Surface contributions to the complex neutron-/sup 208/Pb mean field between -20 and +20 MeV

Phenomenological analyses of the experimental n-/sup 208/Pb differential, total, and polarization cross sections with local optical-model potentials indicate that the radial shape of the surface absorption depends upon energy below 10 MeV: The corresponding diffuseness decreases and the radius parameter increases with decreasing neutron energy. Because of the dispersion relation that connects the real and imaginary parts of the mean field, these features imply that the real potential contains a surface component whose radial shape also depends upon energy. This radial shape is calculated numerically for typical parametrizations of the energy dependence of the surface absorption; it turns out to be quite complicated for neutron energies between 0 and 15 MeV. In this domain, the predicted differential cross sections are sensitive to the radial shapes of both the real and imaginary surface components of the mean field even though their volume integrals are exactly the same in all the investigated models. The best agreement with the experimental data is obtained for parametrizations in which the radial shape of the surface absorption depends only weakly upon energy. It is shown that good fits to the experimental data can also be obtained in the framework of models in which the radial shape of the surface absorption is independent of energy but in which the strength of the surface absorption depends upon the orbital angular momentum of the incoming neutron. Tentative physical interpretations of these features are proposed