Answer to Question #9321 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

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

I'm using a 6 MeV photon energy and have to calibrate a set of TLDs (thermoluminescent dosimeters). What should be the dose suitable for calibration, from 10 to 100 cGy?

The answer to your question depends, at least in part, on what type of TLD you are using, what range of doses you are intending to measure after the TLDs have been calibrated, and what the intensity characteristics of the source are.

If the TLDs are being used for routine, occupational personnel dosimetry, calibration doses in the range from about 0.1 cGy to a few Gy are common. For most TLD types, readings at doses in this range will be sufficient to provide acceptably low uncertainties, the doses are in the range of typical actual interest, and the doses fall in the linear range of dosimeter response.

Also keep in mind that dosimeters being calibrated for personnel dosimetry applications should be mounted on an appropriate phantom to account for increased dosimeter response from secondary radiation effects associated with photon interactions in the body when the dosimeter is worn. Generally, however, backscatter effects that might be significant at lower photon energies are quite small at 6 MeV since most all the scatter is in the forward direction at this high photon energy.

If your interest is in using the TLDs to assess therapeutic dose distributions—e.g., in a phantom, as might be of interest if a linear accelerator is being used for radiation therapy—then the doses of interest might be considerably higher than those of concern in routine personnel dosimetry. Indeed, doses in excess of 1 Gy might be of concern and your range might extend from about one Gy to possibly beyond 10 Gy. In such instances, you must be aware of the TLDs’ linearity characteristics. Many TL materials, including the common TLD-100 (LiF(Mg:Ti)) exhibit supralinear responses beyond a few gray, and it may be necessary to prepare a calibration curve with several measurement points that cover the dose interval of interest so that the response curve is well defined.

If you are interested in environmental doses, as might be the case, for example, if you were trying to assess direct or scattered radiation at a site location at some distance from a source of 6 MeV photons, as might be represented by 16N in the turbine room of a boiling water reactor, then the doses of concern may be relatively low. In such an event, one may select a TLD type that is more sensitive than some of the common types used in personnel dosimetry and, with the enhanced sensitivity, calibrations at doses less than 0.1 cGy may be reliable and appropriate.

I hope this addresses your concerns. Good luck.

George Chabot, PhD, CHP

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