Anomalous fading in TL, OSL and TA - OSL signals of Durango apatite for various grain size fractions; from micro to nano scale

Anomalous fading (AF) of luminescence signals has been studied extensively both experimentally and by simulations. This paper reports a new type of study of anomalous fading in grains of Durango apatite, a naturally occurring luminescent material that yields very intense anomalous fading. Grains of Durango apatite were ball milled (BM) for various durations, up to 48 h. Different ball milling durations resulted in different average grain size fractions as low as 200 nm, as it was indicated by Scanning Electron Microscopy (SEM) measurements. The anomalous fading effect was studied for optically stimulated luminescence (OSL), thermoluminescence (TL), as well as thermally-assisted OSL signals (TA - OSL). Anomalous fading was found to be ubiquitous for all luminescence signals, and for all apatite grain size fractions. The anomalous fading rate is weakly affected by the grain size for the cases of OSL and TL, while the TA - OSL signals were found to fade in a much slower rate than either the TL or the conventional OSL signals. An important experimental result is that the fading rate of TA - OSL decreases as the grain size fraction is decreased. For average grain size fractions between 200 and 450 nm, the TA - OSL signal is unaffected by the AF effect. A differential analysis on the TL glow curves showed that the AF rate decreases with increasing temperature along the glow curve, and also with increasing BM time. Finally, a component resolved de-convolution analysis was performed for both OSL and TA - OSL decay curves and recombination lifetimes are reported for both localized and delocalized components. FTIR analysis indicates that the ball milling procedure does not induce a new phase in this material.