Explanation of the near equality of isoscalar and electromagnetic transition rates in neutron-excess nuclei; representation of the data in terms. beta. /sub. cap alpha cap alpha. '/. beta. /sub em/

Abstract

The ratios of isoscalar and electromagnetic transition rates to the lowest 2/sup +/ state in neutron-excess nuclei are compared to predictions based on the ratios of isovector and isoscalar transition strengths determined from microscopic effective-charge theory in which giant-quadrupole core-polarization contributions are included with a schematic model. The near equality of the isoscalar (determined from (..cap alpha..,..cap alpha..')) and electromagnetic transition rates is explained, and it is shown that shell effects may be large but that they are compensated for in part by core-polarization effects. A comparison to data for single-closed-shell nuclei is consistent with the schematic-model prediction that the remaining shell effects are systematically different for neutron- versus proton-valence nuclei.