Capture cross sections for very heavy systems
In intermediate-mass systems, collective excitations of the target and projectile can greatly enhance the subbarrier capture cross section sigma(cap) by giving rise to a distribution of Coulomb barriers. For such systems, capture essentially leads directly to fusion [formation of a compound nucleus (CN)], which then decays through the emission of light particles (neutrons, protons, and alpha particles). Thus, the evaporation-residue (ER) cross section is essentially equal to sigma(cap). For heavier systems, the experimental situation is significantly more complicated owing to the presence of quasifission (QF) (rapid separation into two fragments before the CN is formed) and by fusion-fission (FF) of the CN itself. Thus, three cross sections need to be measured in order to evaluate sigma(cap). Although the ER essentially recoil along the beam direction, QF and FF fragments are scattered to all angles and require the measurement of angular distributions in order to obtain the excitation function and barrier distribution for capture. Two other approaches to this problem exist. If QF is not important, one can still measure just the ER cross section and try to reconstruct the corresponding sigma(cap) through use of an evaporation -model code that takes account of the FF degree of freedom. Some earlier results on sigma(cap) obtained in this way will be reanalyzed with detailed coupled-channels calculations, and the "extra-push" phenomenon discussed. One may also try to obtain sigma(cap) by exploiting unitarity, that is, by measuring instead the flux of particles corresponding to quasielastic (QE) scattering from the Coulomb barrier. Some new QE results obtained for the Kr-86 + Pb-208 system at iThemba LABS in South Africa will be presented