Nonlocality and Off-Shell Effects in Nuclear Optical Potentials at Medium Energies.

We calculate the microscopic optical potential by starting with the G-matrix for infinite nuclear matter within the framework of the Breuckner theory as modified by Brieva and Rook. Using these nuclear matter results, we develop the optical potential for finite nuclei by taking into account the off-shell effects and the nonforward parts of the amplitudes. These effects give rise to an energy dependent and nonlocal optical potential in configuration space. The equivalent local potential is a sum of terms, the first being the familiar zero range expression which is often used in many practical calculations. The remaining terms of the optical potential contain nonlocality and finite-range effects which arise because of the finite extent of the nucleus. The calculations show that off -shell and nonforward corrections modify the potential in the surface region, hence changing its shape from that of the density distribution. A nonlocal phenomenological potential is also developed which is energy independent and whose local equivalent potential yields an energy dependence seen in phenomenological potentials. Using the nonlocal potential, we calculate the elastic scattering cross sections and the wave functions for the P-('40)Ca system at 400, 550, and 700 MeV energies. Local equivalent potentials are then obtained using cross sections as data in an optical model search code. Our calculations show that the amplitude of the nonlocal wave function in the nuclear interior is much smaller than those of local wave functions. This so-called Perey effect is investigated and is shown to have an important role in nuclear reactions with large momentum transfers.