fixed period of time (typically 1 microsecond) after Td, and before the next data acquisition cycle begins. The timing of pulses on the counter electrode 108 is not depicted because FIG. 2 assumes that the counter electrode 108 merely acts as an uncharged ground plane. However, if the counter electrode 108 was to be charged, a counter electrode pulse of the desired magnitude could be initiated by the trigger pulse PT, and could have sufficient pulse width that the laser pulse PL will occur over its duration. In this manner, the counter electrode pulse would serve as an overvoltage pulse which steps the specimen 104 to a level near its ionization energy, and the laser pulse PL would then supply the energy sufficient for ionization. For efficient ionization, it is useful to accurately know the beam diameter and power density of the laser beam 122 (and thus the amount of ionization energy being delivered by the beam 122). This can be done in a variety of ways. Most preferably, measurements are taken at or shortly after the aperture plane 112 such that the energy input at the apex of the specimen 104 can be inferred. A first way to accomplish this is to situate photosensors adjacent to the specimen mount 102 in FIG. 1a (or on or in place of the specimen mount 102 before it is fully moved into place), and directly measure the output of the beam 122. An exemplary arrangement of this nature is depicted in FIG. 1a, wherein a photosensor array 128 is situated about the edges of the specimen mount 102. Measurements at the aperture plane 112 can be calculated from the measurements taken from the location of the photosensors 128. Another approach is to use mirrors near the specimen mount 102 in FIG. 1a (or in place of the specimen mount 102 before the specimen mount 102 is moved into place), with the mirrors delivering the reflected beam 122 to a photosensor within or outside the vacuum chamber 114 (e.g., through a viewport in the vacuum chamber 114) to allow calculation of the beam power at the aperture plane 112. Other approaches include building a photosensor into the counter electrode 108, or temporarily moving a photosensor immediately above or below the counter electrode 108 (as with the foregoing approach of placing the photosensor 128 on the specimen mount 102), to more directly measure the beam 122 at the aperture plane 112. Other approaches using photosensors inside or outside the chamber 114, and with or without any mirrors or other optical elements between the laser 116 and photosensor, are also possible. As previously noted, the laser 116 is used with second and third harmonic generators, thereby allowing tuning of the mean wavelength of the beam 122 from the ultraviolet to the near-infrared ranges and allowing the wavelength to be adjusted to better induce ionization in specimens 104 of different materials. However, in some cases, a single wavelength does not result in efficient ionization owing to material differences in the specimen 104, with the single wavelength failing to efficiently couple with all components present in the specimen 104. The prototypical atom probe 100 therefore preferably uses a beam 122 containing multiple wavelengths. While this could be done by using multiple lasers 116 directing their beams 122 onto the specimen 104, perhaps after combining their beams 122 with the use of dichroic mirrors or other elements, the use of multiple separate lasers 116 leads to added space and expense, and also leads to added complexity, particularly regarding the need to synchronize pulse timing among different lasers. Therefore, a particularly preferred arrangement is to use the single laser 116 and generate harmonic wavelengths in the same beam 122 by interposing nonlinear crystals or other harmonic-generating optics in the path of the beam 122. In the prototypical atom probe 100 of FIG. 1b, the beam from the laser 116 is focused into a nonlinear crystal (such as a BBO crystal from EKSMA Photonics Components, Vilnius, Lithuania), which is not depicted in the accompanying drawings. The crystal allows production of a second harmonic wavelength in the beam 122 (having half of the wavelength emitted by the laser 116) with approximately 50% conversion efficiency. If desired, subsequent nonlinear crystals may be placed in the path of the beam 122 to produce a third harmonic (having one-quarter of the wavelength emitted by the laser 116), a fourth harmonic, and so forth. Achromatic lenses/collimators and/or other optical components may be used to focus and adjust beam diameters for each wavelength so that when they enter the counter electrode aperture 110 and impinge upon the specimen 104, they will all be focused to the same beam diameter. To summarize, use of the laser 116 provides significant operational advantages over conventional atom probes. One primary advantage is that conventional atom probes are generally limited to analysis of specimens 104 which are at least substantially conductive, since nonconductive specimens 104 require significantly higher boost voltages and overvoltages (and the electric fields of these higher voltages cause substantial stress on the specimen 104, which may then mechanically fracture). Since the laser 116 allows operation at significantly lower voltages, the laser atom probe 100 allows analysis of even significantly nonconductive specimens 104, such as organic specimens 104. As a related advantage, the wavelength(s) of the laser 116 (or lasers) used to generate the beam 122 may be adapted for more efficient ionization of specimens 104 of different types, including those which have nonheterogeneous compositions (e.g., specimens 104 containing both conductive and nonconductive regions, inorganic and organic regions, etc.). Another primary advantage is that a suitable laser 116 can generate pulses having widths on the order of picoseconds or femtoseconds. Since specimen ionization occurs over the very narrow window of the laser pulse, ion departure time may be specified with far greater precision, thereby allowing far greater mass resolution than in conventional atom probes (better than 1 in 500 mass-to-charge units). Further, the pulses can be generated with frequencies of 1 kHz-1 MHz, thereby allowing extremely rapid data collection. However, despite these advantages, the foregoing arrangement introduces several significant challenges, in particular difficulties with accurate focusing of the beam 122 on the specimen 104. Since the laser 116 is situated distantly from the specimen 104, and has a preferred spot size of less than 0.5 mm (with the prototypical atom probe 100 having a spot size of 0.02 mm), focusing the beam 122 onto the apex of the specimen 104—which may itself have a diameter on the order of tenths or hundredths of millimeters—is not trivial, particularly since the beam 122 may drift over time owing to environmental vibration, thermal expansion and contraction of atom probe components, and so forth. Prior laser atom probes (such as that in References 20 and 21 of the accompanying Bibliography) reduced these difficulties by closely situating the laser 116 adjacent the specimen 104 and on the same side of the aperture plane 112, and using a far larger spot/beam diameter, thereby making it easier to direct the beam onto the apex or other area of interest on the specimen. Even then, the process of focusing the beam onto the desired area of the specimen, and keeping it properly aligned, was not a trivial matter; focusing required methods such as visual inspection of fluorescence or field ion microscopy, which are subject to interpretation. These approaches are also time-consuming in that they require that some small amount of gas be introduced into the vacuum chamber, and then pumped out again before atom probing can be initiated. When it is then considered that reducing the beam diameter, and situating the laser more distantly from the specimen, will both make initial focusing more difficult and will also require periodic checking for drift during an atom probe data acquisition session, it would seem that the foregoing arrangement for the atom probe 100 would not be worthwhile—particularly since drift checking would require interruption of data acquisition for time-consuming introduction and removal of gas into the vacuum chamber 114, which would be intolerably tedious. However, many of these disadvantages can be avoided by use of the following beam alignment methodology, which first involves a coarse beam alignment, followed by specimen alignment, and finally a fine beam alignment. The coarse beam alignment may be performed only once upon the first installation and use of the atom probe 100, and only infrequently thereafter between data acquisition sessions. Specimen alignment is then performed prior to each data acquisition session. Fine beam alignment is then preferably performed both prior to and during data acquisition sessions to protect against beam drift. Coarse beam alignment is performed by focusing the beam 122 through the aperture 110 of the counter electrode 108. A preferred method of coarse alignment is as follows: 1. Power on the laser 116 and wait for the position of the beam 122 to stabilize (usually 15 minutes or so is sufficient). 2. Activate the photosensor (e.g., the photosensor array 128 depicted in FIG. 1a). As previously noted, the photosensor is preferably positioned (at least temporarily) generally along the ion travel axis 126 on the specimen side of the aperture plane 112, or may otherwise be situated on the counter electrode 108 or at other locations (including locations outside the vacuum chamber 114) which allow determination of whether the beam 122 is passing through the electrode aperture 110. Again, a particularly preferred arrangement is to have a movable photosensor 128 which moves immediately adjacent the counter electrode 108 on the specimen side of the aperture plane 112, so that the photosensor 128 can be situated at generally the same location at which the apex of the specimen 104 will rest during later data acquisition. 3. Starting with a defocused beam 122, obtain an initial signal on the photosensor 128 by translating the beam using beam steerers (i.e., mirrors, collimators, and the like, which are not depicted in the drawings). Such beam steerers are preferably located outside the viewport 120, though they could instead be located within the vacuum chamber 114. 4. Translate the beam 122 until the signal on the photosensor 128 is maximized, thereby indicating that the entire unfocused beam 122 is passing through the aperture 110 without impinging on the counter electrode 108 at the borders of the aperture 110. 5. Focus the beam 122 to maximize the signal on the photosensor 128. 6. Repeat the foregoing steps 4 and 5 iteratively until the maximum signal is obtained on the photosensor 128. 7. Adjust the power of the beam 122 to the desired level.Coarse beam alignment is then complete, and the settings for the beam steerers, focusing optics, laser power controls, etc. may be secured. The coarse alignment procedure may be performed upon initial startup and installation of the atom probe 100, and is also beneficially performed after maintenance or refitting of the atom probe 100, e.g., if a new counter electrode 108 is installed, or if additional counter electrodes (often referred to as intermediate electrodes) are installed between the counter electrode 108 and the detector 106. Otherwise, the coarse alignment procedure may not need to be repeated after initial startup. However, keeping in mind that preferred versions of the atom probe 100 have apertures 110 of less than 0.1 mm (and preferably on the order of 0.05 mm), it is useful to perform periodic coarse alignment checks after every few data acquisition sessions to verify that gross drift has not misaligned the beam 122 from the aperture 110. If the atom probe is used with counter electrodes 108 having larger apertures 110 (e.g., on the order of 2.5 mm or more, which is more conventional), the need to repeat coarse alignment is not as critical. Other coarse alignment methods are also possible, with use of the photosensor 128 merely being a preferred method. Coarse alignment could instead be performed, for example, by using a long-range microscope and a videocamera. It is also possible to combine photosensors and imaging devices in the coarse alignment process. Note that in essence, the counter electrode aperture 110 serves as the focal point for alignment of the beam 122 with the apex or other area of interest on the specimen 104: aligning the (movable) beam 122 with the (movable) specimen 104 can be challenging and time-consuming, but by using the counter electrode 108—which is at a fixed location within the vacuum chamber 114 of the atom probe 100—as the focal point for the beam 122 (since the specimen 104 will be centered in its aperture 110 during data acquisition), the alignment process is greatly simplified. Further note that when the atom probe 100 is later operated in such a mode that the counter electrode 108 is an uncharged ground plane during data acquisition, alignment of the beam 122 is effectively the primary purpose of the counter electrode 108: if beam alignment was not a concern, the counter electrode 108 could be eliminated. After coarse beam alignment is complete, alignment of the specimen 104 with the counter electrode aperture 110 may be performed in a variety of ways. One preferred method of specimen alignment is to initially use two orthogonal optical microscopes for coarse specimen alignment, and if necessary, follow coarse specimen alignment with field ion microscopy for fine specimen alignment. The coarse specimen alignment process is as follows: 1. Ensure that the laser 116 is off, or that its beam 122 is shuttered. 2. Move the specimen mount 102 until the specimen 104 is roughly aligned with the counter electrode aperture 110. 3. Using 2-axis translation of the specimen 104 (along the plane perpendicular to the ion travel axis 126), move the specimen mount 102 such that the specimen 104 is situated generally along the ion travel axis 126. Optical microscopes orthogonally situated about the ion travel axis 126 at the general location of the aperture plane 112 can be used to verify alignment along both axes of translation. (Such microscopes are not shown in the drawings, but can be usefully provided outside the vacuum chamber 114 to view the specimen at appropriately-located viewports, one such viewport being shown in FIG. 1b at 130.) 4. The specimen mount 102 can then be moved parallel to the ion travel axis 126 until the apex or other area of interest on the specimen 104 is situated generally on the aperture plane 112 (i.e., so that the apex of the specimen 104 is situated just outside or within the counter electrode aperture 110).If necessary, fine specimen alignment can be accomplished using field ion microscopy (FIM): 1. An imaging gas (e.g., neon) is introduced into the vacuum chamber 114 of the atom probe 100. An imaging gas pressure of approximately 5×10−6 mbar is usually sufficient. 2. The gain of the detector 106 is adjusted to an appropriate level for FIM. 3. Voltage is then supplied to the specimen mount 102 (and thus the specimen 104) until an image of the apex of the specimen 104 can be obtained on the detector 106. 4. The specimen 104 is translated along the two axes of the aperture plane 112 until an unobstructed image is obtained on the detector 106. If the specimen 104 is misaligned, the counter electrode 108 will occlude a portion of the image. Fine beam alignment is then preferably performed after specimen alignment, and also periodically during the course of data acquisition to ensure that the beam 122 is still aligned with the apex or other area of interest on the specimen 104. Fine alignment could be performed using FIM methods such as those described by Kellogg et al. and Cerezo et al. (References 10 and 12 in the Bibliography at the end of this document). In these methods, the FIM is driven by both the laser 116 and by the voltage applied to the specimen 104, and the specimen position is constantly adjusted while at the same time adjusting beam power and specimen voltage to obtain an appropriate image. However, since the atom probe 100 preferably uses beam 122 diameters of less than 1 mm, and more preferably less than 0.5 mm (with the prototypical atom probe 100 of FIG. 1b using a preferred beam diameter of approximately 0.02 mm), it is found that prior alignment schemes are exceedingly difficult and tedious because they are extremely sensitive to beam alignment: with a narrow beam, very small changes in beam position require very large corrections in beam power and/or specimen voltage. This is because the sensitivity of the FIM is roughly approximate to the inverse of the beam diameter. As a result, significant time may be needed to accurately align a small-diameter beam to a specimen 104. Further, when it is considered that FIM sensitivity varies with the inverse of the beam diameter, it should be apparent that the atom probe 100 is extremely sensitive to beam drift (which can readily misalign a small-diameter beam 122 from the apex of the specimen 104). Thus, data acquisition may be drastically impeded by beam drift occurring during a data acquisition session. If large-diameter beams of greater than 1 mm are used (as in prior laser atom probes), drift is not a significant problem since the large-diameter beam can still impinge on the apex of the specimen 104 even if drift occurs. However, as the beam diameter is decreased below 0.5 mm—which begins to approach the range of drift that can occur from standard environmental factors (such as vibration, thermal expansion/contraction, etc.)—drift becomes a key concern, particularly as the beam diameter is decreased to the range of hundredths of millimeters (or smaller). Since it would be plainly impractical to halt a data acquisition session to introduce imaging gas, perform FIM, and then re-evacuate the vacuum chamber 114 to resume data acquisition, the utility of the atom probe 100 is greatly enhanced if improved methods of fine beam alignment are used. An exemplary control system for fine beam alignment is then shown in FIG. 3, and is designated generally by the reference numeral 300. A data acquisition control system 302 receives raw data 304 from the atom probe 100, and adjusts the (DC) specimen voltage 306 applied to the specimen mount 102 (and in turn to the specimen 104) in accordance with the data 304. This data acquisition control loop repeats throughout the fine beam alignment process and continually adjusts the specimen voltage 306 to obtain a controlled rate of field ionization. The data acquisition control system 302 also supplies the trigger pulse 308 (also shown as PT in FIG. 2) to the laser 116, and encodes the departure time of the resulting laser beam pulse (via the pulse pickoff signal PP in FIG. 2). A second control loop, which runs synchronously or asynchronously with respect to the control loop of the data acquisition control system 302, is executed by a beam alignment control system 310. The beam alignment control system 310 receives raw and/or conditioned atom probe data 312 from the data acquisition control system 302, and also receives image data 314 from image acquisition hardware 316 (videocameras or other optical imaging devices which monitor the specimen 104 within the vacuum chamber 114), and in turn provides motion commands 318 to (and receives position feedback 320 from) beam alignment hardware 322. The beam alignment hardware 322, which is not illustrated in FIG. 1a or 1b, may be provided by one or more actuators for adjusting the direction of the laser beam 122, and may take the form of actuators which adjust the position of the laser 116 and/or mirrors, lenses, or other optics along the path of the laser beam 122. Within the beam alignment control system 310, the atom probe data 312 and image data 314 are conditioned to generate one or more control parameters which are indicative of the interaction between the laser beam 122 and the specimen 104, and which are used by the beam alignment control system 310 to finely (and automatically) adjust the alignment of the laser beam 122: (1) Evaporation rate (the collection rate of any ions detected by the detector 106): The evaporation rate of the specimen should increase as the laser beam 122 approaches the apex of the specimen 104, since the field strength is also strongest at this area of the specimen 104, and therefore the laser beam 122 should induce ionization more easily at the apex than elsewhere on the specimen 104. Accordingly, if the beam alignment control system 310 seeks the area on the specimen 104 with the maximum evaporation rate, there is a high likelihood that this area will correspond to the specimen apex. (2) The voltage applied to the specimen 104. In similar fashion, as the laser beam 122 approaches the apex of the specimen 104, it should be able to induce evaporation with a lower specimen voltage. Thus, if the beam alignment control system 310 seeks the area on the specimen 104 where evaporation can be maintained with minimum voltage on the specimen 104, there is a high likelihood that this area will correspond to the specimen apex. (3) Mass resolution of detected ions. An ion's arrival time can be determined from the detector 106, and if the ion departure time is well known, the mass/charge ratio of the ion should correlate well with known values to allow identification of the ion. However, as the departure time grows uncertain, the correlation decreases. In the laser atom probe 100, departure time variations will begin to increase if it takes longer for the heat of the laser beam 122 to dissipate (i.e., as the effective width of the laser pulse grows wider). Since sensitivity to heat dissipation should be greatest at the apex of the specimen, if the beam alignment control system 310 adjusts the alignment of the laser beam 122 to find the area on the specimen 104 with the lowest uncertainty in mass resolution, there is a high likelihood that this area will correspond to the specimen apex. (4) Signal-to-noise ratio. Similar to mass resolution (item (3) above), the signal-to-noise ratio of the atom probe data is limited by the quality of the beam alignment: as the laser beam 122 deviates from the apex of the specimen 104, well-timed evaporation will decrease and unplanned evaporation will increase. The signal and noise floors will therefore approach each other as the beam 122 deviates from the apex of the specimen 104, and will diverge as the beam approaches the apex. Thus, if the beam alignment control system 310 adjusts the alignment of the laser beam 122 to find the area on the specimen 104 with the highest signal-to-noise ratio, there is a high likelihood that this area will correspond to the specimen apex. (5) Reflected light from the specimen. The image acquisition hardware 316 (i.e., videocameras or other optical imaging devices which monitor the specimen 104 within the vacuum chamber 114) can monitor the specimen 104. The apex of the specimen 104 will have a greater tendency to reflect and/or fluoresce when illuminated by the laser beam 122. Therefore, the beam alignment control system 310 can adjust the alignment of the laser beam 122 to find the area on the specimen 104 with peak intensity (or other reflection/emission characteristics), and thereby have a higher likelihood of illuminating the apex of the specimen 104. (6) Diffracted light from the specimen. Diffracted light is more usefully monitored to maintain alignment of a beam than to initially align a beam 122. Here, the far-field (Fraunhofer) diffraction pattern produced by the specimen 104 can be monitored by the image acquisition hardware 316, and the beam alignment control system 310 can adjust the alignment of the laser beam 122 to maintain a constant diffraction pattern, thereby helping to ensure that the beam 122 maintains alignment with the apex of the specimen 104 once focused on this location.There are other possible control parameters which are indicative of the interaction between the laser beam 122 and the specimen 104, and which can be used to instruct the beam alignment control system 310 to make alignment corrections. It is also possible (and advisable) to have the beam alignment control system 310 use more than one of these variables, with appropriate weights applied to each selected variable, to better allow the beam alignment control system 310 to more rapidly locate the apex of the specimen 104. A preferred fine alignment process for the laser beam 122 then proceeds in the manner illustrated in FIG. 4. Initially, at step 402, the user verifies to the beam alignment control system 310 that coarse alignment has been performed, thereby providing reasonable assurance that the beam 122 is directed through the counter electrode aperture 110 onto the specimen 104 (or its immediate area). At step 404 in FIG. 4, the user then specifies (or the beam alignment control system 310 defines or recalls) a sweep path—a beam path in the plane perpendicular to the ion travel axis 126—about which the beam 122 will be swept. The beam alignment control system 310 will simultaneously monitor one or more of the aforementioned control parameters (see step 406 in FIG. 4) to seek to meet some predefined alignment criterion, i.e., a standard which is characteristic of the apex of the specimen 104. For example, the beam alignment control system 310 might verify whether the parameter(s) for a swept location has values within a range that would be expected for beam impingement on the apex of the specimen 104; whether the parameter(s) indicates that a location along the sweep is more likely near the apex of the specimen 104 than a prior location; and/or whether the parameter(s) “optimally” indicates the apex of the specimen 104 (e.g., whether a location along the sweep has the highest evaporation rate, which would seem to indicate the apex of the specimen 104). In essence, the objective is to identify the location(s) along the sweep which optimizes the control parameter(s), thereby locating some point(s) or segment along the sweep path which is believed to be closer to the apex of the specimen 104. The sweep area can assume a wide variety of sizes and shapes, with an initial sweep area preferably having a size on the order of the diameter of the counter electrode aperture 110. As examples, the sweep area might be a circular or square area which can be swept in a spiraling, sinuous, or zig-zagging pattern so that much of the sweep area (and thus some portion of the specimen 104) is swept. Alternatively, as will be discussed below, the sweep area might be defined as a narrow lane, and the sweep might simply occur in one dimension to sweep along the sweep area in a straight line. While performing the initial sweep of the sweep area, the beam alignment control system 310 will identify the point or other subset of the sweep which has control parameters which meet the alignment criteria (i.e., which appear to be more promising candidate locations for the apex of the specimen 104). After completion of the initial sweep, the beam alignment control system 310 will then take one of two paths to redefine the initial sweep area (step 408 in FIG. 4): (1) If the beam alignment control system 310 did identify some subset of the sweep having control parameters which best met the alignment criteria—i.e., some single location (a “home location”) was located with control parameters that were optimal in comparison to all locations swept along the sweep area, or if some collection of points most closely met the alignment criteria (e.g., the 10% of sampled locations having the most promising control parameters)—the beam alignment control system 310 will automatically define a new sweep area, one which is reduced in size to encompass at least this subset. As an example, if a single optimal home location is identified, a new sweep area might be defined which is 50% the size of the initial sweep area, and which is preferably centered about the home location. (2) If the beam alignment control system 310 did not identify some subset of the sweep having control parameters which met the alignment criteria—for example, if all sampled locations along the sweep area had control parameters which did not deviate from each other by more than 10%—the sweep area can be increased rather than decreased (for example, its borders might be expanded outwardly by 50%), since such a result would appear to indicate that the apex of the specimen 104 is not within the sweep area. Alternative approaches are possible; for example, the beam alignment control system 310 could simply define another initial sweep area having the same size, and which is offset from the first in some direction in the plane perpendicular to the ion travel axis 126. If this sweep area does not result in at least one location having control parameters which met the alignment criteria, the beam alignment control system 310 can continue to define sweep areas about the initial one until some promising location(s) is found. Once the sweep area is redefined in step 408, the process may continue to step 410, and a new sweep area may be swept by the beam 122 using a sweep path which is finer (in the case of a smaller sweep area) or coarser (in the case of a larger sweep area), in the sense that the paths traversed by the beam 122 will have closer or more distant spacing. The sweep path preferably takes the same form as in the prior sweep, i.e., it preferably uses the same sinuous, zig-zagging, spiral, etc. path which is merely compressed or enlarged in scale to cover much of the area of the new sweep area. During the new sweep, the beam alignment control system 310 again monitors the control parameters versus the alignment criteria to seek the location(s) which optimally indicate the presence of the apex of the specimen 104. Once the new sweep is completed, the sweep area is again redefined (shrunk or expanded) and swept with the control parameters being monitored versus the alignment criteria. The process continuously repeats in this manner, with the sweep area being iteratively shrunk about the home (optimal) location(s) until the alignment criteria are met to some predefined level of accuracy. Once this occurs—for example, once the control parameters of an identified home location do not significantly change between subsequent sweeps—the sweeping may be halted, and it can be assumed that the identified home location corresponds to the apex of the specimen 104. The foregoing process is schematically illustrated in FIGS. 5a-5c. In FIG. 5a, the area 502 is concentrically situated about the apex of the specimen 104 (the location of which is unknown to the beam alignment control system 310). The beam alignment control system 310 defines an arbitrary location 504 about which the sweep area is defined, and further defines the sweep path 506, which in this case is defined as an array of adjacent straight lines/sweeps distributed across the sweep area. The sweep path 506 is traversed by the beam 122, and the control parameters collected during the sweep are measured versus the alignment criteria. (It should be understood that while the sweep path 506 is illustrated as a continuous line, it in fact consists of a series of discrete sample points, each being a location at which a laser pulse from the beam 122 is received.) The locations along the sweep path 506 within the area 502 meet the alignment criteria, and are this identified as optimal locations. A new sweep path center 508 (a home location) is therefore defined within the area 502 (FIG. 5b). The sweep area is then shrunk about the new sweep path center 508, and a finer sweep path 510 is defined and swept to identify a new home location (not shown). Subsequent sweeps are then performed to identify subsequent home locations, with each successive sweep being centered about the prior home location, until the alignment criteria are met to some desired degree of accuracy, indicating that the home location specifies the apex of the specimen 104. In the prototypical atom probe 100 of FIG. 1b, where a 0.02 mm beam 122 is used, it typically takes numerous iterations to “home in” on the apex of the specimen 104. Smaller beams may require more iterations. Regardless of how many iterations are needed, the fine beam alignment process continuously follows the steps of locating the home location and rescaling the sweep area about the home location. Numerous variations of the foregoing fine beam alignment process are possible. As one example, the beam alignment control system 310 could increase the sampling rate along the sweep path if the control parameters are converging on the alignment criteria, and can decrease the sampling rate if divergence occurs. It is also possible that the sweep area and/or sweep path might be immediately redefined once divergence is noted, so that the sweep area is immediately redefined about the area of convergence. Additionally, the sweep areas and sweep paths may take a wide variety of forms, and they need not take identical form from one sweep to the next; for example, one sweep might take the form of a straight line along an X axis, and the next sweep might take the form of a straight line along a Y axis defined about the home location in the prior sweep. It is also possible that the process might occur semiautomatically; for example, a plot of the control parameters might be displayed to the user, who would then have the opportunity to manually define a new home location for the next sweep. Once fine alignment has been achieved, the laser atom probe 100 may begin data acquisition: the specimen 104 and detector 106 may each be charged to levels conducive to ionization of the specimen 104, and the laser 116 may be pulsed to add sufficient energy that ionization occurs. Since the beam 122 of the laser 116 may drift over time, the foregoing beam fine alignment process may periodically be repeated during data acquisition, perhaps after a certain number of data acquisition cycles occur, and/or after certain parameters (such as evaporation rates, mass resolution, etc.) appear to indicate that the beam 122 is no longer centered on the desired area of the specimen 104. Data acquisition need not be ceased during such fine alignment, since the data obtained from data acquisition may be used to generate many of the control parameters. Stated differently, data acquisition from the atom probe 100 may proceed in standard fashion, with the acquired data being monitored versus alignment criteria to verify whether the beam 122 is still directed at the apex of the specimen 104, and if the alignment criteria are not met, sweep areas may be defined and sweeping may be performed to re-locate the apex of the specimen 104. Preferred versions of the laser atom probe 100 are shown in the drawings and described above merely to illustrate possible features of the laser atom probe 100 and the varying ways in which these features may be combined. Modified versions of the laser atom probe 100 are also considered to be within the scope of the invention. Following is an exemplary list of such modifications. First, it is notable that a wide variety of operational modes are possible for the atom probe 100 to induce evaporation of the specimen, with any one or more of the specimen mount 102, counter electrode 108, and laser 116 providing energy to the specimen 104 in constant or pulsed fashion. The laser 116 is preferably pulsed since the narrow pulse widths achievable with a laser 116 are useful to more precisely specify ion departure times (and thus lead to better mass resolution), but steady operation of the laser 116, with pulsing of other components (to provide the overvoltage necessary for ionization), is possible. The use of a pulsing laser 116 with an overvoltage pulse applied to either or both of the counter electrode 108 and/or specimen mount 102 can be beneficial with some types of specimens 104 since this may allow the specimen 104 to remain at a lower boost voltage (and thus a lower field and lower mechanical stress) for the time between pulses, thereby improving the survival of delicate specimens 104 and simultaneously reducing spurious ionization events between pulses (which effectively results in lost data). Second, apart from laser and electron beams 122, other beams bearing energies at different ranges of the electromagnetic spectrum might be used. Similarly, other forms of energy might be used to impart the boost (non-pulsed) energy, such as microwaves. Third, some of the arrangements and methodologies described above might be implemented in a version of the atom probe 100 wherein the counter electrode 108 is omitted (or in a version of the atom probe 100 wherein a counter electrode 108 is included, but the beam 122 is not directed through the counter electrode 108). In either case, the aforementioned methods for coarse alignment of the beam 122 through the counter electrode 108 do not apply. Other methods of coarse alignment (followed by fine alignment) might then be used, though they may require more time and effort. Fourth, it is emphasized that the counter electrode 108 may take a wide variety of sizes and configurations, including configurations which are not conventional to prior atom probes. An example is illustrated in FIGS. 6a and 6b. In FIG. 6a, an alternative to the counter electrode 108 of FIGS. 1a-1b is depicted, with the alternative counter electrode 600 providing certain additional and advantageous features over the basic counter electrode 108. Here, the counter electrode 600 has a converging aperture 602 which is configured such that radiation incident on its aperture walls 604 is reflected away from the aperture plane 606, i.e., away from the larger entry of the aperture 602, and toward the smaller aperture exit 608, whereby incident radiation is thereby collected and concentrated to higher intensity at the aperture exit 608. The configuration of the concentrating aperture 602 may take a variety of shapes, and such shapes may need to be chosen to provide optimal concentration for a given angle between the laser beam 122 and the ion travel axis 126 (i.e., for a given incidence angle). Information regarding concentrator design can be found, for example, in the field of solar energy, where concentrators are commonly used to collect sunlight. Other prior concentrators, and discussions of considerations relevant to concentrator design, can be found in (for example) U.S. Pat. No. 5,572,355 to Cotton et al.; U.S. Pat. No. 5,604,607 to Mirzaoff; U.S. Pat. No. 5,952,645 to Wang et al.; U.S. Pat. No. 5,978,407 to Chang et al.; and U.S. Pat. No. 6,704,341 to Chang, as well as in patents cited in (and citing to) these patents. Other useful references are R. N. Wilson, Reflecting Telescope Optics: Basic Design Theory and Its Historical Development, Springer-Verlag (1996) (for optical concentrators) and T. Wilhein, D. Hambach, B. Niemann, M. Berglund, L. Rymell, and H. M. Hertz, Off-axis reflecting soneplate for quantitative soft x-ray source characterization, Appl. Phys. Lett. 71, 190 (1997) (for concentrators for non-optical radiation). The concentrating aperture 602 can provide several advantages. First, as implied by the arrangement depicted in FIG. 6a, the counter electrode 600 can accommodate a larger incidence angle, since the beam 122 need not necessarily directly impinge on the specimen 104, and can instead merely impinge on the aperture walls 604 to be directed onto the specimen 104 (which is situated immediately at the aperture exit 608 so as to receive the concentrated beam 122). A greater beam angle 122 might in some cases allow closer laser placement, or space-saving atom probe component layouts. Second, the counter electrode 600 can accommodate greater (and lower power) beam diameters, and can automatically concentrate beams of various diameters down to some desired diameter. Third, as implied by the prior two comments, the counter electrode 600 can accommodate a beam 122 which is not as precisely aligned with the specimen 104, since the aperture 602 can largely provide such alignment so long as the tip or other area of interest is itself aligned with the aperture exit 608. FIG. 6b shows an arrangement similar to that of FIG. 6a, but here the laser beam 122 is directed through the aperture 602 and directly onto the area of interest on the specimen 104, as in the arrangements discussed previously. In this case, the concentrating aperture 602 is provided to receive and collect supplemental energy (supplied in pulsed or steady fashion) from one or more supplemental energy sources 610, with such energy being directed onto the specimen 104 for ionizing or other purposes. A supplemental energy source 610 could be a light source such as another laser, LED laser, LED, or a traditional lamp, whether in the visible or invisible portions of the spectrum, and its light can be beamed from a location distant from the electrode 600 or may be piped to the electrode 600 by a fiberoptic cable or other light guide. As an alternative, a supplemental energy source 610 could transmit radio, microwave, or other electromagnetic radiation. The energy supplied by the supplemental energy source 610 could be provided for purposes in addition to or other than ionization, such as for modifying the characteristics of a specimen 104. As an example, a silicon-based specimen 104 might experience a change in conductivity if illuminated by a supplemental energy source 610 emitting infrared light (with such a conductivity perhaps allowing a lower boost voltage to be supplied to the specimen 104). Arrangements are also possible wherein only a non-laser and/or non-ionizing supplemental energy source 610 directs energy through the aperture 602, with no laser being included (i.e., with the concentrating electrode 600 being used in conjunction with a non-laser supplemental energy source 610 in a more conventional atom probe arrangement), or wherein the concentrating electrode 600 and supplemental energy source 610 are used in conjunction with a laser beam which is not directed through the aperture 602. Fifth, other arrangements are possible which yield effects similar to that of the concentrating electrode 600 (with or without a supplemental energy source). As an example, a single laser beam could be split into two or more beams, each of which are directed toward the specimen at a different angle, with the beams perhaps being distributed for more even illumination of the tip or other area of interest; and/or with one or more beams being adjusted to a different wavelength (as by use of the aforementioned nonlinear crystals); and/or with one or more of the beams being directed through the electrode aperture, with others being directed onto the specimens from outside the aperture. Sixth, many other atom probe features known in the field but not noted in the foregoing discussion may be used with the atom probe 100. As an example, a recessed viewport 120 (a viewport 120 situated at the end of a viewing tube 118 extending into the vacuum chamber 114), as described in U.S. Pat. No. 6,762,415 to Strait, may be utilized to reduce the distance between the laser 116 and specimen 104 to reduce alignment burdens. The invention is not intended to be limited to the preferred versions described above, but rather is intended to be limited only by the claims set out below. Thus, the invention encompasses all different versions that fall literally or equivalently within the scope of these claims. 1. U.S. Pat. No. 5,061,850 to Kelly et al. (“High mass resolution local-electrode atom probe”) 2. U.S. Pat. No. 5,440,124 to Kelly et al. (“High-repetition rate position sensitive atom probe”) 3. U.S. Pat. No. 6,576,900 to Kelly et al. (“Methods of sampling specimens for microanalysis”) 4. U.S. Pat. No. 6,580,069 to Cerezo (“Atom probe”) 5. U.S. Pat. No. 6,700,121 to Kelly et al. (“Methods of sampling specimens for microanalysis”) 6. U.S. Pat. No. 6,762,415 to Strait (“Vacuum chamber with recessed viewing tube and imaging device situated therein”) 7. International Publication WO 99/14793 8. International Publication WO 87/00682 9. Cerezo, A. et al., J. Phys. (Orsay) C9 (1984) p. 315. 10. Cerezo, A., C. R. M. Grovenor and G. D. W. Smith. “Pulsed laser atom probe analysis of semiconductor materials.” Journal of Microscopy v 141, pt. 2 (February 1986): 155-170. 11. Drachsel, W., Nishigaki, S., and Block, J. H., “Photon-Induced Field Ionization Mass Spectroscopy.” Int. J. Mass Spectrom. and Ion Phys. Vol. 32 (1980): 333-343. 12. Kellogg, G. L., and T. T. Tsong. “Pulsed-laser atom-probe field-ion microscopy.” Journal of Applied Physics 51 (2) (1980): 1184-1193. 13. Kelly, T. F., Zreiba, N. A., Howell, B. D., and Bradley, F. J., “Energy Deposition and Heat Transfer in a Pulse-Heated Field Emission Tip at High Repetition Rates.” Surface Science, vol. 246 (1991): 377-385. 14. Kelly, T. F., Camus, P. P., Larson, D. J., Holman, L. M., and Bajikar, S. S., Ultramicroscopy 62:29-42 (1996). 15. Kelly, T. F. and Larson, D. J., “Local Electrode Atom Probes.” Materials Characterization 44, 59-85 (2000). 16. Liu, J., and T. T. Tsong. “Kinetic Energy and Mass Analysis of Carbon Cluster Ions in Pulsed-Laser-Stimulated Field Evaporation.” Physical Review (B) 38 (12) (1988): 8490-8493. 17. Liu, J., C. Wu, and T. T. Tsong. “Measurement of the atomic site specific binding energy of surface atoms of metals and alloys.” Surface Science 246 (1991) 157-162. 18. Miller, M. K., A. Cerezo, M. G. Hetherington, and G. D. W. Smith. “Atom Probe Field Ion Microscopy.” Oxford Science Publications, 1996. 19. Tsong, T. T., Photon stimulated field ionization, J. Chem. Phys. 65, 2469 (1976). 20. Tsong, T. T., S. B. McLane, and T. J. Kinkus. “Pulsed-laser time-of-flight atom-probe field ion microscope.” Review of Scientific Instruments 53(9), September 1982. 21. Tsong, T. T. “Pulsed-Laser-Stimulated Field Ion Emission from Metal and Semiconductor Surfaces: A Time-of-Flight Study of the Formation of Atomic, Molecular, and Cluster Ions.” Physical Review (B) 30 (9) (1984): 4946-4961. |
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description | This application is a filing under 35 U.S.C. 371 of international application number PCT/IB2007/001076, filed Apr. 24, 2007, which claims priority to application No. 60/795,784 filed Apr. 28, 2006, in the United States the entire disclosure of which is hereby incorporated by reference. The present invention relates to a method of preparing [18F]F2 from [18F] fluoride by a plasma induced scrambling procedure. The present invention also relates to an apparatus of preparing [18F]F2 by a plasma induced scrambling process. The present invention further relates to kits for producing a method and apparatus of [18F]F2 by a plasma induced scrambling reaction. Additionally, a method of use and use of claims for preparing [18F]F2 from [18F] fluoride through a plasma induced scrambling procedure are also provided. Electrophilic fluorination of organic molecules with F2, or its derivatives is efficient, controllable and fast. F2 is a highly potent chemical agent. It is the most reactive pure element. (Bergman et al., Nucl. Med. Biol., 1997, vol. 24, pgs. 677-683). The fluorine atom is about the same size as the hydrogen atom. This makes it possible for fluorine to mimic hydrogen with respect to steric requirements in molecules as well as at binding sites on receptors and enzymes. Fluorine substitution can also have a profound effect on the lipofilicity and biological activity of small molecules. (Park et al., Drug Metab. Rev., 1994, vol. 26, pgs. 605-643). The Positron Emission Tomography technique makes it possible to follow, in a patient, the binding of radiolabeled ligands to receptor sites, thus making it possible to quantitate the number of binding sites in both healthy and diseased states. Displacement of the labeled ligands makes it possible to measure the affinity to the binding site. Tracer studies with potent and/or toxic neuroreceptor ligands require high specific radioactivity for the tracer, as the amount of mass that can be injected into a human subject is limited by the toxicity of the substance and its affinity for the receptor site. For an injected dose of typically 185 MBq, the amount will be 10 nmol when the specific radioactivity (“SA”) is 185 GBq/micromol. (Bergman et al.). Electrophilic radiofluorine is particularly suitable for the synthesis of fluoroaryl compounds ([18F]Ar—F) by cleavage of aryl-metal bonds of typically Ar-MR (M=Sn, Hg, Si, R═(CH3)n) compounds either with [18F]F2 or [18F]CH3COOF. The factor limiting the more widespread use of the method has been the low specific radioactivity of the labeled fluorine gas available through radionuclide production from gas phase target materials, typically neon or 18O2 mixed with carrier F2. Gas targetry for the production of [18F]F2 has recently been extensively reported. Straatman et al. describe a method where n.c.a. [18F]HF, in an exchange reaction with F2, is converted to [18F]F2 by a microwave discharge. (Straatman et al., Label. Compd. Radiopharm., 1982, vol. 19, pg. 1373). A simplistic way of producing 18F is to irradiate, with a particle accelerator beam, highly 18O-enriched water. Through the 18O(p,n) 18F nuclear reaction, up to several curies of radioactivity can be produced. The radiolabeled fluoride ([18F]F-aq) recovered after charged particle irradiation can be used for the production of numerous labeled neurotracers. (Bergman et al.) The synthetic chemistry, however, with this anion is usually neither simple nor fast, especially if more complex molecules are needed. Furthermore, Bergman et al. developed a method for the routine use of [18F] F2 for synthesis of radiotracers for PET. The SA of the product should be such that studies with receptor ligands are possible. The SA of [18F]F2 achieved by Bergman et al. was about 90 GBq/micromole. Bergman et al. considered four different methods of excitation: a) excitation with a cyclotron particle beam; b) excitation with a hard UV-light; c) excitation via an electric discharge through a gas; and d) excitation with laser light. Methods b and c were considered to be practical, and were therefore tested by Bergman et al. In view of the prior art, a more rapid method to mass produce [18F]F2 by obtaining a higher SA than previously reported is needed. Discussion or citation of a reference herein shall not be construed as an admission that such reference is prior art to the present invention. In view of the needs of the prior art, the present invention provides a method of obtaining [18F]F2 from [18F] fluoride, for use in electrophilic fluorination reactions. Unlike previous methods, [18F]F2 is obtained from [18F] fluoride by using a plasma induced scrambling procedure. This procedure uses [18F] fluoride, which has been isolated from a water target electrolytically, and then converting the [18F] fluoride directly from an electrode to [18F]F2 by adding a carrier gas source such as fluorine to form plasma or a mixture thereof where either F2 is added to the carrier gas source in controlled amounts, or as a metal fluoride, and then inducing the plasma by any of the following ways: a) a fluorescent light tube driver circuit, where components are commercially available and inexpensive; b) a plasma induction by microwaves; or c) a high voltage discharge may also be used. Thereafter the contents within the reactor are emptied into a stream of halogen or noble gas wherein [18F]F2 is trapped. The present invention also depicts an apparatus for preparing [18F]F2 in a plasma reactor system. The apparatus comprises of a [18F]F2 produced from [8F] fluoride wherein said system further comprises a carrier gas or a metal fluoride, a controlled amount of fluorine gas, at least a fluorescent light tube driver circuit, a plasma induction by microwaves, or a high voltage discharge, and a stream of gas wherein [18F]F2 is trapped. Yet another embodiment comprises a kit for preparing [18F]F2 from [18F] fluoride is presented. The kit comprises the steps of: isolating [18F] fluoride from a water target by electrolysis in a reactor; then drying the [18F] fluoride and thereafter filling the reactor with a carrier gas to form plasma or a mixture thereof wherein the reactor also contains a controlled amount of fluorine gas; next igniting the plasma by using at least a fluorescent light tube driver circuit, a plasma induction by microwaves, or a high voltage discharge; and thereafter emptying the contents in the reactor into a stream of gas wherein [18F]F2 is trapped. The ability to produce the plasma required to prepare [18F]F2 from [18F] fluoride is achieved either by using a high voltage charge wherein a voltage is about 5 kV to about 20 kV, a fluorescent light tube driver circuit or by microwave induced plasma. More specifically, in the present invention, [18F] fluoride is isolated from a water target by electrolysis in a reactor. The [18F] fluoride is then dried. Next, the reactor is filled with a carrier gas such as a noble gas or halogen gas to form a plasma or a mixture thereof wherein the reactor also contains a controlled amount of carrier gas such as fluorine gas. A metal fluoride such as Na, K, Mg, Ca or a salt thereof could replace the carrier gas in order to increase the specific radioactivity (“SA”) from about 5 to about 15%. The metal fluoride would be added as either a deposition on the wall of the reactor, or as a powder within the reactor. The plasma is then lit using a modern type of fluorescent light tube driver circuit (FIG. 1), or by plasma induced by microwaves (FIG. 2), or by high voltage discharge. Thereafter this scrambling reaction is completed. The contents within the reactor are then emptied into a stream of gas and the formed [18F]F2 can be trapped and used for further radiolabeling synthesis. The radiopharmaceutical yield at this point is about 90 to about 98% and can be reached within 5 minutes. The current invention sets forth several advantages of producing [18F]F2 in a plasma induced scrambling method. The current method presents an ease of use over the other prior methods and a quicker synthesis time for the production of [18F]F2. Short synthesis times with the use of a metal fluoride to form plasma in the reactor will also yield compounds with higher radiochemical yield and SA due to less decay. Radiochemical purity (RCP) is defined as the amount of radioactivity originating from a specific substance in relation to the total amount of radioactivity in a sample, expressed in %. Additionally, SA is the ratio between the amount of radioactivity originating from a specific substance labeled with a radionuclide and the total amount of that specific substance. Below a detailed description is given of a method for producing [18F]F2 from [18F] fluoride through a plasma induced scrambling procedure. The present invention relates to a method for preparing [18F]F2 from [18F] fluoride, comprising the steps of isolating [18F] fluoride from a water target by electrolysis in a reactor; then drying the [18F] fluoride and thereafter filling the reactor with a carrier gas or a metal fluoride to form plasma or a mixture thereof wherein the reactor also contains a controlled amount of fluorine gas; next igniting the plasma by using at least a fluorescent light tube driver circuit, a plasma induction by microwaves, or a high voltage discharge; and thereafter emptying the contents in the reactor into a stream of gas wherein [18F]F2 is trapped. Another embodiment of the present invention is that the metal fluoride is a deposition on the wall of the reactor or a powder on the wall of the reactor. Yet another embodiment is that the reactor comprises a reaction chamber and an optional microwave cavity. A further embodiment is wherein the carrier gas is a noble gas or a halogen gas. An additional embodiment is when the metal within the metal fluoride is Na, Ca, K, Mg, Mn or a salt thereof. Yet another embodiment of the present invention is when the controlled amount of fluorine gas is about 200 nanomoles to about 10 micromoles. An additional embodiment is when the high voltage discharge has a voltage of about 10 kV to about 50 kV. Another embodiment is when the stream of gas is a halogen gas or a noble gas. Yet another embodiment of the present invention is that the reactor generates no heat. In another embodiment of the present invention of an apparatus for preparing [18]F2 in a plasma reactor system is presented. The apparatus comprises of a [18F] F2 produced from [18F] fluoride wherein said system further comprises a carrier gas or a metal fluoride, a controlled amount of fluorine gas, at least a fluorescent light tube driver circuit, a plasma induction by microwaves, or a high voltage discharge, and a stream of gas wherein [18F]F2 is trapped. Another embodiment of the present invention is that the metal fluoride is a deposition on the wall of the reactor or a powder on the wall of the reactor. Yet another embodiment is that the reactor of the apparatus comprises a reaction chamber and an optional microwave cavity. A further embodiment is wherein the carrier gas is a noble gas or a halogen gas. An additional embodiment is when the metal of the apparatus within the metal fluoride is Na, Ca, K, Mg, Mn or a salt thereof. Yet another embodiment of the present invention is when the controlled amount of fluorine gas of the apparatus is about 200 nanomoles to about 10 micromoles. An additional embodiment of the apparatus is when the high voltage discharge has a voltage of about 10 kV to about 50 kV. Another embodiment of the apparatus is when the stream of gas is a halogen gas or a noble gas. Yet another embodiment of the present invention is that the reactor of the apparatus generates no heat. In a further embodiment of the present invention a kit for preparing [18F]F2 from [18F] fluoride is disclosed. The kit comprises the steps of: isolating [18F] fluoride from a water target by electrolysis in a reactor; then drying the [18F] fluoride and thereafter filling the reactor with a carrier gas or a metal fluoride to form plasma or a mixture thereof wherein the reactor also contains a controlled amount of fluorine gas; next igniting the plasma by using at least a fluorescent light tube driver circuit, a plasma induction by microwaves, or a high voltage discharge; and thereafter emptying the contents in the reactor into a stream of gas wherein [18F]F2 is trapped. Another embodiment of the present invention is that the metal fluoride of the kit is a deposition on the wall of the reactor or a powder on the wall of the reactor. Yet another embodiment of the kit is that the reactor comprises a reaction chamber and an optional microwave cavity. A further embodiment of the kit is wherein the carrier gas is a noble gas or a halogen gas. An additional embodiment of the kit is when the metal within the metal fluoride is Na, Ca, K, Mg, Mn or a salt thereof. Yet another embodiment of the present invention kit is when the controlled amount of fluorine gas is about 200 nanomoles to about 10 micromoles. An additional embodiment of the kit is when the high voltage discharge has a voltage of about 10 kV to about 50 kV. Another embodiment of the kit is when the stream of gas is a halogen gas or a noble gas. Yet another embodiment of the present invention is that the reactor of the kit generates no heat. A further embodiment of the present invention depicts a method of use for preparing [18F]F2 from [18F] fluoride, comprising the steps of: isolating [18F] fluoride from a water target by electrolysis in a reactor; then drying the [18F] fluoride and thereafter filling the reactor with a carrier gas or a metal fluoride to form plasma or a mixture thereof wherein the reactor also contains a controlled amount of fluorine gas; next igniting the plasma by using at least a fluorescent light tube driver circuit, a plasma induction by microwaves, or a high voltage discharge; and thereafter emptying the contents in the reactor into a stream of gas wherein [18F]F2 is trapped. Still a further embodiment of the present invention depicts a method of use for preparing [18F]F2 from [18F] fluoride, wherein the metal fluoride is a deposition on the wall of the reactor or a powder on the wall of the reactor. A further embodiment of the present invention shows a method of use wherein the reactor comprises a reaction chamber and an optional microwave cavity. Yet another embodiment of the present invention shows a method of use wherein the carrier gas is a noble gas or a halogen gas. Still a further embodiment of the invention depicts a method of use wherein the metal within the metal fluoride is Na, Ca, K, Mg, Mn or a salt thereof. Another embodiment of the invention shows a method of use wherein the controlled amount of fluorine gas is about 200 nanomoles to about 10 micromoles. Still a further embodiment of the invention depicts a method of use wherein the high voltage discharge has a voltage of about 10 kV to about 50 kV. A further embodiment shows a method of use wherein the stream of gas is a halogen gas or a noble gas. Another embodiment of the invention shows a method of use wherein the reactor generates no heat. Yet another embodiment depicts a use of preparing [18F]F2 from [18F] fluoride, comprising the steps of: isolating [18F] fluoride from a water target by electrolysis in a reactor; then drying the [18F] fluoride and thereafter filling the reactor with a carrier gas or a metal fluoride to form plasma or a mixture thereof wherein the reactor also contains a controlled amount of fluorine gas; next igniting the plasma by using at least a fluorescent light tube driver circuit, a plasma induction by microwaves, or a high voltage discharge; and thereafter emptying the contents in the reactor into a stream of gas wherein [18F]F2 is trapped. Still another embodiment encompasses a use of preparing [18F]F2 from [18F] fluoride according to the metal fluoride is a deposition on the wall of the reactor or a powder on the wall of the reactor or wherein the reactor comprises a reaction chamber and an optional microwave cavity and the carrier gas is a noble gas or a halogen gas and wherein the metal within the metal fluoride is Na, Ca, K, Mg, Mn or a salt thereof and optionally wherein the controlled amount of fluorine gas is about 200 nanomoles to about 10 micromoles and further wherein the high voltage discharge has an optional voltage of about 10 kV to about 50 kV and wherein the stream of gas is a halogen gas or a noble gas and finally wherein the reactor optionally generates no heat. The invention is further described in the following examples which are in no way intended to limit the scope of the invention. Calculations of Specific Radioactivity and Reaction Yields The specific radioactivity (“SA”) of [18F]F2 was determined by iodometric titration of oxidizing material and measurement of the radioactivity of the [18F]KF formed and indirectly by radiochromatographic determinations of amounts of mass and radioactivity of substances labeled with 18F. Note that the SA of the product in the labeling synthesis is decreased by half as compared to [18F]F2 as there is only a 50% chance for the introduction of the 18F-atom. The decay of the 18F will also decrease the SA in proportion to the half-life of 109.8 minutes. When the amount of methyl fluoride used in the exchange reaction is known, the theoretical SA and yield of [18F]F2 can be calculated and compared to the measured time-corrected specific radioactivity of the synthesis product. From these numbers the yield in the exchange reaction can be determined. The present invention is not to be limited in scope by specific embodiments described herein. Indeed, various modifications of the inventions in addition to those described herein will become apparent to those skilled in the art from the foregoing description and accompanying figures. Such modifications are intended to fall within the scope of the appended claims. Various publications and patent applications are cited herein, the disclosures of which are incorporated by reference in their entireties. |
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abstract | A power module assembly includes a reactor vessel containing a reactor core surrounded by a primary coolant. A containment vessel is adapted to be submerged in a containment cooling pool and to prohibit a release of the primary coolant outside of the containment vessel. A secondary cooling system is configured to remove heat generated by the reactor core. The heat is removed by circulating liquid from the containment cooling pool through the primary coolant. |
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055920276 | claims | 1. A method of compacting, without danger of ignition and/or explosion, waste liable to ignite and/or explode when compacted, said method comprising: loading a container with said waste; injecting an inert gas into said container so that said container is saturated in said inert gas; and compacting the container, which has been loaded with the waste and injected with the inert gas, thereby causing the container to develop cracks and the inert gas to be released from the container through the cracks. said waste is loaded in bulk into said container while the inert gas is being injected therein in order to fill voids within said container between pieces of said waste and between said waste and said container; and in that after loading, said container is provided with a cover, that is optionally sealed, sealing being necessarily required only if the inert gas used is lighter than air; and said loaded and optionally sealed container is then inserted in a compacting skirt to be compacted therein under drive from a piston. 2. A method according to claim 1, further characterized in that: 3. A method according to claim 2, characterized in that prior to compacting, an inert gas is injected around the container to replace air between said container and said compacting skirt with said inert gas. 4. A method according to claim 2, characterized in that the pressure exerted during compacting generates cracks in the structure of said container. 5. A method according to claim 2, characterized in that it is used for compacting radioactive metal waste containing, in particular, zirconium and/or magnesium and/or alloys of said metals. 6. A method according to claim 2, characterized in that argon and/or nitrogen is/are used as the inert gas. 7. A method according to claim 1, characterized in that the pressure exerted during compacting generates cracks in the structure of said container. 8. A method according to claim 1, characterized in that it is used for compacting radioactive metal waste containing, in particular, zirconium and/or magnesium and/or alloys of said metals. 9. A method according to claim 1, characterized in that argon and/or nitrogen is/are used as the inert gas. 10. A method according to claim 2, further characterized in that said inert gas is injected into the container under atmospheric pressure. 11. A method according to claim 10, further characterized in that said container develops cracks caused by kinking under the drive exerted by the piston, not by excess internal pressure within the container. |
description | The present application claims the benefit of U.S. Provisional Application Ser. No. 61/332,934, filed May 10, 2010. The present application also relates to commonly assigned non-provisional U.S. patent application entitled “CHLORIDE SCINTILLATOR FOR RADIATION DETECTION”, filed on the same day as the present application and claiming the benefit of U.S. Provisional Application Ser. No. 61/332,972, filed May 10, 2010 and non-provisional U.S. patent application entitled “IODIDE SCINTILLATOR FOR RADIATION DETECTION”, filed on the same day as the present application and claiming the benefit of U.S. Provisional Application Ser. No. 61/332,945, filed May 10, 2010. All applications are incorporated herein by reference. This disclosure relates to scintillator materials used for detecting ionizing radiation, such as X-rays, gamma rays and thermal neutron radiation, in security, medical imaging, particle physics and other applications. This disclosure relates particularly to halide scintillator materials. Certain arrangements also relate to specific compositions of such scintillator material, method of making the same and devices with such scintillator materials as components. Scintillator materials, which emit light pulses in response to impinging radiation, such as X-rays, gamma rays and thermal neutron radiation, are used in detectors that have a wide range of applications in medical imaging, particle physics, geological exploration, security and other related areas. Considerations in selecting scintillator materials typically include, but are not limited to, luminosity, decay time and emission wavelengths. While a variety of scintillator materials have been made, there is a continuous need for superior scintillator materials. The present disclosure relates generally to halide scintillator materials and method of making such scintillator materials. In one arrangement, a halide scintillator material is single-crystalline and has a composition of the formula A3 MBr6(1-x) Cl6x, 0≦x≦1, wherein A consists essentially of Li, Na K, Rb, Cs or any combination thereof, and M consists essentially of Ce, Sc, Y, La, Lu, Gd, Pr, Tb, Yb, Nd or any combination thereof. In another arrangement, a halide scintillator material is single-crystalline and has a composition of the formula A3MBr7(1-x)Cl7x, 0≦x≦1 wherein A consists essentially of Li, Na K, Rb, Cs or any combination thereof, and M consists essentially of Ce, Sc, Y, La, Lu, Gd, Pr, Tb, Yb, Nd or any combination thereof. Specific examples of these scintillator materials include single-crystalline Cs3CeBr6(1-x)Cl6x and CsCe2Br7(1-x)Cl7x. More specific examples include the end members of the respective formulae: Cs3CeBr6 and CsCe2Br7, i.e., x=0; and Cs3CeCl6 and CsCe2Cl7, i.e., x=1. In arrangement, a halide scintillator material is single-crystalline and has a composition of the formula A3 MBr6(1-x)Cl6x, wherein 0≦x≦1. A further aspect of the present disclosure relates to a method of making halide scintillator materials of the above-mentioned compositions. In one example, high-purity starting halides (such as CsBr, CeBr3, CsCl and CeCl3) are mixed and melted to synthesize a compound of the desired composition of the scintillator material. A single crystal of the scintillator material is then grown from the synthesized compound by the Bridgman method, in which a sealed ampoule containing the synthesized compound is transported from a hot zone to a cold zone through a controlled temperature gradient at a controlled speed to form a single-crystalline scintillator from molten synthesized compound. Inorganic scintillators are commonly used in nuclear and high-energy physics research, medical imaging, homeland security, and geological exploration. These materials typically possess sufficient stopping power for detection, high luminosity, high spectral energy resolution at room temperature and short decay time. Certain cerium-doped halides, such as LaCl3:Ce and LaBr3:Ce, have satisfactory scintillation properties at room temperature for gamma ray detection. Another desirable property of scintillators is a capability of neutron-gamma discrimination that is of importance for nuclear non-proliferation applications. Materials containing gadolinium, lithium and boron are employed to quickly and efficiently discriminate neutrons from gamma rays. In one aspect of present disclosure, a halide scintillator material is single-crystalline and has a composition of the formula A3MB6(1-x)Cl6x, 0≦x≦1, wherein A consists essentially of Li, Na K, Rb, Cs or any combination thereof, and M consists essentially of Ce, Sc, Y, La, Lu, Gd, Pr, Tb, Yb, Nd or any combination thereof. In another arrangement, a halide scintillator material is single-crystalline and has a composition of the formula AM2Br7(1-x)Cl7x, 0≦x≦1, wherein A consists essentially of Li, Na K, Rb, Cs or any combination thereof, and M consists essentially of Ce, Sc, Y, La, Lu, Gd, Pr, Tb, Yb, Nd or any combination thereof. Specific examples of these scintillator materials include single-crystalline Cs3CeBr6(1-x)Cl6x and CsCe2Br7(1-x)Cl7x. More specific examples include the end members of the respective formulae: Cs3CeBr6 and CsCe2Br7, i.e., x=0; and Cs3CeCl6 and CsCe2Cl7, i.e., x=1. Cs3CeCl6, Cs3CeBr6, CsCe2Cl7 and CsCe2Br7 are known to be congruently-melting compounds and therefore good for practical crystal growth from the melt. The above materials have high enough densities and are expected to have fast scintillation decay and high light output due to Ce 5d-4f luminescence, which make them very suitable for applications in gamma ray and/or X-ray detection in such applications as medical imaging and homeland security. A further aspect of the present disclosure relates to a method of making halide scintillator materials of the above-mentioned compositions. In one example, high-purity starting halides (such as CsBr, CeBr3, CsCl and CeCl3) are mixed and melted to synthesize a compound of the desired composition of the scintillator material. A single crystal of the scintillator material is then grown from the synthesized compound by the Bridgman method, in which a sealed ampoule containing the synthesized compound is transported from a hot zone to a cold zone through a controlled temperature gradient at a controlled speed to form a single-crystalline scintillator from molten synthesized compound. In another aspect of the present disclosure, the above-described scintillator materials are used in radiation detection by scintillation. For example, a radiation detector can include a scintillator described above for generating photons in response to the impinging radiation. The scintillator is optically coupled to a photon detector, such as a photomultiplier tube (PMT), arranged to receive the photons generated by the scintillator and adapted to generate a signal indicative of the photon generation. (a) Scintillator Crystal Growth In one arrangement, a modified 24-zone Electro-Dynamic Gradient Mellen furnace with a translation mechanism was used to grow halide single crystals via the Bridgman technique. As a first step, these compounds were synthesized by mixing and melting starting anhydrous halides in quartz ampoules. Quartz ampoules were first baked and freshly cleaned by rinsing with a dilute HF solution and deionized water. High purity, anhydrous beads of starting compounds (e.g., CsCl and CeCl3 for Cs3CeCl6 and CsCe2Cl7; CsBr and CeBr3 for Cs3CeBr6 and CsCe2Br7; CsCl, CeCl3, CsBr and CeBr3 for Cs3CeBr6(1-x)Cl6x and CsCe2Br7(1-x)Cl7x, x≠0) (available from Sigma-Aldrich)) were loaded into the cylindrical quartz ampoules in a nitrogen-purged glove box and sealed under 10−6 mbar vacuum with a hydrogen torch. The relative amounts of the starting compounds in one arrangement were chosen to achieve stoichiometry of the synthesized scintillator material. Examples include 3 CsBr:1 CeBr3 for Cs3CeBr6 and 1 CsBr:2 CeBr3 for CsCe2Br7 (molecular ratios). Other ratios can be used for desired degree of stoichiometry. The ampoule was heated up to a temperature above the melting points of the starting halides. Then the synthesized compound was loaded into a specially designed quartz ampoule of about 15 mm in diameter to grow a single crystal, During the growth, the ampoule travels through the furnace from a hot zone to a cold zone at a rate generally in the range 0.5-2 mm/h. Cooling down was done at a rate of about 10° C./h. After the crystals were grown and removed from the growth ampoules, they were stored in mineral oil to protect from the atmosphere. (b) Characterization of Scintillator Crystals Certain samples were characterized without polishing while for certain others, plates of about 1-2 mm in thickness were cut from the boules and polished using a set of sand papers and mineral oil. To identify the obtained phase, powder X-ray diffraction (XRD) analysis was carried out in air at room temperature. To minimize the effects of self-absorption, small samples (typically 1-2 mm thick, 3 mm×3 mm) were selected for the optical characterization. Photoluminescence spectra were obtained with a Horiba Jobin Yvon Fluorolog3 spectrofluorometer equipped with Xe lamp and monochromator. Scintillation time profiles were recorded using the time-correlated single photon technique and a 137Cs gamma-ray source. Radioluminescence spectra were measured at RT under continuous irradiation from an X-ray generator (35 kV and 0.1 mA) using a PI Acton Spectra Pro SP-2155 monochromator. Light output measurements were carried out on samples covered in mineral oil and directly coupled to a photomultiplier tube (PMT) and covered with Teflon tape. A Hamamatsu 3177-50 PMT was used for absolute light output measurements. Gamma-ray energy spectra were recorded using a 137Cs source with a 2 ms shaping time. The integral quantum efficiency of the PMT according to the emission spectrum of the scintillators was used to calculate the number of photons per unit gamma ray energy. The energy resolution, at 662 keV was determined from the full-width at half-maximum (FWHM) of the 662 keV photopeak. (c) Example Results According to certain aspects of the present disclosure, single crystals of the halide materials suitable for scintillator applications were made, and their scintillation properties were measured. A single crystal of Cs3CeCl6 made using the Bridgman method as described above is shown in the image in FIG. 1. The sample is approximately 1 cm across and slightly translucent. Similar single crystals of Cs3CeBr6, CsCe2Br7 and CsCe2Cl7 were also made. The above samples were shown to be homogeneous by X-ray diffraction analysis. The single crystal scintillators described above have demonstrated high performance under optical, X-rays and gamma rays. These scintillators exhibit Ce 5d-4f luminescence. FIG. 2 shows radioluminescence spectra of (a) Cs3CeCl6, (b) CsCe2Cl7, (c) Cs3CeBr6 and (d) CsCe2Br7 single crystals. The absolute light output and The energy resolution (FWHM) at 662 keV for certain samples are listed in Table I: TABLE ISelected Scintillator PropertiesLight Output,Energy ResolutionCompositionph/MeV(ΔE), % @ 622 keVCs3CeCl6~19,0008.4CsCe2Cl7~26,0007.5Cs3CeBr6~28,0009CsCe2Br7~40,000~8 FIG. 3 shows scintillation decay time spectra of (a) Cs3CeCl6, (b) Cs3CeBr6, (c) CsCe2Cl7 and (d) CsCe2Br7 single crystals. The decay of each crystal can be characterized by a double exponential decay. The decay times for these samples are approximately: (a) 58 ns (52%) and 293 ns (48%) for Cs3CeCl6, (b) 93 ns (45%) and 557 ns (55%) for Cs3CeSr6, (c) 55 ns (43%) and 244 ns (57%) for CsCe2Cl7, and (d) 20 ns (40%) and 95 ns (60%) for CsCe2Br7 FIG. 4 shows energy spectra of (a). CsCe2Cl7 and (h) CsCe2Cl7 crystals, (normalized, with the photopeak of a BGO standard sample at channel no. 100). The photopeaks are located approximately at channels nos. 320 and 540, respectively. FIG. 5 shows an energy spectrum for a single crystal of CsCe2Br7(1-x)Cl7x, x=0 (i.e., CsCe2Br7). The channel number is proportional to the relative light output of the sample. The relative light output (photo peak position) of the reference crystal BGO is 100 on this scale. The relative light output for this sample is thus at least 7 time of that of a BGO crystal. Preliminary tests indicate that the absolute light output of a CsCe2Br7(1-x)Cl7x (x=0) sample is at least 40,000 photons/MeV. FIG. 6 shows a scintillation decay time spectrum of a CsCe2Br7 single crystal; FIG. 7 shows a scintillation decay time spectrum of a single crystal. The spectra were measured using a 137Cs gamma-ray source (662 keV). The scintillation decay times calculated for these sample each consist of two components: 26 ns (65%) and 124 ns (35%) for CsCe2Br7(1-x)Cl7x (x=0), and 94 ns (47%) and 550 ns (53%) for Cs3CeBr6(1-x)Cl6x (x=0). Additional examples of scintillation decay time spectra are shown in FIGS. 11 and 12. FIG. 8 shows an X-ray excited radioluminescence spectrum of CsCe2Br7; FIG. 9 shows an X-ray excited radioluminescence spectrum of Cs3CeBr6. The emission peak are at approximately 421 nm and 406 nm, respectively. The emission wavelengths of both scintillators are in the sensitive wavelength range for many commercial Photo Multiplier Tubes (PMTs). Additional examples of radioluminescence spectra are shown in FIG. 13 FIG. 10 shows energy spectra of Cs3CeBr6 and CsCe2Br7 crystals, respectively (normalized, with the photopeak of a BGO standard sample at channel no. 100); the spectra were measured using 137Cs gamma-ray source (662 keV). Thus, halide scintillator crystals with excellent scintillation properties have been produced according to the present disclosure. Because many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended. |
