A Tool to Study Fusion-Evaporation Reactions

Abstract

<p>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.</p>