Patent Number: 
Section: claims

1. A fissile neutron detection system for detecting incident fissile neutrons, said fissile neutron detection system, comprising:an ionizing thermal neutron detector arrangement including an inner peripheral shape that at least substantially surrounds a moderator region for detecting thermal neutrons that exit the moderator region but is at least generally transparent to the incident fissile neutrons;a moderator arrangement disposed within the moderator region for converting the incident fissile neutrons in the moderator region to thermal neutrons which exit the moderator region to then enter the thermal neutron detector arrangement for detection of at least some of the thermal neutrons to produce an electrical current as a detector output with the moderator arrangement having an outer peripheral shape that is at least generally complementary to said inner peripheral shape and the moderator arrangement includes lateral extents such that any given dimension that bisects the lateral extents includes a length that is greater than any thickness of the moderator arrangement transverse to the lateral extents wherein the thermal neutron detector arrangement and the moderator arrangement are configured to define a pair of opposing major receiving directions for bidirectionally receiving the incident fissile neutrons from the opposing major receiving directions with a fundamental sensitivity that is at least generally equal from each of the major receiving directions; andan additional moderator arrangement at least interposed in each of the opposing major receiving directions outward of the thermal neutron detector arrangement and having a supplemental moderator thickness along each of the opposing major receiving directions that is less than said thickness of the moderator arrangement such that a modified sensitivity is presented from each of the major receiving directions with the modified sensitivity remaining equal from the each of the major receiving directions but different from the fundamental sensitivity. 2. The fissile neutron detection system of claim 1 wherein the additional moderator arrangement at least substantially surrounds the thermal neutron detector arrangement. 3. The fissile neutron detection system of claim 1 wherein the modified sensitivity is greater than said fundamental sensitivity. 4. The fissile neutron detection system of claim 1 wherein the supplemental moderator thickness is in a range from 0.1 cm to 1 cm, inclusively. 5. A fissile neutron detection system for detecting incident fissile neutrons, said fissile neutron detection system, comprising:an ionizing thermal neutron detector arrangement including an inner peripheral shape that at least substantially surrounds a moderator region for detecting thermal neutrons that exit the moderator region but is at least generally transparent to the incident fissile neutrons;a moderator arrangement disposed within the moderator region for converting the incident fissile neutrons in the moderator region to thermal neutrons which exit the moderator region to then enter the thermal neutron detector arrangement for detection of at least some of the thermal neutrons to produce an electrical current as a detector output with the moderator arrangement having an outer peripheral shape that is at least generally complementary to said inner peripheral shape and the moderator arrangement includes lateral extents such that any given dimension that bisects the lateral extents includes a length that is greater than any thickness of the moderator arrangement transverse to the lateral extents wherein the thermal neutron detector arrangement and the moderator arrangement are configured to define a pair of opposing major receiving directions for bidirectionally receiving the incident fissile neutrons from the opposing major receiving directions with a fundamental sensitivity that is at least generally equal from each of the major receiving directions; andan additional moderator arrangement at least interposed in each of the opposing major receiving directions outward of the thermal neutron detector arrangement and having a first supplemental moderator thickness along a first one of the opposing major receiving directions that is less than the thickness of the moderator arrangement and a second supplemental thickness along a second one of the opposing major directions that is greater than said thickness of the moderator arrangement such that a modified sensitivity is presented from each of the major receiving directions with the modified sensitivity being different in each of the major receiving directions as well as different from the fundamental sensitivity. 6. The fissile neutron detection system of claim 5 wherein the first supplemental moderator thickness is in a range from 0.1 cm to 1 cm, inclusively, and the second supplemental moderator thickness is in a range from 1.1 cm to 10 cm, inclusively. 7. The fissile neutron detection system of claim 5 wherein the modified sensitivity is greater than the fundamental sensitivity. 