Optically stimulated luminescence studies on CaF2:N using CW and LM OSL techniques

Natural calcium fluoride (fluorite CaF2:N) exhibits multiple LMOSL peaks when stimulated using blue LED’s, which makes it an unique phosphor. Such a property is not observed experimentally in any other dosimetric phosphors namely Al2O3:C, BeO etc. This makes CaF2:N as an effective tool not only for testing the kinetic models of luminescence but also for investigating OSL using CW and LM stimulation modes. In view of this, CW and LM OSL processes have been investigated for CaF2:N and the effect of optical bleaching and pre-readout annealing on various components of OSL have also been studied. It has been found that CWOSL from CaF2:N could be fitted as a sum of four first order decaying exponentials (components) and the photo-ionization cross-sections for these four components were found to be ~2.77*10-17cm2 (σ1), ~0.90*10-17cm2 (σ2), ~0.16*10-17cm2 (σ3) and ~0.03*10-17cm2 (σ4) respectively. Effect of optical bleaching using blue LEDs on CWOSL curves was also studied. It has been found that for stimulation power of 22.5 mW cm-2 using blue LEDs, the first component of OSL is bleached in ~ 5-15 s. Further for optical bleaching times of ~ 20-25 s, only two components are observed. Similar studies were carried out for LMOSL and it has been found that the first peak in LMOSL from CaF2:N is bleached in 5-15 s using blue LEDs whereas the second peak which is a broader peak, shifts toward higher side with increase in optical bleaching time. CW and LM OSL studies were also carried out after performing pre-readout annealing at 120oC and 200oC for 10 s. It has been found that after optical bleaching or pre-readout thermal annealing, the photo-ionization cross-sections of the remaining components are closer to the corresponding component values obtained by fitting CWOSL curve as a sum of four order exponentials. Transformation of CW to LM OSL curves was carried out as per: i) Bulur’s method, and ii) Bos & Wallinga method. These results indicate that the behavior of the transformed LM OSL curves is nearly similar to the LM OSL curves recorded experimentally.