Radiophotoluminescence and thermoluminescence characteristics of undoped and Mg doped LiF phosphor in the high dose region

Abstract

Radiophotoluminescence (RPL) and thermoluminescence (TL) from undoped and Mg-doped LiF powder samples were investigated for high dose dosimetric applications. F-aggregate centres in LiF are created by radiolysis. The RPL emission at 530 nm and 650 nm from F3+ and F2 centres respectively (on excitation at 450 nm) in undoped commercial LiF powder enhanced with increasing sintering temperature. Its TL glow curve showed peaks at 145, 290 and 365 °C. The RPL-TL correlation studies carried out for the first time showed that the thermal annihilation of F3+ and F2 centes following Fi0 - Va (interstitial-vacancy) recombination cause 290 °C and 365° TL peaks respectively. This showed that F2 centres are thermally more stable than F3+ centres. Its integrated TL response kept increasing nearly linearly up to 14 kGy, in contrast to the response of the TL peaks in dosimetry grade LiF:Mg, Ti (TLD-100). However, the TL from LiF suffers from fading problems. A 100 °C, 15 min pre-read annealing treatment erased most of the 145 °C TL peak without disturbing the other high temperature peaks. A 750° C, 22 h sintering treatment in carbon atmosphere shifted the major TL glow peak of undoped LiF to 200 °C but reduced its TL sensitivity by more than a factor of 10. Mg-doped LiF samples exhibit a TL glow curve peaking at ∼240 °C with satellite peaks typical of Mg doped LiF whose shape did not change significantly with gamma dose. However it showed lower TL and RPL sensitivities as compared to those of undoped LiF. All LiF except LiF alfa (750 °C, 1 h, CB) showed sublinear increase in TL with dose and therefore will cause inaccuracy in dose measurements especially at high doses due to reduced sensitivity. LiF alfa 750 °C, 22 h and 1000 °C, 6 min sintered powder samples are better than others because their RPL sensitivity increases nearly linearly with dose. However, a low fading observed in the latter sample supports its application. The bright yellow RPL emission seen through a 550 nm cut on filter illuminated with a 450 nm blue LED or He-Cd laser light could quickly confirm an irradiated consignment on which LiF samples are affixed. The increase in green to red emission ratio with dose and post-irradiation storage period might be related to slow migration of anion vacancies created by one secondary electron track to F centres created by another track and consequent conversion of F2 → F3+ centres.