A simple model is constructed for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$+${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ elastic angular distribution and excitation function at back angles. A "structure" concentrated at a critical value of angular momentum smaller than the grazing value in $l$ space is suggested as a viable mechanism. Possible connection to $\alpha$-particle transfer contribution is pointed out. A short comparison with other models is made.

NUCLEAR REACTIONS Back angle and 90° scattering of ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$+${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ is discussed.

• Received 24 September 1981

DOI:https://doi.org/10.1103/PhysRevC.25.1896