180° elastic excitation functions for C12 + S32 , C13 + S32 , C12 + Si28 , and C13 + Si28 at low bombarding energies

We have measured the elastic 180\ifmmode^\circ\else\textdegree\fi{} excitation functions for $^{12}\mathrm{C}$ + $^{32}\mathrm{S}$, $^{13}\mathrm{C}$ + $^{32}\mathrm{S}$, $^{12}\mathrm{C}$ + $^{28}\mathrm{Si}$, and $^{13}\mathrm{C}$ + $^{28}\mathrm{Si}$ at low bombarding energies (${V}_{\mathrm{CB}}\ensuremath{\lesssim}{E}_{\mathrm{c}.\mathrm{m}.}\ensuremath{\lesssim}1.8 {V}_{\mathrm{CB}}$). Gross structures with features resembling those observed at higher energies were observed in the $^{12}\mathrm{C}$ + $^{32}\mathrm{S}$ and $^{12}\mathrm{C}$ + $^{28}\mathrm{Si}$ systems but not in the other two accompanying reactions. This systematic trend is consistent with compound nucleus level density and channel competition considerations. Conventional optical model potentials with very shallow absorption can generate gross structures comparable to the data in this energy region but over predict the number of peaks.NUCLEAR REACTIONS $^{12,13}\mathrm{C}$($^{32}\mathrm{S}$,$^{32}\mathrm{S}$), $^{12,13}\mathrm{C}$($^{28}\mathrm{Si}$,$^{28}\mathrm{Si}$), measured elastic $\ensuremath{\sigma}(E;180\ifmmode^\circ\else\textdegree\fi{})$; $^{12}\mathrm{C}$($^{32}\mathrm{S}$,$^{32}\mathrm{S}$) measured elastic $\ensuremath{\sigma}(\ensuremath{\theta})$, $E=55\ensuremath{-}99$ MeV; optical model analysis, parity dependent potential.