The elastic scattering of 25MeV α-particles and neutron shell effects in the A = 50 TO A = 93 mass region

Abstract

Experimental elastic scattering angular distributions of 25 MeV α-particles scattered from 28 nuclei ranging from 50Cr to 93Nb have been measured and then analysed in terms of a regular optical model with standard Woods-Saxon geometries for both the real and imaginary potentials. The experimental distributions are well fitted over the whole angular range from 5° to 175° c.m. by the predictions, provided that a smaller than normal diffuseness is used for the imaginary potentials. The usual families of potentials with volume integrals differing by approximately 100 MeV · fm 3 are found. The family with volume integral ranging from 540 to 420 MeV· fm 3 over the nuclei studied has been chosen as the most appropriate, corresponding to the single family found for α-particle elastic scattering at above 100 MeV. When plotted as functions of the back-angle enhancement parameter γ(θ), the derived parameter at the strong absorption radius, v sar, shows clear grouping into nuclei with similar neutron shell configurations. The derived optical model rms radii also show an abrupt change at the N = 40 neutron sub-shell closure. These effects are shown to be consistent with a contribution to the α-nucleus potential from the neutron pairing interaction.