Simulation of a neutron multiplicity counter and comparison to validation experiments

Neutron coincident counting is a useful tool, both to determine the nature of a neutron source and to extract parameters like the multiplicity, a-ratio and ultimately the mass of uranium or plutonium . For the latter, well-characterized detectors enable the determination in the order of several grams. The multiplicity analysis also allows determining if an unknown neutron source emits fission neutrons and thus possibly contains special nuclear material. The Ortec Fission Meter is an instrument designed for this purpose, equipped with a highly efficient moderated 3He neutron detector. In order to gain deeper understanding of the measured data and to predict the dependence of the analysis on different parameters like additional shielding, Fraunhofer INT performed a Monte-Carlo simulation of the instrument. A MCNP simulation of the source assembly and the instrument results in the arrival times of the neutrons for one single source event. Further software modules allow generating a pulse train and performing data analysis. While the count rate of the simulation and a validation experiment were in agreement, the calculated Feynman-Variance showed a significant deviation. The main cause is presumably a small fraction of double pulsing from the discriminator. The inclusion of this effect in the post-processing results in a substantial improved agreement of measured and simulated data.