Collective magnetic dipole transitions: Dependence of the energies and rates on the nuclear effective interaction

Abstract

The dependence on the properties of the effective interaction of the energies of and excitation strengths to magnetic dipole states in open shell nuclei is studied. In particular the single j-shell for 48Ti is used as an example. In this case there are two 1 + states with isospin T = 2 and one with isospin T = 3. The conditions for having a strong low-lying collective 1 + state are examined. Focus is also given on the excitation strength to the analog T = 3 state since this is of relevance also to β + Gamow-Teller reactions and to double beta decay. It is found that in the rotational limit there is no strength to the T = 3 state and there is an overly strong low-lying 1 + T = 2 state. This is almost also true for a quadrupole-quadrupole interaction. At the other extreme, as shown by Halse, with a pairing interaction all the strength goes to the T = 3 state. Other interactions considered are pairing plus quadrupole, spin-dependent delta and Kuo-Brown bare and renormalized, and matrix elements taken from the spectrum of 42Sc. It is found that the β + strength can be reduced either by making the two-body J = 2 T = 1 matrix element or J = 1 T = 0 matrix element more attractive, just as was shown by others in heavier nuclei. However such parameter changes have effects on other properties of the 1 + spectrum, which can serve as indicators as to whether or not these changes are justified.