Double-beta decay of 48Ca revisited

The 48Ca → 48Ti two neutrino double-beta decay rate is calculated exactly in the full fp shell with the hamiltonian describing well the spectroscopy of the A = 48 system. The resulting half-life is very near the experimental lower limit. In order to find out whether the shell model can at all accommodate longer 2ν half-lives we modify the hamiltonian in several ways, keeping, however, the acceptable agreement with the main spectroscopic indicators intact. We conclude that, within such an approach, the longest possible half-life is about 1020 yr, only a factor of three longer than the present limit. We stress that any possible modifications of the hamiltonian must be restricted by the comparison with available data on energy levels and transitions rates.