Partial γ-Ray Cross Sections for the Reaction 239Pu(n,2nγi) and the 239Pu(n,2n) Cross Section

Abstract

Absolute partial γ-ray cross sections for discrete γ-ray production in the 239Pu(n,2nγi)238Pu reaction have been measured. The experiments were performed at LANSCE/WNR on the 60R neutron flight path. Reaction γ-rays were measured using the large-scale Compton-suppressed array of Ge detectors, GEANIE. The motivation for this experiment, an overview of the partial γ-ray cross-section measurement, and an introduction to the main experimental issues will be presented. The energy resolution of the Ge detectors allowed identification of reaction γ rays above the background of sample radioactivity and fission γ rays. The use of planar Ge detectors with their reduced sensitivity to neutron interactions and improved line shape was also important to the success of this experiment. Absolute partial γ-ray cross sections are presented for the 61+ → 41+ member of the ground state rotational band in 238Pu, together with miscellaneous other γ-ray partial cross sections. The n,2n reaction cross section shape and magnitude as a function of neutron energy were extracted from these partial cross sections using nuclear modeling (enhanced Hauser-Feshbach) to relate partial γ-ray cross sections to the n,2n cross section. The critical nuclear modeling issue is the ratio of a partial cross section to the reaction channel cross section, and not the prediction of the absolute magnitude.