(sup 208)Pb(n,pxn(gamma)) reactions for neutron energies up to 200 MeV

摘要

The prompt gamma-radiation from the interaction of fast neutrons with enriched samples of (sup 208)Pb was measured using the white neutron beam of the WNR facility at Los Alamos National Laboratory. The samples were positioned at about 40 m distance from the neutron production target. The spectra of the emitted gamma-rays were measured with a high-resolution HPGe detector. The incident neutron energy was determined by the time-of-flight method and the neutron fluence was measured with a (sup 238)U fission chamber. In addition to the primary purpose of this experiment, the study of (n,xn(gamma)) reactions leading to various lead isotopes, gamma transitions in the residual nuclei (sup 207,205,203,201)Tl were analyzed. From these data gamma-production cross sections in the neutron energy range from the effective thresholds to 200 MeV were derived. The lines for the analysis had to be chosen carefully as the (n,pnx(gamma)) cross sections are rather small and the interference with unresolved lead lines (even weak ones) would cause significant errors. The effect due to isomers with half-lives exceeding a few nanoseconds was taken into account and corrected for, if necessary. The measured cross sections were compared with the results of nuclear model calculations based on the exciton model for preequilibrium particle emission and the Hauser-Feshbach theory for compound nucleus decay. Unlike in the case of (n,xn(gamma)) reactions the calculated results in general did not give a good description of the measured cross sections.