The 1-(1.9 MeV β-)2+ transition in the decay of148Pm

The non-unique first-forbidden 1^?(1.9 MeVβ ^?)2⁺ transition in the decay of¹⁴⁸Pm is studied. The nuclear matrix elements are evaluated using for the ground state of the odd-odd¹⁴⁸Pm a phenomenological wave function derived in a previous work in the framework of the two-quasiparticle model and assuming that the first excited 2⁺ state in¹⁴⁸Sm can be described as the first quadrupole phonon state. This model provides nuclear matrix elements which are capable of describing experimental data like the spectrum shape factor, the longitudinal polarization of the electrons and a particular set ofβ-γ directional correlation coefficients, but it fails to interpret the large logft value of this transition. In addition, the validity of theξ-approximation is discussed.