The thick-target integrated-over-angle (α,n) yield curve for U_3O_8 over the energy range from 1.5 to 10 MeV and associated specific (α,n) yields of the uranium isotopes

摘要

The high-accuracy (α,n) yield measurements on UO_2 reported in 1982 by West and Sherwood [UKAEA report AERE-R-10501(1982)] are used to derive the yield curve for U_3O_8 needed for certain safeguards applications. By combining the experimental results for UO_2 of West and Sherwood, which cover the energy range from about 3.8 MeV to approximately 10 MeV, with the early calculations of Liskien and Paulsen [Atomkernenergie (ATKE) 30(1)(1977)59-61], we obtain a curve for UO_2 that extends from 1.5 to 10 MeV. This result is then used to calculate the U_3O_8 yield curve according to the continuous-slowing-down approximation. West and Sherwood used two UO_2 targets, and in blending the data, we get some verification of the experimental accuracy. They used cubic splines with cubic end-conditions to place their experimental data onto a regular grid. We find that this approach introduces excessive curvature at the lowest energies, so we prefer parabolic end-conditions. Particular attention is given to estimating the uncertainty introduced by using different choices of mass-stopping powers, which are not well known for heavy metal oxides. From the yield curve we derive specific (α,n) yields for the common uranium nuclides by overlaying the specific α-decay spectra. We conclude that new thick-target yield measurements over a broader energy range, 1.5 to 10 MeV, are needed for both UO_2 and U_3O_8 to adequately validate both the low energy range for UO_2 and the UO_2 to U_3O_8 scaling rules that depend on uncertain stopping-power data. Yields for different oxygen compounds and from oxides of transuranic elements would be especially interesting to measure.