Entrance-Channel Resonance Effects in theMg24+O16Reaction

Backward-angle cross sections for the reactions ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$)${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$(g.s.; 1.78 MeV) and ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$)${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$(g.s.; 1.37 MeV) were obtained for $17 \mathrm{MeV}<~E_{\mathrm{c}.\mathrm{m}.\mathrm{}}<~31 \mathrm{MeV}$, ${\overline{\theta }}_{\mathrm{c}.\mathrm{m}.\mathrm{}}\simeq 176°$. The first reaction shows gross and fine resonant structures; the second shows mainly gross structures. Angular distribution measurements give $L$ values of 15 and 21, respectively, at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=21.6\mathrm{} \mathrm{and} 28.0$ MeV. The resonant gross structures for ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$)${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$(g.s.) are governed by the entrance channel and are discussed from the shaperesonance point of view.