Fine structure, partial widths and spreading widths of isobaric analogue resonances

Formulas for the microscopic calculation of partial widths and spreading widths, derived from several theories of the isobaric analogue resonances, are compared and critically examined. It is shown that the statistical assumptions used in the derivation of these formulas are not in contradiction with the fine-structure data, in contrast to recent speculations. The accuracy with which partial and spreading widths can be calculated is discussed. The theoretical values of the partial widths, and thus the determination of the spectroscopic factors, involve poorly known phenomenological parameters (essentially the shape of the symmetry potential). Thence, only relative values of the spectroscopic factors can at present be accurately determined. The calculation of the spreading width depends critically upon our knowledge of isospin violation in the nucleus. Recent calculations of the spreading widths are re-examined in the light of these results.