Evaluation of the ²³⁸U Neutron Cross Section in the Unresolved Resonance Range

This paper presents a new analysis of the ²³⁸U cross sections in the unresolved resonance range, from 20 to 150 keV. Statistical analysis of the resonance parameters in the resolved resonance range with random-matrix theory provides accurate experimental values of strength function, average radiative width and average level spacing for s- and p-wave resonances. Above 20 keV, the simultaneous fit of selected experimental data (average transmission and capture) is performed with a statistical model of nuclear reactions as implemented in the SAMMY code.

Compared to previous evaluations, such as those described by Fröhner or by Maslov et al., this work benefits from the accurate transmission data measured by Harvey et al. at Oak Ridge Electron Linear Accelerator, which have never been studied before. This new evaluation was written into the current ENDF format for use in practical applications. This work stresses the need for an improved ENDF format to store average resonance parameters and cross sections in the unresolved resonance range.