A New Resonance Region Evaluation of Neutron Cross Sections for ²³⁵U

A new evaluation of the “resolved resonance range” for the neutron cross sections of ²³⁵U is described. Up to 110 eV, the evaluation is based on an R-matrix analysis of several fission, capture, and transmission measurements. Levels above 110 eV are no longer resolved so that many resonances are missed; from 110 to 500 eV, most of the important resonances can be identified and analyzed so that the cross section and transmission data are well represented by the proposed parameters. From 500 to 2250 eV, fictitious parameters are provided that describe fairly well the results of thick sample transmission measurements and recent fission cross-section data. Such a parameterization is likely to yield a better approximation of resonance self-shielding than the current ENDF/B- V unresolved resonance treatment.