Nuclear structure studies of double-charge-exchange Gamow-Teller strength

The double Gamow-Teller strength distributions in even-A calcium isotopes were calculated using the nuclear shell model by applying the single Gamow-Teller operator two times sequentially on the ground state of the parent nucleus. The number of intermediate states actually contributing to the results was determined. The sum rules for the double Gamow-Teller operator in the full calculation were approximately fulfilled. In the case that the symmetry is restored approximately by introducing degeneracies of the f levels, and the p levels in the fp-model space, the agreement with the sum rules was very close.