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, with absorption included. 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 are 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, gamma photons or combinations thereof, for the analysis of a specific fissile sample. It is seen that for larger mass samples photon absorption in the sample strongly decreases the multiplicity of emitted photons, whereas this is not the case for neutrons. The model correctly accounts for this strong self-shielding effect and confirm previous observations 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.