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.

High-resolution [gamma]-ray spectra from the interaction of neutrons in the energy range from 3 to 200 MeV with [sup 207,208]Pb were measured with the white neutron source at the WNR facility at Los Alamos National Laboratory. From these data, excitation functions for prominent [gamma] transitions in [sup 200,202,204,206,207,208]Pb were derived from threshold to 200 MeV incident neutron energy. These [gamma]-production cross sections represent formation cross sections for excited states of the residual nuclei. The results are compared with the predictions of nuclear reaction calculations based on the exciton model for precompound emission, the Hauser-Feshbach theory for compound nuclear decay, and coupled channels calculations to account for direct excitation of collective levels. Good agreement was obtained over the entire energy range covered in the experiment with reasonable model parameters. The results demonstrate that multiple preequilibrium emission has to be taken into account above about 40 MeV, and that the level density model of Ignatyuk should be used instead of the Gilbert-Cameron and back-shifted Fermi-gas models if excitation energies exceed about 30 MeV.

The prompt gamma-radiation from the interaction of fast neutrons with enriched samples of 2°8Pb 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 238U fission chamber. In addition to the primary purpose of this experiment, the study of (n, xn?) 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?) 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?) reactions the calculated results in general did not give a good description of the measured cross sections.

The spectrum of gamma rays emitted in resonant radiative neutron capture permits the study of states which are related to each other by electric or magnetic dipole transitions. It is expected therefore that levels either bound or unbound can appear prominently in resonant radiative capture, which are not readily observable in other reactions. Such resonant capture can be studied in light nuclei and in closed shell regions by means of a pulsed Van de Graaff as neutron spectrometer and a large (9" x 12") NaI crystal as gamma-ray spectrometer. The time resolution at the present state of the art is 30 to 100 ns/meter which, with the Van de Graaff, permits the study of nuclides for which the level spacing is a few kilovolts.