Single and Double Beta-Decay Q Values among the Triplet Zr 96, Nb 96, and Mo 96

The atomic mass relations among the mass triplet Zr96, Nb96, and Mo96 have been determined by means of high-precision mass measurements using the JYFLTRAP mass spectrometer at the IGISOL facility of the University of Jyväskylä. We report Q values for the Zr96 single and double β decays to Nb96 and Mo96, as well as the Q value for the Nb96 single β decay to Mo96, which are Qβ(Zr96)=163.96(13), Qββ(Zr96)=3356.097(86), and Qβ(Nb96)=3192.05(16) keV. Of special importance is the Zr96 single β-decay Q value, which has never been determined directly. The single β decay, whose main branch is fourfold unique forbidden, is an alternative decay path to the Zr96 ββ decay, and its observation can provide one of the most direct tests of the neutrinoless ββ-decay nuclear-matrix-element calculations, as these can be simultaneously performed for both decay paths with no further assumptions. The theoretical single β-decay rate has been re-evaluated using a shell-model approach, which indicates a Zr96 single β-decay lifetime within reach of an experimental verification. The uniqueness of the decay also makes such an experiment interesting for an investigation into the origin of the quenching of the axial-vector coupling constant gA.