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abstract | An apparatus is provided and includes a reactor core, a boom and a shield assembly supportively interposed between the reactor core and the boom, a heat pipe disposed in thermal communication with the reactor core, a thermoelectric power converter operably coupled to the heat pipe, struts supportively coupled to the heat pipe at opposite ends of the power converter and hinge joints to rotatably couple the struts to the boom, at least one of the hinge joints being spring-loaded. |
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abstract | An operating floor confinement has an operating floor, a sidewall that surrounds the operating floor, a ceiling that is provided on an upper portion of the sidewall, a reactor well, a fuel pool, a dryer and separator pit, an equipment hatch that is provided on the sidewall, an air lock that is provided on the sidewall, and an isolation valve that is provided in a penetration line. The operating floor confinement forms a pressure boundary having pressure resistance and a leakage protection function. The operating floor confinement is separated from an equipment area of the reactor building and has no blowout panel. |
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description | This application is a continuation of U.S. patent application Ser. No. 12/575,312, filed Oct. 7, 2009; now U.S. Pat. No. 8,247,769, which claims the benefit of U.S. Provisional Patent Application No. 61/195,639, filed Oct. 9, 2008, U.S. Provisional Patent Application No. 61/236,745, filed Aug. 25, 2009, and U.S. Provisional Patent Application No. 61/240,946, filed Sep. 9, 2009, which are commonly assigned, the disclosures of which are hereby incorporated by reference in their entirety. U.S. patent application Ser. No. 12/575,285, now U.S. Pat. No. 8,203,120 was filed concurrently with U.S. patent application Ser. No. 12/575,312 and the entire disclosure of U.S. patent application Ser. No. 12/575,285 is hereby incorporated by reference into this application for all purposes. The U.S. Government has certain rights in this invention pursuant to Grant No. GM081520 awarded by the National Institutes of Health, Grant No. FA9550-07-1-0484 awarded by the Air Force (AFOSR) and Grant No(s). CHE0549936 & DMR0504854 awarded by the National Science Foundation. Electrons, because of their wave-particle duality, can be accelerated to have picometer wavelength and focused to image in real space. With the impressive advances made in transmission electron microscopy (TEM), STEM, and aberration-corrected TEM, it is now possible to image with high resolution, reaching the sub-Angstrom scale. Together with the progress made in electron crystallography, tomography, and single-particle imaging, today the electron microscope has become a central tool in many fields, from materials science to biology. For many microscopes, the electrons are generated either thermally by heating the cathode or by field emission, and as such the electron beam is made of random electron bursts with no control over the temporal behavior. In these microscopes, time resolution of milliseconds or longer, being limited by the video rate of the detector, can be achieved, while maintaining the high spatial resolution. Despite the advances made in TEM techniques, there is a need in the art for improved methods and novel systems for ultrafast electron microscopy. According to embodiments of the present invention, methods and systems for 4D ultrafast electron microscopy (UEM) are provided—in situ imaging with ultrafast time resolution in TEM. Thus, 4D microscopy provides imaging for the three dimensions of space as well as the dimension of time. In some embodiments, single electron imaging is introduced as a component of the 4D UEM technique. Utilizing one electron packets, resolution issues related to repulsion between electrons (the so-called space-charge problem) are addressed, providing resolution unavailable using conventional techniques. Moreover, other embodiments of the present invention provide methods and systems for convergent beam UEM, focusing the electron beams onto the specimen to measure structural characteristics in three dimensions as a function of time. Additionally, embodiments provide not only 4D imaging of specimens, but characterization of electron energy, performing time resolved electron energy loss spectroscopy (EELS). The potential applications for 4D UEM are demonstrated using examples including gold and graphite, which exhibit very different structural and morphological changes with time. For gold, following thermally induced stress, the atomic structural expansion, the nonthermal lattice temperature, and the ultrafast transients of warping/bulging were determined. In contrast, in graphite, striking coherent transients of the structure were observed in the selected-area image dynamics, and also in diffraction, directly measuring the resonance period of Young's elastic modulus. Measurement of the Young's elastic modulus for the nano-scale dimension, the frequency is found to be as high as 30 gigahertz, hitherto unobserved, with the atomic motions being along the c-axis. Both materials undergo fully reversible dynamical changes, retracing the same evolution after each initiating impulsive stress. Thus, embodiments of the present invention provide methods and systems for performing imaging studies of dynamics using UEM. Other embodiments of the present invention extend four-dimensional (4D) electron imaging to the attosecond time domain. Specifically, embodiments of the present invention are used to generate attosecond electron pulses and in situ probing with electron diffraction. The free electron pulses have a de Broglie wavelength on the order of picometers and a high degree of monochromaticity (ΔE/E0≈10−4); attosecond optical pulses have typically a wavelength of 20 nm and ΔE/E0≈0.5, where E0 is the central energy and ΔE is the energy bandwidth. Diffraction, and tilting of the electron pulses/specimen, permit the direct investigation of electron density changes in molecules and condensed matter. This 4D imaging on the attosecond time scale is a pump-probe approach in free space and with free electrons. As described more fully throughout the present specification, some embodiments of the present invention utilize single electron packets in UEM, referred to as single electron imaging. Conventionally, it was believed that the greater number of electrons per pulse, the better the image produced by the microscope. In other words, as the signal is increased, imaging improves. However, the inventor has determined that by using single electron packets and repeating the imaging process a number of times, images can be achieved without repulsion between electrons. Unlike photons, electrons are charged and repel each other. Thus, as the number of electrons per pulse increases, the divergence of the trajectories increases and resolution decreases. Using single electron imaging techniques, atomic scale resolution of motion is provided once the space-charge problem is addressed. According to an embodiment of the present invention, a four-dimensional electron microscope for imaging a sample is provided. The four-dimensional electron microscope includes a stage assembly configured to support the sample, a first laser source capable of emitting a first optical pulse of less than 1 ps in duration, and a second laser source capable of emitting a second optical pulse of less than 1 ns in duration. The four-dimensional electron microscope also includes a cathode coupled to the first laser source and the second laser source. The cathode is capable of emitting a first electron pulse less than 1 ps in duration in response to the first optical pulse and a second electron pulse of less than 1 ns in response to the second optical pulse. The four-dimensional electron microscope further includes an electron lens assembly configured to focus the electron pulse onto the sample and a detector configured to capture one or more electrons passing through the sample. The detector is configured to provide a data signal associated with the one or more electrons passing through the sample. The four-dimensional electron microscope additionally includes a processor coupled to the detector. The processor is configured to process the data signal associated with the one or more electrons passing through the sample to output information associated with an image of the sample. Moreover, the four-dimensional electron microscope includes an output device coupled to the processor. The output device is configured to output the information associated with the image of the sample. According to another embodiment of the present invention, a convergent beam 4D electron microscope is provided. The convergent beam 4D electron microscope includes a laser system operable to provide a series of optical pulses, a first optical system operable to split the series of optical pulses into a first set of optical pulses and a second set of optical pulses and a first frequency conversion unit operable to frequency double the first set of optical pulses. The convergent beam 4D electron microscope also includes a second optical system operable to direct the frequency doubled first set of optical pulses to impinge on a sample and a second frequency conversion unit operable to frequency triple the second set of optical pulses. The convergent beam 4D electron microscope further includes a third optical system operable to direct the frequency tripled second set of optical pulses to impinge on a cathode, thereby generating a train of electron packets. Moreover, the convergent beam 4D electron microscope includes an accelerator operable to accelerate the train of electron packets, a first electron lens operable to de-magnify the train of electron packets, and a second electron lens operable to focus the train of electron packets onto the sample. According to a specific embodiment of the present invention, a system for generating attosecond electron pulses is provided. The system includes a first laser source operable to provide a laser pulse and a cathode optically coupled to the first laser source and operable to provide an electron pulse at a velocity v0 directed along an electron path. The system also includes a second laser source operable to provide a first optical wave at a first wavelength. The first optical wave propagates in a first direction offset from the electron path by a first angle. The system further includes a third laser source operable to provide a second optical wave at a second wavelength. The second optical wave propagates in a second direction offset from the electron path by a second angle and the interaction between the first optical wave and the second optical wave produce a standing wave copropagating with the electron pulse. According to another specific embodiment of the present invention, a method for generating a series of tilted attosecond pulses is provided. The method includes providing a femtosecond electron packet propagating along an electron path. The femtosecond electron packet has a packet duration and a direction of propagation. The method also includes providing an optical standing wave disposed along the electron path. The optical standing wave is characterized by a peak to peak wavelength measured in a direction tilted at a predetermined angle with respect to the direction of propagation. The method further includes generating the series of tilted attosecond pulses after interaction between the femtosecond electron packet and the optical standing wave. According to a particular embodiment of the present invention, a method of operating an electron energy loss spectroscopy (EELS) system is provided. The method includes providing a train of optical pulses using a pulsed laser source, directing the train of optical pulses along an optical path, frequency doubling a portion of the train of optical pulses to provide a frequency doubled train of optical pulses, and frequency tripling a portion of the frequency doubled train of optical pulses to provide a frequency tripled train of optical pulses. The method also includes optically delaying the frequency doubled train of optical pulses using a variable delay line, impinging the frequency doubled train of optical pulses on a sample, impinging the frequency tripled train of optical pulses on a photocathode, and generating a train of electron pulses along an electron path. The method further includes passing the train of electron pulses through the sample, passing the train of electron pulses through a magnetic lens, and detecting the train of electron pulses at a camera. According to an embodiment of the present invention, a method of imaging a sample is provided. The method includes providing a stage assembly configured to support the sample, generating a train of optical pulses from a laser source, and directing the train of optical pulses along an optical path to impinge on a cathode. The method also includes generating a train of electron pulses in response to the train of optical pulses impinging on the cathode. Each of the electron pulses consists of a single electron. The method further includes directing the train of electron pulses along an imaging path to impinge on the sample, detecting a plurality of the electron pulses after passing through the sample, processing the plurality of electron pulses to form an image of the sample, and outputting the image of the sample to an output device. According to another embodiment of the present invention, a method of capturing a series of time-framed images of a moving nanoscale object is provided. The method includes a) initiating motion of the nanoscale object using an optical clocking pulse, b) directing an optical trigger pulse to impinge on a cathode, and c) generating an electron pulse. The method also includes d) directing the electron pulse to impinge on the sample with a predetermined time delay between the optical clocking pulse and the electron pulse, e) detecting the electron pulse, f) processing the detected electron pulse to form an image, and g) increasing the predetermined time delay between the optical clocking pulse and the electron pulse. The method further includes repeating steps a) through g) to capture the series of time-framed images of the moving nanoscale object. According to a specific embodiment of the present invention, a method of characterizing a sample is provided. The method includes providing a laser wave characterized by an optical wavelength (λ0) and a direction of propagation and directing the laser wave along an optical path to impinge on a test surface of the sample. The test surface of the sample is tilted with respect to the direction of propagation of the laser by a first angle (α). The method also includes providing a train of electron pulses characterized by a propagation velocity (vel), a spacing between pulses ( λ 0 v el c ) ,and a direction of propagation tilted with respect to the direction of propagation of the laser by a second angle (β). The method further includes directing the train of electron pulses along an electron path to impinge on the test surface of the sample. The first angle, the second angle, and the propagation velocity are related by sin ( α ) sin ( α - β ) = c v el . According to another specific embodiment of the present invention, a method of imaging chemical bonding dynamics is provided. The method includes positioning a sample in a reduced atmosphere environment, providing a first train of laser pulses, and directing the first train of laser pulses along a first optical path to impinge on a sample. The method also includes providing a second train of laser pulses, directing the second train of laser pulses along a second optical path to impinge on a photocathode, and generating a train of electron pulses. One or more of the electron pulses consist of a single electron. The method further includes accelerating the train of electron pulses and transmitting a portion of the train of electron pulses through the sample. Numerous benefits are achieved by way of the present invention over conventional techniques. For example, the present systems provide temporal resolution over a wide range of time scales. Additionally, unlike spectroscopic methods, embodiments of the present invention can determine a structure in 3-D space. Such capabilities allow for the investigation of phase transformation in matter, determination of elastic and mechanical properties of materials on the nanoscale, and the time evolution of processes involved in materials and biological function. Depending upon the embodiment, one or more of these benefits may be achieved. These and other benefits will be described in more detail throughout the present specification and more particularly below. These and other objects and features of the present invention and the manner of obtaining them will become apparent to those skilled in the art, and the invention itself will be best understood by reference to the following detailed description read in conjunction with the accompanying drawings. Ultrafast imaging, using pulsed photoelectron packets, provides opportunities for studying, in real space, the elementary processes of structural and morphological changes. In electron diffraction, ultrashort time resolution is possible but the data is recorded in reciprocal space. With space-charge-limited nanosecond (sub-micron) image resolutions ultrashort processes are not possible to observe. In order to achieve the ultrafast resolution in microscopy, the concept of single-electron pulse imaging was realized as a key to the elimination of the Coulomb repulsion between electrons while maintaining the high temporal and spatial resolutions. As long as the number of electrons in each pulse is much below the space-charge limit, the packet can have a few or tens of electrons and the temporal resolution is still determined by the femtosecond (fs) optical pulse duration and the energy uncertainty, on the order of 100 fs, and the spatial resolution is atomic-scale. However, the goal of full-scale dynamic imaging can be attained only when in the microscope the problems of in situ high spatiotemporal resolution for selected image areas, together with heat dissipation, are overcome. FIG. 1 is a simplified diagram of a 4D electron microscope system according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. As illustrated in FIG. 1, a femtosecond laser 110 or a nanosecond laser 105 is directed through a Pockels cell 112, which acts as a controllable shutter. A Glan polarizer 114 is used in some embodiments, to select the laser power propagating in optical path 115. A beam splitter (not shown) is used to provide several laser beams to various portions of the system. Although the system illustrated in FIG. 1 is described with respect to imaging applications, this is not generally required by the present invention. One of skill in the art will appreciate that embodiments of the present invention provide systems and methods for imaging, diffraction, crystallography, electron spectroscopy, and related fields. Particularly, the experimental results discussed below yield insight into the varied applications available using embodiments of the present invention. The femtosecond laser 110 is generally capable of generating a train of optical pulses with predetermined pulse width. One example of such a laser system is a diode-pumped mode-locked titanium sapphire (Ti:Sapphire) laser oscillator operating at 800 nm and generating 100 fs pulses at a repetition rate of 80 MHz and an average power of 1 Watt, resulting in a period between pulses of 12.5 ns. In an embodiment, the spectral bandwidth of the laser pulses is 2.35 nm FWHM. An example of one such laser is a Mai Tai One Box Femtosecond Ti:Sapphire Laser, available from Spectra-Physics Lasers, of Mountain View, Calif. In alternative embodiments, other laser sources generating optical pulses at different wavelengths, with different pulse widths, and at different repetition rates are utilized. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. The nanosecond laser 105 is also generally capable of generating a train of optical pulses with a predetermined pulse width greater than that provided by the femtosecond laser. The use of these two laser systems enables system miniaturization since the size of the nanosecond laser is typically small in comparison to some other laser systems. By moving one or more mirrors, either laser beam is selected for use in the system. The ability to select either laser enables scanning over a broad time scale—from femtoseconds all the way to milliseconds. For short time scale measurement, the femtosecond laser is used and the delay stage (described below) is scanned at corresponding small time scales. For measurement of phenomena over longer time scales, the nanosecond laser is used and the delay stage is scanned at corresponding longer time scales. A first portion of the output of the femtosecond laser 110 is coupled to a second harmonic generation (SHG) device 116, for example a barium borate (BaB2O4) crystal, typically referred to as a BBO crystal and available from a variety of doubling crystal manufacturers. The SHG device frequency doubles the train of optical pulses to generate a train of 400 nm, 100 fs optical pulses at an 80 MHz repetition rate. SHG devices generally utilize a nonlinear crystal to frequency double the input pulse while preserving the pulse width. In some embodiments, the SHG is a frequency tripling device, thereby generating an optical pulse at UV wavelengths. Of course, the desired output wavelength for the optical pulse will depend on the particular application. The doubled optical pulse produced by the SHG device propagates along electron generating path 118. A cw diode laser 120 is combined with the frequency doubled optical pulse using beam splitter 122. The light produce by the cw diode laser, now collinear with the optical pulse produced by the SHG device, serves as an alignment marker beam and is used to track the position of the optical pulse train in the electron generating path. The collinear laser beams enter chamber 130 through entrance window 132. In the embodiment illustrated in FIG. 1, the entrance window is fabricated from materials with high transparency at 400 nm and sufficient thickness to provide mechanical rigidity. For example, BK-7 glass about 6 mm thick with anti-reflection coatings, e.g. MgF2 or sapphire are used in various embodiments. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. An optical system, partly provided outside chamber 130 and partly provided inside chamber 130 is used to direct the frequency doubled optical pulse train along the electron-generating path 134 inside the chamber 130 so that the optical pulses impinge on cathode 140. As illustrated, the optical system includes mirror 144, which serves as a turning mirror inside chamber 130. In embodiments of the present invention, polished metal mirrors are utilized inside the chamber 130 since electron irradiation may damage mirror coatings used on some optical mirrors. In a specific embodiment, mirror 144 is fabricated from an aluminum substrate that is diamond turned to produce a mirror surface. In some embodiments, the aluminum mirror is not coated. In other embodiments, other metal mirrors, such as a mirror fabricated from platinum is used as mirror 144. In an embodiment, the area of interaction on the cathode was selected to be a flat 300 μm in diameter. Moreover, in the embodiment illustrated, the frequency doubled optical pulse was shaped to provide a beam with a beam waist of a predetermined diameter at the surface of the cathode. In a specific embodiment, the beam waist was about 50 μm. In alternative embodiments, the beam waist ranged from about 30 μm to about 200 μm. Of course, the particular dimensions will depend on the particular applications. The frequency doubled optical pulse train was steered inside the chamber using a computer controlled mirror in a specific embodiment. In a specific embodiment, the optical pulse train is directed toward a front-illuminated photocathode where the irradiation of the cathode by the laser results in the generation of electron pulses via the photoelectric effect. Irradiation of a cathode with light having an energy above the work function of the cathode leads to the ejection of photoelectrons. That is, a pulse of electromagnetic energy above the work function of the cathode ejects a pulse of electrons according to a preferred embodiment. Generally, the cathode is maintained at a temperature of 1000 K, well below the thermal emission threshold temperature of about 1500 K, but this is not required by the present invention. In alternative embodiments, the cathode is maintained at room temperature. In some embodiments, the cathode is adapted to provide an electron pulse of predetermined pulse width. The trajectory of the electrons after emission follows the lens design of the TEM, namely the condenser, the objective, and the projector lenses. Depending upon the embodiment, there may also be other configurations. In the embodiment illustrated, the cathode is a Mini-Vogel mount single crystal lanthanum hexaboride (LaB6) cathode shaped as a truncated cone with a flat of 300 μm at the apex and a cone angle of 90°, available from Applied Physics Technologies, Inc., of McMinnville, Oreg. As is often known, LaB6 cathodes are regularly used in transmission and scanning electron microscopes. The quantum efficiency of LaB6 cathodes is about 10−3 and these cathodes are capable of producing electron pulses with temporal pulse widths on the order of 10−13 seconds. In some embodiments, the brightness of electron pulses produced by the cathode is on the order of 109 A/cm2/rad2 and the energy spread of the electron pulses is on the order of 0.1 eV. In other embodiments, the pulse energy of the laser pulse is reduced to about 500 pJ per pulse, resulting in approximately one electron/pulse Generally, the image quality acquired using a TEM is proportional to the number of electrons passing through the sample. That is, as the number of electrons passing through the sample is increased, the image quality increases. Some pulsed lasers, such as some Q-switched lasers, reduce the pulse count to produce a smaller number of pulses characterized by higher peak power per pulse. Thus, some laser amplifiers operate at a 1 kHz repetition rate, producing pulses with energies ranging from about 1 μJ to about 2 mJ per pulse. However, when such high peak power lasers are used to generate electron pulses using the photoelectric effect, among other issues, both spatial and temporal broadening of the electron pulses adversely impact the pulse width of the electron pulse or packet produced. In some embodiments of the present invention, the laser is operated to produce low power pulses at higher repetition rates, for example, 80 MHz. In this mode of operation, benefits available using lower power per pulse are provided, as described below. Additionally, because of the high repetition rate, sufficient numbers of electrons are available to acquire high quality images. In some embodiments of the present invention, the laser power is maintained at a level of less than 500 pJ per pulse to prevent damage to the photocathode. As a benefit, the robustness of the photoemitter is enhanced. Additionally, laser pulses at these power levels prevent space-charge broadening of the electron pulse width during the flight time from the cathode to the sample, thus preserving the desired femtosecond temporal resolution. Additionally, the low electron count per pulse provided by some embodiments of the present invention reduces the effects of space charge repulsion in the electron pulse, thereby enhancing the focusing properties of the system. As one of skill in the art will appreciated, a low electron count per pulse, coupled with a high repetition rate of up to 80 MHz provided by the femtosecond laser, provides a total dose as high as one electron/Å2 as generally utilized in imaging applications. In alternative embodiments, other suitable cathodes capable of providing a ultrafast pulse of electrons in response to an ultrafast optical pulse of appropriate wavelength are utilized. In embodiments of the present invention, the cathode is selected to provide a work function correlated with the wavelength of the optical pulses provided by the SHG device. The wavelength of radiation is related to the energy of the photon by the familiar relation λ(μm)≈1.24÷v (eV), where λ is the wavelength in microns and v is the energy in eV. For example, a LaB6 cathode with a work function of 2.7 eV is matched to optical pulses with a wavelength of 400 nm (v=3.1 eV) in an embodiment of the present invention. As illustrated, the cathode is enclosed in a vacuum chamber 130, for example, a housing for a transmission electron microscope (TEM). In general, the vacuum in the chamber 130 is maintained at a level of less than 1×10−6 torr. In alternative embodiments, the vacuum level varies from about 1×10−6 torr to about 1×10−10 torr. The particular vacuum level will be a function of the varied applications. In embodiments of the present invention, the short duration of the photon pulse leads to ejection of photoelectrons before an appreciable amount of the deposited energy is transferred to the lattice of the cathode. In general, the characteristic time for thermalization of the deposited energy in metals is below a few picoseconds, thus no heating of the cathode takes place using embodiments of the present invention. Electrons produced by the cathode 140 are accelerated past the anode 142 and are collimated and focused by electron lens assembly 146 and directed along electron imaging path 148 toward the sample 150. The electron lens assembly generally contains a number of electromagnetic lenses, apertures, and other elements as will be appreciated by one of skill in the art. Electron lens assemblies suitable for embodiments of the present invention are often used in TEMs. The electron pulse propagating along electron imaging path 148 is controlled in embodiments of the present invention by a controller (not shown, but described in more detail with reference to certain Figures below) to provide an electron beam of predetermined dimensions, the electron beam comprising a train of ultrafast electron pulses. The relationship between the electron wavelength (λdeBroglie) and the accelerating voltage (U) in an electron microscope is given by the relationship λdeBroglie=h/(2m0eU)1/2, where h, m0, e are Planck's constant, the electron mass, and an elementary charge. As an example, the de Broglie wavelength of an electron pulse at 120 kV corresponds to 0.0335 Å, and can be varied depending on the particular application. The bandwidth or energy spread of an electron packet is a function of the photoelectric process and bandwidth of the optical pulse used to generate the electron packet or pulse. Electrons passing through the sample or specimen 150 are focused by electron lens assembly 152 onto a detector 154. Although FIG. 1 illustrates two electron lens assemblies 146 and 152, the present invention is not limited to this arrangement and can have other lens assemblies or lens assembly configurations. In alternative embodiments, additional electromagnets, apertures, other elements, and the like are utilized to focus the electron beam either prior to or after interaction with the sample, or both. Detection of electrons passing through the sample, including single-electron detection, is achieved in one particular embodiment through the use of an ultrahigh sensitivity (UHS) phosphor scintillator detector 154 especially suitable for low-dose applications in conjunction with a digital CCD camera. In a specific embodiment, the CCD camera was an UltraScan™ 1000 UHS camera, manufactured by Gatan, Inc., of Pleasanton, Calif. The UltraScan™ 1000 CCD camera is a 4 mega-pixel (2048×2048) camera with a pixel size of 14 μm×14 μm, 16-bit digitization, and a readout speed of 4 Mpixels/sec. In the embodiment illustrated, the digital CCD camera is mounted under the microscope in an on-axis, below the chamber position. In order to reduce the noise and picture artifacts, in some embodiments, the CCD camera chip is thermoelectrically cooled using a Peltier cooler to a temperature of about −25° C. The images from the CCD camera were obtained with DigitalMicrograph™ software embedded in the Tecnai™ user interface, also available from Gatan, Inc. Of course, there can be other variations to the CCD camera, cooler, and computer software, depending upon the embodiment. FIG. 2 is a simplified perspective diagram of a 4D electron microscope system according to an embodiment of the present invention. The system illustrated in FIG. 2 is also referred to as an ultrafast electron microscope (UEM2) and was built at the present assignee. The integration of two laser systems with a modified electron microscope is illustrated, together with a representative image showing a resolution of 3.4 Å obtained in UEM2 without the field-emission-gun (FEG) arrangement of a conventional TEM. In one embodiment of the system illustrated in FIG. 2, the femtosecond laser system (fs laser system) is used to generate the single-electron packets and the nanosecond laser system (ns laser system) was used both for single-shot and stroboscopic recordings. In the single-electron mode of operation, the coherence volume is well defined and appropriate for image formation in repetitive events. The dynamics are fully reversible, retracing the identical evolution after each initiating laser pulse; each image is constructed stroboscopically, in seconds, from typically 106 pulses and all time-frames are processed to make a movie. The time separation between pulses can be varied to allow complete heat dissipation in the specimen. Without limiting embodiments of the present invention, it is believed that the electrons in the single electron packets have a transverse coherence length that is comparable to the size of the object that is being imaged. Since the subsequent electrons have a coherence length on the order of the size of the object, the electrons “see” the whole object at once. To follow the area-specific changes in the hundreds of images collected for each time scan, we obtained selected-area-image dynamics (SAID) and selected-area-diffraction dynamics (SADD); for the former, in real space, from contrast change and for the latter, in Fourier space, from changes of the Bragg peak separations, amplitudes, and widths. It is the advantage of microscopy that allows us to perform this parallel-imaging dynamics with pixel resolution, when compared with diffraction. As shown below, it would not have been possible to observe the selected temporal changes if the total image were to be averaged over all pixels, in this case 4 millions. As illustrated in FIG. 2, a TEM is modified to provide a train of electron pulses used for imaging in addition to the thermionic emission source used for imaging of samples. Merely by way of example, an FEI Tecnai™ G2 12 TWIN, available from FEI Company in Hillsboro, Oreg., may be modified according to embodiments of the present invention. The Tecnai™ G2 12 TWIN is an all-in-one 120 kV (λdeBroglie=0.0335 Å) high-resolution TEM optimized for 2D and 3D imaging at both room and liquid-nitrogen temperatures. Embodiments of the present invention leverage capabilities provided by commercial TEMs such as automation software, detectors, data transfer technology, and tomography. In particular, in some embodiments of the present invention, a five-axis, motor-driven, precision goniometer is used with computer software to provide automated specimen tilt combined with automated acquisition of images as part of a computerized tomography (CT) imaging system. In these embodiments, a series of 2D images are captured at various specimen positions and combined using computer software to generate a reconstructed 3D image of the specimen. In some embodiments, the CT software is integrated with other TEM software and in other embodiments, the CT software is provided off-line. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. In certain embodiments in which low-electron content electron pulses are used to image the sample, the radiation damage is limited to the transit of the electrons in the electron pulses through the sample. Typically, samples are on the order of 100 nm thick, although other thicknesses would work as long as certain electrons may traverse through the sample. Thus, the impact of radiation damage on these low-electron content electron pulse images is limited to the damage occurring during this transit time. Radiation induced structural damage occurring on longer time scales than the transit time will not impact the collected image, as these damage events will occur after the structural information is collected. Utilizing the apparatus described thus far, embodiments of the present invention provide systems and methods for imaging material and biological specimens both spatially and temporally with atomic-scale spatial resolution on the order of 1 nm and temporal resolution on the order of 100 fs. At these time scales, energy randomization is limited and the atoms are nearly frozen in place, thus methods according to the present invention open the door to time-resolved studies of structural dynamics at the atomic scale in both space and time. Details of the present computer system according to an embodiment of the present invention may be explained according to the description below. Referring to FIG. 2, a photograph of a UEM2 in accordance with embodiments of the present invention is illustrated, together with a high-resolution image of graphitized carbon. As illustrated, two laser systems (fs and ns) are utilized to provide a wide range of temporal scales used in 4D electron imaging. A 200-kV TEM is provided with at least two ports for optical access to the microscope housing. Using one or more mirrors (e.g., two mirrors), it is possible to switch between the laser systems to cover both the fs and ns experiments. The optical pulses are directed to the photocathode to generate electron packets, as well as to the specimen to initiate (clock) the change in images with a well-defined delay time Δt. The time axis is defined by variable delay between the electron generating and clocking pulses using the delay stage 170 illustrated in FIG. 1. Details of development of ultrafast electron microscopy with atomic-scale real-, energy-, and Fourier-space resolutions is now provided. The second generation UEM2 described in FIG. 2 provides images, diffraction patterns, and electron-energy spectra, and has application for nanostructured materials and organometallic crystals. The separation between atoms in direct images, and the Bragg spots/Debye-Scherrer rings in diffraction, are clearly resolved, and the electronic structure and elemental energies in the electron-energy-loss spectra (EELS) and energy-filtered-transmission-electron microscopy (EFTEM) are obtained. The development of 4D ultrafast electron microscopy and diffraction have made possible the study of structural dynamics with atomic-scale spatial resolution, so far in diffraction, and ultrashort time resolution. The scope of applications is wide-ranging with studies spanning diffraction of isolated structures in reactions (gas phase), nanostructures of surfaces and interfaces (crystallography), and imaging of biological cells and materials undergoing first-order phase transitions. Typically, for microscopy the electron was accelerated to 120 keV and for diffraction to 30 keV, respectively, and issues of group velocity mismatch, in situ clocking (time zero) of the change, and frame referencing were addressed. One powerful concept implemented is that of “tilted pulses,” which allow for the optimum resolution to be reached at the specimen. For ultrafast electron microscopy, the concept of “single-electron” imaging is fundamental to some embodiments. The electron packets, which have a well-defined picometer-scale de Broglie wave length, are generated in the microscope by femtosecond optical pulses (photoelectric effect) and synchronized with other optical pulses to initiate the change in a temperature jump or electronic excitation. Because the number of electrons in each packet is one or a few, the Coulomb repulsion (space charge) between electrons is reduced or eliminated and the temporal resolution can reach the ultimate, that of the optical pulse. The excess energy above the work function determines the electron energy spread and this, in principle, can be minimized by tuning the pulse energy. The spatial resolution is then only dependent on the total number of electrons because for each packet the electron “interferes with itself” and a coherent buildup of the image is achievable. The coherence volume, given by:Vc=λdeBroglie2(R/a)2ve(h/ΔE)establishes that the degeneracy factor is much less than one and that each Fermionic electron is independent, without the need of the statistics commonly used for Bosonic photons. The volume is determined by the values of longitudinal and transverse coherences; Vc is on the order of 106 nm3 for typical values of R (distance to the source), a (source dimension), ve (electron velocity), and ΔE (energy spread). Unlike the situation in transmission electron microscopy (TEM), coherence and image resolution in UEM are thus determined by properties of the optical field, the ability to focus electrons on the ultrashort time scale, and the operational current density. For both “single electron” and “single pulse” modes of UEM, these are important considerations for achieving the ultimate spatio-temporal resolutions for studies of materials and biological systems. Atomic-scale resolution in real-space imaging can be achieved utilizing the second generation ultrafast electron microscopy system (UEM2) of FIG. 2. With UEM2, which operates at 200 keV (λdeBroglie=2.507 pm), energy-space (electron-energy-loss spectroscopy, EELS) and Fourier-space (diffraction) patterns of nanostructured materials are possible. The apparatus can operate in the scanning transmission electron microscope (STEM) mode, and is designed to explore the vast parameter space bridging the gap between the two ideal operating modes of single-electron and single-pulse imaging. With these features, UEM2 studies provide new limits of resolution, image mapping, and elemental analysis. Here, demonstrated are the potential by studying gold particles and islands, boron nitride crystallites, and organometallic phthalocyanine crystals. FIG. 2A displays the conceptual design of UEM2, which, as with the first generation (UEM1—described generally in FIG. 1), comprises a femtosecond laser system and an electron microscope modified for pulsed operation with femtosecond electron packets. A schematic representation of optical, electric, and magnetic components are shown. The optical pulse train generated from the laser, in this case having a variable pulse width of 200 fs to 10 ps and a variable repetition rate of 200 kHz to 25 MHz, is divided into two parts, after harmonic generation, and guided toward the entries of the design hybrid electron microscope. The frequency-tripled optical pulses are converted to the corresponding probe electron pulses at the photocathode in the hybrid FEG, whereas the other optical pump beam excites (T-jump or electronic excitation) in the specimen with a well-defined time delay with respect to the probe electron beam. The probe electron beam through the specimen can be recorded as an image (normal or filtered, EFTEM), a diffraction pattern, or an EEL spectrum. The STEM bright-field detector is retractable when it is not in use. The laser in an embodiment is a diode-pumped Yb-doped fiber oscillator/amplifier (Clark-MXR; in development), which produces ultrashort pulses of up to 10 μJ at 1030 nm with variable pulse width (200 fs-10 ps) and repetition rate (200 kHz-25 MHz). The output pulses pass through two successive nonlinear crystals to be frequency doubled (515 nm) and tripled (343 nm). The harmonics are separated from the residual infrared radiation (IR) beam by dichroic mirrors, and the frequency-tripled pulses are introduced to the photocathode of the microscope for generating the electron pulse train. The residual IR fundamental and frequency-doubled beams remain available to heat or excite samples and clock the time through a computer-controlled optical delay line for time-resolved applications. The electron microscope column is that of a designed hybrid 200-kV TEM (Tecnai 20, FEI) integrated with two ports for optical access, one leading to the photocathode and the other to the specimen. The field emission gun (FEG) in the electron-generation assembly adapts a lanthanum hexaboride (LaB6) filament as the cathode, terminating in a conical electron source truncated to leave a flat tip area with a diameter of 16 μm. The tip is located in a field environment controlled by suppressor and extractor electrodes. The gun can be operated as either a thermal emission or a photoemission source. The optical pulses are guided to the photocathode as well as to the specimen by a computer-controlled, fine-steering mirror in an externally-mounted and x-ray-shielded periscope assembly. Each laser beam can be focused to a spot size of <30 μm full width at half maximum (FWHM) at its respective target when the beam is expanded to utilize the available acceptance angle of the optical path. Various pulse-energy, pulse-length, and focusing regimes have been used in the measurements reported here. For UEM measurements, the cathode was heated to a level below that needed to produce detectible thermal emission, as detailed below, and images were obtained using both the TEM and the UEM2 mode of operation. For applications involving EELS and energy-filtered-transmission-electron microscopy (EFTEM), the Gatan Imaging Filter (GIF) Tridiem, of the so-called post-column type, was attached below the camera chamber. The GIF accepts electrons passing through an entrance aperture in the center of the projection chamber. The electron beam passes through a 90° sector magnet as shown in FIG. 2A, which bends the primary beam through a 10 cm bending radius and thereby separates the electrons according to their energy into an energy spectrum. An energy resolution of 0.87 eV was measured for the EELS zero-loss peak in thermal mode operation of the TEM. A retractable slit is located after the magnet followed by a series of lenses. The lenses restore the image or diffraction pattern at the entrance aperture and finally it can be recorded on a charge-coupled device (CCD) camera (UltraScan 1000 FT) at the end of the GIF with the Digital Micrograph software. The digital camera uses a 2,048×2,048 pixel CCD chip with 14 μm square pixels. Readout of the CCD is done as four independent quadrants via four separate digitizing signal chains. This 4-port readout camera combines single-electron sensitivity and 16-bit pixel depth with high-speed sensor readout (4 Mpix/s). Additionally, for scanning-transmission-electron microscopy (STEM), the UEM2 is equipped with a bright-field (BF) detector with a diameter of 7 mm and an annular dark-field (ADF) detector with an inner diameter of 7 mm and an outer diameter of 20 mm. Both detectors are located in the near-axis position underneath the projection chamber. The BF detector usually collects the same signal as the TEM BF image, i.e., the transmitted electrons, while the ADF detector collects an annulus at higher angle where only scattered electrons are detected. The STEM images are recorded with the Tecnai Imaging & Analysis (TIA) software. To observe the diffraction pattern, i.e., the back focal plane of the objective lens, we inserted a selected area aperture into the image plane of the objective lens, thus creating a virtual aperture in the plane of the specimen. The result is a selected area diffraction (SAD) pattern of the region of interest only. Adjustment of the intermediate and projector lens determines the camera length. Diffraction patterns are processed and analyzed for crystal structure determination. Several features of the UEM2 system are worthy of note. First, the high repetition rate amplified laser source allows us to illuminate the cathode with 343 nm pulses of energies above 500 nJ, compared with typical values of 3 nJ near 380 nm for UEM1. Thus, a level of average optical power for electron generation comparable to that of UEM1 operating at 80 MHz, but at much lower repetition rates, was able to be delivered. The pulse energy available in the visible and IR beams is also at least two orders of magnitude greater than for UEM1, allowing for exploration of a much greater range in the choice of sample excitation conditions. Second, the hybrid 200-kV FEG, incorporating an extractor/suppressor assembly providing an extractor potential of up to 4 kV, allows higher resolving power and greater flexibility and control of the conditions of electron generation. Third, with simple variation of optical pulse width, the temporal and spatial resolution can be controlled, depending on the requirements of each experiment. Fourth, with variation of spacing between optical pulses without loss of pulse energy, a wide range of samples can be explored allowing them to fully relax their energy after each excitation pulse and rewind the clock precisely; with enough electrons, below the space-charge limit, single-pulse recording is possible. Finally, by the integration of the EELS spectrometer, the system is empowered with energy resolution in addition to the ultrafast time resolution and atomic-scale space resolution. The following results demonstrate the capabilities of UEM2 in three areas: real-space imaging, diffraction, and electron energy resolution. Applications of the present invention are not limited to these particular examples. First discussed are the images recorded in the UEM mode, of gold particles and gold islands on carbon films. FIGS. 2Ba-f are UEM2 images obtained with ultrafast electron pulses. Shown are gold particles (a, d) and gold islands (c, f) on carbon films. UEM2 background images (b, e) obtained by blocking the photoelectron-extracting femtosecond laser pulses. For the UEM2 images of gold particles, we used the objective (contrast) aperture of 40 μm to eliminate diffracted beams, while no objective aperture was used for the gold-island images. FIGS. 2Ba and 2Bd show gold particles of uniform size dispersed on a carbon film. From the higher magnification image of FIG. 2Bd, corresponding to the area indicated by the black arrow in FIG. 2Ba, it is found that the gold particles have a size of 15 nm, and the minimum particle separation seen in the image is 3 nm. It should be noted that FIGS. 2Bb and 2Be were recorded under identical conditions to FIGS. 2Ba and 2Bd, respectively, but without cathode irradiation by the femtosecond laser pulses. No images were observed, demonstrating that non-optically generated electrons from our warm cathode were negligible. Similar background images with the light pulses blocked were routinely recorded and checked for all cathode conditions used in this study. The waffle (cross line) spacing of the cross grating replica (gold islands) seen in FIG. 2Bc is known to be 463 nm. The gold islands are observed in FIG. 2Bf, where the bright regions correspond to the amorphous carbon support film and the dark regions to the nanocrystalline gold islands. It is found that the islands may be interconnected or isolated, depending on the volume fraction of the nanocrystalline phases. To test the high-resolution capability of UEM utilizing phase contrast imaging, an organometallic compound, chlorinated copper phthalocyanine (hexadecachlorophthalocyanine, C32Cl16CuN8), was investigated. The major spacings of lattice fringes of copper of this molecule in projection along the c-axis are known to be 0.88, 1.30, and 1.46 nm, with atomic spacings of 1.57 and 1.76 nm. FIGS. 2Ca-b are high-resolution, phase-contrast UEM images. Shown are an image in FIG. 2Ca and digital diffractogram in FIG. 2Cb of an organometallic crystal of chlorinated copper phthalocyanine. The diffractogram was obtained by the Fourier transform of the image in FIG. 2Ca. The high-resolution image was taken near the Scherzer focus for optimum contrast, which was calculated to be 90.36 nm for a spherical aberration coefficient Cs of the objective lens of 2.26 mm. The objective aperture was not used. FIG. 2Da exhibits the lattice fringes observed by UEM, where the black lines correspond to copper layers parallel to the c-axis. The Fourier transform of FIG. 2Da is shown in FIG. 2Db, discussed below, and the clear reciprocity (without satellite peaks in the F.T.) indicates the high degree of order in crystal structure. FIG. 2D shows high-resolution, phase-contrast UEM image and structure of chlorinated copper phthalocyanine. The high-resolution image shown in FIG. 2Da is a magnified view of the outlined area in FIG. 2Ca. The representation of the crystal structure shown in FIG. 2Db is shown in projection along the c axis, and the assignment of the copper planes observed in FIG. 2Da is indicated by the gray lines. The spheres are the copper atoms. FIG. 2Da is an enlargement of the area outlined in FIG. 2Ca, clearly showing the lattice fringe spacing of 1.46 nm, corresponding to the copper planes highlighted in gray in FIG. 2Db, in which a unit cell is shown in projection along the c-axis. Regions without lattice fringes are considered to correspond to crystals with unfavorable orientation, or amorphous phases of phthalocyanine, or the carbon substrate. It is known that in high resolution images, the lattice fringes produced by the interference of two waves passing through the back focal plane, i.e., the transmitted and diffracted beams, are observed only in crystals where the lattice spacing is larger than the resolution of the TEM. In the profile inset of FIG. 2Da, it should be noted that the FWHM was measured to be approximately 7 Å, directly indicating that our UEM has the capability of sub-nanometer resolution. The digital diffractogram obtained by the Fourier transform of the observed high-resolution image of FIG. 2Ca is shown in FIG. 2Cb. In the digital diffractogram, the peaks represent the fundamental spatial frequency of the copper layers (0.69 nm−1), and higher harmonics thereof. A more powerful means of obtaining reciprocal-space information such as this is the direct recording of electron diffraction, also available in UEM. FIGS. 2Ea-f show measured and calculated electron diffraction patterns of gold islands and boron nitride (BN) on carbon films, along with the corresponding real-space images of each specimen, all recorded by UEM. Shown are images and measured and calculated electron diffraction patterns of gold islands (a,b,c) and boron nitride (BN) (d,e,f) on carbon films. The incident electron beam is parallel to the [001] direction of the BN. All diffraction patterns were obtained by using the selected-area diffraction (SAD) aperture, which selected an area 6 μm in diameter on the specimen. Representative diffraction spots were indexed as indicated by the arrowheads. In FIG. 2Eb, the electron diffraction patterns exhibit Debye-Scherrer rings formed by numerous diffraction spots from a large number of face-centered gold nanocrystals with random orientations. The rings can be indexed as indicated by the white arrowheads. The diffraction pattern of BN in FIG. 2Ee is indexed by the hexagonal structure projected along the [001] axis as shown in FIG. 2Ef. It can be seen that there are several BN crystals with different crystal orientations, besides that responsible for the main diffraction spots indicated by the white arrowheads. In order to explore the energy resolution of UEM, we investigated the BN specimen in detail by EELS and EFTEM. FIG. 2F shows energy-filtered UEM images and spectrum. FIG. 2F shows a zero-loss filtered image (FIG. 2Fa), boron K-edge mapping image (FIG. 2Fb), thickness mapping image (FIG. 2Fc), and corresponding electron-energy-loss (EEL) spectrum (FIG. 2Fd) of the boron nitride (BN) sample. The 5.0- and 1.0-mm entrance aperture were used for mapping images and EEL spectrum, respectively. The thickness at the point indicated by the asterisk in FIG. 2Fc is estimated to be 41 nm. ZL stands for zero-loss. The boron map was obtained by the so-called three-window method. In the boron map of FIG. 2Fb, image intensity is directly related to areal density of boron. In the thickness map of FIG. 2Fc, the brightness increases with increasing thickness: d (thickness)=λ(β)ln(It/I0), where λ is the mean free path for inelastic scattering under a given collection angle β, I0 is the zero-loss (ZL) peak intensity, and It is the total intensity. The thickness in the region indicated by the asterisk in FIG. 2Fc was estimated to be 41 nm. In the EEL spectrum of FIG. 2Fd, the boron K-edge, carbon K-edge, and nitrogen K-edge are observed at the energy of 188, 284, and 401 eV, respectively. In the boron K-edge spectrum, sharp π* and σ* peaks are visible. The carbon K-edge spectrum is considered to result from the amorphous carbon film due to the existence of small and broad peaks at the position π* and σ*, being quite different from spectra of diamond and graphite. With the capabilities of the UEM2 system described herein, structural dynamics can be studied, as with UEM1, but with the new energy and spatial resolution are achieved here. Specimens will be excited in a T-jump or electronic excitation by the femtosecond laser pulses (FIG. 2A) scanned in time with respect to the electron packets which will probe the changes induced in material properties through diffraction, imaging, or electron energy loss in different regions, including that of Compton scattering. Also planned to be explored is the STEM feature in UEM, particularly the annular dark-field imaging, in which compositional changes are evident in the contrast (Z contrast). Such images are known to offer advantages over high-resolution TEM (relative insensitivity to focusing errors and ease of interpretation). Electron fluxes will be optimized either through changes of the impinging pulse fluence or by designing new photocathode materials. In this regard, with higher brightness the sub-angstrom limit should be able to be reached. The potential for applications in materials and biological research is rich. FIG. 3 is a simplified diagram of a computer system 310 that is used to oversee the system of FIGS. 1 and 2 according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize many other modifications, alternatives, and variations. As shown, the computer system 310 includes display device 320, display screen 330, cabinet 340, keyboard 350, and mouse 370. Mouse 370 and keyboard 350 are representative “user input devices.” Mouse 370 includes buttons 380 for selection of buttons on a graphical user interface device. Other examples of user input devices are a touch screen, light pen, track ball, data glove, microphone, and so forth. The system is merely representative of but one type of system for embodying the present invention. It will be readily apparent to one of ordinary skill in the art that many system types and configurations are suitable for use in conjunction with the present invention. In a preferred embodiment, computer system 310 includes a Pentium™ class based computer, running Windows™ NT, XP, or Vista operating system by Microsoft Corporation. However, the system is easily adapted to other operating systems such as any open source system and architectures by those of ordinary skill in the art without departing from the scope of the present invention. As noted, mouse 370 can have one or more buttons such as buttons 380. Cabinet 340 houses familiar computer components such as disk drives, a processor, storage device, etc. Storage devices include, but are not limited to, disk drives, magnetic tape, solid-state memory, bubble memory, etc. Cabinet 340 can include additional hardware such as input/output (I/O) interface cards for connecting computer system 310 to external devices external storage, other computers or additional peripherals, which are further described below. FIG. 4 is a more detailed diagram of hardware elements in the computer system of FIG. 3 according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize many other modifications, alternatives, and variations. As shown, basic subsystems are included in computer system 310. In specific embodiments, the subsystems are interconnected via a system bus 375. Additional subsystems such as a printer 374, keyboard 378, fixed disk 379, monitor 376, which is coupled to display adapter 382, and others are shown. Peripherals and input/output (I/O) devices, which couple to I/O controller 371, can be connected to the computer system by any number of means known in the art, such as serial port 377. For example, serial port 377 can be used to connect the computer system to a modem 381, which in turn connects to a wide area network such as the Internet, a mouse input device, or a scanner. The interconnection via system bus allows central processor 373 to communicate with each subsystem and to control the execution of instructions from system memory 372 or the fixed disk 379, as well as the exchange of information between subsystems. Other arrangements of subsystems and interconnections are readily achievable by those of ordinary skill in the art. System memory, and the fixed disk are examples of tangible media for storage of computer programs, other types of tangible media include floppy disks, removable hard disks, optical storage media such as CD-ROMS and bar codes, and semiconductor memories such as flash memory, read-only-memories (ROM), and battery backed memory. Although the above has been illustrated in terms of specific hardware features, it would be recognized that many variations, alternatives, and modifications can exist. For example, any of the hardware features can be further combined, or even separated. The features can also be implemented, in part, through software or a combination of hardware and software. The hardware and software can be further integrated or less integrated depending upon the application. Further details of the functionality, which may be carried out using a combination of hardware and/or software elements, of the present invention can be outlined below according to the figures. Embodiments of the present invention enable ultrafast imaging with applications in studies of structural and morphological changes in single-crystal gold and graphite films, which exhibit entirely different dynamics, as discussed below. For both, the changes were initiated by in situ femtosecond impulsive heating, while image frames and diffraction patterns were recorded in the microscope at well-defined times following the temperature-jump. The time axis in the microscope is independent of the response time of the detector, and it is established using a variable delay-line arrangement; a 1-μm change in optical path of the initiating (clocking) pulse corresponds to a time step of 3.3 fs. FIG. 5 illustrates both time-resolved images and diffraction. In this example, the images in FIGS. 5A and 5B were obtained stroboscopically at several time delays after heating with the fs pulse (fluence of 1.7 mJ/cm2). The specimen is a gold single crystal film mounted on a standard 3-mm 400-mesh grid. Shown are the bend contours (dark bands), {111} twins (sharp straight white lines) and holes in the sample (bright white circles). The insets in FIG. 5B are image-difference frames Im(tref; t) with respect to the image taken at −84 ps. The gold thickness was determined to be 8 nm by electron energy loss spectroscopy (EELS). FIG. 5C illustrates the time dependence of image cross-correlations of the full image from four independent scans taken with different time steps. A fit to biexponential rise of the 1 ps step scan is drawn, yielding time constants of 90 ps and 1 ns. FIG. 5D illustrates the time dependence of image cross-correlations at 1 ps time steps for the full image and for selected regions of interest SAI #1, #2, and #3, as shown in FIG. 5A. FIGS. 5E and 5F are diffraction patterns obtained using a single pulse of 6×106 electrons at high peak fluence (40 mJ/cm2) and selected-area aperture of 25 μm diameter. Two frames are given to indicate the change. Diffraction spots were indexed and representative indices are shown as discussed below. FIGS. 5A and 5B illustrate representative time-framed images of the gold nanocrystal using the fs excitation pulses at a repetition rate of 200 kHz and peak excitation fluence of ˜1.7 mJ/cm2. In FIG. 5A, taken at −84 ps, before the clocking pulse (t=0), typical characteristic features of the single crystal gold in the image are observed: twins and bend contours. Bend contours, which appear as broad fuzzy dark lines in the image, are diffraction contrast effects occurring in warped or buckled samples of constant thickness. In the dark regions, the zone axis (the crystal [100]) is well aligned with the incident electron beam and electrons are scattered efficiently, whereas in the lighter regions the alignment of the zone axis deviates more and the scattering efficiency is lower. Because bend contours generally move when deformation causes tilting of the local crystal lattice, they provide in images a sensitive visual indicator of the occurrence of such deformations. At positive times, following t=0, visual dynamical changes are observed in the bend contours with time steps from 0.5 ps to 50 ps. A series of such image frames with equal time steps provide a movie of the morphological dynamics. To more clearly display the temporal evolution, image-difference frames were constructed. Depicted as insets in the images of FIG. 5B, are those obtained when referencing to the −84 ps frame; for t=+66 ps and +151 ps. In the difference images, the regions of white or black directly indicate locations of surface morphology change (bend contour movement), while gray regions are areas where the contrast is unchanged from that of the reference frame. It is noted that the white and black features in the difference images are nm-scale dynamical change, indicating the size of the induced deformations. Care was taken to insure the absence of long-term specimen drifts as they can cause apparent contrast change. To quantify the changes in the image the following method of cross-correlation was used. The normalized cross correlation of an image at time t with respect to that at time t′ is expressed as: γ ( t ) = ∑ x , y C x , y ( t ) C x , y ( t ′ ) ∑ x , y C x , y ( t ) 2 ∑ x , y C x , y ( t ′ ) 2 where the contrast Cx,y(t)=[Ix,y(t)−Ī(t)]/Ī(t); Ix,y(t) and Ix,y(t′) are the intensities of pixels at the position of (x,y) at times t and t′, and Ī(t) and Ī(t′) are the means of Ix,y(t) and Ix,y(t′), respectively. This correlation coefficient γ(t) is a measure of the temporal change in “relief pattern” between the two images being compared, which can be used as a guide to image dynamics as a function of time. Two types of cross-correlation plots were made, those referenced to a fixed image frame before t=0 and others that show correlation between adjacent time points. (Another quantity that shows time dependence qualitatively similar to that of the image cross-correlation is the standard deviation of pixel intensity in difference images). FIGS. 5C and 5D show the cross-correlation values between the image at each measured time point and a reference image recorded before the arrival of the clocking pulse. The experiments were repeated, for different time-delay steps (500 fs, 1 ps, 5 ps, and 50 ps), and similar results were obtained, showing that morphology changes are completely reversible and reproducible over each 5 μs inter-pulse interval. The adjacent-time cross-correlations reveal the timescales for intrinsic changes in the images, which disappear for time steps below 5 ps, consistent with full-image rise in time. Over all pixels, the time scale for image change covers the full range of time delay, from ps to ns, indicating the collective averaging over sites of the specimen; as shown in FIG. 5C the overall response can be fit to two time constants of 90 ps and 1 ns. The power of selected area image dynamics (SAID) is illustrated when the dynamics of the bend contours are followed in different selected areas of the image, noted in the micrographs as SAI #1, 2, and 3. The corresponding image cross-correlations (FIG. 5D) have different shape and amplitude from each other and from the full image correlation. The large differences observed here and for other data sets, including onsets delayed in time and sign reversals, indicate the variation in local deformation dynamics. In FIGS. 5G-L, a time-resolved SAI at higher magnification is depicted. A broad and black “penguin-like” contour is observed as the dominant feature of this area. As shown in the frames, a colossal response to the fs heating is noted. The gray region inside the black contour appears and broadens with time. Also, a new black contour above the large central white hole begins to be evident at 1200 ps, and gains substantial intensity over the following 50 ps. All frames taken can be used to construct a movie of SAID. The observed SAID changes correspond to diffraction contrast (bright-field) effects in bend contours, as mentioned above. It is known that the shape of bend contours can be easily altered by sample tilting or heating inside the microscope. However, here in the ultrafast electron microscope (UEM) measurements, the changes in local tilt are transient in nature, reflecting the temporal changes of morphology and structure. Indeed, when the experiments were repeated in the TEM mode of operation, i.e., for the same heating laser pulse and same scanning time but with continuous electron probe beam, no image change was observed. This is further supported by the change in diffraction observed at high fluences and shown in FIGS. 5E and 5F for two frames, at negative time and at +50 ns; in the latter, additional Bragg spots are visible, a direct evidence of the transient structural change due to bulging at longer times. Whereas real-space imaging shows the time-dependent morphology, the selected area diffraction dynamics (SADD) patterns provide structural changes on the ultrashort timescale. Because the surface normal of the film is parallel to the [100] zone axis, the diffraction pattern of the sample was properly indexed by the face-centered-cubic (fcc) structure projected along the [100] zone axis at zero tilt angle (see FIG. 5E). From the positions of the spots in FIG. 5F, which are reflections from the {113} and {133} planes, forbidden in the [100] zone-axis viewing, we measured the interplanar spacings to be 1.248 and 0.951 Å, respectively. With selected area diffraction, Bragg peak separations, amplitudes, and widths were obtained as a function of time. The results indicate different timescales from those of image dynamics. FIG. 6A illustrates structural dynamics and heat dissipation in gold and FIG. 6B illustrates coherent resonance of graphite. Referring to FIG. 6A, SADD for fs excitation at 1.7 mJ/cm2 peak fluence (519 nm) is illustrated. The Bragg separation for all peaks and the amplitude of the {042} peaks are shown in the main panel; the inset gives the 2.2 μs recovery (by cooling) of the structure obtained by stroboscopic ns excitation at 7 mJ/cm2. The peak amplitude has been normalized to the transmitted beam amplitude, and the time dependence of amplitude and separation is fit as an exponential rise, and a delay with rise, respectively. Referring to FIG. 6B, resonance oscillations are observed for the Bragg (1 22) peak in the diffraction pattern of graphite; the amplitudes are similar in magnitude to those in FIG. 6A. The sample was tilted at 21° angle to the microscope axis and the diffraction pattern was obtained by using the SAD aperture of 6 μm diameter on the specimen. The graphite thickness is 69 nm as determined by EELS; the oscillation period (τp) is measured to be 56.3 ps. For a thickness of 45 nm, the period is found to be τp=35.4 ps. FIGS. 6D-G illustrate, for selected areas, time dependence of intensity difference (dark-field) for graphite. The image change displays the oscillatory behavior with the same τp as that of diffraction. The dark-field (DF) images were obtained by selecting the Bragg (1 22) peak. In FIG. 6H, each line corresponds to the difference in image intensities, Im(t−30 ps; t), for selected areas of 1×100-pixel slices parallel to contrast fringes in the DF image. The average amplitude of {042} diffraction peaks drop significantly; the rise time is 12.9 ps, whereas the change in separations of all planes is delayed by 31 ps and rises in 60 ps. The delay in the onset of separation change with respect to amplitude change is similar to the timescale for the amplitude to reach its plateau value of 15% reduction in the case of the {042} amplitude shown. In order to determine the recovery time of the structure, we carried out stroboscopic (and also single-pulse) experiments over the timescale of microseconds. The recovery transient in the inset of FIG. 6A (at 7 mJ/cm2) gives a time constant of 2.2 μs; we made calculations of 2D lateral heat transport with thermal conductivity (λ=3.17 W/(cm K) at 300 K) and reproduced the observed timescale. For this fluence, the maximum lattice spacing change of 0.08% gives the temperature increase ΔT to be 60 K, knowing the thermal expansion coefficient of gold (α=14.2×10−6 K−1). The atomic-scale motions, which lead to structural and morphological changes, can now be elucidated. Because the specimen is nanoscale in thickness, the initial temperature induced is essentially uniform across the atomic layers and heat can only dissipate laterally. It is known that for metals the lattice temperature is acquired following the large increase in electron temperature. The results in FIG. 6A give the temperature rise to be 13 ps; from the known electron and lattice heat-capacity constants [C1=70 J/(m3 K2) and C2=2.5×106 J/(m3 K), respectively] and the electron-phonon coupling [g=2×1016 W/(m3 K)] we obtained the initial heating time to be ˜10 ps for electron temperature T1=2500 K, in good agreement with the observed rise. Reflectivity measurements do not provide structural information, but they give the temperature rise. For bulk material, the timescale for heating (˜1 ps) is shorter than that of the nano-scale specimen (˜10 ps), due to confinement in the latter, which limits the ballistic motion of electrons in the specimen, and this is evident in the UEM studies. Because the plane separation is 0.4078 nm, the change of the average peak separation (0.043%), at the fluence of 1.7 mJ/cm2, gives a lattice constant change of 0.17 pm. Up to 30 ps the lattice is hot but, because of macroscopic lattice constraint, the atomic stress cannot lead to changes in lateral separations, which are the only separations visible for the [100] zone-axis probing. However, the morphology warping change is correlated with atomic (lateral) displacements in the structure as it relieves the structural constraint. Indeed the time scale of the initial image change is similar to that of plane separations in diffraction (60-90 ps). This initial warping, which changes image contrast, is followed by longer time (ns) minimization of surface energy and bulging, as shown in FIG. 5D. Given the picometer-scale structural change (0.17 pm), the stress over the 8-nanometer specimen gives the total expansion to be 3.4 pm over the whole thickness. Considering the influence of lateral expansion, the maximum bulge reaches 1 to 10 nm depending on the lateral scale. Finally, the calculated Debye-Waller factor for structural changes gives a temperature of 420 K (ΔT=125 K), in excellent agreement with lattice temperature derived under similar conditions, noting that for the nanoscale material the temperature is higher than in the bulk. Graphite was another study in the application of the UEM methodology. In contrast to the dynamics of gold, in graphite, because of its unique 2D structure and physical properties, we observed coherent resonance modulations in the image and also in diffraction. The damped resonance of very high frequency, as shown below, has its origin in the nanoscale dimension of the specimen and its elasticity. The initial fs pulse induces an impulsive stress in the film and the ultrafast electron tracks the change of the transient structure, both in SAID and SADD. In FIG. 6B, the results obtained by measuring changes of the diffraction spot (1 22) are displayed and in FIGS. 6D-G those obtained by dark-field (DF) imaging with the same diffraction spot being selected by the objective aperture and the specimen tilted, as discussed below. For both the image and diffraction, a strong oscillatory behavior is evident, with a well defined period and decaying envelope. When the transients were fitted to a damped resonance function [(cos 2πt/τp)exp(−t/τdecay)], we obtained τp=56.3±1 ps for the period. The decay of the envelope for this particular resonance is significantly longer, τdecay=280 ps. This coherent transient decay, when Fourier transformed, indicates that the length distribution of the film is only ±2 nm as discussed in relation to the equation below. The thickness of the film was determined (L=69 nm) using electron energy loss spectra (EELS). In order to test the validity of this resonance behavior we repeated the experiments for another thickness, L=45 nm. The period indeed scaled with L, giving τp=35.4 ps. These, hitherto unobserved, very high frequency resonances (30 gigahertz range) are unique to the nanoscale length of graphite. They also reflect the (harmonic) motions due to strain along the c-axis direction, because they were not observed when we repeated the experiment for the electron to be along the [001] zone axis. The fact that the period in the image is the same as that of the diffraction indicates the direct correlation between local atomic structure and macroscopic elastic behavior. Following a fs pulse of stress on a freely vibrating nanofilm, the observed oscillations, because of their well-defined periods, are related to the velocity (C) of acoustic waves between specimen boundaries, which in turn can be related to Young's modulus (Y) of the elastic stress-strain profile: 1 τ p = nC 2 L = n 2 L ( Y ρ ) 1 / 2 ,where n is a positive integer, with n=1 being the fundamental resonance frequency (higher n are for overtones). Knowing the measured τp and L, we obtained C=2.5×105 cm/s. For graphite with the density ρ=2.26 g/cm3, Y=14.6 gigapascal for the c-axis