Entrance channel effect for complete fusion of O + C isotopes

The cross section for total fusion of the reaction 18O+12C was studied over a large range of energies from near the Coulomb barrier B/sub C/ to approx.6B/sub C/. Good agreement was found between the critical angular momenta deduced from the experimental results and the predictions of different models. The reaction 17O+13C, leading to the same compound nucleus 30Si, was studied in the second fusion region (above approx.2B/sub 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 18O+12C. 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