Date of Award
Doctor of Philosophy (PhD)
Dr. S. Flibotte
In recent experiments it has been observed that some discrete superdeformed bands in the mass A ~ 150 region are more strongly populated when mass-symmetric target-projectile combinations are used. It is proposed in this Thesis that these population differences result from a modification of the compound-nucleus angular-momentum distribution due to the presence of low-lying vibrational states in the target/projectile nuclei. Testing this hypothesis, for reactions leading to the population of super-deformed bands in the mass A ~ 150 region, is difficult because the fission process begins to compete with the formation of evaporation residues. This, however, is not a problem in the mass A ~ 130 region and a study of the nucleus ¹³⁵Nd was therefore undertaken with the reactions ⁷⁴Ge + ⁶⁴Ni and ²⁶MG + ¹¹²Cd. Angular-momentum distributions, when coupling to vibrational states was not considered, and excitation energies of the compound nucleus ¹³⁸Nd were closely matched for the two reactions. The ⁷⁴Ge-induced mass-symmetric reaction was found to preferentially populate high-spin states, including superdeformed states in ¹³⁵Nd. This is the first report of such an effect in the mass A ~ 130 region and the first time that such measurements have been made as a function of spin for individual superdeformed states. Statistical-model calculations were shown to be in agreement with the present observations provided coupled-channel effects were taken into account. To learn more about population mechanisms in the mass A ~ 150 region, a study of superdeformed band population at very high angular momenta was initiated with the reactions ⁷⁶Ge + ⁷⁶Ge and ²⁸Si + ¹²⁴Sn. Both reactions formed the compound nucleus ¹⁵²Gd at an excitation energy of 87 MeV. Relative to normally deformed states, the yrast superdeformed band in the residual nucleus ¹⁴⁷Gd was found to be population 4.6±0.2 times more strongly for the mass-symmetric reaction compared to the mass-asymmetric reaction. Such a large difference in the population was not expected. Furthermore, the superdeformed continuum feeding the yrast superdeformed states in ¹⁴⁷Gd was found to be at least 12 times stronger, relative to the population of the channel, for the mass-symmetric reaction. Statistical-model calculations, similar to those preformed in the mass A ~ 130 region study, could not reproduce the experimental results. Experiments like the one presented in this Thesis may provide valuable information about the fission barrier at very high angular momenta.
Nieminen, John M., "A Tool to Study Fusion-Evaporation Reactions" (1998). Open Access Dissertations and Theses. Paper 2135.