strain in the natural specimen examined. Pyrolytic graphite has Y values that range from about 10 to 1000 gigapascal depending on the orientation, reaching the lowest value in bulk graphite and the highest one for graphene. The real-time measurements reported here can now be extended to different length scales, specimens of different density of dislocations, and orientations, exploring their influence at the nanoscale on C, Y, and other properties. We note that selected-area imaging was critical as different regions have temporally different amplitudes and phases across the image. Uniting the power of spatial resolution of EM with the ultrafast electron timing in UEM provides an enormous advantage when seeking to unravel the elementary dynamics of structural and morphological changes. With total dissipation of specimen heat between pulses, selected-area dynamics make it possible to study the changes in seconds of recording and for selected pixels of the image. In the applications given here, for both gold and graphite, the difference in timescales for the nonequilibrium temperature (reaching 1013 K/s), the structural (pm scale) and morphological (nm scale) changes, and the ultrafast coherent (resonance) behavior (tens of gigahertz frequency) of materials structure illustrate the potential for other applications, especially when incorporating the different and valuable variants of electron microscopy as we have in our UEM. Embodiments of the present invention extend ultrafast 4D diffraction and microscopy to the attosecond regime. As described herein, embodiments use attosecond electron diffraction to observe attosecond electron motion. Pulses are freely generated, compressed, and tilted. The approach can be implemented to extend previous techniques including, for example phase transformations, chemical reactions, nano-mechanical processes, and surface dynamics, and possibly to other studies of melting processes, coherent phonons, gold particles, and molecular alignment. As described herein, the generation of attosecond resolution pulses and in situ probing through imaging with free electrons. Attosecond diffraction uses near mono-energetic attosecond electron pulses for keV-range of energies in free space and thus space charge effects are considered. Additionally, spatiotemporal synchronization of the electron pulses to the pump pulses is made along the entire sample area and with attosecond precision. Diffraction orders are shown to be sensitive to the effect of electron displacement and conclusive of the four-dimensional dynamics. A component of reaching attosecond resolution with electron diffraction is the generation of attosecond electron pulses in “free space,” so that diffraction from freely chosen samples of interest can take place independent of the mechanisms of pulse generation. Electrons with energies of 30-300 keV are ideal for imaging and diffraction, because of their high scattering cross sections, convenient diffraction angles, and the appropriate de Broglie wavelength (0.02 to 0.07 Å) to resolve atomic-scale changes. Moreover, they have a high degree of monochromaticity. For example, electrons accelerated to E0=30-300 keV with pulse duration of 20 attoseconds (bandwidth of ΔE≈30 eV) have ΔE/E0≈10−3−10−4, making diffraction and imaging possible without a spread in angle and resolution. Optical attosecond pulses have typically ΔE/E0≈0.5 and because of this reach of ΔE to E0, their duration is Fourier-limited to ˜100 attoseconds. Free electron pulses of keV central energy can, in principle, have much shorter duration, down to sub-attoseconds, while still consisting of many wave cycles. Pulses with a large number of electrons suffer from the effect of space charge, which determines both the spatial and the temporal resolutions. This can be avoided by using packets of single, or only a few, electrons in a high repetition rate, as demonstrated in 4D microscopy imaging. FIG. 7A depicts the relation of single electron packets to the effective envelope due to statistics. Each single electron (blue) is a coherent packet consisting of many cycles of the de Broglie wave and has different timing due to the statistics of generation. On average, multiple single electron packets form an effective electron pulse (dotted envelope). It will be appreciated that there is high dispersion for electrons of nonrelativistic energy. The small but unavoidable bandwidth of an attosecond electron pulse causes the pulse to disperse during propagation in free space, even when no space charge forces are present. For example, a 20-attosecond pulse with ΔE/E0≈10−3 would stretch to picoseconds after just a few centimeters of propagation. Embodiments of the present invention provide methods and systems for the suppression of dispersion and the generation of free attosecond electron pulses based on the initial preparation of negatively-chirped electron packets. As described herein, femtosecond electron pulses are generated by photoemission and accelerated to keV energies in a static electric field. Preceding the experimental interaction region, optical fields are used to generate electron packets with a velocity distribution, such that the higher-energy parts are located behind the lower-energy ones. With a proper adjustment of this chirp, the pulse then self-compresses to extremely short durations while propagating towards the point of diffraction. To achieve attosecond pulses, the chirp must be imprinted to the electron pulse on a nanometer length scale. Optical waves provide such fields. However, non-relativistic electrons move significantly slower than the speed of light (e.g. ˜0.3 c for 30 keV). The direct interaction with an optical field will, therefore, cancel out over time and can not be used to accelerate and decelerate electrons for compression. In order to overcome this limitation, we make use of the ponderomotive force, which is proportional to the gradient of the optical intensity to accelerate electrons out of regions with high intensity. By optical wave synthesis, intensity profiles can be made that propagate with less than the speed of light and, therefore, allow for co-propagation with the electrons. FIG. 7B illustrates a schematic of attosecond pulse generation according to an embodiment of the present invention. A synthesized optical field of two counter-propagating waves of different wavelengths results in an effective intensity grating, similar to a standing wave, which moves with a speed slower than the speed of light. Electrons can, therefore, co-propagate with a matched speed and are accelerated or decelerated by the ponderomotive force according to their position within the wave. After the optical fields have faded away, this velocity distribution results in self-compression; the attosecond pulses are formed in free space. Depending on the optical pulse intensity, the electron pulse duration can be made as short as 15 attoseconds, and, in principle, shorter durations are achievable. If the longitudinal spatial width of the initial electron pulse is longer than the wavelength of the intensity grating, multiple attosecond pulses emerge that are located with well-defined spacing at the optical minima. This concept of compression can be rigorously described analytically as a “temporal lens effect.” The temporal version of the Kapitza-Dirac effect has an interesting analogy. Some of our initial work was based on an effective ponderomotive force in a collinear geometry. In order to extend the approach to more complex arrangements, here we generalize the approach and consider the full spatiotemporal (electric and magnetic) fields of two colliding laser waves with an arbitrary angle and polarization. The transversal and longitudinal fields of a Gaussian focus were applied. We simulated electron trajectories by applying the Lorentz force with a fourth-order Runge-Kutta algorithm using steps of 100 attoseconds. Space charge effects were taken into account by calculating the Coulomb interactions between all single electrons for each time step (N-body simulations). FIG. 7B illustrates temporal optical gratings for the generation of free attosecond electron pulses for use in diffraction. (a) A femtosecond electron packet (blue) is made to co-propagate with a moving optical intensity grating (red). (b) The ponderomotive force pushes electron towards the minima and thus creates a temporal lens. (c) The induced electron chirp leads to compression to attosecond duration at later time. (d) The electron pulse duration from 105 trajectories reaches into the domain of few attoseconds. FIG. 7C depicts the compression of single electron packets in the combined field of two counter-propagating laser pulses with durations of 300 fs at wavelengths of 1040 nm and 520 nm. The pulse is shown just before, at, and after the time of best compression; the center along Z is shifted for clarity. The plotted pulse shape is a statistical average over 105 packets of single electrons. The beam diameter of the initial electron packet was 10 μm and the beam diameters of the laser pulses were 60 μm; the resulting compression dynamics is depicted before, just at, and some time after the time of best compression to a duration of 15 attoseconds (see FIG. 7C(b)). These results show that an optical wave with a beam diameter of only several times larger than that of the electron packet is sufficient to result in almost homogeneous compression along the entire electron beam. The characteristic longitudinal spread after the point of best compression, as depicted in FIG. 7C is the result of an “M”-shaped energy spectrum of the electrons after interactions with the sinusoidal intensity grating. Coulomb forces prevent concentration of a large number of electrons in a limited volume, and a compromise between electron flux and laser repetition rate must be found to achieve sufficiently intense diffraction. The laser pulses for compression have energies on the order of 5 μJ and can, therefore, be generated at MHz repetition rates with the resulting flux of 106 electrons/s, which is sufficient for conclusive diffraction. Nevertheless, having more than one electron per attosecond pulse is beneficial for improving the total flux. In order to investigate the influence of space charge on the performance in our attosecond compression scheme, we considered electron packets of increasing electron density and evaluated the resulting pulse durations and effective electron density per attosecond pulse. Two findings are relevant with the results shown in FIG. 8. First, the duration of individual attosecond electron pulses increases relatively insignificantly with the number of electrons contained within. Even for 40 electrons in a single pulse, the duration increases only from 15 to 25 attoseconds (see FIG. 8(a)). The reasons for this are the highly oblate shape of the electron pulses, and the approximate linearity of space charge forces in the longitudinal direction, which are compensated for by somewhat longer interaction in the ponderomotive forces of the optical waves. Secondly, for a train of pulses, there is an effect on synchronization. When the initial femtosecond electron packet covers several optical cycles of the compression wave, a train of attosecond pulses results as shown in FIG. 7B. Perfect synchronization to the optical wave is provided, because all attosecond pulses are located at the same optical phase of the fundamental laser wave. This phase matching relation, which permits attosecond resolution, despite the presence of multiple pulses, is altered under space charge conditions. The attosecond pulses repel each other and a temporal spreading of the comb-like train results. For a train of near 10 attosecond pulses, FIG. 8(b) displays the difference in timing for an adjacent attosecond pulse in relation to the central one, which is always locked to the optical phase because the space charge forces cancel out. The total timing mismatch is the product of the plotted value with the number of attosecond pulses in the entire electron packet (near 10 for this example). In order to keep the total mismatch to the optical wave below 20 attoseconds, 10 electrons per attosecond electron pulse represent an optimum value. The total pulse train then consists of 200 electrons for that group of pulses; of course the total flux of electrons is determined by the repetition rate. Note that mismatch to the compression wave is absent with isolated attosecond electron pulses, which are generated when the initial uncompressed electron packet is shorter than a few femtoseconds, or with optical fields of longer wavelength. Numerous imaging experiments have been successful with single electron packets. In state-of-the-art electron crystallography experiments, typically 500 electrons per pulse were used at a repetition rate of kHz to produce the needed diffraction. This is equivalent to having 5 electrons per attosecond pulse at 100 kHz, which is a convenient repetition rate for optical wave synthesis, and provides enough time for letting the material under investigation to cool back to the initial state. Laser systems with MHz repetition rates will provide even higher fluxes. Applications of attosecond electron pulses for diffraction and microscopy use synchronization of events in the pump-probe arrangement with an accuracy that is equal or better than the individual pulse durations. In contrast to recompression concepts that are based on time-dependent microwave fields, the application of laser waves for attosecond electron pulse generation provides exact temporal synchronization when the pump pulses are derived by phase-locking from the same laser system. Many common optical techniques, such as nonlinear frequency conversion, continuum generation in solids, or high-harmonic generation, all provide a phase lock in the sense that the outcome has the same relative phase and timing in relation to the incoming optical wave for each single pulse of the laser. A second requirement for reaching into the temporal resolution of attoseconds is the realization of spatial delay matching along extended areas of the diffraction. The use of large samples, for example with up to millimeters in size in some electron diffraction experiments, provides enhanced diffraction efficiency and offers the possibility to use electron beams with large diameter, in order to maximize the coherence and flux. In this case, the time resolution is limited by differences in the arrival times of pump and probe pulses at different points within the involved beam diameters (group velocity mismatch). Electron pulses at keV energies travel with significantly less than the speed of light (e.g. vel=0.3 c for 30 keV electrons) and are, therefore, “overtaken” by the laser wave. Embodiments of the present invention provide two arrangements for matching the group velocity of electrons with the phase velocity of optical pulses. Both arrangements are suitable for applications in noncollinear, ultrafast electron microscopy and diffraction. FIG. 9(a) presents a concept for the transmission geometry of diffraction and microscopy in which two angles are introduced, one between the laser beam and the electron beam (β), and another one (α) for the tilt angle of the sample (black) to the phase fronts of the laser wave. Total synchrony is achieved if the relative delay between the optical wave and the attosecond electron pulses is made identical for all points along the entire sample surface. Each small volume of the sample is then subject to an individual pump-probe-type experiment with the same time delay. The above condition is found when we match the coincidence along the entire width of the specimen. The effective surface velocity vsurface of the laser and of the electron pulses must be identical. From FIG. 9A, this requirement is expressed by the following equation: sin ( α ) sin ( α - β ) = c v el . ( 1 ) It follows that an angle of β=10°, for example, results in an optimum angle for the sample tilt of α=14.8°, which are both easily achievable angles in a real experiment. The effective tilt of the sample with respect to the electron direction is then α−β=4.8°. Naturally, if this value is not coincident with a zone axis direction, a complete rocking curve should be obtained in order to optimize α and β with tilt requirements. Although different portions of the laser wavefront impinge on the surface of the sample at different times, this behavior is matched by the electron pulse, resulting in all portions of the surface of the sample being phase matched. As illustrated in FIG. 9(a), the laser beam, also referred to as a laser wave, is used to activate the sample, for example, to heat the sample, cause motion of the sample, or to effect the chemical bonds present in the sample. The timing of the laser wave and the electron pulses are synchronized using the delay stage discussed in relation to FIG. 1. The train of electron pulses can be generated using the configuration illustrated in FIG. 10(a). Another option for synchronization along extended surfaces is the use of tilted electron pulses, for that the electron density makes an angle with respect to the propagation direction. Tilted optical pulses have been used for reaching femtosecond resolution in reflection geometry, but here tilted electron pulses are introduced for effective spatiotemporal synchronization to the phase velocity of the excitation pulses along the entire sample surface. FIG. 9B depicts the concept. If an angle γ is chosen between the laser (red) and the attosecond electron pulses (blue), the electron pulses need to be tilted likewise. The sample is located parallel to the optical phase fronts and its entire surface is illuminated by the attosecond electron pulse at once and at the same time of incidence relative to the optical pulse wave. Because the incidence is delay-free for all points along the surface, velocity matching is provided for the whole probed area. The generation of tilted attosecond electron pulses is outlined in FIG. 10(a). The introduction of an angle between the intensity grating (red) and the electron beam (blue) leads to formation of electron pulses with a tilt. As described above, a femtosecond electron packet (blue) is first generated by conventional photoelectron generation and accelerated in a static electric field. By intersecting the counter-propagating intensity grating at an angle, tilted electron pulses result with attosecond duration. The ponderomotive force accelerates the electrons towards the planes of destructive interference in the intensity wave and they form attosecond pulses that are compressed along the optical beam axis; but the pulses propagate in the original direction. Only a slight adjustment of the electrons' central energy is required to achieve phase matching to the moving optical grating. Based on this concept, we simulated the tilting effect by using 31-keV electron pulses with an initial duration of ˜15 femtoseconds and a spatial beam diameter of ˜10 μm. FIG. 10(b) illustrates the simulation results for an initial packet of 15-femtosecond duration (left) and an intersection angle of 5°. The tilted attosecond pulses have duration of ˜20 attoseconds when measured perpendicular to the tilt (note the different scale of Z and X). The optical intensity wave is synthesized by two counter-propagating laser pulses of 100-fs duration and wavelengths of 1040 and 520 nm. The angle between the electron beam and the laser wave is 5°. The results of compression are shown in FIG. 10(b): The attosecond electron pulses are formed at the minima of the optical intensity wave and, therefore, are tilted by 5° with respect to the electron propagation direction. For other incidence angles of the laser, the electron pulses are tilted accordingly. Perpendicular to the attosecond pulses, the measured duration is ˜20 attoseconds, given as the full width at half maximum. Based on the methodology for generation and synchronization of attosecond electron pulses described above, the diffraction and manifestation of electron dynamics in the patterns are described. By way of two different examples, embodiments of the present invention are utilized to observe electronic motions in molecules and materials with attosecond electron packets. We consider first the physics of electron scattering and the change in scattering factors which characterize individual atoms and the electron density involved. Diffraction from molecular crystals or other crystalline structures provides two distinct advantages over that obtained for gas phase ensembles. First, the sample density is many orders of magnitudes higher (1021 molecules/cm3 as compared to 1010 to 1016/cm3 in gas jets); diffraction is, therefore, more intense. Second, the crystalline order results in Bragg scatterings and they are concentrated into well-defined “spots” for ordered crystals; the patterns become rods for surfaces and narrow rings for amorphous substances. The diffraction results in intensities which are proportional to the square of the diffraction amplitude. As discussed below, coherence in diffraction is used in observing the changes of interest. The diffraction from molecular crystals, or other crystalline materials of interest, is defined by the summation over the contributions of all scatterers in a unit cell. The intensity I of a Bragg spot with the Miller indices (hkl) is determined by the positions (xyz) of the scatterers j in the unit cell: I ( hkl ) ∝ ∑ j f j exp [ - 2 π ⅈ ( hkl ) · ( xyz ) j ] 2 , ( 2 ) where fj are the atomic scattering factors. Electron diffraction is the result of Coulomb interaction between the incoming electrons and the potential formed by nuclei and electrons. The factors fj account for the effective scattering amplitude of atoms and are derived from quantum calculations that take into account the specific electron density distribution around the nuclei, including core electrons. The scattering we are considering here is the elastic one. In order to estimate the influence of electron dynamics on contributions to time-resolved diffraction patterns, we consider typical densities of electrons in chemical bonds, and the possible change. Static electron density maps show that typical covalent bonds consist of about one electron/Å3 and that this density is delocalized over volumes with diameters in the order of 1 Å. For estimating an effective scattering factor of such electron density, we consider a Gaussian sphere with a diameter of 1 Å, consisting of one electron. The electric potential is derived by Gauss' law and results in a radial dependence that is represented in FIG. 11, dotted line. The total scattering amplitude of free charges diverges at small angles, because of the long-range behavior of the potential. Since in real crystals the potential is localized in unit cells, we use a Gaussian distribution of the same magnitude in order to restrict the range to about ±1.5 Å. For potential of spherical symmetry, an effective scattering factor can be calculated from the radial potential Φ(r) according to f el ( s ) = 8 π 2 m e e h 2 ∫ 0 ∞ r 2 Φ ( r ) sin ( 4 π sr ) 4 π sr ⅆ r , ( 3 ) where s=sin(υ/2)/λel is the scattering parameter for a diffraction angle υ and λel is the de Broglie wavelength of the incident electrons. The result for our delocalized electron density is shown in FIG. 11(b); for comparison we plot also the tabulated scattering factor of neutral hydrogen. Both have comparable magnitude, which is expected because of their similar sizes. Here, we consider the iodine molecule as a model case and invoke the transition from a bonding to an anti-bonding orbital. The crystal structure of iodine consists of nearly perpendicular iodine pairs with a bond length of ˜2.7 Å. Two electrons contribute to the intramolecular σ bond; the intermolecular bond is weaker. FIG. 12 depicts the system under study and the two cases considered. The effect of antibonding excitation is made by comparing the Bragg intensities for the iodine structure, including the binding electrons, to a hypothetical iodine crystal consisting only of isolated atoms (see FIG. 12(a)). In Table 1, we give the results of the calculations following equation 2 with the values off tabulated for iodine atoms and from equation (3) for the electronic distribution changes. Despite the large difference in f of the iodine nuclei and the electron (about 50:1), the changes of Bragg spot intensity are significant, being on an order of 10-30%. TABLE 1Effects of Electron Motion on Selected Molecular Bragg SpotsMiller Indices (hkl)(a) ΔITransition(b) ΔIMovement(0.08 Å)100, 010, 001(forbidden)(forbidden)200, 400, 60000002−35%0020 (weak)+100% −17%40000040−18%+13%00400111+15% −2%331−20%+15% In column (a), the mMagnitude of Bragg spot intensity change ΔI of crystalline iodine as a result of bonding to antibonding transition is given. In column (b), the magnitude of Bragg spot intensity change as a result of field interaction with charge density, also in iodine. This large change is for two reasons. First, the bonding electrons are located in-between iodine atoms and contribute, therefore, strongly to the enhancement or suppression of all Bragg spots that project from the inter-atomic distances of the molecular units. Second, the large effect is result of the intrinsic “heterodyne detection” scheme of diffraction; the total intensity of a Bragg spot scales with the square of the coherent sum of individual contributions (see equation (2)). Although the total contribution to the intensity of a Bragg spot from bonding electrons is lower by a factor of several hundreds than the intensity contributions from the iodine atoms, the modulation is on the order of several percents as a result of the coherence of diffraction on a nanometer scale. Symmetry in the crystal is evident in the absence of change in certain Bragg orders. From measurements of the dynamics of multiple spots, it follows that electron density movies could be made. This is best achieved in an electron microscope in diffraction geometry; however conventional diffraction is also suitable to simultaneously monitor many Bragg spots and is advantageous for the study of ordered bulk materials. The example given is not far from an experimental observation made on a metal-to-insulator transition for which a σ*-type excitation was induced with a femtosecond pulse. As a second model case we consider the reaction of bonded electron density to external electric fields, such as the ones from laser fields. Depending on the restoring force and the resonance, an electron density will oscillate with the driving field in phase or with a phase delay. This charge oscillation re-radiates and is responsible for the refractive index of a dielectric material. In order to estimate the magnitude of charge displacement, we must take into account the polarizability, α, and the electric field strength, Elaser. In the limit of only one moving charge, the displacement D is approximately given by D ≈ α e E laser . The polarizability of molecular iodine along the bond is α≈130 ∈0Å3 (˜70 a.u.) in the static limit and a similar magnitude is expected for the crystal for optical frequencies away from the strong absorption bands; the anisotropy of polarizability indicates the role of the bonding electrons. With femtosecond laser pulses, a field of Elaser=109 V/m is possible for intensities below the damage threshold. With these parameters, one expects a charge displacement of D≈0.08 Å, or about 3% of the bond length. FIG. 12(b) is a schematic for the change in charge distribution by an electric field. We assume an active role of only the bonded electrons, and take the polarization of the laser field to be along the b axis of solid iodine. This axis is chosen because it has the least symmetry; a is perpendicular to the bonds. Table 1 gives the intensity changes of selected Bragg spots; the change is in the range of ±20% for some of the indices. The total energy delivered to the molecular system by the laser field is only on the order of 0.01 eV. Nevertheless the changes of charge displacements on sub-angstrom scales are evident. This marks a central advantage of electron diffraction over spectroscopic approaches, which require large energy changes in order to have projections on dynamics. In contrast, diffraction allows for the direct visualization of a variety of ultrafast electron dynamics with combined spatial and temporal resolutions, and independent of the resolution of internal energy levels. The “temporal lens” concept can be used for the focus and magnification of ultrashort electron packets in the time domain. The temporal lenses are created by appropriately synthesizing optical pulses that interact with electrons through the ponderomotive force. With such an arrangement, a temporal lens equation with a form identical to that of conventional light optics is derived. The analog of ray diagrams, but for electrons, are constructed to help the visualization of the process of compressing electron packets. It is shown that such temporal lenses not only compensate for electron pulse broadening due to velocity dispersion but also allow compression of the packets to durations much shorter than their initial widths. With these capabilities ultrafast electron diffraction and microscopy can be extended to new domains, but, as importantly, electron pulses are delivered directly on the target specimen. With electrons, progress has recently been made in imaging structural dynamics with ultrashort time resolution in both microscopy and diffraction. Earlier, nuclear motions in chemical reactions were shown to be resolvable on the femtosecond (fs) time scale using pulses of laser light, and the recent achievement of attosecond (as) light pulses has opened up this temporal regime for possible mapping of electron dynamics. Electron pulses of femtosecond and attosecond duration, if achievable, are powerful tools in imaging. The “electron recombination” techniques used to generate such attosecond electron pulses require the probing electron to be created from the parent ions (to date no attosecond electron pulses have been delivered on an arbitrary target) and for general applications it is essential that the electron pulse be delivered directly to the specimen. In ultrafast electron microscopy (UEM), the electron packet duration is determined by the initiating laser pulse, the dispersion of the electron packet due to an initial energy spread and electron-electron interactions. Since packets with a single electron can be used to image, and the initiating laser pulse can in principle be made very short (sub-10 fs), the limiting factor for the electron pulse duration is the initial energy spread. In photoelectron sources this spread is primarily due to the excess energy above the work function of the cathode, and is inherent to both traditional photocathode sources and optically-induced field emission sources. Energy-time uncertainty will also cause a measurable broadening of the electron energy spread, when the initiating laser pulse is decreased below ˜10 fs. For ultrafast imaging techniques to be advanced into the attosecond temporal regime, methods for dispersion compensation and new techniques to further compress electron pulses to the attosecond regime need to be developed. As described herein techniques for compressing free electron packets, from durations of hundreds of femtoseconds to tens of attoseconds, using spatially-dependent ponderomotive potentials are provided by embodiments of the present invention. Thus, a train of attosecond pulses can be created and used in ultrafast electron imaging. Because they are generated independent of the target they can be delivered to a specimen for studies of transient structures and electronic excitations on the attosecond time scale. The deflection of electrons (as in the Kapitza-Dirac effect) by the ponderomotive potential of intense lasers and the diffraction of electrons in standing waves of laser light have been observed, and so is the possibility (described through computer modeling) of spatial/temporal focusing with combined time-dependent electric and static magnetic fields. The “temporal lens” description analytically expresses how ponderomotive compression can be used to both compensate for the dispersion and magnify, in this case compress, the temporal duration of electron packets. We obtain simple lens equations which have analogies in optics and the results of “electron ray optics” of temporal lenses allows for analytical expressions and for the design of different schemes using geometrical optics. Here, we consider two types of temporal lenses, thin and thick. For the realization of the temporal thin lens, a laser beam with a Laguerre-Gaussian transverse mode, radial index ρ=0 and azimuthal index l=0 (or, in common nomenclature, a “donut” mode, is utilized. In the center of the donut mode, electrons will experience a spatially-varying ponderomotive potential (intensity) that is approximately parabolic. This potential corresponds to a linear spatial force which, for chirped electron pulses, can lead to compression from hundreds of fs to sub-10 fs. The second type, that of a thick lens, is based on the use of two counter-propagating laser beams in order to produce a spatially-dependent standing wave that co-propagates with the electrons. A train of ponderomotive potential wells are produced at the nodes of the standing wave, leading to compression but now with much “tighter focus” (thick lens). Because the electron co-propagates with the laser fields, velocity is matched. Analytical expressions are derived showing that this lens has the potential to reach foci with attosecond duration. Finally, we discuss methods for creating tunable standing waves for attosecond pulse compression, and techniques for measuring the temporal durations of the compressed pulses. Space-charge dispersed packets of electrons that have a linear spatial velocity chirp may also be compressed with the temporal lenses described here. All electron sources, both cw and pulsed, have an initial energy spread. For pulsed electron sources this is particularly relevant as electron packets created in a short time disperse as they propagate. The initial energy spread leads to an initial spread in velocities. These different velocities cause the initial packet to spread temporally, with the faster electrons traveling a further distance and the slower electrons traveling a shorter distance in a given amount of time. The dispersion leads to a correlation between position (along the propagation direction) and electron velocity as described in relation to FIG. 14. The linear spatial velocity “chirp” can be corrected for with a spatially-dependent linear impulsive force (or a parabolic potential). Thus, if a pulsed, spatially-dependent parabolic potential can be made to coincide appropriately with the dispersed electron packet, the slow trailing electrons can be sped up and the faster leading electrons can be slowed down. The trailing electrons, now traveling faster, can catch the leading electrons and the electron pulse will thus be compressed. FIG. 13 illustrates dispersion of an ultrashort electron packet. At t=to the packet is created from a photocathode and travels with a velocity v0. As it propagates along the x-axis it disperses, with the faster electrons traveling further, and the slower ones trailing for a given propagation time t. At t=0 a parabolic potential is pulsed on, giving an impulsive “kick” to the dispersed electron packet. After the potential is turned off, t>τ, the trailing electrons now have a greater velocity than the leading electrons. After a propagation time t=ti, the pulse is fully compressed. Consider a packet of electrons, propagating at a speed v0 along the x-axis, with a spread in positions of Δxo=v0Δto, at time t=to. At t=0, a potential of the form U(x)=½Kx2 interacts with the electron packet for a duration τ in the lab frame. The waist, or spatial extent of the potential (temporal lens) is chosen to be w, while the duration τ is chosen such that it is short compared to w/v0. When this condition is met the impulse approximation holds, and the change in velocity is Δv=−τ/m(dU(x)/dx)=−τKx/m, for |x|<w, where m is the electron mass. After the potential is turned off, t>τ, the electrons will pass through the same position, xf−x=(v0+Δv)tf, at the focal time tf=−x/Δv=m/(Kτ). To include an initial velocity spread around v0 (due to an initial ΔE), consider electrons that all emanate from a source located at a fixed position on the x-axis. An electron traveling exactly at v0 will take a time t0 to reach the center of the potential well at x=0. Electrons leaving the source with other velocities v0+vk will reach a location x=vkto at t=0. The image is formed at a location where electrons traveling with a velocity v0 and a velocity v0+vk intersect, this is, when v0ti=x+(v0+Δv+vk)ti. The image time ti is then ti=−x/(Δv+vk). FIG. 14 illustrates ray diagrams for spatial and temporal lenses. The top figure in FIG. 14 depicts three primary rays for an optical thin spatial lens. The object is located at yo, and the spatial lens has a focal length, f. A real image of the object is created at the image plane, position yi. The bottom figure in FIG. 14 is a ray diagram for a temporal thin lens. The diagram is drawn in a frame moving with the average speed v0 of the electron packet. The slopes of the different rays in the temporal diagram correspond to different initial velocities that are present in the electron packet. As shown in the diagram, a temporal image of the original electron packet is created at the image time ti. The initial packet (object) is created at a time to with Δto=Δxo/v0, where the spatial extend of the pulse is directly related to the temporal duration of the object. The lens is pulsed on at t=0 and the temporal focal length of the lens is tf. The lens represents the ponderomotive potential and in this case is on for the very short time τ. For the object time, to=x/vk, image time ti=−x/(Δv+vk) and the focal time tf=−x/Δv, the temporal lens equation holds, 1 t o + 1 t i = 1 t f . ( 4 ) Ray tracing for optical lenses is often used to visualize how different ray paths form an image, and is also useful for visualizing how temporal lenses work as shown in FIG. 14. As derived in later sections, the magnification M is defined as the ratio of the electron pulse duration (Δti) at the image position to the electron pulse duration (Δto), and is directly proportional to the ratio of the object and image times (−ti/to) and distances (−xi/x0). In polar coordinates, a Laguerre-Gaussian (LG01) mode has a transverse intensity profile given by, I(r, φ)=I0exp(1)2r2exp(−2(r/w)2)/w2 where w is the waist of the focus and I0 the maximum intensity. This “donut” mode has an intensity maximum located at r=√{square root over (2)}w/2 with a value of I0=2EP(√{square root over (ln 2/π3)}/(w2τ) where EP is the energy of the laser pulse and τ is the full-width-at-half-maximum of the pulse duration, assuming a Gaussian temporal profile given by exp(−4 ln 2(t/τ)2). The ponderomotive energy UP(x) is proportional to intensity, U P ( x ) = 1 2 [ e 2 λ 2 exp ( 1 ) I 0 2 π 2 m ɛ 0 c 3 w 2 ln 2 π ] x 2 ≡ 1 2 K x 2 , ( 5 ) where m is the electron mass, e is the electron charge and λ the central wavelength of the laser radiation and replacing r with x. Near the center of the donut mode focus (or x<<w) the intensity distribution is approximately parabolic, and hence the ponderomotive energy near the donut center is also parabolic. In analogy with a mechanical harmonic oscillator, the quantity in the square brackets of equation (5) can be referred to as the stiffness K; it has units of J/m2=N/m, and at 800 nm has the numerical value of, K≈3.1×10−36EP/(w4τ). For this parabolic approximation to be applicable, the spatial extent of the dispersed electron pulse, at t=0, Δx(0)=v0Δto+Δvoto must be much smaller than the laser waist, where the object velocity spread is Δvo=ΔE/√{square root over (2mE)}. The effect of this parabolic potential on an ensemble of electrons emitted from a source will now be analyzed. The velocity distribution of the ensemble is centered around v0, with an emission time distribution centered on −to, where all electrons are emitted from the same location xo=−v0to. Assuming a single donut-shaped laser pulse is applied at t=0, and centered at x=0, the electron ensemble is then influenced by the potential U(x)=½Kx2. The kth electron in the ensemble has an initial velocity v0+vk and emission time −to+tk. Using a Galilean transformation to a frame moving with velocity v0, the propagation coordinate x (lab frame) is replaced with the moving frame coordinate {tilde over (x)}=x−v0t. At t=0 the potential exists for the ultrashort laser pulse duration τ, giving the electron an impulse (or “kick”) dependent on its instantaneous position in the parabolic potential. In both frames, the position of the electron at t=0 is xk(0)={tilde over (x)}k(0)≡−v0tk+vkto−vktk, where xk(t) and {tilde over (x)}k (t) are in the lab and moving frames, respectively. Using the impulse approximation the electron trajectory immediately after the potential is turned off becomes,{tilde over (x)}k(t)=vkt+{tilde over (x)}k(0)(1−t/tf), (6)where tf=m/(Kτ) is the focal time. The electron trajectories, before and after t=0, can be plotted in both frames to give the equivalent of a ray diagram as illustrated in FIG. 15. Electrons emitted at the same time, i.e. tk=0, but with different velocities, will meet at the image position, {tilde over (x)}k=0 in the moving frame at the image time ti. The image time is found by setting {tilde over (x)}k(ti)=0, from equation (6), with tk=0, {tilde over (x)}k(ti)=vkti+vkto(1−ti/tf)=0 which is equivalent to the lens equation, equation (4): to−1+ti−1=tf−1. An expression for the magnification can be obtained when electrons that are emitted at different times tk and different velocities vk are considered. If the magnification is defined as M=−ti/to then the temporal duration at the image time becomes,Δti=MΔto, (7)where Δto and Δti are the duration of the electron packet at the object and image time, respectively. Durations achievable with a thin temporal lens follow from equation (7). An experimentally realistic temporal lens would use a 50 fs, 800 nm laser pulse with 350 μJ energy, focused to a waist of w=25 μm. These values result in a stiffness of K=5.5×10−8 N/m and a focal time of tf=0.3 ns; tf=m/(Kτ). If the lens is applied 10 cm from the source, electrons emitted at V0=c/10 (3 keV) would have an object time of to=xo/v0=0.1/(c/10)=3.0 ns. Using the temporal lens equation, equation (4), ti is obtained to be 0.33 ns. Hence, a magnification of M=−ti/to=0.1. Consequently, a thin temporal lens can compress an electron packet with an initial temporal duration of Δto≈100 fs, after it has dispersed, to an image duration of Δti≈10 fs. While the example presented here is for 3 keV electrons, the thin lens approximation holds for higher energy electrons as long as τ is chosen to be short compared to w/v0. Experimentally, the thin temporal lens can be utilized in ultrafast diffraction experiments which operate at kHz repetition rates with lasers that typically possess power that exceeds the value needed for the ponderomotive compression. Referring to FIG. 15, thin lens temporal ray diagrams for the lab and co-propagating frames are illustrated. The upper left panel is a ray diagram drawn in the lab frame showing how different initial velocities can be imaged to a single position/time. The gray lines are rays representing electrons with different velocities. The lower left panel is a ray diagram drawn in a frame moving with the average velocity V0 of the electron packet. The rays represent velocities of V0/67, V0/100 and 0. In the co-propagating frame, the relationship between Δto and Δti can be visualized as Δti=Δtoti/to. One major difference between the lab frame and the moving frame is that in the latter the position of the object and image are moving. The lines representing the object and the image positions are drawn with slopes of −V0. The upper right panel depicts the experimental geometry for the implementation of a thin temporal lens. Note that the laser pulse and electron packet propagate perpendicular to each other, and that the interception point between the electrons and photons is at x=0 and t=0. The lower right panel shows how the parabolic (idealized) potential compares to the experimentally realizable donut potential. The colored dots indicate the position of electrons following the rays indicated in the left bottom diagram. Above, it was analytically shown that free electron packets can be compressed from hundreds to tens of femtoseconds using a temporal thin lens, which would correspond to a magnification of ˜0.1. Co-propagating standing wave can be created by using two different optical frequencies, constructed by having a higher frequency (ω1) optical pulse traveling in the same direction as the electron packet and a lower frequency (ω2) traveling in the opposite direction. When the optical frequencies ω1, ω2, and the electron velocity v0 are chosen according to v0=c(ω1−ω2)/(ω1+ω2), a standing wave is produced in the rest frame of the electron as illustrated in FIG. 16. If the electron has a velocity v0=c/3, and ω1=2ω2 then the co-propagating standing wave has a ponderomotive potential of the form, U P ( x ) = 1 2 ( e 2 λ ~ 2 E 0 2 8 π 2 m c 2 ) cos 2 ( k ~ x ) , ( 8 ) where E0 is the peak electric field, {tilde over (λ)} the Doppler shifted wavelength. The envelopes of the laser pulses are ignored in this derivation, but they can be engineered so that the standing wave contrast is optimized. The standing waves can be provided outside the microscope housing or inside the microscope housing. The presence of the standing wave copropagating with the electron pulse or packet inside the microscope housing can produce a series of attosecond electron pulses as illustrated in FIG. 7B and FIG. 16. Depending on the geometry with which the laser beams interact, the standing wave and the electron pulse can overlap adjacent to the sample, providing attosecond electron pulse generation at distances close to the sample. The attosecond electron pulses can be single electron pulses. To find an analytic solution in the thick lens geometry, each individual potential well in the standing wave is approximated by a parabolic potential that matches the curvature of the sinusoidal potential, UP(x)=½[e2E02/(2 mc2)]x2≡½Kx2. Using the exact solution to the harmonic oscillator the focal time is,tf=cot(ωPτ)/ωP+τ (9)where ωp=√{square root over (Km)} and τ is the duration that the lens is on. For τ→0, tf→m/(Kτ), which is identical to the thin lens definition. The image time, ti, has a form,ti=(1/ωP2+totf−tfτ+τ2)/(to−tf+τ) (10)and after the two assumptions, τ→0 and to>>1/(tfωP2) becomes equivalent to equation (4), the lens equation: to−1+ti−1=tf−1. The standard deviation of the compressed electron pulse at arbitrary time ta is, Δ t a = t f 2 ( λ ~ 2 + 4 t a 2 Δ v o 2 ) + t a 2 λ ~ 2 - 2 t f t a λ ~ 2 48 t f 2 v 0 2 , ( 11 ) which is valid for an individual well. The time when the minimum pulse duration occurs is ta=tf{tilde over (λ)}2/({tilde over (λ)}2+4tf2Δvo2)≈tf and for experimentally realistic parameters is equal to tf. This implies that the thick lens does not image the initial temporal pulse; it temporally focuses the electrons that enter each individual well. Since there is no image in the thick lens regime, the minimum temporal duration is not determined by the magnification M as in the thin lens section, but is a given by, Δ t f = t f 2 λ ~ 2 Δ v o 2 12 v o 2 ( λ ~ 2 + 4 t f 2 Δ v o 2 ) ≅ t f Δ v o v o 2 3 ( 12 ) It should be noted that neither the temporal focal length nor the temporal duration are directly dependent on the Doppler shifted wavelength {tilde over (λ)}, as long as the condition to<v0Δto/Δvo is met. An example illustrates what temporal foci are obtainable. A source emits electrons with an energy distribution of 1 eV and a temporal distribution of 100 fs. Electrons traveling at v0=c/3 and having an energy E=31 keV gives a velocity distribution of Δv0=1670 m/s. If the distance between the source and the temporal lens is 10 cm, to=1.0 ns is less than v0Δto/Δv0≈6.0 ns, satisfying the condition to<v0Δto/Δvo and equation (12) is then valid. If the two colors used for the laser beams are 520 nm and 1040 nm, the Doppler-shifted wavelength is {tilde over (λ)}=740 nm. For a laser intensity of 3×1012Wcm−2 (available with repetition rates up to megahertz), the oscillation frequency in the potential well is ωp≈2×1012 rad/s, which gives a focal time of tf≈1 ps. With these parameters, equation (12) gives a temporal duration at the focus of Δtf≈5 as. To support this ˜5 as electron pulse, time-energy uncertainty demands an energy spread of ˜50 eV. The ponderomotive compression imparts an energy spread to the electron pulse which can be estimated from ΔE˜mv0{tilde over (λ)}(2tf), giving ˜50 eV similar to the uncertainty limit. This ΔE is very small relative to the accelerating voltage in microscopy (200 keV) and only contributes to a decrease of the temporal coherence. In optical spectroscopy such pulses can still be used as attosecond probes despite the relatively large ΔE when the chirp is well characterized. Combining the anharmonicity broadening of 15 as, we conclude that ultimately temporal pulse durations in the attosecond regime can be reached. In the temporal thick lens case, the use of ω and 2ω to create a co-propagating standing wave requires v0=c/3. However, the velocity of the electrons, v0, can be tuned by changing the angle of the two laser pulses. A co-propagating standing wave can still be obtained by forcing the Doppler-shifted frequencies of both tilted laser pulses to be equal. A laser pulse that propagates at an angle θ with the respect to the electron propagation direction has a Doppler-shifted frequency {tilde over (ω)}γω(1±(v/c)cos θ), where ω is the angular frequency in the lab frame, {right arrow over (v)}=v{circumflex over (x)} is the electron velocity, and γ=1/√{square root over (1−v2/c2)}. When the two laser pulses are directed as shown in FIG. 16, a co-propagating standing wave occurs for an electron with a velocity v0=c(k1−k2)/(k1 cos θ1+k2 cos θ2), where the laser pulse travelling with the electron packet has a wave vector of magnitude k1 and makes an angle of θ1 with the electron propagation axis; the second laser pulse traveling against has a wave vector magnitude of k2 and angle θ2, in the lab frame. An electron moving at v0 will see a standing wave with an angular frequency, ω ~ = 2 ( cos θ 1 + cos θ 2 ) 2 cos θ 1 + cos θ 2 γ ω ( 1 - β ) , ( 13 ) where 2k=k1=2k2 for experimental convenience, ω=kc, and the wavelength is {tilde over (λ)}=2πc/{tilde over (ω)}=2π/{tilde over (k)}. The standing wave created with arbitrary angles θ1 and θ2 will be tilted with respect to the electron propagation direction, which will temporally smear the electron pulse. This tilting of the standing wave can be corrected for by constraining the angles θ1 and θ2 to be: θ2=arcsin(2 sin θ1). For θ1=15° (forcing θ2≈31°), electrons with velocity v0=0.36c (E≈33 keV) see a standing wave. A 1 eV electron energy distribution at the source gives a velocity distribution of Δv0≈1630 m/s, at 33 keV. Using the same laser intensity as in the thick lens case, and the new v0 and Δvo, the condition to<v0Δto/Δv0 is still satisfied, allowing equation (13) to be used, resulting in a duration at the focus of Δtf≈4.6 as. Using the tunable thick lens makes the experimental realization more practical, allowing for easy optical access and electron energy tuning, while at the same time keeping Δtf approximately the same. For additional tunability, an optical parametric amplifier can be used so that the laser pulse frequencies are not restricted to ω and 2ω. The ability to create electron pulses with duration from ˜10 fs to ˜10 as raises a challenge regarding the measuring of their duration and shape. Two different schemes are presented here for measuring pulses compressed by thick and thin temporal lenses. For measuring the thin lens compressed electron packet, the focused packet could be intersected by a laser pulse with a Gaussian spatial focus as illustrated in FIG. 17. An optical delay line would control the time delay between the measuring laser pulse and the compressed electron packet. As the time delay, Δt, is varied, so is the average energy of the electrons, as shown in FIG. 17. If the delay time is zero, then the average electron energy will be unaffected, as there is no force. If the delay line is changed so that the Gaussian pulse arrives early (late), then the average energy will decrease (increase). The change in the average energy is dependent on the duration of the electron pulse, and the intensity of the probing laser pulse. If the electron pulse is longer than the duration of the measuring laser pulse, then the change in the average energy will be reduced. The steepness of the average energy as a function of delay time, Ē (Δt), is a direct measure of the electron pulse duration, and using fs-pulsed electron energy loss spectra this scheme can be realized. For the thick lens a similar method is described here. At the focal position and time of the compressed temporal electron packet, a second co-propagating potential is introduced. The positions of the individual wells in the second co-propagating standing wave can be moved by phase shifting one of the two laser beams that create the probing potential (FIG. 17). By varying the phase shift, the potential slope (and hence the force) that the electrons encounter at the focus is changed. If no phase shift is given to the probing standing wave, no average energy shift results. When a phase shift is introduced, the electrons will be accelerated (or decelerated) by the slope of an individual well in the standing wave, and as long as the phase stability between the electrons and the probing standing wave is appropriate, attosecond resolution can be achieved. As the electron pulse duration becomes less than the period of the standing wave, the average electron energy change increases. The electron temporal duration of the compressed electron packet can be determined directly by the steepness of the Ē (φ) curve. Diffraction with focused electron probes is among the most powerful tools for the study of time-averaged nanoscale structures in condensed matter. Embodiments of the present invention provide methods and systems for four-dimensional (4D) nanoscale diffraction, probing specific-site dynamics with ten orders of magnitude improvement in time resolution, in convergent-beam ultrafast electron microscopy (CB-UEM). For applications, we measured the change of diffraction intensities in laser-heated crystalline silicon as a function of time and fluence. The structural dynamics (change in 7.3±3.5 ps), the temperatures (up to 366 K), and the amplitudes of atomic vibrations (up to 0.084 angstroms) are determined for atoms strictly localized within the confined probe area of 50-300 nm; the thickness was varied from 2 to 100 nm. A broad range of applications for CB-UEM and its variants are possible, especially in the studies of single-particles and heterogeneous structures. In fields ranging from cell biology to materials science, structures can be imaged in real-space using electron microscopy. Atomic-scale resolution of structures is usually available from Fourier-space diffraction data, but this approach suffers from the averaging over the selected specimen area which is typically on the micrometer scale. Significant progress in techniques has enabled localization of diffraction to nanometer and even angstrom-sized areas by focusing a condensed electron beam onto the specimen. Parallel illumination with a single electron wavevector is reshaped to a convergent beam with a span of incident wavevectors. This method of convergent beam electron diffraction (CBED), or electron microdiffraction, and with energy filtering, has made possible determination of structures in 3 dimensions with highly precise localization to areas reaching below one unit cell. The applications have been wide-ranging, from revealing bonding charge distribution and local defects and strains in solids to detecting local atomic vibrations and correlations. Today, aberration-corrected, atomic-sized convergent electron beams enable analytical probing using electron-energy-loss spectroscopy (EELS) and scanning transmission electron microscopy (STEM). In order to resolve structural dynamics with appropriate spatiotemporal resolution, femtosecond (fs) and picosecond (ps) electron pulses are ideal probes because of their picometer wavelength and their large cross section, resulting from the effective Coulomb interaction with atomic nuclei and core/valence electrons of matter. Typically, ultrafast electron diffraction is achieved by initiating the physical or chemical change with a pulse of photons (pump) and observing the ensuing dynamics with electron pulses (probe) at later times. By recording sequentially delayed diffraction frames a “movie” can be produced to reveal the temporal evolution of the transient structures involved in the processes under study. FIG. 18 is a simplified schematic diagram of a CB-UEM set-up (top), and observed low-angle diffraction discs according to an embodiment of the present invention. Femtosecond electron pulses are focused on the specimen to form a nanometer-sized electron beam. Structural dynamics are determined by initiating a change with a laser pulse and then observing the consequences using electron packets delayed in time. Insets (right) show the CB-UEM patterns taken along the Si [011] zone axis at different magnifications. At the high camera length used, only the ZOLZ discs indexed in the figure are visible; the kinematically-forbidden 200 disc appears as a result of dynamic scattering. In the reciprocal space representation of the diffraction process (bottom) the Ewald sphere has an effective thickness of 2α, the convergence angle of the electron beam. The diamond structure of Si forbids any reflections from odd numbered Laue planes when the zone axis is [011]. Embodiments of the present invention provide CB-UEM methods and systems with applications in the study of nanoscale, site-selected structural dynamics initiated by ultrafast laser heating (1014 K/s). Because of the femtosecond pulsed-electron capability, the time resolution is ten orders of magnitude improved from that of conventional TEM, which is milliseconds; and because of beam convergence, high-angle Bragg scatterings are visible with their intensities being very sensitive to both the 3D structural changes and amplitudes of atomic vibrations. The CB-UEM configuration is shown in FIG. 18; our chosen specimen is a crystalline silicon slab, a prototype material for such investigations. From these experiments, it is found that the structural change within the locally probed site occurs with a time constant of 7.3±3.5 ps, which is on the time scale of the rise of lattice temperature known for bulk silicon. For these local sites, the temperatures measured at different laser fluences range from 299° K to 366° K, corresponding to vibrational amplitude changes from 0.077 Å to 0.084 Å, respectively. The reported results would be impossible to obtain with conventional, parallel beam diffraction. The electron microscope is integrated with a fs oscillator/amplifier laser system. The fundamental mode of the laser at 1036 nm was split into two beams: the first was frequency doubled to 518 nm and used to initiate the heating of the specimen, whereas the second, which was frequency tripled, was directed to the microscope for extracting electrons from the cathode. The time delay between pump and probe was adjusted by changing the relative optical path lengths of these two pulses. The pulses were sufficiently separated in time (5 μs) to allow for cooling of the specimen. The electron packets were accelerated to 200 keV (corresponding to a de Broglie wavevector of 39.9 Å−1), de-magnified, and finally focused (with a 6 mrad convergence angle) to an area of 50-300 nm diameter on the wedge-shaped specimen, as shown in FIG. 18. A wide range of thicknesses, starting from ˜2 nm was accessible simply by moving the electron beam laterally. The silicon specimen was prepared by mechanical polishing of a wafer along the (011) planes, followed by Ar ion-milling for final thinning/smoothing; the wedge angle was 2°. In the microscope, Kikuchi lines were observed and used as a guide to orient the specimen with the [011] zone axis either parallel or tilted relative to the incident electron beam direction. FIG. 18 display the typical high-magnification (high-value camera length) CB-UEM patterns of Si obtained when the specimen is unexcited and the zone axis is very close to [011]; the magnification (>10×) cab be seen by comparing the disc length scale in FIG. 18 and ring radius in FIG. 19. Unlike parallel-beam diffraction which yields spots, convergent-beam diffraction produces discs in reciprocal space (back focal plane of the objective lens) with their diameter given by the convergence angle (2α) of the electron pulses. These discs form the Zero Order Laue Zone (ZOLZ) of the pattern; they show white contrast with thin specimens and exhibit the interference patterns displayed in FIG. 18 when the thickness is increased. In the reciprocal space, the effective thickness of the Ewald sphere is 2α (bottom panel of FIG. 18), giving rise to multiple spheres that can intersect with Higher Order Laue Zones (HOLZ) reflections, the focus of this study (see FIG. 19) and the key to 3D structural information; the first and second zones, FOLZ and SOLZ, are examples of such zones or rings. The interference patterns in the disks are the result of dynamical scattering in silicon and are reproduced in our CB-UEM patterns (FIG. 18). The scattering vectors of HOLZ rings (R) are related to the inter-zone spacing in the reciprocal space (hz in Å−1) by the tilt angle from the zone axis (η) and by the magnitude of the incident electron's wavevector (k0). In the plane of the detector and for our tilt geometry, the HOLZ ring scattering vector is given by (equation (14)):R≅(k02 sin2(η)+2k0hz)1/2−k0 sin(η), (14)where, for our case of the [011] zone axis, hz=n/(a√{square root over (2)})) with n=1, 2, 3 . . . for the different Laue zones. Additionally, for this zone axis, k+l=n, where (hkl) are the Miller indices of the reciprocal space. When k+l=1, for FOLZ, k and l must have different parity, which is forbidden by the symmetry of the diamond Si structure. Therefore, the FOLZ along the [011] zone axis should be absent and the first visible ring should belong to SOLZ; in general, all odd numbered zones will be forbidden. Here, HOLZ indexing is defined according to the fcc unit cell and not to the primitive one [1]. FIG. 19 illustrates temporal frames obtained using CBUED. In FIG. 19(a) high angle SOLZ ring obtained for a tilt angle of 5.15° from the [011] zone axis are shown. Besides SOLZ, Kikuchi lines and periodic bands (due to atomic correlations) are visible. The ZOLZ discs are blocked (top left) to enhance the dynamic range in the area of interest; the disc of the direct beam (the center one in FIG. 18 discs) is indicated by a circle. The intensity scale is logarithmic. In FIG. 19(b,c,d) time frames of the SOLZ ring are shown by color mapping for visualization of dynamics. The intensity of the ring changes within picoseconds, but the surrounding background remains at the same level. FIG. 19(a) presents the HOLZ ring taken with the CB-UEM. In order to reduce the strong on-zone-axis dynamic scattering (and to bring the high scattering angles into the range of the recording camera), the slab was tilted 5.15° away from the [011] zone axis, along the [02 2] direction. The scattering vector of the Bragg points of the ring, from the direct beam position, was measured to be 2.2 Å−1, close to the value of 2.22 Å−1 obtained by using equation (14) for n=2, which identifies the spots shown as part of the SOLZ. From this value, the know lattice separation of 5.4 Å was obtained for silicon. In addition to the SOLZ ring, Kikuchi lines and some oscillatory bands are also visible in the CB-UEM, as seen in FIG. 19(a). Kikuchi lines arise from elastic scatterings of the inelastically scattered electrons, whereas the oscillatory bands in the thermal diffuse scattering (TDS) background result from correlations between the atoms. We also observed deficit HOLZ lines and interference fringes in ZOLZ discs for a two-beam condition. The temporal behavior is displayed in FIG. 19, with three CB-UEM frames taken at time delays of t=−14.8 ps, +5.2 ps, and +38.2 ps, together with a static image; the zero of time is defined by the coincidence of the pump and probe pulses in space and time. The frame at negative time has higher ring intensity than that observed at +38.2 ps, whereas the +5.2 ps frame shows an intermediate intensity value. It is clear from the results that the intensity change is visible within the first 5 ps of the structural dynamics. For quantification, the intensities in each frame were normalized to the area of azimuthally integrated background. The normalization of the HOLZ ring intensities to the TDS background makes the atomic vibration estimations insensitive to the thickness changes of the probed area, which may result from slight beam jittering. FIG. 20 illustrates diffraction intensities at different times and fluences. Normalized, azimuthally-integrated intensity changes of the SOLZ ring are shown with time ranging from −20 ps to +100 ps, for two different laser powers. Whereas the 10 mW response does not show noticeable dynamics, the 107 mW transient has a clear intensity change with a characteristic time of 7.3±3.5 ps. The range of fluences studied was 1.7 to 21 mJ/cm2 (see FIG. 21). The red curve is a mono-exponential fit based on the Debye-Waller effect. The red dashed line through the 10 mW data is an average of the points after +20 ps. The dependence on fluence is given in FIG. 21. FIG. 20 depicts the transient behavior of the SOLZ ring intensity for two different laser power, 10 mW and 107 mW, corresponding to pulse fluence of 1.7 and 19 mJ/cm2, respectively; the heating laser beam diameter on the specimen is 60 μm. The intensities were normalized to the average value obtained at negative times. Whereas the intensity change is essentially absent in the 10 mW data, the results for the 107 mW set shows a transient behavior with a characteristic time of 7.3±3.5 ps, obtained from the mono-exponential fit shown in red in the figure. The temporal response of UEM-2 is on the fs time scale, as obtained by EELS, and it is much shorter than the 7 ps illustrated here. The local heating of the lattice is responsible for the SOLZ intensity change with time. A pump laser, in our case at 518 nm (2.4 eV), excites the valance electrons of Si to the conduction band; one-photon absorption occurs through the indirect bandgap at 1.1 eV, and multi-photon absorption excites electron-hole pairs through the direct gap. The excited carriers thermalize within 100 fs, via carrier-carrier scatterings, and then electron cooling takes place in ˜1 ps, by electron-phonon coupling. During this time lattice heating occurs through increased atomic vibration, reducing SOLZ intensity. The effective lattice temperature is ultimately established with a time constant of a few picoseconds depending on density of carriers or fluence. However, in CB-UEM measurements the lattice-temperature rise could be slower than in bulk depending on the dimension of the specimen relative to the mean free path of electrons in the solid. The dynamical change can be quantified by considering a time-dependent Debye-Waller factor with an effective temperature describing the decrease in the Bragg spot intensity with time. If the root-mean-square (rms) displacement of the atoms, ux21/2, along one of the three principle axes is denoted by ux for simplicity, and the scattering vector by s, then the HOLZ ring intensity can be expressed as (equation (15)):IRingF(t)=I0(t_)exp[−4π2s2ux2(t)], (15)where IRingF(t) is the measured intensity for a given fluence, F, and the vibrational amplitude is now time dependent. Note that ux is ⅓ of the total, utotal. In the Einstein model of atomic vibrations, which has been used successfully for silicon, the atoms are treated as independent harmonic oscillators, with the three orthogonal components of the vibrations decoupled. As a result, a single frequency (ω) is sufficient to specify the energy eigenstates of the oscillators. The relationship of the vibrational amplitude to temperature can be established by simply considering the Boltzmann average over the populated eigenstates. Consequently, the probability distribution of atomic displacements is derived to be of Gaussian form, with a standard deviation corresponding to the rms (ux) of the vibration involved (equation (16)):ux=[(ℏ/2ωm)cot h(ℏω/2kBTeff)]1/2 (16)where ℏ is Planck's constant, kB the Boltzmann constant, Teff in our case the effective temperature, and m the mass of the oscillator. In the high temperature limit, i.e. when ℏω/2kBT<<1, eq. 3 simplifies to mω2ux2=kBT, which is the classical limit for a harmonic oscillator; the zero-point energy, which contributes almost half of the mean vibration amplitude at room temperature, is included in equation (16). The value of ℏ w is 25.3 meV. Despite its simplicity, the Einstein model in equation (16) was remarkably successful in predicting the HOLZ rings and TDS intensities by multi-slice simulations. FIG. 21 illustrates the amplitudes of atomic vibrations (rms) plotted against the observed intensity change at different fluences. The inset shows the mono-exponential temporal behavior, with the asymptotes highlighted (circles) for their values at different fluences. The fluence was varied from 1.7 to 21 mJ/cm2. This comparative study of the effect of the fluence was performed at a slightly different sample tilt (corresponding to s=2.7 Å−1), corresponding to a thickness of ˜80 nm. For each fluence, the temperature represents the effective value for the lattice structural change. The error bars given were obtained from the fits at the asymptotes shown in the inset, and they are determined by the noise level of temporal scans. In FIG. 21, we present the change in the asymptotic intensity with fluence (inset), and the derived vibrational amplitudes for the different temperatures. The amplitudes are directly obtained from equation (15), as s is experimentally measured. The relative temperature change (from t− to t+) is then derived from equation (16), taking the value of ux at room temperature (297° K) to be 0.076 Å. The amplitude of atomic vibrations, and hence the temperature, increases as the fluence of the initiating pulse increases. Although the trend is expected for an increased ux with temperature, the absolute values, from 0.077 to 0.084 Å, correspond to a large 3.2% to 3.6% change in nearest neighbor separation; these values are still well below the 15% criterion for a melting phase transition. The linear thermal expansion coefficient has been accurately determined for silicon, and for a value of 2.6×10−6 K−1 at room temperature the vibrational amplitudes reported here are much higher than the equilibrium thermal values at the same temperature. This is because the effective temperature applies to a lattice arrested in a picosecond time window; at longer times, the vibrations equilibrate to a lower temperature. As such, measuring nanoscale local temperatures on the ultrashort time scale enhances the sensitivity of the probe thermometer by orders of magnitude. Moreover, the excitation per site is significantly enhanced. For a single-photon absorption at the fluence used, we estimate, for a 60 nm-thick specimen, the number of absorbed photons per Si atom (for the fs pulse employed) to be ˜0.01, as opposed to 10−9 photons per atom if the experiments were conducted in the time-averaged mode. The achievement of nanoscale diffraction with convergent-beam ultrafast electron microscopy opens the door to exploration of different structural, morphological, and electronic phenomena. The spatially focused and timed electron packets enable studies of single particles and structures of heterogeneous media. Extending the methodology reported here to other variants, such as EELS, STEM and nanotomography, promises possibilities for mapping individual unit cells and atoms on the ultrashort time scale of structural dynamics. With 4D electron microscopy, in situ imaging of the mechanical drumming of a nanoscale material is measured. The single crystal graphite film is found to exhibit global resonance motion that is fully reversible and follows the same evolution after each initiating stress pulse. At early times, the motion appears “chaotic” showing the different mechanical modes present over the micron scale. At longer time, the motion of the thin film collapses into a well defined fundamental frequency of 0.54 MHz, a behavior reminiscent of mode locking; the mechanical motion damps out after ˜200 μs and the oscillation has a “cavity” quality factor of 150. The resonance time is determined by the stiffness of the material and for the 53-nm thick and 55-μm wide specimen used here we determined Young's modulus to be 0.8 TPa, for the in-plane stress-strain profile. Because of its real-time dimension, this 4D microscopy has applications in the study of these and other types of materials structures. Structural, morphological, and mechanical properties of materials have different length and time scales. The elementary structural dynamics, which involve atomic movements, are typically of picometer length scale and occur on the time scale of femto (fs) to picoseconds (ps). Collective phenomena of such atomic motions, which define morphological changes, are observed on somewhat longer time scale, spanning the ps to nanosecond (ns) time domain, and the length scale encompasses up to sub-micrometers. These microscopic structures are very different in behavior from those involved in the mechanical properties. On the nanoscale, when the membrane-like mechanical properties have high frequencies and complex spatial-mode structures, imaging becomes of great value in displaying the spatiotemporal behavior of the material under stress. Utilizing embodiments of the present invention, we have visualized nanoscale vibrations of mechanical drumming in a single-crystalline graphite film (53-nm thick). To study the transient structures, in both space and time, our method of choice has been 4D ultrafast electron microscopy (UEM). This microscope enables investigation of the atomic structural and morphological changes in graphite on the fs to ns time scale and for nm-scale resolution. Additionally, mechanical properties can be determined in real time, which are evident on the ns and microsecond (μs) time scale. The stress is introduced impulsively using a ns laser pulse while observing the motions in real space (in situ) in the microscope using the stroboscopic electron pulses. Remarkably, at times immediately following the initiating pulse the motion appears “chaotic” in the full image transients, showing the different mechanical modes present in graphite. However, after several μs the motion of the nanofilm collapses into a final global resonance of 0.54 MHz. From this resonance of mechanical drumming of the whole plate, we obtained the in-plane Young's modulus of 0.8 terapascal (Tpa). The reported coherent resonance represents the in-phase build up of a mechanical drumming, which is directly imaged without invasive probes. Graphite was chosen because of its unique material properties; it is made of stacked layers of 2D graphene sheets, in which the atoms of each sheet are covalently bonded in a honeycomb lattice, and the sheets separated by 0.335 nm are weakly held together by van der Waals forces. It displays anisotropic electromechanical properties of high strength, stiffness, and thermal/electric conductivity along the 2D basal planes. More recently, with the rise of graphene, a new type of nano-electromechanical system (NEMS) has been highlighted with a prototypical NEMS being a nanoscale resonator, a beam of material that vibrates in response to an applied external force. With the thicknesses reaching the one atomic layer, graphene remains in a high crystalline order, resulting in a NEMS with extraordinary thinness, large surface area, low mass density, and high Young's modulus. Briefly, the setup for ultrafast (and fast) electron imaging involves the integration of laser optical systems into a modified transmission electron microscope (TEM). Upon the initiation of a structural change by either heating of the specimen or through electronic excitation by the laser pulses, an electron pulse generated by the photoelectric effect is used to probe the specimen with a well-defined time delay. A microscopy image or a diffraction pattern is then taken. A series of time-framed snapshots of the image or the diffraction pattern recorded at a number of delay times provides a movie, which displays the temporal evolution of the structural (morphological) and mechanical motions, using either the fs or ns laser system. Because here the visualization is that of the mechanical modes with resonances on the MHz scale, the ns resolution was sufficient. The electrons are accelerated to 200 kV with a de Broglie wavelength of 2.5079 pm. Two laser pulses were used to generate the clocking, excitation pulse at 532 nm and another at 355 nm for the generation of the electron pulse for imaging. The time delay was controlled by changing the trigger time for electron pulses with respect to that of clocking pulses. The delay can be made arbitrarily long and the repetition rate varies from a single shot to 200 kHz, to allow complete heat dissipation in the specimen. The experiments were carried out with a natural single crystal of graphite flakes on a TEM grid. Graphite flakes were left on the surface, covering some of the grid squares completely. The observed dynamics are fully reversible, retracing the identical evolution after each initiating pulse; each image is constructed stroboscopically, in a half second, from typically 2500 pulses of electrons and completing all time-frames (movies) in twenty minutes. FIG. 22 illustrates images and the diffraction pattern of graphite. (A), an image shows features of fringes in contrast (scale bar: 5 μm). Sample thickness was measured to be 53 nm using electron energy loss spectroscopy (EELS). (B) Magnified view of the indicated square of panel A (scale bar: 1 μm). (C) Diffraction pattern obtained by using a selected area diffraction aperture (SAD), which covered an area of 6 μm in diameter on the specimen. The incident electron beam is parallel to the [001] zone axis. Bragg spots are indexed as indicated for some representative ones. Panels A and B of FIG. 22 show the UEM (bright field) images of graphite, and in panel C, a typical electron diffraction pattern is given. The Bragg spots are indexed according to the hexagonal structure of graphite along the [001] zone axis, with the lattice dimension of a=b=2.46 Å (c=6.71 Å). In FIG. 22A, and at higher magnification in FIG. 22B, contrast fringes are clear, typically consisting of linear fringes having ˜1 μm length and a few hundred-nm spacing. These contrast fringes are the result of physical bucking of the graphene layers by constraints or by nanoscale defects within the film. In the dark regions, the zone axis (the crystal [001]) is well aligned with the incident electron beam and electrons are scattered efficiently, whereas in the lighter regions the alignment of the zone axis deviates more and the scattering efficiency is lower. With these contrast