Entrance channel effect for complete fusion of O + C isotopes

The cross section for total fusion of the reaction ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ was studied over a large range of energies from near the Coulomb barrier $B_C$ to $\sim 6B_C$. Good agreement was found between the critical angular momenta deduced from the experimental results and the predictions of different models. The reaction ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, leading to the same compound nucleus ${}_{}{}^{30}{}_{}{}^{}\mathrm{Si}$, was studied in the second fusion region (above $\sim 2B_C$). By comparing the relative cross sections for fusion-evaporation to each isotope it is shown that for different entrance channels, even at the highest energies studied, the reactions appear to pass by the formation of a compound nucleus. The critical angular momenta were found to be systematically different from ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$. This difference, which may be due partially to the entrance channel spin, is interpreted as arising from the effect of direct reactions diverting flux from the compound nuclear processes.