Fusion, transfer, and elastic scattering at sub-barrier energies for16,18O ions onPb208andN15andO16ions onBi209

The fusion of the heavy asymmetric systems of ${}_{}{}^{16}{}_{}{}^{}$O,${\mathrm{}}^{18}$O${+\mathrm{}}^{208}$Pb and ${}_{}{}^{15}{}_{}{}^{}$N,${\mathrm{}}^{16}$O${+\mathrm{}}^{209}$Bi was studied at bombarding energies near and below the Coulomb barrier. The fusion-fission, transfer, and elastic angular distributions were measured for all four systems. The evaporation residue component of the fusion was measured for only the ${}_{}{}^{16}{}_{}{}^{}$O${+\mathrm{}}^{208}$Pb→${\mathrm{}}^{224}$Th reaction; in agreement with statistical model calculations, the cross section for the evaporation residues was found to be negligible compared to the fission, and hence can be ignored in all the systems. The shapes of the fission fragment angular distributions are then a direct measure of the entrance channel spins which lead to fusion. The experimental sub-barrier fusion cross sections are underpredicted by the one-dimensional barrier penetration model in all cases. However, if the zero point motion of the targets is included in the barrier penetration model, the energy dependence of both the total fusion and the fission angular distributions can be explained for the ${}_{}{}^{15}{}_{}{}^{}$N${+\mathrm{}}^{209}$Bi system alone. To fit the low-energy fusion of the oxygen reactions requires considerably more zero point motion—this being inconsistent with the fact that this effect should be the same in all the reactions. Also, the cross sections measured for the transfer channels were found to exhibit the same trend as the fusion at the lower energies. This correlation suggests that some of the sub-barrier fusion in the oxygen-induced reactions may be due to transfer-related processes.