Effect of Doping with Monovalent Cation Impurites on the Thermoluminescence Response of CaSO4:Dy Phosphor

The effect on thermoluminescence (TL) of CaSO4:Dy on additionally doping with Li+ or Na+ (as charge compensators) is studied as a function of Li+ (or Na+) concentration. Doping with either of these species is found to reduce the peak heights of the 225 as well as the 375 °C TL peak; the reduction being concentration dependent. Doping of CaSO4:Dy (0.05 mol%) with 1.13 mol% of Li+ gives a reduction in the 225 as well as the 375 °C TL peak by factors of 25 and 6.9, respectively. This indicates that Ca2+ vacancies are involved in the formation of traps giving rise to these TL peaks. The removal of Ca2+ vacancies (due to Dy3+ doping) in CaSO4: Dy by the addition of Li+ (or Na+) destroys the traps corresponding to the dosimetry peak (225 °C) as well as to the high temperature TL peak (375 °C). This result agrees with the earlier suggestion that SO42− radicals closely associated with Ca2+ vacancies act as hole traps for the 225 °C peak. The simultaneous reduction in the sensitivity of the 225 and 375 °C TL peaks, and the dependence of sensitization of the 225 on the 375 °C peak, indicate that the traps giving rise to these two peaks are similar in nature and are possibly spatially correlated. The addition of Li+ to CaSO4: Dy (i) induces additional TL peaks in the regions 275 to 325 °C and 430 to 475 °C and (ii) enhances the low temperature peak at about 131 °C. Li+ doped CaSO4:Dy samples show enhanced photo-transferred TL (PTTL). The results on supralinearity are also reported.