Answer to Question #13252 Submitted to "Ask the Experts"

Regarding thermoluminescent dosimeters (TLDs) such as LiF(Mg, Ti) and LiF(Mg, Cu, P), is there any difference in the response to a single exposure compared to repeated single readings of multiple exposures? If we expose the TLD with multiple exposures for one read, will the average response (for one exposure) be the same as a single exposure? If not, how is that possible? Can you explain how scattered radiation affects the TLD response?

The short answer to your initial question is "Yes, it is possible that multiple exposures will result in a final dose readout that is somewhat different from that for a single exposure that delivers the same total dose." The more complete answers are not so straightforward since the dosimetric response that one encounters using these TLDs depends on several factors, including the temperature profile being used in the readout cycles, the annealing cycle used, the lengths of times between irradiations when multiple irradiations are performed, and the ambient storage conditions in which the dosimeters are maintained during, pre- and post-irradiations. I shall attempt briefly to address these.

The TLD LiF(Mg,Ti) dosimeters have a complex glow curve structure with about 10 different glow peaks. The major dosimetry glow peak for routine use is peak 5, which peaks at somewhat less than 200°C; however, it does exhibit considerable overlap with peak 4 and some of peaks 3 and 6. The readout cycle used should be such that the low temperature peaks do not contribute significantly to the recorded readout. Any contribution of such peaks to the recorded readout may change because of fading effects both prior to and following irradiations. Thus, if such peaks are contributing, and there is always some contribution for any practical readout regimens, and as you perform multiple exposures, the dosimetry signal may be reduced compared to what would be observed for a single exposure of the same theoretical dose as the sum of the multiple exposures.

For TLD LiF(Mg,Cu,P) the glow curve is simpler than that for LiF(Mg,Ti), having five glow peaks with the major dosimetry peak 4 at about 220°C. Problems with residual dose resulting from the high temperature peak 5 and possible reductions in sensitivity when readout temperatures above 240°C are used can be a problem that also might affect multiple exposures differently from single exposures. Whether you are performing in-reader annealing or manual external annealing of dosimeters it is important to maintain a fixed and acceptable protocol, attempting to use protocols that minimally influence changes in sensitivity. The readout cycle should be chosen to minimize recorded readout from low energy peaks 1, 2, and 3. To the extent that these contribute to the recorded readout they, as in the LiF(Mg,Ti) case, may lead to different results for multiple exposures compared to single exposures with possible fading effects.

Fading effects are naturally enhanced when the ambient temperature or the storage time between exposures increase. It is often possible to control these variables to minimize their dose impact, although it may be impossible to avoid natural temperature excursions and time requirements that augment negative fading effects.

As mentioned above, depending on the readout conditions, the multiple exposures may yield a different apparent dose from that for a single exposure of the same total delivered dose magnitude. The dose interpreted for a single exposure out of the multiple exposures (e.g., for five exposures of equal delivered doses, the expected average dose for a single exposure would be the total interpreted dose divided by 5) would presumably yield the same dose value as that for a single exposure of the same duration if the prehistory of the dosimeters used were the same; that is, subjected to the same previous readout and annealing cycles, stored for similar durations and under similar ambient conditions prior to use. Differences in such conditions could, however, lead to differences between the interpreted doses, and we would expect that there would be some differences since you have said that the dose was delivered using multiple exposures, making the prehistory for each exposure likely different and different from that that would apply to a single exposure. Assuming you are using recommended readout protocols, I would not expect the difference to be very high, possibly a few percent.

Regarding scattered radiation, generally the most important source of scattered radiation reaching the dosimeter is the body of the dosimeter wearer. Radiation entering the body may undergo single or multiple scattering events in the body with some of the scattered radiation finding its way to the dosimeter. In general, if photons are the major concern, the degree of backscatter that contributes to dosimeter response increases as photon energy decreases. Of course, depending on the nature of the radiation sources of concern and the physical characteristics of the worker environment, other external objects may also contribute to scattered radiation reaching the body and the dosimeter.

George Chabot, PhD, CHP