Optical bleaching of radiation-induced colour centres in fluorapatite

Abstract

One of the important steps in thermoluminescence (TL) dating studies is the assessment of the total natural radiation dose that a sample has received and retained. This, in turn, depends on the thermal and temporal stabilities as well as the optical bleaching of the radiation-induced colour centres. In this paper, we report the effect of selective optical bleaching, after X-irradiation, on the TL glow curves of synthetic fluorapatite. X-irradiated fluorapatite exhibits three unresolved TL peaks around 150, 250, and 345°C. Diffuse reflectance studies of the polycrystalline fluorapatite show that X-irradiation induces absorption bands at 370 and 450 nm. Furthermore, exposure to light through 320–400 nm and 400–480 nm band filters effectively bleaches the TL glow from the first two peaks at around 150 and 250°C. By correlating the wavelength of light used for bleaching the TL with the radiation-induced absorption bands, the type of centres involved in the ∽ 150°C TL glow peak in fluorapatite has been identified as E(II) or A centres, i.e. halogen ion vacancies with trapped electrons. Although the ∽ 250°C TL glow peak is also bleached along with the 150°C TL peak, its temperature stability does not agree with that of the A centres.