patterns, changes in image provide a sensitive visual indicator of the occurrence of mechanical motions. The black spots are natural graphite particles. FIG. 23 illustrates representative image snapshots and difference frames. (A) Images recorded stroboscopically at different time delays, indicated at the top right corner of each image (t1, t2, t3, t4, and t5), after heating with the initiating pulse (fluence=7 mJ/cm2); t1=200 ns; t2=500 ns; t3=10 μs; t4=30 μs; t5=60 μs; and the negative time frame was taken at −1000 ns. Note the change in position of fringes with time, an effect that can be clearly seen in FIG. 23B. (B) Image difference frames with respect to the image taken at −1 μs, i.e., Im(−1 μs; t), which show the image change with time. The reversal in contrast clearly displays the oscillatory (resonance) behavior. In FIG. 23(A), we display several time-framed images of graphite taken at a repetition rate of 5 kHz and at delay times indicated with respect to the clocking (heating) pulse with the fluence of 7 mJ/cm2. At positive times, following t=0, visual changes are seen in the contrast fringes. With time, the contrast fringes change their location in the images, and with these and other micrographs of equal time steps we made a movie of the mechanical motions of graphite following the ns excitation impulse. To more clearly display the temporal evolution on the nanoscale, image-difference frames were constructed. In FIG. 23(B), depicted are the images obtained when referencing to the −1 μs frame, i.e., Im(−1 μs; t). In the difference images, the regions of white or black indicate locations of surface morphology change (contrast pattern movement), while gray regions are areas where the contrast is unchanged from that of the reference frame. Care was taken to insure the absence of long-term specimen drifts as they can cause apparent contrast change; note that in the difference images, the static features are not present. The image changes, reported in this study, are fully reproducible, retracing the identical evolution after each initiating laser pulse, as mentioned above. The reversal of contrast with time in FIG. 23(B) directly images the oscillatory behavior of the drumming. The image change was quantified by using the method of cross-correlation. The normalized cross correlation of an image at time t with respect to that at time t′ is expressed as γ ( t ) = ∑ x , y C x , y ( t ) C x , y ( t ′ ) ∑ x , y C x , y ( t ) 2 ∑ x , y C x , y ( t ′ ) 2 ( 17 ) where the contrast Cx,y(t) is given by [Ix,y(t)−Ī(t)]/Ī(t), and Ix,y(t) and Ix,y(t′) are the intensities of pixels at the position of (x,y) at times t and t′; Ī(t) and Ī(t′) are the means of Ix,y(t) and Ix,y(t′), respectively. This correlation coefficient γ(t) is a measure of the temporal change in “relief pattern” between the two images being compared, which can be used as a guide to image dynamics as a function of time. Shown in fug 24 are cross-correlation values between the image at each measured time point and a reference image recorded before the arrival of the clocking pulse. FIG. 24 illustrates the time dependence of image cross correlation. The whole scan for 100 μs is made of 2000 images taken at 50-ns steps. Also depicted are the zoomed-in image cross-correlations of three representative time regimes (I, II, and III). In each zoomed-in panel, the selected-area image dynamics of five different regions are included. Note the evolution from the “chaotic” to the global resonance (drumming) behavior at long times. Over all pixels, the time scale for image change covers the full range of time delays, from tens of ns to hundreds of μs, indicating the collective averaging over the sites of the specimen. Upon impulsive heating at t=0, the image cross-correlation changes considerably with an appearance of a “chaotic” behavior, in the ˜5 μs range (regime I in FIG. 24). After 10 μs, e.g., regime II, the cross correlation change begins to exhibit periodicity (regime II), and at longer time, a well-defined resonance oscillation emerges (regime III). This is also evident in the selected-area image dynamics (SAID) in several regions (noted as 1 to 5) where the temporal behavior is of different shapes at early time but converges into a single resonance transient after several tens of μs. The shape of image cross correlation dynamics was robust at different fluences, from 2 to ˜10 mJ/cm2, but the amplitude varies. The overall decay of the transients is on a time scale shorter than the separation between pulses. In fact, we have verified the influence of repetition rate and could establish the full recovery at the time intervals indicated. Heat transfer must occur laterally. With an initial z-independent heat profile by absorption of the heating pulse in graphite, we estimated, using a 2D heat diffusion in a homogeneous medium, the time scale for an in-plane transfer, with thermal conductivity λ=5300 W/(m·K), density ρ=2260 kg/m3, and specific heat cV=707 J/(K·kg). For the radius at half height of the initial pulse heat distribution r0=30 μm, t1/2, the time for the axial temperature to drop to a half of its initial value, is deduced to be ˜720 ns, certainly much shorter than the 200-μs time interval between pulses. It follows that the decay of the oscillation [Q/(π·f0)], as derived below, is determined by the damping of mechanical motions. When the specimen absorbs intense laser light, the lattice energy, converted from carriers (electron energy) by electron-phonon coupling, in a few ps, builds up in the illuminated spot on the surface within the duration of the laser pulse. As a consequence, the irradiated volume will expand rapidly following phonon-phonon interaction on the time scale of tens of ps. The resulting thermal stress can induce mechanical vibration in the material, but a coherent oscillatory behavior, due to the thermoelastic stress, will only emerge in the image if the impulsive stress is on a time scale shorter than the period; probing of images should be over the entire time scale of the process, in this case 100 μs. On the ultrashort time scale we have observed the structural and morphological elastic changes. FIG. 25 illustrates resonance dynamics and FFT of graphite. (Left) Time dependences of image cross correlation of full image (A) and image intensity on the selected area of 4×6 pixels as indicated by the arrowhead (B) in FIG. 24. (Right) Fast Fourier transforms of image cross-correlation (C: 0-100 μs; D: 60-100 μs) and image intensity (E: 0-100 μs; F: 60-100 μs). Asterisks in the panels indicate overtones. Note the emergence of the resonance near 1 MHz in panel F. The resonance modes in graphite are highlighted in FIG. 25 by taking the fast Fourier transform (FFT) of image cross-correlation in the time regime of 0-100 μs. The FFT (FIG. 25C) shows several peaks of different frequencies, among which the strongest one around 2.13 MHz is attributed to the overtone of 1.08 MHz. The overtones, due to the truncated nature of cross-correlation close to the value of 1, are greatly reduced in the FFT of image intensity change (FIGS. 25E and 25F). In a few tens of μs, various local mechanical modes observed at early time damp out and one global mode around 1 MHz survives. The peak when fitted to a Lorentzian yields a resonant frequency of 1.08 MHz, and a “cavity” quality factor Q (=f0/Δf)=150±30. This dominant peak gives the fundamental vibration mode of the plate in graphite. For a period of vibration, the contrast pattern of image would recur twice to its initial feature giving the observed frequency to be twice that of structural vibration; the fundamental frequency is, thus, obtained to be 0.54 MHz. A square mechanical resonator clamped at four edges without tension has a fundamental resonance mode of f0 which is given by f 0 = A d L 2 [ Y ( 1 - v 2 ) ρ ] 1 / 2 + f ( T ) ( 18 ) where f(T) due to tension T is zero in this case. Y is the Young's modulus; ρ is the mass density; v is the Poisson's ratio; L is the dimension of a grid square; d is the thickness of the graphite; and A is a constant, for this case equal to 1.655. We measured d to be 53 nm from EELS. Knowing ρ=2260 kg/m3 (300 K), v=0.16 for graphite, and L=55 μm, we obtained from the observed resonance frequency the Young's modulus to be 0.8 TPa, which is in good agreement with the in-plane value of 0.92 TPa, obtained using stress-strain measurements. This value is different by more than an order of magnitude from the c-axis value we measured using the microscope in the ultrafast mode of operation. Thus, using embodiments of the present invention, we have demonstrated a very sensitive 4D microscopy method for the study of nanoscale mechanical motions in space and time. With selected-area-imaging dynamics, the evolution of multimode oscillations to a coherent resonance (global) mode at long time provides the mapping of local regions in the image and on the nanoscale. The time scale of the resonance is directly related to materials anisotropic elasticity (Young's modulus), density, and tension, and as such the reported real-time observation in imaging can be extended to study mechanical properties of membranes (graphene in the present case) and other nanostructures with noninvasive probing. The emergent properties resolved here are of special interest to us as they represent a well-defined “self-organization” in complex macroscopic systems. The function of many nano and microscale systems is revealed when they are visualized in both space and time. Here, four-dimensional (4D) electron microscopy provided in accordance with an embodiment of the present invention is used to measure nanomechanical motions of cantilevers. From the observed oscillations of nanometer displacements as a function of time, for free-standing beams, we are able to measure the frequency of modes of motion, and determine Young's elastic modulus, the force and energy stored during the optomechanical expansions. The motion of the cantilever is triggered by molecular charge redistribution as the material, single-crystal organic semiconductor, switches from the equilibrium to the expanded structure. For these material structures, the expansion is colossal, typically reaching the micron scale, the modulus is 2 GPa, the force is 600 μN, and the energy is 200 pJ. These values translate to a large optomechanical efficiency (minimum of 1% and up to 10% or more), and a pressure of nearly 1,500 atm. We note that the observables here are real-material changes in time, in contrast to those based on changes of optical/contrast intensity or diffraction. As the physical dimensions of a structure approach the coherence length of carriers, phenomena not observed on the macroscopic scale (e.g., quantization of transport properties) become apparent. The discovery and understanding of these quantization effects requires continued advances in methods of fabrication of atomic-scale structures and, as importantly, in the determination of their structural dynamics in real-time when stimulated into a configuration of a nonequilibrium state. Of particular importance are techniques that are noninvasive and capable of nanoscale visualization in real-time. Examples of the rapid progress in the study of nanoscale structures are numerous in the field of micro and nanoelectromechanical systems (i.e., MEMS and NEMS, respectively). Recent advancements have resulted in structures having single-atom mass detection limits and binding specificities on the molecular level, and especially for biological systems. Beyond mass measurement and analyte detection, changes in the dynamics of these nanoscale structures have been shown to be sensitive to very weak external fields, including electron and nuclear spins, electron charge, and electron and ion magnetization. The response to external stimuli is manifested in deflections of the nanoscale, and a variety of techniques have been used to both actuate and detect the small-amplitude deflections. Optical interference is often used for measurement purposes, wherein the deflections of the structure cause a phase shift in the path-stabilized laser light thus providing detection sensitivities that are much less than the radius of a hydrogen atom. High spatiotemporal resolutions (atomic-scale) can be achieved in 4D ultrafast electron microscopy (UEM). Thus it is possible to image structures, morphologies, as well as nanomechanical motions (e.g., nanogating and nanodrumming) in real-time. Using embodiments of the present invention, we direct visualized nano and microscale cantilevers, and the (resonance) oscillations of their mechanical motions. The static images were constructed from a tomographic tilt series of images, whereas the in situ temporal evolution was determined using the stroboscopic configuration of UEM, which is comprised of an initiating (clocking) laser pulse and a precisely-timed packet of electrons for imaging. The pseudo-one-dimensional molecular material (copper 7,7,8,8-tetracyanoquinodimethane, [Cu(TCNQ)]), which forms single crystals of nanometer and micrometer length scale, is used as a prototype. The optomechanical motions are triggered by charge transfer from the TCNQ radical anion (TCNQ−) to copper (Cu+). More than a thousand frames were recorded to provide a movie of the 3D movements of cantilevers in time. As shown below, the expansions are colossal, reaching the micrometer scale, and the spatial modes are resolved on the nanoscale in the images (and angstrom-scale in diffraction) with resonances of megahertz frequencies for the fixed-free cantilevers. From these results, we obtained the Young's modulus, and force and energy stored in the cantilevers. Here, different crystals were studied and generally are of two types: (1) those “standing”, which are free at one end (cantilevers), and (2) those which are “sleeping” on the substrate bed; the latter will be the subject of another report. For cantilevers, the dimensions of the two crystals studied are 300 nm thick by 4.6 μm long and 2.0 μm thick by 10 μm long (see FIG. 26). As such, they define an Euler-Bernoulli beam, for which we expect the fundamental flexural modes to be prominent, besides the longitudinal one(s) which are parallel to the long axis of the crystal. Our interest in Cu(TCNQ) stems from its highly anisotropic electrical and optical properties, which arise from the nature of molecular stacking in the structure. As illustrated in FIG. 26, Cu(TCNQ) consists of an interpenetrating network of discrete columns of Cu+ and TCNQ− running parallel to the crystallographic a-axis. The TCNQ molecules organize so that the π-systems of the benzoid rings are strongly overlapped, and the favorable interaction between stacked TCNQ molecules makes the spacing between the benzoid rings only 3.24 Å, significantly less than that expected from purely van der Waals-type interactions. It is this strong π-stacking that results in the pseudo-one-dimensional macroscale crystal structure and is responsible for the anisotropic properties of the material. With electric field or light, the material becomes mixed in valence with both Cu+(TCNQ−) and)Cu° (TCNQ°) in the stacks, weakening the interactions and causing the expansion. At high fluences, the reversible structural changes become irreversible due to the reduction of copper from the +1 oxidation state to copper metal and subsequent formation of discrete islands of copper metal driven by Ostwald ripening. The methodology we used here for synthesis resulted in the production of single crystals of phase I. FIG. 26 illustrates atomic to macro-scale structure of phase I Cu(TCNQ). Shown in the upper panel is the crystal structure as viewed along the a-axis (i.e., π-stacking axis) and c-axis. The unit cell is essentially tetragonal (cf. ref 19) with dimensions: a=3.8878 Å, b=c=11.266 Å, α=γ=90°, β=90.00(3)°; gray corresponds to carbon, blue corresponds to nitrogen, and yellow corresponds to copper. The hydrogen atoms on the six-membered rings are not shown for clarity. The lower panel displays a typical selected-area diffraction pattern from Cu(TCNQ) single crystals as viewed down the [011] zone axis along with a micrograph taken in our UEM. The rod-like crystal habit characteristic of phase I Cu(TCNQ) is clearly visible. FIG. 27 illustrates a tomographic tilt series of images. The frames show images (i.e., 2D-projections) of the Cu(TCNQ) single crystals acquired at different tilt angles of the specimen substrate. The highlighted region illustrates a large change in the position of the free-standing mircoscale crystal relative to another, which is lying flat on the substrate, as we change the tilt angle. The scale bar in the lower left corner measures two micrometers. The tilt angle at which each image was acquired is shown in the lower right corner of each frame in degrees. The tilt angle is defined as zero when the specimen substrate is normal to the direction of electron propagation in the UEM column. The tilt series images shown in FIG. 27 provide the 3D coordinates of the cantilevers. The dimensions and protrusion angles of these free-standing crystals were characterized by taking static frames at different rotational angles of the substrate. By placing the crystal projections into a laboratory frame orthogonal basis and measuring the length of the projections in the x-y (substrate) plane as the crystal is rotated by an angle α about the x-axis, the measured projections were obtained to be Θ of 37.8° and φ of 25.3°, where Θ is the angle the material beam makes with respect to substrate-surface normal and φ is the azimuthal angle with respect to the tilt axis, respectively. Note that the movie of the tilt series clearly shows the anchor point of the crystal to be the substrate. The dimensions and geometries of the crystals are determined from the tilt series images with 5% precision. To visualize real-time and space motions, the microscope was operated at 120 kV and the electron pulses were photoelectrically generated by laser light of 355 nm. The clocking optical pulses (671 nm laser), which are well-suited to induce the charge transfer in Cu(TCNQ), were held constant at 3 μJ, giving a maximum fluence of 160 mJ/cm2. Because the relevant resonance frequencies are on the MHz scale, the ns pulse arrangement of the UEM was more than enough for resolving the temporal changes. The time delay between the initiating laser pulse and probe electron pulse was controlled with precision, and the repetition rate of 100 Hz ensured recovery of the structure between pulses. A typical static image and selected-area diffraction are displayed in FIG. 26. From the selected-area diffraction and macroscopic expansion we could establish the nature of correlation between unit cell and the crystal change. The 4D space-time evolution of cantilevers is shown in FIGS. 28 and 29. The referenced (to negative time, tref=−10 ns; i.e., before the arrival of the clocking pulse) difference images of the microscale (FIG. 28) and nanoscale (FIG. 29) free-standing single crystal clearly display modes of expansion on the MHz scale. Each image illustrates how the spatial location of the crystal has changed relative to the reference image as a function of the time delay, elucidating both the longitudinal and transverse displacements from the at-rest position. In order to accurately measure the positions in space we used a reference particle in the image. These reference particles, which are fixed to the surface of the substrate, do not appear in frame-reference images if drift is absent or corrected for. This is an important indication that the observed crystal dynamics do not arise from motion of the substrate due to thermal drift or photothermal effects. Moreover, there is no significant movement observed in images obtained before the arrival of the excitation pulse, indicating that, during the time of pulse separation, the motion has completely damped out and the crystal has returned to its original spatial configuration. The thermal, charging, and radiation effects of the electron pulses are negligible here and in our previous studies made at higher doses. This is evidenced in the lack of blurring of the images or diffraction patterns; no beam deflection due to sample charging was observed. Lastly, no signs of structural fatigue or plasticity were observed during the course of observation, showing the function of the cantilever to be robust for at least 107 pulse cycles. Shown in FIG. 30 is the displacement of the microscale single crystal as a function of time, in both the longitudinal and transverse directions, along with the fast Fourier transforms (FFT) of the observed spatial oscillations for the time range shown (i.e., 0 to 3.3 μs). The motions in both directions of measurement are characterized by a large initial displacement from the at-rest position. The scale of expansion is enormous. The maximum longitudinal expansion possible (after accounting for the protrusion angle) for the 10 μm crystal would be 720 nm or over 7% of the total length. For comparison, a piezoelectric material such as lead zirconate titanate has typical displacements of less than 1% from the relaxed position, but it is known that molecular materials can show enormous optically-induced elastic structural changes on the order of 10% or more. The large initial motion is transferred into flexural modes in the z and x-y directions, and these modes persist over the microsecond (or longer) scale. The overall relaxation of the crystal to its initial position is not complete until several milliseconds after excitation. From the FFTs of the measured displacements, we obtained the frequency of longitudinal oscillation to be 3.3 MHz, whereas the transverse oscillations are found at 2.5 and 3.3 MHz (FIG. 30). We note that the motion represents coupling of modes with dephasing, so it is not surprising that the FFT gives more than one frequency. In fact, from an analysis consisting of a decomposition of the motion via rotation of a principle axes coordinate system relative to the laboratory frame, we found that the plane of lateral oscillation of the crystal was tilted by 18° relative to the plane of the substrate. The nature of contact with the substrate influences not only the mode structure but also the damping of cantilevers. Because of the boundary conditions of a fixed-free beam, the vibration nodes are not evenly spaced and the overtones are not simple integer multiples of the fundamental flexural frequency (f1), but rather occur at 6.26, 17.5, and 34.4 f2, f3, and f4, respectively. This is in stark contrast to the integer multiples of the fundamental frequency of a fixed-fixed beam. Taking 3 MHz to be the main fundamental flexural frequency of the microscale crystal, we can deduce Young's elastic modulus of the crystal. The expression for the frequencies of transverse (flexural) vibrations of a fixed-free beam is given by, f n = η π κ 8 L 2 c ≡ η π κ 8 L 2 Y ρ ( 19 ) where fn is the frequency of the nth mode in Hz, L is the beam length at rest, Y is Young's modulus, and ρ is the density. The radius of gyration of the beam cross section is κ and is given as t/√{square root over (12)}, where t is the thickness of the beam with rectangular cross section. The value of η for the beam is: 1.1942; 2.9882; 52; 72; . . . ; (2n−1)2, approaching whole numbers for higher η values. The overtones are not harmonics of the fundamental, and the numerical terms for f1 and f2, which result from the trigonometric solutions involved in the derivation, must be used without rounding. For the longitudinal modes of fixed-free beam, fn=(2n−1)c/4 L. From the above equation, and knowing ρ=1.802 g·cm−3, we obtained Young's modulus to be 2 GPa, with the speed of sound, therefore, being 1,100 m·s−1; we estimate a 12% uncertainty in Y due to errors in t, L, and f. This value of Young's modulus (N·m−2) is very similar to that measured for TTF-TCNQ single crystals using a mm-length vibrating reed under an alternating voltage. Both materials are pseudo-one-dimensional, and the value of the modulus is indicative of the elastic nature along the stacking axis in the direction of weak intercolumn interactions. Young's modulus slowly varies in value in the temperature range of 50 to 300 K but, when extrapolated to higher temperatures, decreases for both TTF-TCNQ and K(TCNQ). From the absorbed laser pulse energy (30 nJ), the amount of material (7.2×10−14 kg), and assuming the heat capacity to be similar to TTF-TCNQ (430 J·K−1·mol−1), the temperature rise in the microscale crystal is expected to be at most 260 K. Finally, we note that for the same modulus reported here, the frequency of longitudinal mode expansion [f=c/4 L; n=1] should be nearly 25 MHz, which is not seen in the FFT with the reported resolution, thus suggesting that the observed frequencies in the longitudinal direction are those due to cantilever motion in the z direction; the longitudinal expansion of the crystal is about 1 to 2% of its length, which in this case will be 100 to 200 nm. The potential energy stored in the crystal and the force exerted by the crystal at the moment of full extension along the long axis just after time zero [cf. FIG. 30(A)] can be estimated from the amplitudes and using Hooke's law: V = 1 2 ( YA L ) Δ L 2 ( 20 a ) F = ( YA L ) Δ L ( 20 b ) where V and F are the potential energy and force, respectively, and A is the cross-sectional area of the crystal. The bracketed term in equation (20) is the spring constant (assuming harmonic elasticity, and not the plasticity range), and by simple substitution of the values, we obtained 200 pJ and 600 μN for the potential energy and force, respectively, considering the maximum possible expansion of 720 nm; even when the amplitude is at its half value [see FIG. 30(A)], the force is very large (˜300 μN). For comparison, the average force produced by a single myosin molecule acting on an actin filament, which was anchored by two polystyrene beads, was measured to be a few piconewtons. In other words, because of molecular stacking, the force is huge. Also because of the microscale cross-section, the pressure of expansion translates to 0.1 GPa, only a few orders of magnitude less than pressures exerted by a diamond anvil. Based on the laser fluence, crystal dimensions, and absorptivity of Cu(TCNQ) at 671 nm (3.5×106 m−1), the maximum pulse energy absorbed by the crystal is 30 nJ. This means that, of the initial optical energy, a minimum of ˜1% is converted into mechanical motion of the crystal. But in fact, it could reach 10 or more percent as determined by the projection of the electric field of light on the crystal. In order to verify the trend in frequency shifts, the above studies were extended to another set of crystal beams, namely those of reduced dimensions. Because the resonant frequencies of a fixed-free beam are determined, in part, by the beam dimensions [cf. equation (19)], a Cu(TCNQ) crystal of different length than that shown in FIG. 30 should change the oscillation frequencies by the κ/L2 dependence. With a smaller cantilever beam we measured the oscillation frequencies for a crystal of 300 nm thickness and 4.6 μm length, using the same laser parameters as for the larger crystals, and found them to be at higher values (FIG. 31). This is confirmed by the FFTs of the displacement spanning the range 0 to 3.3 μs [FIGS. 31 (C) and (D)]; a strong resonance near 9 MHz with another weaker resonance at 3.6 MHz in the longitudinal direction [FIG. 31 (C)] is evident. Within a few microseconds, the only observed frequency in the FFT was near 9 MHz. This oscillation persists up to the time scans of 30 μs, at which point the amplitude was still roughly 40% of the leveling value near 2 μs. By taking this duration (30 μs) to be the decay time (τ) required for the amplitude to fall to 1/e of the original value, the quality factor (Q=πfτ) of the crystal free oscillator becomes near 1,000. However, on longer time scales, and with less step resolution, the crystal recovers to the initial state in a few milliseconds, and if the mechanical motion persists, Q would increase by an order of magnitude. It is clear from the resonance value of the flexural frequency at 9 MHz that as the beam reduces in size, the frequency increases, as expected from equation (19). However, if we use this frequency to predict Young's modulus we will obtain a value of 30 GPa, which is an order of magnitude larger than that for the larger microscale crystal. The discrepancy points to the real differences in modes structure as we reach nanometer-scale cantilevers. One must consider, among other things, the anchor-point(s) of the crystals, the frictional force with substrate and other crystals, and the curvature of the beam (see movie in supporting information). This curvature will cause the crystal to deviate from ideal Euler-Bernoulli beam dynamics, thus shifting resonance frequencies from their expected positions. Interestingly, by using the value of 30 GPa for Young's modulus, the minimum conversion efficiency increases by a factor of 15. These dependencies and the extent of displacement in different directions, together with the physics of modes coupling (dephasing and rephasing), will be the subject of our full account of this work. Thus, with 4D electron microscopy it is possible to visualize in real space and time the functional nanomechanical motions of cantilevers. From tomographic tilt series of images, the crystalline beam stands on the substrate as defined by the polar and azimuthal angles. The resonance oscillations of two beams, micro and nanocantilevers, were observed in situ giving Young's elastic modulus, the force, and the potential energy stored. The systems studied are unique 1D molecular structures, which provide anisotropic and colossal expansions. The cantilever motions are fundamentally of two types, longitudinal and transverse, and have resonance Q factors that make them persist for up to a millisecond. The function is robust, at least for 107 continuous pulse cycles (˜1011 oscillations for the recorded frames), with no damage or plasticity. With these imaging methods in real-time' and with other variants, it is now possible to test the various theoretical models involved in MEMS and NEMS. Electron energy loss spectroscopy (EELS) is a powerful tool in the study of valency, bonding and structure of solids. Using our 4D electron microscope, we have performed ultrafast EELS, taking the time resolution in the energy-time space into the femtosecond regime, a 10 order of magnitude increase, and for a table-top apparatus. It is shown that the energy-time-amplitude space of graphite is selective to changes, especially in the electron density of the π+σ plasmon of the collective oscillation of the four electrons of carbon. Embodiments of the present invention related to EELS enable the microscope to be used as an analytical tool. As electrons pass through the specimen, each type of material (e.g., gold, copper, or zinc) will have a different electron energy. Thus, it is possible to “tune” into a particular element and study the dynamic behavior of the material itself. In microscopy, EELS provides rich characteristics of energy bands describing modes of surface atoms, valence- and core-electron excitations, and interferences due to local structural bonding. The scope of applications thus spans surface and bulk elemental analysis, chemical characterization and electronic structure of solids. The static, time-integrated, EEL spectra do not provide direct dynamic information, and with video-rate scanning in the microscope could changes be recorded only with a time resolution of millisecond or longer. Dedicated time-resolved EELS apparatus, without imaging, have obtained millisecond resolution, being determined primarily by detector response and electron counts. However, for studies of dynamics of electronic structure, valency and bonding, the time resolution must increase by at least nine orders of magnitude. We have performed femtosecond resolved EELS (FEELS) using our ultrafast electron microscope (UEM), developed for 4D imaging of structures and morphology. Embodiments of the present invention are conceptually different from time-resolved EELS (termed TREELS) as the time resolution in FEELS is not limited by detector response and sweep rate. Moreover, both real-space images and energy spectra can be recorded in situ in UEM and with energy filtering the temporal resolution can be made optimum. We demonstrate the method in the study of graphite which displays changes on the femtosecond (fs) time scale with the delay steps being 250 fs. Near the photon energy of 2.4 eV (away from the zero energy loss peak), and similarly for the π+σ plasmon band, the change is observed, but it is not as significant for the π plasmon band. Thus it is possible to chart the change from zero to thousands of eV and in 3D plots of time, energy and amplitude; the decrease in EELS intensity at higher energies becomes the limiting factor. This table-top approach using electrons is discussed in relation to recent achievements using soft and hard (optical) X-rays in laboratory and large-scale facilities of synchrotrons and free electron lasers. According to embodiments of the present invention, the probing electrons and the initiating light pulses are generated by a fs laser, and the EEL spectra of the transmitted electrons are recorded in a stroboscopic mode by adjusting the time delay between the pump photons and the probe electron bunches. The concept of single-electron packet used before in imaging is utilized in this approach. When each ultrafast electron packet contains at most one electron, “the single-electron mode,” space-charge broadening of the zero-loss energy peak, which decreases the spectrometer's resolution, is absent. FIG. 32 is a schematic diagram of a microscope used in embodiments of the present invention. A train of 220 fs laser pulses at 1.2 eV was frequency doubled and tripled and then split into two beams. In other embodiments, a range of laser pulse widths could be used, for example from about 10 femtoseconds to about 10 microseconds. The frequency tripled light at 3.6 eV was directed to the microscope photocathode, and the photoelectron probe pulse was accelerated to 200 keV. The 2.4 eV pulses were steered to the specimen, and provided the excitation at a fluence of 5.3 mJ/cm2. In other embodiments, a fluence ranging from about 1 mJ/cm2 to about 20 mJ/cm2 could be utilized. By varying the delay time between the electron and optical pulses, the time dependence of the associated EEL spectrum was followed. The electrons pass through the sample and a set of magnetic lenses to illuminate the CCD camera, forming either a high resolution image of the specimen, a diffraction image, or they can be energy dispersed to provide the EEL spectra. The apparatus is equipped with a Gatan imaging filter (GIF) Tridiem, of the postcolumn type, which is attached below the transmission microscope camera chamber. The energy width of near 1 eV was measured for the EELS zero-loss peak and it is comparable to that obtained in thermal-mode operation of the TEM, but increases significantly in the space-charge limited regime. The experiments were performed at repetition rates of 100 kHz and 1 MHz, and no difference in the EEL spectra or the temporal behavior was observed, signifying a complete recovery of electronic structure changes between subsequent pulses. The reported temporal changes were missed when the scan resolution exceeded 250 fs, and the entire profile of the transient is complete in 2 ps. The electron beam passes through the graphite sample perpendicular to the sample surface while the laser light polarization was parallel to the graphene layers. Finally, the zero of time was determined to the precision of the reported steps, and was observed to track the voltage change in the FEG module of the microscope. The semi-metal graphite is a layered structure, which was prepared as free-standing film. The thickness of the graphite film was estimated from the EEL spectrum to be 106 nm (inelastic mean-free path of ˜150 nm), and the crystallinity of the specimen was verified by observing the diffraction pattern which was indexed as reported. FIG. 33 shows a static EEL spectrum of graphite taken in UEM. The distinct features are observed in the spectrum and indeed are typical of the electronic structure bands of graphite; the in-plane π plasmon is found near 7 eV, while at higher energy, the peak at 27 eV is observed with a shoulder at 15 eV. These latter peaks correspond to the π+σ oscillation of the bulk and surface plasmons, respectively. The results are in agreement with those of literature reports. The bands displayed in different colors (FIG. 33) are the simulations of the profiles with peak positions reproducing the theoretical values near 7, 15 and 27 eV. The 3D FEELS map of the time-energy evolution of the amplitude of the plasmon portion of the spectrum (up to 35 eV) is shown in FIG. 34, together with the EEL spectrum taken at negative time. The spectra were taken at 1 MHz repetition-rate, for a pump fluence of 5.3 mJ/cm2 at room temperature and for ts=250 fs for each difference frame. The map reflects the difference for all energies and as a function of time, made by subtracting a reference EEL spectrum at negative time from subsequent ones. The relatively strong enhancement of the energy loss in the low energy (electron-hole carriers) region is visible and the change is near the energy of the laser excitation. This feature represents the energy loss enhancement due to the creation of carriers by the fs laser excitation in the ππ* band structure, as discussed below. At higher energy, the 7 eV π plasmon peak remains nearly unperturbed by the excitation, and no new features are observed at the corresponding energies. For the 27 eV π+σ bulk plasmon an increased spectral weight at positive time is visible as a peak in the time-resolved spectrum. In order to obtain details of the temporal evolution of the different spectroscopic energy bands, we divided the spectrum into three regions: the low energy region between 2 and 5 eV, the π plasmon region between 6 and 8 eV, and the π+σ plasmon region between 20 and 30 eV. The 3D data are integrated in energy within the specified regions of the spectrum, and the temporal evolution of the different loss features are obtained; see FIG. 35. For regions where changes occur, the time scales involved in the rise and subsequent decay are similar. In FEELS, the shortest decay is 700 fs taken with the steps of 250 fs. The duration of the optical pulse is ˜220 fs, but we generate the UV pulse for electron generation through a non-linear response, and it is possible that the pulses involved are asymmetric in shape and that multiphotons are part of the process; full analysis will be made later. We note that the observed ˜700 fs response indeed reflects the joint response from both the optical and electron pulses and it is an upper limit for the electronic change. It is remarkable that, in FIG. 35, the temporal evolution of the interlayer spacing of graphite obtained by ultrafast electron crystallography (UEC) at a similar fluence, i.e. 3.5 mJ/cm2, the timescale of the ultrafast compression corresponds well to the period in which the bulk plasmon is out of equilibrium; in this plot the zero of time is defined by the change of signal amplitude. In graphite, the characteristic time for the thermalization of photo-excited electrons is known to be near 500 fs at low fluences (a few μJ/cm2). When excited by an intense laser pulse, a strong electrostatic force between graphene layers is induced by the generated electron-hole (carrier) plasma. This causes the structure to be out of equilibrium for nearly 1 ps; a stressful structural rearrangement is imposed on the crystal, which, at very high fluences (above 70 mJ/cm2), has been proposed as a cause of the phase transformation into diamond. Because graphite is a quasi two-dimensional structure, distinct spectral features are visible in EELS. The most prominent and studied peaks are those at 7 eV and the much stronger one at 27 eV. From the solution of the in-plane and out-of-plane components of the dielectric tensor it was shown, for graphite, that the 7 eV band is a π plasmon, resulting from interband ππ* transitions in the energy range of 2-5 eV, whereas the 27 eV band is a π+σ plasmon dominated by σσ* transitions beyond 10 eV (FIG. 5). We note that in this case the plasmon frequencies are not directly given by the ππ* and σσ* transition energies as they constitute tensorial quantities. For example ϖ π + σ 2 = ϖ p 2 + 1 4 ( Ω π 2 + 3 Ω σ 2 ) ,where ωp=npe2/∈0m)1/2 is the free electron gas plasma frequency; Ωπ and Ωσ are the excitation energies for ππ* and σσ* transitions, respectively. For ωp, the electron density is np, n is the number of valence electrons per atom and p is the density of atoms, and ∈0 is the vacuum dielectric constant. It follows that the density of occupied and empty (π, σ, π*, and σ*) states is critical, and that the π Plasmon is from the collective excitation of the π electrons (one electron in the p-orbital, with screening corrections) whereas the π+σ plasmon is the result of all 4 valence electrons collectively excited over the coherent length scale of bulk graphite; there are also surface plasmons but at different energies. Recently it was demonstrated, both theoretically and experimentally, that the π and π+σ plasmons are sensitive to the inter-layer separation, but while the former shows some shift of peaks the latter is dramatically reduced in intensity, and, when reaching the grapheme limit, only a relatively small peak at ˜15 eV survives. This is particularly evident when the momentum transfer is perpendicular to the c-axis, the case at hand and for which the EEL spectrum is very similar to ours. With the above in mind, it is now possible to provide, in a preliminary picture, a connection between the selective fs atomic motions, which are responsible for the structural dynamics, and changes in the dielectric properties of Plasmon resonances, the electronic structure. The temporal behavior, and coherent oscillation (shear modes of ˜1 ps), of c-axis expansion display both contraction and expansion on the picometer length scale per unit cell. The contraction precedes the expansion, as shown in FIG. 35, with velocity that depends on the fluence, i.e., the density of carriers. With fs excitation, the electronic bands are populated anisotropically, and, because of energy and momentum conservation, the carriers transiently excite large-momentum phonons, so called strongly coupled phonons. They are formed on the fs time scale (electron-phonon coupling) but decay in ˜7 ps. The initial compression suggests that the process is a cooperative motion and is guided by the out-of-equilibrium structure change dictated by the potential of excited carriers; in this case ππ* excitation which weakens c-axis bonding. The initial atomic compression, when plotted with transient EELS data (FIG. 35), shows that it is nearly in synchrony with the initial change, suggesting that the spacing between layers (c-axis separation) is the rate determining step, and that in the first 1 ps, the compressed ‘hard graphite’ effect is what causes the increase in the amplitude of the π+σ plasmon peak. In other words, the decrease of the spectral weight due to the change of electronic structure upon increasing the interlayer separation (to form graphene) becomes an increase when the plates are compressed, because of the enhanced collectiveness of all four valence electrons of carbon. The change involves shear motions and it is not surprising that the π+σ peak (dominated by σσ* excitation) is very sensitive to such changes. The π peak is less influenced as only one electron is involved, as discussed above, and the amplitude change is relatively small. The faster recovery of EEL peaks in 700 fs is, accordingly, the consequence of expansion which ‘decouples’ the π and σ system. Lastly, the relatively large increase in EEL near the photon energy is due to carrier excitation (π*) which leads to a loss of electron energy at near 3 eV, possibly by electronic excitation involving the cy system (FIG. 36). The created carriers cause an increase in the Drude band as evidenced in the decrease in optical transmission. The demonstration of ultrafast EELS in electron microscopy opens the door to experiments that can follow the ultrafast dynamics of the electronic structure in materials. The fs resolution demonstrates the ability of UEM to probe transients on the relevant sub-picosecond time scale, while keeping the energy resolution of EELS. Moreover, the selectivity of change in the collective electron density (for graphite) suggests future experiments, including those with changes in polarization, shorter optical pulses, core excitation and oxidation sites. We believe that this table-top UEM-EELS should provide the methodology for studies which have traditionally been made using synchrotrons (and free electron lasers) especially in the UV and soft X-ray regions. Chemical bonding dynamics are important to the understanding of properties and behavior of materials and molecules. Utilizing embodiments of the present invention, we have demonstrated the potential of time-resolved, femtosecond electron energy loss spectroscopy (EELS) for mapping electronic structural changes in the course o nuclear motions. For graphite, it is found that changes of milli-electron volts in the energy range of up to 50 electron volts reveal the compression and expansion of layers on the subpicometer scale (for surface and bulk atoms). These nonequilibrium structural features are correlated with the direction of change from sp2 [two-dimensional (2D) grapheme] to sp3 (3D-diamond) electronic hybridization, and the results are compared with theoretical charge-density calculations. The reported femtosecond time resolution of four-dimensional (4D) electron microscopy represents an advance of 10 orders of magnitude over that of conventional EELS method. Bonding in molecules and materials is determined by the nature of electron density distribution between the atoms. The dynamics involve the evolution of electron density in space and the motion of nuclei that occur on the attosecond and femtosecond time scale, respectively. Such changes of the charge distribution with time are responsible for the outcome of chemical reactivity and for phenomena in the condensed phase, including those of phase transitions and nanoscale quantum effects. With convergent-beam electron diffraction, the static pattern of charge-density difference maps can be visualized, and using x-ray absorption and photoemission spectroscopy substantial progress has been made in the study of electronic-state dynamics in bulks and on surfaces. Electron energy loss spectroscopy (EELS) is a powerful method in the study of electronic structure on the atomic scale, using aberration-corrected microscopy, and in chemical analysis of selected sites; the comparison with synchrotron-based near-edge x-ray absorption spectroscopy is impressive. The time and energy resolutions of ultrafast electron microscopy (UEM) provide the means for the study of (combined) structural and bonding dynamics. Here, time-resolved EELS is demonstrated in the mapping of chemical bonding dynamics, which require nearly 10 orders of magnitude increase in resolution from the detector-limited millisecond response. By following the evolution of the energy spectra (up to 50 eV) with femtosecond (fs) resolution, it was possible to resolve in graphite the dynamical changes on a millielectronvolt (subpicometer motion) scale. In this way, we examined the influence of surface and bulk atoms motion and observed correlations with electronic structural changes: contraction, expansion, and recurrences. Because the EEL spectra of a specimen in this energy range contain information about plasmonic properties of bonding carriers, their observed changes reveal the collective dynamics of valence electrons. Graphite is an ideal test case for investigating the correlation between structural and electronic dynamics. Single-layered grapheme, the first two-dimensional (2D) solid to be isolated and the strongest material known, has the orbitals on carbon as sp2 hybrids, and in graphite the π-electron is perpendicular to the molecular plane. Strongly compressed graphite transforms into diamond, whose charge density pattern is a 3D network of covalent bonds with sp3 hybrid orbitals. Thus, any structural perturbation on the ultra-short time scale of the motion will lead to changes in the chemical bonding and should be observable in UEM. Moreover, surface atoms have unique binding, and they too should be distinguishable in their influence from bulk atom dynamics. The experiments were performed on a nm-thick single crystal of natural hexagonal graphite. The sample was cleaved repeatedly until a transparent film was obtained, and then deposited on a transmission electron microscopy (TEM) grid; the thickness was determined from EELS to be 108 nm. The fs-resolved EELS (or FEELS) data were recorded in our UEM, operating in the single-electron per pulse mode to eliminate Boersch's space charge effect. A train of 220 fs infrared laser pulses (λ=1038 nm) was split into two paths, one was frequency-doubled and used to excite the specimen with a fluence of 1.5 mJ/cm2, and the other was frequency-tripled into the UV and directed to the photoemissive cathode to generate the electron packets. These pulses were accelerated in the TEM column and dispersed after transmission through the sample in order to provide the energy loss spectrum of the material. The experimental, static EEL spectra of graphite in our UEM, with grapheme for comparison, are displayed in FIG. 37A; FIG. 37B shows the results of theoretical calculations. The spectral feature around 7 eV is the π Plasmon, the strong peak centered around 26.9 eV is the π+σ bulk plasmon, and the weaker peak on its low energy tail is due to the surface Plasmon. The agreement between the calculated EEL spectra and the experimental ones is satisfactory both for graphite and grapheme. Of relevance to our studies of dynamics is the simulation of the spectra for different c-axis separations, ranging from twice as large as naturally occurring (2c/a; a and c are lattice constraints) to 5 times as large. This thickness dependence is displayed in FIG. 37B. As displayed in FIG. 37, the surface and bulk Plasmon bands (between 13 and 35 eV) can be analyzed using two Voigt functions, thus defining the central position, intensity, and width. At different delay times, we monitored the changes and found that they occur in the intensity and position; the width and shape of the two spectral components are relatively unchanged. FIGS. 37C and 37D, show the temporal changes of the intensity for both the surface and bulk plasmons. As noted, the behavior of bulk dynamics is “out of phase” with that of the surface dynamics, corresponding to an increase in intensity for the former and a decrease for the latter. Each time point represents a 500-fs change. Within the first 1 ps, the bulk Plasmon gains spectral weight with the increase in intensity. With time, the intensity is found to return to its original (equilibrium) value. At longer times, a reverse in sign occurs, corresponding to a decrease and then an increase in intensity—an apparent recurrence or echo occurring with dispersion. The intensity change of the surface plasmon in FIGS. 37C and 37D, shows a π phase-shifted temporal evolution with respect to that of the bulk plasmon. The time dependence of the energy position of the different spectral bands is displayed in FIG. 38. The least-squares fit converges for a value of the surface plasmon energy at 14.3 eV and of the bulk plasmon at 26.9 eV. The temporal evolution of the surface plasmon gives no sign of energy dispersion, whereas the bulk plasmon is found to undergo first a blueshift and then a redshift at longer times (FIGS. 38A and 38B). The overall energy-time changes in the FEEL spectra are displayed in FIG. 39. To make the changes more apparent, the difference between the spectra after the arrival of the initiating laser pulse (time zero) and a reference spectrum taken at −20 ps before time zero is shown. The most pronounced changes are observed in the region near the energy of the laser itself (2.39 eV), representing the energy-loss enhancement due to the creation of carriers by the laser excitation, and in the region dominated by the surface and bulk plasmons (between 13 and 35 eV). Clearly evident in the 3D plot are the energy dependence as a function of time, the echoes, and the shift in phase. A wealth of information has been obtained on the spectroscopy and structural dynamics of graphite. Of particular relevance here are the results concerning contraction and expansion of layers probed by diffraction on the ultra-short time scale. Knowing the amplitude of contraction/expansion, which is 0.6 pm at the fluence of 1.5 mJ/cm2, and from the charge of plasmon energy with interlayer distance (FIG. 37), we obtained the results shown in FIG. 38C. The diffraction data, when now translated into energy change, reproduce the pattern in FIG. 38A, with the amplitude being within a factor of two. When the layers are fully separated, that is, reaching grapheme, the bulk plasmon, as expected, is completely suppressed. The dynamics of chemical bonding can now be pictured. The fs optical excitation of graphite generates carriers in the nonequilibrium state. They thermalize by electron-electron and electron-phonon interactions on a time scale found to be less than 1 ps, less than 500 fs, and −200 fs. From our FEELS, we obtained a rise of bulk plasmons in ˜180 fs (FIG. 39). The carriers generated induce a strong electrostatic force between grapheme layers, and ultrafast interlayer contraction occurs as a consequence. In FIG. 37D, the increase of the bulk plasmon spectral weight on the fs time scale reflects this structural dynamics of bond-length shortening because it originates from a denser and more 3D charge distribution. After the compression, a sequence of dilatations and successive expansions along the c axis follows, but, at longer times lattice thermalization dephases the coherent atomic motions; at a higher fluence, strong interlayer distance variations occur, and grapheme sheets can be detached as a result of these interlayer collisions. Thus, the observations reported here reflect the change in electronic structure: contraction toward diamond and expansion toward grapheme. The energy change with time correlates well with the EELS change calculated for different interlayer distances (FIG. 37). We have calculated the charge density distribution for the three relevant structures. The self-consistent density functional theory calculations were made using the linear muffin-tin orbital approximation, and the results are displayed in FIG. 40. To emphasize the nature of the changes observed in FEELS, and their connection to the dynamics of chemical bonding, we pictorially display the evolution of the charge distribution in a natural graphite crystal, a highly compressed one, and the extreme case of diamond. Once can see the transition from a 2D to a 3D electronic structure. The compressed and expanded graphite can pictorially be visualized to deduce the change in electron density as interlayer separations change. With image, energy, and time resolution in 4D UEM, it is possible to visualize dynamical changes of structure and electronic distribution. Such stroboscopic observations require time and energy resolutions of fs and meV, respectively, as evidenced in the case study (graphite) reported here, and for which the dynamics manifest compression/expansion of atomic planes and electronic sp2/sp3-type hybridization change. The application demonstrates the potential for examining the nature of charge density and chemical bonding in the course of physical/chemical or materials phase change. It would be of interest to extend the scale of energy from ˜1 eV, with 100 meV resolution, to the hundreds of eV for exploring other dynamical processes of bonding. The following articles are hereby incorporated by reference for all purposes: 4D imaging of transient structures and morphologies in ultrafast electron microscopy, Brett Barwick, et al., Science, Vol. 322, Nov. 21, 2008, p. 1227. Temporal lenses for attosecond and femtosecond electron pulses, Shawn A. Hibert, et al., PNAS, Vol. 106, No. 26, Jun. 30, 2009, p. 10558. Nanoscale mechanical drumming visualized by 4D electron microscopy, Oh-Hoon Kwon, et al., Nanoletters, Vol. 8, No. 11, November 2008, p. 3557. Nanomechanical motions of cantilevers: direct imaging in real space and time with 4D electron microscopy, David J. Flannigan, et al., Nanoletters, Vol. 9, No. 2 (2009), p. 875. EELS femtosecond resolved in 4D ultrafast electron microscopy, Fabrizio Carbone, et al., Chemical Physics Letters, 468 (2009), p. 107. Dynamics of chemical bonding mapped by energy-resolved 4D electron microscopy, Fabrizio Carbone, et al., Science, Vol. 325, Jul. 10, 2009, p. 181. Atomic-scale imaging in real and energy space developed in ultrafast electron microscopy, Hyun Soon Park, et al., Nanoletters, Vol. 7, No. 9, September 2007, p. 2545. It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. |
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abstract | The invention relates to an optical device intended to treat an incident X-ray beam. The optical device comprises a monochromator and an optical element for conditioning the incident beam. The reflective surface of the optical element is able to produce a two-dimensional optical effect in order to adapt a beam in destination of the monochromator. The reflective surface of the optical element comprises a multilayer structure type surface that is reflective to X-rays. In particular, the reflective surface consists of a single surface shaped according to two curvatures corresponding to two different directions. |
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048274933 | claims | 1. A method of making a radiography source of a radioactive material, said method comprising the steps of; providing a plurality of radioactive pellets, providing an open capsule of a rigid metal having sufficient tensile strength to resist substantial deformation under pressure and selected from a group including elements of the periodic table displaced in density by an amount at least on the order of 2.0 gm/cc from the density of the radioactive pellet material, sufficient to be resolvable by X-ray radiography, disposing the radioactive pellets in said capsule, compacting the radioactive pellets to reduce source focal size, providing a rigid metal plug, inserting the plug in the open capsule, and welding the plug to said capsule. 2. A method as set forth in claim 1 wherein the compacting step comprises, providing a ram dimensioned for close tolerance fit in said capsule, and compacting the pellets by inserting the ram into the capsule and applying predetermined pressure to the ram. 3. A method as set forth in claim 2 wherein said predetermined pressure is a direct function of the area of pellets to be compacted. 4. A method as set forth in claim 3 including providing a die and disposing the capsule in the die for subsequent compaction of pellets. 5. A method as set forth in claim 4 wherein the capsule is disposed in the die in a manner to hold the capsule so as to prevent any substantial deformation of the capsule due to pellet compaction. 6. A method as set forth in claim 5 wherein the die is provided as a split die including die halves that are closed to retain the capsule. 7. A method as set forth in claim 1 including constructing said capsule of titanium. 8. A method as set forth in claim 7 including constructing said plug of titanium. 9. A method as set forth in claim 1 including providing at least one insert of the same material as said capsule and disposing the insert in the capsule after compacting the pellets and before inserting the plug. 10. A method as set forth in claim 9 including constructing the capsule, plug and insert all of titanium. 11. A method as set forth in claim 1 including providing a cobalt-60 source and providing a capsule of a tensile strength greater than on the order of 45,000 p.s.i. 12. A radiography source of a radioactive material, said source comprising, a plurality of radioactive pellets, an open capsule of a rigid metal having sufficient tensile strength to resist substantial deformation under pressure and selected from a group including elements of the periodic table displaced in density by an amount at least on the order of 2.0 gm/cc from the density of the radioactive pellet material, sufficient to be resolvable by X-ray radiography, said pellets being disposed in said capsule and compacted therein to reduce the source focal size, and a plug disposed in the open capsule and sealed therewith. 13. A radiography source as set forth in claim 12 wherein said predetermined pressure is a direct function of the cross-sectional area of pellets to be compacted. 14. A radiography source as set forth in claim 12 wherein said capsule is constructed of titanium. 15. A radiography source as set forth in claim 14 wherein said plug is constructed of titanium. 16. A radiography source as set forth in claim 12 including at least one insert of the same material as said capsule, said insert disposed in the capsule. 17. A radiography source as set forth in claim 16 wherein said capsule, plug and insert are all constructed of titanium. 18. A radiography source as set forth in claim 12 wherein said pellets comprise cobalt-60 pellets and said capsule has a tensile strength greater than on the order of 45,000 p.s.i. |
047160055 | summary | BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to devices and a method for forming a pressure seal between two planar sealing surfaces and more particularly to such a device and method which include a toroidal, crushable seal member and a spacer unit for positioning the seal member between the planar sealing surfaces and for controlling the deformation of the seal member as it is crushed between the sealing surfaces. 2. Prior Art There are many types of equipment which require the formation of a pressure tight seal between confronting planar sealing surfaces. Such equipment includes, for example, pump housings, high pressure fluid conduits, and pressure vessels. Often the confronting planar sealing surfaces are formed on annular flanges which are bolted together. It is common practice to use organic or inorganic gaskets or "O" rings to form such seals and to provide annular grooves in one or both of the confronting planar sealing surfaces to position the seal member. There is a tendency in many such installations for the flanges to bend about the pivot formed by the edge of the gasket or the "O" ring. One type of installation in which a pressure tight seal is required between confronting planar sealing surfaces is the pressure vessel of a nuclear reactor. The upright cylindrical pressure vessel terminates at its upper end in a radially outwardly extending flange to which is bolted a complimentary flange on the hemispherical vessel head. In order to provide a seal to withstand the two to three thousand psi pressures developed in the pressure vessel with adequate safety margins, a crushable, tubular, metallic torus is typically inserted between the flanges. As the bolts are tightened to draw the flanges toward each other, the crushable tubular metallic torus is deformed to form the pressure tight seal. Annular grooves in the flanges position the torrus and prevent it from slipping as the bolts are tightened. Holes in the inner surface of the tubular torrus, admit reactor coolant into the tubular seal to pressurize it. SUMMARY OF THE INVENTION In accordance with the invention, a sealing assembly and a method for forming a seal between confronting annular, planar sealing surfaces include a toroidal, crushable seal member and an annular spacer unit having a resilient annular inner portion with an inner diameter smaller than the outer diameter of the seal member which surrounds the seal member and positions it between the sealing surfaces. The resilient portion of the spacer unit also applies uniform resistance to the periphery of the seal member as it is being crushed between the sealing surfaces so that the seal member remains centered as it is deformed and expands radially. The spacer unit is thinner than the crushable seal member so that it can be used to set the distance that the crushable seal member is axially compressed. Preferably, the spacer unit includes a substantially rigid, flat annular member so that the sealing surfaces can be clamped down firmly on the spacer unit. Where the sealing surfaces are drawn together by elongated fasteners, the spacer unit engages these fasteners to fix the lateral position of the seal member. Preferably, the elongated fasteners pass through axial bores in the flat annular member of the spacer unit. Also preferably, the flat annular member has a radially extending annular recess in its inner surface in which the resilient portion of the spacer unit, in the form of an annular resilient member, is seated. In the specific embodiment of the invention disclosed, this recess is V-shaped and the annular resilient member is a toroidal coil spring. The preferred seal member is a toroidal, metallic tubular member with radial holes in the inner surface through which the seal may be pressurized. |
abstract | The present invention, in one form, is a natural circulation reactor having, in one embodiment, a layer of high aluminate cement concrete disposed between an uninsulated steel liner and a prestressed concrete reactor vessel. The prestressed concrete reactor vessel includes a concrete shell having a cavity therein. The steel liner is positioned in the cavity and spaced from the concrete shell so that an insulating chamber is formed between the steel liner and the concrete shell. The insulating chamber is filled with high aluminate cement concrete which is configured to substantially insulate the concrete shell from the steel liner and to transfer loads such as pressure from the liner to the concrete shell. |
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abstract | A substrate cover includes a frame-like member configured to be placed on a substrate which is to be written using a charged particle beam, and to have an outer perimeter dimension larger than a perimeter end of the substrate and an inner perimeter dimension, being a border between the frame-like member and an inner opening portion, smaller than the perimeter end of the substrate, and a contact point part configured to be provided on an undersurface of the frame-like member, in order to be electrically connected to the substrate. |
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description | The present application claims the benefit of Japanese Patent Application No. 2020-166727, filed on Oct. 1, 2020, the entire contents of which are hereby incorporated by reference. The present disclosure relates to an extreme ultraviolet light generation system and an electronic device manufacturing method. Recently, miniaturization of a transfer pattern in optical lithography of a semiconductor process has been rapidly proceeding along with miniaturization of the semiconductor process. In the next generation, fine processing at 70 to 45 nm and further at 32 nm or less will be required. Therefore, in order to meet the demand for fine processing of, for example, 32 nm or less, the development of an exposure apparatus that combines an extreme ultraviolet (EUV) light generation apparatus that generates EUV light having a wavelength of about 13 nm and reduced projection reflection optics is expected. As an EUV light generation apparatus, three types of apparatuses have been proposed: a laser produced plasma (LPP) type apparatus using plasma generated by irradiating a target substance with pulse laser light, a discharge produced plasma (DPP) type apparatus using plasma generated by discharge, and a synchrotron radiation (SR) type apparatus using synchrotron radiation. Patent Document 1: US Patent Application Publication No. 2008/0143989 An extreme ultraviolet light generation system according to an aspect of the present disclosure includes a laser device configured to emit pulse laser light, an EUV light concentrating mirror configured to reflect and concentrate extreme ultraviolet light generated by irradiating a target with the pulse laser light, and a processor configured to receive a first energy parameter of the extreme ultraviolet light and control an irradiation frequency of the pulse laser light with which the target is irradiated so that change in a second energy parameter related to energy per unit time of the extreme ultraviolet light reflected by the EUV light concentrating mirror is suppressed. An electronic device manufacturing method according to an aspect of the present disclosure includes generating extreme laser light in an extreme ultraviolet light generation system, emitting the extreme ultraviolet light to an exposure apparatus, and exposing a photosensitive substrate to the extreme ultraviolet light in the exposure apparatus to manufacture an electronic device. Here, the extreme ultraviolet light generation system include a laser device configured to emit pulse laser light, an EUV light concentrating mirror configured to reflect and concentrate the extreme ultraviolet light generated by irradiating a target with the pulse laser light, and a processor configured to receive a first energy parameter of the extreme ultraviolet light and control an irradiation frequency of the pulse laser light with which the target is irradiated so that change in a second energy parameter related to energy per unit time of the extreme ultraviolet light reflected by the EUV light concentrating mirror is suppressed. A method of manufacturing an electronic device according to an aspect of the present disclosure includes inspecting a defect of a mask by irradiating the mask with extreme ultraviolet light generated in an extreme ultraviolet light generation system, selecting a mask using a result of the inspection, and exposing and transferring a pattern formed on the selected mask onto a photosensitive substrate. Here, the extreme ultraviolet light generation system includes a laser device configured to emit pulse laser light, an EUV light concentrating mirror configured to reflect and concentrate the extreme ultraviolet light generated by irradiating a target with the pulse laser light, and a processor configured to receive a first energy parameter of the extreme ultraviolet light and control an irradiation frequency of the pulse laser light with which the target is irradiated so that change in a second energy parameter related to energy per unit time of the extreme ultraviolet light reflected by the EUV light concentrating mirror is suppressed. <Contents> 1. EUV light generation system 11 according to comparative example 1.1 Configuration 1.2 Operation 1.3 Problems of comparative example 2. EUV light generation system 11a that controls irradiation frequency FI to suppress change in second energy parameter P2 2.1 Configuration 2.2 Operation 2.2.1 First operation example for increasing irradiation frequency FI 2.2.2 Second operation example for increasing irradiation frequency FI 2.3 Effect 3. EUV light generation system 11a to stepwise increase irradiation frequency FI 3.1 Configuration and operation 3.2 Effect 4. EUV light generation system 11a for changing pulse energy E of pulse laser light 33 4.1 Configuration and operation 4.2 Effect 5. Others Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The embodiments described below show some examples of the present disclosure and do not limit the contents of the present disclosure. Also, all configurations and operation described in the embodiments are not necessarily essential as configurations and operation of the present disclosure. Here, the same components are denoted by the same reference numerals, and duplicate description thereof is omitted. 1. EUV Light Generation System 11 According to Comparative Example 1.1 Configuration FIG. 1 schematically shows the configuration of an LPP-type EUV light generation system 11 according to a comparative example. An EUV light generation apparatus 1 is used together with a laser device 3. In the present disclosure, a system including the EUV light generation apparatus 1 and the laser device 3 is referred to as the EUV light generation system 11. The EUV light generation apparatus 1 includes a chamber 2 and a target supply unit 26. The chamber 2 is a sealable container. The target supply unit 26 supplies a target substance into the chamber 2. The material of the target substance may include tin, terbium, gadolinium, lithium, xenon, or a combination of any two or more thereof. A through hole is formed in a wall of the chamber 2. The through hole is blocked by a window 21 through which pulse laser light 32 emitted from the laser device 3 passes. An EUV light concentrating mirror 23 having a spheroidal reflection surface is arranged in the chamber 2. The EUV light concentrating mirror 23 has first and second focal points. A multilayer reflection film in which molybdenum and silicon are alternately stacked is formed on a surface of the EUV light concentrating mirror 23. The EUV light concentrating mirror 23 is arranged such that the first focal point is located in a plasma generation region 25 and the second focal point is located at an intermediate focal point 292. A through hole 24 is formed at the center of the EUV light concentrating mirror 23, and pulse laser light 33 passes through the through hole 24. The EUV light generation apparatus 1 includes a processor 5, a target sensor 4, and the like. The processor 5 is a processing device including a memory 501 in which a control program is stored, and a central processing unit (CPU) 502 for executing the control program. The processor 5 is specifically configured or programmed to perform various processes included in the present disclosure. The target sensor 4 detects at least one of the presence, trajectory, position, and velocity of a target 27. The target sensor 4 may have an imaging function. Further, the EUV light generation apparatus 1 includes a connection portion 29 providing communication between the internal space of the chamber 2 and the internal space of an EUV light utilization apparatus 6. An example of the EUV light utilization apparatus 6 will be described later with reference to FIGS. 15 and 16. A wall 291 in which an aperture is formed is arranged in the connection portion 29. The wall 291 is arranged such that the aperture is located at the second focal point of the EUV light concentrating mirror 23. Furthermore, the EUV light generation apparatus 1 includes a laser light transmission device 34, a laser light concentrating mirror 22, a target collection unit 28 for collecting the target 27, and the like. The laser light transmission device 34 includes an optical element for defining a transmission state of laser light, and an actuator for adjusting the position, posture, and the like of the optical element. 1.2 Operation Operation of the EUV light generation system 11 will be described with reference to FIG. 1. Pulse laser light 31 emitted from the laser device 3 enters, via the laser light transmission device 34, the chamber 2 through the window 21 as the pulse laser light 32. The pulse laser light 32 travels along a laser light path in the chamber 2, is reflected by the laser light concentrating mirror 22, and is radiated to the target 27 as the pulse laser light 33. The target supply unit 26 supplies the target 27 containing the target substance to the plasma generation region 25 in the chamber 2. The target 27 is irradiated with the pulse laser light 33. The target 27 irradiated with the pulse laser light 33 is turned into plasma, and radiation light 251 is radiated from the plasma. EUV light contained in the radiation light 251 is reflected by the EUV light concentrating mirror 23 with higher reflectance than light in other wavelength ranges. Reflection light 252 including the EUV light reflected by the EUV light concentrating mirror 23 is concentrated at the intermediate focal point 292 and output to the EUV light utilization apparatus 6. When the target 27 includes a plurality of droplets, one droplet may be irradiated with a plurality of pulses included in the pulse laser light 33. The processor 5 controls the entire EUV light generation system 11. The processor 5 processes a detection result of the target sensor 4. Based on the detection result of the target sensor 4, the processor 5 controls the timing at which the target 27 is output, the output direction of the target 27, and the like. Further, the processor 5 controls oscillation timing of the laser device 3, a travel direction of the pulse laser light 32, the concentration position of the pulse laser light 33, and the like. Such various kinds of control described above are merely exemplary, and other control may be added as necessary. 1.3 Problems of Comparative Example FIG. 2 is a graph showing change in an EUV emission efficiency and a second energy parameter P2 in the comparative example. In the graph of the present disclosure, the horizontal axis represents the operation time, and the alternate long and short dash lines indicate that events on each alternate long and short dash line occur at the same timing. The second energy parameter P2 is a parameter related to the energy per unit time of the EUV light reflected by the EUV light concentrating mirror 23 and includes one of EUV power, EUV power density, and EUV radiation brightness. The EUV power is the energy per unit time at the focal point of the EUV light, and the unit is watts (W). The focal point is the intermediate focal point 292 or a focal point on the downstream side on the optical path of the EUV light from the intermediate focal point 292. The EUV power density is a value obtained by dividing the EUV power by the optical path cross section at the focal point of the EUV light, and the unit is W/mm2. The EUV radiation brightness is a value obtained by dividing the EUV power density by a solid angle formed before and after the focal point of the EUV light, and the unit is W/mm2sr. A first energy parameter P1 will be described later. When the EUV light is generated by the EUV light generation system 11, the EUV emission efficiency may gradually decrease due to deterioration of the EUV collector mirror 23 or the like. When the EUV emission efficiency decreases, the second energy parameter P2 decreases even when various conditions such as the pulse energy E of the pulse laser light 33 are the same. The EUV light having a low second energy parameter P2 may not be suitable for use in the EUV light utilization apparatus 6. As one solution, the EUV light generation system 11 may be designed to output the EUV light having a second energy parameter P2 significantly higher than the lower limit of the second energy parameter P2 required by the EUV light utilization apparatus 6 when the EUV light generation system 11 is new. Accordingly, the EUV light having the second energy parameter P2 higher than the lower limit can be obtained for a long period of time. However, when it is new, the second energy parameter P2 may be too high. In some examples of the present disclosure, an irradiation frequency FI of the pulse laser light 33 with which the target 27 is irradiated is controlled so as to suppress change in the second energy parameter P2. The irradiation frequency FI refers to the number per second of times the target 27 is irradiated with the pulse laser light 33 and turned into plasma. 