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Luminescence from high-purity synthetic silicas, Spectrosil, Spectrosil WF, Suprasil 1, and Suprasil W1 , has been monitored in situ by an optical detection system during 50 eV synchrotron photon exposure at room temperature in ultra-high vacuum. Three luminescence bands were found, centered approximately at 290, 450, and 650 nm. Samples were divided into two sets. One set was irradiated by 137Cs gamma rays and another set was virgin samples. Then all the samples were exposed under 50 eV light from a synchrotron source. Photons, 50 eV, stimulate more intense luminescence in pre-irradiated silicas than in virgin silicas. Increasing the exposure time to UV light on the same spot, initially irradiated, increases the luminescence intensity for both set of samples. After an exposure of 25 minutes, the intensities of the 290 and 650 nm bands increased by a factor <1 while the 450 nm band increased by a factor of three. Electron Paramagnetic Resonance spectroscopy was used to determine the correlation between the concentrations of paramagnetic centers, e.g. E' centers, intro-duced by gamma rays or UV light and the total dose of the 50 eV photons. We found that E' centers increase linearly with the increasing dose of 50 eV photons. The rate of increase of intensities of the luminescent bands decreased with increasing exposure. 290 and 650 nm bands increase slowly with increasing exposure, while intensity of 450 nm band increases three times with increase of exposure time from 5 to 25 minutes in Suprasil 1. These results show that defects created either by gamma-ray radiation or by UV light enhance dynamic luminescence processes in silica. We tentatively attribute the more intense luminescence in gamma-ray irradiated samples under 50 eV light exposure to E' centers introduced by the gamma-ray irradiation. The increase of luminescence intensity with exposure time to 50 eV light is also attributed to the introduction of the E' centers by 50 eV photons in silicas. Possible defect-enhanced luminescence mechanisms will be discussed through electron-hole interactions with defect states in the bandgap.
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