Direct measurement of the mass difference of ${}^{72}$As-${}^{72}$Ge rules out ${}^{72}$As as a promising $\beta$-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 ${}^{72}$As to ${}^{72}$Ge 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 50-fold 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 $\sigma$) lower. With the significant reduction of the uncertainty of the ground-state to ground-state $Q$-value value combined with the level scheme of ${}^{72}$Ge from $\gamma$-ray spectroscopy, we confirm that the five potential ultra-low $Q$-value ${\beta }^{+}$-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 ${}^{72}$As for the study of virtual $\beta$-$\gamma$ transitions.

8 pages, 6 figures, nuclear experiment

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