Neutron multiplicity assay device and experiment progress on Pu-U components

To verify nuclear components dismantled from a nuclear warhead or stored in a nuclear facility, a neutron multiplicity counter with 32 3He tubes and high density polythene cavity was built. Eight channel circuits were used to process the signals from 3He tubes. Five 252Cf neutron sources were used to calibrate parameters of the counter. To improve the mass assay accuracy, analysis algorithm for weighted point model and double Am-Li neutron sources neutron multiplicity counting were studied. The neutron multiplicity distribution for six weapon-grade plutonium components and seven highly enriched uranium components were measured with the neutron multiplicity counter, assay results show that the mass assay bias is below 1.3% for plutonium components with the weighted neutron multiplicity model, while the value is below 5% for uranium components with calibration curve method.