AN ACTIVE SYSTEM FOR THE DETECTION OF SPECIAL FISSILE MATERIAL IN SMALL WATERCRAFT

An MCNP calculation is presented to examine the feasibility of using active neutron interrogation to detect the presence of special fissile material onboard recreational watercraft. The calculation simulates using the differential die-away technique for detecting delayed fission neutrons via recoil protons resulting from the elastic scattering of neutrons with hydrogen nuclei. To achieve this, time and energy bins are used in conjunction with a tally-multiplier card. The time bins are used to be able to discern between source and fission neutrons. Energy bins are used because in practical applications, only neutrons with at least 0.1 keV are realistically detectable. Lastly, an elastic scattering cross section multiplier card is used to recreate the probability that a neutron will elastically scatter with a hydrogen nucleus, thus creating a recoil proton. To recreate the physical environment, a model consisting of a simplified boat design floating in water, a neutron source, and a detector are created within MCNP. A 14 MeV neutron source represents the neutrons yielded from the deuterium-tritium reaction. Such sources are commonly used in pulsed neutron generators, which are useful for the detection of fissile materials via counting delayed fission neutrons. A comparison of tallies for cases with no fissile material and those with fissile material present over similar time spans shows that after a certain period, any detected neutrons indicate the presence of fissile material.