Fully relaxed heavy ion reactions in the mass 80 region
This thesis describes a set of experiments to investigate the properties of fully relaxed reactions in the mass 80 region. Symmetric and asymmetric entrance channels, leading to similar compound nuclei, have been compared in order to determine the characteristics of the reaction mechanism. The symmetric system 40Ca + 40Ca has been studied at laboratory energies of 197.3 and 230.5 MeV. The fissionlike products were detected by a series of six time of flight telescopes, placed at angles between 10° and 52.4° to the beam, and a Bragg curve spectrometer-multiwire proportional chamber telescope, situated at 30°. Data were collected in both singles and coincidence modes. To complement this measurement, fully damped yields from 28Si + 50,52,54 Cr at 130 MeV and from 28Si + 50Cr at 150 MeV were measured using an array of four avalanche counter-silicon surface barrier detector telescopes spanning the angular range between 20° and 50°. The avalanche counters used in this experiment are of a new design which was developed in Oxford. We present the features of this design and discuss the detectors' operational characteristics. Several possible interpretations of the data are considered. It is shown that diffusion models of deep-inelastic scattering tend to underpredict the measured fully relaxed cross sections. Rotating finite range model calculations of fission barriers point to the possibility of fast-fission in this mass region; a process hitherto associated with much heavier systems. Furthermore, such models also predict that these systems may fission asymmetrically. The more commonly employed rotating liquid drop model of the nucleus is unable to predict either of these effects. The data are compared with those reported in the literature for other systems in this mass region. It is argued that all these results are consistent with fast-fission and asymmetric fission, thus obviating the need for the previously proposed interpretation in terms of intermediate mechanisms.