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.

NUCLEAR REACTIONS ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$, and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ beams on ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ targets, respectively; natural and enriched targets; $E_{\mathrm{lab}}=32\mathrm{} \mathrm{to} 140$ MeV; time of flight with $Z$ identification technique; fusion evaporation and direct cross section measurements from $Z=5\mathrm{} \mathrm{to} 14$ and $A=10\mathrm{} \mathrm{to} 29$; statistical model calculations; entrance channel effects discussed; macroscopic model results for total fusion cross sections.

• Received 1 December 1981

DOI:https://doi.org/10.1103/PhysRevC.26.542