Resonant heavy-ion elastic scattering froms−dshell nuclei

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.