The role of alkali ions in the 190 K TSL peak in quartz

The role of alkali ions in the creation of the thermally stimulated luminescence (TSL) peak at 190 K in quartz has been investigated by wavelength resolved TSL and thermally stimulated current (TSC) measurements performed on synthetic crystals, both as grown and hydrogen swept, characterized by alkali content of about 1-3 and 0.1 ppm respectively. The 190 K peak has been efficiently produced in as-grown crystals by a double irradiation procedure consisting of a first x-ray irradiation at 90 K followed by heating in the 170-300 K temperature range and a second irradiation at 90 K; this effect has not been observed in the hydrogen swept crystal. Moreover, the study of the spectral composition of the emitted light has shown the existence of two emission bands, one peaking at 450 nm (T<150 K) and the second one evidenced for T>150 K and peaking at around 380 nm. In as grown samples, TSC peaks at 205, 260 and at around 350 K (composite structure) have been detected: their intensities are much stronger in the as grown crystal and with the electric field oriented along the z-axis indicating that they have an ionic character. By taking into account the mechanism of formation of the [SiO4/M+]0 (M+ = Li+, Na+) traps (previously found to be responsible for the 190 K TSL peak), the 205 K TSC peak can be attributed to the radiation induced dissociation of alkalis from [AlO4/M+]0 defect centres and subsequent migration near to Si sites; on the other hand, the 260 K TSC peak can be related to the subsequent disintegration of [SiO4/M+]0 defects involving the migration of alkalis to different ionic traps.