Measurement of the excitation spectra of ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$ following the decays of ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ permits the comparison of mirror Gamow-Teller transitions over a wide range of ${W_0}^{+}+{W_0}^{-}$, largely free of uncertainties associated with possible $T_z$ dependencies of nuclear overlap integrals. We find no evidence for second-class currents: Using the conventional parametrization we find that $\left|g_{\mathbf{I}\mathbf{T}}\right|<\mathrm{}7\mathrm{}\times {10}^{-4\mathrm{}}$ at the 99% confidence level as against $g_{\mathbf{I}\mathbf{T}}\approx 2\times {10}^{-3\mathrm{}}$ that comes from the direct comparison of absolute $\mathrm{ft}$ values.

• Received 10 March 1971

DOI:https://doi.org/10.1103/PhysRevLett.26.1127