Structural relaxation of E_gamma centers in amorphous silica

We report experimental evidence of the existence of two variants of the E-gamma(') centers induced in silica by gamma rays at room temperature. The two variants are distinguishable by the fine features of their line shapes in paramagnetic resonance spectra. These features suggest that the two E-gamma(') differ in their topology. We find a thermally induced interconversion between the centers with an activation energy of about 34 meV. Hints are also found for the existence of a structural configuration of minimum energy and of a metastable state.