Low-background detection of fission neutrons produced by pulsed neutron interrogation

Measurements designed to detect shielded Special Nuclear Materials (SNM) have been carried out using a pulsed 8.5-MeV neutron source. Fission-neutron counts were detected as a function of time in the intervals between 100-μs neutron bursts at burst frequencies of 500, 1000, and 2000 Hz. The pulse timing sequences were chosen to optimize detection of fission neutrons produced by thermal-neutron-induced fission in the SNM. Fission neutrons were detected directly as proton, carbon, and silicon recoils in silicon carbide (SiC) semiconductor fast neutron detectors. SiC detectors recorded neutron counts during and immediately following the source neutron bursts, allowing detection of fission neutrons with short (120 μs) die-away times. The SiC detectors demonstrated excellent background discrimination with more than 2000 neutron counts observed in time intervals where zero background counts were detected.