We have measured the $\gamma$-ray branching ratios of the lowest $T=\frac{3}{2}$ states in the isospin mirror nuclei ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ and ${}_{}{}^9{}_{}{}^{}\mathrm{B}$. The ratio of reduced $M1\mathrm{}$ transition strengths $\frac{B_{{\gamma }_2}\mathrm{}\left(M1\mathrm{}\right)\mathrm{}}{B_{{\gamma }_0}\mathrm{}\left(M1\mathrm{}\right)\mathrm{}}=1.97\mathrm{}\pm 0.12$ in ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ and 1.76±0.21 in ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ and hence shows no charge asymmetry, in agreement with the usual selection rules for $\Delta T=1\mathrm{}$ electromagnetic transitions. A value of (1.9±2.1)% is derived for the relative size of any isotensor amplitudes for the $M1\mathrm{}$ transitions. For the total widths of the $T=\frac{3}{2}$ levels we find $\Gamma \left({}_{}{}^9{}_{}{}^{}\mathrm{Be}\right)=381\mathrm{}\pm 33$ eV, $\Gamma \left({}_{}{}^9{}_{}{}^{}\mathrm{B}\right)=395\mathrm{}\pm 42$ eV, and $\frac{\Gamma \left({}_{}{}^9{}_{}{}^{}\mathrm{B}\right)}{\Gamma \left({}_{}{}^9{}_{}{}^{}\mathrm{Be}\right)}=1.04\mathrm{}\pm 0.10$, assuming $B_{{\gamma }_0}(M1, {}_{}{}^9{}_{}{}^{}\mathrm{B})=B_{{\gamma }_0}(M1, {}_{}{}^9{}_{}{}^{}\mathrm{Be})$ and ${\Gamma }_{{\gamma }_0}=6.9\mathrm{}\pm 0.5$ eV in ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$. The $\gamma$-ray transition strengths are compared with theoretical calculations and with the analogous $\beta$ decay of ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$.

NUCLEAR REACTIONS ${}_{}{}^7{}_{}{}^{}\mathrm{Li}({}_{}{}^3{}_{}{}^{}\mathrm{He}, p\gamma )$, ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(\mathrm{He}, n\gamma )$, particle-$\gamma$ coincidence; measured $\frac{{\Gamma }_{{\gamma }_i}}{\Gamma }$ and deduced ${\Gamma }_{{\gamma }_i}$ and $\Gamma$ for ${}_{}{}^9{}_{}{}^{}\mathrm{Be}(T=\frac{3}{2})$ and ${}_{}{}^9{}_{}{}^{}\mathrm{Be}(T=\frac{3}{2})$; symmetry of mirror transitions.

• Received 8 May 1978

DOI:https://doi.org/10.1103/PhysRevC.18.1973