2. EUV Light Generation System 11a that Controls Irradiation Frequency FI to Suppress Change in Second Energy Parameter P2 2.1 Configuration FIG. 3 schematically shows the configuration of an EUV light generation system 11a according to a first embodiment. The EUV light utilization apparatus 6 that receives the EUV light generated in the EUV light generation system 11a includes a measurement device 61. The processor 5 included in the EUV light generation system 11a is connected to the measurement device 61 by a signal line through a processor (not shown) of the EUV light utilization apparatus 6. The present disclosure is not limited to the case where the measurement device 61 is arranged in the EUV light utilization apparatus 6. The measurement device 61 may be arranged in the chamber 2. The measurement device 61 measures the first energy parameter P1 of the EUV light. The first energy parameter P1 includes one of EUV pulse energy, EUV power, EUV power density, and EUV radiation brightness. The EUV pulse energy is the energy per pulse of the EUV light at the intermediate focal point 292, and the unit is joule (J). The first energy parameter P1 may include a combination of one of the EUV pulse energy and the EUV power and one of EUV light concentration size and EUV emission size. The EUV light concentration size is a spot diameter when the EUV light is concentrated at the focal point. The EUV emission size is a plasma diameter in the plasma generation region 25. The EUV light concentration size can be calculated based on the EUV emission size. The light concentration size at the intermediate focal point 292 can be calculated based on the EUV light concentration size. The EUV power can be calculated based on the EUV pulse energy. The EUV power density and the EUV radiation brightness can be calculated based on the combination of the EUV light concentration size and the EUV power. The processor 5 receives the first energy parameter P1 from the EUV light utilization apparatus 6. The processor 5 calculates the second energy parameter P2 based on the first energy parameter P1. As the first energy parameter P1, one of the EUV power, the EUV power density, and the EUV radiation brightness may be measured by the measurement device 61 and received by the processor 5. In this case, since the second energy parameter P2 is received as the first energy parameter P1, the processor 5 may not calculate the second energy parameter P2. 2.2 Operation FIG. 4 is a graph showing decrease in the EUV emission efficiency in the first embodiment, control of the irradiation frequency FI and the pulse energy E of the pulse laser light 33 in accordance with the decrease, and change in the second energy parameter P2. In contrast to the comparative example in which the second energy parameter P2 decreases as the EUV emission efficiency decreases, in the first embodiment, the irradiation frequency FI of the pulse laser light 33 with which the target 27 is irradiated is controlled so that the change in the second energy parameter P2 is suppressed. That is, in the first embodiment, the irradiation frequency FI is controlled so that the irradiation frequency FI increases as the EUV emission efficiency decreases. The pulse energy E of the pulse laser light 33 may not be changed. When the EUV emission efficiency decreases, the EUV pulse energy decreases. However, by increasing the irradiation frequency FI instead, the change in the second energy parameter P2 can be suppressed. 2.2.1 First Operation Example for Increasing Irradiation Frequency FI FIG. 5 is a flowchart showing a first operation example for increasing the irradiation frequency FI in the first embodiment. FIG. 6 shows the shape of the target 27 when the target supply unit 26 generates and supplies a jet-shaped target 27 to the optical path of the pulse laser light 33 in the first embodiment. The processing shown in FIG. 5 is suitable for the case where the target supply unit 26 generates the jet-shaped target 27. In S10 of FIG. 5, the processor 5 receives the first energy parameter P1 measured by the measurement device 61 from the EUV light utilization apparatus 6. In S11, the processor 5 calculates the second energy parameter P2 based on the first energy parameter P1. In S12, the processor 5 calculates the difference ΔP2 between the second energy parameter P2 and the target value. In S13, the processor 5 determines the emission frequency FL of the pulse laser light 33. For example, when the second energy parameter P2 is lower than the target value, an emission frequency FL of the pulse laser light 33 is increased in accordance with the difference ΔP2 from the target value. The emission frequency FL may be controlled by proportional-integral-differential (PID) control. The emission frequency FL refers to the number per second of pulses of the pulse laser light 33 output from the laser device 3. By increasing the emission frequency FL of the pulse laser light 33, the irradiation frequency FI of the pulse laser light 33 with which the target 27 is irradiated increases. When all pulses of the pulse laser light 33 are radiated to the target 27 and a part of the target 27 is turned into plasma for each pulse, the emission frequency FL and the irradiation frequency FI are the same. In S14, the processor 5 changes the emission frequency FL of the pulse laser light 33 to the value determined in S13. After S14, the processor 5 ends the processing of the flowchart. The processing of the flowchart is repeated each time the operation time of the EUV light generation system 11a or the number of output pulses of the EUV light reaches a predetermined value. 2.2.2 Second Operation Example for Increasing Irradiation Frequency FI FIG. 7 is a flowchart showing a second operation example for increasing the irradiation frequency FI in the first embodiment. FIG. 8 shows an arrangement of droplets 27a when the target supply unit 26 sequentially generates and supplies a plurality of the droplets 27a as the target 27 to the optical path of the pulse laser light 33 in the first embodiment. The processing shown in FIG. 7 is suitable for the case where the target supply unit 26 sequentially generates the droplet 27a. The processes of S10 to S12 in FIG. 7 are similar to those described with reference to FIG. 5. After S12, in S13a, the processor 5 determines both the emission frequency FL of the pulse laser light 33 and a generation frequency FD of the droplets 27a. The generation frequency FD refers to the number per second of the droplets 27a generated by the target supply unit 26. When the second energy parameter P2 is lower than the target value, both the emission frequency FL of the pulse laser light 33 and the generation frequency FD of the droplets 27a are increased in accordance with the difference ΔP2 from the target value. By increasing both the emission frequency FL of the pulse laser light 33 and the generation frequency FD of the droplets 27a, the irradiation frequency FI of the pulse laser light 33 with which the target 27 is irradiated increases. In S14a, the processor 5 changes both the emission frequency FL of the pulse laser light 33 and the generation frequency FD of the droplets 27a to the values determined in S13a. After S14a, the processor 5 ends the processing of the flowchart. The processing of the flowchart is repeated each time the operation time of the EUV light generation system 11a or the number of output pulses of the EUV light reaches a predetermined value. 2.3 Effect (1) According to the first embodiment, the EUV light generation system 11a includes the laser device 3, the EUV light concentrating mirror 23, and the processor 5. The laser device 3 emits the pulse laser light 33. The EUV light concentrating mirror 23 reflects and concentrates the EUV light generated by irradiating the target 27 with the pulse laser light 33. The processor 5 receives the first energy parameter P1 of the EUV light and controls the irradiation frequency FI of the pulse laser light 33 with which the target 27 is irradiated so that the change in the second energy parameter P2 related to the energy per unit time of the EUV light reflected by EUV light concentrating mirror 23 is suppressed. Accordingly, by controlling the irradiation frequency FI, it is possible to suppress the second energy parameter P2 from decreasing. Further, when the irradiation frequency FI is decreased, the number of times of generation of plasma is reduced, so that deterioration of the EUV light concentrating mirror 23 can be delayed and the life of the EUV light concentrating mirror 23 can be improved. (2) According to the first embodiment, the first energy parameter P1 includes one of the EUV pulse energy, the EUV power, the EUV power density, and the EUV radiation brightness, and the second energy parameter P2 includes one of the EUV power, the EUV power density, and the EUV radiation brightness. Accordingly, the EUV power can be calculated as the second energy parameter P2 based on the EUV pulse energy. Alternatively, one of the EUV power, the EUV power density, and the EUV radiation brightness can be used as the second energy parameter P2. By stabilizing the second energy parameter P2, it is possible to maintain preferable EUV light characteristics for the EUV light utilization apparatus 6. (3) According to the first embodiment, the first energy parameter P1 includes the combination of one of the EUV pulse energy and the EUV power and one of the EUV light concentration size and the EUV emission size, and the second energy parameter P2 includes one of the EUV power, the EUV power density, and the EUV radiation brightness. Based on the combinations, the EUV power density or the EUV radiation brightness can be calculated as the second energy parameter P2. By stabilizing the second energy parameter P2, it is possible to maintain preferable EUV light characteristics for the EUV light utilization apparatus 6. (4) According to the first embodiment, the processor 5 is connected to the EUV light utilization apparatus 6 that receives the EUV light generated in the EUV light generation system 11a, and receives the first energy parameter P1 from the EUV light utilization apparatus 6. Accordingly, it is possible to suppress the change in the second energy parameter P2 without arranging the measurement device of the first energy parameter P1 in the EUV light generation system 11a. (5) According to the first embodiment, the processor 5 calculates the second energy parameter P2 based on the first energy parameter P1. Accordingly, even without directly measuring the second energy parameter P2, the second energy parameter P2 is calculated based on the first energy parameter P1, and the change in the second energy parameter P2 can be suppressed. (6) According to the first embodiment, the processor 5 receives the second energy parameter P2 as the first energy parameter P1. Accordingly, even without calculating the second energy parameter P2, the change in the second energy parameter P2 can be suppressed. (7) According to the first embodiment, the processor 5 increases the irradiation frequency FI by increasing the emission frequency FL of the pulse laser light 33 from the laser device 3. Accordingly, even when the pulse energy E of the pulse laser light 33 is not changed, the second energy parameter P2 can be suppressed from decreasing. (8) According to the first embodiment, the EUV light generation system 11a includes the target supply unit 26 that sequentially generates and supplies the plurality of droplets 27a as the target 27 to the optical path of the pulse laser light 33. The processor 5 increases the irradiation frequency FI by increasing both the generation frequency FD of the plurality of droplets 27a from the target supply unit 26 and the emission frequency FL of the pulse laser light 33 from the laser device 3. Accordingly, even when the target 27 includes the plurality of droplets 27a, it is possible to suppress the second energy parameter P2 from decreasing. In other respects, the first embodiment is similar to the comparative example. 3. EUV Light Generation System 11a to Stepwise Increase Irradiation Frequency FI 3.1 Configuration and Operation FIGS. 9 and 10 show the arrangement of droplets 27b, 27c when the target supply unit 26 sequentially generates and supplies the plurality of droplets 27b, 27c as the target 27 to the optical path of the pulse laser light 33 in a second embodiment. The processing in the second embodiment is suitable for the case where the target supply unit 26 sequentially generates the droplets 27b, 27c. The configuration of the EUV light generation system 11a in the second embodiment is similar to that described with reference to FIG. 3. The droplets 27b, 27c include the droplet 27c not to be irradiated with the pulse laser light 33 and the droplet 27b to be irradiated with the pulse laser light 33. In FIGS. 9 and 10, one droplet 27b among the N droplets 27b, 27c generated in succession is irradiated with the pulse laser light 33. The ratio of the number of droplets 27b irradiated with the pulse laser light 33 to the number of droplets 27b, 27c is 1/N. N is an integer of 1 or larger, and preferably 5 or larger and 20 or smaller. In FIG. 9, N is 10, and in FIG. 10, N is 9. The relationship between the generation frequency FD of the droplets 27b, 27c and the emission frequency FL of the pulse laser light 33 from the laser device 3 is set to FL=FD/N. The generation frequency FD is, for example, 100 kHz or higher and 200 kHz or lower, and the emission frequency FL is, for example, 10 kHz or higher and 20 kHz or lower. By changing the value of N from a large value to a small value, the ratio 1/N of the number of the droplets 27b irradiated with the pulse laser light 33 to the number of the droplets 27b, 27c can be increased. When all pulses of the pulse laser light 33 are radiated to different droplets 27b, the emission frequency FL and the irradiation frequency FI are the same. By increasing the ratio 1/N without changing the generation frequency FD, the irradiation frequency FI can be increased. The irradiation frequency FI varies stepwise in accordance with the value of N. FIG. 11 is a graph showing decrease in the EUV emission efficiency in the second embodiment, control of the irradiation frequency FI and the pulse energy E of the pulse laser light 33 in accordance with the decrease, and change in the second energy parameter P2. In the second embodiment, the irradiation frequency FI is increased stepwise as the EUV emission efficiency decreases. That is, the irradiation frequency FI is increased at certain timing, and the irradiation frequency FI is maintained without being changed in a period other than the certain timing. The second energy parameter P2 also increases at the timing at which the irradiation frequency FI is increased. During the period in which the irradiation frequency FI is maintained without being changed, the second energy parameter P2 gradually decreases as the EUV emission efficiency decreases. Thus, the second energy parameter P2 varies in a sawtooth waveform manner. FIG. 12 is a flowchart showing an operation example for increasing the irradiation frequency FI stepwise in the second embodiment. The processes of S10 and S11 in FIG. 12 are similar to those described with reference to FIG. 5. After S11, in S15, the processor 5 determines whether the second energy parameter P2 is equal to or larger than a threshold value P2th. The threshold value P2th is set to a value larger than the lower limit of the second energy parameter P2 required by the EUV light utilization apparatus 6. When the second energy parameter P2 is equal to or larger than the threshold value P2th (S15: YES), the processor 5 ends the processing of the flowchart. When the second energy parameter P2 is smaller than the threshold value P2th (S15: NO), the processor 5 proceeds to S19. In S19, the processor 5 subtracts 1 from the current value of N to update the value of N. For example, when the current value of N is N1 and the new value of N is N2, the value of N is updated so that N2=N1−1 is satisfied. However, the present disclosure is not limited to the case of subtracting 1 each from the current value of N. An integer of 2 or larger may be subtracted. In S20, the processor 5 increases the emission frequency FL of the pulse laser light 33 by the equation FL=FD/N. As a result, the irradiation frequency FI of the pulse laser light 33 with which the target 27 is irradiated can be increased. After S20, the processor 5 ends the processing of the flowchart. The processing of the flowchart is repeated each time the operation time of the EUV light generation system 11a or the number of output pulses of the EUV light reaches a predetermined value. In the second embodiment, the case where the irradiation frequency FI is increased when the second energy parameter P2 is lower than the threshold value P2th has been described, but the present disclosure is not limited thereto. The irradiation frequency FI may be increased when the operation time of the EUV light generation system 11a reaches a predetermined value. Alternatively, the irradiation frequency FI may be increased when the number of output pulses of the EUV light reaches a predetermined value. These predetermined values are set in advance based on the prediction of the EUV emission efficiency. In this case, the second energy parameter P2 may temporarily become lower than the second energy parameter P2 shown in FIG. 11, but the second energy parameter P2 may be recovered by increasing the irradiation frequency FI at the timing based on the operation time of the EUV light generation system 11a or the number of output pulses of the EUV light. When the irradiation frequency FI is increased at the timing based on the operation time of the EUV light generation system 11a or the number of output pulses of the EUV light, the number to be subtracted from the current value of N may be determined based on the second energy parameter P2. For example, in the case where the second energy parameter P2 is significantly lower than the threshold value P2th when the operation time of the EUV light generation system 11a reaches the predetermined value, the irradiation frequency FI may be significantly increased by subtracting an integer of 2 or larger from the current value of N. 3.2 Effect (9) According to the second embodiment, the target 27 includes the plurality of droplets 27b, 27c that are sequentially generated and supplied to the optical path of the pulse laser light 33. The plurality of droplets 27b, 27c include the droplet 27c not to be irradiated with the pulse laser light 33 and the droplet 27b to be irradiated with the pulse laser light 33. The processor 5 increases the irradiation frequency FI by increasing the ratio of the number of the droplets 27b irradiated with the pulse laser light 33 to the number of the plurality of droplets 27b, 27c. Accordingly, it is possible to stepwise increase the irradiation frequency FI without changing the generation frequency FD of the droplets 27b, 27c. (10) According to the second embodiment, the EUV light generation system 11a includes the target supply unit 26 that sequentially generates and supplies the plurality of droplets 27b, 27c as the target 27 to the optical path of the pulse laser light 33. The processor 5 controls the irradiation frequency FI by setting the relationship between the generation frequency FD of the plurality of droplets 27b, 27c from the target supply unit 26 and the emission frequency FL of the pulse laser light 33 from the laser device 3 to FL=FD/N1. Thereafter, FL=FD/N2 is set to increase the irradiation frequency FI. N1 is an integer of 2 or larger, and N2 is an integer of 1 or larger and smaller than N1. Accordingly, it is possible to calculate the appropriate emission frequency FL based on the generation frequency FD of the droplets 27b, 27c. (11) According to the second embodiment, the processor 5 increases the irradiation frequency FI when the second energy parameter P2 is lower than the threshold value P2th. This suppresses the second energy parameter P2 from being further decreased. (12) According to the second embodiment, the processor 5 increases the irradiation frequency FI when the operation time of the EUV light generation system 11a reaches the predetermined value. Accordingly, even when the second energy parameter P2 is not monitored, it is possible to determine the timing of increasing the irradiation frequency FI. (13) According to the second embodiment, the processor 5 increases the irradiation frequency FI when the number of output pulses of the EUV light reaches the predetermined value. Accordingly, even when the second energy parameter P2 is not monitored, it is possible to determine the timing of increasing the irradiation frequency FI. In other respects, the second embodiment is similar to the first embodiment. 4. EUV Light Generation System 11a for Changing Pulse Energy E of Pulse Laser Light 33 4.1 Configuration and Operation FIG. 13 is a graph showing decrease in the EUV emission efficiency in a third embodiment, control of the irradiation frequency FI and the pulse energy E of the pulse laser light 33 in accordance with the decrease, and change in the second energy parameter P2. The processing in the third embodiment is suitable for the case where the target supply unit 26 sequentially generates the droplets 27b, 27c (see FIGS. 9 and 10). The configuration of the EUV light generation system 11a in the third embodiment is similar to that described with reference to FIG. 3. In the third embodiment, the pulse energy E of the pulse laser light 33 is controlled so that the change in the second energy parameter P2 is suppressed. That is, the pulse energy E of the pulse laser light 33 is increased as the EUV emission efficiency decreases. It is desirable that the pulse energy E of the pulse laser light 33 does not exceed a threshold value Eth. In the third embodiment, when the target pulse energy Et of the pulse laser light 33 is higher than the threshold value Eth, the irradiation frequency FI is increased instead of increasing the pulse energy E of the pulse laser light 33. When the irradiation frequency FI is increased, the pulse energy E of the pulse laser light 33 required to obtain the desired second energy parameter P2 is decreased. Therefore, the pulse energy E of the pulse laser light 33 is decreased at the timing at which the irradiation frequency FI is increased. FIG. 14 is a flowchart showing an operation example for controlling the pulse energy E and the irradiation frequency FI of the pulse laser light 33 in the third embodiment. The processes of S10 to S12 in FIG. 14 are similar to those described with reference to FIG. 5. After S12, in S16, the processor 5 determines the target pulse energy Et of the pulse laser light 33. For example, when the second energy parameter P2 is lower than the target value, the target pulse energy Et of the pulse laser light 33 is increased in accordance with the difference ΔP2 from the target value. In S17, the processor 5 determines whether or not the target pulse energy Et of the pulse laser light 33 determined in S16 is equal to or smaller than the threshold value Eth. When the target pulse energy Et of the pulse laser light 33 is equal to or smaller than the threshold value Eth (S17: YES), the processor 5 proceeds to S18. In S18, the processor 5 changes the pulse energy E of the pulse laser light 33 to approach the target pulse energy Et determined in S16. The pulse energy E may be controlled by PID control. In the case that the target pulse energy Et of the pulse laser light 33 is increased in S16, the pulse energy E of the pulse laser light 33 is increased when the process of S18 is performed. By increasing the pulse energy E of the pulse laser light 33, the ratio of atoms excited by the pulse laser light 33 among atoms constituting the droplet 27b increases. Therefore, it is possible to suppress the second energy parameter P2 from decreasing as the EUV emission efficiency decreases. The processes of S16 to S18 correspond to the first processing of the present disclosure. After S18, the processor 5 ends the processing of the flowchart. When the target pulse energy Et of the pulse laser light 33 is higher than the threshold value Eth (S17: NO), the processor 5 proceeds to S19 without changing the pulse energy E so as to approach the target pulse energy Et determined in S16. The processes of S19 and S20 are similar to those described with reference to FIG. 12. After S20, in S21, the processor 5 decreases the pulse energy E of the pulse laser light 33. Specifically, after the target pulse energy Et of the pulse laser light 33 is decreased, the pulse energy E is controlled so as to approach the target pulse energy Et. The pulse energy E of the pulse laser light 33 is controlled so as to suppress the change in the second energy parameter P2 before and after the emission frequency FL of the pulse laser light 33 is increased in S20. The processes of S19 to S21 correspond to the second processing of the present disclosure. After S21, the processor 5 ends the processing of the flowchart. The processing of the flowchart is repeated each time the operation time of the EUV light generation system 11a or the number of output pulses of the EUV light reaches a predetermined value. In the third embodiment, the case where the irradiation frequency FI is increased when the target pulse energy Et of the pulse laser light 33 is higher than the threshold value Eth has been described, but the present disclosure is not limited thereto. The irradiation frequency FI may be increased when the operation time of the EUV light generation system 11a reaches a predetermined value. Alternatively, the irradiation frequency FI may be increased when the number of output pulses of the EUV light reaches a predetermined value. These predetermined values are set in advance based on the prediction of the EUV emission efficiency. In this case, there is a possibility that the pulse energy E of the pulse laser light 33 temporarily becomes higher than the threshold value Eth. However, increase in the pulse energy E can be suppressed by increasing the irradiation frequency FI at the timing based on the operation time of the EUV light generation system 11a or the number of output pulses of the EUV light. Here, the threshold value Eth in this case is set to a value lower than a designed upper limit of the pulse energy E of the pulse laser light 33 in the EUV light generation system 11a. 4.2 Effect (14) According to the third embodiment, the processor 5 performs the first processing (S16 to S18) of controlling the pulse energy E of the pulse laser light 33 so as to suppress the change in the second energy parameter P2 and the second processing (S19 to S21) of increasing the irradiation frequency FI and decreasing the pulse energy E of the pulse laser light 33. Accordingly, even when the irradiation frequency FI is increased stepwise, the pulse energy E of the pulse laser light 33 is adjusted to suppress the fluctuation in the second energy parameter P2. The dynamic range of the pulse energy E of the pulse laser light 33 may not be enough to compensate for the drop in the EUV emission efficiency, but in combination with the stepwise adjustment of the irradiation frequency FI, decrease in the EUV emission efficiency can be compensated. (15) According to the third embodiment, in the first processing (S16 to S18), the processor 5 increases the target pulse energy Et of the pulse laser light 33 when the second energy parameter P2 is lower than the target value. Accordingly, the fluctuation of the second energy parameter P2 can be suppressed. (16) According to the third embodiment, the processor 5 performs the second processing (S19 to S21) when the target pulse energy Et of the pulse laser light 33 is higher than the threshold value Eth Accordingly, it is possible to suppress the increase of the pulse energy E. (17) According to the third embodiment, the target 27 includes the plurality of droplets 27b, 27c that are sequentially generated and supplied to the optical path of the pulse laser light 33. The plurality of droplets 27b, 27c include the droplet 27c not to be irradiated with the pulse laser light 33 and the droplet 27b to be irradiated with the pulse laser light 33. In the second processing (S19 to S21), the processor 5 increases the irradiation frequency FI by increasing the ratio of the number of the droplets 27b irradiated with the pulse laser light 33 to the number of the plurality of droplets 27b, 27c. Accordingly, it is possible to stepwise increase the irradiation frequency FI without changing the generation frequency FD of the droplets 27b, 27c. (18) According to the third embodiment, the EUV light generation system 11a includes the target supply unit 26 that sequentially generates and supplies the plurality of droplets 27b, 27c as the target 27 to the optical path of the pulse laser light 33. The processor 5 controls the irradiation frequency FI by setting the relationship between the generation frequency FD of the plurality of droplets 27b, 27c from the target supply unit 26 and the emission frequency FL of the pulse laser light 33 from the laser device 3 to FL=FD/N1. Then, in the second processing (S19 to S21), FL=FD/N2 is set to increase the irradiation frequency FI. N1 is an integer of 2 or larger, and N2 is an integer of 1 or larger and smaller than N1. Accordingly, it is possible to calculate the appropriate emission frequency FL based on the generation frequency FD of the droplets 27b, 27c. In other respects, the third embodiment is similar to the second embodiment. 5. Others FIG. 15 schematically shows the configuration of an exposure apparatus 6a connected to the EUV light generation system 11a. In FIG. 15, the exposure apparatus 6a as the EUV light utilization apparatus 6 (see FIG. 3) includes a mask irradiation unit 68 and a workpiece irradiation unit 69. The mask irradiation unit 68 illuminates, via a reflection optical system, a mask pattern of a mask table MT with the EUV light incident from the EUV light generation system 11a. The workpiece irradiation unit 69 images the EUV light reflected by the mask table MT onto a workpiece (not shown) arranged on a workpiece table WT via the reflection optical system. The workpiece is a photosensitive substrate such as a semiconductor wafer on which photoresist is applied. The exposure apparatus 6a synchronously translates the mask table MT and the workpiece table WT to expose the workpiece to the EUV light reflecting the mask pattern. Through the exposure process as described above, a device pattern is transferred onto the semiconductor wafer, thereby an electronic device can be manufactured. FIG. 16 schematically shows the configuration of an inspection apparatus 6b connected to the EUV light generation system 11a. In FIG. 16, the inspection apparatus 6b as the EUV light utilization apparatus 6 (see FIG. 3) includes an illumination optical system 63 and a detection optical system 66. The illumination optical system 63 reflects the EUV light incident from the EUV light generation system 11a to illuminate a mask 65 placed on a mask stage 64. Here, the mask 65 conceptually includes a mask blank before a pattern is formed. The detection optical system 66 reflects the EUV light from the illuminated mask 65 and forms an image on a light receiving surface of a detector 67. The detector 67 having received the EUV light obtains an image of the mask 65. The detector 67 is, for example, a time delay integration (TDI) camera. Defects of the mask 65 are inspected based on the image of the mask 65 obtained by the above-described process, and a mask suitable for manufacturing an electronic device is selected using the inspection result. Then, the electronic device can be manufactured by exposing and transferring the pattern formed on the selected mask onto the photosensitive substrate using the exposure apparatus 6a. The description above is intended to be illustrative and the present disclosure is not limited thereto. Therefore, it would be obvious to those skilled in the art that various modifications to the embodiments of the present disclosure would be possible without departing from the spirit and the scope of the appended claims. Further, it would be also obvious to those skilled in the art that embodiments of the present disclosure would be appropriately combined. The terms used throughout the present specification and the appended claims should be interpreted as non-limiting terms unless clearly described. For example, terms such as “comprise”, “include”, “have”, and “contain” should not be interpreted to be exclusive of other structural elements. Further, indefinite articles “a/an” described in the present specification and the appended claims should be interpreted to mean “at least one” or “one or more.” Further, “at least one of A, B, and C” should be interpreted to mean any of A, B, C, A+B, A+C, B+C, and A+B+C as well as to include combinations of any thereof and any other than A, B, and C. |
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claims | 1. A method comprising:installing a reactor coolant pump comprising a pump motor, a driveshaft, an impeller, and a mounting flange on a pressurized water reactor (PWR) comprising a pressure vessel assembly comprising a vessel body and a vessel head defining an integrated pressurizer volume secured thereto, and a nuclear reactor core disposed in the pressure vessel assembly, the installing including:pre-assembling the pump motor, the driveshaft, the impeller, and the mounting flange outside of the pressure vessel assembly to form a pump assembly as a unit disposed outside of the pressure vessel assembly in which the pump motor is connected with the impeller by the driveshaft,inserting the impeller and the driveshaft of the pump assembly through an opening of the vessel head while the pump motor remains outside of the pressure vessel assembly, andsecuring the flange of the pump assembly to an outside of the vessel head to mount the pump assembly on the pressure vessel assembly,wherein the inserting and securing mounts the pump assembly on the pressure vessel assembly with the drive shaft of the pump assembly oriented vertically so that no portion of the pump assembly is disposed in the pressurizer volume. 2. The method of claim 1, wherein the reactor coolant pump further comprises a pump diffuser that is not a component of the unitary pump assembly formed by the pre-assembling, and the installing further comprises:disposing the pump diffuser inside the pressure vessel assembly in an operation other than the inserting and the securing operations. 3. The method of claim 2, wherein the installing includes disposing the pump diffuser inside the pressure vessel assembly before performing the inserting, and the inserting includes inserting the impeller and the driveshaft through the opening of the vessel head so as to position the impeller inside the pump diffuser. 4. The method of claim 2, wherein the opening of the vessel head through which the impeller and drive shaft are inserted is too small for the pump diffuser to pass through, and the disposing comprises:inserting the pump diffuser into the pressure vessel assembly through a manway of the vessel body that is separate from the opening of the vessel head through which the impeller and drive shaft are inserted. 5. The method of claim 2, wherein the disposing of the pump diffuser inside the pressure vessel assembly is performed before the inserting of the impeller and the driveshaft of the pump assembly through the opening of the vessel head. 6. The method of claim 2, further comprising:subsequent to installing the reactor coolant pump on the PWR, removing the pump assembly from the PWR while leaving the pump diffuser disposed inside the pressure vessel assembly. 7. The method of claim 1, wherein the inserting and securing mounts the pump assembly on the pressure vessel assembly with the drive shaft of the pump assembly oriented vertically and the pump motor disposed above the impeller. 8. The method of claim 1, wherein the vessel head includes a first mating flange and the vessel body includes a second mating flange, and the vessel head is secured to the vessel body by a step of securing the first mating flange to the second mating flange. 9. The method of claim 1, wherein the PWR further comprises a separator plate that separates the pressure vessel assembly into a pressurizer volume disposed above the separator plate and a reactor volume disposed below the separator plate, and at least a portion of the pump motor of the reactor coolant pump is disposed above the separator plate. |
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abstract | A system of shields designed to provide substantially greater protection, head to toe, against radiation exposure to health care workers in a hospital room during procedures which require real-time imaging. The shields are placed around the patient and the x-ray table and provide protection even when the x-ray tube is moved to various angles around the patient. |
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abstract | The present invention is directed to a system and method for a quality assurance tool generating test plans and identifying new test requirements for a new version of a product. Old versions of the product may be previously tested and test plan documents associated with previously tested versions of the product may be stored in a database. The database may store test plans, test configurations, test scopes, and the like for previously tested versions of the product. Product design requirements may be determined based on received customer desired features for the new version. The database may be updated by adding new tests for new features of the new version. A test plan document for the product may be generated based on the updated database. The generated test document may be verified through automatically generating a general test plan for the new version of the product by querying updated database with the product design requirements. |
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053902181 | description | DESCRIPTION OF THE PREFERRED EMBODIMENT As the result of applying himself to research for attaining this object, the present inventor has found that, as a method of obtaining a easily press moldable soft dry gel particle, that one can prevent the gel particle becoming a particle in the hard glass state by sufficiently washing the gel particle using an organic solvent miscible with water such as alcohol to substitute water therewith and thereafter removing the organ ic solvent used, and that the press moldability can be improved by moistening again the dry heat treated gel, and has invented the present invention based on this knowledge. According to the drying method of the present invention, a low density of soft gel particle as below 29% T.D. can be obtained while the density of dry gel particle obtained by the prior drying method is about 35% T.D. And, according to the method of the present invention in which a dry gel particle is moistened again before press molding, the water content in the moistened gel particle plays the part of lubricant which is added for the purpose of fluidity and adhesive in case of ordinary powder molding, and further plays a part of caking agent, and the moldability can be improved. The present invention will be further explained with Examples by which the present invention is not limited. EXAMPLE 1 [Preparation and gelation of nuclear fuel sol] A sol of 1.0 mol/1 in thorium concentration was prepared by a method described in Literature 6 in which ammonia gas is added to a heated aqueous solution of thorium nitrate to obtain a sol of colloidal thorium oxide. This sol was divided into droplets of 0.3 mm in diameter using a vibration nozzle in air and was gellated in ammonia gas and continuously in ammonia water to obtain microspherical gel particles in which ammonium nitrate contained was washed out with dilute ammonia water. [Drying of gel particle] The water content in the gel particle was substituted with isopropyl alcohol to below 1 wt. % of balance water content concentration in isopropyl alcohol and then the gel particle was separated from isopropyl alcohol. And, after removing most of isopropyl alcohol by vacuum vaporization, the gel particle was dried in air of 80.degree. C. The water content in gel particle in this stage was 10% by weight. This particle was heated in air in 450.degree. C. for 3 hours to make the water content about 0%. The density of this particle was about 29% T.D. This dry particle was used as a raw material for press moldability. [Moistening of particle - Press molding - Sintering] The particles once dried were moistened to each value described in Table 2 (density of thorium oxide pellet -unit : T.D. ) using a thermo-hygrostat. TABLE 2 ______________________________________ Pressure Water content of moisture added (Wt %) (MPa) 0 5 10.about.12 15.about.16 ______________________________________ 150 29.4 33.5 33.7 84.6 94.1 93.8 200 32.5 38.7 90.1 96.8 300 37.6 [43.4]*.sup.2 42.1 39.2 [94.9]*.sup.1 [97.6]*.sup.2 96.7 96.0 400 42.1 98.6 500 [45.8]*.sup.2 43.4 [97.8]*.sup.2 98.6 ______________________________________ *.sup.1 : For comparison the weight of water content of moisture added wa left out of account in the calculation of green density. *.sup.2 : [ ] shows one cracked. The die used for the press mold is 7 or 10 mm in diameter and slightly tapered for easily drawing out therefrom. An alcohol solution of stearic acid was used as a lubricant for dies. It was press molded within the range of 0.about.15-16% in water content added for moistening and the range of 150-500 MPa. This green density is shown in the upper column of Table 2. This green pellet was heated to 500.degree. C. in moistened air and then to 1300.degree. C. in air and, after keeping for 3 hours, was cooled. The density of sintered pellet so obtained is shown in the lower column of Table 2. EXAMPLE 2 A dry gel particle of about 25% T.D. in density and 3% in remained water content was prepared according to the same method as Example 1 exept using uranium for 10% of thorium. This particle was moistened to about 15% water content and press molded under 300 MPa. This green pellet was 40% T.D. in density. After sintering under the same condition as Example 1, it was reduced in a mixed gas of argon and hydrogen at 1300.degree. C. to obtain a pellet of mixed thorium and uraniumm oxides (Th.sub.0.9 U.sub.0.1 O.sub.2) of 97% T.D. in density. The dry gel particle obtained by the process of the present invention is not only low in density and soft but also is so excellent in sinterability that it can be sintered to a high density of particle even in case of sintering as it is. Therefore, even a low density of green pellet as below 40% T.D. press molded under a low pressure as 300 MPa is so effective that it can be sintered to a high density of pellet as above 95% T.D. at a low temperature as 1300.degree. C. And it is effective that it can be press molded to a high density of pellet, comparing with the case of 0% in water content under the same pressure, even without adding an additive, according to the process of the present invention in which, after moistening again a dry gel particle to a water content of about 10-15%, it is press molded (refer to the case of below 300 MPa in Table 2). And the water content of moisture plays a part of caking agent which prevents the occurrence of crack on mold. In Table 2, a remarkable effect is recognized in case of 300 MPa and 500 MPa. Even in low pressure in which such effect is low finally a high density of sintered pellet can be obtained. |
description | This application is a continuation-in-part of application Ser. No. 12/495,873, filed Jul. 1, 2009. 1. Field of the Invention This invention pertains generally to a nuclear reactor fuel assembly and more particularly to a nuclear fuel assembly that employs a spacer grid that minimizes flow induced vibration. 2. Description of the Related Art The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated and in heat exchange relationship with a secondary circuit for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump, and a system of pipes which are connected to the vessel form a loop of the primary side. For the purpose of illustration, FIG. 1 shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel 10 having a closure head 12 enclosing a nuclear core 14. A liquid reactor coolant, such as water, is pumped into the vessel 10 by pump 16 through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18, typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump 16, completing the primary loop. Typically, a plurality of the above-described loops are connected to a single reactor vessel 10 by reactor coolant piping 20. An exemplary reactor design is shown in more detail in FIG. 2. In addition to the core 14 comprised of a plurality of parallel, vertical, co-extending fuel assemblies 22, for purposes of this description, the other vessel internal structures can be divided into the lower internals 24 and the upper internals 26. In conventional designs, the lower internals' function is to support, align and guide core components and instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assemblies 22 (only two of which are shown for simplicity in FIG. 2), and support and guide instrumentation and components, such as control rods 28. In the exemplary reactor shown in FIG. 2, coolant enters the reactor vessel 10 through one or more inlet nozzles 30, flows down through an annulus between the vessel and the core barrel 32, is turned 180° in a lower plenum 34, passes upwardly through a lower support plate 37 and a lower core plate 36 upon which the fuel assemblies are seated and through and about the assemblies. In some designs, the lower support plate 37 and the lower core plate 36 are replaced by a single structure, a lower core support plate having the same elevation as 37. The coolant flow through the core and surrounding area 38 is typically large on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals, including a circular upper core plate 40. Coolant exiting the core 14 flows along the underside of the upper core plate 40 and upwardly through a plurality of perforations 42. The coolant then flows upwardly and radially to one or more outlet nozzles 44. The upper internals 26 can be supported from the vessel or the vessel head and include an upper support assembly 46. Loads are transmitted between the upper support assembly 46 and the upper core plate 40, primarily by a plurality of support columns 48. A support column is aligned above a selected fuel assembly 22 and perforations 42 in the upper core plate 40. Rectilinearly movable control rods 28, which typically include a drive shaft 50 and a spider assembly 52 of neutron poison rods, are guided through the upper internals 26 and into aligned fuel assemblies 22 by control rod guide tubes 54. The guide tubes are fixedly joined to the upper support assembly 46 and the top of the upper core plate 40. The support column 48 arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally affect control rod insertion capability. FIG. 3 is an elevational view, represented in vertically shortened form, of a fuel assembly being generally designated by reference character 22. The fuel assembly 22 is the type used in a pressurized water reactor and has a structural skeleton which, at its lower end includes a bottom nozzle 58. The bottom nozzle 58 supports the fuel assembly 22 on a lower core plate 60 in the core region of the nuclear reactor (the lower core plate 60 is represented by reference character 36 in FIG. 2). In addition to the bottom nozzle 58, the structural skeleton of the fuel assembly 22 also includes a top nozzle 62 at its upper end and a number of guide tubes or thimbles 84 which align with the guide tubes 54 in the upper internals. The guide tubes or thimbles 84 extend longitudinally between the bottom and top nozzles 58 and 62 and at opposite ends are rigidly attached thereto. The fuel assembly 22 further includes a plurality of transverse grids 64 axially spaced along and mounted to the guide thimbles 84 and an organized array of elongated fuel rods 66 transversely spaced and supported by the grids 64. A plan view of a grid 64 without the guide thimbles 84 and fuel rod 66 is shown in FIG. 4. The guide thimbles 84 pass through the cells labeled 96 and the fuel rods occupy the cells 94. As can be seen from FIG. 4, the grids 64 are conventionally formed from an array of orthogonal straps 86 and 88 that are interleaved in an egg-crate pattern with the adjacent interface of four straps defining approximately square support cells through which the fuel rod 66 are supported in the cells 94 in transverse, spaced relationship with each other. In many designs, springs 90 and dimples 92 are stamped into the opposite walls of the straps that form the support cells 94. The springs and dimples extend radially into the support cells and capture the fuel rod 66 therebetween; exerting pressure on the fuel rod cladding to hold the rods in position. The orthogonal array of straps 86 and 88 is welded at each strap end to a bordering strap 98 to complete the grid structure 64. Also, the assembly 22, as shown in FIG. 3, has an instrumentation tube 68 located in the center thereof that extends between and is captured by the bottom and top nozzles 58 and 62. With such an arrangement of parts, fuel assembly 22 forms an integrally unit capable of being conveniently handled without damaging the assembly of parts. As mentioned above, the fuel rod 66 in the array thereof in the assembly 22 are held in spaced relationship with one another by the grids 64 spaced along the fuel assembly length. Each fuel rod 66 includes a plurality of nuclear fuel pellets 70 and is closed at its opposite ends by upper and lower end plugs 72 and 74. The pellets 70 are maintained in a stack by a plenum spring 76 disposed between the upper end plug 72 and the top of the pellet stack. The fuel pellets 70, composed of fissile material, are responsible for creating the reactive power of the reactor. The cladding which surrounds the pellets functions as a barrier to prevent the fission by-products from entering the coolant and further contaminating the reactor system. To control the fission process, a number of control rods 78 are reciprocally moveable in the guide thimbles 84 located at predetermined positions in the fuel assembly 22. The guide thimble locations can be specifically seen in FIG. 4 represented by reference character 96, except for the center location which is occupied by the instrumentation tube 68. Specifically, a rod cluster control mechanism 80, positioned above the top nozzle 62, supports a plurality of the control rods 78. The control mechanism has an internally threaded cylindrical hub member 82 with a plurality of radially extending flukes or arms 52 that form the spider previously noted with regard to FIG. 2. Each arm 52 is interconnected to a control rod 78 such that the control rod mechanism 80 is operable to move the control rods vertically in the guide thimbles 84 to thereby control the fission process in the fuel assembly 22, under the motive power of a control rod drive shaft 50 which is coupled to the control rod hub 80, all in a well known manner. As mentioned above, the fuel assemblies are subject to hydraulic forces that exceed the weight of the fuel rods and thereby exert significant forces on the fuel rods and the assemblies. In addition, there is significant turbulence in the coolant in the core caused by mixing vanes on the upper surfaces of the straps of many grids which promote the transfer of heat from the fuel rod cladding to the coolant. The substantial flow forces and turbulence can result in resonant vibration of the grid straps which results from vortex shedding lock-in vibration when the shedding frequency is close to the natural frequency of the strap. The resonant vibration can cause severe fretting of the fuel rod cladding if the relative motion between the grid strap and the fuel rod is not restrained. Fretting of the fuel rod cladding can lead to a breach and expose the coolant to the radioactive byproduct within the fuel rods. Another potential problem with resonant grid strap vibration is that fatigue could occur in the grid straps causing grid strap cracking (or other damage to the straps). Thus, an improved means of supporting the fuel rods within a fuel assembly grid is desired that will better resist resonant vibration of the grid straps. This invention achieves the foregoing objective by providing an enhanced nuclear fuel assembly for supporting a spaced, parallel array of a plurality of elongated nuclear fuel rods between a lower nozzle and upper nozzle. A plurality of improved support grids are arranged in tandem spaced along the axial length of the fuel rods between the upper nozzle and the lower nozzle, at least partially enclosing an axial portion of the circumference of each fuel rod within a support cell of the support grids to maintain a lateral spacing between fuel rods. At least one of the support grids comprises a plurality of elongated, intersecting straps that define the support cells at the intersection of each four adjacent straps that surround the nuclear fuel rods. A length of each strap along its elongated dimension, between the intersections of the four adjacent straps, forms a wall of the corresponding support cell, with each wall of the cells that surround fuel rods having a lower leading edge and an upper trailing edge that are substantially in the plane of the corresponding strap. At least one of the leading edge and the trailing edge extends in between the intersections of adjacent straps, at an angle substantially deviating from an angle of an axis of the elongated dimension of the straps. In one preferred embodiment, the improved grid structure of this invention has the grid straps interleaved in an egg-crate arrangement and preferably the walls of the intersecting straps are at substantially the same height at the intersection. Desirably, the angle of deviation of the leading edge and trailing edge includes a first angle and a second angle and the lower leading edge and the upper trailing edge extend from the intersection between straps, respectively, at the first and second angles. In one embodiment, the first and second angles are in the same direction. In a second embodiment, the first and second angles are in opposite directions and in still another embodiment the first and second angles are equal. In another preferred embodiment, one or both of the leading edge or trailing edge of at least one wall of at least some of the cells that surround fuel rods starts at a first elevation at a first of the intersections between straps and extends along the elongated dimension of the corresponding strap to a second elevation before intersecting with an adjacent orthogonal strap. Desirably, the second elevation is either higher or lower than the first elevation. In an alternate embodiment, wherein one or both of the leading edge or the trailing edge extends from the second elevation to a third elevation before the intersection with an adjacent orthogonal strap, the third elevation is different than the second elevation. In one embodiment, the second elevation is higher than the first and third elevations on the leading edge and the second elevation is lower than the first and third elevations on the trailing edge. In still another embodiment, the second elevation is at the intersection of the adjacent orthogonal strap and the second elevation is one of either greater or lesser than the first elevation. Desirably, one or both of the leading edge or the trailing edge extends along the elongated dimension of the strap from the adjacent orthogonal intersecting strap to a third elevation at the intersection of a third orthogonal strap wherein the third elevation is the other of either the greater or lesser than the second elevation. Preferably, the first and third elevations are substantially the same. Preferably, either one or both of the leading edge or the trailing edge on adjacent, opposing, parallel straps have the same undulating pattern, but opposing walls are 180° out of phase. Desirably, at least some of the walls of the support cells include a dimple that has a saw tooth opening cut in the wall below the dimple and preferably the dimple is substantially rounded at a plurality of corners at which it changes it direction into the support cell. In still another embodiment, at least some of the walls of the support cells that include a dimple that has a saw tooth opening below the dimple also include a saw tooth ligament as the trailing edge. In yet another embodiment either the leading edge, the trailing edge or both the leading edge and the trailing edge are formed from a cutout in the wall of the support cell. The term “cutout” is used in a very general sense to refer to an opening in the cell wall regardless of how formed. The cutout has at least a first side that is inclined towards a second side with the first and second sides connected at the bottom with a smooth curved transition. Preferably the cutout extends substantially over the width of the wall and in one embodiment the leading edge is above a bottom edge of the grid strap. In the case where the support cell wall has either a dimple or spring protruding into the support cell, the cutout that forms the leading edge is formed between the bottom edge of the strap and the dimple or spring. In one embodiment the cutout is in the form of a triangle having a relatively flat horizontal base at the top and a rounded lower tip. Preferably the angle of the first and second sides with each other is between 20 and 160 degrees with the optimal angle between 60 and 90 degrees. With the radius at the rounded lower tip defined by a ratio of the radius of the curvature of the lower tip to the depth (height) of the triangle, the ratio is preferably between 0.1 and 0.9 and most desirably between 0.5 and 0.7. Preferably the ratio of the width of the cutout to the width of the wall is between 0.1 and 0.9 and most desirably between 0.5 and 0.85. In still another embodiment the leading edge, the trailing edge or both the leading edge and the trailing edge are formed substantially as a half circle with the open end at the top. Preferably, the ratio of the height of the half circle to the width of the half circle at its widest point is approximately 0.5; and the ratio of the width of the half circle at its widest point to the width of the support cell is between 0.2 to 0.9 with an optimal range of 0.4 to 0.6. This invention provides a new fuel assembly for a nuclear reactor and more particularly an improved spacer grid design for a nuclear fuel assembly. The improved grid is generally formed from a matrix of approximately square (or hexagonal) cells, some of which 94 support fuel rods while others of which 96 are connected to guide thimbles and a central instrumentation tube. The plan view shown in FIG. 4 looks very much like the prior art grids since the contour of the individual grid straps 86 and 88 are not regularly apparent from this view, but can be better appreciated from the view shown in FIGS. 5-12. The grid of this embodiment is formed from two orthogonally positioned sets of parallel, spaced straps 86 and 88, that are interleaved in a conventional manner and surrounded by an outer strap 98 to form the structural makeup of the grid 64. Though orthogonal straps 86 and 88 forming substantially square fuel rod support cells are shown in this embodiment, it should be appreciated that this invention can be applied equally as well to other grid configurations, e.g., hexagonal grids. The orthogonal straps 86 and 88, and in the case of the outer rows, the outer strap 98 define the support cells 94 at the intersection of each four adjacent straps that surround the nuclear fuel rods 66. A length of each strap along the straps' elongated dimension, between the intersections of four adjacent straps, forms a wall 100 of the fuel rod support cells 94. Due to the high velocity of the coolant passing upwardly through the core and the turbulence that is generally, intentionally created to promote heat transfer from among the fuel assemblies to the coolant, the nuclear fuel rod grid straps 86 and 88 have a potential to experience vortex shedding lock-in vibration when the shedding frequency is close to the natural frequency of the strap. If the vibration reaches the natural vibration frequency of the strap, the relative vibratory motion between the grid contacts (the dimples and springs) and the fuel rod cladding can cause fretting of the cladding and can eventually result in a breach of the cladding and release of the fission by-products into the coolant. The resonant vibration can also cause cracks or other failures in the grid straps which could also lead to a cladding breach. This invention employs straps with angled trailing and leading edges that are designed to break the correlation of the vortices shed from the trailing and leading edges of the grid straps by varying the phase of the vortices to avoid strap vortex shedding lock-in vibration. The improved grid strap of this invention can better be appreciated from the views shown in FIGS. 5-11. Note FIG. 12 provides an alternate embodiment for breaking the correlation using half circle cutouts instead of angled edges. For simplicity, the portions of the straps that are shown have walls 100 which only support fuel rods and do not border the cells 96 through which the guide thimbles and instrumentation tube extend. FIG. 5 shows a perspective view of a portion of one of the straps 86 and 88 which borders on the cells that support fuel rods. A wall 100 of each cell 94 is defined between the vertical slits 102 and between the vertical slits 102 and the ends of the straps. The vertical slits 102 in the straps 86 which extend from the lower edge 104 of the strap to partially up the straps' height mate with a corresponding slit in the straps 88 which extend from the upper edge and extend partially down to form the intersection between straps at the interleaved joint. The lower edge 104 of the straps 86, 88 is hereafter referred to as the leading edge and the upper edge 106 of the straps 86, 88 is hereafter referred to as the trailing edge in as much as the coolant traverses the core from the lower edge to the upper edge. In accordance with this invention, either one or both the leading edge 104 or the trailing edge 106 is provided with an angled contour that varies the elevation of either or both the leading edge 104 or the trailing edge 106 as the edges extend along the walls 100, longitudinally along the length of the straps. In the embodiment illustrated in FIG. 5, the leading edge is provided with a flat contour that does not vary in elevation along the wall 100 while the trailing edge 106 is provided with a curved contour that will break the correlation between the vortices, i.e., the vortices will not reinforce each other. During reactor operation in the core, the high velocity coolant flow across the grid straps causes vortex shedding lock-in vibration if the shedding frequency is close to the natural frequency of the grid straps. With the angled trailing and/or leading edges the vortex will still form along the trailing edge at each wall 100 along the strap. The vortex at each wall will have the same shedding frequency if the coolant flow velocity is the same. However, the moment (i.e., timing) of the vortices coming out of the angled edges will not be correlated because they will be out of phase. With the angled trailing edge, the delta pressure oscillations due to vortex shedding act at the different phases to cancel each other and no uniform resulting oscillating force will be formed to excite the strap. In the embodiment shown in FIG. 5, the trailing edge 106 of the wall 100 of the strap 86 is inclined at an approximately constant 45° angle between the intersections of adjacent orthogonal straps and makes a smooth rounded transition at the intersection 102 changing directions approximately 90° and similarly reversing directions at each subsequent interface with the orthogonal straps 88. Though the angle of incline was described as being approximately 45°, it should be appreciated that it can vary between 10 and 80° without detracting from the invention. The dimples 108 and 108′ illustrated in FIG. 5 protrude into adjacent cells and contact and support the fuel rods. The dimples 108 and 108′ are also designed at an angle to reduce the dimples' stiffness and contribute to changing the correlation of a vortices to prevent the formation of uniform oscillation forces that could result in resonant vibration of the grid strap. A side view of the dimples 108 and 108′ is shown in FIG. 6. FIG. 7 shows a perspective grid section constructed from an interleaved arrangement of two parallel arrays of orthogonal straps 86 and 88 with each strap formed with the angled trailing edge pattern illustrated in FIG. 5. It should be noted that the straps 86 and 88 are the same height at the intersections 102 and the straps that form the opposing walls of each cell are angled in different directions. Again, for simplicity, the strap arrangement illustrated in FIG. 7 defines cells that support fuel rods and does not illustrate the cells through which the guide thimbles and instrumentation tube pass through. While a trailing edge 106 undulating pattern has been illustrated so far, it should be appreciated that the benefits of this invention can be achieved employing other trailing and leading edge patterns. For example, as illustrated in FIG. 9 by the wall pattern 110, the cell wall 100 can have a straight horizontal leading edge 104 and an inclined trailing edge 106 similar to that described with regard to FIGS. 5-8, except that adjacent cells may be inclined in the same direction as the pattern 110. The angled edge pattern may also vary between adjacent cells. In still another embodiment represented by cell wall pattern 112, the leading edge 104 has a straight horizontal contour while the trailing edge is formed in a saw tooth pattern which is repeated cell to cell. In another embodiment illustrated by wall pattern 114 the cell wall 100 has a leading edge 104 which is slanted parallel to the trailing edge 106 at an angle similar to that described for the embodiment illustrated in FIG. 5. Similar to that described previously, the adjacent cell walls on the same strap may be inclined in the opposite direction or they could be inclined in the same direction. In still another embodiment, both the leading edge 104 and the trailing edge 106 may be formed in a saw tooth pattern inclined in opposite directions. Preferably in each case, the opposing walls of each cell would be inclined 180° out of phase (where 360° is the full extent of the pattern of the angled edge before it repeats itself) similar to that illustrated in FIG. 8. Thus, employing this invention of angled trailing and leading edges in a reactor core, the vortices will still form along the trailing edge. Each vortex along the edge of the strap will have the same shedding frequency if the flow velocity is the same. However, the phases of those vortices coming out of the edge are not correlated due to the shape of the edges. Therefore, the delta pressure oscillations due to vortex shedding act at different phases. Due to the phase difference, the delta forces will cancel each other and will not form uniform resultant oscillating forces. The preferred embodiment is shown in FIG. 10. FIG. 10 shows another embodiment that employs horizontal dimples 108 and 108′ with a straight horizontal lower strap edge 104 and a saw tooth trailing edge 106. A saw tooth cut-out 118 is stamped below the dimple 108 to form a trailing edge for this lower strap region that alters the correlation of the vortices from this strap region. In addition, the dimples 108 and 108′ are softened with gently rounded curves to further alter the vortices, as shown in FIG. 11. FIG. 10 only shows one fuel support cell wall in the grid strap. When looking at the entire grid strap, the top edge looks like a saw tooth design. The “saw tooth” term is intended to refer to the second and fourth designs from the left in FIG. 9 and the design shown in FIG. 10, i.e., with or without the ledge 120. The two cutouts 106 and 118, one at the top edge of the strap and the other below the lower dimple, are defined by the angle of the cutout and the radius at the bottom of the triangle cutouts, as shown in FIG. 10. A third parameter, the cutout width to overall strap cell width ratio, is also a parameter affecting the performance of this invention. The angle θ of the cutouts and top trailing edges (the enclosed angle), as defined in FIG. 10 is between 20 and 160 degrees, with the optimal angle between 60 and 90 degrees. The radius ratio at the bottom of the triangle cutouts is defined by the ratio of the radius of the curve to the depth of the cutout. Using this definition, the radius ratio (R/D) range can extend from 0.1 to 0.9, with an optimal range between 0.5 and 0.7. Experimentation has shown that breaking up the profile of a fuel support cell wall edge (like 106 or 118) can be beneficial for vibration reduction. For instance, the top cutout shown in FIG. 10 has been shown to provide for less vibration than one where the W/P (width of the cutout at its widest point to the overall width of the cell wall) ratio approached 1.0. The top cutout geometry shown in FIG. 10 has a small ledge 120 on either side of the angled trailing edges with a reasonably large radius at the bottom. This geometry removes the long horizontal edge of the prior art design which has shown to exhibit significant vibration. In addition, this geometry provides three different edge configurations, horizontal, angled and radius, which in testing has been shown to be a preferred geometry to prevent vortex correlation, thus preventing high amplitude vibration. The cutout width to overall cell width ratio is between 0.1 and 0.9, with the optimal range 0.5 to 0.85. Thus, the preferred embodiment is described by a cutout angle θ, a bottom radius ratio (R/D), and a cutout width ratio (W/P). FIG. 12 shows an alternate configuration for the top and bottom cutout. This configuration has been tried experimentally and has been shown to provide significant vibration mitigation relative to the prior art design. This configuration uses a semicircular cutout instead of a saw tooth cutout as the trailing edge for coolant flow. The concept behind this configuration is that there is no straight edge along the entire width of the trailing edge cutout, which will further prevent coolant vortices from correlating and thus reduce vibration. The geometry of this configuration should be such that the depth (D) to width (W) ratio approaches 0.5 to maximize the curvature of the cutout and reduce vortex correlation. The cutout width (W) to overall cell width (P) is also important for this alternate configuration. As shown in FIG. 12, the W/P ration should be 0.2 to 0.9, with the optimal range 0.4 to 0.6. FIG. 13 illustrates the experimental results that were obtained that demonstrate the reduction in grid strap vibration that can be achieved by employing this invention. This figure illustrates the reduction in vibration for the variations of the saw tooth design of FIG. 10. This data shows that for different values of the cutout angle θ, a bottom radius ration (R/D), and a cutout width ration (W/P), the vibration reduction will be different. This data, and other data like it, was used to define the ranges for these parameters set forth above, including the optimal ranges. Accordingly, while specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breath of the appended claims and any and all equivalents thereof. |
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claims | 1. A process for preparation of the phosphor panel formed on a substrate by a gas phase-accumulation method, wherein the phosphor layer comprises a large number of phosphor columns standing parallel to each other, in which the phosphor columns have a mean diameter in the range of 0.1 to 50 μm at a top surface thereof, and there are no phosphor columns having a diameter larger than 200 μm at the top surface,comprising the steps of:placing, in a vacuum evaporation-deposition apparatus, a container containing an evaporation source containing phosphor or a constitutional materials thereof,evacuating the evaporation-deposition apparatus to set an inner atmosphere thereof at a pressure in the range of 0.1 to 10 Pa,preheating the container to melt whole evaporation source at the above-mentioned pressure range, andvaporizing the evaporation source to deposit on a substrate a phosphor layer having predetermined thickness. 2. The process of claim 1 wherein at least two evaporation sources are used, in which one of the sources contains compounds for forming matrix of the phosphor and the other contains an activator of the phosphor, comprising the steps of:preheating at least one evaporation source for forming matrix to melt completely the evaporation source, andvaporizing both of the evaporation source for forming matrix and evaporation source containing an activator to deposit on the substrate a phosphor layer having predetermined thickness. 3. The process of claim 1, wherein the step of preheating the container to melt whole evaporation source is performed under such condition that a temperature of the evaporation source at a center area differs from a temperature of the evaporation source at a peripheral area by not larger than 30° C. 4. The process of claim 1, wherein the step of vaporizing the evaporation source is performed by means of a resistance-heater. 5. The process of claim 1, wherein particles of the vaporized evaporation source come into collision with an atmospheric gas molecule in the evaporation-deposition apparatus at 1 to 1,000 times before the particles are deposited on the substrate. |
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058624945 | claims | 1. A method of entrapping the hazardous components of moisture-bearing low-level radioactive and mixed waste material which comprises the steps of: selecting an encapsulating material from the group consisting of polyacrylic acids and the salts thereof; determining the amount of encapsulating material which is required to form a gel-like matrix with the waste material; and blending the encapsulating material with the moisture-bearing waste material to react the encapsulating material with the moisture in the waste material until a gel-like matrix is formed which entraps the hazardous components of the waste material and provides a final waste material product having no freestanding liquids. 2. A method according to claim 1 wherein the encapsulating material selected is polyacrylate. 3. A method according to claim 1 wherein said encapsulating material is placed within a pressurized vessel and blended with said moisture-bearing waste material by being discharged from the vessel into the moisture-bearing waste material. |
summary | ||
