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

Cross sections for the fusion of /sup 28/Si+/sup 12/C and /sup 30/Si+/sup 12/C 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 (l/sub cr//sup max/) 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 l/sub cr//sup max/ using the finite range model of Sierk for diffuse-surface nuclei.