Absolute measurement of thePu242neutron-capture cross section

Abstract

The absolute neutron-capture cross section of $^{242}\mathrm{Pu}$ was measured at the Los Alamos Neutron Science Center using the Detector for Advanced Neutron-Capture Experiments array along with a compact parallel-plate avalanche counter for fission-fragment detection. The first direct measurement of the $^{242}\mathrm{Pu}(n,\ensuremath{\gamma})$ cross section was made over the incident neutron energy range from thermal to $\ensuremath{\approx}6$ keV, and the absolute scale of the $(n,\ensuremath{\gamma})$ cross section was set according to the known $^{239}\mathrm{Pu}(n,f)$ resonance at ${E}_{n,R}=7.83$ eV. This was accomplished by adding a small quantity of $^{239}\mathrm{Pu}$ to the $^{242}\mathrm{Pu}$ sample. The relative scale of the cross section, with a range of four orders of magnitude, was determined for incident neutron energies from thermal to $\ensuremath{\approx}40$ keV. Our data, in general, are in agreement with previous measurements and those reported in ENDF/B-VII.1; the $^{242}\mathrm{Pu}(n,\ensuremath{\gamma})$ cross section at the ${E}_{n,R}=2.68$ eV resonance is within $2.4%$ of the evaluated value. However, discrepancies exist at higher energies; our data are $\ensuremath{\approx}30%$ lower than the evaluated data at ${E}_{n}\ensuremath{\approx}1$ keV and are approximately $2\ensuremath{\sigma}$ away from the previous measurement at ${E}_{n}\ensuremath{\approx}20$ keV.