Observation of an ultralow-Q-value electron-capture channel decaying to As-75 via a high-precision mass measurement

A precise determination of the atomic mass of As-75 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 AtomicMass Evaluation (AME2020). This value has been used to determine the ground-state-to-ground-state electron-capture decay Q value of Se-75 and ss(-) decay Q value of Ge-75, 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 ss(-) decay) for the excited states of As-75(*), we have determined the ground-state-to-excited-state Q values for two transitions of Se-75 -> As-75(*) and one transition of Ge-75 -> As-75(*). 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 Se-75 -> As-75(*) [865.40(50) keV] is revealed to be ultralow (<1 keV) and the first-ever confirmed electron capture transition possessing an ultralow Q value from direct measurements.