Absolute measurement of the Pu 242 neutron-capture cross section

The absolute neutron-capture cross section of Pu242 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 Pu242(n,γ) cross section was made over the incident neutron energy range from thermal to ≈6 keV, and the absolute scale of the (n,γ) cross section was set according to the known Pu239(n,f) resonance at En,R=7.83 eV. This was accomplished by adding a small quantity of Pu239 to the Pu242 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 ≈40 keV. Our data, in general, are in agreement with previous measurements and those reported in ENDF/B-VII.1; the Pu242(n,γ) cross section at the En,R=2.68 eV resonance is within 2.4% of the evaluated value. However, discrepancies exist at higher energies; our data are ≈30% lower than the evaluated data at En≈1 keV and are approximately 2σ away from the previous measurement at En≈20 keV.