Observation of an ultra-low $Q$-value electron-capture channel decaying to $^{75}$As via high-precision mass measurement

Abstract

A precise determination of the atomic mass of 75 As has been performed utilizing the double Penning trap mass spectrometer, JYFLTRAP. The mass excess is measured to be -73035.519(42) keV/c 2 , which is a factor of 21 more precise and 1.3(9) keV/c 2 lower than the adopted value in the newest Atomic Mass Evaluation (AME2020). This value has been used to determine the ground-state-to-ground-state electron-capture decay Q value of 75 Se and β − decay Q value of 75 Ge, which are derived to be 866.041(81) keV and 1178.561(65) keV, respectively. Using the nuclear energy-level data of 860.00(40) keV, 865.40( 50) keV (final states of electron capture) and 1172.00(60) keV (final state of β − decay) for the excited states of 75 As * , we have determined the ground-state-to-excited-state Q values for two transitions of 75 Se → 75 As * and one transition of 75 Ge → 75 As * . The ground-state-to-excited-state Q values are determined to be 6.04(41) keV, 0.64(51) keV and 6.56(60) keV, respectively, thus confirming that the three low Q-value transitions are all energetically valid and one of them is a possible candidate channel for antineutrino mass determination. Furthermore, the ground-state-to-excited-state Q value of transition 75 Se → 75 As * (865.40(50) keV) is revealed to be ultra-low (< 1 keV) and the first-ever confirmed EC transition possessing an ultra-low Q value from direct measurements.