Dopant role in phosphor material is to enhance the luminescence efficiency or properties such as TL, PL and creates more defects inside the host material. As per my knowledge, If you increase your dopant ratio, defect levels should be increased. So, might change their peak position or shift.

see the following article, they have also observed same results after doping the Dy.

http://link.springer.com/article/10.1007/s10853-011-6102-7

I agree with  Mahesh S Bhadane  because do pant create more defects and so new trap levels that produce a new stats for liberated electrons due to the effect of radiation. Pure LIF has no TL property but it should be doped to create TL property of it.

Dear Deepti,

It would be better to answer if you gave some details about your problem. Still, I like to add a few points:

1. The increase of concentration of the dopant does not always increase the defect states. If you increase the concentration of the dopant, up to some level, the defect states increase which can produce new TL peak at any position. However, after some specific value of the concentration, the number of defect states decreases. This is called concentration quenching. For more details, please see “Thermoluminescence in solids by S.W.S. Mckeever”.

2. Sometimes, the increase of the concentration of dopant may produce some delocalized energy states at the same energy depth. If this happens, there will be an increase in the intensity of the TL peak. If the intensity of the main peak increase significantly, one cannot see the shoulder peak and therefore assume that the peak vanishes due to the increase of concentration of the dopant.

There are some other possibilities but without having any idea of the host material, it is not the best idea to add all those.

J.M. Kalita

Shoulder peak in TL glow curve fades or vanishes when we increase the concentration of dopant (e.g. Dy) in phosphor:

With (too much beyond optimization)  increase in dopant concentration, the intensity/glow peak height or integral counts decrease/vanishes due to concentration quenching as explained by @Jitu_Kalita. This is true for every glow peak - however optamized value of dopant conc. will be different for different peaks. Sometimes fading also increases as dopants especially Dy may act as trap as well as emission/luminescence centre as is seen in Localized transitions where retrapping and recombination are within same defect complex. . At high conc., Dy are very close to each other,  the wave functions overlap, leading to enhanced probability of athermal fading.

for lower concentration of dopant there is single peak in TL glow curve but on increasing the concentration of dopant a shoulder peak appears:

At lower/lowest concentration, number of defects actually participating in luminescence process may be less, so no trapping, or negligible trapping, no luminescence during heating or even if trapping is there, no radiative emission as number of Dy which act as luminescence centre (emitting 480 or 570 nm or ...) also are less. With increase in Dy, either number of electron traps increases or lot of Luminescence centre are there or both things happen. we see TL peak. Please note that traps are generally property of host, dopant only changes their no. or concentration. So defect responsible for shoulder peak was inherently present, but it was not actively participating in luminescence process either due to no trapping (no traps) or luminescence losses.  

Thank you, Dr. Munish Kumar sir and J. M. Kalita Sir for the valuable answered. I totally agree with your answered.

Because optimization of doping concentration is most important parameters in TL properties.

Thanks again.

thank you kumar sir and kalita sir for valuable guidance.

i would like to add some more details regarding my question:

i hav prepared phosphors with different concentrations of dopant to observe the effect of dopant concentration on TL and PL properties. when Dy is used as dopant , for increasing concentration of dopant the intensity of shoulder peak increases. After a certain concentration of dopant the shoulder peak intensity decreases and finally vanishes.

When Sm is used as dopant again the same behaviour is observed and at a certain concentration of Sm the height of main TL peak and shoulder peak becomes nearly equal and then , for higher Sm concentrations, shoulder peak intensity decreases and finally vanishes to give one main broad TL peak.

Thank you sharma sir for explaination

Dear Deepti Singh,

I am sorry to say that your question is incomplete. In absence of any information about the host material it is difficult to comment. Moreover, it has also not been mentioned whether the shoulder is on which side (low temperature or high temperature side) of the main peak. Also it has not been mentioned about the level of the doping range, purity of the host material, etc. TL in general is a very complex phenomenon. A TLD phosphor could behave differently with heat treatments, doping levels, radiation dose and even with different dose rates. Some redox reactions could also take place. Therefore, a detailed study on the kinds of defects generated is needed before commenting on your question. Good luck!

--- P D Sahare