High-fidelity passive neutron multiplicity measurements and simulations of uranium oxide

When measuring low-self-multiplication uranium samples by passive neutron multiplicity counting, a technique not commonly used on uranium, there should be a decrease in the doubles counts with increasing enrichment. The reason for this decrease is explained as being due to the reduction in 238U mass, which has a large specific spontaneous fission rate in comparison to the other naturally occurring uranium isotopes. However, this is an over simplification of the actual nuclear physics which is occurring in bulk oxide items. The neutron multiplicity production rates are affected by many factors, such as the items self-multiplication, uranium isotopic composition which controls the spontaneous fission to (α,n) neutron production ratio, and the energy distribution of the emitted neutrons which includes induced fission neutrons. This paper presents a high-fidelity analysis of the passive neutron multiplicity distributions for varying enrichments of known uranium oxide standards. This analysis was performed through physical measurements of standards using the Los Alamos Epithermal Neutron Multiplicity Counter (ENMC) and detailed Monte Carlo N Particle Extended (MCNPX) neutron transport simulations supplemented with best available source term data. The MCNPX simulations reproduced the generic trends observed experimentally but absolute agreement is modest, especially for the triples counting. This suggests improvements in basic nuclear data are needed.