Cross-sections and resonance parameters for 235U, 233U, 239Pu and 241Pu between cadmium cutoff and 10 keV

Fission and capture cross-sections between Cd cutoff and 10 keV have been evaluated for the fissile isotopes, based on the available experimental differential cross-sections, capture-to-fission ratios and total fission and capture resonance integrals. In particular, the recommended cross-sections contained in the 1965 edition of BNL-325 were compared with some other evaluations and with more recent results of cross-section measurements using nuclear explosives in space and underground. As a result of this evaluation, it is felt that even for the most intensively studied nucleus, 235U, the uncertainties in the fission cross-section are about ±5% and in the capture cross-section ±20%. For 239Pu, σf is known to ±10% and σc to ±25%. For 233U and 241Pu, the uncertainty in σf is ±20% and in σc, ±30%. Different sets of resonance parameters for the resolved resonances, which are recommended in BNL-325 or by other research laboratories, were compared by calculating infinitely dilute total and partial resonance integrals. Calculations were performed with the resonance integral code TRIX-1 which was developed at Atomics International and is based on the single-level Breit Wigner model. The well-known limitations of this model for nuclei, such as the fissile, with narrowly spaced interfering levels result in calculated fission resonance integrals, which are 10- 20% too small when compared with integrated measured differential cross-sections. This result was found for energy regions where the cross-sections are sufficiently well known to make such a comparison possible. Average parameters for the unresolved resonances have been extrapolated from resolved resonance parameters. Calculated partial resonance integrals, using these parameters, were compared with integrated measured cross-sections where these are available. As no published capture cross-section measurements exist in the resonance region for 233U, 239Pu, and 241Pu these cross-sections were calculated with the determined average parameters and appropriate statistical distributions. (author)