Pulsed neutron differential die away analysis for detection of nuclear materials

Differential die away analysis (DDAA) is a very sensitive technique for detecting the presence of fissile materials such as 235U and 239Pu. DDAA is based on conventional pulsed neutron source interrogation, employing typically low cost 14 MeV (d, T) generators. In DDAA, a neutron generator produces repetitive pulses of neutrons that are directed into an inspected cargo. As each pulse passes through the cargo, the neutrons are thermalized and absorbed. The population of thermal neutrons decays with the diffusion decay time of the inspected medium (the so called thermal neutron “die-away” time) on the order of hundreds of microseconds. If SNM is present, the thermalized neutrons from the source cause fissions that produce a new source of fast 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 is greatly affected by the size, geometry, density and composition of the inspected cargo. The sensitivity is also affected by the reflection of neutrons from surfaces such as the ground, nearby walls or nearby cargo; these effects were studied. The ability of DDAA to detect enriched uranium sample in dense hydrogenous cargo, such as paper and wood is shown.