Answer to Question #14410 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

Usually, dosimeters are calibrated using cesium (137Cs) and americium (241Am) sources using ~660 kiloelectron (keV) and ~60 keV photon peaks. How effective/efficient are those dosimeters in assessing the dose received by a much broader range of photon energies, say 50 keV–2,600 keV? Can such a dosimeter suffice, or should it be also calibrated using other gamma emitters?

A

Generally speaking, thermoluminiscent (TL) dosimeters (TLDs) calibrated using a relatively high energy source (for example, a 137Cs source) can be used to assess the doses received by a broad range of photon energies, from a few hundred keV to a few million electron volts (MeV).

The response of the TLDs at lower energies is a bit complicated. The photon energy dependence of a dosimeter is usually determined by the ratio of the mass energy absorption coefficient of the TL material to that of a reference material (air or water). For photons with energy below a few hundred keV (300 keV), this ratio increases with decreasing energy because the photoelectric (PE) interaction cross section increases significantly as photon energy decreases and, because most TL materials have effective atomic numbers greater than that of soft tissue, the PE interaction cross section, which typically varies as the 4th to 5th power of the atomic number, increases to a greater extent in the TL material compared to soft tissue. As a result, most TL material typically overrespond for photons with energy below a few hundred keV. This overresponse of TLDs is relatively low (~30%) for tissue-equivalent TL materials such as lithium fluoride (LiF), but can be very high for materials such as calcium sulfate with dysproprium CaSO4(Dy).

Appropriate filters can be used to make the TLDs more energy-independent, and to a wider range of photon energies (30 keV to 10 MeV). In addition, it may be ideal that TLDs that significantly overrespond at low energies be calibrated at the energies that they will be exposed to.

Deepesh Poudel, PhD, CHP

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