V. M. Maslov

Neutron-Induced Fission of 233U, 238U, 232Th, 239Pu, 237Np, natPb and 209Bi Relative to 235U in the Energy Range 1-200 MeV

Fission cross section ratios of 233U, 238U, 232Th, 239Pu, 237Np, natural Pb and 209Bi to 235U have been measured in a wide energy range of incident neutrons from 1 MeV to 200 MeV using a time-of-flight technique at the neutron spectrometer GNEIS based on the 1-GeV proton synchrocyclotron of PNPI. For actinide targets, the threshold cross section method and evaluated data below 14 MeV were used for normalization of the shape measurement data, while the evaluated and recommended fission cross sections of 235U were used to convert the ratio data to absolute fission cross sections. For Pb and Bi targets, an absolute normalization of the measured cross section ratios has been done using the thickness of the targets and detection efficiencies.

Neutron Capture Cross Section of 232Th

Abstract Consistent evaluation of the 232Th capture cross section is performed in the energy range from 4 keV to 5 MeV. The Hauser-Feshbach-Moldauer theory and coupled channel optical model are employed. Total, differential scattering, fission, and inelastic scattering data are consistently reproduced as a major constraint for the capture cross-section estimate. Average resonance parameters are obtained, which are consistent with the capture cross-section database.

Neutron Scattering on 238U and 232Th

Consistent analysis of neutron scattering data of 238U and 232Th with Hauser-Feshbach-Moldauer theory and coupled channel model is carried out. Collective bands structures are reproduced within soft deformable rotator model. The direct excitation of ground state and vibrational band levels is calculated within rigid rotator and soft (deformable) rotator models, respectively. Structures evident in measured neutron emission spectra are correlated with excitation of vibrational levels of Kπ = 0− and Kπ = 0+, 2+ bands. Prompt fission neutron spectra data are described. Contributions of neutrons, emitted in (n, nf) reactions are shown to be strongly dependent on the emissive fission contribution to the observed fission cross section.

Actinide Neutron-Induced Fission up to 200 MeV

Neutron-induced fission cross sections of U, Np and Pu target nuclides are analyzed in fission/evaporation approximation up to 200 MeV excitation energy. Damping of collective modes contribution to the level density at excitation energies higher than ~20 MeV for saddle and equilibrium deformations is shown to be essential. Effective estimates of intrinsic level densities are obtained. Actinide first-chance fission cross sections of 238U(n, f) and 238U(p, f) reactions are found to be much different. Differences of measured proton- and neutron-induced fission cross sections of 238U are attributed to the influence of the isovector term of the nucleon-nucleus optical potential.

Nuclear Data Evaluation for U-233

Cross section of neutron-induced reactions on U-233 have been evaluated between 10-5 eV and 20 MeV for the BROND data library. In the thermal and resonance regions evaluation relies on experimental data. Fission and total cross sections have been evaluated on the basis of recent experimental data. A deformed optical potential that fits total, elastic and inelastic cross sections, s- and p-wave strength functions was used to calculate direct inelastic and other cross sections. The available data on total, fission, capture and inelastic scattering are reproduced quite well. Above (n,n’f)reaction threshold preequilibrium emission has been taken into account. The calculated (n,2n) reaction cross section disagrees with available fission spectrum-averaged data and other evaluations. Partial energy distributions of secondary neutrons are given in tabulated form. The evaluated data are compared with ENDL-78 and JENDL evaluations.