Observation of an ultralow-$Q$-value electron-capture channel decaying to ${}^{75}\mathrm{As}$ via a high-precision mass measurement

A precise determination of the atomic mass of ${}^{75}\mathrm{As}$ has been performed utilizing the double Penning trap mass spectrometer, JYFLTRAP. The mass excess is measured to be $-73035.519\left(42\right)\mathrm{keV}/c^2$, which is a factor of 21 more precise and $1.3\left(9\right)\mathrm{keV}/c^2$ 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 ${}^{75}\mathrm{Se}$ and ${\beta }^{-}$ decay $Q$ value of ${}^{75}\mathrm{Ge}$, 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 ${\beta }^{-}$ decay) for the excited states of ${}^{75}\mathrm{As}^{*}$, we have determined the ground-state–to–excited-state $Q$ values for two transitions of ${}^{75}\mathrm{Se}\to {}^{75}\mathrm{As}^{*}$ and one transition of ${}^{75}\mathrm{Ge}\to {}^{75}\mathrm{As}^{*}$. 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 ${}^{75}\mathrm{Se}\to {}^{75}\mathrm{As}^{*}$ [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.

Figure 1

(a) Schematic view of ion production and mass measurements with PI-ICR technique. The stable ${}^{75}\mathrm{As}^{+}$ and ${}^{76}\mathrm{Ge}^{+}$ ions were simultaneously produced with an offline glow-discharge ion source, where the ions were produced and transported with an He gas flow and electric fields. Ions having mass number of 75 or 76 were selected with a dipole magnet and transported to the JYFLTRAP PTMS for final ion species selection in the preparation trap by means of a buffer-gas cooling and cyclotron frequency determination using the phase-imaging technique at the measurement trap. A position-sensitive MCP detector was used to register the images of the phases. (b) An illustration of the radial-motion (”magnetron”, ”cyclotron”, and ”center”) projection of the ${}^{75}\mathrm{As}^{+}$ ions onto the position-sensitive MCP detector. The magnetron phase spot is displayed on the left side and the cyclotron phase spot on the right. The angle difference between the two spots relative to the center spot is utilized to deduce the cyclotron frequency of the measured ion. The number of ions in each pixel is illustrated by color bars.

Figure 2

The measured cyclotron frequency ratios $R({\nu }_c\left({}^{76}\mathrm{Ge}^{+}\right)/{\nu }_c({}^{75}\mathrm{As}^{+}$) (left axis) and mass excess (right axis) in this work compared to values adopted from AME2020. The red dots with uncertainties are the measured PI-ICR single ratios in five time slots, which are separated with vertical brown dashed lines. The weighted average value in this work $\overline{R}=0.98683089652\left(53\right)$ represents by the solid red line and its $1\sigma$ uncertainty band is shaded in red. The dashed blue line indicates the value adopted from AME2020 with its $1\sigma$ uncertainty area shaded in blue.

Figure 3

Partial decay scheme of ${}^{75}\mathrm{Se}$ and ${}^{75}\mathrm{Ge}$ to ${}^{75}\mathrm{As}$ using $Q$ values from AME2020 [40, 45] in comparison to this work. The energies of the excited states and spin-parities indicated in the figure are from [41]. The solid lines demonstrate the levels of the excited states with the $Q$ values from AME2020 and dashed lines from the refined $Q$ values in this work. The shaded or hatched areas (in blue for ${\beta }^{-}$ decay or in red for EC) illustrate the corresponding 1 $\sigma$ uncertainties of the $Q$ values. See Table 2 for more details on the $Q$ values.