Dear Dr Suriyamurthy, This is realy an interesting observation. One possible reason may be that the nanophosphors when heated might have both agglomeration and defects inclusion. When ionizing particles reaches the nanophosphors depending upon its interaction with the sites it may reemit and hence its sennsitivity may be affected.

Dear Suriyamurthy,

I do not think that the above comments made by Dr. Chandramohan and Dr. S. K. Sharma are very correct. In many TLD nanophosphor systems that we studied we found that there is change in the glow curve structures which indicate that there is reorganization of the energy levels, For example, CaSO4:Dy nanoparticles glow peaks are found at very high temperatures also. So, forming more shallow traps (defects) due to surface to volume ratio is not correct. Moreover, agglomeration due to heating has also not been observed.

The plausible reasons for low sensitivity could be i) there is a large surface to volume ratio and the traps formed on irradiation may not lie on the surface so the production of such traps will be less or at least slow for a particular exposure as compared to that of a microcrystalline phosphor. That is how the nanocrystalline phosphors show very wide dose range than the microcrystalline ones. ii) According to the track interaction model, some of the radiation (projectiles) may miss the target (the impurity cites) during irradiation, also the track lengths would also be shorter as compared to the same in microcrystalline phosphors. There would also be more overlapping of the tracks.

The overall effect would be slow production of the total number of traps for a particular dose and thus the TL yield would also be less resulting in less sensitivity but wider dose range.