Abstract
Advanced modelling of neutron induced reactions on the 238 U nucleus is aimed at improving our knowledge of neutron scattering. Capture and fission channels are well constrained by available experimental data and neutron standard evaluation. A focus of this contribution is on elastic and inelastic scattering cross sections. The employed nuclear reaction model includes - a new rotational-vibrational dispersive optical model potential coupling the low-lying collective bands of vibrational character observed in even-even actinides; - the Engelbrecht-Weidenmuller transformation allowing for inclu- sion of compound-direct interference effects; - and a multi-humped fission barrier with absorption in the secondary well described within the optical model for fission. Impact of the advanced modelling on elastic and inelastic scattering cross sections including an- gular distributions and emission spectra is assessed both by comparison with selected microscopic experimental data and integral criticality benchmarks including measured reaction rates (e.g. JEMIMA, FLAPTOP and BIG TEN). Benchmark calculations pro- vided feedback to improve the reaction modelling. Improvement of existing libraries will be discussed.
Highlights
While neutron scattering on the 238U ground-state rotational band below 600 keV is well described by a rigid rotor model, above 600 keV several vibrational bands are observed (e.g. see estimated scattering cross sections on 1− (680 keV) and 3− (732 keV) states of the octupole vibrational band in Fig.1(b) compared with available experimental data from the EXFOR database [8])
We have identified that the calculated inelastic scattering cross sections depend on the compound nucleus decay and on the optical model potential that defines the direct contribution
Interference between the direct and compound reactions predicted by Moldauer is shown to increase the calculated inelastic cross sections [34]; the cross-section increase is significant in the energy range from 100 to 500 keV
Summary
While neutron scattering on the 238U ground-state rotational band below 600 keV is well described by a rigid rotor model, above 600 keV several vibrational bands are observed (e.g. see estimated scattering cross sections on 1− (680 keV) and 3− (732 keV) states of the octupole vibrational band in Fig.1(b) compared with available experimental data from the EXFOR database [8]) Those vibrational bands need to be considered in the coupled-channel description as initially suggested by the University of Lowell group [9,10,11], and later used in evaluations by Kawano et al [12], Maslov et al [13, 14] and López Jimenez et al [15]. An updated 238U evaluated file in the fast neutron region will be validated vs integral criticality benchmarks from the ICBESP Handbook [18] aiming at reducing uncertainties in input model parameters
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