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Fully energy-damped yields from the S+ Mg reaction have been measured at center-of-mass energies of E, =51.6 and 60.5 MeV with the use of an experimental arrangement where both of the resulting heavy fragments could be detected in coincidence. Energy, velocity, and angular distributions 'of the reaction fragments have been determined. The cross sections prior to secondary light-particle emission have been deduced for the breakup of the compound system into different mass channels. These data are discussed in terms of two possible reaction mechanisms: fusion followed by fission and deep-inelastic orbiting.

Cross sections for the two-body channels populated in the 32 S+ 24 Mg reaction at E c . m . =60.8 MeV have been measured by use of a coincidence technique which allows correction for secondary lightparticle evaporation. The data show reaction yields with full equilibration of energy and massasymmetry coordinates. These results suggest an asymmetric fission mechanism and are contrary to what is expected from the previously proposed "orbiting" mechanism in light systems.

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Velocity distributions of mass-identified evaporation residues produced in the Si+ Ca reaction have been measured at bombarding energies of 309, 397, and 452 MeV using time-of-flight techniques. These distributions were used to identify evaporation residues and to separate the complete-fusion and incomplete-fusion components. Angular distributions and upper limits for the total evaporation-residue and complete-fusion evaporation-residue cross sections were extracted at all three bombarding energies. The complete-fusion evaporation-residue cross sections and the deduced critical angular momenta are compared with earlier measurements and the predictions of existing models. The ratios of the complete-fusion evaporation-residue cross section to the total evaporation-residue cross section, along with those measured for the Si+ ' C and Si+ Si systems at the same energies, support the entrancechannel mass-asymmetry dependence of the incomplete-fusion evaporation-residue process reported earlier.PACS number(s): 25.70.Jj

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Evaporation residues following the interaction of 12 C with targets of 197 Au, 160 Gd, and 120 Sn were measured in the energy range of isiab^S.S-lO MeV/nucleon. The experimental system allowed the determination of the cross sections of the complete fusion (CF) and the incomplete fusion (ICF) processes with a sensitivity to ICF processes below the percent level. The ICF processes were observed at all bombarding energies studied, down to energies slightly above the Coulomb barrier. The two processes have very different angular distributions, with the ICF peaked at a significantly larger angle than the CF.

The total fusion cross sections for the systems 12 C + 12 C, 18, 0 + l2 C, and 19 F+ 12 C have been measured as a function of bombarding energy and compared with results for other light heavy-ion systems. The presence or absence of oscillations in the fusion excitation function and the overall magnitude of the cross section at high energies appear to depend on the structure of the colliding nuclei.The energy dependence of the cross section for the complete fusion of two complex nuclei has received considerable recent attention. 1 In most cases the measured excitation functions are smooth, as is consistent with macroscopic models of such processes. In a recent study of the 16 0 + 12 C system, unexpected oscillatory structure was seen in the energy dependence of the fusion cross section. 2 In the present study we have bombarded 12 C targets with several other ions, near in mass to le O, in order to establish the range of nuclei over which structure is seen in the fusion excitation function, and also to delineate the macroscopic features of the fusion process.The fusion cross sections for the systems 12 C + 12 C, 18 0+ 12 C, and 19 F+ 12 C were measured as a function of bombarding energy using beams obtained from the Argonne National Laboratory FN tandem accelerator. The experimental procedure and uncertainties have been described previously. 2 All reaction products with higher Z than the incident ions were assumed to be evaporation residues and were included in defining the fusion cross section.The fusion cross sections [o fus (E)] for the three systems studied are shown in Fig. 1 as a function of cm. energy. The solid lines in Fig. 1 are calculated using the model of Glas and Mosel 3 which is based on the assumption that fusion occurs whenever the nuclei reach a critical separation distance, R c . The parameters used to fit the data are listed in Table I. The experimental results confirm the qualitative prediction of the model that a fus for such a light system should saturate at some critical bombarding energy and then decrease or remain roughly constant at higher energies.There are two qualitative features in our exper-1.2 H -Q 0.61 1.0 0.8 0.6h 0.41