Answer to Question #10738 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

Presently I have a thermoluminescent dosimeter (TLD) reader in our department. We are reading CaSO4 only. In case I wanted to read other crystals, is it possible? For example, I want to read an NaCl crystal.

A

Yes, in principle it is possible to use a TLD reader for reading the thermoluminescent (TL) output from different phosphors, but your success will depend on the characteristics of the reader and those of the phosphor. A number of commonly used phosphors have major dosimetry glow peaks that appear at around 200°C—CaSO4(Dy) peaks at about 220°C—and you might get away with using the same readout cycle (i.e., the same heating rates and maximum temperatures for the preread, read, and postread portion of the heating cycle), although this would often not give the best results because of differences in some of the electron trapping depths in the different phosphors.

Some phosphors have very different temperature-TL profiles and require different readout cycles in order to yield acceptable results. For example, LiF(Mg,Ti) has its major dosimetry peak at about 200°C, whereas NaCl(Ca) has its major peak around 150°C. If the NaCl is doped with terbium, NaCl(Tb), the major glow peak shifts to about 200°C. Thus, many TLD readers have programmable readout cycles so that you can select and set the preread, read, and postread temperature profiles that suit the phosphor. If your reader has this capability, you should be able to make the adjustments to optimize results. If it does not, you might still get results but you may have to evaluate sensitivity, reproducibility, and effects of fading to determine adequacy. If you are using an NaCl phosphor, such as NaCl(Ca) with a rather low temperature major TL peak, the preread cycle used for CaSO4 would not be appropriate since it would relieve many of the NaCl(Ca) major peak traps.

George Chabot, PhD, CHP

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