description | This application claims the benefit of U.S. Provisional Application Ser. No. 60/504,779, filed Sep. 17, 2003, the contents of which are incorporated herein by reference. Contained herein is material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction of the patent disclosure by any person as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all rights to the copyright whatsoever. Copyright© 2004 S. M. Stoller Corporation. Embodiments of the present invention relate generally to packaging to ship hazardous materials. More particularly, embodiments of the present invention relate to packaging to ship radioactive material and radioactive liquids. In the field of hazardous material transport, the packaging used for transport must comply with strict international and national regulations. For instance, a Department of Transportation (“DOT”) 7A Type A package must be used to ship radioactive liquids. Such shipped packages must comply with the DOT regulations for Hazardous Materials, of which 49 C.F.R. §§ 100–178 are herein incorporated by reference. Packages used for transport of hazardous materials by plane must comply with the International Air Transport Association (“IATA”) Dangerous Goods Regulations (“DGR”), of which Sections 5, 6, and 10 are herein incorporated by reference. Consequently, a packaging design was necessary that complied with both sets of hazardous material transport regulations. The only currently available packaging qualified to ship radioactive liquids up to an A2 quantity is known as a “bean pot,” and will only hold one liter of liquid. The shipping costs to ship large quantities of samples needing analysis, one sample bottle at a time, can be prohibitive. Additionally, many samples can be taken in very radioactively contaminated environments, and/or need to be analyzed for organic content, so they must be sealed at the time of capture to preserve all of the constituents for accurate analysis. Consequently, a new packaging design was necessary that would accommodate shipping multiple sample containers in one packaging. Often, the samples needing analysis contain multiple hazardous materials from more than one hazard class. This necessitates that the package being used meet the requirements for all hazards being shipped. For example, the IATA DGR and DOT regulations for the transport of radioactive materials require the packaging to withstand various tests, including: a water spray test, a free drop test, a stacking test, an internal pressure test, and a penetration test. Apparatus and methods are described for shipping hazardous materials, such as radioactive materials and radioactive liquids. According to one embodiment, the package design meets International Air Transport Association (IATA) Dangerous Goods Regulations (DGR) and/or Department of Transportation (DOT) regulations for Hazardous Materials (49 C.F.R. §§ 100–178) for multiple and preferably all hazard classes as a Packing Group I package, and a 7A Type A package for radioactive material, including radioactive liquids. According to another embodiment, the package design accommodates shipping a plurality of sample containers in one packaging and provides flexibility in terms of the mixing and matching of various sized sample containers. According to one embodiment, the container used for transport has several components. The container is a box with a foam insert with cutouts for six secondary container cylinders, and seven slots for holding ice packs. Hazardous materials such as radioactive liquids and/or solids are placed into a glass bottle. The lid of the glass bottle is then closed. The glass bottle is double contained in plastic and secured with plastic tape. The glass bottle is then inserted into a secondary container, along with layers of foam inserts separating the glass bottle from the inside of the secondary container and from other glass bottles within the secondary container. The lid of the cylindrical secondary container is then closed, and the lid is secured with plastic tape. The secondary container is inserted into one of the six cutouts in the box. Then the lid for the box is closed and secured. The package is then ready for shipping. Other features of embodiments of the present invention will be apparent from the accompanying drawings and from the detailed description that follows. Apparatus and methods are described for shipping hazardous materials, such as radioactive materials and radioactive liquids. According to one embodiment, the package design meets International Air Transport Association (IATA) Dangerous Goods Regulations (DGR) and/or Department of Transportation (DOT) regulations for Hazardous Materials (49 C.F.R. §§ 100–178) for all hazard classes as a Packing Group I package, and a 7A Type A package for radioactive material, including radioactive liquids. According to another embodiment, the package design accommodates shipping a plurality of sample containers in one packaging and provides flexibility in terms of the mixing and matching of various sized sample containers. For the purposes of explanation, specific details regarding particular commercial embodiments are set forth herein in order to provide a thorough understanding of various aspects of the present invention. It will be apparent, however, to one skilled in the art that embodiments of the present invention may be practiced without some of these specific details. While, for convenience, embodiments of the present invention are described with reference to particular commercial embodiments formed of particular materials, parts, hardware, sealants, foams, sample container configurations, embodiments of the present invention are equally applicable to various other industry-recognized equivalents and alternative configurations. Further information regarding specific embodiments of the present invention are described below. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. Further, like reference numbers are used to refer to the same element throughout all drawing figures. A particular configuration chosen to satisfy 49 C.F.R. §§ 173.201 and 173.211, along with IATA DGR for UN Specification Packaging, is a solid plastic outer container 120 containing glass inner containers 720. This configuration may also meet the requirements for a Type A package as defined in 49 C.F.R. § 173.412. The outer packaging may be manufactured by Viking Packing Specialist. The glass inner containers 720 may be obtained from Eagle Picher. Looking first to FIGS. 1–5, the outer wall 127 of the outer container 120 is a 3/16 inch thick high density polyethylene (“HDPE”) material. The outer wall 127 is held together with extruded aluminum hardware and closed-end aluminum and steel rivets 122, with Hybraflex sealant applied to seal the seams. Bonded to the outer wall is a ½ inch thick layer of 1.7# density HDPE open cell foam. Bonded to the layer of open cell foam is a ½ inch thick cloth-covered 1.7# HDPE foam pad with Formflex HDPE backing. The extruded aluminum hardware includes several different elements. The lid 128 of the outer container 120 is attached with a full-length piano hinge (depicted in FIGS. 17A–C). The lid 128 is latched closed by four valance spanning latches 121, two on the front and one on each side of the outer container 120. There is a recessed handle 125 on each end of the outer container 120 for handling the outer container 120, and D-rings on the outer container 120 and lid 128 on the front side for applying sealing and tamper indicating devices. Four-hole clamps 124, also depicted in FIG. 19, hold together the outer walls 127 of the outer container 120. Combination corner-clamps 126, also depicted in FIG. 20, are attached to the top corners of the lid 128. Flat corners 123, also depicted in FIG. 22, are attached to the bottom corners of the outer container 120. One embodiment of the outer container 120 has outside dimensions of 28.25 inches wide, 18.375 inches high, and 18.375 inches deep. Inside the outer container 120 is a custom insert 421 made of the 1.7# density HDPE open cell foam. The custom insert 421 features six cylindrical cutouts 422, and seven slots 520 for retaining ice packs. Depending upon the number and size of sample containers that need to be accommodated, more or fewer cylindrical cutouts 422 may be employed. Further, depending on the cooling needs, more or fewer slots 520 may be present. Inside each cutout 422 fits a secondary container 620. As seen in FIG. 6, the secondary container 620 has a threaded lid 621 and a cup portion 622. In one embodiment, the secondary container 620 has a diameter of five inches and a height of twelve inches. Referring now to FIGS. 7–16, the inner containers 720, also seen in FIGS. 10 and 12, are amber glass, and are commonly available in seven sizes, as tested: 20 mL, 40 mL, and 60 mL vials with Teflon® septa; and 125 mL, 250 mL, 500 mL, and 1 L wide mouth bottles with solid plastic lids. All seven sizes have been tested within the context of the preferred embodiment and meet the 95 kPa internal pressure test prescribed by 49 C.F.R. § 173.27(c), and Section 5.0.2.9 of the IATA Dangerous Goods Regulations. The inner containers 720 are placed into the secondary containers 620 along with various foam inserts, as seen in FIGS. 7, 8, and 10–16. These foam inserts are of different shapes and sizes. Also constructed of 1.7# HDPE open cell foam, the foam inserts provide cushioning for the various sized inner containers 720. The foam inserts provide cushioning for the inner containers 720 by filling the space between the inner containers 720 and the inside of the secondary container 620. This is accomplished by stacking layers of different foam inserts, wherein each foam insert is customized to conform to and fit around inner containers 720 of a certain size. Inner containers 720 may be stacked with the foam inserts inside secondary containers 620 to allow one to three stacks of inner containers 720 separated by foam inserts. Without limitation, some various configurations for single, double, and triple-stacked inner containers 720 are depicted in FIGS. 14–16. According to the embodiment depicted, the foam inserts all have an outer diameter that conforms to the inner diameter of the secondary container 620. As seen in FIGS. 14A–C, different variations of the foam inserts may include: a one-inch tall cylindrical foam insert 1420, a taller cylindrical foam insert 1421, a hollow foam insert 1422 with an inner diameter that conforms to the top of a 1 L inner container 720, and a hollow foam insert 1423 that conforms to the outer diameter of the bottom portion of a 1 L inner container 720. Foam insert 1424 features three longitudinal, non-overlapping cylindrical holes wherein the diameter of each of the holes conforms to the outer diameter of a 60 mL, 40 mL, or 20 mL inner container 720. FIGS. 15A–C depicts a foam insert 1521 with a height smaller than one inch. Foam insert 1520 has an inner diameter that conforms to the outer diameter of a 125 mL inner container 720. Foam insert 1522 has an inner diameter that conforms to the outer diameter of a 250 mL inner container 720. Using and stacking the various foam inserts 1420, 1421, 1422, 1423, 1424, 1520, 1521, and 1522 as depicted in FIGS. 14–16, the following combinations of inner containers 720 will each fit within foam inserts in a single secondary container 620: one 1 L bottle (FIG. 14A) one 500 mL bottle (FIG. 14B) three 60 mL bottles (FIG. 14C) six 40 mL bottles (FIG. 15A) three 40 mL and three 20 mL bottles (FIG. 15B) three 60 mL and one 125 mL bottle (FIG. 15C) three 60 mL and three 40 mL bottles (FIG. 15D) three 60 mL and three 20 mL bottles (FIG. 15E) two 125 mL bottles (FIG. 15F) one 125 mL and three 40 mL bottles (FIG. 15G) one 125 mL and three 20 mL bottles (FIG. 15H) two 250 mL bottles (FIG. 15I) one 250 mL and one 125 mL bottle (FIG. 15J) one 250 mL and three 40 mL bottles (FIG. 15K) one 250 mL and three 20 mL bottles (FIG. 15L) one 500 mL and one 125 mL bottle (FIG. 15M) one 500 mL and three 40 mL bottles (FIG. 15N) one 500 mL and three 20 mL bottles (FIG. 15O) one 250 mL and six 20 mL bottles (FIG. 16A) one 125 mL and six 20 mL bottles (FIG. 16B) three 40 mL and six 20 mL bottles (FIG. 16C) nine 20 mL bottles (FIG. 16D) As seen in FIGS. 7, 8, and 10–13, the inner containers 720 remain separated from each other and from the inside of the secondary container 620 due to the foam inserts. The foam insert configurations depicted in FIGS. 14–16 do not comprise an exhaustive depiction of the foam insert embodiments according to the present invention. Those skilled in the art will appreciate that virtually any shape or number of foam inserts may be used to fit bottles or inner containers 720 of various sizes. Additionally, various other shapes of secondary containers 620 may be used, including, but not limited to: square and rectangular. In additional to HDPE foam, any other suitable cushioning material may be used to make the inserts. Also, the outer container 120 may be constructed of other rigid materials, such as metal or wood or other plastics, such that the outer container 120 complies with international and national regulations for the shipping of hazardous materials. The sides of the outer container 120 may be joined using fasteners and hinges made of different materials, or fasteners and hinges of different kinds, than the extruded aluminum elements of FIGS. 17–22. To prepare a sample of radioactive or hazardous material for shipment in one embodiment of the present invention, the sample is first inserted into an inner container 720. Then the lid of the inner container 720 is closed, such as by turning the threaded lid of an inner container 720 to tighten it. The inner container 720 is then wrapped, or “double contained,” in plastic and secured with plastic tape. Next, foam inserts are inserted into the cup portion 622 of the secondary container 620, as shown in FIG. 7. As the foam inserts are inserted, the inner container 720 is placed into the corresponding foam insert or inserts that conform to the outside of the inner container 720. Once the inner container 720 has been placed into its surrounding foam inserts, the remaining foam inserts are placed over the top of the inner container 720. The foam inserts and inner container 720 thus fill the space within the cup portion 622 of the secondary container 620. FIG. 8 shows a cutaway view of what the inside of the secondary container 620 would look like following this step. Next, the lid 621 of the secondary container 620 is releasably secured to the cup portion 622, such as by screwing a threaded lid 621 onto the cup portion 622. Once the lid 621 has been secured to the cup portion 622, tape 920 is placed over the lid 621 and attached to the cup portion 622 in the configuration shown in FIG. 9. The secondary container 620 is then inserted into one of the six cylindrical cutouts 422 in the custom insert 421 in the outer container 120. Ice packs 420 may then be inserted into the slots 520 of the custom insert 421 to maintain the samples at a cooler temperature. The lid 128 of the outer container 120 is then closed, and secured with the valance spanning catches 121. Following this step, the outer container 120 is ready for safely shipping the radioactive or hazardous materials. Under one embodiment, up to six liters of radioactive liquid may be shipped in a single package, not to exceed a specific gravity of 1.4, or up to 8.55 kg of solids, or any combination of solids and liquids not to exceed 8.55 kg net weight. |
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abstract | Apparatus for amplifying low level signals within a nuclear plant's containment building, derived from the ex-core nuclear instrumentation system. The system employs vacuum micro-electronic devices in place of conventional pre-amplifier assemblies to position the pre-amplifier assemblies closer to and within the vicinity of the ex-core detector outputs. |
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056065825 | summary | FIELD OF THE INVENTION The present invention relates to a disconnection device between a means for absorbing neutrons in the core of a pressurized nuclear reactor and a control rod for the same, disconnection taking place automatically during the phase leading the reactor to its cold shutdown state. The invention has a particularly important application in pressurized water reactors, by making it possible to reduce the duration of the core reloading operations and the like, as well as the exposure doses of personnel to radiation in this connection. PRIOR ART AND SET PROBLEMS For the control of pressurized water reactors, use is presently made of fuel rod clusters having an absorbing power with respect to neutrons and placed vertically in the reactor core, between the fuel elements, which, when grouped, is called a "fuel assembly", so as to regulate the reactivity of said core and therefore the power supplied by the reactor. These means are referred to as "absorber clusters" within the present application. The displacement of each of the absorber clusters respectively attached to a control rod is obtained by displacement mechanisms, e.g. of the screw-nut type, cooperating with the control rod and placed within tight tubular enclosures linked with the reactor vessel and arranged vertically above the reactor vessel sealing cover. The lower portion of the control rod is terminated by linking means, which engage in a gripping pommel of the absorber cluster and which can be remotely manually unlocked with the aid of a special tool, once the vessel sealing cover has been removed. The operation of a nuclear reactor requires the periodic replacement of the fuel assemblies of the core. Such an operation requires the removal of the sealing cover from the vessel, together with the control rod drive mechanisms carried by the same. Following the removal of the vessel cover, the control rods are manually disconnected individually from their respective absorber clusters with the aid of the special tool. The internal equipments of the reactor placed above the core are then removed at the same time as the group of control rods, the rods being raised by internal equipments. The fuel assemblies are then uncovered and can be removed from the reactor core. Obviously, the reactor is shutdown during the core reloading operations and the maintenance in said state and which is referred to as the "cold shutdown state", so that the absorber clusters must remain fully within the fuel assemblies in order to satisfy safety standards. The manual manipulation of each of the links, in order to disconnect the control rod from its associated absorber cluster, takes a relatively long time. Thus, the total duration of the manual disconnection operations is relatively long and during said time personnel is exposed to radiation. U.S. Pat. No. 2,261,595 discloses a linking device between a control rod or bar and its associated absorber cluster. This device permits disconnection prior to the removal of the sealing cover from the vessel making use of the displacement mechanism of the control rod. Disconnection and reconnection are carried out by lowering and then raising the control rod, whilst the absorber cluster is completely inserted in the fuel assembly. Such a device has the advantage of avoiding manual disconnection of the links following the opening of the vessel and therefore reduces the time taken for the core reloading operations and reduces the radiation exposure dose for personnel. However, the means for coupling the control rod base remain engaged in the absorber cluster gripping member, when they are in the inoperative position. Thus, it is necessary to fear the accidental raising of one or more absorber clusters during the removal of internal equipments covering the core if, in conventional manner, said equipments are above the control rods. This may e.g. be due to an alignment deficiency between the base of the control rods and the gripping member of their associated absorber cluster, producing an attachment or fastening between said elements. Such an event can produce a rise in the core reactivity, which can be completely incompatible with the safety criteria which provide for the maintenance of the absorber clusters in the fuel assemblies when the reactor vessel is open. Moreover, there is no system for disconnecting the link between the control rod and the absorber cluster in the case of the jamming of the device when the coupling means are in the operative position. German patent 42 12284 discloses a thermal control device automatically ensuring the uncoupling of a control rod from its associated absorber cluster and the propulsion in the core of said cluster in the case of an abnormal temperature rise within the reactor. This device has coupling means of the ball type maintained in the operative position by a bolt connected to a thermal expansion module constituted by one or more metal bellows and ancillary tanks communicating with one another and filled with primary fluid. The increase in the length of the bellows under the effect of the expansion of the fluid trapped in the expansion module displaces the bolt in such a way as to firstly authorize the disconnection and then pushes back the absorber cluster. However, such a device which has the advantage of requiring no external manipulation, only ensures the disconnection and propulsion into the core of the absorber cluster. The position of the bolt which conditions the putting into the inoperative or operative state of the coupling means is directly dependent on the temperature of the reactor and in fact the temperature of the expansion module. AIMS OF THE INVENTION The aim of the invention is to equip the link between the control rod and its associated absorber cluster with a device making it possible to automatically separate the rod from the cluster during the bringing of the reactor into the cold shutdown state prior to the removal of the vessel sealing cover in order to eliminate, without any external intervention and without encountering the difficulties which arise with the invention of U.S. Pat. No. 2,261,595, the manual manipulation in conventional form of the joints and consequently the reduction of the duration of the operations performed when the vessel is opened, whilst reducing the radiation exposure doses for personnel. The aim of the invention is also to give said same link between the control rod and the absorber cluster an automatic separating device, which can be neutralized, unlike that of German patent 42 12 284. The neutralization of the automatic separation makes it possible to avoid on the one hand the needless uncoupling if the reactor is brought into the cold shutdown state for a reason other than an opening of the vessel and on the other the capacity to recouple, e.g. as in the prior art, during the cold shutdown prior to the sealing of the vessel and the reactor power rise. SUMMARY OF THE INVENTION To this end, the main object of the invention is an automatic uncoupling device between the pommel of an absorber cluster intended to be introduced between the fuel elements of the core of a pressurized water nuclear reactor and a control rod transmitting to the absorber cluster movements induced by a control mechanism located in a tight enclosure placed above the reactor vessel sealing cover, said uncoupling device incorporating an attachment head and a mobile locking member, able to occupy a locked state and an unlocked state, whereof an axial displacement leads to a change of state, characterized in that it also comprises a thermal module axially deforming under the effect of the temperature and which is positioned between the attachment head and the locking member to form an axially mobile assembly. In a preferred embodiment of the invention, the thermal module comprises a water-filled, tight bellows. The attachment head comprises at least one radial attachment finger projecting as a result of a small spring and sliding within a cylinder fitted into the cladding or shell of the control rod, said cylinder having in an inner portion and from top to bottom, in order to receive the attachment finger a chamfered widening and an axial notch, having a U-shaped cross-section, machined over a portion of its height. Above the attachment head is located a second spring, which pushes it permanently downwards. The locking member is a sliding cylindrical member coaxially mounted in the control rod cladding, said locking member having an annular locking groove which can be positioned in front of the balls, located in the control rod cladding, when the locking member is in a high position. The balls are received in radial, truncated cone-shaped cavities, located at the bottom of the cladding and whose thickness is less than the diameter of the balls, said cavities being shaped so as to prevent a radial, outwardly directed escape of the balls. An extractor tube is mounted in sliding manner on the lower end of the cladding and is pushed downwards by a third spring sufficiently stiff to raise the control rod. This extractor tube has in its upper portion a slot having an axial portion followed by a helical portion and which, by sliding on a guide pin integral with the cylinder, guides the displacement of the latter. Level with the guide pin, the control rod cladding has a horizontal slot allowing the rotation by a fraction of a turn of the cylinder with respect to the control rod, thus permitting the freeing of the attachment finger from the U-shaped, inner notch and, consequently, the free displacement of the attachment head in the downwards direction. The automatic uncoupling of the absorber cluster with respect to the control rod, controlled by the device according to the invention, occurs in the case of a drop in the temperature of the reactor vessel water below a certain threshold (e.g. approximately 80.degree. C.). The attachment finger then abuts in the U-shaped axial notch and the locking member rises under the effect of the shortening of the bellows, due to the contraction of the water, up to its upper position, which permits the radial engagement of the balls in the annular groove of the locking member. In this uncoupled position, when a pressure is exerted from the top on the control rod, the latter is firstly displaced downwards by a height equal to that of the axial portion of the cylinder slot and then, under the action of the helical portion of the slot on the guide pin, rotates the cylinder by a fraction of a turn disengagement the attachment finger from the U-shaped notch, thus permitting a downward displacement of the attachment head-thermal module-locking member assembly and consequently the recoupling of the control rod with the pommel of the absorber cluster. In a constructional variant, it is possible to complete the device with a barometric module positioned below the lower end of the locking member, which is then subdivided into two sections, which are interconnected by a fourth spring, whose displacement is limited by a guide and stop pin-axial slot system, said barometric module, kept under a vacuum, axially deforming as a function of the pressure variations prevailing in the reactor core giving rise to a change of state of the locking member. The barometric module is preferably constituted by a lower base surmounted by an axial bush, an upper cap covering the axial bush, a metal bellows tightly connecting the bush and the cap and a fifth spring placed within the bush. In this constructional variant, the automatic uncoupling is obtained on the one hand when the pressure within the reactor vessel is sufficiently reduced and on the other when the temperature within the vessel is sufficiently lowered. LIST OF DRAWINGS The invention and its various technical features will be better understood from a study of the following drawings, wherein show: FIG. 1 A sectional view of the upper portion of a reactor vessel where the invention is installed. FIG. 2 A sectional view of the device according to the invention in two parts. FIG. 3 A part front view of the notch permitting the rotation of the cylinder in order to retract the abutment in the device according to the invention. FIG. 3A A horizontal section along line A--A of FIG. 3. FIG. 3B A horizontal section along B--B of FIG. 3. FIG. 4 In section, the lower portion of the device according to the invention. FIGS. 5A to 5D Four simplified sectional views of the device according to the invention during four operating phases. FIG. 6 A frontal section of a variant of the device according to the invention. |
052290689 | abstract | In a fuel bundle for use in the core of a boiling water nuclear reactor, part length rods having a tendency to reduce pressure drop are used in combination with spacers and spacer attached devices tending to restore pressure drop to improve critical power. The addition of the part length rods has the advantage of lowering the pressure drop. Attached devices substantially recapture the pressure drop. Exemplary spacer attached mechanisms for the recapture of pressure drop are set forth including vanes--preferably swirl vanes on the spacers, decreasing the spacer pitch to increase the total number of spacers in the upper two phase region of the fuel bundle, increasing the vertical height of the spacers, and increasing the thickness of the metal from which the spacers are constructed. Two classes of separation devices are disclosed for placement in the volume overlying the end of the partial length fuel rods. A first type of device fits to the end of the part length rods and is primarily intended for preventing water passing along the surface of the part length rod adjacent the end of the part length rod from entering the volume overlying the part length fuel rod. A second type of device resides in the volume overlying the part length rod. In either case, critical power is improved. |
054003733 | summary | BACKGROUND OF THE INVENTION This invention relates to an assembly fixture and method used for the fabrication of metal grids. It relates particularly to apparatus and a method used for the fabrication of metal grids used to support and position the fissionable fuel rods used in a nuclear reactor core. Most nuclear reactors use as fuel, elongated rods of fissionable material arranged and supported in a spaced parallel array between upper and lower core support plates. To provide integrity within the supports, the fuel rods are divided into groups and the fuel rods in each group are formed as an integral fuel rod assembly prior to placement between the reactor core support plates. More specifically, the fuel rods in each group have been typically in the past arranged in spaced, parallel arrangements with each other in supporting and spacing frames or grids. The grids are formed of a plurality of interconnected metal grid straps to provide a square or rectangular structural network of interconnected open grid cells, similar to an "egg crate divider". U.S. Pat. Nos. 3,379,617 and 3,379,619 issued Apr. 23, 1968 to Andrew et al. and assigned to the assignee of the present invention, disclose typical supporting grid assemblies for fuel rods in which the fuel rods are held laterally and longitudinally in the grid cells by resilient, spring-like retainers in selected grid cells of the grid structure. In the past the assembly of the grids has been difficult and time consuming due to the large number of grid strap components, their small size, the flexibility of the individual grid straps and the large number of welds required to connect all the grid straps and components together into an integral structural grid network. In addition, the nuclear fuel assembly grid specifications require that the that the grid straps and other grid components be very accurately aligned, both during and after assembly and welding, in order to hold the group of fuel rods making up a fuel rod assembly in a precise, parallel alignment. Such accurate alignment was often difficult and time consuming to achieve with the previously known multi-step grid assembly and welding fixtures and practices used prior to this invention. SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a grid assembly fixture and assembly method useful for the assembly and welding of grids used to support a fuel rod assembly used in a nuclear reactor. It is a further object of this invention to provide a grid assembly fixture and assembly method useful for the assembly and welding of grids that provides for an easy but precise and accurate alignment and positioning of the thin grid straps, especially those used for the sides of the grid, during its assembly and welding. It is a still further object of this invention to provide a grid assembly fixture and assembly method that allows for faster and more efficient grid assembly and welding procedures. Other and further objects of this invention will become apparent from the following detailed description and the accompanying drawings and claims. It has been discovered that the foregoing objects can be attained by a grid assembly fixture or apparatus for the assembly of a grid structure used to support elongate fuel rods for a nuclear reactor comprising a rectangular grid assembly plate supported above a work surface and having a flange on each four sides of the rectangular grid assembly plate. A clamp support plate attached to each side flange supports a toggle action clamp assembly that is able to move and hold an elongated clamping pad against each side of the grid assembly plate to hold temporarily hold the four border grid straps along the four sides of the grid assembly plate. A rectangular strap retention assembly is then placed around the border grid straps to hold the border grid straps in place against the four sides of the grid assembly plate until they can be welded together. The toggle clamps and associated clamping pads are then released. |
062927515 | abstract | A method is provided for correcting errors in position derived from an inertial measurement unit (IMU), by performing a first zero velocity update at a time when the IMU is at rest, recording the time and position of the IMU at a subsequent start of a period of interest after the first zero velocity update, recording the time and position of the IMU at the end of the period of interest, performing a second zero velocity update at the end of the period of interest with the IMU at rest, and then recording a velocity indicated from the IMU, and deriving an accumulated error in position from the recorded data, by approximating errors in velocity by a function of time with a parameter determined from the recorded indicated velocity, and integrating the function over the period of interest to determine the accumulated error in position during the period of interest. |
abstract | A rotatable and replaceable plunger type multi-source radiator is provided for equipment correlation and radiation experiment. The rotatable and replaceable plunger type multi-source radiator comprising a radiation source rod, a rotatable radiation source selection device, staircases radiation aperture, radiation shields, and a mechanic operation arm for replacing radiation source. The plunger type rotatable and replaceable multi-source radiator of the present invention is capable of preventing radiation leakage and mutual interferences among radiation sources, providing precise positioning, safety operation, and simple replacement of radiation sources. |
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claims | 1. A reactor containment vessel basket configured for placement in a reactor containment vessel that includes a spray device having a spray ring mounted near an inside ceiling of the reactor containment vessel so that the basket is subject to spray from the spray ring, the basket comprising:an upper surface opening which is formed on an upper surface of the basket;a lower surface opening which is formed on a lower surface of the basket and is covered by a mesh wire net; anda plurality of containment units arranged with predetermined first spaces in a stacked manner in a length direction, each of the containment units being configured to contain a pH adjuster and to allow for said spray from the spray ring to contact each of said plurality of containment units. 2. The basket according to claim 1, further comprising a plurality of partition plates respectively provided in the first spaces between the containment units. 3. The basket according to claim 2, wherein the partition plates are inclined with respect to a lateral direction. 4. The basket according to claim 3, further comprising a plurality of inflow guide plates which are formed in a plate shape extending upward in the height direction, respectively provided on an upper side end of the inclined partition plates, each of the inflow guide plates guiding the solvent that flows into each of the containment units. 5. The basket according to claim 3, further comprising a plurality of outflow guide plates which are formed in a plate shape extending downward in the height direction, respectively provided on a lower side end of the inclined partition plates, each of the outflow guide plates guiding the pH adjuster solution that flows out from each of the containment units. 6. The basket according to claim 1, wherein each of the containment units includes a plurality of divided containment units divided at predetermined second spaces perpendicular to the first spaces. 7. The basket according to claim 6, wherein the second spaces between the divided containment units are formed to extend along a flow direction of the pH adjuster solution. 8. A pH adjusting device including the basket according to claim 1 comprising:a cooling water inflow vessel that is configured to contain the basket therein and that is configured to store cooling water therein; anda cooling water outflow unit that causes the pH adjuster solution produced from the pH adjuster dissolved in the cooling water in the cooling water inflow vessel to flow out. 9. The basket according to claim 1, whereinthe upper surface opening is covered with a coarse mesh wire net, andthe mesh wire net of the lower surface opening is finer than the coarse mesh wire net. |
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abstract | Provided is an electrostatic coating apparatus capable of insulating an electric motor electrically from a member, to which an electrostatic high voltage is applied, and reducing the size and weight of the electrostatic coating apparatus. This electrostatic coating apparatus comprises a rotary atomizing head, to which high voltage is electrostatically applied, an electrostatically grounded AC servomotor, and a spindle and a fixed insulating member for insulating the AC servomotor electrically from the rotary atomizing head and a speed-increasing device to be set at the same potential as that of the former. The spindle and the fixed insulating member have insulation distance enlarging portions and of the mode, in which the creepage insulation distances from the speed-increasing device to the AC servomotor are enlarged. |
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044949871 | description | DETAILED DESCRIPTION OF THE INVENTION In order to demonstrate the advantages of the present invention, alloys having the nominal compositions shown in Table I were melted. It will be noted upon review of Table I that a gamma prime hardening austenitic base composition was selected and then additions of about 0.2 weight percent scandium, yttrium or hafnium were made to the base composition while varying the silicon content of the base composition between about 1.0 weight percent and about zero (i.e., impurity levels). In this manner seven alloys having the nominal compositions shown in Table I were melted into ingots. While it is desired to hold the levels of the other alloying elements constant from ingot to ingot, normal ingot to ingot variability in chemistry did occur. Examples of the variability observed are indicated by the chemical analyses shown in Table II. This variability is not believed to have had a significant affect on the determination of the effect of additions of scandium, yttrium and hafnium, with and without silicon, on the swelling resistance and post irradiation ductility of the alloys studied. The ingots representing the alloys shown in Table I were first hot worked to an intermediate size to improve the chemical homogeneity within the ingot and substantially remove the as cast microstructure of each ingot. After hot working, the intermediate size products were cold worked to final size in a series of steps having intermediate solution anneals between each cold working step. For example, the Base, #7 and #8 alloy ingots were intially soaked for about 1 to 11/2 hours at about 1150.degree. C. They were then press forged at about 1150.degree. C. to a flat bar having a nominal thickness of about 5/8 inch. Subsequently, each ingot received a homogenization treatment which entailed soaking the ingot at about 1225.degree. C. for about one hour followed by about a 2 hour soak at about 1275.degree. C. and then furnace cooling. Intermediate product from each of these three ingots was then cold worked in steps to substantially final size. The reductions utilized in each step typically varied from about 25 to 45 percent. Intermediate solution anneals at about 1150.degree. C. for about 3/4 hour followed by furnace cooling were performed between each cold working step. The last cold working step comprised about a 25% reduction. After the last cold working step, material from each of the heats shown in Table I were solution treated and aged as follows: 1. Solution treating was performed by soaking at about 1050.degree. C. for about 1/2 hour and was followed by air cooling. 2. Aging was then performed by soaking at about 800.degree. C. for about 11 hours followed by air cooling. A secondary aging treatment was then performed by soaking at about 700.degree. C. for about 8 hours followed by air cooling. Samples of the fully fabricated and heat treated alloys were then irradiated in fast neutron fluxes to various fluences and at various temperatures. The addition of hafnium and yttrium to the base alloy were found to significantly improve swelling resistance as demonstrated in Table III. Scandium, however, had no significant affect on swelling resistance. TABLE III ______________________________________ SWELLING CHARACTERISTICS Irrad- iation Neutron Fluence Percent Volume Expansion* Temp. .times. 10.sup.22 n (E >0.1 Base .degree.C. MeV)/cm.sup.2 Alloy Alloy #7 Alloy #8 ______________________________________ 400 5.9 0.72 -0.49 -0.47 427 6.8 1.29 -0.32 -0.33 454 5.7 0.73 -- -- 482 6.7 0.86 -- -- 510 7.4 1.36 -- -- 538 7.4 2.23 0.91 -- 593 7.8 0.61 -0.20 -0.06 650 7.7 0.41 -0.37 -0.44 ______________________________________ *Negative values indicate a volume contraction Additional samples irradiated at selected temperatures and fluences indicated in Table III were characterized as to their post irradiation ductility. These ductility results are shown in Table IV. TABLE IV __________________________________________________________________________ POST IRRADIATION DUCTILITY AS MEASURED BY DISC BEND TESTING Percent Strain, Irradiation Test Base Alloy #2 Alloy #6 Alloy #3 Alloy #7 Alloy #4 Alloy #8 Temp. .degree.C. Temp. .degree.C. Alloy (Sc no si) (Sc) (Y no si) (Y) (Hf no si) (Hf) __________________________________________________________________________ 454 564 0.7 0.2 -- -- -- 0.7 -- 482 592 -- 1.0* V.D.** 0.1 V.D.** -- V.D.** 510 620 0.4 0.5 2.1 0.9 5.0 0.3 2.2 538 648 0.3 0.5 1.2 0.3 1.1 0.5 0.8 593 703 0.6 -- -- 0.3 -- -- -- __________________________________________________________________________ *tested at 564.degree. C. rather than 592.degree. C. **V.D. = very ductile, i.e., ductility greater than 5% The disc bend ductility test used to test these alloys is a specially designed microductility test in which an indentor is pushed through a thin disc-shaped sample of the test material. The strain, .epsilon., or measure of ductility provided by this test has been correlated with tensile test results. The correlation between these two tests is accurate for low ductility materials. The discs are typically about 1/8 inch or 3 mm. in diameter and approximately 0.009-0.014 inch thick. The ductility test results shown in Table IV indicate that a significant improvement in the gamma prime hardened base alloy post irradiation ductility is obtained by the addition of scandium, yttrium or hafnium to the base alloy composition. However, it is also indicated that where the alloy contains no significant quantity of silicon, these additions did not enhance ductility. It is therefore believed that when about 0.05 to 0.5 weight percent of scandium, yttrium and/or hafnium is added to a gamma prime hardening austenitic alloy containing an effective amount of silicon the post irradiation ductility of the alloy should be enhanced. It is preferred that silicon be present at a level of about 0.5 to 1.5 weight percent. In addition it is believed that lanthanum may be substituted for all or part of the scandium, yttrium and hafnium. It is also believed that the benefits of the present invention are also applicable to gamma prime hardening austenitics which are placed in pile in a cold worked and aged condition or a cold worked condition. Typical of the treatments that may be utilized are as follows: TREATMENT I 1. Solution treat at about 950.degree. to 1150.degree. C. 2. Cold work 20-80%, preferably 30 to 60% 3. Age at one or more temperatures. TREATMENT II 1. Solution treat at about 950.degree. to 1150.degree. C. 2. Cold work 10-60%, preferably 15 to 30%. The present invention provides an improved precipitation strengthening austenitic superalloy for liquid metal fast breeder reactor ducts and fuel pin cladding. While the invention has been described in connection with specific embodiments, it will be readily apparent to those skilled in the art that various changes in compositional limits and heat treatments can be made to suit arrangements without departing from the spirit and scope of the invention. |
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046577335 | summary | FIELD OF THE INVENTION The invention relates to a fuel assembly for a nuclear reactor, in which the fuel rods are fixed on the lower end plate of the assembly. BACKGROUND OF THE INVENTION Fuel assemblies for nuclear reactors and in particular the fuel assemblies for water nuclear reactors consist of a bundle of parallel fuel rods held by a structure consisting of spacer grids, two end plates, and guide tubes which connect the spacer grids and the end plates. The fuel rods, which are cylindrical and of great length, are engaged in the spacer grids which hold them in position in the bundle. The spacer grids generally provide both the transverse positioning and the axial positioning of the rods which are shorter than the guide tubes which are substituted for certain rods of the assembly. The ends of the rods are therefore free and are located at a certain distance from the end plates. In order to provide both the axial positioning and the transversal positioning of the rods by virtue of the spacer grids, it is necessary to use springs which exert a high transversal force on the rods. PRIOR ART To improve the neutron yield of the reactor core, use is generally made of a zirconium alloy with a low neutron absorption for forming the grids of the assembly. However, the springs must be made of a nickel alloy to make it possible to obtain adequate elastic and mechanical properties of the spring under irradiation. Thought has therefore been given to separating the functions of transverse positioning and of axial positioning of the rods by fixing the latter at one of their ends on the lower end plate of the assembly. This, however, has the disadvantage of complicating the structure of the lower end plate, of making its disassembly more awkward, and of reducing the cross-section of flow of the primary fluid in the assembly. SUMMARY OF THE INVENTION The aim of the invention is therefore to propose a fuel assembly for a nuclear reactor, which consists of a bundle of fuel rods which are cylindrical over at least a part of their length, are parallel and are held by a structure which consists of spacer grids which are transverse relative to the rods, by two transverse end plates and by guide tubes which are connected to the spacer grids and to the end plates and which are substituted for certain rods of the bundle, the rods being fixed at one of their ends on one of the end plates arranged in the lower part of the assembly, when this assembly is in a vertical working position in the reactor core, this fuel assembly needing to have a lower end plate of a simple structure which permits easy dismantling and an adequate flow cross-section of primary fluid, while permitting efficient axial and transverse positioning of the rods. To this end, the assembly comprises, in addition an attachment plate which is superimposed on the lower end plate and attached to the latter against its upper face, comprising openings throughout its thickness each of which corresponds, in diameter and position, to a fuel rod, and opening into parallel grooves, each corresponding to a row of rods, which are machined on the lower surface of the plate in contact with the end plate which is itself pierced with holes opposite each of the rods, and each rod comprises a lower part the transverse section of which has three branches at 120.degree. angles to each other, each of the corresponding radial expansions terminating at the bottom in a shoulder projecting in the radial direction relative to the outline of the cylindrical part of the rod, the rods passing through the attachment plate in such a way that their lower part projecting by a height which is substantially equal to the depth of the grooves is arranged in the corresponding groove for their locking in rotation and their axial fixing between the end plate and the attachment plate. |
abstract | Disclosed is a heat exchanger for a passive residual heat removal system, which improves heat transfer efficiency by expanding a heat transfer area. A heat exchange tube includes a first member connected to a steam pipe through which steam generated from a steam generator of a nuclear reactor circulates, and a second member connected to both of the first member and a feed water pipe used to supply water to the steam generator provided in the nuclear reactor, and the first member has the shape different from that of the second member, thereby expanding the heat transfer area so that the heat transfer efficiency is improved. |
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abstract | In a radiation image storage panel composed of a phosphor layer produced by vapor phase deposition, the phosphor layer is composed of an europium activated cesium halide stimulable phosphor and exhibits an ultraviolet light-excited emission spectrum satisfying the condition of:0<S(400–420)<0.20in which S(400–420) represents a ratio of an amount of a light emitted by a luminous component giving an emission peak in the region of 400 to 420 nm based on the total amount of emitted light. |
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claims | 1. A window for allowing transmission of x-rays, comprising:a) a support frame defining a perimeter and an aperture;b) a plurality of ribs extending across the aperture of the support frame and carried by the support frame;c) openings between the plurality of ribs;d) a film disposed over, carried by, and spanning the plurality of ribs and openings and configured to pass radiation therethrough;e) the plurality of ribs having at least two different cross-sectional sizes including at least one larger sized rib and at least one smaller sized rib;f) the at least one larger sized rib has a widthwise cross-sectional area across the aperture of the support frame that is at least 5% larger than a widthwise cross-sectional area of the at least one smaller sized rib; andg) the window being hermetically sealed to an enclosure configured to enclose an x-ray source or detection device in order to separate air from a vacuum within the enclosure. 2. The window of claim 1, wherein the at least one larger sized rib has a cross-sectional area that is at least 50% larger than a cross-sectional area of the at least one smaller sized rib. 3. The window of claim 1, wherein the at least one larger sized rib has a cross-sectional area that is at least twice as large as a cross-sectional area of the at least one smaller sized rib. 4. The window of claim 1, wherein the plurality of ribs include at least three different sizes and each larger size has a cross-sectional area that is at least 5% larger than a cross-sectional area of a smaller sized rib. 5. The window of claim 1, wherein the plurality of ribs include at least four different sizes and each larger size has a cross-sectional area that is at least 5% larger than a cross-sectional area of a smaller sized rib. 6. The window of claim 1, wherein the plurality of ribs form multiple hexagonal-shaped structures and define hexagonal-shaped openings. 7. The window of claim 1, wherein the plurality of ribs extend from one side of the support frame to an opposing side and are substantially parallel with respect to each other. 8. The window of claim 1, wherein the plurality of ribs intersect one another. 9. The window of claim 8, wherein the plurality of ribs are oriented non-perpendicularly with respect to each other and define non-rectangular openings. 10. The window of claim 1, wherein at least one larger sized rib has a longer length than all smaller sized ribs. 11. The window of claim 1, wherein at least one larger sized rib spans a greater distance across the aperture of the support frame than all smaller sized ribs. 12. The window of claim 1, wherein the plurality of ribs extend non-linearly across the aperture of the support frame. 13. The window of claim 1, wherein the at least one larger sized rib along with the support frame separate the at least one smaller sized rib into separate and discrete sections. 14. The window of claim 1, wherein tops of the plurality of ribs terminate substantially in a common plane. 15. The window of claim 1, wherein a pattern of the at least one larger sized rib is aligned with a portion of a pattern of the at least one smaller sized rib. 16. The window of claim 1, wherein a portion of a pattern of the at least one larger sized rib is aligned with a portion of a pattern of the at least one smaller sized rib. 17. The window of claim 1, wherein the at least one larger sized rib has a larger width than the at least one smaller sized rib. 18. The window of claim 1, wherein a portion of the support frame and a portion of the plurality of ribs are disposed in a single plane, having a thickness of less than 5 micrometers, which is substantially parallel with the film. 19. The window of claim 1, wherein the film contacts the plurality of ribs. 20. The window of claim 1, wherein:a) the window is hermetically sealed to a mount;b) the mount is attached to an x-ray detector; andc) the window is configured to allow x-rays to impinge upon the detector. 21. The window of claim 1, wherein:a) the window is hermetically sealed to an enclosure including an x-ray source, the enclosure being partially formed by the window and an x-ray tube; andb) the window is configured to allow x-rays to exit the x-ray source. 22. The window of claim 1, wherein the cross-section of the at least one larger sized rib extends along an entire length of the at least one larger sized rib. 23. The window of claim 1, wherein a larger cross-section of the at least one larger sized rib is larger than a smaller cross-section of the at least one smaller sized rib along a majority of a length of the at least one smaller sized rib across the aperture of the support frame. 24. The window of claim 1, wherein the at least one smaller sized rib has a smaller cross-section along at least a majority of a length of the at least one smaller sized rib across the aperture of the support frame. 25. A window for allowing transmission of x-rays, comprising:a) a support frame defining a perimeter and an aperture;b) a plurality of ribs extending across the aperture of the support frame and carried by the support frame, the plurality of ribs having openings therebetween;c) the plurality of ribs having tops that terminate substantially in a common plane;d) a film disposed over and spanning the plurality of ribs and openings and configured to pass radiation therethrough;e) the plurality of ribs having at least two different cross-sectional sizes including at least one larger sized rib and at least one smaller sized rib;f) the at least one larger sized rib has a widthwise cross-sectional area across the aperture of the support frame that is at least 50% larger than a widthwise cross-sectional area across the aperture of the support frame of the at least one smaller sized rib;g) the at least one larger sized rib has a longer length than all of the smaller sized ribs;h) the at least one larger sized rib spans a greater distance across an aperture of the support frame than at least one of the smaller sized ribs; andi) the window being hermetically sealed to an enclosure configured to enclose an x-ray source or detection device in order to separate air from a vacuum within the enclosure. 26. A window for allowing transmission of x-rays, the window comprising:a) a support frame defining a perimeter and an aperture;b) a plurality of ribs extending across the aperture of the support frame and carried by the support frame, the plurality of ribs having openings therebetween;c) the plurality of ribs terminate substantially in a common plane;d) a film disposed over and spanning the plurality of ribs and openings and configured to pass radiation therethrough;e) the plurality of ribs having at least two different cross-sectional sizes including at least one larger sized rib and at least one smaller sized rib;f) the at least one larger sized rib has a widthwise cross-sectional area that is at least 5% larger than a widthwise cross-sectional area of the at least one smaller sized rib, a larger widthwise cross-section of the at least one larger sized rib extending across the aperture of the support frame and along an entire length of the at least one larger sized rib, a smaller widthwise cross-section of the smaller sized rib being smaller along at least a majority of a length of the at least one smaller sized rib across the aperture of the support frame; andg) the window being hermetically sealed to a mount, the mount being further hermetically sealed to either an x-ray source or a detector in order to form a hermetically sealed enclosure. |
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060375175 | claims | 1. An apparatus for treating waste materials, the apparatus comprising: (a) a molten metal reactor including a reaction chamber charged with a reactant metal, and further including a heating arrangement for placing the reactant metal in a molten state; (b) a waste material input structure through which waste material may be introduced into the reaction chamber to contact the molten reactant metal; (c) field generating means for generating a unidirectional electromagnetic field through the molten reactant metal and through a first target area, the electromagnetic field directing beta particles toward the first target area; and (d) a first radiation absorbing module positioned in the first target area, the first radiation absorbing module including a radiation absorbing material. (a) a field generating coil; and (b) a voltage supply for directing a field generating electrical current through the field generating coil. (a) cooling means for cooling the field generating coil; and (b) a protective material encasing the field generating coil and protecting the coil from the reactant metal. (a) the field generating coil is made from a tubular conductor; and (b) the cooling means comprises a cooling fluid supply and a pump for directing cooling fluid from the cooling fluid supply through the tubular conductor. (a) a waste material submerging arrangement for submerging the waste material in the molten reactant metal. (a) a plurality of layers of tungsten, each layer of tungsten being separated from each adjacent layer of tungsten by a layer of lead; (b) a spacing arrangement for maintaining the spacing between the layers of tungsten; and (c) a protective material encasing the tungsten and lead layers to protect the tungsten and lead from the reactant metal. (a) a positioning structure for positioning the first radiation absorbing module in the reaction chamber and for selectively withdrawing the first radiation absorbing module from the reaction chamber. (a) a second radiation absorbing module positioned in the electromagnetic field at an end of the electromagnetic field opposite to an end in which the first radiation absorbing module is positioned. (a) a plurality of layers of tungsten, each layer of tungsten being separated from each adjacent layer of tungsten by a layer of lead; (b) a spacing arrangement for maintaining the spacing between the layers of tungsten; and (c) a protective material encasing the tungsten and lead layers to protect the tungsten and lead from the reactant metal. (a) placing a reactant metal in a molten state and substantially isolating the molten reactant metal from oxygen; (b) producing a unidirectional electromagnetic field through the molten reactant metal and through a first target area, the electromagnetic field directing beta particles toward the first target area; (c) introducing the waste material into the molten reactant metal; (d) circulating the molten reactant metal to direct constituents of the waste material into the area of the molten reactant metal traversed by the electromagnetic field; and (e) intercepting the electromagnetic field in the first target area with a radiation absorbing material. (a) directing a field generating current through an electrically conductive coil made from a tubular material and positioned in the molten reactant metal, the coil being encased in a protective coating material which protects the coil from the molten reactant metal; (b) cooling the coil by directing a coolant fluid through the tubular coil material. (a) intercepting the electromagnetic field with a plurality of layers of tungsten and lead, each layer of tungsten being separated from each adjacent layer of tungsten by a layer of lead. (a) intercepting the electromagnetic field with a radiation absorbing material in a second target area traversed by the electromagnetic field. 2. The apparatus of claim 1 wherein the field generating means comprises: 3. The apparatus of claim 2 wherein the field generating current comprises a pulsed current. 4. The apparatus of claim 2 wherein the field generating coil is adapted to be positioned within the molten reactant metal and further comprising: 5. The apparatus of claim 4 wherein: 6. The apparatus of claim 1 wherein the reactant metal comprises an alloy including aluminum and further including at least one additional metal chosen from a group consisting of cadmium, palladium, tungsten, lead, dysprosium, and europium. 7. The apparatus of claim 1 further comprising: 8. The apparatus of claim 1 wherein the first target area comprises an area in which the electromagnetic field strength is substantially greatest. 9. The apparatus of claim 1 wherein the first radiation absorbing module comprises: 10. The apparatus of claim 1 further comprising: 11. The apparatus of claim 1 further comprising: 12. The apparatus of claim 11 wherein the second radiation absorbing module comprises: 13. A method for treating waste materials, the method comprising the steps of: 14. The method of claim 13 wherein the step of producing the unidirectional electromagnetic field includes: 15. The method of claim 13 wherein the step of intercepting the electromagnetic field with a radiation absorbing material includes: 16. The method of claim 13 wherein the first target area is located within the reactant metal bath and further comprising the step of protecting the radiation absorbing material from the reactant metal with a protective coating material. 17. The method of claim 13 further comprising the step of: |
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abstract | A core spray T-box attachment assembly for a core spray nozzle includes a primary cruciform wedge and a secondary cruciform wedge in contact with the primary cruciform wedge to form a cruciform wedge subassembly adapted for insertion within a bore of the core spray nozzle to sealingly engage an interior converging portion of a safe end of the core spray nozzle. The assembly includes a spider in contact with the cruciform wedge subassembly, and a draw bolt engaging an axial bore of a center portion of the cruciform wedge subassembly and the spider to the T-box. |
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description | This patent application is a continuation of U.S. application Ser. No. 14/910,737 filed on Feb. 8, 2016, which is a 371 of International Application No. PCT/IB2014/063371 filed on Jul. 24, 2014, which claims the benefit of U.S. Provisional Application No. 61/863,466 filed on Aug. 8, 2013, this U.S. Provisional patent application incorporated by reference in its entirety herein. The invention is related to the field of x-ray imaging and more particularly to the field of controlling x-ray radiation amount during multiple frames imaging. In a typical multiple frames imaging (MFI) system the x-ray tube generates x-ray radiation over a relatively wide solid angle. To avoid unnecessary exposure to both the patient and the medical team, collimators of x-ray absorbing materials such as lead are used to block the redundant radiation. This way only the necessary solid angle of useful radiation exits the x-ray tube to expose only the necessary elements. Such collimators are used typically in a static mode but may assume a variety of designs and x-ray radiation geometry. Collimators can be set up manually or automatically using as input, for example, the dimensions of the organ environment that is involved in the procedure. In multiple frames imaging, where typically a series of images are taken automatically one after the other, the situation is more dynamic than in a single exposure x-ray. For such cases also collimators with partially-transparent to x-ray materials are used to manipulate the x-ray energy distribution as described in details below. It is desired to change the distribution of the x-ray energy during the MFI session so that for at least 2 different frames of the MFI session the distribution of the x-ray beam will be different. In MFI the x-ray radiation is active for a relatively long period and the treating physician typically has to stand near the patient, therefore near the x-ray radiation. As a result, it is desired to provide methods to minimize exposure to the medical team. Methods for reducing x-ray radiation intensity have been suggested where the resultant reduced signal to noise ratio (S/N) of the x-ray image is compensated by digital image enhancement. Other methods suggest a collimator limiting the solid angle of the x-ray radiation to a fraction of the image intensifier area and periodically moving the collimator to expose the entire input area of the image intensifier so that the Region of Interest (ROI) is exposed more than the rest of the area. This way, the ROI gets high enough x-ray radiation to generate a good S/N image while the rest of the image is exposed with low x-ray intensity, providing a relatively low S/N image or reduced real-time imaging, as per the collimator and method used. The ROI size and position can be determined in a plurality of methods. For example, it can be a fixed area in the center of the image or it can be centered automatically about the most active area in the image, this activity is determined by temporal image analysis of s sequence of cine images received from the video camera of the multiple frames imaging system. It is desired to provide collimators solutions to enable reduction of dose during MFI. It is also desired to provide a method to move the collimator elements so as to support best imaging results. It is desired to provide a method to handle the effects of the motion on image quality. The term session is used here to encompass the x-ray activity from the moment it is activated to the moment it is stopped. Typically it would be similar to the time from pressing an x-ray system pedal to the time of releasing the pedal. Although the x-ray energy generation period for a session is typically not identical to the pedal pressing period, the term “session” is used in a broader sense that covers also the time from pressing a pedal to releasing it, pressing a button to releasing it and in the general concept of the time from turning x-ray on to turning it off. According to an example of the present invention there is provided an x-ray system incorporating an x-ray source, a detector, a monitor for displaying an x-ray image of a field of view and an input device such as an eye tracker to indicate focus of attention coordinates in the image area of one or more users of the system; said system configured to determine at least one Region of Interest (ROI) so that the focus of attention is contained in said at least one ROI; and to optimize the image displayed on said monitor according to the image part that is contained in said at least one ROI. The image optimization may be made by controlling and modifying any of the following parameters: signal processing algorithms which process the video stream, x-ray tube current (whether in continuous or pulse modes); x-ray tube Peak Kilo Voltage (PKV); x-ray pulse length; AGC (Automatic Gain Control), whether analog or digital; Tone reproduction of the image implemented in brightness function; Tone reproduction of the image implemented in contrast function; Tone reproduction of the image implemented in brightness function; Tone reproduction of the image implemented in gamma function; Tone reproduction of the image implemented in offset function; Tone reproduction of the image implemented in n-degree linear function; and Tone reproduction of the image implemented in a non-linear function. The x-ray system may further include a collimator, which may be configured to modify the x-ray radiation dose per pixel (DPP) in the field of view according to the location of the gazing point. The x-ray system may further include a collimator, which may be configured to modify the dose per pixel (DPP) in the field of view according to the location of the gazing point. The x-ray system may further include multiple filament elements to generate multiple and simultaneous X Ray beams a subset of which may be selected and may be configured to modify the x-ray radiation in order to aim at the desired ROIs in the field of view according to the location of the gazing point. The x-ray system may further include a matrix/array of x ray tubes/sources to generate multiple and simultaneous X Ray beams a subset of which may be selected and may be configured to modify the x-ray radiation in order to aim at the desired ROIs in the field of view according to the location of the gazing point. The x-ray system may further include a rotatable and translatable cathodes and or anodes to generate multiple and simultaneous X Ray beams a subset of which may be selected and may be configured to modify the x-ray radiation in order to aim at the desired ROIs in the field of view according to the location of the gazing point. According to an example of the present invention there is provided an x-ray system incorporating an x-ray source, a detector, a monitor for displaying an x-ray image and a collimator; said collimator is configured to expose different areas to varying radiation levels; and said system configured to process the different areas to become similar to a reference area using a tone-correction function. In a further more specific example of the present invention said collimator is configured to expose a first area to a first radiation level and a second area to a second radiation level; and said system is configured to process said second area to become similar to said first area using a tone-correction function. The tone-correction functions may be one of at least two tone-correction functions, each of the tone-correction functions is associated with a specific PKV. The system may further be configured to create a tone-correction function by interpolation of two other tone-correction functions, each of the other tone-correction functions associated with a specific PKV. The system may further be configured to estimate a tone-correction function for a third area from the tone-correction function used for said second area. The estimation may use exponential calculation. The system may further be configured to adjust the input scale of the tone-correction function to fit changes in x-ray current. The adjustment may be made using a factor equal to the relative change of the x-ray current. According to an example of the present invention there is provided a method of calculating a tone-correction function including: exposing at least two areas to different radiation levels, wherein at least a part of said at least two radiations is exposed through a variable absorption phantom so that for each designated transmission level of said phantom there is at least one area exposed by each of said at least two radiations; for each such designated transmission level calculating the average pixel value; calculating the ratio of said at least two average pixel values for all designated absorption levels; and fitting a function to the said calculated ratios to be used as the tone-correction function. The variable absorption phantom may be a step wedge. The variable absorption phantom may be a variable thickness phantom of continuous slope function. According to an example of the present invention there is provided a method of calculating a tone-correction function including: exposing an area to a first x-ray radiation and exposing said area to a second x-ray radiation, wherein said first and second radiation is through a human tissue in said area; calculating the ratio of at least one pixel value in said area corresponding to said first radiation to the corresponding pixel value in said area corresponding to said second radiation; and fitting a function to the said at least one calculated ratio and pixel value in said area corresponding to said second radiation to be used as a first tone-correction function. More than one area may be used. A second tone-correction function may be calculated, using also data that was acquired after the acquisition of the data used to calculate said first tone-correction function. The data used to calculate said first tone-correction function may be from at least 2 patients. According to an example of the present invention there is provided an x-ray system incorporating an x-ray source, a collimator, a detector and a monitor, means for moving said collimator in a plane generally parallel to the plane of said collimator; said collimator comprising one or more apertures that allows all the radiation to pass through, and made of material that reduces the radiation passing through at an amount depending on the material and the thickness of said collimator. In a further more specific example of the present invention there is provided an x-ray system incorporating an x-ray source, a collimator, a detector and a monitor, means for moving said collimator in a plane generally parallel to the plane of said collimator; said collimator comprising an aperture that allows all the radiation to pass through, an outer annulus that reduces the radiation passing through at an amount depending on the material and the thickness of the said outer annulus and an inner annulus between said aperture and said outer annulus, with thickness changing as a function of the distance from the said aperture, starting at a low thickness on the side of the aperture and ending at the thickness of the outer annulus on the side of the outer annulus; and the system configured to modify image data so as to essentially adjust the image acquired through the inner annulus and the image acquired through the outer annulus to appear visually similar to the image acquired through said aperture, wherein parameters used for said adjustments depend on the position of said collimator. The system may be configured to acquire said parameters by a calibration procedure, said calibration procedure includes measurements made at a variety of said collimator positions. The variety of collimator positions may include a variety of positions in the collimator plane. The variety of collimator positions may include a variety of distances from the x-ray source. The internal annulus thickness may be essentially symmetrical relative to a plane that is located essentially midway between the two external surfaces of said outer annulus. The system may include a layer of material that is different from said material of the outer annulus, said layer located at said aperture area. The layer may overlaps at least a part of said inner annulus. According to an aspect of the present invention there is provided an x-ray system incorporating an x-ray source, a detector, a monitor for displaying an x-ray image, a collimator and an input device; wherein said input device is configured to provide coordinates relative to the x-ray image; the system configured to select at least one region of the image according to said coordinates; and adjust at least one of the following parameters according to said coordinates: said at least one region's shape; and said at least one region's position. The system may further be configured to adjust at least one of the following parameters according to said at least one region: x-ray tube mA; x-ray tube mAs; x-ray tube KVp; said x-ray image brightness; said image contrast; and said image tone. The input device may be at least one of: an eye tracker; a joy-stick; a keyboard; an interactive display, a gesture reading device; and a voice interpreter. According to an example of the present invention, two or more partially overlapping, partially transparent blades with non-parallel elongated apertures are provided wherein the plates can move on non-parallel tracks so that at the cross of the non-parallel elongated apertures the full x-ray energy passes and only part of the x-ray energy passes through the rest of the area. According to an example of the present invention, two or more partially overlapping, partially transparent, rotating shapes with elongated apertures are provided wherein the shapes can rotate so that at the cross of the elongated apertures the full x-ray energy passes and only part of the x-ray energy passes through the rest of the area. According to an example of the present invention, a partially transparent shape having an aperture is connected to non-parallel tracks so as to be moved across a cross section of the x-ray beam. At the aperture the full x-ray energy passes and only part of the x-ray energy passes through the rest of the area. The methods also include solutions for the case wherein the partially transparent parts of the filters are typical x-ray blocking parts. According to an example of the present invention, methods are provided to drive a collimator motion in a way that will provide a support to produce better frames. The methods also include solutions for the case wherein the partially transparent parts of the filters are typical x-ray highly attenuating parts. Reference is made now to FIG. 1A which presents a typical layout of a multiple frames imaging clinical environment X-ray tube 100 generates x-ray radiation 102 directed upward occupying a relatively large solid angle towards collimator 104. Collimator 104 blocks a part of the radiation allowing a smaller solid angle of radiation to continue in the upward direction, go through bed 108 that is typically made of material that is relatively transparent to x-ray radiation and through patient 110 who is laying on bed 108. Part of the radiation is absorbed and scattered by the patient and the remaining radiation arrives at the typically round input area 112 of image intensifier 114. The input area of the image intensifier is typically in the order of 300 mm in diameter but may vary per the model and technology. The image generated by image intensifier 114 is captured by camera 116, processed by image processor 117 and then displayed on monitor 118 as image 120. Although the invention is described mainly in reference to the combination of image intensifier 114 and camera 116 it would be appreciated that both these elements can be replaced by a digital radiography sensor of any technology such as CCD or CMOS flat panels or other technologies such as Amorphous Silicon with scintillators located at plane 112. One such example is CXDI-50RF Available from Canon U.S.A., Inc., Lake Success, N.Y. The term “detector” will be used to include any of these technologies, including the combination of any image intensifier with any camera and including any type of a flat panel sensor or any other device converting x-ray to electronic signal. The terms “area” and “region” are used alternatively in the detailed description of the invention and they mean the same and are used as synonyms. The term “x-ray source” is used to provide a wide interpretation for a device having x-ray point source that does not necessarily have the shape of a tube. Although the term x-ray tube is used in the examples of the invention in convention with common terminology in the art, it is represented here that the examples of the invention are not limited to a narrow interpretation of x-ray tube and that any x-ray source can be used in these examples (for example even radioactive material configured to function as a point source). Operator 122 is standing by the patient to perform the medical procedure while watching image 120. The operator has a foot-switch 124. When pressing the switch, continuous x-ray radiation (or relatively high frequency pulsed x-ray as explained below) is emitted to provide a cine imaging 120. The intensity of x-ray radiation is typically optimized in a tradeoff of low intensity that is desired to reduce exposure to the patient and the operator and high intensity radiation that is desired to enable a high quality image 120 (high S/N). With low intensity x-ray radiation and thus low exposure of the image intensifier input area, the S/N of image 120 might be so low that image 120 becomes useless. Coordinate system 126 is a reference Cartesian coordinate system with Y axis pointing into the page and X-Y is a plane parallel to planes such as that of collimator 104 and image intensifier input plane 112. It is a purpose of the present invention to provide high exposure at the input area of the image intensifier in the desired one or more ROIs that will provide therefore a high S/N image there while reducing the exposure of other sections of the image intensifier area, at the cost of lower image quality (lower S/N). With this arrangement the operator can see a clear image in the one or more ROIs and get a good enough image for general orientation in the rest of the image area. It is also a purpose of this invention to provide more complex map of segments in the image where each segment results from a different level of x-ray radiation as desired by the specific application. It is also the purpose of the current invention to provide various methods to read the data off the image sensor. According to some embodiments, the x-ray system may include multiple filament elements to generate multiple and simultaneous X Ray beams, a subset of which may be selected and may be configured to modify the x-ray radiation in order to aim at the desired ROIs in the field of view according to the location of the operator's focus of attention. According to some embodiments, the x-ray system may include a matrix/array of x ray tubes/sources to generate multiple and simultaneous X Ray beams, a subset of which may be selected and may be configured to modify the x-ray radiation in order to aim at the desired ROIs in the field of view according to the location of the operator's focus