The ${\beta }^{-}$ decay $Q$ value of ${}^{136}\mathrm{Cs}$ $(J^{\pi }=5^{+}, t_{1/2}\approx 13 \mathrm{d})$ was measured with the JYFLTRAP Penning trap setup at the Ion Guide Isotope Separator On-Line facility of the University of Jyväskylä, Finland. The monoisotopic samples required in the measurements were prepared with a new scheme utilized for the cleaning, based on the coupling of dipolar excitation with Ramsey's method of time-separated oscillatory fields and the phase-imaging ion-cyclotron-resonance technique. The $Q$ value is determined to be 2536.83(45) keV, which is $\approx 4$ times more precise and 11.4(20) keV $(\approx 6\sigma )$ smaller than the adopted value in the most recent Atomic Mass Evaluation AME2020. The daughter, ${}^{136}\mathrm{Ba}$, has a $4^{+}$ state at 2544.481(24) keV and a $3^{-}$ state at 2532.653(23) keV, both of which can potentially be ultralow $Q$-value end states for the ${}^{136}\mathrm{Cs}$ decay. With our new ground-to-ground state $Q$ value, the decay energies to these two states become $-7.65\left(45\right)$ keV and 4.18(45) keV, respectively. The former is confirmed to be negative at the level of $\approx 17\sigma$, which verifies that this transition is not a suitable candidate for neutrino mass determination. On the other hand, the slightly negative $Q$ value makes this transition an interesting candidate for the study of virtual $\beta \text{−}\gamma$ transitions. The decay to the $3^{-}$ state is validated to have a positive low $Q$ value which makes it a viable candidate for neutrino mass determination. For this transition, we obtained a shell-model-based half-life estimate of $2.1_{-0.8}^{+1.6}\times {10}^{12}$ yr. Furthermore, the newly determined low reaction threshold of 79.08(54) keV for the charged-current ${\nu }_e+{}^{136}\mathrm{Xe}$ $\left(0^{+}\right)\to {}^{136}\mathrm{Cs}^{*}+e^{-}$ neutrino capture process is used to update the cross sections for a set of neutrino energies relevant to solar ${}^7\mathrm{Be}$, pep, and CNO neutrinos. Based on our shell-model calculations, the new lower threshold shows event rates of 2–4 percent higher than the old threshold for several final states reached by the different species of solar neutrinos.

• Received 7 June 2023
• Accepted 19 September 2023

DOI:https://doi.org/10.1103/PhysRevC.108.045502