SIMULATIONS FOR DEVELOPING A FLAG-BASED ACTIVE NEUTRON INTERROGATION METHOD FOR EXPLOSIVES DETECTION IN SEA-LAND CARGO CONTAINERS

Fast neutron interrogation for explosives detection has shown potential for the screening of sea-land cargo containers. Simulations were completed investigating the neutron scatter behavior of 14.1-MeV fast neutrons in such screening scenarios. Earlier efforts centered on Monte Carlo (MCNP5) simulations to identify flags or on specific calculations based on photons or neutrons produced as a result of fast neutron interaction that signal the presence of the explosive RDX (C2H6N6O6). Those simulations consisted of simplified target geometry; artificially collimated neutron source; and generalized organic, hydrogenous, or metallic types of cargo materials. In this study, the MCNP5 simulation was expanded to include a more accurate representation of the neutron source, target geometry, detector response, and realistic and varied container contents. The flags found using the earlier simulations were applied to the more realistic scenario models in order to determine the feasibility of the use of flags in a detection algorithm. Additional flags utilizing the simulated detector response were also investigated. The conditions under which specific flags were preferable were also examined. It was found that many flags performed well independent of the cargo type while others, such as those using only neutron backscatter, were more highly dependent on cargo type. Furthermore, many of the best-performing flags were those that did not require stringent neutron spectroscopy and would therefore be feasible with existing technology.