of attention. According to some embodiments, the x-ray system may further include rotatable and translatable cathodes and/or anodes to generate multiple and simultaneous X Ray beams, a subset of which may be selected and may be configured to modify the x-ray radiation in order to aim at the desired ROIs in the field of view according to the location of the operator's focus of attention. In the context of the examples provided throughout the detailed description of the invention, when S/N of one area is compared to S/N in another area the S/N are compared for pixels that have the same object (such as patient and operators hands and tools) transmittance. For example, when an area A is described as having lower S/N than area B it is assumed that the transmission of x-ray by the object to both areas is uniform over the area and is the same. For example, if at the center of the area A only ½ of the radiation arriving at the object is transmitted through to the image intensifier then, S/N in area B is compared to area A for an area B in which also only ½ of the radiation arriving at the object is transmitted through to the image intensifier. The S (signal) of area A is the average reading value of the area A (average over time or over the area if it includes enough pixels in the statistical sense). The S (signal) of area B is the average reading value of the area B (average over time or over the area if it includes enough pixels in the statistical sense). To simplify the discussion scattered radiation is not considered in the detailed description of the invention. The effect of scattered radiation and means to reduce it are well known in the art. In the examples below the noise statistics is assumed to be of Gaussian distribution which satisfies most practical aspects of implementation of the invention and serves well clear presentations of examples of the detailed description of the invention. This is not a limitation of the invention and, if desired, the mathematics presented in association to Gaussian statistics can be replaced by that of Poisson statistics (or other statistics) without degrading the scope of the invention. The noise values associated with each signal are represented by the standard deviation of the Poisson statistics for that signal, known in the art as Poisson Noise. Also dose per pixel (DPP) throughout the detailed description of the invention is discussed in the same sense, i.e. when the DPP of pixel A is compared to DPP of pixel B it is assumed the object transmission for both pixels is the same. An example of a more detailed layout of a multiple frames imaging clinical environment according to the present invention is described in FIGS. 1B and 27. Operator 122 presses foot switch 124 to activate x-ray (step 2724). Eye tracker 128 (such as EyeLink 1000 available from SR Research Ltd., Kanata, Ontario, Canada) or any alternative input device provides indication where one or more operators (or users) 122 are focusing their attention (step 2728). This information is typically provided relative to monitor 118. This information, the at least one desired center of ROI, may be provided for example in terms of (X,Z) coordinates, in the plane of monitor 118, using coordinate system 126. It would be appreciated that in this example the plane of monitor 118 and therefore also image 120 are parallel to the (X,Z) plane of coordinate system 126. Other coordinate systems are possible, including coordinate systems that are bundled to monitor 118 and rotate with monitor 118 when it is rotated relative to coordinate system 126. The data from input 128 is provided to controller 127 which is basically a computer, such as any PC computer. If the controller 127 determines that the operator's focus of attention is not fixed on the image 120, the x-ray tube 100 is not activated (step 2700). Otherwise, in step 2710, x-ray tube 100 is activated and x-ray radiation is emitted towards collimator 104 (and/or 150/150A). Box 150 in FIG. 1B represents a collimator according to the present invention, for example, the collimator of FIG. 5, FIG. 10A through FIG. 10C, FIG. 11A through FIG. 11D, FIG. 12A through 12B, FIG. 13A through FIG. 13B, FIG. 14A through 14B, FIG. 15A through 15D, FIG. 16A through 16D, FIG. 18A through 18C, FIG. 20A through 20B, FIG. 24A through 24B, FIG. 25, FIG. 34, FIG. 35, FIG. 37, FIG. 38 and FIG. 40. Box 150 can be located under collimator 104, above collimator 104 as shown by numerical reference 150A or instead of collimator 104 (not shown in FIG. 1B). The collimators represented by boxes 150 and 150A are controlled by controller 127. X-ray emission is also controlled by controller 127, typically through x-ray controller 130. In one example, x-ray can be stopped even if operator 122 presses foot-switch 124 if at least one of the users' desired center of ROI is not within image 120 area. The collimator partially blocks radiation, depending on the determined at least one desired center of ROI (step 2720). Part of the x-rays are absorbed by the patient 110 (step 2730) and the remaining radiation arrives at the image intensifier 114 (step 2740). In step 2750 the image is intensified and captured by a camera 116 and in step 2760 the captured image is transferred to the image processor 117 and in step 2770 the processed image is displayed on monitor 120. Image processor 117 may assume many forms and may be incorporated in the current invention in different ways. In the example of FIG. 1B, image processor 117 includes two main sub units: 117A provides basic image correction such as pixel non-uniformity (dark offset, sensitivity, reconstruction of dead pixels etc), 117C provides image enhancement processing (such as noise reduction, un-sharp masking, gamma correction etc). In conventional systems, the image from sub-unit 117A is transferred for further processing in sub-unit 117C. The sub-units of image processor 117 can be supported each by a dedicated hardware but they can also be logical sub-units that are supported by any hardware. In the example of FIG. 1B the image from camera 116 is corrected by image processing sub-unit 117A and then transferred to controller 127. Controller 127 processes the image as required from using any of the collimators represented by box 150 and returns the processed image to sub-unit 117C for image enhancement. It would be appreciated that the image processing of controller 127 does not have to take place in controller 127 and it can be executed by a third sub-unit 117B (not shown in FIG. 1B) located between 117A and 117C. Sub-unit 117B can also be only a logical process performed anywhere in image processor 117. It would also be appreciated that x-ray controller 130 is presented here in the broad sense of system controller. As such it may also communicate with image processor 117 to determine its operating parameters and receive information as shown by communication line 132, It may control image intensifier 114, for example for zoom parameters (communication line not shown), it may control camera 116 parameters (communication line not shown), it may control the c-arm and bed position (communication line not shown) and it may control x-ray tube 100 and collimator 104 operation parameters (communication line not shown). There may be a user interface for operator 122 or other staff members to input requests or any other needs to x-ray controller 130 (not shown). Physically, part or all of image processor 117, controller 127 and x-ray generator (the electrical unit that drives x-ray tube 100) may all be included in x-ray controller 130. X-ray controller 130 may contain one or more computers and suitable software to support the required functionality. An example for such a system with an x-ray controller is mobile c-arm OEC 9900 Elite available from GE OEC Medical Systems, Inc., Salt Lake City, Utah USA. It would be appreciated that the exemplary system is not identical to the system of FIG. 1B and is only provided as a general example. Some of these features are shown in FIG. 26. Reference is made now to FIG. 2 illustrating an example of an image 120 displayed on monitor 118. In this example dashed circle line 204 indicates the border between segment 200 of the image and segment 202 of the image, both segments constitute the entire image 120. In this example it is desired to get a good image quality in segment 200, meaning higher x-ray DPP for segment 200 and it is acceptable to have a lower image quality in segment 202, meaning lower DPP for segment 202. It would be appreciated that the two segments 200 and 202 are provided here only as one example of an embodiment of the invention that is not limited to this example and that image 120 can be divided to any set of segments by controlling the shape of the apertures in the collimators and mode of motion of the collimators. Such examples will be provided below. It would be appreciated that DPP should be interpreted as the x-ray dose delivered towards a segment representing one pixel of image 120 to generate the pixel readout value used to construct image 120 (excluding absorption by the patient or other elements which are not a part of the system, such as the hands and tools of the operator). Reference is made now to FIG. 3. A typical collimator 104 having a round aperture 304 is introduced to the x-ray path so that only x-rays 106 that are projected from focal point 306 of x-ray tube 100 and pass through aperture 304 arrive at the round input surface 112 of image intensifier 114 while other x-rays 102 are blocked by the collimator. This arrangement exposes the entire input area 112 of the image intensifier to generally the same DPP. Such an arrangement does not provide the function of one DPP to segment 300 that correlates with segment 200 of FIG. 2 and another DPP to segment 302 that correlates with segment 202 of FIG. 2. The diameter of input area 112 is B as indicated in FIG. 3. D1 represents the distance from the x-ray focal point 306 to aperture 104. D2 represents the distance from the x-ray focal point 306 to image intensifier input surface 112. Reference is made now to FIG. 4 that defined the segments of the current example of the image intensifier input surface 112 to support an example of the invention. In this example segment 300 is a circular area of radius R1 centered on circular input area 112 of the image intensifier. Segment 302 has an annulus shape with internal radius R1 and external radius R2. R2 is also typically the radius of the input area of the image intensifier. Reference is made now to FIG. 5 that provides one embodiment of a collimator that functions to provide one DPP for segment 300 and another DPP for segment 302. Collimator 500 is constructed basically as a round plate of x-ray absorbing material (such as lead, typically 1-4 mm thick), of a radius larger than r2. Aperture 502 of collimator 500 is constructed as a circular cut-out 504 of radius r1 at the center of the collimator and a sector cut-out 506 of radius r2 and angle 508. It would be appreciated that the term sector is used both to indicate a sector of a circular area and a sector of an annulus shaped area, as per the context. In this example, r1 and r2 of aperture 502 are designed to provide R1 and R2 of FIG. 4. When collimator 500 is positioned in the location of collimator 104 of FIG. 4 r1 and r2 can be calculated using the following equations:r1=R1/(D2/D1)r2=R2/(D2/D1) In this example angular span 508 is 36 degrees, 1/10 of a circle. Collimator 500 can rotate about its center as shown by arrow 512. Weight 510 can be added to balance collimator 500 and ensure that the center of gravity coordinates in the plane of the collimator coincide with the center of rotation, thus avoiding vibrations of the system that might result from an un-balanced collimator. Following a completion of one 360 degrees rotation, DPP for segment 302 is 1/10 of the DPP of segment 300. It would be appreciated that angle 508 can be designed to achieve any desired DPP ratio. For example, if angle 508 is designed to be 18 degrees, following one complete rotation of aperture 500 the DPP for segment 302 will be 1/20 of the DPP for segment 300. The discussion of the current example will be made in reference to angle 508 being 36 degrees. Following the completion of one rotation of collimator 500, camera 116 captures one frame of the data integrated by the sensor over the one complete rotation time of collimator 500, such a frame consists of the values read from the set of pixels of the camera sensor. This will be described in more details now, providing as an example a camera based on a CCD (charge coupled device) sensor such as TH 8730 CCD Camera available from THALES ELECTRON DEVICES, Vélizy Cedex, France. In this example, synchronization of the camera 116 with collimator 500 rotation is made using tab 514 constructed on collimator 500 that passes through photo-sensor 516 such as EE-SX3070 available from OMRON Management Center of America, Inc., Schaumburg, Ill., U.S.A. When tab 514 interruption signal is received from photo sensor 516, the lines of camera 116 sensor are transferred to their shift registers and the pixels start a new integration cycle. The data of the previous integration cycle is read out from the camera. When tab 514 interrupts photo sensor 516 again, the accumulated signals are transferred again to the shift registers of camera sensor 116 to be read out as the next frame. Through this method, one frame is generated for each collimator complete round. For each frame the DPP in segment 202 of image 120 is 1/10 the DPP in segment 200 of image 120. To provide additional view of the above, reference is made to FIG. 6 that describes the exposure map of image intensifier input 112 at a momentary position of the rotating collimator 500. In this position circular area 600 and sector area 602 are exposed to radiation while the complementary sector 604 is not exposed to radiation being blocked by collimator 500. As collimator 500 rotates, sector area 602 and 604 rotate with it while circular area 600 remains unchanged. During one cycle of constant speed of rotation of collimator 500, each pixel outside of area 600 is exposed to x-ray for 1/10 of the time of a pixel in area 600 and thus, receives DPP that is 1/10 than a pixel of area 600. In FIG. 7 the equivalent optical image projected on the camera sensor 710 is shown, where area 700 of FIG. 7 is the equivalent of area 600 of FIG. 6 and area 702 of FIG. 7 is the equivalent of area 602 of FIG. 6. The output image of image intensifier projection on sensor 710 is indicated by numerical indicator 712. 714 is a typical sensor area that is outside the range of the image intensifier output image. For each frame, in addition to typical offset and gain correction to compensate per pixel linear response characteristics, a multiplication by a factor of 10 of the signal from pixels of segment 202 would be needed to generate an image 120 so that the brightness and contrast appearance of segment 202 would be similar to that of segment 200. This method described here in reference to a specific example will be called “normalization” of the pixels. Normalization scheme is made in accordance with the x-ray exposure scheme (i.e., collimator shape, speed and position). To generate a cine of 10 frames per second (fps) collimator 500 has to be rotated as a speed of 10 rounds per second (rps). To generate a cine of 16 fps collimator 500 has to be rotated as a speed of 16 rps. With each such rotation of 360 degrees a complete exposure of input area 112 is completed. An Exposure Cycle (EC) is therefore defined to be the smallest amount of rotation of collimator 500 to provide the minimal complete designed exposure of input area 112. In the example of collimator 500 of FIG. 5, EC requires a rotation of 360 degrees. For other collimator designs such as the one of FIG. 13A EC requires 180 degrees rotation and the one of FIG. 13B EC requires 120 degrees rotation. It would be appreciated that the examples of collimators, x-ray projections on image intensifier input area 112, the images projected on the camera sensor (or flat panel sensor) and the images displayed on monitor 118 are described in a general way ignoring possible geometrical issues such as image up-side down due to lens imaging that might be different if a mirror is also used or the direction of rotation that is shown clockwise throughout the description but depending on the specific design and orientation of the observer might be different. It is appreciated that a person skilled in the art understands these options and has the proper interpretation for any specific system design. It would be appreciated that the camera frames reading scheme described above in reference to collimator 500 can be different: 1. The reading of the frame does not have to be at the instant that tab 514 interrupts photo sensor 516. This can be done at any phase of collimator 500 rotation as long as it is done at the same phase for every EC. 2. Reading more than one frame during one EC. It is desired however, that for each EC, an integer number of frames is read. By doing so, the read frames include the complete data of one EC which makes it easier to build one display-frame that can be presented on monitor 118 in a number of ways: a. Reference is made to FIG. 28A. In step 2800 a new EC begins. In step 2805 pixels from the current frame are normalized and added to the pixels sum (step 2810). In step 2815 next frame is considered. If the end of the EC has been reached, the displayed image is refreshed (step 2825) and the process returns to the beginning of a new EC. This process sums up the pixel values of all the frames of one EC to generate one complete exposure image. Then sums up the pixel values of all the frames of the next EC to generate next complete exposure image. This way, the picture on the monitor is replaced by a temporally successive image each time an EC is completed. Normalization of pixel values can be made for each frame separately or once only for the sum of the frames, as shown in FIG. 28B, or any other combination of frames. b. Reference is made to FIG. 28C. For the example of this method we shall assume that the camera provides 8 frames during one EC. In step 2830 a new EC begins. In this example, all 8 frames numbered 1 to 8 are stored in frames storage (steps 2835-2845 and a first display-frame is generated from these frames as described above (summing the frames in step 2850 and normalizing pixel values in step 2855). The resultant image is then displayed on monitor 118. When frame 9 is acquired (after ⅛ EC), frame 1 is replaced by frame 9 in the frames storage (step 2870) and frames 9,2,3,4,5,6,7,8 are processed (summing, normalizing) to generate the second display-frame that can now be displayed on monitor 118 after ⅛ EC. After another ⅛ of an EC the next frame (frame 10) is acquired in step 2875 and stored in position of frame 2. Frames 9,10,3,4,5,6,7,8 are now processed to generate the third display-frame. This way, using a frames storage managed in the method of FIFO (first in first out) and generating display-frames with each new frame acquired from the sensor, a sequence of cine images are displayed for the user on monitor 118. c. In another embodiment of the invention, summing the pixels of frames is made only for pixels that have been exposed to x-ray according to the criteria of collimator shape and motion during the integration time of the acquired frame. In example b above this would be ⅛ of the EC time. The pixels to be summed to create the image are (1) those from area 700 and (2) those in a sector of angle in the order of 2×(the angular span 508 of the collimator sector 506). The reason for 2× is that during ⅛ of the integration time the collimator rotates ⅛ of EC. A sector angle somewhat larger than 2× angle 508 may be desired to compensate for accuracy limitations. This summing method reduces considerably the amount of pixels involved in the summing process and thus reduces calculation time and computing resources. d. In another embodiment of the invention, the pixel processing is limited to those pixels specified in c above. This processing method reduces considerably the amount of pixels involved in the processing and thus reduces calculation time and computing resources. e. In another embodiment of the invention, the storing of pixels is limited to those pixels specified in c above. This storing method reduces considerably the amount of pixels involved in the storage and thus reduces storage needs. f. In another embodiment of the invention, any of the methods described in this section (a—as a general concept, b—as a specific example of a, c, d and e) can be combined to an implementation that uses any combination of the methods or few of them. 3. Reading one frame during more than one EC. In yet another embodiment, the collimator can be operated to provide an integer number of EC per one frame received from the sensor. For example, after 2 EC made by the collimator, one frame is read from the sensor. After normalizing pixel values of this frame, it can be displayed on monitor 118. It would be appreciated that in many designs the frame rate provided from the sensor is dictated by the sensor and associated electronics and firmware. In such cases the speed of rotation of collimator 500 can be adjusted to the sensor characteristics so that one EC time is the same as the time of receiving an integer number of frames from the sensor (one frame or more). It is also possible to set the rotation speed of the collimator so that an integer number of EC is completed during the time cycle for acquiring one frame from the sensor. The description of frames reading above is particularly adequate to CCD-like sensors, whether CCD cameras mounted on image intensifier or flat panel sensors are used instead of image intensifiers and cameras and located generally at plane 112 of FIG. 3. The specific feature of CCD is capturing the values of the complete frame, all the pixels of the sensor, at once. This is followed by sequential transfer of the analog values to an analog to digital convertor (A/D). Other sensors such as CMOS imaging sensors read the frame pixels typically one by one in what is known as a rolling shutter method. The methods of reading the sensor frames in synchronization with the collimator EC is applicable to such sensors as well regardless of the frames reading methods. The “random access” capability to read pixels of sensors such as CMOS sensors provides for yet another embodiment of the present invention. Unlike a CCD sensor, the order of reading pixels from a CMOS sensor can be any order as desired by the designer of the system. The following embodiment uses this capability. In this context, CMOS sensor represents any sensor that supports pixel reading in any order. Reference is made now to FIGS. 8 and 29. The embodiment of FIG. 8 is also described using an example of image intensifier and a CMOS camera but it would be appreciated that the method of this embodiment is applicable also to flat panel sensors and other sensors capable of random access for pixel reading. In step 2900 the output image of image intensifier 114 is projected on area 712 of sensor 710. In accordance with the momentary position of rotating collimator 500, circle 700 and sector 702 are momentary illuminated in conjunction with collimator 500 position and sector 704 and sector 714 are not illuminated. Sectors 702 and 704 rotate as shown by arrow 706 in conjunction with the rotation of collimator 500. For the purpose of this example, pixels before a radial line such as 702A or 800A are pixels with centers on the radial line or in direction clockwise from the radial line. Pixels that are after the radial line are pixels with centers in direction anticlockwise from the radial line. Sector 702 for example includes pixels that are after radial line 702A and also before radial line 702B. For example, in an embodiment mode where frame is read from the sensor once in an EC, the pixels adjacent to radial line 702A have just started to be exposed to the output image of the image intensifier and pixels adjacent to radial line 702B have just completed to be exposed to the output image of the image intensifier. Pixels in sector 702 are partially exposed per their location between 702A and 702B. In this example, the pixels in sector between radial lines 702B and 800B has not been read yet after being exposed to the image intensifier output. In the current example of this embodiment, the instant angular position of radial line 702A is K·360 degrees (K times 360, K is an integer indicating the number of ECs from the beginning of rotation). Angular span of section 702 is 36 degrees per the example of collimator 500. Therefore radial line 702B is at angle K·360-36 degrees. At this position of the collimator, a reading cycle of the pixels of sector 800 starts (step 2910). Radial line 800A is defined to ensure that all pixels after this radial line have been fully exposed. This angle can be determined using R1 of FIG. 5 and the pixel size projected on FIG. 5. To calculate a theoretical minimum angular gap between 702B and 800A to ensure that also the pixels adjacent to 800A have been fully exposed one should consider an arch of radius R1 in the length that has a chord of ½ pixel diagonal in length. This determines the minimum angular span between 702B and 800A to ensure full exposure to all the pixels in sector 800. In a more practical implementation, assuming that area 712 is about 1,000 pixels vertically and 1,000 pixels horizontally, and that R1 is in the order of ¼÷½ of R2 (see FIG. 4) and considering tolerances of such designs and implementation, a useful arch length of radius R1 would be, for example, the length of 5 pixels diagonal. This means the angular span between 702B and 800A is about 2.5 degrees. That is, at the instant of FIG. 8 the angular position of radial line 800A is K·360−(36+2.5) degrees. In this specific example of the present embodiment, the angular span of sector 800 is also selected to be 36 degrees. Therefore, at the instant of FIG. 8 the angular position of radial line 800B is K·360−(36+2.5+36) degrees. In FIG. 8 the angular span of sector 800 is drawn to demonstrate a smaller angle than the angular span of sector 702 to emphasize that the angles need not to be the same and they are the same in the example provided here in the text just for the purpose of the specific example of the embodiment. Having determined the geometry of sector 800, the pixels of that sector are read now from the camera sensor. In a typical CMOS sensor the reading of each pixel is followed by a reset to that pixel (step 2920) so that the pixel can start integration signal from zero again. In another embodiment, in a first phase all the pixels of sector 800 are readout and in a second phase the pixels are reset. The reading and reset cycle of sector 800 has to be finished within the time it takes to sector 702 rotate an angular distance equal to the angular span of sector 800 (step 2950) to enable the system to be ready on time to read the next sector of the same angular span as sector 800 but is rotated clockwise the amount of angular span of sector 800 relative to the angular position of sector 800. In this example: 36 degrees. In the above example, with collimator 500 rotating at 10 rps, sector 800 of 36 degrees span assumes 10 orientations through one EC, the orientations are 36 degrees apart and pixel reading and resetting cycles are made at a rate of 10 cps (cycles per second). It would be appreciated that this embodiment can be implemented in different specific designs. For example, the angular span of sector 800 may be designed to 18 degrees while that of sector 702 is still 36 degrees and collimator 500 is rotating at 10 rps. In this example, sector 800 assumes 20 orientations through one EC, the orientations are 18 degrees apart and pixel reading and resetting cycles are made at a rate of 20 cps (cycles per second). In yet another embodiment, the dark noise accumulated by the pixels in sector 704 that are after radial line 800B and before radial line 802A is removed by another reset cycle of the pixels located in sector 802 (after radial line 802A and before radial line 802B). This reset process is ideally made in a sector 802 specified near and before sector 702. The reset of all pixels of sector 802 has to be completed before radial line 702A of rotating sector 702 reaches pixels of sector 802. Otherwise, the angular span and angular position of reset sector 802 are designed in methods and considerations analogous to those used to determine sector 800. Pixels read from sector 800 should be processed for normalization (step 2930) and can be used to generate display-frames (step 2940) in ways similar to those described in section 2 above “Reading more than one frame during one EC”. In the current embodiment only the sector pixels are read, stored and processed and not the complete sensor frame. In this embodiment, after pixel normalization of the last sector read, the processed pixels can be used to replace directly the corresponding pixels in the display-frame. This way the display-frame is refreshed in a mode similar to a radar beam sweep, each time the next sector of the image is refreshed. Following 360/(angular span of the readout sector) refreshments, the entire display-frame is refreshed. This provides a simple image refreshment scheme. Attention is made now to FIG. 9. Unlike FIG. 8 where the reading sector included the complete set of pixels located after radial line 800A and before radial line 800B, in the present embodiment the reading area geometry is divided into two parts: circular area 700 and sector 900. Sector 900 of the embodiment of FIG. 9 contains the pixels that are after radial line 900A and are also before radial line 900B and are also located after radiuses R-1 and before R-2. In this example pixels before a radius are those with distance from the center smaller or equal to radius R and pixels after a radius R are those with distance from the center larger then R. The pixels of area 700 are all those pixels located before R-1. In this embodiment, the pixels of section 900 are read and handled in using the same methods described in reference to the embodiment of FIG. 8. The same holds also for reset sector 802. The pixels of area 700 are handled differently. In one implementation of the current embodiment, The pixels in area 700 can be read once or more during one EC and handled as described above for the embodiment of reading the entire CMOS sensor or area 700 can be read once in more than one EC and handled accordingly as described above for the embodiment of reading the entire CMOS sensor. It would be appreciated that for each reading method the normalization process of the pixels must be executed to get a display-frame where all the pixels values represent same sensitivity to exposure. Attention is made now to FIG. 10 that provides one example for the design of a collimator of the present invention combined with a motion system aimed to provide the rotation function of collimator 500. FIG. 10A is a top view of the collimator and the rotation system of this example. FIG. 10B is a bottom view of the collimator and the rotation system of this example. FIG. 10C is a view of cross-section a-a of FIG. 10A. FIG. 10A is showing collimator 500 and aperture 502 (other details are removed for clarity). Pulley 1000 is mounted on top of collimator 500 in a concentric location to the collimator. Pulley 1002 is mounted on motor 1012 (see motor in FIG. 10B and FIG. 10C). Belt 1004 connects pulley 1000 with pulley 1002 to transfer the rotation of pulley 1002 to pulley 1000 and thus to provide the desired rotation of collimator 500. The belt and pulley system example 1000, 1002 and 1004 presents a flat belt system but it would be appreciated that any other belt system can be used including round belts, V-belts, multi-groove belts, ribbed belt, film belts and timing belts systems. FIG. 10B showing the bottom side of FIG. 10A displays more components not shown before. V-shape circular track 1006 concentric with collimator 500 is shown (see a-a cross section of 1006 in FIG. 10C). Three wheels 1008, 1010 and 1012 are in contact with the V-groove of track 1006. The rotation axes of the 3 wheels are mounted on an annulus shaped static part 1016 (not shown in FIG. 10B) that is fixed to the reference frame of the x-ray tube. This structure provides a support of collimator 500 in a desired position in reference to the x-ray tube (for example the position of collimator 104 of FIG. 3) while, at the same time provides 3 wheels 1008, 1010 and 1012 with track 1006 for collimator 500 to rotate as desired. The rotation of motor 1014 is transferred to collimator 500 by pulley 1002, through belt 1004 and pulley 1006. Collimator 500 then rotates being supported by track 1006 that slides on wheels 1008, 1010 and 1022. It would be appreciated that the rotation mechanism described here is just one example for a possible implementation of rotation mechanism for a rotating collimator. Rotation mechanism may instead use gear transmission of any kind including spur, helical, bevel, hypoid, crown and worm gears. The rotation mechanism can use for 1002 a high friction surface cylinder and bring 1002 in direct contact with the rim of collimator 500 so that belt 1004 and pulley 1000 are not required. Another implementation may configure collimator 500 as also a rotor of a motor with the addition of a stator built around it. In the description of the collimator of FIG. 5, tab 514 and photo sensor 516 were presented as elements providing tracing of the angular position of collimator 500 for the purpose of synchronization between the collimator angular position and the sensor reading process. These elements were presented as one implementation example. The means for tracing the rotational position can be implemented in many other ways. In the example of FIG. 10, motor 1002 may have an attached encoder such as available from Maxon Precision Motors, Inc, Fall River, Mass., USA. Simple encoder can be constructed by taping a black and white binary coded strip to the circumference of collimator 500 and reading the strip using optical sensors such as TCRT5000 Reflective Optical sensor available from Newark, http://www.newark.com. Collimator 500 was described hereinabove as having a fixed aperture that cannot be modified after manufacturing of the collimator. It would be appreciated that in other embodiment of the inventions, mechanical designs of collimator assemblies can be made to accommodate exchangeable collimators. This way, different apertures can be mounted to the collimator assembly per the needs of the specific application. In additional implementation example of the invention, the collimator can be designed to have a variable aperture within the collimator assembly. This is demonstrated in reference to FIG. 11. The collimator of FIG. 11 is constructed from two superimposed collimators shown in FIG. 11A. One collimator is 1100 with aperture 1104 and balancing weight 510 to bring the center of gravity of this collimator to the center of rotation of this collimator. The second collimator is 1102 with aperture 1105 and balancing weight 511 to bring the center of gravity of this collimator to the center of rotation of this collimator. In both collimators the aperture geometry is the combination of central circular hole of radius r1 and sector hole of radius r2 and sector angular span of 180 degrees. Actually, collimator 1102 is of the same general design as collimator 1100 and it is flipped upside down. When collimators 1100 and 1102 are placed concentrically one on top of the other as shown in FIG. 11B we get a combined aperture which is the same as that in collimator 500 of FIG. 5. By rotating collimator 1100 relative to collimator 1102, the angular span of sector 508 can be increased or decreased. In this example the angular span of sector 508 can be set in the range of 0÷180 degrees. In this example, ring 1108 holds collimators 1100 and 1102 together as shown also in FIG. 11C which is cross-section b-b of FIG. 11B. Reference is made now to FIG. 11C (weights 510 and 511 are not shown in this cross-section drawing). In this example of the invention, ring 1108 is shown holding together collimators 1100 and 1102, allowing them to be rotated one relative to the other to set the angular span 508 of sector 506 as desired. An example for a locking mechanism to hold collimators 1100 and 1102 is the relative desired orientation is described in FIG. 11D. In FIG. 11D ring 1108 is shown without collimators 1100 and 1102 for clarity. A section 1110 is cut-out in the drawing to expose the u-shape 1112 of ring 1108, inside which collimators 1100 and 1102 are held. Screw 1114 that fits into threaded hole 1116 is used to lock collimators 1100 and 1102 in position after the desired angular span 508 has been set. To change angular span 508 the operator can release screw 1114, re-adjust the orientation of collimators 1100 and/or 1102 and fasten screw 1114 again to set the collimators position. The example of FIG. 11, including the manual adjustment of angular span 508 is provided as one example of implementation of the invention. Many other options are available. One more example is shown in reference to FIG. 12. In this example, angular span 508 can be controlled by a computer. The mechanism of FIG. 12 is mainly a structure containing two units similar to the unit of FIG. 10 with a few changes including the removal of pulley 1000, using instead the rim of the collimator as a pulley. Balance weights 510 and 511 are not shown here for clarification of the drawing. In FIG. 12A, the bottom unit that includes collimator 500 is essentially the assembly of FIG. 10 with pulley 1000 removed and using instead the rim of the collimator 500 as a pulley. In the top unit that includes collimator 1200, the assembly is same as the bottom assembly when the bottom assembly is rotated 180 degrees about an axis vertical to the page with the exception that motor 1214 was rotated another 180 degrees so that it is below the pulley, same as motor 1014. This is not compulsory of this example but in some design cases it may help to keep the space above the assembly of FIG. 12 clear of unwanted objects. FIG. 12B shows these 2 assemblies brought together so that collimators 500 and 1200 are near each other and concentric. In the assembly of FIG. 12B each of the collimators 500 and 1200 can be rotated independently. For each collimator the angular position is known through any encoding system, including the examples provided above. In one example of usage of the assembly of FIG. 12B, angular span 508 is set when collimator 500 is at rest and collimator 1200 is rotated until the desired angle 508 is reached. Then both collimators are rotated at the same speed to provide the x-ray exposure pattern examples as described above. It would be appreciated that it is not required to stop any of the collimators to adjust angle 508. Instead, during the rotation of both collimators, the rotation speed of one collimator relative to the other can be changed until the desired angle 508 is achieved and then continue rotation of both collimators at the same speed. It would be appreciated that a mechanism with capabilities such as the example of FIG. 12B can be used to introduce more sophisticated exposure patterns. With such mechanisms angle 508 can be changed during an EC to generate multiple exposure patterns. For example angle 508 may be increased for the first half of the EC and decreased for the second half of the EC. This creates an exposure pattern of 3 different exposures (it is appreciated that the borders between the areas exposed through sector 506 is not sharp and the width of these borders depend on angle 508 and the speed of changing this angle relative to the speed of rotation of the collimators. It would also be appreciated that any of the collimators of the invention can be rotated at a variable speed through the EC and affect the geometry of exposure. For example, collimator 500 of FIG. 5 can rotate at one speed over the first 180 degrees of the EC and twice as fast during the other 180 degrees of the EC. In this example the area exposed through sector 506 during the first half of the EC has twice the DPP than the area exposed through sector 506 during the second half of the EC, with gradual DPP change over the boundary between these two halves. The central area exposed through circular aperture 504 has a 3rd level of DPP. Other rotation speed profiles can generate other exposure geometries. For example 3 different rotation speeds over 3 different parts of the EC will generate 4 areas with different DPP. The examples provided above presented collimators with apertures having similar basic shapes consisting of central round opening combined with a sector-shaped opening. These examples were used to present many aspects of the invention but the invention is not limited to these examples. Reference is made now to FIG. 13A showing another example of an aperture of the invention. In this example the aperture of collimator 1300 is constructed of a circular hole 1302 concentric with the collimator rim, a sector-shaped hole 1304 and a sector shaped hole 1306 in opposite direction to 1304 (the two sectors are 180 degrees apart). If it is desired, for example, that annulus area of FIG. 6 (that includes sectors 602 and 604) will be exposed to DPP that is 1/10 than the DPP of area 600 of FIG. 6, then each of the sectors 1304 and 1306 can be set to 18 degrees and then one EC can be achieved with only 180 degrees rotation of collimator 1300 compared to 360 degrees required for the collimator of FIG. 5. Also, for 10 fps the rotation speed of collimator 1300 should be 5 rps and not 10 rps as in the case of collimator 500 of FIG. 5. Furthermore, balance weight such as 510 of FIG. 5 is not required for collimator 1300 of FIG. 13A since it is balanced by its geometry. Another example of a collimator according to the invention is provided in FIG. 13B. The aperture of collimator 1310 is constructed of a circular hole 1312 concentric with the collimator rim, a sector-shaped hole 1314, a sector-shaped hole 1316 and a sector shaped hole 1318 the three sectors are 120 degrees apart. If it is desired, for example that annulus area of FIG. 6 (that includes sectors 602 and 604) will be exposed to DPP that is 1/10 than the DPP of area 600 of FIG. 6, then each of the sectors 1314, 1316 and 1318 can be set to 12 degrees and then one EC can be achieved with only 120 degrees rotation of collimator 1310 compared to 360 degrees required for the collimator of FIG. 5. Also, for 10 fps the rotation speed of collimator 1300 should be 10/3 rps and not 10 rps as in the case of collimator 500 of FIG. 5. Furthermore, balance weight such as 510 of FIG. 5 is not required for collimator 1310 of FIG. 13B since it is balanced by its geometry. It would be appreciated that relations and methods for rotating the collimator examples of FIG. 13A and FIG. 13B and reading pixel values from the image sensor described above in relation to the collimator example of FIG. 5 are fully implantable with the examples of the collimators of FIG. 13A and FIG. 13B with adjustments that are obvious for a person skilled in the art. For example, for the collimator of FIG. 13B and a CMOS camera pixel reading sector 800 of FIG. 8 can be complemented by additional 2 pixel reading sectors, each in conjunction to one of the 2 additional aperture sectors of FIG. 13B. Some of these changes and comparison are indicated in the following table that presents an example of differences in features and implementation between the 3 different examples of collimators. CollimatorFIG. 5FIG. 13AFIG. 13BCommentsCentral roundYesYesYesaperture# of aperture 12 3sectorsSectors angular36deg18deg12degFor 1:10spanDPP ratioSectors angularNA180deg120degseparationEC rotation360deg180deg120degrps10510/3For 10 fpsfps at 10 rps1020 30 FIG. 11 and FIG. 12 provide an example of how collimator 500 of FIG. 5 can be constructed in a way that enables variable angle span 508 of sector 506. FIG. 14 provides an example of how the collimator of FIG. 13A can be constructed so that the angle span of sectors 1304 and 1306 can be adjusted as desired. FIG. 14A presents an example of 2 collimators 1400 and 1402. The gray background rectangle is provided for a better visualization of the collimators' solid areas and the aperture holes and are not part of the structure. Same is for FIG. 14B. Each of the collimators have an aperture made of a circular hole concentric with the collimator rim and two sector holes, each sector has an angular span of 90 degrees and the sectors are 180 degrees apart. When collimators 1400 and 1402 are placed one on top of the other and concentric, the combined collimator of FIG. 14B is provided. The aperture size and shape of the collimator in FIG. 14B is the same as the size and shape of the aperture of the collimator of FIG. 13A. In the case of the assembly of FIG. 14B however, the angular span of aperture sectors 1404 and 1406 can be modified by rotating collimators 1400 and 1402 relative to each other. This can be done using any of the methods described above in reference to FIG. 11 and FIG. 12. It would be appreciated that similar designs can provide for variable angular span of the aperture sectors of collimator 1310 of FIG. 13B and other aperture designs. In the aperture design above, the aperture shape was designed to provide, at a constant rotation speed two areas with two different DPP. FIG. 15A represents such a collimator and also a qualitative exposure profile showing two levels of DPP for different distances from the center—r. Other apertures can be designed to provide any desired exposure profiles. Some examples are shown in FIG. 15B, FIG. 15C and FIG. 15D. All the collimators of FIG. 15 have aperture design aimed at rotation of 360 degrees for one EC. The features of the apertures in the collimators of FIG. 15 can be combined with the features of the apertures in the collimators of FIG. 13. Examples for such combinations are shown in FIG. 16 showing 4 collimators with 4 different aperture designs. In FIG. 16A the left and right halves of the aperture are not symmetrical and one EC requires 360 degrees rotation. FIG. 16B offers a collimator with an aperture providing an exposure profile similar (but not identical) to that of FIG. 15C but one EC consists of 90 degrees rotation only. FIG. 16C offers a collimator with an aperture providing an exposure profile similar (but not identical) to that of FIG. 15D but one EC consists of 360/8=45 degrees rotation only. FIG. 16D offers a collimator with an aperture providing an exposure profile similar (but not identical) to that of FIG. 15D also but one EC consists of 180 degrees rotation only. Following these examples it is appreciated the invention may be implemented in many designs and it is not limited to a particular design provided hereinabove as an example. Pixel Correction: As explained above, pixels with different DPP per the collimator design and use are normalized to provide a proper display-frame. Normalization scheme is made in accordance with the x-ray exposure scheme (i.e., collimator shape, speed and position). Such normalization can be done on the basis of theoretical parameters. For example, in reference to FIG. 7 and FIG. 5, with collimator 500 rotating at a constant speed, the pixels of the annulus incorporating sectors 702 and 704 receive 1/10 the dose of circular area 700 (in this example the angular span 508 of sector 506 is 36 degrees). For simplicity of this example it is assumed that one frame is read from the sensor every time an EC is completed (i.e., collimator 500 completes a rotation of 360 degrees). It is also assumed that all sensor pixels are of the same response to the image intensifier output and that the image intensifier has uniform response and the x-ray beam from the x-ray tube is uniform. The only built-in (i.e. system level) source of differences between the pixels is from the collimator and the way it is operated. In this example the normalization based on the system design would be a multiplication of pixels by one or 2 factors that will compensate for the difference in DPP. In one normalization example the values from the pixels of the annulus incorporating sectors 702 and 704 can be multiplied by 10. In another normalization example the values from the pixels of circular area 700 can be multiplied by 1/10. In yet another normalization example the values from the pixels of the annulus incorporating sectors 702 and 704 can be multiplied by 5 and the values from the pixels of circular area 700 can be multiplied by ½. It would be appreciated that in the description, explanations and examples of this invention, multiplication and division are completely equivalent and expressions like “multiplying by 1/10” is completely equivalent to expressions like “divide by 10” and whenever multiplication by a value is mentioned it means also the division by reciprocal value alternative and vise-versa. The same holds for multiplication and division symbols used in equations. For example A/B represents also A·C where C=1/B. The example above is relatively simple since the normalization scheme incorporates 2 knows areas with two known DPP. The situation can become relatively more complicated with different collimators or collimator motion scheme. In the following example a change is introduced to the rotation of collimator 500. Instead of constant rotation speed a variable rotation speed is used as presented in the following table for one EC (in the case of collimator 500: 360 degrees): Sector #EC range (degrees)Angular rotation status1 0-150Constant speed 12150-180Constant positive acceleration3180-330Constant speed 24330-360Constant negative acceleration This rotation pattern together with the convolution with the image pixels, especially in the acceleration sectors, makes it more difficult to estimate normalization factors. In the example of the collimators of FIG. 15C and FIG. 15D, many “pixel rings” (pixels at a certain distance from the center) need a suitable normalization factor. Production tolerances of the system that are not included in the theoretical estimation of the normalization factors may result in errors that will show up as ring patterns in the image displayed on monitor 118. The following calibration method provides calibration that removes the need for theoretical estimation of the factors and also compensates for production tolerances. In this example any collimator of the invention can be used and any rotation pattern that is fixed per EC can be used. The multiple frames imaging system is set to include all the fixed element relevant to the imaging process (x-ray tube, the desired x-ray operation mode i.e. voltage and current, possible x-ray filter, collimator, patient bed, image intensifier, camera) but none of the variable parts (the patient, the operator's hands and tools). According to this calibration method, the desired collimator is rotated in the desired pattern. A set of raw frames is acquired (using any of the example methods mentioned above). A raw frame is a frame resulting from an integer number of one or more EC with all the pixels of area 712 (FIG. 7), without any manipulation of the pixels. The number of raw frames acquired should be enough to get a relatively good S/N on an average raw frame that is the average of the acquired raw frames. An average raw frame with S/N that is 10 times higher than that of the raw frame is typically sufficient and this can be achieved by averaging 100 raw frames. It would be appreciated that more or less raw frames can be used, depending on the desired quality of the normalized frame. One average raw frame is created with x-ray off and another with x-ray on. For this example we assume that the brightness value for each pixel for display purpose ranges from zero to 255. We also select to display a theoretical noiseless frame in the range 5÷250 (darkest noiseless pixel is displayed at value 5 and the brightest exposed noiseless pixel is displayed at value 250. This enables noise that brings the pixel values to the range 0÷4 and 251÷255 contribute its statistics appearance to the displayed frame). The correction for each pixel i of raw frames j, Pij (j is a frame number index in this example) is calculated using the values of the pixels of the average raw frame made with x-ray radiation on, Ai, and values of the pixels of the average raw frame made with x-ray radiation off, Bi, to produce the corrected pixel Dij as follows:Dij=(Pij−Bi)·(245/Ai)+5 (Equation 1) In yet a somewhat more simple approach the correction may ignore noise visual aspects at the dark and bright level and simply correct to the display range 0÷255 as follows:Dij=(Pij−Bi)·(255/Ai) (Equation 2) It would be appreciated that the correction suggested above is linear and works best for systems with relatively linear response of the image intensifier and the camera. For systems with non linear response, more complicated correction schemes may be used such as bi-linear correction. In this example the range of the values of the pixels is divided roughly into 2 ranges. The current of the x-ray can be reduced, for example to ½ its normal operation mode so that the DPP is reduced to ½. It is appreciated that the reduced current level depends on the nature of the non linearity and optimal bi-linear correction may require other than ½ of the x-ray current. It would also be appreciated that DPP can be reduced also in other ways such as aluminum plates placed right after the collimator. In this example, with ½ the x-ray current, another set of raw frames is acquired. It would be appreciated that the S/N of these raw frames is lower than that of the raw frames of the standard x-ray current for the specific application. This can be compensated by using more raw frames to generate the average raw frame for ½ the x-ray current, for example 200 raw frames. Let Mi represent the values of the pixels of the average raw frame made with ½ x-ray current radiation on. The correction example of Equation 2 is implemented in this example as follows: For Pij with values less or equal 127Dij=(Pij−Bi)·(127/Mi) (Equation 3) For Pij with values higher than 127Dij=(Pij−Bi)·(255/Ai) (Equation 4) It would be appreciated that the x-ray current for Mi may be set to a different level (for example ¼ of the standard current for the specific application) and the equations will assume the form: For Pij with values less or equal 63Dij=(Pij−Bi)·(63/Mi) (Equation 5) For Pij with values higher than 63Dij=(Pij−Bi)·(255/Ai) (Equation 6) It would also be appreciated that if the non-linearity of the pixels is similar between the different pixels within the operating range of the system (that is differences in non linear response are relatively small) correction for non linearity, in most cases is not required. If the application does not require linear response and it is only desired to reduce pixels response non uniformity effects on the displayed frame, then one may skip non-linearity correction. All pixels corrections can be skipped if the noise pattern resulting from this does not disturb the application. The correction can be made at different sophistication levels (linear, bi-linear, tri-linear, polynomial interpolation and so on) or not at all, as suitable for the application. Variable ROIs and variable rotation speed profiles: In the above examples different rotation profiles with different rotation speeds were described. In the following example rotation profiles of variable speed will be described in the context of ROI in the image. In the examples of the collimators above, a central circular area (such as 600 of FIG. 6 and 700 of FIG. 7) was presented as the ROI and therefore receiving more DPP than the annulus of sectors 702 and 704 that receive lower DPP. This is the trivial case and typically the central area of the image is also the ROI, where the more important part of the image is located. The higher DPP results in higher S/N in this area and therefore provides a better image quality in that area (such as better distinguishable details). Normally, during, for example a catheter insertion procedure, the patient's bed is moved during the process to keep the tip of the catheter in the range of area 700. Yet, sometimes the area of highest interest in the image moves out of area 700. For example, in reference to FIG. 17A, to the area denoted by numerical indicator 1700. This may be a result of many reasons such as (1) the catheter tip has moved to area 1700 and the patient has not been moved to bring the catheter tip to area 700 (2) the operator is looking at area 1700 for any reason. This new ROI information can be fed as input to the system in many ways including automatic follow-up of the catheter tip or follow-up of the area at which the operator looks using an input device such as an eye tracker device (such as EyeLink 1000 available from SR Research Ltd., Kanata, Ontario, Canada) to indicate focus of attention of one or more users of the system or by using a computer mouse to indicate the desired one or more ROI locations. With angular span of aperture sector 702 and at a constant rotation speed of collimator 500, the DPP in the annulus outside area 700 is 1/10 of the DPP inside circular area 700 and S/N in the annulus outside area 700 is 1/101/2 of that of area 700, resulting in a lower image quality. To overcome this and maintain refreshment rate of the displayed frames of 10 fps with collimator 500 EC of 1/10 of a second as in the basic example of the invention, the rotation profile can be modified so that the collimator rotation speed in sector 1702 (FIG. 17B) that contains area 700 is reduced to 1/10 of the uniform speed and the rotation speed at the rest of the EC is increased to maintain EC of 1/10 of a second. This will be explained now in reference to FIG. 17B and the corresponding flowchart in FIG. 30 with example of actual numbers. Let us assume that the angular span of sector 1702 that just contains area 1700 is 54 degrees (step 3000). The first edge of sector 1702 is 1702A and is located at angular position 63 degrees and the second edge of sector 1702 is 1702B and is located at angular position 117 degrees. That is, sector 1700 is centered on angular position 90 degrees. In step 3010, the reduced rotation speed of collimator 500 is calculated for area 1702, that will result in area 1702 having similar S/N to that of area 702. In this example, when edge 702A of sector 702 approaches angle 63 degrees (the location of 1702A) the rotation speed of collimator 500 is reduced to 1 rps. This rotation speed is maintained until edge 702B of sector 702 reaches the position of edge 1702B (117 degrees). From this point the rotation speed of collimator 500 is increased again. In step 3020 the increased rotation speed of collimator 500 in area 704 is calculated, that will compensate for the change of speed in area 1702, to leave the total rps unchanged. For simplicity it will be assumed that acceleration and deceleration are extremely high and therefore acceleration and deceleration times are definitely negligible for this example. Per the explanation above, collimator 500 rotation profile then includes 54+36=90 degrees (¼ of the EC rotation) at a speed of 1 rps. To compensate for this and complete the EC at an average of 10 rps the rotation speed of collimator 500 at the rest ¾ of the EC rotation must be increased to Xrps, satisfying the following equation:1 rps·¼+X rps·¾=10 rps (Equation 7)Therefore:X rps=(10 rps−1 rps·¼)/(¾) (Equation 8) That is, during the rest of the 270 degrees rotation of the EC, the rotation speed should be 13 rps. With this rotation profile sector 1702 is exposed to the same DPP as area 700 and the S/N of area 1700 is also the same as area 700 as desired. It would be appreciated that in the sector range outside sector 1702, for which the collimator rotation speed is increased to 13 rps, the DPP is reduced below that of the DPP of constant rotation speed to 1/13 the DPP of area 700. It would also be appreciated that area 1700 was presented here as an example to demonstrate the design of rotation profile according to different ROI geometries. Area 1700 may be different in shape and location and it may be possible to add more than one ROI to the basic ROI of circle 700. Such variations are handled with profile variations of the same concept described above. It would also be appreciated that acceleration and deceleration mentioned above may take unreliable part of the EC and must be accounted for. Let us assume in the next example that acceleration and deceleration occupy 45 degrees of rotation each and that they are uniform. In this case acceleration has to start 45 degrees before edge 702A arrives at the position of edge 1702A and deceleration starts when edge 702B arrives at the position of 1702B. All other parameters of the system are the same. If X indicates the rotation speed during the 180 degrees of EC and Y is the average rotation speed during each of the 45 degrees acceleration deceleration sectors then the following equation needs to be satisfied to maintain EC of 0.1 s (or average rotation speed of 10 rps):1 rps·¼+2·Y rps·⅛+X rps·½=10 rps (Equation 9) Given constant acceleration and deceleration between 1 rps and 10 rps, Y=(1+10)/2=5.5 and the high rotation during 180 degrees is 16.75 rps. It would be appreciated that this approach presented through the example above is applicable also to other acceleration profiles, other collimators and other operation schemes (such as different fps rates). It would also be appreciated that pixel correction methods described above are fully applicable also to variable rotation speed profiles, Different Refreshment Rates for Different Areas of the Image: It has been presented above (with the example of collimator 500 of FIG. 5 and operation mode of constant rotation speed of the collimator at 10 rps and display-frame refreshment rate of 10 fps) that the DPP of circular area 700 of FIG. 7 is 10 times higher than the DPP of the annular area constructed of sectors 702 and 704 (to be denoted “annulus” for short). Therefore the S/N in area 700 is also 101/2 better than the S/N in the annulus area. The refreshment rate of the entire image 120 (FIG. 2) is the same: 10 fps. The temporal resolution of the entire frame is 0.1 second (s). In the previous example, each display-frame was constructed from the data of one frame acquired from camera 116. Area 200 on the display 118 is equivalent to area 700 on the sensor. Area 200 is exposed to 10 times the DPP of area 202 and the S/N in area 200 is 101/2 better than the S/N the annulus area 202. With each EC of collimator 500 the data is read from sensor 714, processed and displayed on monitor 118. The complete image 120 is refreshed then every 0.1 s. In the following example of the invention it is desired to improve the S/N of annulus 202. In a first example, while area 200 is refreshed every 0.1 s with the data read from sensor 714, annulus 202 is refreshed only every 1 s. During this 1 s, the data received from sensor 714 for pixels of annulus 202 is used to generate an annulus image that is the sum of the 10 previous frames. In a simplified form, all 10 frames indexed j=1 to 10 are stored. Then for each pixels i in the range of annulus 202 the sum of values is calculated: Pni=Σpij. Pni are then corrected and displayed where n is index number for every set of 10 frames. Therefore for j=1 to 10, the pixels of the sum frames is P1i. For frames j=11 to 20, the pixels of the sum frames is P2i. For frames j=21 to 30, the pixels of the sum frames is P3i and so on. With this example therefore we get a display of image 120 where the S/N of annulus 202 is similar to that of area 200 although annulus 202 receives 1/10 of the DPP in every unit time of area 200. The compromise is that annulus 202 is refreshing every is comparing to every 0.1 s of area 200 and the temporal resolution of annulus 202 is 1 s comparing to 0.1 s of area 200. In a second example, after the first 10 frames indexed j=1 to 10 were acquired and stored and displayed as the sum of the pixels for annulus 202, refreshment of annulus 202 is made in a different way. Instead of keeping the display of annulus 202 for 1 s until j=11 to 20 are acquired, the displayed image is refreshed after 0.1 s as follows: Frame j=11 is acquired and stored instead of frame 1. Therefore the previously stored frames 1,2,3,4,5,6,7,8,9,10 the following frames are stored: 11,2,3,4,5,6,7,8,9,10. This set of frames is handled in the same way as the pervious set and annulus 202 is refreshed. After additional 0.1 s frame indexed 12 is acquired and is stored instead of the frame indexed 2: 11,12,3,4,5,6,7,8,9,10. The set is now processed in the same way and annulus 202 display is refreshed. This process repeat itself and as a result annulus area is refreshed every 0.1 s, same as area 200. The temporal resolution of annulus 202 is still is comparing to area 200 with temporal resolution of 0.1 s. The S/N in annulus 202 is similar to the S/N of area 200. In a third example, an intermediate approach is presented. Following the first example, instead of summing pixels of 10 frames and refreshing annulus 202 every 1 s, summing can be done every 5 frames and refreshment of annulus 202 can be made every 0.5 s. The S/N of annulus 202 is now ½1/2 of the S/N of Area 200 but still better than 1/101/2 of the basic example of collimator 500 and the temporal resolution is only 0.5 s comparing to is of the first example of this method. It would be appreciated that also in the second example an intermediate approach can be used where, instead of replacing each time one of 10 frames, the replacement is of one frame in a set of 5 frames: 1,2,3,4,5 then 6,2,3,4,5 then 7,6,3,4,5 and so on. Here we gain again the refreshment of annulus 202 every 0.1 s but with temporal resolution of 0.5 s and S/N of annulus 202 is now ½1/2 of the S/N of Area 200 but still better than 1/101/2 of the basic example of collimator 500. It will be appreciated that this method can be implemented also for collimators that are not rotating collimators such as the one of FIG. 18. FIG. 18A provides a top view of the collimator and FIG. 18B is cross section c-c of FIG. 18A. Collimator 1800 provides a similar function of x-ray reduction as other collimators of the invention. It has an aperture 1802 that allows all the radiation in that area to pass through, annulus 1806 that reduces the radiation passing through the area at amount depending on the material (typically aluminum) and the thickness of the material and annulus 1804 with thickness changing as a function of the distance from the center, starting at thickness zero on the side of aperture 1802 ending at the thickness of annulus 1806 on the side of annulus 1806. FIG. 18C provides a schematic DPP graph as a function of distance from the center: r. It is assumed that beyond annulus 1806 radiation is fully blocked. For the purpose of the description of this example radiation that is scattered from collimator 1800 is ignored. For this example it is also assumed that DPP passing through annulus 1806 is 1/10 the DPP passing through aperture 1802. Frame rate is 10 fps and display-frame refreshment rate is 10/s. As described in the above examples S/N of the image part associated with annulus 1806 is 1/101/2 of the S/N associated with aperture 1802. To display an image where the S/N of the area associated with annulus 1806 is similar to the S/N in the area associated with aperture 1802 any of the methods above can be used. FIG. 18D provides a representation of monitor 118 with the displayed frame associated with collimator 1800. Circle 1822 is the area associated with radiation arriving through aperture 1802 of collimator 1800. Annulus 1824 is the area associated with radiation arriving through annulus 1804 of collimator 1800. Annulus 1826 is the area associated with radiation arriving through annulus 1806 of collimator 1800. It would be appreciated that while the example of annulus 1804 in FIG. 18B is linear change of thickness, the example of change in radiation of 1814 in FIG. 18C is of a non-linear thickness change. That is, many different functions can be used to generate gradient in thickness 1804 to suit the desired gradual change in radiation between annulus 1800 and annulus 1806 of FIG. 18B. In a first example, while area 1822 is refreshed every 0.1 s with the data read from sensor 714, annulus 1826 is refreshed only every 1 s. During this 1 s, the data received from sensor 714 for pixels of annulus 1826 is used to generate an annulus image that is the sum of the 10 previous frames. In a simplified form, all 10 frames indexed j=1 to 10 are stored. Then for each pixels i in the range of annulus 1826 the sum of values is calculated: Pni=Σpij. Pni are then corrected and displayed where n is index number for every set of 10 frames. Therefore for j=1 to 10, the pixels of the sum frames is P1i. For frames j=11 to 20, the pixels of the sum frames is P2i. For frames j=21 to 30, the pixels of the sum frames is P3i and so on. With this example therefore we get a display of image 120 where the S/N of annulus 1826 is similar to that of area 1822 although annulus 1826 receives 1/10 of the DPP in every unit time of area 1822. The compromise is that annulus 1826 is refreshing every is comparing to every 0.1 s of area 1822 and the temporal resolution of annulus 1826 is 1 s comparing to 0.1 s of area 1822. For annulus 1824, we shall use here the example where the DPP decreases linearly over the width of annulus 1824 from DPP of 1822 to 1/10 of this DPP, the DPP of annulus 1826. In this example one may divide annulus 1824 to 8 annuluses of equal radius step so that the average DPP in the smallest annulus #1 is 9/10 of 1822, the average DPP in the next annulus #2 is 8/10 of 1822, annulus #3 is 7/10 and so on until the last annulus #8 that has 2/10 DPP of 1822. Whenever a value is mentioned in reference to the above segments (annuluses #1 through #8) the value is the average value of that segment in consideration of the thickness variation of the collimator over that segment. When the purpose is to provide on the entire displayed image 120 the same S/N and keep temporal resolution of up to 1 s, it can be done in a simple way for annulus #5 (½ DPP than in area 1822) and annulus #8 (⅕ DPP of area 1822) since the ratio of DPP in area 1822 and the DPP in annulus #5 is an integer. The same is the case for annulus #2. In the case of annulus #5 adding 2 temporally successive frames as described in any of the above methods (with adequate pixel correction as described above) provides S/N similar to area 1822. Temporal resolution in this example is 0.2 s. In the case of annulus #8 adding 5 temporally successive frames as described in any of the above methods (with adequate pixel correction as described above) provides S/N similar to area 1822. Temporal resolution in this example is 0.5 s. For other annuluses (#1, #3, #4, #6, #7 and #8) the ratio of DPP in area 1822 and the DPP in any of these annuluses is not an integer. Therefore adding pixels of an integer number of frames (up to 10 considering the desired limit of not more than is temporal resolution) will exceed the desired S/N or be less than the desired S/N. To achieve the desired S/N under the requirements of this example, the following method (which is described by flowchart in FIG. 31) can be applied: 1. For each annulus #m add the minimum number of pixels of temporally successive frames that provide S/N equal or higher to the S/N of area 1822 (steps 3100-3120). 2. Execute pixel correction (offset, normalization and so on as described above) (step 3130). 3. Add noise to each pixel in annulus #m to compensate for the cases of S/N higher than in area 1822 (steps 3140-3150). The above steps will be discussed in more details in reference to annulus #1. The DPP in annulus #1 is 9/10 the DPP of area 1822. The S/N in annulus #1 is ( 9/10)1/2 of the S/N in area 1822. Therefore, according to step 1 above we need to add pixels of 2 temporally successive frames in the area of annulus #1 to make the S/N of the pixels in annulus #1 equal or higher than that of area 1822. By adding the pixels of 2 temporally successive frames in the area of annulus #1 the effective DPP in the resultant frame in annulus 1 is 18/10 of the DPP in area 1822. The S/N in annulus #1 is now (18/10)1/2 of the S/N in area 1822. To compensate for the too high S/N (and therefore result in possible visual artifacts in image 120, a Gaussian noise is added to each pixel to satisfy the equation:(N1822)2=(N#1)2+(Nadd)2 (Equation 10) Where N1822 is the noise associated with a specific pixel in area 1822 for a specific object transmission, N#1 is the noise associated with the pixel that is the sum of 2 temporally successive pixels in annulus #1 (sum-pixel), having the same object transmission and after the sum-pixel has gone through pixel correction process (including, in the simplest correction form, dividing the value of the summed pixels by 1.8 to bring the effective DPP from 18/10 to 10/10—the same as in area 1822) and Nadd is the noise to be added to the sum-pixel to bring its S/N to the same level as the equivalent pixel in area 1822. In the example above, since the number of x-ray photons in the sum pixel of annulus #1 is 1.8 of the equivalent pixel (same object transmission) of area 1822, the noise of the sum-pixel is (1.8)1/2 of the equivalent pixel in area 1822 and the S/N is also (1.8)1/2 of the equivalent pixel in area 1822. To calculate the amount of Nadd we use equation 10 in the form:Nadd=((N1822)2−(N#1)2)1/2 (Equation 11)with the pixel correction division by 1.8. Using numbers:Nadd=(12−((1.81/2)/1.8)2)1/2 Nadd=0.667 Therefore, by adding this poisson noise to the sum pixel we provide to that pixel a noise that is similar to the equivalent pixel in area 1822. It is appreciated that all examples are calculated on a relative basis and therefore the pixel of area 1822 is 1. It would be appreciated that the noise values in equation 10 are dependent on the pixel value and are typically the square root of the pixel average level. The same correction method is applicable to all the segments of annulus 1824 with suitable adjustments. It would be appreciated that adding pixels of successive frames can be done by adding new frames each time before display-frame refreshment or using the FIFO method as described above. It would be appreciated that dividing annulus 1824 into 8 segments (Annulus #1 through annulus #8) is provided as an example only. The higher the number of segments, the more uniform the S/N is over annulus 1824. Yet, the visibility of the non uniformity of the S/N adjustment is obscured by the S/N of the image therefore, above a certain number of segments the visual contribution of more segments is low and may be undistinguishable to the operator. Therefore one may limit the number of annulus segments in accordance with the S/N statistics of the image in the specific procedure. The same methods for handling the non-uniform DPP regions such as annulus 1824 of the collimator example 1800 can also be used for collimators of the present invention such as those of FIG. 15C, FIG. 15D and all the collimators of FIG. 16 that also produce non-uniform DPP regions. These methods can be used with any collimator that generates different exposure regions, regardless of the method used by the collimator, whether the different exposure regions are generated by the shape of the collimator, by a motion of the collimator or by combining shape and motion. In all cases of motion of the collimator, cycles of the same motion pattern simplify the image enhancement as described above but it is not a requirement to allow the image enhancement described above. In the above example, in reference to the image area 1826 (FIG. 18D) corresponding with annulus 1806 (FIG. 18A), the discussion referred to basic processing of image area 1826: since the radiation there is 1/10 of the radiation in area 1822, one can sum last 10 frames in area 1826 to generate a processed 1826 area with S/N similar to that of area 1822. In another approach, one may compromise S/N goal in area 1826 for adding less frames. For example, one may prefer summing only 5 frames and get S/N that is 0.71 of the S/N of area 1822 but, by doing so, improve temporal resolution of area 1826 by a factor of 2 compared to the case of summing 10 frames. To compensate for the resulting ½ brightness in this example, each pixel value in area 1826 can be multiplied by 2. More generally, if one needs to sum M frames to get the brightness that is in conjunction with the brightness of area 1822, and instead m frames are summed (m can be any positive number), the pixel values of the pixels in area 1826 should be multiplied by M/m. It would also be appreciated that the number of summed frames does not have to be an integer. For example, 4.5 frames can be summed. In this example FRMn is the last frame, FRMn−1 is the previous frame and so on. Summing last 4.5 frames can assume the form (for each pixel):SUM=(FRMn)+(FRMn−1)+(FRMn−2)+(FRMn−3)+0.5×(FRMn−4) Brightness adjustment then uses the factor 10/4.5. In some cases, due to the spectral change in radiation that goes through annulus 1806 (and also 1804), the x-ray in that area experiences a lower absorption coefficient when passing through the patient. Therefore, although when no patient or other absorbing matter is present the radiation for area 1826 is 1/10 that of the radiation for area 1822, when an absorbing object is present the effective radiation for area 1826 relative to that for area 1822 is higher than 1/10. It may be, for example, ⅛. In such a case, adding 8 last frames satisfies both the S/N and brightness criteria (being similar to that of area 1822). This can be used to sum less frames, especially in dark areas (high absorption coefficient). In yet another example of the invention when the ROI shifts to area 1700 as presented in FIG. 17A, instead of adjusting the rotation profile of collimator 500 as explained in reference to FIG. 17B, the whole collimator can be displaced linearly, in direction parallel to the plane of collimator 500, so that the x-ray radiation passing through circular aperture 504 of FIG. 5 is now centered about area 1700 as shown in FIG. 19A on camera sensor 710. It is assumed that the only radiation that can arrive at the collimator input surface 112 is radiation that passes through the aperture of collimator 500 (circular hole 505 and sector hole 506). Therefore area 1902 in the sensor is shadowed out in FIG. 19A (no radiation arrives at the corresponding area of image intensifier input 112) and only the area including 700, 702 and 704 limited by boundary 712 is exposed. The exposed area is then the overlap between two circles with centers shifted one relative to the other and indicated in FIG. 19A by the numerical indicator 1900. This desired function of the invention is provided here within area 1900 by circular hole 504 that enables higher DPP in area 700 and sector hole 506 associated with the rest of the image area enabling only 1/10 of the DPP of hole 504. FIG. 19B illustrates the appearance version of FIG. 2 according to the example of FIG. 19A. Collimator 500 can be moved in X-Y plane (see coordinate system 126 of FIG. 1A) using any common X-Y mechanical system. For example, annulus shaped static part 1016 of FIG. 10C is connected to an X-Y system instead of being connected to the x-ray tube structure and the X-Y system is connected to the structure of the x-ray tube, thus enabling the collimator of FIG. 10C, in this example, to move in X-Y plane as desired for the example of FIG. 19A. It would be appreciated that the above methods such as pixel correction, S/N adjustments, adding pixels of different frames are fully applicable to the example of FIG. 19A with the adjustment to the displacement of the collimator. The X-Y shift method is applicable to any of the collimators of this invention. It would also be appreciated that displacement along a line (X axis for example) instead of X-Y can be applied in the same way with the limitation of ROI areas that can be handled this way over image 120 area. X-Y mechanical systems can assume many designs, including such as Motorized XY Table ZXW050HA02 available from Shanghai ZhengXin Ltd, Shanghai, China. The custom design of X-Y mechanical systems is common in the art and is often made to optimally suit the needs of the application. One such provider of custom designed X-Y mechanical systems is LinTech, Monrovia, Calif., USA. It would be appreciated that the diameter of collimator 500 can be increased so that the length of sector 702 is increased to r3 as shown in FIG. 20B. FIG. 20A is the collimator of FIG. 5 provided here as FIG. 20A for easy comparison with the collimator of FIG. 20B. Angle 508 is the same (36 degrees in this example), the diameter of circular hole 504 is the same (r1). R3 is large enough to incorporate the complete field of view of image intensifier input 112 also when the collimator is displaced laterally as explained in reference to FIG. 19. With this design, the complete image area 120 of FIG. 19B remains active without any shadowed area such as 1902 in the example of FIG. 19. This collimator enlargement can be implemented in any collimator of the invention. For the example of FIG. 19, where the maximum displacement desired is up to the point that the edge circular hole 700 is just in one point contact anywhere on the edge of image 712 edge (such one example point is point 1904 in FIG. 19A) the required radius r3 of the sector hole can be calculated as follows, in reference to FIG. 20B:r3=A−r1 (Equation 12) Where A is the diameter of the image intensifier input 112 B (see FIG. 3) scaled to its projection in the collimator plane. That is:A=B·(D1/D2) (Equation 13) In the process of moving the collimator in the X-Y plane, pixels that have been exposed to full DPP (through area 504) may change status to be exposed at 1/10 DPP since area 504 has moved and such pixels are not included in that area anymore. It would be appreciated that for the first frame in which a pixel has changes status from being included in area 504 and full DPP to be outside area 504 and 1/10 DPP, considering the operation mode of this example, 10 frames of 1/10 DPP have been already acquired and the processing of this pixel for display is made in any of the methods described above that use last 10 frames to provide S/N same as within area 504 (or 5 frames after 0.5 s in another example). During the 1 s transition another handling is required to keep the S/N of this pixel the same as it was when it was included in area 504. Reference is made to FIG. 32. In step 3200, pixels from the current frame are added to the pixel sum, and the next frame is considered (step 3210). The frames summed thus far in the transition period are combined in a weighted sum with the full DPP data (step 3220), where the full DPP data is weighted in order to compensate for the lower DPP of the new frames and retain a consistent S/N. For instance, one frame into the transition, the display will be a weighed sum of 90% full DPP and the one new frame. After two frames, 80% full DPP and the two new frames, and so on. The weight of the full DPP attenuates its effective dose to represent the DPP necessary to keep the S/N of the pixels the same as it was when they were included in area 504. In step 3230, normalization is performed, and then the updated image is displayed (step 3240). The process continues for a full EC, where the new frames progressively receive more weight compared to the old full DPP data. When 10 frames have passed, the transition period is over and methods such as the one described in FIG. 28C can begin to operate (step 3260). An example is provided below for further clarification: In this example, with refreshment rate of 0.1 s and temporal resolution that varies from 0.1 s to 1 s the following procedure is implemented, where N is the index of the last full DPP frame for that pixel: 1. At time 0 display for the pixel 100% the last full DPP data of frame N. Temporal resolution is 0.1 s. 2. At time 0.1 s display for the pixel a weighted sum of 90% the last full DPP data of frame N and 100% of the new DPP data of frame N+1. 