Investigations of active interrogation techniques to detect special nuclear material in maritime environments: Standoff interrogation of small- and medium-sized cargo ships

In this work, several active interrogation (AI) sources are evaluated to determine their usefulness in detecting the presence of special nuclear material (SNM) in fishing trawlers, small cargo transport ships, and luxury yachts at large standoff distances from the AI source and detector. This evaluation is performed via computational analysis applying Monte Carlo methods with advanced variance reduction techniques. The goal is to determine the AI source strength required to detect the presence of SNM. The general conclusion of this study is that AI is not reliable when SNM is heavily shielded and not tightly coupled geometrically with the source and detector, to the point that AI should not be considered a via interrogation option in these scenarios. More specifically, when SNM is shielded by hydrogenous material large AI source strengths are required if detection is based on neutrons, which is not surprising. However, if the SNM is shielded by high-Z material the required AI source strengths are not significantly different if detection is based on neutrons or photons, which is somewhat surprising. Furthermore, some of the required AI source strengths that were calculated are very large. These results coupled with the realities of two ships moving independently at sea and other assumptions made during this analysis make the use of standoff AI in the maritime environment impractical.