Empirical density-dependent effective interaction for nucleon-nucleus scattering at 500 MeV

We report new cross-section and analyzing-power data for the excitation by 498-MeV protons of all narrow normal-parity states of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ below 12.1-MeV excitation. In addition, spin-rotation measurements for elastic scattering and depolarization measurements for the $1_1^{\mathrm{-}}$, $2_1^{+}$, and $3_1^{\mathrm{-}}$ states of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ have been performed. These data are used in conjunction with existing data for ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ to study medium corrections to the effective interaction for nucleon-nucleus scattering at 500 MeV. Systematic differences between the data and nonrelativistic impulse approximation calculations based upon either the free t matrix or a recent density-dependent effective interaction are interpreted within the framework of the local-density approximation. An empirical effective interaction has been constructed which parametrizes the density dependence of the medium modifications in a simple form amenable to phenomenological analysis of data.