R&D of Passive Fast Neutron Multiplicity Counting System

A passive fast neutron multiplicity counting system has been developed in the School of Nuclear Science and Engineering, North China Electrical Power University, Beijing, China. The system mainly consists of 32 custom-made EJ-309 liquid scintillation detectors and a custom-made multichannel readout electronics. These detectors are symmetrically mounted on a patented spherical supporting structure. The simulation results show that the detection efficiency of the system is about 12% for 252Cf neutrons at an energy threshold of 100 keVee and not affected by the size and shape of the samples if their location and distribution within a radius of 10 cm in the system which is due to the symmetrical arrangement of these detectors. The experimental results further show that the system has a uniform detection efficiency of ~11% for 252Cf neutrons at an energy threshold of 100 keVee with a relative efficiency deviation of less than 5% as samples are located or distributed within a radius of 6 cm in the system. The readout electronics integrates two combined neutron-gamma discrimination circuits for the realization of pulse-shape and pulse-width discrimination algorithms and a FPGA(Field Programmable Gate Array)-based data acquisition system for picking out all neutron events and registering their global arriving time. By combining pulse-shape and pulse-width discrimination methods, the detection system can achieve a neutron-gamma discrimination performance with FOM>1 even at a lower energy threshold of 100 keVee. The spontaneous rates of seven different 252Cf sources measured with our system are within less than 5% of the results using a Plutonium Scrap Multiplicity Counter (PSMC). The system is going to be tested with Plutonium samples and the corresponding results will be reported on the conference.