Answer to Question #7966 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 want to use TLDs (thermoluminescent dosimeters) to measure the dose to patients (skin, thyroid, and eye) under angiography. What are the types and shapes of TLDs? What is the loose pellet shape and can I use all shapes of TLDs in a reader? I want to use a SOLARO2A reader and it has a circular tray, but I have a square-shaped TLD-100.
 

A

TLDs come in a variety of shapes, sizes, and physical arrangements. Among the most common have been square-shaped TLDs, exemplified by the common LiF (Mg,Ti) chips, originally manufactured by Harshaw, with typical dimensions of about 3.2 mm square and a density thickness of about 235 mg cm-2, although thinner versions are available.

Dosimeters are available in various other configurations, including longitudinal rods, circular disc shapes, and crystalline powders. The latter are often contained between two layers of heat-resistant plastic with at least one plastic face being optically transparent. The dosimeter elements may be sandwiched between plastic layers that are held as windows within metal or plastic cards; the readers are adapted to accommodate these cards, which are often desirable for users who are processing large numbers of personnel dosimeters since the cards are readily adapted to automated reader operation.

The powder types may be fabricated so as to be relatively thin, and this can be an advantage especially when trying to measure low-penetrating radiation. Dosimeters have also been fabricated using a mixture of the TL material in Teflon® to form sintered products of various shapes and sizes for a number of possible applications (CaSO4 in Teflon® dosimeters, manufactured by Teledyne, were among the most common of this type in the past).

It is not uncommon for TLD readers to have the capability of receiving various TLD geometries. Sometimes this may require manipulation in changing out a holding tray that serves to hold the TLD and to transfer heat to the TLD. A tray with a circular-shaped recess to receive the TLD element may often be used for a square element as long as the TLD will fit within the circular area and sit flat against the surface to ensure good heat transfer to the TLD (especially in the case where resistive heating is used).

In such a case, one should try to position the TLDs in about the same location on the tray each time to avoid possible physical/optical effects that might show slight position dependence. I do not have direct experience with the Solaro reader that you wish to use, but I believe the circular tray may be adequate as long as the square elements fit within the circular area so that they lie flat and can be located in a reasonably reproducible fashion.

I would test for reproducibility by irradiating a group of the TLDs (at least five) to a known dose and reading them out individually in the reader. Evaluate the reproducibility to see whether it is acceptable to you. If you find it is not adequate, you may have the option of changing out the heating tray with another designed specifically for the square profile. You would have to determine such availability from the manufacturer or supplier of the reader. Good luck.

George Chabot, PhD, CHP
 

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