The limitations of the one-channel phenomenological optical model

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Abstract
A phenomenological analysis of n-208Pb differential cross sections for elastic scattering has been performed over the neutron energy range 7.5-13.5 MeV using an optical model with a spherically symmetric central potential. The limitations of this model are explored by comparing the present results with those of a recent analysis of total cross sections. The authors demonstrate that with constant geometry this model fails to describe n-208Pb data adequately over this energy range and that this failure is associated only with the imaginary part of the central potential. Evidence is presented that this failure is a consequence of strong coupling between the ground state and collective excitations of 208Pb. In agreement with several structure calculations in the 208Pb region, the influence of low-lying collective states appears to be particularly important.