GENERATION OF E' CENTERS AND OXYGEN HOLE-CENTERS IN SYNTHETIC SILICA GLASSES BY GAMMA-IRRADIATION

Generation of typical paramagnetic centers by gamma irradiation was studied for various kinds of synthetic silica glasses. Growth behavior of E' centers and nonbridging oxygen hole centers (NBOHC's) with gamma-ray dose depends on contained preexisting point defects. By irradiation at room temperature, E' centers grow linearly and show a saturating tendency in silicas that contain precursors for E' centers, such as oxygen-deficient centers (ODC's), Si-H bonds or Si-Cl bonds. In silicas that contain very few precursors for E' centers, the growth of E' centers with the dose substantially equals that of NBOHC's and both follow a sublinear growth law in which the concentration of the defects is proportional to the square root of the dose. These results lead to the conclusion that gamma irradiation fundamentally creates defect pairs of an E' center and an oxygen hole center from an intrinsic Si-O network of amorphous silica; this is in addition to a large amount of E' centers induced from preexisting point defects as precursors. By irradiation at 77 K, E' centers and self-trapped holes are suggested to be formed as the defect pair from the Si-O network, while the conversion of the precursors into E' centers is probably lessened. Concentration of E' centers generated by gamma irradiation is almost the same as that of irradiation of ultraviolet lasers and x rays if they are compared based on the absorbed energy. Thus, defect generation with gamma rays is suggested to involve similar fundamental processes as those with photons, which have much smaller energy than gamma rays.