Benchmarking Monte Carlo Simulations of Thermal Neutron Kinetics and Differential Die-away Analysis

Differential Die-Away Analysis (DDAA) is a very effective technique for detection of special nuclear material (SNM). It is based on fission neutrons that are detected with a time constant characteristic of the thermal neutron that created them. The presence of fast neutrons with this slow die-away time is a positive indication of the presence of SNM in the inspected cargo. Results are presented from MCNPX simulations of pulsed neutron interrogation of hydrogenous cargo. MCNPX is first used to simulate the thermal neutron die-away times of the cargo. Next, it is used to simulate the fast-neutron die-away times of the cargo with SNM present, and the gross signal of a full DDAA measurement modeled against a set of experiments. These results are benchmarked against these experimental measurements. MCNPX is in good agreement with measurement of thermal die-away times. The differential die-away signature, which appears on the order of tens to hundreds of microseconds after a neutron pulse, can be modeled accurately for hydrogenous cargoes.