Some model calculations of Coulomb mixing

Abstract

The nucleus 56Co offers a good case for testing various mechanisms of isospin mixing. The analog state is bound making the mixing calculation easier than for isobaric analog resonances. There is, however, a direct correspondence between the mechanisms in the bound case and in the continuum case. The analog states of this nucleus are at low excitation so there is hope of making detailed model calculations of all the states involved in the mixing. For these calculations we consider the J = 0 + T = 2 state of 56Co as a simple 2p-2h shell-model state. Four possibilities are offered for a nearby T = 1 0 + state. The external mixing mechanism of continuum calculations is a second-order perturbation in this case which gives matrix elements several keV in magnitude. The internal mixing caused by the off-diagonal matrix elements of the Coulomb force has been neglected in continuum calculations, but we find that in this case it can be comparatively large, of the same order of magnitude as the other mechanisms. There is sufficient disparity between the matrix elements calculated for the different models to distinguish among the models experimentally. For 56Co, experimental data can be interpreted to give a mixing of 33 keV with a nearby state. This is large enough to exclude two of the models.