Direct determination of the atomic mass difference of the pairs ${}^{76}\mathrm{As}\text{−}{}^{76}\mathrm{Se}$ and ${}^{155}\mathrm{Tb}\text{−}{}^{155}\mathrm{Gd}$ rules out ${}^{76}\mathrm{As}$ and ${}^{155}\mathrm{Tb}$ as possible candidates for electron (anti)neutrino mass measurements

The first direct determination of the ground-state–to–ground-state $Q$ values of the ${\beta }^{-}$ decay ${}^{76}\mathrm{As}\to {}^{76}\mathrm{Se}$ and the electron-capture decay ${}^{155}\mathrm{Tb}\to {}^{155}\mathrm{Gd}$ 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 ${}^{76}\mathrm{As}\to {}^{76}\mathrm{Se}$ and ${}^{155}\mathrm{Tb}\to {}^{155}\mathrm{Gd}$ 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 $\gamma$-ray spectroscopy, the $Q$ values for ground-state–to–excited-state transitions ${}^{76}\mathrm{As}$ (ground state) $\to {}^{76}\mathrm{Se}^{*}$ (2968.4(7) keV) and ${}^{155}\mathrm{Tb}$ (ground state) $\to {}^{155}\mathrm{Gd}^{*}$ (815.731(3) keV) were derived to be $-9.13\left(70\right)$ keV and $-0.79\left(18\right)$ keV. Thus we have confirmed that both of the ${\beta }^{-}$-decay and EC-decay candidate transitions are energetically forbidden at a level of at least $4\sigma$, thus definitely excluding these two cases from the list of potential candidates for the search of low-$Q$-value ${\beta }^{-}$ or EC decays to determine the electron-(anti)neutrino mass.

Figure 1

Schematic overview of the IGISOL facility. The ions were produced at the IGISOL target chamber (1). ${}^{76}\mathrm{As}^{+}$ and ${}^{76}\mathrm{Se}^{+}$ were produced with deuteron-induced fusion reactions, while ${}^{155}\mathrm{Tb}^{+}$ and ${}^{155}\mathrm{Gd}^{+}$ were produced with proton-induced fusion reactions. After production and extraction from the gas cell, the ions were guided through an electrostatic bender (2), mass number selected with a dipole magnet (3), cooled and bunched in the RFQ cooler-buncher (4), and finally injected to the JYFLTRAP double Penning trap setup (5) for the mass-difference measurement.

Figure 2

Center, cyclotron phase and magnetron phase spots of ${}^{155}\mathrm{Tb}^{+}$ on the position-sensitive MCP detector after the PI-ICR excitation pattern with an accumulation time of 620 ms composed of one set of data recorded in the experiment. The cyclotron phase spot is displayed on the left panel (a) and the magnetron phase spot with a center spot on the right (b). The color bar illustrates the number of ions in each pixel.

Figure 3

Comparison of results from this work and AME2020 for (a) ${}^{76}\mathrm{As}^{+}\text{−}{}^{76}\mathrm{Se}^{+}$ and (b) ${}^{155}\mathrm{Tb}^{+}\text{−}{}^{155}\mathrm{Gd}^{+}$ measurements. The left axis shows the frequency ratio deviation from the measured value and the right axis the corresponding $Q$ value. The red points are the data points and the solid horizontal red line with the shaded area ($1\sigma$ uncertainty band) the final value. The dashed blue line is the value derived from AME2020 (shaded area is the $1\sigma$ uncertainty band).

Figure 4

Partial decay scheme of (a) ${\beta }^{-}$ decay of the ${}^{76}\mathrm{As}$ ground state to excited states in ${}^{76}\mathrm{Se}$ and (b) EC decay of the ${}^{155}\mathrm{Tb}$ ground state to excited states in ${}^{155}\mathrm{Gd}$. $Q_{{\beta }^{-}}/Q_{EC}$ for both decays is the ground-state–to–ground-state $Q$ value. The energies of the excited states are taken from [23]. The uncertain spin-parities are denoted with parentheses. The hatched bands in blue show the $1\sigma$ uncertainty in the $Q$ value as derived from the AME2020 (a cut-off is indicated for EC of ${}^{155}\mathrm{Tb}$ due to its large uncertainty of 10 keV). The shaded bands in red show the $1\sigma$ uncertainty in the $Q$ value deduced from the newly determined $Q_0$ from this work. The decay $Q$ values and excitation energies are in units of keV. See also Tables 1, 3, and 4.