Direct determination of the ${}^{138}$La ${\hat{I}}^2$-decay $Q$ value using Penning trap mass spectrometry

Abstract

Background: The understanding and description of forbidden decays provides interesting challenges for nuclear theory. These calculations could help to test underlying nuclear models and interpret experimental data. Purpose: Compare a direct measurement of the ${}^{138}$La ${\hat{I}}^2$-decay $Q$ value with the ${\hat{I}}^2$-decay spectrum end-point energy measured by Quarati et al. using LaBr${}_3$ detectors [Appl. Radiat. Isot. 108, 30 (2016)]. Use new precise measurements of the ${}^{138}$La ${\hat{I}}^2$-decay and electron capture (EC) $Q$ values to improve theoretical calculations of the ${\hat{I}}^2$-decay spectrum and EC probabilities. Method: High-precision Penning trap mass spectrometry was used to measure cyclotron frequency ratios of ${}^{138}$La, ${}^{138}$Ce and ${}^{138}$Ba ions from which ${\hat{I}}^2$-decay and EC $Q$ values for ${}^{138}$La were obtained. Results: The ${}^{138}$La ${\hat{I}}^2$-decay and EC $Q$ values were measured to be $Q$ = 1052.42(41) keV and $Q_{EC}$ = 1748.41(34) keV, improving the precision compared to the values obtained in the most recent atomic mass evaluation [Wang, et al., Chin. Phys. C 41, 030003 (2017)] by an order of magnitude. These results are used for improved calculations of the ${}^{138}$La ${\hat{I}}^2$-decay shape factor and EC probabilities. New determinations for the ${}^{138}$Ce 2EC $Q$ value and the atomic masses of ${}^{138}$La, ${}^{138}$Ce, and ${}^{138}$Ba are also reported. Conclusion: The ${}^{138}$La ${\hat{I}}^2$-decay $Q$ value measured by Quarati et al. is in excellent agreement with our new result, which is an order of magnitude more precise. Uncertainties in the shape factor calculations for ${}^{138}$La beta-decay using our new $Q$ value are reduced by an order of magnitude. Uncertainties in the EC probability ratios are also reduced and show improved agreement with experimental data.

Nine pages, eight figures