Thermoluminescence in Lapis Lazuli crystal: Glow peaks and their connection with F-centers estimated by ESR analysis

Abstract

The mineral Lapis Lazuli exhibits three thermoluminescence (TL) peaks approximately at 140, 260 and 370 degrees C. The TL intensity of the three peaks grows with radiation dose and it saturates only beyond 9 kGy. Electron Spin Resonance (ESR) studies have been carried out to identify the defect centers responsible for the thermoluminescence peaks. The ESR spectrum of the as-received natural sample presented the typical six signals due to Mn2+ ion and a large signal around g = 2.030 due to Fe3+ ion. Room temperature ESR spectrum of irradiated Lapis Lazuli shows the formation of a defect center. This center (center I) is identified as an F-center (singly ionized oxygen vacancy) and seems to correlate with 140 and 250 degrees C TL peaks. An additional defect center is observed during thermal annealing experiments and this center (center II - assigned to F+-center) appears to originate from an F-center (oxygen vacancy with two electrons). This F-center, precursor of center II, may be related to the observed high temperature 360 degrees C TL peak in Lapis Lazuli.