Investigation of the potential ultralow $Q$-value $\beta$-decay candidates ${}^{89}\mathrm{Sr}$ and ${}^{139}\mathrm{Ba}$ using Penning trap mass spectrometry

Background: Ultralow $Q$-value $\beta$ decays are interesting processes to study with potential applications to nuclear $\beta$-decay theory and neutrino physics. While a number of potential ultralow $Q$-value $\beta$-decay candidates exist, improved mass measurements are necessary to determine which of these are energetically allowed.

Purpose: To perform precise atomic mass measurements of ${}^{89}\mathrm{Y}$ and ${}^{139}\mathrm{La}$. Use these new measurements along with the precisely known atomic masses of ${}^{89}\mathrm{Sr}$ and ${}^{139}\mathrm{Ba}$ and nuclear energy level data for ${}^{89}\mathrm{Y}$ and ${}^{139}\mathrm{La}$ to determine if there could be an ultralow $Q$-value decay branch in the $\beta$ decay of ${}^{89}\mathrm{Sr}\to {}^{89}\mathrm{Y}$ or ${}^{139}\mathrm{Ba}\to {}^{139}\mathrm{La}$.

Method: High-precision Penning trap mass spectrometry was used to determine the atomic mass of ${}^{89}\mathrm{Y}$ and ${}^{139}\mathrm{La}$, from which $\beta$-decay $Q$ values for ${}^{89}\mathrm{Sr}$ and ${}^{139}\mathrm{Ba}$ were obtained.

Results: The ${}^{89}\mathrm{Sr}\to {}^{89}\mathrm{Y}$ and ${}^{139}\mathrm{Ba}\to {}^{139}\mathrm{La}\beta$-decay $Q$ values were measured to be $Q_{\mathrm{Sr}}=1502.20\left(0.35\right)$ keV and $Q_{\mathrm{Ba}}=2308.37\left(0.68\right)$ keV. These results were compared to energies of excited states in ${}^{89}\mathrm{Y}$ at 1507.4(0.1) keV, and in ${}^{139}\mathrm{La}$ at 2310(19) keV and 2313(1) keV to determine $Q$ values of $-5.20\left(0.37\right)$ keV for the potential ultralow $\beta$-decay branch of ${}^{89}\mathrm{Sr}$ and $-1.6\left(19.0\right)$ keV and $-4.6\left(1.2\right)$ keV for those of ${}^{139}\mathrm{Ba}$.

Conclusion: The potential ultralow $Q$-value decay branch of ${}^{89}\mathrm{Sr}$ to the ${}^{89}\mathrm{Y}$ ($3/2^{-}$, 1507.4 keV) state is energetically forbidden and has been ruled out. The potential ultralow $Q$-value decay branch of ${}^{139}\mathrm{Ba}$ to the 2313 keV state in ${}^{139}\mathrm{La}$ with unknown $J^{\pi }$ has also been ruled out at the $4\sigma$ level, while more precise energy level data is needed for the ${}^{139}\mathrm{La}$ ($1/2^{+}$, 2310 keV) state to determine if an ultralow $Q$-value $\beta$-decay branch to this state is energetically allowed.

Figure 1

Decay schemes for ${}^{89}\mathrm{Sr}$ and ${}^{139}\mathrm{Ba}$ showing the main $\beta$-decay branches (solid black arrows) and the potential ultralow $Q$-value decay branches (dashed blue arrows) investigated in this work. The ground-state to ground-state $Q$ values are obtained using data from the AME2016 [17]. All values are given in units of keV.

Figure 2

A $t_{rf}=1$ s cyclotron frequency resonance curve for ${}^{89}\mathrm{Y}$ (see text for details). The solid red line is a fit to the theoretical line shape [31].

Figure 3

The mass excesses measured in this work. The red bands show the AME2016 uncertainty and the black dots are the measured values.