Entrance channel limit on the fusion of Si28 with C12 at high energy

Cross sections for the fusion of C28 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 Z12 and mass number A24 were studied at several angles between thetalab=2°and 13°. Separation of fusion from other processes was made by comparison of kinematic calculations to measured spectra. The extracted critical angular momentum (lcrfus) of 22Latin small letter h with stroke indicates the existence of an entrance-channel-imposed limit on the high energy fusion cross section. More symmetric systems forming the same compound nucleus, Ca40, have considerably larger limiting angular momenta. This result is consistent with the previous observation of a maximum angular momentum of 22Latin small letter h with stroke (relative to the entrance channel) for the orbiting dinuclear complex found in studies of back-angle deeply inelastic scattering of Si28 and C12.