Passive neutron multiplicity device for ²⁴⁰Pu measurement based on FPGA

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. FH-NCM/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  \({\upmu }\) $\mathrm{\mu }$ s 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 ( \(\varepsilon\) $\epsilon$ ) and double gate fraction ( \(f_{\rm d}\) $f_{\mathrm{d}}$ ), resulting in corrected double rates ( \(D_{\rm c}\) $D_{\mathrm{c}}$ ), 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.