Passive Neutron Multiplicity Device for 240Pu Measurement Based on FPGA

passive neutron multiplicity measurement device, FH-NCM/S1, based on field-programmable gate arrays<br />(FPGAs), is developed specifically for measuring the mass of plutonium-240 (240Pu) in mixed oxide fuel. FHNCM/<br />S1 adopts an integrated approach, combining the shift-register analysis mode with the pulse-position<br />timestamp mode using an FPGA. The optimal effective length of the 3He neutron detector was determined to<br />be 30 cm, and the thickness of the graphite reflector was ascertained to be 15 cm through MCNP simulations.<br />After fabricating the device, calibration measurements were performed using a 252Cf neutron source; a detection<br />efficiency of 43.07% and detector die-away time of 55.79 μs were observed. Nine samples of plutonium<br />oxide were measured under identical conditions using the FH-NCM/S1 in shift-register analysis mode and a<br />plutonium-waste multiplicity counter. The obtained double rates underwent corrections for detection efficiency<br />(ε) and double gate fraction (f d), resulting in corrected double rates (Dc), which were used to validate the accuracy<br />of the shift-register analysis mode. Furthermore, the device exhibited fluctuations in the measurement<br />results, and within a single 20-s measurement, these fluctuations remained below 10%. After 30 cycles, the<br />relative error in the mass of 240Pu was less than 5%. Finally, correlation calculations confirmed the robust<br />consistency of both measurement modes. This study holds specific significance for the subsequent design and<br />development of neutron multiplicity devices.