Generation of E' centers in high-purity silica glasses was studied for irradiation with γ-rays and 6.4- and 5.0-eV excimer lasers. The linear growth and a saturating tendency of the concentration of E' centers in oxygen-deficient "dry" silica with the dose are explained by a first-order reaction of pre-existing precursors, Si-Si bonds. The sublinear growth of the E' center concentration in oxygen-excess "dry" silica is proposed to be ascribed to the intrinsic defect formation involving cleavage of the intrinsic amorphous network. If concentrations of E' centers generated in these "dry" silicas are compared based on the absorbed energy, the growth under γ-irradiation consisting of 1.25-MeV photons is similar to those under the two-photon excitation by excimer laser lights, which have 6.4- or 5.0-eV photons. The results suggest that the generation process of E' centers has the same mechanism irrespective of incident photon energy for either the transformation of precursors or the intrinsic defect formation.
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