Do you consider fade when you are calibrating your TLD system and performing readings for dose?
If you are using TLD-100, 600 or 700 (LiF:Mg,Ti), you need to ensure this is part of your readout protocol. For dosimetry systems using TLD-100H, 600H or 700H (LiF:Mg,Cu,P) this is not a concern when using the factory recommended time temperature profile (TTP).
There are several ways that fade in LiF:Mg,Ti may be addressed.
Reducing Short-Term Uncertainty
First, let’s look at calibrations for the reader calibration factor (RCF) or the TLD card element correction coefficient (ECC). The Harshaw TLD recommendation is to wait 24 - 48 hours after annealing before irradiation. Then, after irradiation to wait 24 – 48 hours before readout.
The purpose of waiting the 24 – 48 hours pre and post irradiation is that you are allowing the low temperature / short-term peaks to fade away. This procedure allows the more stable high temperature peaks to remain which reduces short-term uncertainty.
This procedure would also apply to readout for dose. While less of a concern in the case of field dosimeters due to transit times, one should be aware of fade when doing exposures for experiments, performance testing or blind irradiations.
A Common Method Leading to Uncertainty
A common procedure for some users is to use a preheat or to select a partial calibration region instead of 1 – 200 on LiF:Mg,Ti to reduce or eliminate the low temperature peaks. Fade reduction effectiveness may be reduced as TLD cards age since the glow curve may shift to the right due to less efficient heating when the Teflon™ encapsulation weakens or air pockets form. This causes more of the low temperature peaks to become part of the total integral increasing the uncertainty of the measurement.
Advantage of Harshaw TLD Model 6600 & 8800
If you are using the automatic calibration feature in the Model 6600 or 8800 with LiF:Mg,Ti material then this issue is not of concern because the timing is kept consistent between the anneal, exposure and readout.
The Harshaw TLD dose calculation algorithms for LiF:Mg,Ti based dosimeters use an 8-day pre and post fade factor in the fade calculations and your Cs137 Relative Response must take this into account. More on this in a future post.
Come back soon for more information on the difference between pre and post fade.
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