We prepared silica glasses having various concentrations of chlorine, fluorine, hydrogen (SiO2−x), and oxygen (SiO2+x) to examine the precursors of paramagnetic centers induced by &ggr; rays. In the case of glasses sintered under chlorine and hydrogen ambients, the concentration of theE’center induced by &ggr;‐ray irradiation scaled with the partial pressure of chlorine and hydrogen. In contrast, theE’center was suppressed in glasses doped with fluorine. Stress in the glasses was also found to enhance formation of theE’center. Planar ring structures in the glass are influenced by stress and are proposed as precursors to theE’center.