Monte Carlo simulation of the passive neutron multiplicity counting system

The neutron multiplicity counting (NMC) technique is a reliable method for quantifying special nuclear material (SNM). Simulations of these NMC instruments using the Monte Carlo codes can aid in improving and redesigning these instruments. The Geant4 toolkit can provide multiplicity distribution information for spontaneous fission events and can be used to determine the mass of plutonium material. This study constructed a complete passive neutron multiplicity counting (PNMC) system model to verify this capability for the Geant4. The performance parameters extracted from the simulation were compared to the experiments performed using a plutonium scrap multiplicity counter (PSMC) at the State Nuclear Security Technology Center (SNSTC). The comparison results indicated the validated accuracy and reliability of the Geant4 toolkit in PNMC studies, despite slight deviations from simulation results. The difference in moment distribution between the R+A- and A-gate exerts a strong influence on the multiplicity counting rate, particularly to double and triple neutron events. Therefore, variations in moment distributions resulting from source and counter modeling may be major contributing factors to bias in the results. Furthermore, variations in the SNM geometry will result in different self-shielding and multiplication effects, while different positions of the SNM will change the detection efficiency of the counter. However, these effects will ultimately manifest as a change in the multiplicity count rate.