The Detection Statistics of Neutrons and Photons Emitted from a Fissile Sample

The purpose of this paper is to present new analytical derivations to describe the emission and detection statistics of neutrons and photons generated in and emitted from fissile samples. The results of the analytical approach are compared with and validated by corresponding Monte Carlo simulations. The joint statistics of the generated and detected neutrons and photons is also described. The analytical model described in this paper accounts for absorption and detection, thus extending the model presented in previous studies. By using this new, improved model, one can investigate the relative feasibilities of measuring neutrons or gamma photons for the analysis of a specific fissile sample. In fact, larger mass will lead to larger self-shielding for gamma photons, whereas for neutrons a larger mass will lead to increased multiplicities due to an increased probability to induce fission for each neutron, with absorption playing a minor role. The results suggest that although photons have a larger initial (source) multiplication, neutrons might be more favourable to measure in the case of large samples because of the increasing self-shielding effect for gamma photons.