Complex nonlocal optical potential for neutron scattering fromPb208

Abstract

A nonlocal energy-dependent imaginary optical potential is calculated for neutron scattering from $^{208}\mathrm{Pb}$ in the intermediate structure model with weak particle-vibration coupling. The energy range studied is 0-12 MeV and the partial waves considered are $l=0\ensuremath{-}4$. The corresponding contribution to the real potential has also been obtained and is relatively small; this potential is presented for $s$ waves. The imaginary potential is used to calculate the absorption cross section in this energy range for each partial wave. Both compound-nucleus and inelastic contributions to the potential and the absorption cross section are included. Below 5 MeV compound-nucleus contributions are dominant. Above this energy inelastic excitations based on single-particle resonances and compound-nucleus states based on giant resonances contribute, with the former being more significant. A comparison of the calculated absorption to experiment for $s$, $p$, $d$, and $f$ waves is made below the inelastic threshold of 2.6 MeV. The agreement, except for $p$ waves, is quite good in terms of the number of resonances and other significant details of the cross section. The calculated absorption cross sections up to 12 MeV are compared to the results from a phenomenological local surface-peaked imaginary potential. The nonlocal potential is also surface peaked and the details of its radial behavior for an arbitrary energy are given in a contour plot.[NUCLEAR REACTIONS $^{208}\mathrm{Pb}(n,n)$, $E=0\ensuremath{-}12$ MeV; calculated nonlocal imaginary optical potential and absorption. Comparison with experiment and with a local imaginary potential. Particle-vibration doorways.]