Optimization of OSL/IRSL experimental measurement parameters on various types of naturally occurring CaSO4 samples; sensitization pattern versus temperature, bleaching properties and slurry zeroing effect

Abstract

Calcium sulphate (CaSO4) geological material is the main constituent of art objects and geological formations, mostly in the form of gypsum. Nevertheless, investigations of either optically or infrared stimulated luminescence (OSL or IRSL respectively) properties are scarce for this material towards age assessment applications. The present work studies for both IRSL and OSL signals of CaSO4 (a) the effect of the preheating to the sensitivity, towards selecting the optimum preheating conditions, (b) the bleaching ability of both signals via the corresponding residual TL signal levels, towards selecting the appropriate stimulation duration and (c) whether mixing with water acts as a zeroing mechanism for these signals. IRSL and OSL yield different sensitization threshold temperatures, being 250 °C or 300 °C, depending on the water content of the mineral (hemihydrate, anhydrous and the most usual dihydrate). However, for the cases of protocols involving either a single IRSL measurement or post IRSL OSL measurements, preheating temperature should not exceed 225 °C. The optimum stimulation duration is 100 s for the blue OSL and 60 s for the IRSL. Finally, mixing with water affects (i) insignificantly the zeroing of IRSL signal in anhydrites, (ii) partially the zeroing of OSL in all groups and (iii) substantially the zeroing of IRSL signal in gypsum and bassanite.