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When materials are subjected to irradiation by energetic particles such as fast neutrons, the primary radiation damage event is the formation of vacancies, interstitials, and more complex defects along the trajectory of the recoil lattice atoms. Defects are also formed by purely ionizing radiations such as gamma rays. Several different mechanisms have been proposed to account for this type of defect formation and these are discussed. One of the fundamental problems of radiation damage is to determine the various types of defects formed, their concentration, spatial distribution, rate of formation, and so forth, and how the properties of each type of defect depend on parameters such as the energy of the recoiling atom, the sample temperature, etc. In nonmetals, some of these defects are, or can be, converted into centers that absorb light, or color centers. Each of these centers gives rise to different optical absorption bands, and from absorption measurements the center or defect concentration can be computed. Unfortunately, numerous absorption bands have been found in many crystals that have not been unequivocally attributed to specific defects. In order to use this technique to determine defect concentrations accurately, it is essential to ensure that at least a known fraction of the defects have captured electrons or holes. In many instances the conditions required to do this can be determined by studying the conversion of existing defects into color centers by purely ionizing radiation, such as X-rays. The techniques described in this paper have been used to show that the number of defects produced in sodium chloride by fast reactor neutrons are consistent with existing theories on radiation damage. They have also been used to study the growth and annealing of the defects formed in aluminum oxide, and fused silica by reactor irradiations.
Levy, P. W.
Physicist, Brookhaven National Laboratory, Upton, N. Y.