Calculation of Neutron Die-Away Times in a Large-Vehicle Portal Monitor

Abstract

Monte Carlo methods have been used to calculate neutron die-away times in a large-vehicle portal monitor. These calculations were performed to investigate the adequacy of using neutron die-away time measurements to detect the clandestine movement of shielded nuclear materials. The geometry consisted of a large tunnel lined with ³He proportional counters. The time behavior of the (n,p) capture reaction in these counters was calculated when the tunnel contained a number of different tractor-trailer load configurations. Neutron die-away times obtained from weighted least-squares fits to these data were compared. The change in neutron die-away time due to the replacement of cargo in a fully loaded truck with a spherical shell containing 240 kg of borated polyethylene was calculated to be <3%. This result, together with the overall behavior of neutron die-away time versus mass inside the tunnel, strongly suggests that measurements of this type do not provide a reliable means of detecting shielded nuclear materials in a large vehicle.