Effects of the Two-Fission-Neutron Distributions in Reactivity Measurements with the ²⁵²Cf Source

Abstract

Stochastic descriptors based on the first and second moments of the probability distribution of the neutrons in phase space $\bar{\mu }{\mkern 1mu} {\mkern 1mu} = {\mkern 1mu} {\mkern 1mu} \left( {\bar{r},{\mkern 1mu} \bar{v}} \right)$ are extensively used in the nuclear industry for the non-intrusive measurement of nuclear parameters. One of these methods is based on the ²⁵²Cf source which has been used in an unprecedented range of situations and reactivities; the interpretation of this method is sensitive to the distribution of two neutrons coming from the same fission, $\gamma \left( {\bar{v},{\mkern 1mu} \bar{v}'} \right)$. The few direct measurements of this n − n correlation have shown that γ depends on $\cos \theta '' = \bar{v}.\bar{v}'|vv'$ and the neutron energies, the available experimental data was used to quantify the sensitivity of the ²⁵²Cf method to the anisotropies of the n − n correlation. It was found that it affects the interpretation of the reactivity measurement for low values of the multiplication constant.