Answer to Question #5413 Submitted to "Ask the Experts"

Q

I've calibrated some TLD100s using an x-ray beam and a calibrated ion chamber, but need to investigate the energy dependence for ¹³¹I (364 keV gammas) and ^99mTc (140 keV gammas).

What would be the best way to go about doing this? That is, what is the correct way to find the correction factors for the above gamma energies?

A

Lithium fluoride, LiF, is a desirable dosimetric material because its effective atomic number is close to that of soft tissue, being just slightly higher. Its response, light output per unit tissue dose, has a weak energy dependence. If the material is calibrated at ¹³⁷Cs or ⁶⁰Co photon energies (662 keV and 1.25 Mev, respectively), it will typically show some over-response as energy decreases below a couple of hundred keV, being perhaps 2% high at 200 keV, about 15% high at 50 keV, and 30 to 40% high at 10 keV. You can find a typical energy response curve on page 734 of the text by Knoll (Radiation Detection and Measurement, 3^rd ed., Wiley and Sons, 2000).

If you calibrated with x rays, the response at other energies will depend on the energies of the x rays used. If the effective x-ray energy was less than about 200 keV, which I suspect it would be unless you used a medical therapy machine for calibration, then you will somewhat underestimate doses at ¹³¹I energies when you apply the x-ray calibration factor. The response at ^99mTc energy would depend on the x-ray energy used, but would expectedly be somewhat low if the effective x-ray energy used was notably less than 140 keV.

You could apply corrections based on data such as that used for the Knoll figure, but you should be aware that the energy response is also affected by the particular characteristics of the badge holder in which the TLD element is held. Depending on how much accuracy you require, this method might not be suitable. In such event you have a couple of possible options.

One option depends on the availability of higher-energy sealed sources and whether you are willing to accept some error on the conservative side in low-energy measurements. If so, then you could calibrate with a source such as ¹³⁷Cs. The response at ¹³¹I energies would not be significantly different from that at ¹³⁷Cs energy. If you don't have a source available you might be able to arrange with some other facility that uses ¹³⁷Cs for calibration purposes to irradiate some TLDs for you. If you have funds you always have the option of having a standards laboratory such as NIST do some irradiations for you. The response at 140 keV would probably be 1 to 2 percent high if you use the ¹³⁷Cs calibration factor.

A second option, desirable if you require high accuracy (higher than what is normally required for personnel dosimetry) is to actually perform calibrations with ¹³¹I and ^99mTc. I have done such things in the past when evaluating responses of selected instruments at specific energies. It requires purchasing the radionuclides from a supplier and having a calibrated detector available (usually a calibrated ionization chamber) that you can use to establish the dose rates from the sources. If you were to take this approach, keep in mind that the sources will be in liquid form, and things will be simpler if you order them in amounts and in container types that will not require opening the container (with the idea of removing an aliquot) for use. The source volumes should be low and container walls sufficiently thin to minimize self absorption; low volumes also ensure that the sources will reasonably approximate point isotropic sources (not a necessity but often a convenience).

I hope the above is useful to you. Good luck.

George Chabot, PhD, CHP