Neutron-neutron angular and energy-angle correlations of plutonium samples with varying α-ratio

Abstract

Abstract Accurate and timely characterization of physical properties pertinent to plutonium bearing materials is important for fulfilling nuclear nonproliferation and safeguards goals. Physical properties include the fissile mass, leakage multiplication, and the α -ratio, defined as the ratio of ( α ,n) neutrons to spontaneous fission neutrons. Traditionally, these properties can be inferred by relating the measured neutron multiplicity count rates to the well-established point kinetics moments equations; the current state-of-the-art utilizes 3 He-based detection systems. Organic scintillators have been used extensively to study and measure characteristic signatures in the neutron angular and energy distributions. Previous work has proposed techniques that independently leverage the energy and angle sensitivity of organic scintillators to estimate the α -ratio of plutonium bearing material; however, it is expected that the energy and angular distributions are correlated to one another due to the underlying physics of fission and ( α ,n) neutron emissions. This work presents experimental results that characterize neutron-neutron angular distribution and subsequently the neutron-neutron energy-angle correlations for plutonium samples of similar mass and multiplication, but varying α -ratio due to the type of low-Z impurity. Full neutron-neutron angular distributions are presented using a low-energy detection threshold of 0.10, 0.15, and 0.20 MeVee (0.73, 0.96, 1.16 MeV neutron-equivalent energy). Neutron anisotropy was quantified by taking the ratio of neutron-neutron coincidences at 180°to those at 90°, where a value of unity indicates a purely isotropic source. The results show that the observed neutron-neutron correlations transition away from fission-induced signal to the cross-talk signal associated with single ( α ,n) neutrons with increasing α -ratio. Energy-angle correlations are characterized by calculating the neutron anisotropy at various detection thresholds and show positive correlation between the observed anisotropy and the energy of the neutrons.