Fusion cross sections for four heavy-ion entrance channels leading to theNa23compound nucleus

Excitation functions for the yields of residual nuclei from the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$+${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$+${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ reactions have been measured for ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ laboratory bombarding energies of 12 to 34 MeV and 15 to 63 MeV, respectively. Beginning at Coulomb barrier energies, we find that the total fusion cross section diverges from the total reaction cross section obtained from optical model fits to elastic scattering data. This indicates that the process responsible for fusion cross section limitations in this mass region begins near the Coulomb barrier and becomes progressively more important as the bombarding energy is increased. Comparison of the critical angular momenta deduced from the total fusion cross sections for these and two previously investigated entrance channels which form the ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ compound nucleus, ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$+${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$+${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, rules out a compound nucleus limitation at these energies. A systematic study of fusion cross sections in this mass and energy region, together with the results of a previous study of the light particles produced in the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$+${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$+${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ reactions, suggests that competition for entrance channel flux by reactions producing light particles with projectilelike velocities is primarily responsible for the fusion cross section limitations.