207,208Pb(n,xn gamma ) reactions for neutron energies from 3 to 200 MeV.

Abstract

High-resolution \ensuremath{\gamma}-ray spectra from the interaction of neutrons in the energy range from 3 to 200 MeV with $^{207,208}\mathrm{Pb}$ were measured with the white neutron source at the weapons neutron research (WNR) facility at Los Alamos National Laboratory. From these data, excitation functions for prominent \ensuremath{\gamma} transitions in $^{200,202,204,206,207,208}\mathrm{Pb}$ were derived from threshold to 200 MeV incident neutron energy. These \ensuremath{\gamma}-production cross sections reflect the excitation cross sections for the respective 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 nucleus 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 of this work clearly demonstrate that multiple preequilibrium emission has to be taken into account above about 40 MeV, and that the level density model of Ignatyuk, which accounts for the gradual disappearance of shell effects with increasing excitation energy, should be used instead of the Gilbert-Cameron and backshifted Fermi-gas models if excitation energies exceed about 30 MeV. No indication for a reduction of the nuclear moment of inertia below the rigid body value was found.