Effect of entrance channel asymmetry on fusion reactions leading to compound nuclei with A=40,42

Cross sections for the fusion of Si28 + C12 and Si30 + C12 have been measured at energies between 6.4 and 9.4 MeV/nucleon and 5.2 and 8.3 MeV/nucleon, respectively. Comparison with existing data for compound systems with A=40 and 42 shows that the extracted critical angular momentum at saturation (lcrmax) depends strongly on the entrance channel mass asymmetry. This effect is not accounted for by the standard entrance channel or compound nucleus models. A calculation of the conditional saddle-point shapes for touching spheroidal nuclei indicates that this mass asymmetry effect is a consequence of the dynamical fusion thresholds at high angular momentum. Quantitative agreement is obtained with the experimentally measured lcrmax using the finite range model of Sierk for diffuse-surface nuclei.