Electron-spin-resonance spectra have been obtained at 9.1 and 35 GHz for $E^{\prime }$ centers induced by $\gamma$ irradiation in a number of high-purity commercial fused silicas and in a suite of glassy Si${\mathrm{O}}_2$ samples of varying degrees of enrichment (or depletion) in the magnetic isotopes ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$. Computer simulation of the spectra of the ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$-enriched samples indicates the existence of a weak superhyperfine (shf) interaction ($\frac{a_{\mathrm{meas}}}{a_{\mathrm{atomic}}}\approx 5\times {10}^{-4\mathrm{}}$) with oxygens in three chemically equivalent near-neighbor sites. "Weak" shf structure characterized by $a\approx 8$ G observed in samples containing ≤50 at.% ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ is shown to be due to protons present as impurities. An unresolved broadening of the spectra of samples containing ≥50 at.% ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ is ascribed to "very weak" shf interactions with next-nearest-neighbor silicons. These findings, together with a comparison of the $g$ matrices of bulk and surface $E^{\prime }$ centers, support the notion that the structure of the $E^{\prime }$ center in glassy silica is essentially identical to that in $\alpha$ quartz; i.e., both comprise asymmetrically relaxed oxygen vacancy defects.

• Received 25 March 1980

DOI:https://doi.org/10.1103/PhysRevB.22.4192