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${}^{\ast }$, we have determined the ground-state-to-excited-state $Q$ values for two transitions of ${}^{75}$Se $\to$ ${}^{75}$As${}^{\ast }$ and one transition of ${}^{75}$Ge $\to$ ${}^{75}$As${}^{\ast }$. 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 $\to$ ${}^{75}$As${}^{\ast }$ (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.

7 pages, 3 figures