Direct measurement of the mass difference of 72As−72Ge rules out 72As as a promising β-decay candidate to determine the neutrino mass

Abstract

We report the first direct determination of the ground-state to ground-state electron-capture Q value for the 72As to 72Ge decay by measuring their atomic mass difference utilizing the double Penning trap mass spectrometer, JYFLTRAP. The Q value was measured to be 4343.596(75) keV, which is more than a fiftyfold improvement in precision compared to the value in the most recent Atomic Mass Evaluation 2020. Furthermore, the new Q value was found to be 12.4(40) keV (3.1 σ) lower. With the significant reduction of the uncertainty of the ground-state to ground-state Q value combined with the level scheme of 72Ge from γ-ray spectroscopy, we confirm that the five potential ultralow Q-value β+ decay or electron capture transitions are energetically forbidden, thus precluding all the transitions as possible candidates for the electron neutrino mass determination. However, the discovery of small negative Q values opens up the possibility to use 72As for the study of virtual β−γ transitions.