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 BC to ~6BC. 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 ~2BC). 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.NUCLEAR REACTIONS ¹⁶O, ¹⁷O, and ¹⁸O beams on ¹⁴C, ¹³C, and ¹²C targets, respectively; natural and enriched targets; Elab=32 to 140 MeV; time of flight with Z identification technique; fusion evaporation and direct cross section measurements from Z=5 to 14 and A=10 to 29; statistical model calculations; entrance channel effects discussed; macroscopic model results for total fusion cross sections.