Location of Thermal Neutron Sources Using Narrow-Field-of-View Detectors

Conventional wisdom has assumed that the location of sources of thermal neutrons in confined spaces is difficult because of the multiple scattering the neutrons undergo before reaching the detector. Monte Carlo calculations and laboratory tests have demonstrated that a useful fraction of the thermal neutrons leaving a specific source will travel with no scattering and be detected by a narrow-field-ofview detector. We demonstrate that a highly directional neutron detector made from unmoderated 3He gas proportional counters can locate a thermal neutron source at distances up to 24 m. The effects of room scattering and interference from moderators away from the source are minimal. Applications for narrow-field-of-view thermal neutron detectors include the monitoring of warheads on delivery vehicles or monitoring storage vaults containing special nuclear materials.