β− decay Q-value measurement of 136Cs and its implications for neutrino studies

The β− decay Q value of 136Cs (Jπ = 5+, t1/2 ≈ 13 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 ≈4 times more precise and 11.4(20) keV (≈6σ) smaller than the adopted value in the most recent Atomic Mass Evaluation AME2020. The daughter, 136Ba, 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 136Cs decay. With our new ground-to-ground state Q value, the decay energies to these two states become −7.65(45) keV and 4.18(45) keV, respectively. The former is confirmed to be negative at the level of ≈17σ, 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 β-γ 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+1.6 −0.8 × 1012 yr. Furthermore, the newly determined low reaction threshold of 79.08(54) keV for the charged-current νe + 136Xe (0+) → 136Cs∗ + e− neutrino capture process is used to update the cross sections for a set of neutrino energies relevant to solar 7 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.