Direct determination of the atomic mass difference of the pairs $^{76}$As-$^{76}$Se and $^{155}$Tb-$^{155}$Gd rules out $^{76}$As 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 76 As → 76 Se and the electron-capture decay 155 Tb → 155 Gd was performed utilizing the double Penning trap mass spectrometer JYFLTRAP. By measuring the atomic mass difference of the decay pairs via the phase-imaging ioncyclotron-resonance (PI-ICR) technique, the Q values of 76 As → 76 Se and 155 Tb → 155 Gd 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 groundstate-to-ground-state Q values combined with the excitation energy from γ-ray spectroscopy, the Q values for ground-state-to-excited-state transitions 76 As (ground state) → 76 Se * (2968.4(7) keV) and 155 Tb (ground state) → 155 Gd * (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.