A new and model-independent way of analyzing the data on magnetic moments and $\beta$ decays of mirror nuclei gives firm evidence for a quenching of the axial-vector coupling constant in nuclei. Specifically, $R=\left|\frac{C_A}{C_V}\right|=1.00\mathrm{}\pm 0.02$ is found, compared with the bare nucleon value $R=1.25\mathrm{}\pm 0.01$. The same analysis yields effective orbital $g$ factors ${\tilde{g}}_p=0.94\mathrm{}\pm 0.05$ and ${\tilde{g}}_n=0.10\mathrm{}\pm 0.04$ for proton and neutron, values which deviate from the bare values $g_p=1\mathrm{}$ and $g_n=0\mathrm{}$ in a direction opposite to that generally accepted.

• Received 21 April 1983

DOI:https://doi.org/10.1103/PhysRevLett.50.1975