Detection of 235U in hydrogenous cargo with Differential Die-Away Analysis and optimized neutron detectors

Differential Die-Away Analysis (DDAA) is a sensitive technique for detecting the presence of fissile materials such as 235U and 239Pu. In DDAA, a neutron generator produces repetitive pulses of neutrons that are directed into a cargo being inspected. As each pulse passes through the cargo, the neutrons are thermalized and absorbed. The thermalization process is very rapid and the population of the source and epithermal neutrons decays away within microseconds. The population of thermal neutrons, however, decays much slower with the diffusion decay time of the inspected medium (thermal neutron “die away” time), on the order of hundreds of microseconds. If Special Nuclear Material (SNM) is present, the thermalized neutrons from the source will cause fissions that produce a new source of neutrons. These fast fission neutrons decay with a time very similar to that of the thermal neutron die-away of the surrounding cargo. The sensitivity of DDAA for a given source of neutrons critically depends on optimizing the fast/epithermal neutron detection system. Optimization involves both time response and detection efficiency. Optimized detectors were able to detect fissile material throughout pallets of cargo.