Neutron detectors for active interrogation of highly enriched uranium

We describe the results of our effort to optimize three neutron detector systems for active interrogation of highly enriched uranium: 1) a large-area detector for maximum absolute efficiency; 2) modular detectors for maximum flexibility in configuring a detector system for an application; and 3) a portable detector. All three systems contained /sup 3/He tubes, polyethylene to moderate the neutrons, and cadmium to filter out room-return thermal neutrons. The back and sides of the detectors were shielded with additional polyethylene or borated polyethylene. The electronics gated off the data acquisition during the interrogating pulse, either bremsstrahlung photons from an electron linac or neutrons from a DT generator. The sensitivity of each detector system depends on the distance between the detector and nuclear material as well as on the intervening material. We present representative data for several configurations showing the performance of each system.