Answer to Question #12579 Submitted to "Ask the Experts"

Is it possible for me to calibrate thermoluminescent dosimeters (TLDs) using under-the-couch x-ray tube type x-ray machine? How do I manage the penetration/attenuation/scatter in this type of calibration? How do you suggest for me to do calibration for this type of facility?

While it may be possible to calibrate TLDs using such an x-ray machine it may not be the most appropriate approach depending on a number of considerations, including (1) are the TLDs going to be used to monitor doses to people working around this machine or similar machines or are they intended for some other use? (2) what types of TLDs are being used (e.g., LiF, CaSO₄), (3) if the TLDs are to be used for personnel dose assessments, will you be able to mount the dosimeters properly on an appropriate phantom that can be held in place when using this x-ray delivery system? (4) will there be sufficient space above the couch for you to place the dosimeters on a phantom and at an appropriate distance where the radiation field will be acceptably uniform over the field to be irradiated? and (5) do you have access to appropriately calibrated instruments (e.g., calibrated ionization chambers) to measure the beam intensity at the proposed calibration locations and the necessary information to convert the instrument response to the dose units of interest?

I shall return to some of these points, but I will first address your question about handling scatter and attenuation associated with the under-couch geometry. These are factors that may influence the energy quality and, to some extent, the directionality of photons arriving at the calibration location. Such effects may be acceptable if the instrument(s) you are using to establish the intensity at the calibration location has good energy dependence and directional dependence characteristics; a good quality ionization chamber with wall thickness appropriate for the x-ray energies involved may be suitable. Such instruments should have been calibrated using appropriate radiation sources (usually including x-ray techniques that will cover the energy range of interest).

I do not know what country you are from or what the specific requirements are there, but if the TLDs are to be used for assessment of doses to personnel, and these measurements will represent the doses of legal record, the requirements for calibrations of the TLDs are often quite specific and relatively stringent. You should make sure that your intended procedures are consistent with such requirements if necessary.

Some TL phosphors, such as CaSO₄, which have an effective atomic number appreciably greater than that for soft tissue, have responses that vary rather markedly with photon energy. At the relatively low energies associated with diagnostic x rays, they may exhibit a significant overresponse relative to tissue. If you are using such dosimeters it is necessary to calibrate them to cover the typical range of expected photon energies. They can present problems, especially if the dosimeters will be exposed to a variety of situations in which x-ray energies may be changing—e.g., with differing machine voltages.

Usual calibration of TLDs for personnel dose assessment requires placement of dosimeters on the face of an appropriate phantom, typically something similar to a 30 cm × 30 cm × 15 cm deep polymethylmethacrylate rectangular parallelepiped. Such a phantom is necessary to attempt to simulate the photon scatter that will occur in the body and influence the TLD readings.

Depending on the specific characteristics of the x-ray machine you intend to use, there may be restrictions as to how far above the couch you will be able to place the TLDs and phantom (e.g., imaging system components may interfere), and this may be a problem. Most users in the United States abide by recommendations that, for photon calibrations, the source-to-phantom surface (to which dosimeters are attached) distance should be at least 1 m. Naturally, the radiation field area at the phantom face must include all of the TLDs that you place on the phantom, and TLDs should not be placed too close to the edge of the phantom (usually no closer than about 5 to 10 cm from the edge).

The instrument you use to establish the actual dose (rate) at the dosimeter location must have been properly calibrated to allow accurate measurements of the photon fields of interest. Such an instrument is usually referred to as a transfer standard. Its reliability results from use of highly characterized sources and instruments to establish an appropriate calibration factor for the instrument. Such calibrations are usually performed by duly authorized primary or secondary calibration laboratories. Such an instrument may be calibrated to yield a specific quantity, such as air kerma. You may then have to employ appropriate conversion factors to convert to the dose units of interest.

There are numerous other considerations that might apply to the calibration process. I have noted some of the more cogent ones as regard your question. I should also note that if the TLDs are not being used for personnel dose assessment but rather for other experimental purposes, you may be able to abide less severe requirements. I cannot comment further without more specific knowledge of your intentions

George Chabot, PhD, CHP