We have measured the $\gamma$-ray branching ratios of the lowest $T=\frac{3}{2}$ levels in ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{13}{}_{}{}^{}\mathrm{N}$, and the absolute strength of the ${\gamma }_0$ transition in ${}_{}{}^{13}{}_{}{}^{}\mathrm{N}$. The mirror electromagnetic selection rule is obeyed by the $M1\mathrm{}$ (${\gamma }_0$ and ${\gamma }_2$) transitions. However the $E1\mathrm{}$ (${\gamma }_1$) transitions exhibit a surprisingly large charge asymmetry. Charge-dependent differences in the radial wave functions do not account for a similar asymmetry in strong $T=\frac{1}{2}\to T=\frac{1}{2}$ $E1\mathrm{}$ transitions in mass 13.

• Received 7 April 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.202