Angular distributions at angles $130°<{\theta }_{\mathrm{cm}}<180\mathrm{}°$ have been measured for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ as well as ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$, ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ in the energy range $20 \mathrm{MeV}<E_{\mathrm{cm}}<35\mathrm{} \mathrm{MeV}$. Cross sections rising towards 180° are observed for all reactions. Excitation functions for the back angle enhancement show distinct structures, most pronounced for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$. Angular distributions for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, especially those corresponding to peaks in the excitation function show oscillations of the type $P_J^2\left(cos\theta \right)$. The ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ back angle enhancement decreases with target mass. Backscattering of the non-$\alpha$ nuclei ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ and ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ is reduced by about two orders of magnitude in comparison with ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$. Standard theoretical approaches fail to explain all the observed effects.

NUCLEAR REACTIONS ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, measured $\sigma (\theta ,E)$ elastic scattering, $130°<{\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}<180\mathrm{}°$, $20 \mathrm{MeV}<E_{\mathrm{c}.\mathrm{m}.\mathrm{}}<35\mathrm{} \mathrm{MeV}$, deduced $J^{\pi }$ of resonances.

• Received 18 August 1978

DOI:https://doi.org/10.1103/PhysRevC.19.740