The heavy-ion radiative capture ${}^{12}\mathrm{C}$$({}^{16}\mathrm{O}$$,\gamma {}^{28}\mathrm{Si})$ was measured at the sub-Coulomb barrier bombarding energy $E_{\mathrm{lab}}=15.7$ MeV, which corresponds to the lowest important resonance observed in the ${}^{12}\mathrm{C}+$ ${}^{16}\mathrm{O}$ fusion excitation function. Thanks to combination of the bismuth germanate (BGO) $\gamma$-ray array and the $0^{\circ }$ DRAGON electromagnetic spectrometer at TRIUMF, the $\gamma$-decay spectrum from the entrance channel down to the ground state of ${}^{28}$Si was measured. Comparisons of the experimental spectrum to $\gamma$ spectrum extracted from Monte Carlo simulations of the complete setup suggest a $J^{\pi }=2^{+}$ spin-parity assignment to the entrance channel and yield the radiative capture cross section ${\sigma }_{RC}=0.22\pm 0.04\mu$b. Combining this present spin assignment with previous data on radiative capture, a $J(J+1)$ systematics was constructed, and it indicated a moment of inertia commensurate with the ${}^{12}\mathrm{C}+$${}^{16}\mathrm{O}$ grazing angular momentum. Strong dipole transitions are observed from the entrance channel to $T=1$ states around 11.5 MeV and are found to result from enhanced $M{1}_{IV}$ transitions to states exhausting a large part of the $M1$ sum rule built on the ground state of ${}^{28}$Si. This specific decay was also reported at bombarding energies close to the Coulomb barrier in our previous study of the ${}^{12}\mathrm{C}$$({}^{12}\mathrm{C}$$,{\gamma }^{24}$Mg) heavy-ion radiative capture reaction. Similarities between both systems are investigated.

• Received 6 September 2013
• Revised 19 November 2013

DOI:https://doi.org/10.1103/PhysRevC.89.014305