8. A fissile neutron detection system for detecting incident fissile neutrons, said fissile neutron detection system, comprising:an ionizing thermal neutron detector arrangement including an inner peripheral shape that at least substantially surrounds a moderator region for detecting thermal neutrons that exit the moderator region but is at least generally transparent to the incident fissile neutrons;a moderator arrangement disposed within the moderator region for converting the incident fissile neutrons in the moderator region to thermal neutrons which exit the moderator region to then enter the thermal neutron detector arrangement for detection of at least some of the thermal neutrons to produce an electrical current as a detector output with the moderator arrangement having an outer peripheral shape that is at least generally complementary to said inner peripheral shape and the moderator arrangement includes lateral extents such that any given dimension that bisects the lateral extents includes a length that is greater than any thickness of the moderator arrangement transverse to the lateral extents wherein the thermal neutron detector arrangement and the moderator arrangement are configured to define a pair of opposing major receiving directions for bidirectionally receiving the incident fissile neutrons at least from the opposing major receiving directions with a fundamental sensitivity that is at least generally equal from each of the major receiving directions; andan additional moderator arrangement interposed in at least in a first one of the opposing major receiving directions outward of the thermal neutron detector arrangement and having a first supplemental moderator thickness along the first one of the opposing major receiving directions sufficient to produce a first modified sensitivity in the first major receiving direction that is less than the fundamental sensitivity and a second modified sensitivity in the second major receiving direction that is greater than the fundamental sensitivity to form a unidirectional fissile neutron detection system. 9. The fissile neutron detection system of claim 8 wherein the first supplemental moderator thickness is configured to backscatter and further thermalize at least some fissile neutrons, that initially pass through the thermal neutron detector arrangement and the moderator arrangement, to return the backscattered further thermalized neutrons for detection by the thermal neutron detector arrangement to provide an additional electric current that contributes to the detector output. 10. The fissile neutron detection system of claim 8 wherein said first supplemental moderator thickness is greater than said thickness of the moderator arrangement. 11. The fissile neutron detection system of claim 8 wherein the additional moderator arrangement further includes a second supplemental moderator thickness imposed in a second, opposite one of the opposing major receiving directions having a second supplemental moderator thickness that is less than the first supplemental moderator thickness to further increase the second modified sensitivity. 12. The fissile neutron detection system of claim 11 wherein the second supplemental moderator thickness is less than the thickness of the moderator arrangement. 13. The fissile neutron detection system of claim 11 wherein the second supplemental moderator thickness further moderates at least some undetected ones of the thermal neutrons, that exit the moderator arrangement and initially pass undetected through the thermal neutron detector arrangement, to backscatter at least a portion of the undetected ones of the thermal neutrons to return to the thermal neutron detector arrangement for detection to contribute an additional electrical current to the detector output. 14. The fissile neutron detection system of claim 11 wherein the first supplemental moderator thickness is in one range from 0.1 cm to 1 cm and the second supplemental moderator thickness is in another range from 1.1 cm to 10 cm. 15. The fissile neutron detection system of claim 8 wherein the lateral extents are at least generally rectangular with a widthwise dimension and a lengthwise dimension and the thickness of the moderator arrangement is less than the widthwise dimension and the lengthwise dimension. 16. The fissile neutron detection system of claim 8 wherein the lateral extents define a symmetrical edge configuration. 17. The fissile neutron detection system of claim 8 wherein the moderator arrangement includes a pair of opposing major sides that cooperatively define the lateral extents. 18. The fissile neutron detection system of claim 8 wherein the thermal neutron detector arrangement sealingly contains a readout gas and supports an active sheet layer arrangement that is in gaseous communication with the readout gas and the active sheet layer arrangement spans at least a majority of said lateral extents such that (i) a majority of the incident fissile neutrons pass through the active sheet layer arrangement prior to impinging on said moderator arrangement, and (ii) a majority of thermal neutrons impinge on the active sheet layer arrangement after exiting the moderator arrangement, wherein the active sheet layer arrangement emits ionizing particles responsive to said thermal neutrons, and the ionizing particles initiate an avalanche of ions, within said readout gas, to produce said electrical current. 19. The fissile neutron detection system of claim 8 wherein the moderator arrangement includes at least one moderator sheet material having a pair of opposing major sides that are spaced apart by said thickness. 20. The fissile neutron detection system of claim 19 wherein said thermal neutron detector arrangement includes at least first and second thermal neutron detectors in a spaced apart confronting relationship with said moderator arrangement disposed therebetween. 