Generation of E' centers and oxygen hole centers in synthetic silica glasses by γ irradiation

Generation of typical paramagnetic centers by γ irradiation was studied for various kinds of synthetic silica glasses. Growth behavior of E' centers and nonbridging oxygen hole centers (NBOHC's) with γ-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 γ 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 γ 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 γ rays is suggested to involve similar fundamental processes as those with photons, which have much smaller energy than γ rays.