Excitation functions and angular distributions of the reactions ${}_{}{}^{20,22}{}_{}{}^{}\mathrm{Ne}(\alpha , {\gamma }_0)$ were used to study the distribution of $E2\mathrm{}$ strength in the energy regions 14-22 MeV of ${}_{}{}^{24,26}{}_{}{}^{}\mathrm{Mg}$, as well as to probe the importance of isospin in the decay of the $E1\mathrm{}$ giant resonance in these nuclei. It was found that the $E2\mathrm{}$ strength in the ($\gamma , {\alpha }_0$) channel is rather uniformly distributed and accounts for about 10% of the energy weighted sum rule in each nucleus. Together with the $E2\mathrm{}$ strength observed in lower resonances, about 45% of the sum rule is accounted for. No evidence for a compact $E2\mathrm{}$ resonance is found. A comparison of the $E1\mathrm{}$ distribution and total strength in the ($\gamma , {\alpha }_0$) channel of the giant dipole resonances in ${}_{}{}^{24,26}{}_{}{}^{}\mathrm{Mg}$ indicates that isospin conservation may be important in these reactions. Information was also obtained on the reactions ${}_{}{}^{20,22}{}_{}{}^{}\mathrm{Ne}(\alpha , {\gamma }_1)$.

NUCLEAR REACTIONS ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}(\alpha , \gamma )$, $E=2.5-20\mathrm{}$ MeV; ${}_{}{}^{22}{}_{}{}^{}\mathrm{Ne}(\alpha , \gamma )$, $E=4.5-16\mathrm{}$ MeV; measured $\sigma (E, E_{\gamma }, {\theta }_{\gamma })$. ${}_{}{}^{24,26}{}_{}{}^{}\mathrm{Mg}$ deduced $E1\mathrm{}$, $E2\mathrm{}$ strength. Enriched targets.

• Received 23 December 1974

DOI:https://doi.org/10.1103/PhysRevC.11.1525