Measurement of U238(n,n′γ ) cross section data and their impact on reaction models

Abstract

A better knowledge of $(n,xn)$ reaction cross sections is important for both reaction modeling and energy applications. This article focuses on inelastic scattering of neutrons off $^{238}\mathrm{U}$ for which improvements are needed to better constrain evaluations and solve inconsistencies in nuclear power reactor calculations. A new precise measurement of $(n,xn\ensuremath{\gamma})$ reaction cross sections on $^{238}\mathrm{U}$ has been performed at the GELINA (Geel Electron LINear Accelerator) neutron facility operated by EC-JRC-Geel (Belgium) with the GRAPhEME (GeRmanium array for Actinides PrEcise MEasurements) setup. The prompt $\ensuremath{\gamma}$-ray spectroscopy method coupled to time-of-flight measurements is used to extract $(n,xn\ensuremath{\gamma})$ cross section values which can be further combined to infer the total neutron inelastic scattering cross section. Cross section data for 18 $\ensuremath{\gamma}$ transitions (five never measured before) are presented and compared to the data in the literature. Emphasis is especially given to the uncertainty determination to produce partial cross section data as accurate as possible. Due to intrinsic limitations of the experimental method, the use of additional nuclear structure information coupled with theoretical modeling is required to determine the total $(n,{n}^{\ensuremath{'}})$ cross section over the whole neutron energy range. We have investigated modeling aspects of the $^{238}\mathrm{U}(n,{n}^{\ensuremath{'}}\ensuremath{\gamma})$ cross sections related to the description of compound nucleus and preequilibirum mechanisms as well as the discrete part of nuclear structure. Through comparison between experimental and calculated $(n,{n}^{\ensuremath{'}}\ensuremath{\gamma})$ cross sections, we pinpoint inaccuracies in the description of specific reaction mechanisms and challenge recently implemented models. This helps improving the whole modeling of the $(n,{n}^{\ensuremath{'}})$ reaction.