Shape and log f t values of forbidden β decays within the realistic shell model

We describe the electron energy spectra and half-lives of forbidden β decays for nuclear systems outside the 78Ni core in the framework of the realistic shell model, starting from a realistic interaction. The effective shell-model Hamiltonians and decay operators are derived using many-body perturbation theory. In particular, we examine how sensitive the calculated electron energy spectra and half-lives are to the renormalization of forbidden β-decay operators. The study focuses on the first-forbidden decays of 89Sr and 90Y, on the second-forbidden decays of 94Nb and 99Tc, as well as the fourth-forbidden decays of 113Cd and 115In.