Pb+n208reaction and the mean nuclear field near threshold

Measurement of the total and differential elastic cross sections for the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$+n reaction are reported for the neutron energy range E=50–1005 keV. The data have been analyzed in terms of partial waves using the R-matrix formalism. The results were used to deduce optical model potentials near threshold. When the real part is expressed solely in terms of an ‘‘effective’’ volume and surface spin-orbit components, it is found that the depth of the ‘‘effective’’ volume potential exhibits an lJ dependence. It is noted that because of differences in the continuum wave functions in the surface region, such an effect could arise if the actual potential contained a real surface term. Relevance to the ‘‘Fermi surface anomaly’’ is mentioned. The observed reduced neutron strengths are compared with predictions from three microscopic models and good agreement is found.