We have investigated the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$,$\gamma$)${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ reaction, populating low-lying states of ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$, with ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ c.m. energies between 5 and 11 MeV. High energy $\gamma$ rays were detected in a large volume NaI spectrometer which had sufficient resolution to distinguish clearly the ${\gamma }_0$, ${\gamma }_1$, and ${\gamma }_{2,3}$ transitions. Several resonances were observed in each of the decay channels. In particular, the ${\gamma }_0$ excitation function has four narrow resonances at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=5.6, 6.0, 6.8, \mathrm{and} 8.0$ MeV, which are unambiguously assigned $J^{\pi }=2^{+}$. The upper three of these resonances are correlated in all three decay channels but are not correlated either with previously identified $2^{+}$ structures in the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ system or with structure in the distribution of ground-state $E2\mathrm{}$ strength in ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$. Furthermore, calculations demonstrate that these resonances have large nonstatistical ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$- and photon-partial widths. We argue that the data are suggestive of a new and unusual form of intermediate structure, namely highly clustered ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ configurations built on the ground-state band of ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$.

NUCLEAR REACTIONS ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$,$\gamma$)${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$, $5<~E_{\mathrm{c}.\mathrm{m}.\mathrm{}}<~11$ MeV; measured $E\gamma$, $\frac{d\sigma }{d\Omega }(45\mathrm{}°,E_{{\gamma }_0})$, $\frac{d\sigma }{d\Omega }(45\mathrm{}°,E{\gamma }_1)$, $\frac{d\sigma }{d\Omega }(45\mathrm{}°,E{\gamma }_{2,3})$; deduced $\Gamma$, ${\Gamma }_{\gamma } \frac{{\Gamma }_{{}_{}{}^{12}{}_{}{}^{}\mathrm{C}}}{\Gamma }$, and $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}$ for isolated resonances.

• Received 3 April 1981

DOI:https://doi.org/10.1103/PhysRevC.24.932