A correction method for point model equations in neutron multiplicity assay of spherical shell plutonium samples

To decrease the significant assay mass bias of spherical shell plutonium samples in nuclear disarmament verification and increase the confidence of verification results, the failure of assumptions in point model equations was studied. Doubles factorial and triples factorial correction factors for spontaneous fission neutrons and oxide generated neutrons were then introduced into the point model equations. The relationships between the correction factors and the sample mass and multiplication were derived with Monte Carlo calculation, and the corrected equations were solved by an iteration process. The calculation results show that: compared with the point model assay, the corrected point model assay decreases the average assay mass bias from -4.70% to -0.70%, and average assay alpha bias from 11.45% to 2.31%.