Background: Measurements of the $ft$ values for $T=1/2$ mirror ${\beta }^{+}$ decays offer a method to test the conserved vector current hypothesis and to determine $V_{\mathrm{ud}}$, the up-down matrix element of the Cabibbo-Kobayashi-Maskawa matrix. In most mirror decays used for these tests, uncertainties in the $ft$ values are dominated by the uncertainties in the half-lives.

Purpose: Two precision half-life measurements were performed for the $T=1/2{\beta }^{+}$ emitters, ${}^{17}\mathrm{F}$ and ${}^{33}\mathrm{Cl}$, in order to eliminate the half-life as the leading source of uncertainty in their $ft$ values.

Method: Half-lives of ${}^{17}\mathrm{F}$ and ${}^{33}\mathrm{Cl}$ were determined using $\beta$ counting of implanted radioactive ion beam samples on a moving tape transport system at the Système de Production d'Ions Radioactifs Accélérés en Ligne low-energy identification station at the Grand Accélérateur National d'Ions Lourds.

Results: The ${}^{17}\mathrm{F}$ half-life result, 64.347 (35) s, precise to $\pm 0.05\%$, is a factor of 5 times more precise than the previous world average. The half-life of ${}^{33}\mathrm{Cl}$ was determined to be 2.5038 (22) s. The current precision of $\pm 0.09\%$ is nearly 2 times more precise compared to the previous world average.

Conclusions: The precision achieved during the present measurements implies that the half-life no longer dominates the uncertainty of the $ft$ values for both $T=1/2$ mirror decays ${}^{17}\mathrm{F}$ and ${}^{33}\mathrm{Cl}$.

• Received 11 June 2015
• Revised 27 August 2015

DOI:https://doi.org/10.1103/PhysRevC.92.045503

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Published by the American Physical Society