Fine structure in isobaric analogue resonances

Abstract

Isobaric analogue resonances in nuclei near mass-90 exhibit observed total widths of approximately 50 keV. This paper is concerned with the observation of fine structure of the order of 3 keV near the isobaric analogue resonances. These measurements consist of excitation functions over the 5.3 and 5.9 MeV s-wave resonances in proton elastic scattering on 92Mo and 90Zr, respectively. In both cases the neutron exit channel is closed and other competing reactions are relatively unimportant. It is found that the magnitude of the fine structure becomes about 25 % of the average cross section on resonance and fades away off resonance. The variation of the magnitude of the fine structure is asymmetric with respect to the resonance energy. The cross section averaged over the fine structure has been analysed using an opticalmodel-plus-single-level formula. It is found that both resonances exhibit a large spreading width, which is a measure of the absorption of protons out of the isobaric analogue state into the more complicated states which are essentially degenerate with the analogue state. The asymmetry of the fluctuations away from the average is explained in terms of the Robson “external” mixing formulation.