Neutron multiplicity counting for mass determination of nuclear materials in large-volume containers

Abstract Accurate mass determination of nuclear materials in large-volume containers is hindered by geometric constraints that preclude conventional 4 π detector encapsulation. This study optimizes the design of a semi-wrapped neutron multiplicity counter tailored for kilogram-scale material assay under restricted access conditions. Based on Geant4 Monte-Carlo simulations, a detection system comprising 60 3 He proportional tubes was optimized. Detector pressure, active length, and moderator assembly were systematically evaluated to maximize the figure of merit (FOM). System performance was assessed using material samples ranging from 1000 g to 5000 g. The optimized detector achieved a detection efficiency of 12.85% and a die-away time of 35 μs, yielding a FOM of 2.17. Mass inversion results exhibited stable relative errors ranging 4.76% to 5.60% across the entire mass range. This work confirms that a semi-wrapped configuration can achieve performance metrics comparable to those of traditional $\\mathrm{4\\pi }$ 4 π systems. The proposed methodology offers a viable solution for passive neutron multiplicity counting in geometrically constrained storage and reprocessing scenarios.