High-Precision $Q$-Value Measurement Confirms the Potential of ${}^{135}\mathrm{Cs}$ for Absolute Antineutrino Mass Scale Determination

The ground-state-to-ground-state $\beta$-decay $Q$ value of ${}^{135}\mathrm{Cs}(7/2^{+})\to {}^{135}\mathrm{Ba}(3/2^{+})$ has been directly measured for the first time. The measurement was done utilizing both the phase-imaging ion-cyclotron resonance technique and the time-of-flight ion-cyclotron resonance technique at the JYFLTRAP Penning-trap setup and yielded a mass difference of 268.66(30) keV between ${}^{135}\mathrm{Cs}(7/2^{+})$ and ${}^{135}\mathrm{Ba}(3/2^{+})$. With this very small uncertainty, this measurement is a factor of 3 more precise than the currently adopted $Q$ value in the Atomic Mass Evaluation 2016. The measurement confirms that the first-forbidden unique ${\beta }^{-}$-decay transition ${}^{135}\mathrm{Cs}(7/2^{+})\to {}^{135}\mathrm{Ba}(11/2^{-})$ is a candidate for antineutrino mass measurements with an ultralow $Q$ value of 0.44(31) keV. This $Q$ value is almost an order of magnitude smaller than those of nuclides presently used in running or planned direct (anti)neutrino mass experiment.

Figure 1

${\beta }^{-}$ decay of the ground state of ${}^{135}\mathrm{Cs}$ to the ground state and first two excited states in ${}^{135}\mathrm{Ba}$. The as yet undetected transition to the second excited state ($11/2^{-}$) is an ultralow $Q$-value transition relevant for the effective antineutrino mass measurements. The transition to the first excited state ($1/2^{+}$) is greatly hindered by the large change in angular momentum. The numbers to the right of the energy levels are excitation energies in megaelectron-volt.

Figure 2

Layout of the IGISOL facility. The radioactive ${}^{135}{\mathrm{Cs}}^{+}$ ions were produced with proton-induced fission reactions (1), the stable ${}^{135}{\mathrm{Ba}}^{+}$ ions with an off-line source (2). The beam from either source was selected with an electrostatic kicker (3). The mass number selection was performed with a dipole magnet (4), the ion bunching with the cooler buncher (5), and finally the mass-difference measurement with the JYFLTRAP Penning trap setup (6) [18].

Figure 3

Ramsey TOF-ICR spectrum for ${}^{135}{\mathrm{Cs}}^{+}$ ions using 25-350–25 ms (on-off-on) excitation pattern. The mean data points are shown in black, the fit of the theoretical curve [27] in red. The blue-shaded squares indicate the number of ions in each time-of-flight bin.

Figure 4

The three different ion spots (center, magnetron phase, and cyclotron phase) of ${}^{135}{\mathrm{Cs}}^{+}$ on the 2D position-sensitive microchannel plate detector after a typical PI-ICR excitation pattern. On the left, the magnetron phase spot is shown, on the right the cyclotron phase spot. The angle difference between the two spots is related to the cyclotron frequency of the ion species. The number of ions in each pixel is indicated by color bars (colors in the online version).

Figure 5

Difference between the cyclotron frequency ratios ${\nu }_c({}^{135}{\mathrm{Cs}}^{+})/{\nu }_c({}^{135}{\mathrm{Ba}}^{+})$ measured in this work, shown as black data points, and the weighted average value from this work $⟨R⟩=1.0000021380(24)$ represented by the solid red line and its uncertainty in red dashed lines. The dotted gray lines represent the difference between our new value and the one referred to in the AME2016 [11] with its uncertainty.

Figure 6

Shell-model calculated partial half-life of the decay of ${}^{135}\mathrm{Cs}$ to the second excited state in ${}^{135}\mathrm{Ba}$ as a function of the $Q$ value newly obtained: The red band corresponds to the range $g_A^{\text{eff}}=0.8–1.2$ of values of the axial coupling. The gray horizontal stripe gives the half-life assuming the best estimate $Q=0.44(31)\mathrm{keV}$ from the present work.