Neutron multiplicity measurements are a useful technique for the characterization of special nuclear
material. This technique relies on the detection of correlated neutrons from fission events. As correlated
events are detected it is possible to determine the neutron multiplicity distribution for the sample. This
distribution is useful for identifying the material and estimating the mass. This work focuses on the
ability of the Monte Carlo code MCNP-PoliMi to simulate measured distributions. The experiment used
as the basis of comparison consisted of a 4.5 kg plutonium metal sphere surrounded by up to 6 in. of
polyethylene. A bank of 15 (3 He) detectors was used to detect the correlated neutron events. MCNP-
PoliMi was used to simulate the particle transport and a post-processing algorithm was developed to
apply detector deadtime effects and to determine the neutron multiplicity distributions. These
simulated distributions were then compared to the measured results. The simulation provided an
adequate estimation of the measured data. However, we observed a systematic over-prediction in both
the mean and the variance of the measured distribution.
Simulation of polyethylene-moderated plutonium neutron multiplicity measurements