F.C. Difilippo

Measurement and Resonance Analysis of Neutron Transmission Through Uranium-238

Neutron transmissions through 0.076-, 0.254-, 1.080-, and 3.620-cm-thick samples of isotopically enriched /sup 238/U were measured from 0.88 to 100.0 keV by means of a time-of-flight technique over a path length of 150 m, the ORELA pulsed neutron source, and a 13-mm-thick lithium-glass detector. To obtain resonance parameters, these transmissions from 0.88 to 4.00 keV were simultaneously least-squares shape-fitted with a multilevel Breil--Wigner cross-section formalism. In general, large neutron widths were obtained, resulting in an s-wave strength function of (1.208 +- 0.045) x 10/sup -4/ over the interval from 0.0 to 4.0 keV. An absolute energy scale accurate to 2 parts in 10,000 was established. 11 figures, 16 tables.

Evaluation of the 238U neutron cross-sections for incident neutron energies up to 4 keV☆

Abstract This report describes an evaluation of the 238U cross-sections below 4 keV. Recent measurements and re-analyses of older data are discussed. Evaluated resonance parameters are obtained for 164 s-wave and 280 p-wave levels. The capture widths of the first three s-wave levels are significantly lower than in the ENDF/B-IV evaluation. The s-wave strength function above 1.5 keV is systematically larger than in ENDF/ B-IV. Statistical and systematic uncertainties are evaluated for the resonance parameters and for the smooth backgrounds. The statistical distributions of the resonance parameters are compared with theoretically expected distributions.

Measurement of the Uranium-238 Subthreshold Fission Cross Section for Incident Neutron Energies Between 0.6 and 100 keV

The neutron-induced /sup 238/U subthreshold fission cross section has been measured in the neutron energy range between 0.6 and 100 keV. A total of 28 fission clusters were identified. The well-known clusters at 721 and 1210 eV appeared resolved into their Class I components. Average /sup 238/U subthreshold fission cross sections were determined and compared with available results in the literature. The measurement is interpreted in terms of fission doorway (Class II levels) arising from the assumption of the existence of a double-humped fission barrier for the (/sup 238/U + n) compound nucleus at large deformations. On the basis of this model, several fission barrier parameters were determined.