Excitation functions for the dominant fusion channels in the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ reactions have been measured from 7 to 25 MeV (c.m.) using $\gamma$-ray spectroscopy. Comparison with the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ system shows that gross structure persists in the $\alpha$-out channel for ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ but is not apparent in this channel for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$, although observed in the inelastic channels for the latter. ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ is the first system not comprised of $\alpha$-cluster nuclei to exhibit gross structures in fusion channels.

• Received 26 December 1978

DOI:https://doi.org/10.1103/PhysRevLett.42.687