Corrections on Point Model Equations in Spherical and Hemispherical Shell Weapons-Grade Plutonium Mass Assay with Neutron Multiplicity Counting

In future disarmament verification regime, china may directly or indirectly participate in the disarmament activities in many fields.An original designed cavity neutron counter, including neutron detecting chamber, integrated amplifier circuit, data acquisition system, data analysis software, has been being developed in new pile laboratory of China Academy of Engineering Physics. After this neutron counter being used to assay the weapons-grade plutonium metal spherical shell and hemispherical shell samples with neutron multiplicity analysis, a large assay mass bias has been observed. To solve this significant assay mass bias in these plutonium samples, the weighted point model neutron multiplicity equations were adopted. Doubles and triples factorial counting rates correction factors wD, waD, wT and waT for spontaneous fission neutrons and (?, n) neutrons, were used in the point model equations. The quantitative relationship between factorial correction factors, wD, waD, wT and waT, with samples mass and leakage multiplication were derived with Monte Carlo calculations, then the corrected equations were solved by iteration. The calculation results show that: compared with point model assay, the corrected point model decreases the average assay mass bias from -4.70% to -0.70%, and average assay alpha bias from 11.45% to 2.31%.