FISSION DECAY OF CR-48 AT E-ASTERISK(CN)APPROXIMATE-TO-60 MEV

The fully energy-damped yields for the Ar-36 + C-12 and Ne-20 + Si-28 reactions at E(c.m.)=47.0 MeV and 45.5 MeV, respectively, are explored using particle-particle-gamma coincidence data. These reactions reach a similar excitation energy of E(CN)=59.5 MeV in the Cr-48 compound nucleus as was obtained in an earlier particle-particle coincidence st udy of the Mg-24 + Mg-24 reaction. The overall mass and total kinetic energy distributions of the fission fragments are found to be well rep roduced by statistical-model calculations. These calculations are also found to reproduce structure seen in the excitation-energy spectra fo r the Ne-20 + Si-28 and Mg-24 + Mg-24 exit channels for all three reac tions. In previous excitation-function measurements, strong heavy-ion resonance behavior has been observed in elastic and inelastic cross se ctions for the Mg-24 + Mg-24 system. There has been speculation that p eaks observed in the corresponding excitation-energy spectra at more n egative Q values may also be a consequence of this resonance phenomeno n. The observation of very similar behavior with the asymmetric-mass e ntrance channels makes it less likely, though, that the peaks arise fr om any special configuration of the compound system. Instead, an analy sis of the gamma-ray data and the results of statistical-model calcula tions support the conclusion that most of the observed high-lying stru cture can be accounted for in terms of statistical fission from a full y energy- and shape-equilibrated compound nucleus. For the Mg-24 + Mg- 24 entrance channel, however, comparisons with the statistical model i ndicate a reduction of high-angular-momentum partial cross sections, l eading to the Mg-24 + Mg-24 fission channel. For the first time, we ar e able to deduce the nature of the competition between the resonance a nd statistical-fission mechanisms in this mass region.