Preliminary simulations in the use of fast neutrons to detect explosives hidden in cargo containers

Fast neutrons have the potential to be a powerful tool in explosives detection. Methods using elastically scattered neutrons would likely require fewer incident neutrons than those that depend on neutron absorption or inelastic scatter due to an increased interaction probability. This reduction would dramatically reduce dose rates and induced target radioactivity. In this study, a series of Monte Carlo (MCNP5) simulations were completed in order to investigate the use of elastically scattered neutrons in explosives detection, specifically explosives hidden inside standard-sized cargo containers. Fast neutrons were simulated with energies of 2.4 MeV, 14.1 MeV, or equivalent to that of a Cf-252 neutron source. Neutrons and gamma rays were tallied in seven idealized detectors arranged around a cargo container. Initial simulations characterized neutron scattering behavior in several different materials, including water, oil, steel, air, soil, and the explosive, RDX (C₂H₆N₆O₆). Further simulations dealt with a theoretical scenario in which a 1.87 m diameter solid sphere of RDX was placed inside a cargo container, shielded with water, oil, or steel, and probed using 14.1 MeV neutrons. Next, the presence of the ground was incorporated into this scenario. Several strategies were then used to determine possible flags that could determine the presence of the RDX. One strategy consisted of searching for changes in the backscattered neutron tallies due to changes in hydrogen concentration when RDX was present. A second strategy involved investigating the changes in scattered neutron flux at specific energies as a function of angle. Other potential flags were found by comparing the ratios of elastic and inelastic peaks in the neutron spectra at seven different angles. Preliminary analysis using this method showed several promising possibilities for detecting hidden explosives, such as an almost 20 fold decrease in the ratio of 9.8 MeV - and 9.2 MeV neutrons at 60deg when RDX is present in an isolated oil-filled cargo container.