Answer to Question #10237 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Personnel Monitoring (PM)

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

Q

I have some questions regarding thermoluminescent dosimeter (TLD) measurements of personal dose, Hp(d). Our TLD-100Hs are calibrated at a secondary standards laboratory for one beam quality on an ISO water slab phantom.

The calibration factors we have received are Hp(10) and Hp(0.07).

If I make a measurement with these TLDs on a person by putting the TLD in a plastic bag and taping it to the skin, what do I measure? Can I directly use the calibration factor for Hp(10) or would I have to put a 10-mm tissue-equivalent material on top of the TLD when putting the TLDs on the person?

A

It appears from what you say that the TLD-100H element that you refer to is normally in a bare configuration—i.e., not contained within a badge designed to measure personal dose. If this is so, the question then arises as to what directions regarding dosimeter covering you gave the secondary standards laboratory that determined the calibration factors for you, or what action did they take on their own.

In order to make meaningful assessments of Hp(10) and Hp(0.07) calibration factors it is usual to have the dosimeter elements covered by 1 cm of soft-tissue-equivalent material and 0.007 cm of tissue-equivalent material, respectively. I would expect the laboratory to provide the required covering thicknesses, but if this was not the case, and the laboratory irradiated the dosimeter elements in a bare configuration, the major uncertainties that arise are whether secondary charged particle equilibrium existed at the location of the dosimeter elements and to what extent would the 10-mm thickness have reduced the photon dose. For the shallow value, Hp(0.07), you may have to live with any effects of nonequilibrium anyway because it may defeat the purpose to add sufficient thickness to ensure equilibrium, but for the Hp(10) value adding the required thickness is desirable. If the dosimeter elements were irradiated in a bare configuration, you may be able to ascertain from the secondary standards laboratory whether secondary charged particle equilibrium existed at the irradiation locations. Even if charged particle equilibrium existed in the laboratory setting, you cannot assume that such is the case for your facility.

You would have to make or have made measurements under various thicknesses of tissue-equivalent material to observe the effect of possible lack of equilibrium and the additional effects of photon attenuation through the 1-cm tissue-equivalent thickness. Putting a bare TLD element in a plastic bag and taping it to the body may give a reasonable estimation of Hp(0.07), but the estimation of Hp(10) may be in error. Depending on what source you are using, the departure from secondary charged particle equilibrium may be quite noticeable, often with excess electrons and/or photons resulting from scatter in the source holder, collimator, etc., but at times with a deficiency of secondary charged particles.

The bottom line is that the laboratory that determined the calibration factors should have provided the required thicknesses above the respective dosimeters, and you should use the same required thicknesses when using the dosimeters for dose assessments.

George Chabot, PhD, CHP

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