Total-Reaction-Cross-Section Measurements for 30-60-MeV Protons and the Imaginary Optical Potential

We have measured total reaction cross sections for 30-, 40-, 49.5-, and 60.8-MeV protons incident on thin, separated isotopes covering the range from ¹²C to ²⁰⁸Pb. Our results are consistent with previous data at 30 MeV, but disagree with earlier data at 60 MeV. We find a strong dependence of the reaction cross section on neutron excess for a series of Fe and Ni isotopes. Little, if any, such dependence is observed for the $N=28$ isotones. The data are well represented by the relation σR=π(r₀A1/3+λ)² with r₀=1.23±0.01 F. When analyzed with the conventional optical model, our data require the volume absorption to increase and the surface absorption to decrease with increasing proton energy Eₚ. The analysis reveals a striking (N-Z)A dependence for the product WDa^'. Using the Oak Ridge parameters for the real and spin-orbit potentials, we arrive at the following parametrization for the imaginary potential: volume absorption potential, W₀=(1.2±0.09Eₚ) MeV; surface absorption potential, WD=[4.2-0.05Eₚ+15.5(N-Z)A] MeV; imaginary diffusivity, a^'=[0.74-0.008Eₚ+1.0×(N-Z)A] F.