Half-lives and branching ratios for the Fermi $\beta$ decays of ${}_{}{}^{18}{}_{}{}^{}\mathrm{Ne}$, ${}_{}{}^{22}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{26}{}_{}{}^{}\mathrm{Si}$, and ${}_{}{}^{34}{}_{}{}^{}\mathrm{Ar}$ have been studied in experiments which employed a gas-transport system. The $\mathrm{ft}$ values for the last two are compared with the mirror decays of ${}_{}{}^{26}{}_{}{}^{}\mathrm{Al}_{}^m{}_{}{}^{}$ and ${}_{}{}^{34}{}_{}{}^{}\mathrm{Cl}$. A difference of (4.6 ± 1.7)% is observed for mass 34 and may be attributable to unexpectedly large charge-dependent mixing. Possible implications are discussed with respect to Cabibbo theory and the existence of an intermediate vector boson.

• Received 30 August 1972

DOI:https://doi.org/10.1103/PhysRevLett.29.1027