It considered due to fact that trap depth decreases with increase in the concentration.

if the trap depth decrease there may be a corresponding decrease in the temperature.

Without observing the experimental glow curve it is not a good choice to answer this question. But still I like to add a possible reason for the shifting of the peak.

In general, non-first order glow peak may be considered as some combination of closely overlap first order peaks. We all aware that each glow peak correspond for a unique set of some physical parameters (E, s etc.). Therefore it might be important to study the nature of the trap distribution. Very professional deconvolution analysis along with commonly known Tm-Tstop analysis and Fractional Glow analysis may give the proper nature of the trap distribution in the crystal.

One possible reason for the shifting of peak maximum temperature with irradiation dose is that if there is a continuum trap distribution in the sample then different closely spaced trapping sites may be filled at different level during irradiation. Therefore during TL recording the peak maximum temperature may be shifted.

I wonder if you observe a peak with first-order features that shifts to a lower temperature at higher doses. The usual explanation of the effect for a single peak is, indeed, that it is a non-first-order peak, and it is easiest to demonstrate it for second-order peaks. However, if the peak you observe is a convolution of several components, the explanation may be different. If the higher-temperature components saturate earlier than the low-temperature ones, the behavior of the envelope peak may be such that its temperature decreases with the applied dose.

Thank you for the answers. Really helpful. Now, I can think for another reasons why my material behave like that.

And Prof Chen, from eye-view, my peak is one isolated single peak with lorentzian characteristic, and at the highest dose, suddenly the peak shift  to lower temperature. At low dose, the peak varies with error of 1 degree celcius only.

Another question here, is it possible for same batch sample to have different kinetic order at different dose delivered? i. e., at dose 3Gy, it shows first order of kinetic, at dose 3.5Gy, it shows general order of kinetics

In my opinion, this result may indicate that the peak is not a single peak but rather, it is composed of a number of components. In order to check that, you can try to perform a thermal bleaching experiment. Suppose you have a peak observed at, say, 500K. Apply a dose of, say, 3.5Gy and heat the sample to 400 K. Cool it back and without further irradiation heat it all the way up to see the remaining peak. In the next step, apply again 3.5Gy and heat the sample to 410K and cool it back. Again, without another irradiation, heat the sample all the way up and record the peak. repeat this with initial heatings to 420K, 430K etc. If the peak is a single first-order peak, the peaks measured in the different stages will look the same but with decreasing intensity. However, if you have a convolution of a number of components, the initial heating will cause thermal bleaching of the low-temperature components and the successive peaks will shift gradually to higher temperatures.