A fissile neutron detection system (200"; 200′′′; 1100G; 1100H; 11001; 1100N; 1100R), for detecting incident fissile neutrons, said fissile neutron detection system, comprising: an ionizing thermal neutron detector arrangement (102A, 102B; 105) 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; and 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 (102A, 102B) 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 (102A, 102B) 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 and the fissile neutron detection system further comprising: an additional moderator arrangement (150B, 150C) interposed in at least a first one of the opposing major receiving directions outward of the thermal neutron detector arrangement (102A, 102B); the system being characterized in that said additional moderator arrangement (150B, 150C) comprises 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. an ionizing thermal neutron detector arrangement (102A, 102B; 105) 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; and 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 (102A, 102B) 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 (102A, 102B) 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 and the fissile neutron detection system further comprising: an additional moderator arrangement (150B, 150C) interposed in at least a first one of the opposing major receiving directions outward of the thermal neutron detector arrangement (102A, 102B); the system being characterized in that said additional moderator arrangement (150B, 150C) comprises 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. an additional moderator arrangement (150B, 150C) interposed in at least a first one of the opposing major receiving directions outward of the thermal neutron detector arrangement (102A, 102B); the system being characterized in that said additional moderator arrangement (150B, 150C) comprises 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.