  We report a study by electron paramagnetic resonance (EPR) on the E'_\alpha
point defect in amorphous silicon dioxide (a-SiO2). Our experiments were
performed on gamma-ray irradiated oxygen-deficient materials and pointed out
that the 29Si hyperfine structure of the E'_alpha consists in a pair of lines
split by 49 mT. On the basis of the experimental results a microscopic model is
proposed for the E'_alpha center, consisting in a hole trapped in an oxygen
vacancy with the unpaired electron sp3 orbital pointing away from the vacancy
in a back-projected configuration and interacting with an extra oxygen atom of
the a-SiO2 matrix.
