Fast Neutron Multiplicity Counting for High Mass Plutonium Assay

The fast neutron multiplicity counting (FNMC) apparatus was consisted of 36 pieces of liquid scintillator detectors in three rings cylindrical layout, multi-modules of 500M14bit digitizers based on PXIe platform, and mechanical structure. Its center cavity was 35 cm in radius and 50 cm in height for assaying the plutonium item in standard container. The digitizers implemented detector’s signal acquisition, real-time digital data processing and data throughput. Each detector’s light output was calibration with Cs-137 source for nearly equal response. The apparatus detection efficiency for fast neutron calibrated with Cf-252 source was about 3.2%. The fast neutron multiplicity counting analysis was verified with Cf-252 neutron source, then experiments were performed with a serial of different high mass plutonium metals. Two-parameter calibration procedure was used for mass resolving. The mass assay results were consistent with the sample nominal values within maximum bias of 6%. This research achievement demonstrated a promising application for nuclear material assay and verification.