Thermal Neutron Die-Away Studies in a 14 MeV Neutron-Based Active Interrogation System

The ability to classify cargos is a critical step in developing cargo dependent algorithms to improve the detection of threats like explosive or nuclear materials concealed in cargo. In a conventional pulsed neutron based active interrogation system, fast neutrons (e.g., 14 MeV) are thermalized as they travel through a moderating cargo. Once they are thermalized, they can be absorbed by the surrounding cargo. The rate at which they are absorbed (the inverse of the thermal neutron die-away) depends on the composition of the moderating cargo, especially for large cargo (e.g., pallet size). If an assessment of thermal neutron die-away times in cargos can be done externally, improved threat (e.g., explosives or SNM) detection can be achieved via different algorithms for different types of cargos. This paper shows that this can be done by measuring the time dependence of capture gamma ray production in the cargo.