Radiation annealing

After the recoil atoms have come to rest, subsequent reactions are stopped. Further reactions including recombination of reactive atoms or molecular fragments are possible after diffusion of the reactive species or after dissolution of the solid. Diffusion can be enhanced by increasing the temperature (thermal annealing) or by iiTadiation with y rays or electrons (radiation annealing). Dissolution may lead to recombination, reaction with other species or reaction with the solvent. Due to these processes secondary retention may increase or decrease. As an example, the retention of in the form of sulfate after neutron irradiation of ammonium sulfate and dissolution is plotted in Fig. 9.13 as a function of the time of thermal annealing at... 

Green and Maddock (S3) subsequently found that the retention of chromium-51 in potassium chromate could be increased by heating the neutron-irradiated crystals. Thermal and radiation annealing have since been observed in a nmnber of systems (4, S, 9, 20, 21, 31, 35, 39, 4I, 50, 52, 53, 54, 60, 66, 67, 74, 77, 91). In general, annealing returns a part of the initially separable recoil atoms to the foim of the parent compound. 

The problem suggested by these data is whether radiation annealing is due to the local high temperatures in the hot spots or to the enhanced probability of recombination owing to the increased supply of ions, free radicals, or electrons produced hy the radiation. 

G. Luck, R. Sizmann, The radiation annealing of Frenkel defects — an approach based on a simple statistical model, Phys. Status Solidi 5 (1964) 683—691. 

The results in this field, including the effects of ambient gases, hydration, and radiation annealing, are explained in terms of the reactivity of the Co" complex and the reversible nature of the formation of the bridged oxygen adducts. ... 

Dema, I. and Zaitseva, N. G. Radiation Annealing Behavior of the Radio-Iodine Recoils Formed by High-Energy Proton Irradiation of Cesium Chloride. J. Inorg. Nucl. Chem. 28, 2491 (1966). 21 4561... 

The energy available in various forms of irradiation (ultraviolet, X-rays, 7-rays) may be sufficient to produce in the reactant effects comparable with those which result from mechanical treatment. A continuous exposure of the crystal to radiation of appropriate intensity will result in radiolysis [394] (or photolysis [29]). Shorter exposures can influence the kinetics of subsequent thermal decomposition since the products of the initial reaction can act as nuclei in the pyrolysis process. Irradiation during heating (co-irradiation [395,396]) may exert an appreciable effect on rate behaviour. The consequences of pre-irradiation can often be reduced or eliminated by annealing [397], If it is demonstrated that irradiation can produce or can destroy a particular defect structure (from EPR measurements [398], for example), and if decomposition of pre-irradiated material differs from the behaviour of untreated solid, then it is a reasonable supposition that the defect concerned participates in the normal decomposition mechanism. 

In a more general application, thermoluminescence is used to study mechanisms of defect annealing in crystals. Electron holes and traps, crystal defects, and color-centers are generated in crystals by isotope or X-ray irradiation at low temperatures. Thermoluminescent emission during the warmup can be interpreted in terms of the microenvironments around the various radiation induced defects and the dynamics of the annealing process (117-118). ... 

It is likely that the answers to these questions will come only from more selective and sophisticated experiments than have been done hitherto, although some useful directions have been established. The use of high-sensitivity electron spin resonance for the study in situ of anticipated radical species will likely be possible, if the background signals from other radiation-produced species are not too intense. Studies of the chemistry of implanted atoms and ions in solid organometallic substrates will make it possible to start with totally unbound atoms which suffer no Auger ionization and thus to simulate the extreme of the total recoil. Careful studies of the thermal annealing effects, especially in the presence of reactive atmospheres, will... 

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