Absolute Partial γ-ray Cross Sections in 238U(n, xny) Reactions

Abstract

Measurements of discrete γ-ray spectra have been carried out as a function of incident-neutron energy for nuclei populated in 238U(n, xnγ (x ≤ 4) reactions. The GEANIE spectrometer, comprised of 26 Ge detectors (11 planar and 15 coaxial), was used to detect γ-rays. Neutrons were provided by the “white” neutron source of the Los Alamos Neutron Science Center’s WNR facility. The energy of the incident neutrons was determined using the time-of-flight technique. Absolute cross sections were determined for emission of several γ-rays from low-lying states of 235-238U isotopes (spin up to 10ħ and excitation energy up to ~1 MeV) as a function of incident-neutron energy (1 MeV < En 100 MeV). Uncertainties associated with the spectroscopic analysis of the data are discussed. Predictions of partial γ-ray cross sections from GNASH calculations up to neutron energy En = 30 MeV are compared to the experimental results and are generally in good agreement. Combining the experimental results with the predictions of the nuclear reaction modeling provides a measurement of the 238U(n, n′) reaction cross section and validate this technique for determining reaction cross sections using γ-ray spectroscopy.