Evaluation of neutron inelastic scattering cross sections of uranium-238 from neutron spectrum measurements in fast media

Systematic discrepancies between experimental and calculated neutron spectra in different fast-neutron media with high $sup 238$U content were observed, allowing the uncertainties in certain $sup 238$U neutron cross sections to be considered as the source of such discrepancies. Sensitivity studies, as well as the effect of the former uncertainties on such fast-neutron spectra showed that $sup 238$U (n,n') inelastic scattering is by far the main parameter determining the spectrum over a large energy range. Uranium-238 (n,$gamma$) neutron capture is important only at low energies, below the $sup 238$U (n,n') threshold. The (n,$gamma$n') reaction in $sup 238$U using Fricke's data leads to an increase in the discrepancies in neutron spectra though this cross section could be considered negligible according to other arguments. A general least- squares adjustment of the $sup 238$U total inelastic cross sections, on the neutron spectrum discrepancies, was carried out between 70 keV and 3.6 MeV, using two different inelastic probability matrices (those of ENDF/B-III and UKNDL-DFN 401), which were chosen among several evaluations of this reaction. The adjusted cross sections obtained in an energy mesh corresponding to lethargy intervals of 0.5 imply a systematic reduction of 20 to 30 percent relative to the initial values in bothmore » files, but they appear to be in good agreement with other recent (coarse-mesh) adjusted data, including the KFK-INR set. 15 tables. (auth)« less