Practical Formalisms for Nuclear Data Representation in Evaluated Nuclear Data Files in the Unresolved Resonance Energy Region

The currently used formalism for neutron cross-section representation in the unresolved resonance energy range is based on the statistical parameters of the population of Breit-Wigner resonances. The present work introduces practical formalisms, based on parametric representation of the shielding factor curves, by which the values of effective cross sections can be obtained simply and quickly in the unresolved range, and suggests their use for neutron data representation. These formalisms were found to be compatible with such existing codes as MC², ETOX, HAMMER, ENDRUN, and MIGROS, and with such existing nuclear data files as ENDF/B and KEDAK. Each formalism is based on one interpolation scheme in temperature and one in σ₀. The accuracy of four schemes in temperature and three schemes in σ₀ was checked. Of these, three temperature schemes and one σ₀ scheme were found to have better than 1% accuracy in the entire unresolved region, thus yielding a formalism with better than 2% accuracy for representation. Observed spatially dependent self-shielding factors are transformed into pseudo-background cross-section-dependent (Bondarenko-type) self-shielding factors. Numerical values of the transformation for ²³⁵U and ²³⁹Pu self-shielding factors are given. It is shown that the formalisms can be used for the preprocessing of current nuclear data files in the unresolved range.