Sample-induced perturbations in neutron field

Abstract

The present work focuses on the complexity of the neutron transport phenomenon, which throws its shadows on every physical system wherever neutron is produced or used. The Monte Carlo N-Particle Transport Code (MCNP) investigated flux perturbations in the neutron field caused by an absorber. The geometry of the present experiment was designed to reach an isotropic neutron field. The neutron source was a 241AmBe with physics dependent only on alpha-beryllium interaction as a physical constraint. The neutron production rate is independent of what happens after it is generated. The geometries have been designed to get a volume of uniform neutron densities within a spherical volume of radius 15 cm in every neutron energy group up to 1 MeV, with a slight central tendency of higher energy neutrons making the field isotropic. Absorbers of different dimensions were placed within the volume to investigate the field perturbation. Two types of neutron absorbers were used, indium and gold. Flux density inside and outside the absorber samples was determined, while the spatial neutron flux distribution produced by the AmBe source was taken as a reference. This study displayed that absorbers of various dimensions perturb the neutron field in a way that is dependent on the absorption and scattering cross-sections, particularly in the neutron resonance. Unlike the simple picture of reducing the number density of neutrons, the perturbation was found to influence the moderation of neutrons in the medium, significantly above 1 MeV.