21. The fissile neutron detection system of claim 20 wherein each thermal neutron detector supports an active sheet layer that spans at least said majority of the lateral extents, and each of said thermal neutron detectors includes first and second arrangements of electrodes with the electrodes of the first arrangement of electrodes laterally spaced apart proximate to one of a pair of opposing major surfaces of each active sheet layer and the electrodes of the second arrangement of electrodes laterally spaced apart proximate to the other, opposite one of the opposing pair of major surfaces such that a projection of each of the first and second arrangements of electrodes onto the active sheet layer defines an area that substantially covers one of the major surfaces of the active sheet material layer such that each electrode is in operative communication with said readout gas. 22. The fissile neutron detection system of claim 21 wherein one major surface of the first thermal neutron detector confronts one of said major sides of said moderator arrangement, and another major surface of said second thermal neutron detector confronts the other one of said major sides of said moderator arrangement, and said first and second thermal neutron detectors along with said moderator arrangement forming an at least generally planar layered structure such that each one of a majority of the incident fissile neutrons passes through one of said first and second thermal neutron detectors before impinging on said moderator arrangement. 23. The fissile neutron detection system of claim 22 wherein said major surfaces of said thermal neutron detectors cooperatively define said moderator region therebetween having a volume and the moderator arrangement substantially fills said volume. 24. The fissile neutron detection system of claim 23 wherein the moderator arrangement fills at least 60 percent of the volume of the moderator region. 25. The fissile neutron detection system of claim 22 wherein said major surfaces of said thermal neutron detectors cooperatively define said moderator region therebetween having a volume and the moderator arrangement substantially fills said volume. 26. The fissile neutron detection system of claim 8 wherein said thickness is an at least approximately uniform thickness. 27. The fissile neutron detection system of claim 8 wherein said thermal neutron detector arrangement includes at least one group of thermal neutron detectors with the thermal neutron detectors of each group in a side-by-side relationship. 28. The fissile neutron detection system of claim 27 wherein each thermal neutron detector sealingly contains a readout gas and each thermal neutron detector supports an active sheet material layer in gaseous communication with the readout gas for detecting thermal neutrons that are incident on the active sheet material layer and the active sheet material layers of the group of thermal neutron detectors cooperate to form an arrangement of active sheet material layers that spans at least a majority of said lateral extents of the moderator arrangement such that a majority of the thermal neutrons that exit the moderator arrangement thereafter impinge on the arrangement of active sheet material layers to cause the active sheet material layer arrangement to emit ionizing particles responsive to the thermal neutrons that initiates an avalanche of ions in the readout gas to produce said electrical current. 29. The fissile neutron detection system of claim 28 wherein each of said thermal neutron detectors of said group includes a first set of electrodes and a second set of electrodes in a spaced apart confronting relationship, with said active sheet material layer supported therebetween, with the electrodes of the first set of electrodes laterally spaced apart proximate to one of a pair of opposing major surfaces of the active sheet material layer and the electrodes of the second set of electrodes laterally spaced apart proximate to the other, opposite one of the opposing pair of major surfaces such that a projection of each of the first and second sets of electrodes onto the active sheet material layer defines an area that substantially covers one of the major surfaces of the active sheet material. 30. The fissile neutron detection system of claim 27 wherein said thermal neutron detector arrangement includes at least two groups of thermal neutron detectors. 31. The fissile neutron detection system of claim 30 wherein the moderator arrangement defines first and second opposing major surfaces with a first one of the two groups of thermal neutron detectors adjacent to one of the major surfaces and a second one of the two groups of thermal neutron detectors adjacent to the other one of the major surfaces. 32. The fissile neutron detection system of claim 27 wherein said major surfaces of said thermal neutron detectors cooperatively define said moderator region therebetween having a volume and the moderator arrangement substantially fills said volume. 33. The fissile neutron detection system of claim 32 wherein the moderator arrangement fills at least 60 percent of the volume of the moderator region. 34. The fissile neutron detection system of claim 27 wherein said moderator arrangement further comprises a side moderator disposed between adjacent ones of the thermal neutron detectors in the side-by-side relationship outside of the moderator region. 