Inelastic neutron scattering on U 238 with γ -ray coincidence spectroscopy

Le résumé fourni par la source

Improvement of the accuracy of neutronics simulation of actual or future reactor designs is essential and requires a better knowledge of the core neutron population. The reactor neutron population is partially driven by (n,xn) reactions that change the number and decrease the energy of the neutrons. However, the cross sections of these reactions are currently not measured with sufficient precision. This is particularly the case for the neutron inelastic-scattering cross section of U238, which represents around 90%–95% of the mass of fuel in present and many future reactor core designs. It therefore features on the High Priority Request List for measurement improvement. This cross section can be measured using prompt γ-ray spectroscopy coupled to neutron time-of-flight measurements, where the total (n,n′) cross section can be deduced from a combination of the measured (n,n′γ) partial cross sections and the level scheme information. The knowledge of the U238 level scheme is still very incomplete, inducing significant uncertainties on the resulting inelastic-scattering cross section. Therefore, an effort has recently been carried out to experimentally revisit the U238 decay scheme by performing high-resolution coincidence spectroscopy of the U238(n,n′) reaction. To obtain detailed level scheme information, the ν-Ball γ spectrometer was coupled to the LICORNE directional neutron source of the ALTO facility, allowing the study of inelastic scattering of fast neutrons (0.5–3 MeV) on U238 via γ-γ coincidence spectroscopy. Coincidence matrices obtained during the first and the second ν-Ball campaigns were analyzed using the RadWare ESCL8R software. 110 γ transitions and 60 levels registered in the Evaluated Nuclear Structure Data File database have been confirmed, and 158 new γ transitions and 58 new levels have been found. Although not exhaustive, the U238 level scheme information has been significantly improved, and its completeness has been estimated to be up to 1.7214 MeV compared with 1.27854 MeV at present. It is hence expected to decrease the uncertainties of U238 (n,n′) cross sections, which is needed for accurate reactor core modeling.

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