Entrance channel limit on the fusion of /sup 28/Si with /sup 12/C at high energy

Cross sections for the fusion of /sup 28/Si+/sup 12/C have been measured at center of mass bombarding energies of 39, 43.5, 48, and 54 MeV. Velocity and energy spectra of reaction products having charge Z > or =12 and mass number A > or =24 were studied at several angles between theta/sub lab/ = 2/sup 0/ and 13/sup 0/. Separation of fusion from other processes was made by comparison of kinematic calculations to measured spectra. The extracted critical angular momentum (l/sub cr//sup fus/) of 22h-dash-bar indicates the existence of an entrance-channel-imposed limit on the high energy fusion cross section. More symmetric systems forming the same compound nucleus, /sup 40/Ca, have considerably larger limiting angular momenta. This result is consistent with the previous observation of a maximum angular momentum of 22h-dash-bar (relative to the entrance channel) for the orbiting dinuclear complex found in studies of back-angle deeply inelastic scattering of /sup 28/Si and /sup 12/C.