Resonances in low energy elastic scattering of α-particles from²⁸Si

Abstract

The elastic scattering of α-particles from²⁸Si at incident energies between 5 and 12 MeV (4.4 and 10.5 MeV in the centre-of-mass system) was studied to ascertain whether quasimolecular states exist for the α+²⁸Si system, as predicted in the literature in terms of a microscopic theory based on the generator coordinate method. Angular distributions were obtained for angles larger than 70°, in energy steps of 100 keV. Analysis of the experimental data was performed by coherently adding specific resonances to an underlying optical model which describes the background portion of the direct elastic scattering amplitude. As is usual, it was furthermore assumed that the average compound elastic contribution adds incoherently to the total cross section. In spite of the presence of a resonance characterized by an angular momentum value ofJ=3 in the energy region, as required by theory, no evidence could be found for the existence of other resonances with the expectedJ-sequence. We conclude that our experiment and analysis do not support the predictions of the microscopic theory.