$β^-$ decay $Q$-value measurement of ${}^{136}$Cs and its implications to neutrino studies

Abstract

The $β^-$ decay $Q$-value of ${}^{136}$Cs ($J^{\ddot{I}}€=5^{+}$, $t_{1/2}\approx 13$~days) was measured with the JYFLTRAP Penning trap setup at the Ion Guide Isotope Separator On-Line (IGISOL) facility of the University of Jyväskylä, Finland. The mono-isotopic samples required in the measurements were prepared with a new scheme utilised 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 (PI-ICR) technique. The $Q$ value is determined to be 2536.83(45) keV, which is $\sim$4 times more precise and 11.4(20) keV ($\sim$ 6$\ddot{I}ƒ$) smaller than the adopted value in the most recent Atomic Mass Evaluation AME2020. The daughter, ${}^{136}$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 ${}^{136}$Cs 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 $\sim$ 17$\ddot{I}ƒ$, 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 ${\hat{I}}^2$-${\hat{I}}^3$ 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.

9 pages, 8 figures