Influence of physical parameter on the properties of thermoluminescence phosphors

Abstract

Recently, there is great interest in determining the role of primary defects involved in the production of thermoluminescence (TL) emitted from lithium fluoride (LiF) phosphors. Since the spatial localization of magnesium (Mg) and titanium (Ti) defects is seen to be of fundamental importance in describing certain aspects of the LiF:Mg,Ti glow curve and related parameters, in this work TL response of differently doped LiF phosphors (both single crystals and extruded chips) has been studied under various experimental conditions. After annealing at 400°C and fast cooling down the behaviour of the low-temperature peaks could be investigated, while a slow cooling procedure following standard annealing was considered appropriate for analyzing the high-temperature TL response. Irradiations have been performed under influence of heating, optical illumination by blue (λpeak ~ 470 nm), green (λpeak ~ 530 nm) and red (λpeak ~ 625 nm) light, and magnetic fields (B ~ 0.6 to 1.6 T). Typical glow curves from LiF single crystals with natural Li isotopic ratio and different Mg (37.5 to 1,200 ppm) and Ti (11.5 to 23 ppm) dopant concentrations indicate that the low-temperature TL peaks are caused by Mg dipole/Ti complexes, while Mg trimers/Ti complexes can mainly give rise to TL peaks at higher temperatures. Since the glow curves were always normalized to peak 5, exact properties of peak 5 itself could not be derived. The results of heating experiments during dosimeter irradiation indicate the existence of two regions in the glow curve with distinctly different behaviour: (i) the low-temperature peaks 2 to 4, and (ii) the high-temperature structure. The Ti concentration plays an important role for the trapping centres associated with the low-temperature peaks, while the presence of Ti coupled to OH dipoles influences the properties of some of the high-temperature peaks. From the exposure to magnetic fields during sample irradiation it can be concluded that Ti complexes act as electron trapping centres responsible for peaks 2 and 3 as well as some high-temperature peaks. Finally, results from sample illumination during irradiation suggest that peaks 3 and 4 are formed by clusters of trapping centres usually associated with peak 2. The effect of red light on LiF crystals with increased Ti dopant concentration confirms the role of Ti complexes in peak 2 and 3 trapping centres. The main conclusions obtained from these results suggest: (i) low-temperature peaks are associated with Ti ion centres; (ii) as moving to higher temperatures (peak 4), Mg ions have some influence by forming highly complex dipoles with Ti ions; (iii) for even higher temperatures, Mg ions are more dominate in dipole formation in addition to Ti(OH) centres.