Shell-Structure Effect on Elastic $\alpha$ Scattering

Back-angle enhancements of elastic $\alpha$-scattering cross sections have been observed for nuclei at the ends of the $1p$, $2s-1d$, and $f_{\frac{7}{2}}$ shells. Strong reduction of this enhancement occurs if excess neutrons enter the next open major shell. The results are discussed in terms of intermediate $\alpha$ structure.