A Triggering Mechanism for the Promotion of Thermal Annealing in Crystalline Hexabromoethane by Radiation-Produced Defects

Thermal annealing of recoil-Br82 to the parent form in hexabromoethane appears to be due to at least five different processes. Several of these occur as independent processes in the absence of radiation-produced defects; the over-all kinetics of these processes are resolvable into first order components. However, a substantial fraction of the recoil-Br82 does not anneal to the parent hexabromoethane form, even at temperatures as high as 120°C, in the absence of radiation- produced defects. When defects are produced in hexabromoethane crystals by ionizing radiation at -196°C prior to the introduction of the recoil-Br82 atoms, these defects tend to promote the subsequent thermal annealing of an additional fraction of the recoil-Br82 atoms. This fraction is a logarithmic function of the total dose. The promotion of the ''nonrannealing'' Br82 species to species that anneal at temperatures in the 0° to 120°C range has recently been found to occur by a sequence of steps, whereby increasing doses of ionizing radiation first promote one kind of annealing process up to a point of saturation, then proceed to promote other annealing processes. A model is introduced to account for the observed changes in the distribution of recoil-Br82 among the various annealing and non-annealing components as a function of the prior-inadiation dose. The model assumes a radiation-produced species that is stable in the crystalline hexabromoethane at -196°C. This species can diffuse at annealing temperatures to interact with recoil-Br82 sites and promote the thermal annealing of these sites. The kinetics of thermal annealing in hexabromoethane at several temperatures and at several dose levels are discussed in terms of this model. The possible extension of this model to radiation annealing in hexabromoethane and to other crystalline compounds is also discussed. (author)