Thick-Target Integrated Over-Angle (a,n) Yield Curve for U3O8 Over the Energy Range from 1.5 to 10 MeV and Associated Specific (a,n) Yields of the Uranium Isotopes.

摘要

The high-accuracy (a,n) yield measurements on UO2 reported in 1982 by West and Sherwood (UKAEA report AERE-R-10501(1982)) are used to derive the yield curve for U3O8 needed for certain safeguards applications. By combining the experimental results for UO2 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)5961), we obtain a curve for UO2 that extends from 1.5 to 10 MeV. This result is then used to calculate the U3O8 yield curve according to the continuous-slowing-down approximation. West and Sherwood used two UO2 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 (a,n) yields for the common uranium nuclides by overlaying the specific a-decay spectra. We conclude that new thick-target yield measurements over a broader energy range, 1.5 to 10 MeV, are needed for both UO2 and U3O8 to adequately validate both the low energy range for UO2 and the UO2 to U3O8 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.