Answer to Question #7746 Submitted to "Ask the Experts"
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You are correct about the interest in measurement of radiation levels associated with granite countertops. Geiger-Mueller (GM) detectors come in a variety of shapes and sizes and have windows of varying thicknesses, and the interpretation of the reading of the detector depends on the detector characteristics and how it was calibrated. Many times GM detectors are calibrated to read exposure rate from a standard source, most often 137Cs, which emits 662 keV photons in its decay through 137mBa. When a thin-window detector that is capable of sensing alpha, beta, and gamma radiation is placed at the surface of a granite slab that likely contains uranium, thorium (along with their decay progeny), and 40K as the major radionuclides, the detector may respond to the three radiations, although it will not likely be possible to make an accurate interpretation of what the reading signifies. If a thick-walled detector is used, the responses to alpha and beta radiation may be eliminated, and the reading will be associated with the gamma radiation response.
The expected gamma exposure response above normal background is on the order of 10 microroentgens per hour. The expected energies of photons emitted from the radionuclides in granite are numerous and range from less than 100 keV to more than 2.5 MeV; if the thick-walled GM detector were calibrated with 137Cs photons it would not likely provide an accurate reading of exposure rate at the surface of the granite. If the detector had been energy compensated (usually accomplished by wrapping the detector in a moderate- to high-atomic number metal to reduce response to the lower-energy photons), one might expect the reading to be within about 30 percent of the actual exposure rate.
If there is a concern with external dose from the granite, the gamma radiation would be the most significant contributor to the dose, and there are other detector types that are preferable over the GM detector. In addition to the possible problems noted above with regard to response to different radiation types and photon-energy dependence, the gamma sensitivity of the GM detector is not as good as some other nongas detectors that are available. My preference would be to use one of the instruments that use solid detectors, often marketed as micro-R (or micro-rem) meters. Some such instruments use plastic-scintillator material as the detector. These have an advantage of being more similar to air (or tissue) in atomic composition so that their energy response is good over a wide range of photon energies. Some of the micro-R meters use inorganic scintillators, the most common being NaI(Tl); these types have effective atomic numbers considerably greater than air or soft tissue and usually exhibit a notable energy-dependent response; they likely will overrespond appreciably when exposed to gamma radiations from the granite. Both the GM detector and the typical scintillator detectors are pulse-type detectors, in which the recorded event rate is correlated with the exposure or dose rate. The increased sensitivity provided by the solid detectors provides for appreciably higher count rates per unit exposure or dose rate and provide commensurately greater statistical certainty in the results.
While gamma exposure rates on the order of 10 microroentgens per hour above background directly above the countertop might be expected, both lower and higher values are possible. Granite for countertops and other applications is being produced in a number of countries throughout the world, and the uranium and thorium contents of the granites may vary considerably. It must be kept in mind that a simple measurement of gamma exposure rate at the countertop surface is not a measure of the average effective dose rate to occupants of the home. The latter dose rate and integral dose depend on proximity to the countertop, body orientation, and occupancy time, as well as other physical factors, and will expectedly be much less than the surface dose (rate).
Naturally, any external gamma exposure or dose measurements made do not provide any direct information regarding the potential radon emission rate and airborne concentration in the home; calculations and measurements that have been made have generally confirmed that the contribution to internal dose from such radon (and its progeny) is insignificant compared to the usual background levels in the home.
Hope this is helpful.George Chabot, PhD, CHP