Direct determination of the atomic mass difference of the pairs <math><mrow><mmultiscripts><mi>As</mi><mprescripts/><none/><mn>76</mn></mmultiscripts><mtext>-</mtext><mmultiscripts><mi>Se</mi><mprescripts/><none/><mn>76</mn></mmultiscripts></mrow></math> and <math><mrow><mmultiscripts><mi>Tb</mi><mprescripts/><none/><mn>155</mn></mmultiscripts><mtext>-</mtext><mmultiscripts><mi>Gd</mi><mprescripts/><none/><mn>155</mn></mmultiscripts></mrow></math> rules out <math><mmultiscripts><mi>As</mi><mprescripts/><none/><mn>76</mn></mmultiscripts></math> and <math><mmultiscripts><mi>Tb</mi><mprescripts/><none/><mn>155</mn></mmultiscripts></math> as possible candidates for electron (anti)neutrino mass measurements

International audienceThe first direct determination of the ground-state–to–ground-state Q values of the β− decay As76→Se76 and the electron-capture decay Tb155→Gd155 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 As76→Se76 and Tb155→Gd155 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 As76 (ground state) →Se*76 (2968.4(7) keV) and Tb155 (ground state) →Gd*155 (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