Deuteron effects in nucleon-nucleus scattering at intermediate energies

We investigate the role of the full dynamical dependence of the free off-shell nucleon-nucleon t matrix on the optical potential for proton-nucleus elastic scattering in the 100--400 MeV incident energy range within a full-folding model context. Particular emphasis is placed on the effects of deuteron formation by explicitly taking into account pole singularities in the free nucleon-nucleon t matrix. The full-folding model for the optical potential provides a flexible framework for this purpose as it allows the sampling of the internucleon effective force both off shell and as a function of the energy available in the center of mass for the interacting nucleon pair. A comparison of calculated and measured scattering observables for proton elastic scattering on ⁴⁰Ca and ²⁰⁸Pb leads to the conclusion that the full off-shell free t matrix is a poor approximation for that part of the nucleon-nucleon effective force required for calculating optical potentials below {~}250 MeV. Medium effects and higher order corrections to the optical potential are necessary to improve our understanding of nucleon scattering.