35. The fissile neutron detection system of claim 34 wherein the side moderator includes a thickness dimension between the side-by-side adjacent ones of the thermal neutron detectors that is no more than 5 cm. 36. A fissile neutron detection system for detecting incident fissile neutrons, said fissile neutron detection system, comprising:an ionizing thermal neutron detector arrangement including an inner peripheral shape that at least substantially surrounds a moderator region for detecting thermal neutrons that exit the moderator region but is at least generally transparent to the incident fissile neutrons;a moderator arrangement disposed within the moderator region for converting the incident fissile neutrons in the moderator region to thermal neutrons which exit the moderator region to then enter the thermal neutron detector arrangement for detection of at least some of the thermal neutrons to produce an electrical current as a detector output with the moderator arrangement having an outer peripheral shape that is at least generally complementary to said inner peripheral shape and the moderator arrangement includes lateral extents such that any given dimension that bisects the lateral extents includes a length that is greater than any thickness of the moderator arrangement transverse to the lateral extents wherein the thermal neutron detector arrangement and the moderator arrangement are configured to define a pair of opposing major receiving directions for bidirectionally receiving the incident fissile neutrons from the opposing major receiving directions with a fundamental sensitivity that is at least generally equal from each of the major receiving directions; andan additional moderator arrangement interposed at least in a first one of the opposing major receiving directions outward of the thermal neutron detector arrangement and having a supplemental moderator thickness that further moderates at least some undetected ones of the thermal neutrons, that exit the moderator arrangement and initially pass undetected through the thermal neutron detector arrangement, to backscatter at least a portion of the undetected ones of the thermal neutrons to return to the thermal neutron detector arrangement for detection to contribute an additional electrical current to the detector output. 37. A fissile neutron detection system for detecting incident fissile neutrons, said fissile neutron detection system, comprising:an ionizing thermal neutron detector arrangement including an inner peripheral shape that at least substantially surrounds a moderator region for detecting thermal neutrons that exit the moderator region but is at least generally transparent to the incident fissile neutrons; anda moderator arrangement disposed within the moderator region for converting the incident fissile neutrons in the moderator region to thermal neutrons which exit the moderator region to then enter the thermal neutron detector arrangement for detection to produce an electrical current as a detector output with the moderator arrangement having an outer peripheral shape that is at least generally complementary to said inner peripheral shape and the moderator arrangement includes widthwise and lengthwise lateral extents such that any given dimension across the lengthwise and widthwise lateral extents includes a length that is greater than any thickness of the moderator arrangement transverse to the lateral extents. 38. A fissile neutron detection system for detecting incident fissile neutrons, said fissile neutron detection system, comprising:an ionizing thermal neutron detector arrangement including an inner peripheral shape that at least substantially surrounds a moderator region for detecting thermal neutrons that exit the moderator region but is at least generally transparent to the incident fissile neutrons; anda moderator arrangement disposed within the moderator region for converting the incident fissile neutrons in the moderator region to thermal neutrons which exit the moderator region to then enter the thermal neutron detector arrangement for detection of at least some of the thermal neutrons to produce an electrical current as a detector output and the moderator arrangement cooperates with the thermal neutron detector arrangement in a layered structural relationship to define major lateral extents defining a first major receiving direction and a second, opposing major receiving direction transverse to the major lateral extents for bidirectionally receiving the incident fissile neutrons at least from the opposing major receiving directions with a fundamental sensitivity that is at least generally equal from each of the major receiving directions; andan additional moderator arrangement interposed in a first one of the opposing major receiving directions outward of the thermal neutron detector arrangement and having a first supplemental moderator thickness along the first one of the opposing major receiving directions sufficient to produce a first modified sensitivity in the first major receiving direction that is less than the fundamental sensitivity and a second modified sensitivity in the second major receiving direction that is greater than the fundamental sensitivity to form a unidirectional fissile neutron detection system. 39. The fissile neutron detection system of claim 38 wherein the additional moderator arrangement further includes a second supplemental moderator thickness imposed in a second, opposite one of the opposing major receiving directions, said second supplemental moderator thickness configured to further moderate at least some undetected ones of the thermal neutrons, that exit the moderator arrangement and initially pass undetected through the thermal neutron detector arrangement, to backscatter at least a portion of the undetected ones of the thermal neutrons to return to the thermal neutron detector arrangement for detection to contribute an additional electrical current to the detector output.