Quick and accurate detection of the effective quality of ²⁴⁰Pu in special nuclear materials (SNM) is of great significance for preventing nuclear proliferation whilst traditional thermal/epithermal neutron multiplicity counting systems based on ³He tubes have relatively low measurement accuracy and high development costs. Fast neutron multiplicity counting (FNMC) technique based on liquid scintillation detectors has become a research hotspot. However, FNMC systems currently built are still in the experimental research stage.

This study aims to develop a passive FNMC system that can be used for practical measurement.

Firstly, a passive FNMC system was built using 32 self-developed EJ-309 liquid scintillation detectors. To improve the uniformity of neutron detection efficiency of the system, these detectors were arranged in a spherically symmetrical geometry. A self-designed and developed 32-channel high-voltage power supply and a user-friendly application software were integrated for the system. Then, the Doubles parameter fitting method and the three-parameter fast neutron multiplicity analytical method were employed to evaluate the spontaneous fission rates of the ²⁵²Cf neutron sources respectively. Finally, the performance of the system under different neutron radiation backgrounds (with Singles rates of 111.42±0.46, 73.97±0.28, and 47.27±0.29, respectively) was also investigatted.

The experimental results show that the Doubles parameter fitting method can accurately evaluate the spontaneous fission rates of ²⁵²Cf neutron sources. The relative differences of the spontaneous fission rates of the ²⁵²Cf neutron sources evaluated by using the fast neutron multiplicity analytical method are less than ±4%. The measurement results under different neutron radiation backgrounds are basically consistent with those under natural environment background (the Singles rates were 0.51±0.02).

The FNMC system developed in this study can accurately determine the spontaneous fission rate of the ²⁵²Cf neutron source (with an equivalent ²⁴⁰Pu_eff mass range of 5.37~209.62 g), and has very good anti-background interference capabilities.