Prompt fission neutron anisotropy in low-multiplying subcritical plutonium metal assemblies

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Fission neutron anisotropy, due to the kinematics of the fission process, has been studied for non-multiplying sources and highly multiplying subcritical plutonium metal assemblies (i.e. relatively long fission chains). The studies on highly multiplying assemblies show that the observed neutron–neutron angular distribution appear isotropic, while the studies on non-multiplying sources show that the neutron–neutronangular distribution appear anisotropic. No measured data exists, however, that investigates the dependence of neutron anisotropy on multiplication for low-multiplying assemblies. We have experimentally characterized the dependence of fission neutron anisotropy on multiplication for low-multiplying plutonium metal assemblies. Here, an array of 16 organic scintillators was used to measure plutonium metal assemblies (95% 239Pu, by mass) exhibiting a leakage multiplication of 1.0722(3) to 1.6006(4). Full neutron–neutron angular distributions were measured, and the fission neutron anisotropy was quantified with the ratio of neutron–neutron coincidences observed at 180°and 90°. The results show that the neutron–neutron angular distribution becomes more isotropic as the multiplication increases. Additionally, energy–angle correlations were also characterized showing that the angular distributions are more anisotropic when observing neutrons of higher energy.