High energy gamma-rays from highly excited thorium, californium, and meitnerium

This dissertation focuses on an understanding of fusion-fission dynamics based upon high energy giant dipole resonance gamma-rays emitted from highly excited compound nuclei. The excited nucleus 224Th was formed with the reaction 16O+208Pb at five bombarding energies ranging from 100 MeV to 177 MeV. The high energy gamma-rays were measured using a small BaF2 array and a large NaI detector. The nucleus 240Cf was formed with the reaction 32S+ 208Pb at seven bombarding energies ranging from 180 to 285 MeV; the gamma-rays were measured using a large ORNL-TAMU-MSU-SB BaF2 array containing up to 154 individual elements. The superheavy nucleus 275Mt was formed with the reaction 37Cl+238U at four bombarding energies ranging from 222 MeV to 300 MeV using the same experimental setup as the 240Cf investigation. In all cases four multi-wire avalanche counters were used to detect fission fragments in coincidence with gamma-rays. The absolute gamma/fission multiplicities were obtained for each of these reactions.;An extensive theoretical analysis of the data obtained from these systems was performed in the context of the statistical model with the inclusion of nuclear dissipation. Significant model improvements involving level densities and effects related to nuclear dissipation were made as part of this dissertation. The improved model is capable of describing all available data for each of these systems within a consistent set of model parameters.;The 224Th data are equally described by either a temperature or deformation dependent dissipation. In both cases strongly overdamped motion is indicated for the higher bombarding energies. The 240Cf and 275Mt data can be described by a constant saddle to scission lifetime on the order of 50 zs (1 zs = 10-21 s) and 20 zs, respectively. For all three systems a temperature dependent level density parameter was required to fit the high energy gamma-ray spectra. The body of data analyzed here suggests very strongly that a one- body mechanism is responsible for nuclear dissipation.