Gamma-Ray Induced Luminescence of Silica Gel

Abstract

Abstract The thermoluminescence glow curve of γ-irradiated silica gel investigated between 77 °K and 600 °K is a broad band extending from 80 °K to ~ 400 °к. The emission occurs at ~ 450 nm. Shallow traps (0.1 - 0.2 eV) are responsible for most of the light emitted. The distribution of trap depth and the increase of trap density toward the band edge is tentatively attributed to the dis-ordered structure of silica gel in accordance with previous suggestions concerning glasses. In contrast to the growth with radiation dose of color-and paramagnetic center concentration, luminescence is already saturated at about 1 Mrad. The luminescence decay follows the hyperbolic relation derived by Adirowich for a bimolecular process in the case of retrapping for which evidence is presented. At very low radiation doses the initial intensity of luminescence was proportional to the square of the dose rate as expected for a bimolecular reaction. Removal of surface OH groups as accomplished by raising the degassing temperature of the gel results in an increase of luminescence intensity by several orders of magnitude. Hydrogen and oxygen when adsorbed prior to or subsequent to irradiation reduce the luminescence intensity to about one per cent of that observed in vacuum or in the presence of helium. The results are dis-cussed in terms of surface reactions involving free and trapped charge carriers generated during irradiation.