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, the thermal neutron detector arrangement further including at least one group of thermal neutron detectors with the thermal neutron detectors of each group in a side-by-side relationship and spaced apart in the side-by-side relationship to form a gap between adjacent ones of the thermal neutron detectors;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; andat least one side moderator disposed in one of the gaps outside of the moderator region. 2. The fissile neutron detection system of claim 1 further comprising:a group of said side moderators with one of the side moderators positioned in the gap between each adjacent pair of the thermal neutron detectors outside of the moderator region. 3. The fissile neutron detection system of claim 1 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. 4. The fissile neutron detection system of claim 3 wherein the thickness dimension of the side moderator is in a range from 1 cm to 5 cm, inclusively. 5. The fissile neutron detection system of claim 1 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. 6. 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, the thermal neutron detector arrangement further including at least one group of thermal neutron detectors with the thermal neutron detectors of each group in a side-by-side relationship and spaced apart in the side-by-side relationship to form a gap between adjacent ones of the thermal neutron detectors;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; andat least one external moderator disposed outside the moderator region proximate to the ionizing thermal neutron detector arrangement. 7. The fissile neutron detection system of claim 6 wherein the external moderator extends at least partially transverse to said lateral extents. 8. The fissile neutron detection system of claim 6 wherein the ionizing thermal neutron detector arrangement is at least partially enclosed by the external moderator. 9. The fissile neutron detection system of claim 6 wherein the external moderator at least partially encapsulates at least a portion of the thermal neutron detector arrangement.