Spin distribution of the compound nucleus in heavy ion reactions at near-barrier energies

This thesis describes the first measurements characterizing the spin distribution of compound nuclei at near-barrier energies. The α + 154Sm, 12C + 154Sm and 16O + 154Sm systems have been studied. The fusion cross section (the zeroth moment of the spin distribution) was obtained from high resolution gamma ray measurements of the yield of discrete gamma transitions in the evaporation residues. The first moment (mean angular momentum) was obtained from gamma ray multiplicity information deduced from the gamma-gamma coincidence rate. Both the cross sections and the mean angular momenta show increasingly large deviations from the sharp-cutoff model as the bombarding energy decreases. A study of the dependence of these quantities on projectile mass show the expected increase in the deviations from the sharp-cutoff model as the projectile mass increases. A number of theoretical models for understanding the effects observed are explored in this thesis. A near-quantitative reproduction of all of the observed effects and dependencies has been obtained by a model which takes into account both the quantum mechanical barrier penetration and reflection and also averages over the orientation of the deformed target nucleus. These results have important implications for the interpretation of fission fragment angular distributions and neutron-fission competition at near-barrier energies. 55 refs., 43 figs., 10 tabs