Abstract
An approximate semiclassical formula for the imaginary part of the scattering phase shift due to Feshbach's second-order optical potential has been obtained. This formula has been applied to the α- 40Ca system and the imaginary phases compared with those calculated from phenomenological optical potentials fitted to the experimental data. The target nucleus was described by a harmonic oscillator shell model and the α-nucleus interaction was obtained from a phenomenological α-nucleon interaction. This model gives an absorption which is stronger than that calculated from phenomenological potentials by a factor of two for a low α-particle incident energy ( E lab = 29 MeV). The discrepancy is less for higher energies. The results are very sensitive to the strength of the α-nucleon interaction. It was found that the giant quadrupole and monopole collective states make only a small contribution to the total absorption.
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