Redox reactions in Cu-activated nanocrystalline LiF TLD phosphor

Samples of cubic shaped nanocrystalline LiF:Mg, LiF:Cu, LiF:P and LiF:Mg, Cu, P materials were synthesized through the chemical coprecipitation route. In case of LiF:Cu, it was predominantly doped with Cu2+ and Cu+ impurities using CuCl2 and Cu2Cl2 salts, respectively, as dopants. The samples were characterized by XRD, SEM and TEM techniques. The particle size observed from TEM photographs and XRD analysis was found to be in the range of 50–80 nm. Samples were annealed at different ambient temperatures in the range of 373–773 K. The pellets of these materials were exposed to γ-rays in the dose range (0.1 Gy–100 kGy) from 137Cs source for further studies. Thermoluminescence (TL) glow curves of these nanocrystalline samples show two main peaks one at around 404 K and another broad peak centered at around 490 K. The relative intensities of these peaks were found to change on doping with CuCl2 and Cu2Cl2. Comparison with commercially available TLD phosphors showed that TLD-700H and TLD-900 are more sensitive while, TLD-100 is less sensitive than LiF:Cu+ phosphor. The change in glow curve structures and the photoluminescence (PL) spectra revealed that the relative concentrations of the Cu2+ ↔ Cu+ ↔ Cu0 change irreversibly during irradiation, while taking TL readouts and during annealing beyond 523 K. It seems that such redox reactions are responsible for the changes in the sensitivity and the loss of the reusability of the phosphor as Cu+ is necessary for the high sensitivity of the phosphor. Further detailed studies by taking glow curve readouts up to different temperatures, irradiating again and taking TL readouts show that the remaining traps after the first readout could change the Cu2+/Cu+ ratio altering the glow curve structure. The PL spectra and ESR measurements of the irradiated samples also support these findings. It may be concluded that redox reactions are mainly responsible for the loss of sensitivity and reusability on taking TL after 523 K in case of LiF:Cu+ and LiF:Mg, Cu, P TLD phosphors. A model representing the redox reactions illustrates the TL phenomenon occurring in LiF:Cu+ phosphor.