Passive Neutron Multiplicity Device for 240Pu Measurement Based on FPGA

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