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Samples of vitreous silica with OH content <2 ppm (‘‘dry ’’) or 1200 ppm (‘‘wet ’’) were irradiated with x rays from Cr, Cu, and Mo tubes, and with γ rays from a 137Cs source (661 keV). Front surface doses ranged from 51 to 1800 Mrad. For each energy of the photons the number of E′ defects produced per megarad of radiation was higher for the ‘‘wet ’’ samples than for the ‘‘dry ’’ samples. The spatial distribution (depth profile) of the E′ defects was measured by spectral-spatial electron paramagnetic-resonance (EPR) imaging. Defects produced by the high-energy 137Cs γ rays were uniformly distributed through the 1.0 mm thickness of the samples. For the x rays, the EPR signal intensity decayed with distance into the sample, the decay being strongest for the lowest photon energies. The EPR profile was compared with the spatial distribution of the energy deposition (dose profile). The basic features of the EPR profile are determined by the dose profile, but in some cases there appears to be enhanced defect production near the surface of the sample.

From experiments using synchrotron radiation, we present evidence that the spin-active defects created in v-SiO2 by x irradiation are predominantly the result of energetic electron damage, rather than the relaxation of the photoionized atoms.

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