Origins of Gamma-induced darkening of BaSO4

Abstract

Optical properties of insulators can be dominated by trace-level impurities and/or other defects that may be difficult to identify using direct chemical analysis. The present work investigates the origins of coloration in BaSO${}_4$ powders following exposure to high energy photons (1.33 and 1.17 MeV $\gamma$ from a ⁶⁰Co source)using a through the combination ofexperimental and computational predictions of defect energies and a suite of complementary experimental techniques. We conclude that a slight greying observed in some BaSO${}_4$ powders is caused by activation of sulfate radicals that are stabilized by nearby barium vacancies or trace amounts of sodium substitution on the barium site. Electron–hole pairs activated by exposure to gamma irradiation and trapped by these defect couples lead to visible absorption centered around $\sim$650 nm. Such trapped electronic defects are stable at room temperature but can be ‘healed’ byeither thermal annealing above 300 °Cor ultraviolet bleaching. Such gamma-induced colorationwill therefore have has no impact on the mass density that dominates radiopacity nor on the chemical or physical properties of BaSO${}_4$and is not an indication of any sort of toxic contamination.