Answer to Question #11262 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
We are planning to measure environmental natural background radiation using calcium fluoride dysprosium (CaF2: Dy) thermoluminescent dosimeters (TLDs).
For calibration of the TLDs, can we calibrate using cobalt-60 (60Co) sources at dose rates of 1 cGy, 2 cGy, 3 cGy, 4 cGy, and 5 cGy and then extrapolate for measurement of low natural background radiation? We don't have any other calibration facility below 1 cGy.
For exposing these TLDs in the environment, can we measure the radiation without using an energy filter such as a 0.1-mm copper (Cu) filter?
If such measurements are carried out using an ionization-based survey meter, which one would be more reliable?
As you know, CaF2:Dy (TLD-200) has been used for many years for environmental monitoring by various facilities. I shall attempt to address each part of your question.
The dose range you cite, from 1 cGy to 5 cGy, may suffice, although I would recommend that you attempt to obtain one or two points in the interval from 0.1 cGy to 0.5 cGy just to confirm the extension of the expected dose response linearity to lower levels. I expect your actual environmental results will include relatively low levels, depending on how long you intend to keep the dosimeters in the field before processing. Most facilities use cesium-137/barium-137m (137Cs/137mBa) as the typical photon calibration source, but the use of 60Co gamma radiation should be technically acceptable. Make sure you perform the irradiations with the badges in the same physical packaging and configuration that you intend to use for the environmental measurements.
The use of metallic filters to reduce excess energy at lower photon energies has been popular among many users of TLD-200, whose relatively high effective atomic number produces some excessive dose response (compared to soft tissue). Some facilities, however, have opted against using such filtration since it notably curtails response below about 70 keV; in such cases one can choose to accept the uncorrected overresponse or attempt to correct for it (this can sometimes be done by making simultaneous environmental measurements with an instrument or dosimeter that exhibits no appreciable energy dependence). Whatever you decide, however, make sure that the dosimeters are properly enclosed in an opaque packaging to prevent exposure to light and ultraviolet radiation.
Reasonable environmental measurements of gamma dose (rates) may be carried out using sufficiently sensitive instruments. Ionization chambers, most especially pressurized ion chambers that provide enhanced sensitivity through the increased gas pressure in the chamber, are appropriate. Having an instrument that is capable of integrating the dose is desirable so that averages over reasonable times can be obtained. This is a common feature on current digital display instruments. Some plastic scintillator-based microdose (rate) measuring instruments are also suitable for reliable environmental dose measurements. There are some microdose meters that use inorganic scintillators, e.g., sodium iodide with thallium, NaI(Tl), or cesium iodide with thallium, CsI(Tl). I do not recommend these because of the very large photon energy dependence that they exhibit. You can find listings of various instruments on the Internet. Check under micro-R instruments or pressurized ion chambers or related terms. You can also find references to a number of manufacturers/providers through the Buyer's Guide list on the Health Physics Society's website.
I wish you well in establishing your environmental surveillance program.
George Chabot, PhD