3. At time 0.2 s display for the pixel a weighted sum of 80% of the last full DPP data of frame N, 100% of the DPP data of frame N+1 and 100% of the DPP data of frame N+2. 4. . . . . 5. . . . . 6. . . . . 7. . . . . 8. . . . . 9. . . . . 10. At time 0.9 s display for the pixel a weighted sum of 10% the last full DPP data of frame N and 100% of the new DPP data of each of frames N+1, N+2, . . . , N+9. 11. At time 1.0 s display for the pixel a weighted sum of 0% the last full DPP data of frame N and 100% of the new DPP data of each of frames N+1, N+2, . . . , N+9, N+10. Temporal resolution has now changed to 1 s. 12. Continue with methods described above for image improvement for 1/10 DPP regions. Temporal resolution is 1 s. It would be appreciated that in the case of the method of refreshing the pixels of 1/10 DPP in a rate of only 1 fps the last full DPP data is presented for is after the change of the pixel to 1/10 DPP exposure and afterwards the average of the last 10 frames of 1/10 DPP will be used to refresh the pixel. In the case that a pixel changes status in the opposite direction, that is changing from 1/10 DPP area to full DPP area, this transition is instant and in the first 0.1 s after the status change the displayed image is refreshed with the first 0.1 s frame of the full DPP. It would be appreciated as explained in reference to FIG. 1A, that the above methods are applicable also for relatively high frequency pulse x-ray. The term “relatively high frequency” is relative to the collimator design and operation mode. In the example of collimator 500 of FIG. 5, that has a sector angular span of 36 degrees and rotates at 10 rps, the pulse frequency should be at least at a frequency of 100/s so that there is at least one x-ray pulse per each 36 degrees area of a frame. To simplify pixel correction scheme, it is also desired that the x-ray pulse frequency would be a positive integer multiplication of minimum frequency. In this example: 200/s, 300/s, 400/s and so on. In this example 1,000/s (10 times the minimum frequency can be considered relatively high frequency. It is appreciated that no collimator is totally opaque to x-ray and collimators are constructed to block most of the x-ray in the opaque regions. With HVL (half value layer) of 0.25 mm (similar to that of lead), 3 mm thick collimator allows 0.5(3/0.25)= 1/4096 of the incident x-ray radiation to pass through (without scatter). The term “essentially opaque” will be used to describe these practical collimators. Most of the collimators described hereinabove are constructed of essentially opaque region such as 518 of FIG. 5 and apertures or holes as 504 and 506 of FIG. 5. Collimators such as the example of FIG. 18 are different since, in addition to the essentially opaque region 1806 and the aperture 1802 they include semi-opaque regions such as 1804 of FIG. 18A. Collimators according to this invention can be mounted on an x-ray system as stand-alone or together with another collimator, for example, such that is designed to limit the x-ray to a part of input area 112 of the image intensifier. Collimators of the invention and other collimators may be placed in any order along the x-ray path. The exposed part of area 112 is the remaining of the superposition of the area of all the collimators in the path of the x-ray block. In the design of such successive arrangement, the distances of each of the collimators from the x-ray source and distance to area 112 needs to be considered with the geometry of the collimators, as described above, to get the desired functionality. Image Optimization Using Dynamic ROI and Focus of Attention Input Device In another example, any of the above examples of collimators and examples of image processing (and also examples that are not described hereinabove) can be used with an input device such as an eye tracker to indicate focus of attention of one or more users of the system, to further enhance the image perceived by the users. In a typical multiple frames imaging system an area is defined, typically centered about the center of the image, to determine what may generally be called the brightness of the image. Sometimes contrast of the image is also determined based on this area. Typically the area is smaller than the entire image but it can also be an area of a size similar to the entire image. Based on the image content in this area, various parameters related to the image quality may be determined to optimize the image for the users, such as: 1. x-ray tube current (whether in continuous or pulse modes) 2. x-ray tube Peak Kilo Voltage (PKV) 3. x-ray pulse length 4. AGC (Automatic Gain Control), whether analog or digital 5. Tone correction or tone-adjustment of the image implemented in various functions such and brightness, contrast, gamma, offset, gain, n-degree linear functions, non linear functions etc. One example of optimizing the image according to the image content in this area is to identify the 10% brightest pixels in this area, calculate the average value of these pixels and adjust the gain (multiply each pixel value by a constant factor) so that the average value is set to level 240 in an 8 bit display system that provides display levels 0 through 255. The typical result of such parameters changes using the image data of the defined area is that the image in that area is optimized to the content of said image for visual perception of the users while image parts outside this area might not be optimized for visual perception of the users. For example, a lung may be present in the optimization area. Since the lung is relatively transparent to x-ray radiation, the optimization operates to reduce radiation to make the lung appear at a desired brightness. As a result, the spine that is nearby, but outside the optimization area, will appear dark and visibility of details might be lost. To overcome this with the present art, the patient is moved until the spine is in the optimization area and optimization is made for the spine, brightening it up. But now, the lung is too bright and lung details in the image are degraded. This conflict can be resolved by using x-ray manipulating collimators such as those described above with the e.g. eye tracker. In the present example, the at least one ROI becomes the area used for image optimization. The at least one ROI is not static but instead it follows the coordinates of the focus of attention of the user(s). The input device, e.g. eye tracker, provides a stream of (x,y) of the focus of attention coordinates of the at least one user on the screen. The ROIs are moved to these coordinates, with a complementary adjustment of the collimator and the optimization is made for the image included in the ROIs—where the at least one user is gazing at. As a result, the image is optimized in the area where the at least one user is looking and where they need the best image at any time without a need for any manual adjustments or compensations for the automatic image optimization function. It would be appreciated that this function can be used throughout the procedure or only during desired intervals of the procedure. The image may be optimized per the ROIs' content using any of the above mentioned parameters or any other parameter that modifies the displayed value of a pixel in the image. It would also be appreciated that the ROIs do not need to be centered at the focuses of attention. The desired optimization can be made also when the ROIs are selected so that they contains the desired centers of the ROIs. It would be appreciated that the above optimization method can be applied also without using any of the above examples of collimators and examples of image processing. This method can be applied to a multiple frames imaging system that employs generally uniform DPP over the field of view of image intensifier input 112. An input device such as an eye tracker is added to such a multiple frames imaging system to detect the desired centers of the ROIs in the image area. The above optimization is made then for an image area that contains this gazing point as described above. Background Image Processing Using Tone-Correction Function One of the effects of using a collimator of the type described in reference to FIG. 18A through FIG. 18D is the change of the spectrum of the x-ray radiation in the background (annulus 1806) Vs the ROI (Annulus 1802). The result of reducing x-ray DPP through the background filter (annulus 1806) is a change of the x-ray spectrum in that area of the image comparing to the ROI areas of the image (ROI in short). This in turn results in different absorption characteristics of the x-ray in human tissue (or any other material) in the background area Vs the ROI. In the example of the collimator associated with FIG. 18A through FIG. 18D and considering also the example that the background area photon count per pixel is 10% of the photon count per pixel in the ROI in no presence of patient or phantom (110 of FIG. 1A), one might suggest that by multiplying each background pixel value by 10 (or by summing last 10 background images as described above), the background image will become similar to the image in the ROI. This is not the typical case. Typically, a more complex tone reproduction function is required to make the background image look more similar to the ROI image. This is explained in more details in reference to FIG. 21A through FIG. 21C. It would be appreciated that the selection of 10% hereinabove is arbitrary and made only as an example. Other values between 1% and 90% can be selected as well as any value higher than zero and lower than 100%. The adjustment of the description for values other than 10% is obvious for those skilled in the art. A typical tool used in the x-ray field for image research, measurements, calibrations and evaluations is the 10 step wedge as shown in FIG. 21A. It can be constructed of many materials. By placing such a step wedge in the x-ray path instead of patient 110 of FIG. 1A, a stripes-image is acquired, the pixels of each rectangular stripe are of a relatively similar value comparing to the difference between pixels of neighboring steps (assuming relatively high S/N). Average value in each stripe can be measured to produce the values of the 11 dotted bars 2104 in FIG. 21B. The horizontal axis represents the relative step thickness, number zero represents no absorption (a strip of air only), number 1 represents the thinnest step of step wedge 2100 and number 10 represents the thickest step of step wedge 2100, being 10 times thicker than the thinnest step, in this example. The vertical axis represents a pixel value. In this example a 12 bit system was selected providing a dynamic range of 0÷4095. A 12 bit system was selected for this example since it is a popular system in this field for digital image processing but it would be appreciated that that any system can be used to realize the invention, that the adoption of the invention to other system is simple for a person skilled in the art and the scope of the invention is not limited by this example. Also, in this example, the average pixel level in air was set to 4000, allowing 95 additional levels for pixel noise and avoiding high noise digital cutoff at 4095. This selection is made as an example and it is appreciated that noise depends in such systems on the x-ray DPP and the value for air transmission should be made according to the preferred x-ray characteristics. In this example, the filtering of x-ray intensity in the background, that results in change of spectral distribution of the x-ray radiation in the background, will change characteristics of absorption coefficient through the same step wedge 2100. The resultant pixel values for the background radiation for each step are shown as 11 black bars 2106 in FIG. 21B. When implementing a first process of the background by adding last 10 background frames as described above (or multiplying each background pixel by 10), the initially-processed background pixel value in step zero becomes similar to ROI pixel value in step zero as shown by the leftmost girded bar in the 11 girded bars 2108, representing the average value of the steps in the background, after adding last 10 frames. By examining bars 2108 Vs bars 2104 it becomes evident that, except for step zero, all 10 remaining 2108 bars are of higher values than the 10 remaining bars 2104. This results from the different absorption in the background due to spectral change made by annulus 1806 of the filter of FIG. 18A. For example, the average pixel value of step 5 in the ROI is 1419 but in the initially-processed background it is 2005. This results in apparent difference between the initially-processed background image and the ROI image. To resolve this, an additional processing step is required for the background image area (background in short). Such a correction function, in reference to the ROI areas and the background area of the steps of FIG. 21B is shown as function 2112 of FIG. 21C and it will be called here tone-correction function. The process of changing an image using the tone-correction function will be called here tone-correction. Tone-correction function 2112 is created by calculating the tone-correction factors for each of the 11 strips to bring the average value of the backgrounds strip to the same average pixel value of the strip in the ROI areas. Each such factor is the ratio of the average step pixel value in the ROI to the average step pixel value in the background. Factors for pixel values between these calculated values can be obtained using interpolation of any kind such as a cubical interpolation or fitting of any function to the 11 calculated points such as exponential or n-dimensional linear function. It is evident, in this example, that the lower the pixel value is, in the background area, the lower is the correction factor. For example, for initially-processed background pixel value 762 the correction factor is 0.44 (2114 in FIG. 21C), while for initially-processed background pixel value 2524 the correction factor is 0.79 (2116 in FIG. 21C). Tone-correction in this example refers to the multiplication of each pixel in the initially-processed background by the associated factor per the example of FIG. 21C. The tone-correction function of FIG. 21C is used to further process the initially-processed background by multiplying each of the initially-processed background pixels by the associated factor (Background pixel correction factor) provided by tone-reproduction function 2112. It would be appreciated that although, in this example, background was processed to become similar to the ROI, it is also possible to use the same approach to process the one or more ROI areas to become similar to the background. It is also possible to execute the initial-processing on the background and execute the tone-reproduction on the ROI area(s) relative to the background. One only needs to exchange the words background and ROI in the example above to get a description of such a tone-correction. It is also appreciated that initial-processing that results in similar step values for ROI and background is not a requirement for tone-correction. The tone correction can be executed without the above initial-processing or with initial processing that, for example, is designed to bring the background step zero to be half the value of the ROI step zero. This can be done, for example, by adding 5 last images instead of 10 last images in the present 10% background radiation. The tone-correction process is the same, only the tone-correction function (calculated in the same way) is different. Tone-Correction Function Calculation Using Step Wedge In the following example a method is presented in more details, for generating a tone-correction function for background image so as to make it appear similar to that of the ROI. In this method reference is made to FIG. 33A. The first phase of this method is the data collection. To collect the data, variable absorption phantom is used to provide for different absorption levels through the image area. Such a phantom may consist of a step wedge (such as the one of FIG. 21A), a linear wedge phantom, a variable thickness phantom of continuous slope function, a random thickness phantom or any other variable absorption phantom that will provide enough measurement points over the dynamic range of the image (0÷4095 in a 12 bit system), reasonably spread throughout the dynamic range to provide the desired accuracy. It would be appreciated that the more steps that are more evenly spread through the dynamic range—the more accurate the tone-correction function will be. A step wedge of 10 steps would be a reasonable choice for reasonable accuracy. The variable absorption phantom (VAP) preferred material would be a material that behaves similar to live tissue. It is common to assume that water is a reasonable representation of a live soft tissue. There are materials that are considered water-equivalent that are used to produce such phantoms such as Plastic Water available from Supertech, Elkhart, Ind., USA. By using such materials the data collection better resembles the response to live soft tissue of the filtered background radiation spectrum and ROI radiation spectrum. Materials that are bone equivalent can also be used in such a variable absorption phantom but anyone skilled in the art would understand that it is merely an extension of the soft tissue discussion and therefore it will not be discussed in more details here. The variable absorption phantom (VAP) is placed in the system of FIG. 1A instead of patient 110. An image, or a set of images are acquired for a given PKV1. The reason for PKV being a parameter is the PKV dependent spectrum of the x-ray and thus, each tone-correction curve is calculated for a given PKV. The acquired images are designed so that, in the example of step wedge, each step is acquired with each of the x-ray spectrums of the ROI and the background. That is, either a part of the step is in the ROI and another part is in the background or, in one image the step is in the ROI and in another image the step is in the background. Now, in this example, we choose to modify the values of the background pixels and to use the ROI as reference and adjust the background to appear similar to the ROI. It would be appreciated that the value of the pixels of the ROI can be adjusted to bring the ROI to look like the background (or other alternatives can be used as discussed above) but since the technique is completely analogous to the present example it will not be discussed here in more details. To do so, for each step i (including step zero of air) the average of 2 pixel groups are calculated: 1. (step 3300) pixels of step i that are in the ROI: AVGri 2. (step 3305) pixels of step i that are in the background: AVGbi These two numbers are used (step 3310) now to calculate the tone-correction function value for background pixel having the level AVGbi: F(AVGbi):F(AVGbi)=AVGri/AVGbi In the example of 10 steps step wedge+one step of air, a set of 11 tone-correction function values is provided:{F(AVGb0),F(AVGb1),F(AVGb2), . . . ,F(AVGb10)} In the example of 12 bit display system, 4096 correction values are desired so that each possible value of a pixel in the background has a correction tone-correction function value. Such values beyond the 11 values calculated above can be estimated using any interpolation and extrapolation approaches (step 3320) such as linear, 2nd degree or any n-degree linear function fit or exponential function fit etc. The concept is the same, the difference is in the accuracy of the calculated tone-correction function, evaluated typically by how similar the background becomes to the ROI after the correction. This can be demonstrated using the following example. An exemplary table is provided for a step wedge used to measure the function values for each of the 10 steps plus the air step: StepAVGriAVGbiF(AVGbi)04000.004000.001.0013733.023819.970.9823251.323483.850.9332642.773034.310.8742004.752523.830.7951419.252004.750.716937.691520.760.627578.181101.690.528332.71762.180.449178.67503.570.351089.55317.730.28 In this example, step zero is an area without absorption, an area outside of the VAP. In this example, the background has gone also through the initial processing (such as adding last 10 frames to compensate for 10% background radiation as described in details above). For this reason AVGr0=AVGb0. In this example also the exposure has been set so that AVGr0=4000. For a given PKV this is done by determining, for example, the mA (milliampere) in a continuous multiple frames imaging system or determining the charge per pulse in a pulsed x-ray system (milliampere-second: mAs). For the purpose of the following discussion we shall refer to mA-0 as indicating the x-ray current setup to get AVGr0=4000. Therefore, to get the correction factors for 0÷4095 an interpolation is needed for the range 319÷3999 and extrapolation is needed for the ranges 0÷317 and 4001÷4095. This can be done using one of the many curve fitting methods provided, for example, by MatLab, available from MathWorks, Inc., Natick, Mass., USA. The specific fitting method typically depends on the data. It would be appreciated that not all steps must be used to calculate the tone-correction function but, typically, using more steps supports a better tone-correction function. It would be appreciated that for the purpose of curve fitting, such a curve is always expected to pass also through the point (AVGb, F(AVGb))=(0,0). That is, when the absorber thickness is so high that the radiation is fully blocked by this thickness, the tone-correction value at this point is zero. In accordance with the above example it can be illustrated with 2 additional lines in the table, presenting relative thickness of 200 and infinity: StepAVGriAVGbiF(AVGbi)2000.000.030.0068∞0.000.000.00 This additional point (0,0) can therefore be additionally used, with any set of measurements, for a better curve fitting. It would also be appreciated that more than just 2 image area types such as ROI (1822 in FIG. 18D corresponding to filter section 1802 in FIG. 18A and FIG. 18B) and background (1826 in FIG. 18D corresponding to filter section 1806 in FIG. 18A and FIG. 18B) are relevant to the tone-correction described above. Other image area types such as transition area 1824 in FIG. 18D corresponding to filter section 1804 in FIG. 18A and FIG. 18B are relevant. In the example of transition area 1824 the spectrum of the x-ray changes gradually as a function of distance from the ROI center due to the variable change in the filter thickness over annulus 1804. It would be appreciated that the tone-correction curve designed for background 1826 will not be optimal for transition area 1824. It is desired therefore to divide transition area 1824 into a number of transition sub-areas, each transition sub-area has a relatively uniform x-ray spectrum after filtering. For each such transition sub-area a tone correction function is calculated (for each PKV) and is used to tone-correct the associated transition sub-area. In another approach, the tone-correction function for a specific sub-area can be estimated from the tone-correction function of the background, taking into account the filter thickness in the specific transition sub-area. For example, for a transition sub-area thickness near that of the background, the tone correction function will be close to the tone-correction function of the background. One example for such an estimation of tone-correction values is provided in the following table: NearNearStepbackgroundbackgroundROI01.001.001.0010.980.981.0020.930.941.0030.870.890.9940.790.820.9850.710.750.9860.620.660.9770.520.580.9680.440.490.9490.350.410.93100.280.340.92 The values for “Near background” and “Near ROI” are estimated from the background values using exponential evaluation in the form:Estimated_value=Background_valueE Where E=0.85 for the “Near background” values estimation and E=0.07 for the “Near ROI” values estimation. Many other estimations can be used; the exponential estimation reasonably supports the exponential absorption characteristics of x-ray in matter. The above method is executed for a range of PKV values to generate a tone-correction function for each such PKV value. For example, in the range of 50 PKV to 150 PKV, 5 tone-correction functions can be generated for 50, 75, 100, 125 and 150 PKV. In case, for example, that 90 PKV is used with a patient, the tone-correction function can be interpolated from the tone-correction functions calculated for 75 PKV and 100 PKV using linear interpolation or any other interpolation. The interpolated tone-correction function can now be used for tone correction of the background generated with 90 PKV radiation. A common situation that may be encountered after executing the above tone-correction function calculation is that the actual image in use does not contain air sections and also maybe does not contain objects equivalent to steps 1, 2, 3 and 4. It is possible, for example, that the most “x-ray transparent” part in the examined object (patient 110 in FIG. 1A) reaches only level 2000 out or the 0÷4095 dynamic range. In such a case the x-ray current may be doubled to increase the DPP so that this area is brightened-up and arrives to level 4000. In such a case, the tone-correction value originally designed to 4000 is not suitable anymore since the current 4000 is generated after absorption equivalent to the 2000 level of the tone-correction function. To handle this situation, if the x-ray mA is doubled so that current mA is 2×(mA−0), one can modify the x-axis units of the tone-correction function of FIG. 21C by also multiplying by 2 the x-axis values, to get the modified tone-correction function of FIG. 21D. The dynamic range to be used in the tone-correction function of FIG. 21D is still 0÷4095 (4095 is indicated by dashed line 2120 in FIG. 21D). In this range the actual tone-correction values range from 0.00 to about 0.71 and not up to 1.00 as before. Therefore, when the x-ray current during usage is changed relative to the x-ray current during the calculation of the tone-correction function, the x-scale (the “input scale”) of the tone-correction function can be adjusted as described above, at the same proportions as the change in mA, and then be used to provide the required tone corrections under the new x-ray current. It would be appreciated that what more precisely determines this scale adjustment is the change in number of x-ray photons emitted from the x-ray tube towards the inspected object. Since this is generally considered to be reasonably proportional to the change in mA, mA is commonly used for this purpose. As explained above, in reference to using tone-correction function, the tone correction function can be used without initial-processing of the background. In such a case the calculation of the tone-correction function should be made under the same conditions, that is, without implementation of initial-processing to the data used for the calculation of the tone-correction function. Background Image Correction Function Calculation Using the Patient's Body In another example of the invention, the calculation of the tone-correction function can be based on real time patient data (instead of a phantom as described above) and be optimized to the specific patient. To describe this example reference is made to FIG. 22A though FIG. 22B. The figures present the display layer but this is made only for convenience. The discussion made in reference to these figures refers also to the image processing and image memory data layers that are typical handled in 12 bit and also to the x-ray distribution and detector layers (either a flat detector or image intensifier or any theoretical x-ray detector), the geometries related to these layers are completely analogous to those described in reference to FIG. 22A and FIG. 22B and the corresponding flowchart in FIG. 33B. This example is provided with the same parameters selected for the description of the above examples such as: calculation of tone-correction function is made for a specific PKV and mA, background radiation is designed to be 10% of the ROI radiation when no patient or phantom is present as 110 of FIG. 1A etc. Deviations important for the explanation of this example will be presented explicitly. The following description will also adopt the above simplification of the collimator of FIG. 18A where annulus 1804 width is zero and only central hole 1802 and annulus 1806 are considered. Expansion to the case of annulus 1804 is completely analogous to the expansions described above. Reference is made now to FIG. 22A. During the time represented by FIG. 22A, the operator is gazing at point 2202. ROI 2204 is set, as described above, around gazing point 2202 (step 3330). High radiation level is directed now at ROI 2204 while background 2206 is exposed to 1/10 the radiation of the ROI. In the main flow the data is processed as described above (typically initial-processing of adding frames, optionally adjusting brightness using a multiplication factor and second processing using stored tone-correction function. Other image enhancement processes such as spatial filters may also be applied). In a background flow, calculation of a tone-correction curve takes place, based on the data acquired from the image of patient 110. From FIG. 22A, 2 types of data are acquired: 1. In ROI 2202 images data is acquired and stored (step 3335) (preferably in 12 bit but possible also in other accuracy such as 8 bit) for the x-ray spectrum that is not filtered by annulus 1806 of FIG. 18A. 2. In background 2206 images data is acquired and stored (step 3340) (preferably in 12 bit but possible also in other accuracy such as 8 bit) for the x-ray spectrum that is filtered by annulus 1806 of FIG. 18A. Now, after some time, the gazing point of the operator moves to point 2208 of FIG. 22B. The ROI follows the gazing point and is now shown as ROI 2210 (step 3345). From FIG. 22B, 2 types of data are acquired: 3. In ROI 2210 images data is acquired and stored (step 3350) (preferably in 12 bit but possible also on other accuracy such as 8 bit) for the x-ray spectrum that is not filtered by annulus 1806 of FIG. 18A. 4. In background 2206 (that includes now also area 2214 that was previously ROI 2204) images data is acquired and stored (step 3360) (preferably in 12 bit but possible also on other accuracy such as 8 bit) for the x-ray spectrum that is filtered by annulus 1806 of FIG. 18A. With this collected data a tone-correction function can be calculated. In one approach, for each of the frames initial-processing is performed (frames summing and brightness adjustment). The other approach of calculating tone-correction function without initial processing will not be discussed as it is already well explained in above examples. At this stage, using the initially-processed data, values of pixels (part or all) from ROI 2204 are divided by values of the corresponding pixels from background area 2214 (step 3370) to provide the tone-correction background pixel correction factor (output) of FIG. 21C for the corresponding values of background pixels (input) in area 2214 of FIG. 22B. Also, using the initially-processed data, values of pixels (part or all) from ROI 2210 are divided by values of the corresponding pixels from the corresponding background area 2206 of the data acquired at the stage of FIG. 22A to provide the tone-correction background pixel correction factor of FIG. 21C for the corresponding values of background pixels in area 2206 of FIG. 22A. This provides a set of multiple input points for the tone-correction function that have corresponding calculated background pixel correction factor. Due to noise, this set typically includes also input values, of the same value, that have different output values. This statistical distribution of output values can be resolved by any method, including averaging of the output values, the median or any other method. In this example the average approach is adopted. This way, the multiple input values of possible different output values are reduced to a single input value with a single output value. Having this set of points, a curve fitting can be performed to fit this set (and preferably also the (0,0) point) to calculate the tone-correction function based on real patient data (step 3380). It would be appreciated that only one ROI position can be used for this purpose as well and more than 2 ROI positions demonstrated in the above example. It would also be appreciated that the more different ROI locations are used, it is more probable to get more points in the set used for curve fitting and thus, a more accurate tone correction function. It would also be appreciated that more data can be used for the calculation to improve accuracy. For example, if the example is based on 10 fps and the position of the ROI in FIG. 22A lasted for more than 5 seconds, then the ROI and background data could be collected from all the frames made during the last 5 seconds before moving the gazing point to the position of FIG. 22B. In the same manner, if ROI position in FIG. 22B lasted for more than 3 seconds, then the ROI and background data could be collected from all the frames made during the first 3 seconds after moving the gazing point to the position of FIG. 22B. Every such data can be temporally averaged, thus reduce noise errors and provide more accurate values for the curve fitting of the tone-correction function. It would also be appreciated that such calculation of tone-correction function can be done during a clinical procedure with a patient, where the first calculation is made right after the ROI moved first from one location to another and the tone-correction function can be re-calculated in any time interval using the additionally accumulated data. At the beginning of the process a default tone-correction function can be used and replaced by the first calculated tone-correction function right after its calculation and further, replace each tone-correction function by the successively calculated tone-correction function that is improved due to the additional data. It would be appreciated that tone correction calculation data collected from multiple patients can be used to generate one or more “general patient” tone-correction functions that can be used for future patients. Such data can be improved with every additional patient whose data is added to the already stored data and processed together. As explained above, in reference to using tone-correction function, the tone correction function can be used without initial-processing of the background. In such a case the calculation of the tone-correction function should be made under the same conditions, that is, without implementation of initial-processing to the data used for the calculation of the tone-correction function. It has been provided above, as an example that one may divide annulus 1824 of FIG. 18 into 8 annuluses of equal radius step so that the average DPP in the smallest annulus #1 is 9/10 of 1822, the average DPP in the next annulus #2 is 8/10 of 1822, annulus #3 is 7/10 and so on until the last annulus #8 that has 2/10 DPP of 1822. In this example it is assumed that each annulus, having a specific internal and external radius, provides DPP that is independent of angle 1828 of FIG. 18A. This method works accurately when the x-ray source is abeam the center of the aperture as shown in FIG. 23A. In FIG. 23A, dashed line 2302 marks the middle layer of collimator 1800 (half thickness). X-ray rays (rays) 2304 and 2306 cross the upper surface of annulus 1804 at the same point line 2302 crosses the upper surface of annulus 1804. This represents that the rays pass the collimator at the same radius but at different angles. Since the x-ray source is abeam the center of aperture 1802 and annulus 1804, the symmetry implies that in the path of each of rays 2304 and 2306 the material of collimator 1800 is the same. Therefore attenuation is the same and independent of angle 1828 of FIG. 18A. FIG. 23B presents the situation where collimator 1800 has moved to the right. Rays 2308 that is analogous to ray 2304 and ray 2310 that is analogous to ray 2306, although passing collimator 1800 at the same radius, do not have the same incidence angle at the collimator surface. The path inside the collimator for rays 2308 and 2310 is different and therefore they have different attenuation. To overcome this, a consideration of the phenomena is made and introduced to the DPP calculations. In one approach, a correction is made to the DPP as a parameter of collimator 1800 position. This can be done using x-ray absorption coefficients of the collimator material and collimator geometry. Since the distance from source 306 to collimator 1800 also affects DPP Vs collimator position, this distance can also be considered in the calculations to further enhance accuracy. As an alternative to DPP calculation, the DPP can be measured for different positions of collimator 1800 and be used as attenuation data. Accuracy can be further increased by measurement of DPP also as a function of source 306 to collimator 1800 distance. Reduction of the sensitivity of attenuation to the incidence angle of the ray can be provided by a symmetric or nearly symmetric aperture edge as shown in FIG. 24A, numerical reference 2312. With this design, the difference of the path in the collimator material between ray 2308 and ray 2310 is smaller than with the aperture edge of FIGS. 23A and 23B. It would be appreciated that design optimization can be made to each side of the aperture edge that is not symmetric to line 2302 to minimize the sensitivity of the attenuation to the ray angle of incidence. The result of such optimization would be two surfaces of the aperture edge (upper and lower surfaces of annulus 2312 in FIGS. 24A and 24B) that are not symmetric to line 2302 as demonstrated in 2400. Reference is made now to FIG. 25 illustrating a modified example of collimator 1800. It is common to filter x-ray radiation, using layers such as aluminum (Al) layers of various thicknesses to change the spectral distribution of the x-ray radiation. Such filtering typically (but not limited to) reduces low energy part of the x-ray spectrum. The collimator of FIG. 18, with most materials, would do the same. Now, if the collimator of FIG. 18 is used with another layer of filter, the other layer of filter which is designed to cover the complete x-ray beam cross section, does not only provide the desired results in the area of aperture 1802 but it adds this effect also to the area outside the aperture, 1806, on top of what the collimator already does. This may be undesired. To overcome this, instead of using a filter that covers the complete x-ray beam cross section, a smaller filter 2500 is added in the aperture area only, as a part of collimator 1800. This way the filter acts in the aperture area 1802 as desired but does not add additional undesired filtering in area 1806. Attention is drawn now to FIG. 26 which presents an exemplary system for carrying out the invention. Typically in x-ray systems, an ROI that is centered in image 120 (such as ROI 200 of FIG. 2) and has a fixed position is used for image analysis and for generating parameters to drive x-ray tube 100 and modify image 120. Parameters such as average value, maximum value and contrast may be calculated for this area. Such parameters are typically used to optimize the x-ray tube operation (such as mA, mAs and KVp). In this example an input device such as an eye tracker 128 is used to provide x-ray controller 130 with the focus of attention coordinates of one or more users 122. Instead of using a fixed position ROI as in the prior art, the one or more ROIs move according to the focus of attention so that they include the desired centers of the ROIs or are near the desired centers of the ROIs. With this adjustment of the ROIs position as a function of the focus of attention, the analysis and parameters calculated from the ROIs to drive the x-ray tube and modify image 120 are made from at least one ROI that is located according to the focus of attention instead of a fixed ROI, that may sometimes be at a distance from the focus of attention and not represent the image information that is relevant to the focus of attention. For example, the center of the image may include mainly bones (such as vertebrae and sternum) that constitute a dark part of the image and the side of image 120 may include mainly lung which is a bright part of the image. With a fixed center ROI, x-ray parameters and image adjustment (such as brightness, contrast and tone-correction) will be adjusted so that the central image will come out clear. This adjustment will drive excess x-ray to the lung area which is outside the ROI and also may increase the brightness of the lungs area beyond an acceptable image quality, resulting in unusable lungs imaging. When the user looks at the lung, the image quality may be useless. In such cases the user may move the patient or the c-arm system to a new position so that the lung enters the centered fixed position ROI. With the current example of moving one or more ROIs, as a function of the focus of attention, when the user gazes at the lung, at the side of image 120, the ROI is moved also to the lungs area and the x-ray parameters and image adjustment are made according to the displaced ROI, as required for the lungs. This would also, in this example, typically reduce x-ray intensity and reduce patient's exposure according to the focus of attention. It would be appreciated that many relations between the focus of attention and the ROI are available. Such relations may include ROI position relative to the focus of attention, ROI size relative to the focus of attention, ROI shape relative to the focus of attention (in a rectangular image the ROI may be circular in the central area and rectangular near the corners of the image or assume any other shape, including a combination of an arch and 90 degrees straight edges). Also, the ROI may be centered about the focus of attention but also may have a variable location relative to the focus of attention. Such a variable location may be dependent on any combination of the focus of attention location, the dynamics of the focus of attention and the fixed or variable shape of the ROI. The ROI may be fixed in position and only change size as a function of the focus of attention. One such example is a circular ROI centered about image 120, where the diameter of the ROI changes according to the focus of attention. In one example the ROI diameter may increase when the focus of attention distance from the center of image 120 increases. In the example of FIG. 26, the input device can be any input device that affects the position and/or the shape of the ROI. For example, an eye tracker, a joy-stick, a keyboard, an interactive display, a gesture reading device, a voice interpreter or any other suitable device can be used to determine coordinates relative to image 120, and the ROI position and/or shape will change according to such input. Tone changes are described above using the terms tone-correction. Although in many examples the term tone-correction is used this does not limit the examples to the sense of “correction” and all these examples can be interpreted in the sense of any tone changes of the image, including such that may include any desired image modification. Tone-correction term should be interpreted as a tone change that may include any desired image modification. Multiple ROI Collimator FIGS. 18A and 18B disclose a collimator with partially transparent background (1806) and fully transparent ROI section (1802) where the diameter of the ROI exposes area 1822 (FIG. 18D) and partially transparent area 1806 exposes area 1826 in FIG. 18D. For simplicity, transition area 1804 in FIG. 18A and 1824 in FIG. 18D are ignored in this example but it will be appreciated that this example can also include a version with the transition area. For example, in FIG. 34A, at the input plane of Image Intensifier 112, the fully exposed area 3402 by x-ray beam 106 may be 12″ in diameter. The ROI exposed area may be designed, for example, to ⅓ of the full area 3402, 4″, as shown by numerical indicator 3404. In an example of 1024×1024 pixels imaging area, the 12″ diameter is imaged on nearly 1024 pixels (1826 AND 1824 of FIG. 18D) and the ROI is imaged on nearly 1024/3=341 pixels in diameter (1822 of FIG. 18D). In some cases, the user may activate zoom function of image intensifier 114 so that only a part of input area 112 is imaged onto camera 116 sensor. For example, instead of 12″ only 9″ diameter from input area 112 is imaged onto camera 116 sensor. In such example an area of 9″ is imaged onto 1024×1024 pixels of the image. The user may expect that the ROI area will still be a ⅓ of displayed image 120. In this case 3″ diameter area of input 112 should be exposed onto 341 pixels as ROI and not 4″ as before, as shown by numerical indicator 3406. In one example, the adjustment of the ROI radiation area from 4″ to 3″ can be done by moving collimator 1800, with ROI area 1802 designed for 4″ towards image intensifier 114 to create a new distance of collimator 1800 from x-ray focal point 306. If D1 is the distance of collimator 1800 from focal point 306 for ROI of 4″, then to get an ROI of 3″ the new distance of collimator 1800 from focal point 306 should be for this example D1×4/3. This proportion calculation example can be used also for other ROI diameters. Collimator 1800 can be moved away or closer to focal point 306 using any motorized mechanical system. It would be appreciated that in this example collimator 1800 is represented in FIG. 34A by a more general collimator 150 and also, other collimators represented by collimator 150 (as explained above) can be used according to this example. In another example, instead of moving collimator 1800 away or towards focal point 306, collimators such as collimator 1800 can be designed according to the example of FIG. 34B and shown by collimator 3410. This collimator has 3 holes for ROI of 3 different sizes. For example, each ROI hole diameter is designed to project ⅓ of the exposed area diameter. For example, if image intensifier input 112 is 12″ and it has 2 zoom options 9″ and 6″ then hole 3414 will be 9/12 of hole 3412 and hole 3416 will be 6/12 of hole 3412. For each zoom of image intensifier 114 the corresponding area of collimator 3410 is used so that the ROI is maintained ⅓ in diameter of image 120 diameter. Collimator 3420 of FIG. 34C is another example enabling adjustment of ROI hole to the zoom options of the image intensifier 114 in a similar manner to collimator 3410 but with a different geometry. Rectangular hole 3428 (that can also be a relatively large circular hole) provides a collimator area which does not limit the x-ray and enables conventional usage of such system. It would be appreciated that collimators with a plurality of holes such as those of FIGS. 34B and 35C can also be moved perpendicularly to the collimator plane to provide variable size ROI onto input area 112. By combining more than one hole size and movement perpendicularly to the collimator area, more ROI sizes can be provided with reduced vertical movement range comparing to one hole. As the variety of holes dimensions increases, a smaller motion range is required perpendicularly to the collimator plane to cover more ROI sizes. It would also be appreciated that the examples of FIG. 34 can be combined with any of the hole edges as shown in reference to FIGS. 23, 24 and 25. Reference is made now to FIG. 35A providing another example of collimator 3500 of the present invention. Coordinate system 126 is present in FIG. 35A to provide orientation in reference to FIG. 1B. X-ray focal point 306 is shown and a cone-shaped x-ray beam 106 is projected upwards towards input area 112 (not shown in FIG. 35A—see FIG. 34A). Plates 3501, 3502, 3503 and 3504 are partially transparent to x-ray. In this example we shall assume that each such plate transmits 30% of beam 106 but it would be appreciated that other transmission levels are available. Plates 3501, 3502, 3503 and 3504 can be made from any suitable material, considering the desired effect of the spectral distribution of the transmitted x-ray beam. For example, copper plates can be used. Dashed circle 106A represents x-ray cone 106 cross section at generally the plane of collimator 3500. Except for a rectangular shaped x-ray beam, 3510, the rest of the beam (106B) intensity is reduced due to plates 3501, 3502, 3503 and 3504. Where there is only one layer of plates the x-ray beam is reduced to 30% of its original intensity. In areas where two plates overlap the x-ray beam is reduced to 9% of its original intensity (30%×30%). With this example ROI 3510 is now rectangular. Motors can move plates 3501, 3502, 3503 and 3504 as explained in FIG. 35B. It would be appreciated that due to x-ray spectral changes depending on thickness of filtering material, the result of 2 layers, each allowing 30% of the incident x-ray to pass, is typically not 9% but depends on the original x-ray spectrum and the material of the filter. Yet, in the disclosure of this invention we shall assume such relations (30%×30%=9%) to simplify the description of the invention. Actual absorbance of one layer Vs 2 layers can be designed per the needs of any specific application and it will be ignored in this disclosure. In FIG. 35B the components of the motorizing elements are detailed in reference to plate 3501. The other 3 plates mechanism is analogous. Motor 3501A drives screw 3501C that moves nut 3501B. Nut 3501B is connected to plate 3501 therefore enables plate 3501 to move in directions of arrow 3501D. Therefore, each plate can move independently of the other plates as indicated by dual-head arrow for each plate. Rails that may be used to support the plates and enable motion are not shown in this figure. It would be appreciated that the specific motion mechanism described here is provided to explain the invention and that the scope of the invention is not limited to this motion mechanism. In the example of FIG. 35B hole 3512 is at the center of beam 106 (as shown by the beam cross section 106A) and it has a certain size. In the example of FIG. 35C plates 3503 and 3504 were moved to the right without changing the distance between these plates. Plates 3501 and 3502 were moved upwards without changing the distance between these plates. As a result hole 3512 moved towards the top-right edge of x-ray beam cross-section 106A but without changing its dimensions. In the example of FIG. 35D hole 3512 is also generally at the top-right area of x-ray beam cross-section 106A but the distance between plates 3501 and 3502 was reduced and also the distance between plates 3503 and 3504 was reduced. As a result the size of hole 3512 was reduced and the resultant ROI is smaller now. In FIG. 35E the hole is still in the upper-right area of x-ray beam cross-section 106A but the distance between plates was changed again to produce a large rectangle that is also particularly longer in the Y direction than in the X direction. The ROI therefore becomes larger and also of a different shape. With this example of collimator 3500 therefore the ROI of image 120 can not only be moved across the area of image 120 to the desired location but also the size and aspect ratio of the ROI can be changed as desired, to compensate for zoom in image intensifier 114 or for other reasons. It would be appreciated that although FIG. 35 implies that pairs of collimator plates are arranged in the same plane, this is not a limitation of the invention and each of the plates of a plates pair can be positioned in a different plane. Reference is made now to FIG. 35F. In this example only 2 plates are used: 3521 and 3524. Each plate has an elongated aperture (3522 and 3525). The plates are driven by motors and screws as shown here and explained in reference to FIG. 35B. The plate itself is partially transparent to x-ray. FIG. 35G illustrates the plates position where plate 3521 is above plate 3524 and elongated apertures 3522 and 3525 partially overlap while being non-parallel to each other. 106A provides an illustration of the x-ray beam cross section at approximately the level of the plates. Plate 3521 can move in the direction of arrow 3530 and plate 3524 can move in direction of arrow 3532. By moving each plate independently aperture 3512 can be positioned in various positions in x-ray beam cross section 106A. The effect of this collimator as well as the effect of the collimator of FIG. 35A on the image and correction of the image thereby is described in reference to FIG. 36 and also above. Reference is made now to FIG. 36, illustrating the x-ray intensity distribution in different areas of image 120 when the ROI is in the position presented in FIG. 35B. In this example there is no object (patient) between collimator 3500 and input area 112 so, ideally, without collimator 3500 the x-ray radiation over input area 112 would be uniform. In this example, as a result of collimator 3500 the area of image 120 is divided into 3 intensity areas: 3602, the ROI, where the original 100% intensity is, 3604 (4 such areas) where the intensity is 30% of the ROI and 3606 (4 such areas) where the intensity is 9% of the ROI. The above described methods to correct background are fully applicable to correct the background of the present example where each of areas 3604 and 3606 require its own correction parameters. It would be appreciated therefore that the current example can be used together with the above described correction methods. It would also be appreciated that edge transition concepts such as those associated with FIGS. 18 and 24 are applicable also to the edges of the plates of collimator 3500 that are facing hole 3512. Attention is made to FIG. 37. X-ray partially transparent plate 3724 having an elongated aperture 3722 is positioned over x-ray partially transparent plate 3721 having an elongated aperture 3725. The x-ray beam is perpendicular to the plane of the drawing and its cross section near the plates is illustrated by circle 106a. Plate 3724 is connected to carriage 3730 that can move along track 3731 as illustrated by arrow 3732. The motion of plate 3724 is enabled by motor 3728 and transmission system 3729. Plate 3721 can move along track 3737 in a similar way, as enabled by motor 3734. As a result of independent motion of motors 3728 and 3734, overlapping aperture 3712 can be positioned in the desired location at x-ray beam 106a, allowing 100% of the radiation to go through aperture 3712, smaller part of the radiation to go through overlapping area of aperture 3722 with plate 3721 and through the overlapping area of plate 3724 with aperture 3725 and further less radiation to pass through the overlapping area of plates 3724 and 3721. During a MFI session, the motors can drive the plates so as to position aperture 3712 at any desired location. In another mode of operation, when the x-ray is operated at a pulsed mode, both motors can be operated to rotate continually in one direction only. The angular speed of the motors is designed so that the rotation frequency of the motors is the same as the frequency of the x-ray pulses. This way, whenever an x-ray pulse is present, aperture 3712 appears in the same position, providing to the images a virtually non-moving aperture in the desired location. By momentary slowing down or accelerating one or both motors, the location of aperture 3712 at the moment of x-ray pulse firing, can be changed. In FIG. 37 apertures 3722 and 3725 appear to be perpendicular to each other and so do tracks 3731 and 3737, but it would be appreciated that this is illustrated this way only to simplify the explanation and such a design will work also with apertures and tracks that are not perpendicular to each other, as long as they are also not parallel to each other. Attention is made now to FIG. 38A. Disk 3802 is partially transparent to x-ray, it can rotate about axis 3806 and has an elongated aperture 3804, generally along a radius of disk 3802. FIG. 38B illustrates disk 3802 connected to motor 3812 through driving belt 3814 so that motor 3812 can be utilized to control the rotation of disk 3802. In the same way, disk 3808 has a similar structure to disk 3802; it has an elongated aperture 3810 along a radius and rotation axis 3818. It is connected to motor 3820 through driving belt 3822. In the arrangement of FIG. 38B the two disks partially overlap so that when elongated apertures 3804 and 3810 have any overlapping part, the long axes of the apertures are not parallel to each other, so as to create a transparent area such as 3812B that is smaller in its maximal dimension than the maximal dimension of the smaller long aperture dimension among apertures 3804 and 3810. The overlapping of the disks, the radius of the disks and the geometry of the apertures is designed so that for an x-ray beam cross section 106A, each of disks 3802 and 3808 can be positioned at an angular position so that aperture 3812B can be centered at any desired point within x-ray beam 106A. In one mode of operation, the disks are rotated to a desired angle so that aperture 3812B is located at the desired position during an x-ray session or a part of it. In another mode of operation, when the x-ray is operated at a pulsed mode, both motors can be operated to rotate continually in one direction only. The angular speed of the motors is designed so that the rotation frequency of the motors is the same as the frequency of the x-ray pulses. This way, whenever an x-ray pulse is present, aperture 3712B appears in the same position, providing to the images a virtually non-moving aperture in the desired location. By momentary slowing down or accelerating one or both motors, the location of aperture 3712B at the time the pulse is fired can be changed. In this mode of rotation at the x-ray pulse frequency, a balancing material removal may be desired in the disks, generally in a location opposite to the elongated aperture. Disks 3802 and 3808 can be engaged directly onto the axis of motors 3812 and 3820 to provide direct drive. FIGS. 38C and 38D illustrate another example similar the example of FIGS. 38A and 38B. The difference is in the shape of the x-ray partially transparent plate 3802C that is now not in the shape of a disk. Motors and driving belts are not shown. The explanation of how it works is analogous to the explanation made above in reference to FIGS. 38A and 30B. It would be appreciated that other shapes are available for the x-ray partially transparent plates. For the mode of rotation at the x-ray pulse frequency, a balancing weight may be desired in the example of FIGS. 38C and 38D. Such a balancing weight should be added on the side opposite to the center of gravity relative to the axis of rotation 3806C and 3818D. Reference is made now to FIG. 39, illustrating the x-ray intensity distribution in different areas of image 120 when the ROI is in the position presented in FIGS. 38B and 38D. Reference will be limited now to FIG. 38D, the explanation for FIG. 38B being analogous. In this example there is no object (patient) between collimator 3800 and input area 112 so, ideally, without collimator 3800 the x-ray radiation over input area 112 would be uniform. In this example, as a result of collimator 3800, the area of image 120 is divided into 3 intensity areas: 3602, the ROI, where the original 100% intensity is, 3604 (4 such areas) where the intensity is 30% of the ROI and 3606 (4 such areas) where the intensity is 9% of the ROI. The same, except for the geometry of the examples, is true also for the collimator of FIG. 37. The above described methods to correct background are fully applicable to correct the background of the present example where each of areas 3604 and 3606 requires its own correction parameters. It would be appreciated therefore that the current example can be used together with the above described correction methods. It would also be appreciated that edge transition concepts such as those associated with FIGS. 18 and 24 are applicable also to the edges of the plates of collimator 3500 that are facing hole 3512. Reference is made now to FIG. 40A illustrating an example for moving collimator 1800 (see FIG. 18) in a 2D (two dimensional) plane. Collimator 1800 is rigidly connected to bar 4010 through interface 4012. Bar 4010 is connected to carriage 4006 that can move in the X direction (relative to coordinate system 126) on track 4008 to provide collimator 1800 motion capability in the X direction. Track 4008 is connected to carriage 4004 that can move on track 4002 in the Y direction (relative to coordinate system 126). This provides collimator 1800 a freedom to move in the Y direction. Through the combination of these two movements collimator 1800 can be moved within the x-ray beam. Motors and driving gears are not shown in this drawing. It would be appreciated that track 4002, track 4008 and bar 4010 do not need to be mutually perpendicular or parallel. In this example the mass that needs to be driven in the Y direction includes not only collimator 1800, but also track 4008 and carriage 4006. In the example of FIG. 40B this is avoided. In the example of FIG. 40B collimator 1800 is connected on one side to carriage 4018 that can slide along track 4020. Track 4020 is assembled onto carriage 4004B that can move along track 4002B to actuate the motion of collimator 1800 in Y direction without preventing it from moving in X direction. Similarly, collimator 1800 is connected on another side to carriage 4014 that can slide along track 4016. Track 4016 is assembled onto carriage 4006B that can move along track 4008B to actuate the motion of collimator 1800 in X direction without preventing it from moving in Y direction. Motors and suitable gear (such as driving screws) that are not shown can drive carriage 4006B along track 4008B and carriage 4004B along track 4002B. It would be appreciated that track 4002B, track 4008B, track 4016 and track 4020 do not need to be mutually perpendicular or parallel. According to embodiments of the present invention, the collimator of FIG. 40 may comprise more than one aperture that allows all the radiation to pass through. FIGS. 41A through 41D illustrate another embodiment of the collimator 1800, in which the aperture 1802 size may be changed. FIG. 41A shows disk 4100, made of x-ray relatively highly transparent material 4102 such as polycarbonate. A plurality of smaller disks (4 are shown) are attached to disk 4100 back face. Each small disk (e.g. 4104, 4106) is made of x-ray attenuating material, e.g. copper, surrounding an aperture (e.g. 4108). The small disks are equal in diameter, which is equal to the diameter of aperture 1802 of collimator 1800. The various disks' apertures differ from one another. FIG. 41B shows the disk 4100 mounted on the collimator motion device of FIG. 40B, by attaching a motor 4114 to interface 4012, for rotating disk 4100 around center of rotation 4104, so that each one of the smaller disks, in turn, covers aperture 1802, thus creating a variable aperture collimator, as demonstrated in FIG. 41C. Disk 4100 may be mounted in friction proximity to collimator 1800, so that when one of the smaller disks (e.g. 4106) arrives at a superposition of aperture 1802, the small disk is pressed inside aperture 1802. Alternatively, motor 4114 may be mounted so as to create a gap between disk 4100 and collimator 1800, whereby disk 4100 may be rotated to bring the selected smaller disk to the required horizontal position and then moved towards collimator 1800 to place the smaller disk inside aperture 1802. FIG. 41D shows a section view of small disk 4108 mounted in aperture 1802 as shown in FIG. 41. According to embodiments of the invention, disk 4100 may have a gradient thickness rim, to minimize visually affecting the x-ray image. Another embodiment, similar in concept to the embodiment of FIG. 41 but occupying less space, is depicted in FIGS. 42A through 42E. According to the embodiment of FIG. 42. Disk 4100 of FIG. 41 is replaced by an anchor shaped body 4220, made of x-ray relatively highly transparent material such as polycarbonate. A plurality of smaller disks (2 are shown) are attached to body 4220 back face. Each small disk (e.g. 4204, 4206) is made of x-ray attenuating material, e.g. copper, surrounding an aperture. The small disks are equal in diameter, which is equal to the diameter of aperture 1802 of collimator 1800. The various disks' apertures differ from one another. Anchor shaped body 4220 is mounted on the collimator motion device of FIG. 40B, by attaching a motor 4202 to interface 4210 at body 4220 center of rotation, so that each one of the smaller disks, in turn, covers aperture 1802, thus creating a variable aperture collimator. Body 4220 may be mounted in friction proximity to collimator 1800, so that when one of the smaller disks (e.g. 4206) arrives at a superposition of aperture 1802, the small disk is pressed inside aperture 1802. Alternatively, motor 4202 may be mounted so as to create a gap between body 4220 and collimator 1800, whereby body 4220 may be rotated to bring the selected smaller disk to the required horizontal position and then moved towards collimator 1800 to place the smaller disk inside aperture 1802. FIGS. 42A through 42D show the exemplary anchor shaped body 4220 in various position, whereby a variable aperture collimator is attained. According to embodiments of the invention, body 4220 may have a gradient thickness rim, to minimize visually affecting the x-ray image. In yet another embodiment of the invention, a round collimator 4300, as depicted in FIG. 43, may comprise a plurality of apertures 4310, 4320 of similar or different diameters, to allow for more than one ROI at any given time. In yet another embodiment of the invention, a round collimator 4400, as depicted in FIG. 44, comprises a disk having an aperture 4410 and an annulus 4420 having a pattern of material thickness which does not necessarily define a gradient like that of collimator 1800. Combining the collimator 4400 with the motion technique such as demonstrated in FIG. 40, enables full control over the attenuation level of each radiation beam at each point in time. It would be appreciated that although the above was described in reference to an image intensifier it is applicable to any detector, including a flat panel detector. The geometry of the detector, the zoom area and the ROI can be of a mixed nature and do not need to be of the same nature (i.e. circular or rectangular or another geometry). It would be appreciated that throughout the description that when, for example, the term aperture is used in the context of elongated aperture, the intention is to an elongated aperture. It would be appreciated that “partially transparent” and “attenuating” are equivalent and the role of such a term is dependent on the amount of transparency or attenuation. In the above description the role of such terms is provided by the context of the description with specific value examples where needed. The structure examples provided in this disclosure can be implemented with different degrees of transparency to x-ray (or, equivalently, with different degrees of attenuation of x-ray), as preferred for specific implementations. As such they can be highly transitive to x-ray (low attenuation) or poorly transmisive to x-ray (high attenuation). High attenuation also refers to “x-ray blocking” terms since x-ray cannot be 100% blocked and “blocking” is used in the field of the invention to indicate high attenuation. It would be appreciated by those skilled in the art that the above described methods and technologies are not limited to the configurations and methods mentioned herein above as examples. These are provided as examples and other configurations and methods can be used to optimize final result, depending on the specific design and the set of technologies implemented in the production of the design, including combinations of various embodiments described separately. The herein above embodiments are described in a way of example only and do not specify a limited scope of the invention. The scope of the invention is defined solely by the claims provided herein below: |
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summary | ||
claims | 1. A support frame for a radiation shield garment, comprising:an elongated upper vertical back member, having an upper top end and an upper bottom end;a lower vertical back member, having a lower top end and a lower bottom end, the lower top end of the lower vertical back member slideably attached to the upper bottom end of the elongated upper vertical back member;a lower back support panel coupled to the lower bottom end of the lower vertical back member; anda pair of shoulder members attached to the upper top end of the elongated vertical back member;wherein the upper top end of the elongated upper vertical back member has a V-shaped configuration and the lower bottom end of the lower vertical back member has an upside down T-shaped configuration. 2. The support frame of claim 1 wherein each shoulder member comprises:a base; anda shoulder bracket projecting upwardly in a curved manner from the base for extending over shoulders of a wearer. 3. The support frame of claim 2 wherein the base includes one or more slots providing angular adjustment of the pair of shoulder members and wherein one or more fasteners extend through the one or more slots slideably attaching the each shoulder member to the upper top end of the elongated upper vertical back member. 4. The support frame of claim 2, further comprising, one or more inwardly facing upper padded members coupled to the base of the each of the pair of shoulder members and positioned to rest upon a shoulder area of the wearer. 5. The support frame of claim 1 wherein the upper bottom end of the elongated upper vertical back member includes one or more longitudinal slots; and wherein one or more fastening means extend through the elongated slots providing vertical height adjustment of the elongated upper vertical back member. 6. The support frame of claim 1 wherein the lower back support member is positioned to rest upon a lower back area of a wearer. 7. The support frame of claim 6, further comprising, a strap integrally connected to the lower back support member. 8. The support frame of claim 7 wherein the strap comprises a first end and a second end, the second end comprising an attaching means integral thereto. 9. The support frame of claim 1, further comprising, a inwardly facing center padded member attached to the upper bottom end of the lower vertical back member and positioned to rest upon a lower back area of a wearer. 10. The support frame of claim 1, wherein the elongated upper vertical back member, the lower vertical back member, and the pair of shoulder members attached to the upper top end of the elongated vertical back member are comprised of a material selected from the group consisting of aluminum and polypropylene. 11. A support frame for a radiation shield garment, comprising:an elongated upper vertical back member, having an upper top end and an upper bottom end, the upper top end branching into a left portion and a right portion;a lower vertical back member, having a lower top end and a lower bottom end, the lower top end of the lower vertical back member slideably attached to the upper bottom end of the elongated upper vertical back member;a lower back support member attached to the lower bottom end of the lower vertical back member; anda pair of shoulder members attached to the upper top end of the elongated vertical back member, each of the pair of should members comprising:a base; anda rigid attachment strap projecting upwardly in a curved manner from the base for extending over shoulders of a wearer;wherein the upper top end of the elongated upper vertical back member has a V-shaped configuration and the lower bottom end of the lower vertical back member has an upside down T-shaped configuration. 12. The support frame of claim 11 wherein the base includes one or more slots providing angular adjustment of the pair of shoulder members and wherein one or more fasteners extend through the one or more slots slideably attaching the each shoulder member to the upper top end of the elongated upper vertical back member. 13. The support frame of claim 11 wherein the upper bottom end of the elongated vertical back member includes one or more longitudinal slots; and wherein one or more fastening means extend through the elongated slots providing vertical height adjustment of the elongated upper vertical back member. 14. The support frame of claim 11, further comprising a strap integrally connected to the lower back lower padded member. 15. The support frame of claim 14, wherein the strap comprises a first end and a second end, the second end comprising an attaching means integral thereto. 16. The support frame of claim 11, further comprising an inwardly facing center padded member attached the upper bottom end of the lower vertical back member and positioned to rest upon a lower back area of the wearer. 17. The support frame of claim 11, wherein the elongated upper vertical back member, the lower vertical back member, and the pair of shoulder members attached to the upper top end of the elongated vertical back member are comprised of aluminum. 18. A support frame for a radiation shield garment, comprising:an elongated upper vertical back member, having an upper top end and an upper bottom end, the upper top end having a V-shaped configuration;a lower vertical back member, having a lower top end and a lower bottom end, the lower top end of the lower vertical back member slideably coupled to the upper bottom end of the elongated upper vertical back member;a lower back support member attached to the lower bottom end of the lower vertical back member, the lower bottom end having an upside down T-shaped configuration;an inwardly facing center padded member attached to the upper bottom end of the lower vertical back member and positioned to rest upon the lower back of the wearer;a pair of shoulder members attached to the upper top end of the elongated vertical back member, each of the pair of should members comprising:a base;a shoulder bracket projecting upwardly in a curved manner from the base for extending over shoulders of a wearer; andan inwardly facing upper padded member coupled to the base the each of the pair of shoulder members and positioned to rest upon a shoulder area of the wearer. |
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043158315 | abstract | Process for the conditioning of solid radioactive waste with large dimensions, constituted by contaminated objects such as cartridge filters, metal chips, tools etc., wherein said waste is incorporated into an ambient temperature-thermosetting resin to which has previously been added at least one inert filler, and the said resin is then cross-linked. One application is the encasing of solid radioactive waste below water, particularly at the bottom of a pond. |
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