Fusion-fission in the 16O + /sup 40,44/Ca and 32S + 24Mg reactions

In measurements of fully-damped yields for the 16O + /sup 40,44Ca and 32S + 24Mg reactions, evidence for a fusion-fission reaction mechanism were found. The experiments were performed at the Argonne tandem-linac booster facility. Reaction products with masses 22 < A < 36 for the 16O + /sup 40,44Ca reactions and 10 < A < 28 for the 32S + 24Mg reaction were detected and identified using the time-of-flight technique. The 16O + /sup 40,44Ca reactions were measured at several center-of-mass energies from 49.5 to 62.5 MeV, while the 32S + 24Mg data were obtained at a single energy of E/sub cm/ = 60 MeV. The resulting total kinetic energies (calculated assuming two-body kinematics), angular distributions, and mass distributions of the fragments are found to be consistent with a fusion-fission reaction mechanism. An alternative explanation of the yields in terms of a deep-inelastic scattering mechanism is ruled out by the asymmetric nature of the mass distributions: the maximum cross section for the 32S + 224Mg reaction is detected in the mass 12 channel whereas the maximum cross sections for deep-inelastic scattering would be expected to occur near the target-projectile mass partition. For the 32S + 4Mg reaction at E/sub cm/ = 60 MeV, the fission cross sections is found to be approximately 6% of the evaporation-residue cross section. These results indicate that at higher energies fission competes with light particle evaporation in compound nuclear decay - even in these light systems. 8 refs