Investigation of potential ultra-low $Q$-value ${\hat{I}}^2$-decay candidates ${}^{89}$Sr and ${}^{139}$Ba using Penning trap mass spectrometry

Abstract

Background: Ultra-low $Q$-value ${\hat{I}}^2$-decays are interesting processes to study with potential applications to nuclear ${\hat{I}}^2$-decay theory and neutrino physics. While a number of potential ultra-low $Q$-value ${\hat{I}}^2$-decay candidates exist, improved mass measurements are necessary to determine which are energetically allowed. Method: Penning trap mass spectrometry was used to determine the atomic mass of ${}^{89}$Y and ${}^{139}$La, from which ${\hat{I}}^2$-decay $Q$-values for ${}^{89}$Sr and ${}^{139}$Ba were obtained to determine if there could be an ultra-low $Q$-value decay branch in the ${\hat{I}}^2$-decay of ${}^{89}$Sr $\to$ ${}^{89}$Y or ${}^{139}$Ba $\to$ ${}^{139}$La. Results: The ${}^{89}$Sr $\to$ ${}^{89}$Y and ${}^{139}$Ba $\to$ ${}^{139}$La ${\hat{I}}^2$-decay $Q$-values were measured to be $Q_{\mathrm{Sr}}$ = 1502.20(0.35) keV and $Q_{\mathrm{Ba}}$ = 2308.37(68) keV. These were compared to energies of excited states in ${}^{89}$Y at 1507.4(1) keV, and in ${}^{139}$La at 2310(19) keV and 2313(1) keV to determine $Q$-values of -5.20(37) keV for the potential ultra-low ${\hat{I}}^2$-decay branch of ${}^{89}$Sr and -1.6(19.0) keV and -4.6(1.2) keV for those of ${}^{139}$Ba. Conclusion: The potential ultra-low $Q$-value decay branch of ${}^{89}$Sr to the ${}^{89}$Y (3/2${}^{-}$, 1507.4 keV) state is energetically forbidden and has been ruled out. The potential ultra-low $Q$-value decay branch of ${}^{139}$Ba to the 2313 keV state in ${}^{139}$La with unknown J${}^{\ddot{I}}€$ has also been ruled out at the 4$\ddot{I}ƒ$ level, while more precise energy level data is needed for the ${}^{139}$La (1/2${}^{+}$, 2310 keV) state to determine if an ultra-low $Q$-value ${\hat{I}}^2$-decay branch to this state is energetically allowed.

5 pages, 3 figures