NONLINEAR GAMMA-RAY ACTIVATION OF DEFECT SPINS IN VITREOUS SILICA

Previously published data showing electron spin resonance spin count versus ionizing radiation dose in bulk vitreous SiO2 are compared for the two cases of approximately 1.25 MeV gamma-rays and approximately 8 keV X-rays. For both Suprasil 1 and Suprasil W1 materials, the non-linear production of paramagnetic states with increasing dose for gamma-rays at doses below approximately 175 Mrad is shown to have nearly identical forms to those for X-rays at doses below approximately 8 Mrad. It is then argued that the non-linearity of the gamma-ray data has the same physical origin as previously ascribed to the non-linearity of the X-ray data: an exponentially saturating 'activation' of pre-existing defects, accompanied by a linearly increasing number of newly created defects. The data for E' and non-bridging oxygen hole center defects show that approximately 1.25 MeV gamma-rays are at least an order of magnitude more efficient per unit of dose for activating pre-existing defects than are approximately 8 keV X-rays.