Monte Carlo simulation of an accounting system for small amounts of special nuclear material using fast neutron multiplicity counting

This study explores the use of fast neutron multiplicity counting (FNMC) to estimate the effective mass of pure plutonium-240 ( 2 ⁴⁰Pu). A pixelated stilbene scintillator detector system was designed and simulated using the MCNPX-Polimi code to assess the performance of different geometric arrangements, including 4π, cubic, and ring configurations. Neutron multiplicity counting techniques were employed to detect spontaneous fission neutrons, and Monte Carlo simulations were conducted to evaluate the counting efficiency and estimate the mass of 2 ⁴⁰Pu samples ranging from 5 to 100 g. The results demonstrate that FNMC systems offer significant advantages in neutron counting due to shorter die-away times and the capability to perform accurate measurements with organic scintillator detectors. This study highlights the importance of optimizing detector geometry, as the ring-type system showed superior performance with lower relative errors, especially in configurations with smaller source-to-detector distances. The application of point models and parameter assays further validated the accuracy of mass estimates, with the two-parameter assay achieving relative errors below 1 % in most masses. This work contributes to improving nuclear material verification techniques, with potential applications in safeguards and non-destructive analysis of special nuclear materials.