Improving FNMC for the matrix effect of spherical shell plutonium samples

The fissile mass deduced from fast neutron multiplicity counting (FNMC) measurement is underestimated if the matrix self-absorption effect of the radioactive source is not taken into account. Based on the analysis of FNMC equations, a set of FNMC system was built to simulate and study the mass attribute of the hollow sphere (spherical shell) plutonium under different shapes and different masses conditions. Geant4 simulation shows that an appropriate parameter correction successfully removes the bias because of the matrix effect. Consequently, the self-multiplication factor, alpha coefficient and scattering crosstalk of the simulation result were corrected after analyzing the detection efficiency and multiplicity counting rate, and the corresponding polynomial fitting equation was obtained. The corrected mass deviation of samples was less than +/- 1% in this interval. The results show that the combination of the FNMC and parameter correction can accurately measure the sample mass attribute, which provides a new method for solving similar problems.