The dose dependence of the defect concentration produced by radiation was studied for many types of silica glass in order to discuss the contribution of 'extrinsic' and 'intrinsic' processes to the paramagnetic defect formation. A linearly increasing concentration of paramagnetic defects with dose accompanied by a saturating tendency is observed for the 'extrinsic' defect formation due to transformation of pre-existing precursors. The concentration of E′ centers substantially equals that of non-bridging oxygen hole centers, and both are approximately proportional to the square-root of the accumulated dose for the 'intrinsic' defect formation involving cleavage of the SiO network. Since the dose dependence of the defects is independent of the incident photon energy, electron-hole pairs having the band gap energy of silica are implied to have an essential role for either 'extrinsic' or 'intrinsic' defect formation.
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