Direct determination of the atomic mass difference of the pairs 76As−76Se and 155Tb−155Gd rules out 76As and 155 Tb as possible candidates for electron (anti)neutrino mass measurements

Abstract

The first direct determination of the ground-state–to–ground-state Q values of the β− decay 76As→76Se and the electron-capture decay 155Tb→155Gd was performed utilizing the double Penning trap mass spectrometer JYFLTRAP. By measuring the atomic mass difference of the decay pairs via the phase-imaging ion-cyclotron-resonance technique, the Q values of 76As→76Se and 155Tb→155Gd were determined to be 2959.265(74) keV and 814.94(18) keV, respectively. The precision was increased relative to earlier measurements by factors of 12 and 57, respectively. The new Q values are 1.33 keV and 5 keV lower compared to the values adopted in the most recent Atomic Mass Evaluation 2020. With the newly determined ground-state–to–ground-state Q values combined with the excitation energy from γ-ray spectroscopy, the Q values for ground-state–to–excited-state transitions 76As (ground state) →76Se∗ (2968.4(7) keV) and 155Tb (ground state) →155Gd∗ (815.731(3) keV) were derived to be −9.13(70) keV and −0.79(18) keV. Thus we have confirmed that both of the β−-decay and EC-decay candidate transitions are energetically forbidden at a level of at least 4σ, thus definitely excluding these two cases from the list of potential candidates for the search of low-Q-value β− or EC decays to determine the electron-(anti)neutrino mass.