Answer to Question #312 Submitted to "Ask the Experts"

Q

Is the accuracy of a TLD (containing LiF) personnel radiation monitor affected by environmental temperature changes? If so, at what temperature (F/C) will the TLD's accuracy begin to be compromised?

A

Since I don't know the educational background of the person who asked, I will give a little summary of the basic physics of thermoluminescent dosimetry (TLD) and of LiF(Mg, Ti), commonly known as TLD-100, the name given by Harshaw many years ago. This TLD phosphor is not fully understood on the basis of its composition but many studies indicate that when it is irradiated, electrons are trapped in metastable states which empty at a rate determined by the "depth" of the trap below the conduction band and the temperature of the TLD. This depth is typically about 1 eV. LiF (TLD-100) has at least six traps above room temperature. (I'm sure it has many more below room temperature.) The first significant peak in the glow curve if it is heated at about 10°/s occurs at about 105° C, followed by several closely spaced peaking at about 190°C. (If the heating is faster, the peaks occur at higher temperatures, and if the heating rate is slower, the peaks occur at lower temperatures.) The low-tempearature (105°) peak fades in about a day. Most TLD readers are programmed not to integrate any of the emitted TL until the temperature is above 100°, which eliminates most of this low-temperature peak if they are read soon after exposure. The main peaks near 190° are not affected significantly by normal ambient temperatures.The fading rate seems to vary with the sample. A fading rate of 5 percent a month is commonly used but each TLD probably has slightly different composition so the fading is probably different. A study of annealing the TLDs at 80° for a day showed no detectable fading of the 190° peak. Annealing the TLDs at 125° C for 48 hours before exposing them reduces the sensitivity of the main peak to about 10 percent of normal! The point of mentioning the latter is to alert the reader that TLD-100s (and probably other types of TLDs) are sensitive to their thermal history. The solid-state physics of high-temperature TLD phosphors is not understood. Nature did not provide radiation scientists with a "perfect" TLD. It requires great care to get the best accuracy. They are still very convenient if you have the patience to use them correctly. If you have further questions, please contact me by e-mail. John R. Cameron, Ph.D. Professor Emeritus UW-Madison