Answer to Question #12034 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
On a granite countertop, are radiation measurements of 785 counts per minute (cpm) and 1.7 microsieverts per hour (µSv h-1) safe? These measurements were performed with Mazur Instruments' Nuclear Radiation Contamination Detector, PRM-9000. What are acceptable radiation values for a countertop?
The values you give for the thin-window Geiger-Mueller (GM) detector, such as the Mazur model you cite, are consistent with typical expectations for such measurements. You can be assured that the values do not represent a health risk. You can find more information on such measurements on the Ask the Experts website; check out the answers to questions under "Granite and Stone Countertops."
The problem with attempting an exact interpretation of such readings is that the thin-window detector responds to beta radiation and possibly alpha radiation, as well as some gamma radiation, from natural radioactive species in the granite. The response conversion factors that the manufacturer provides, however, are valid only for gamma radiation, in particular gamma radiation from cesium-137 (137Cs), a standard radionuclide used for instrument calibrations.
If you wanted to attempt another measurement of just the gamma radiation, you could place a piece of plastic, about one centimeter (cm) in thickness, between the granite surface and the face of the detector. The plastic would attenuate the beta and possibly the alpha radiation and would provide a sufficient thickness so that the gamma-energy response would be acceptably uniform for dose estimation purposes. The reading in such a case would likely be considerably less than the value you obtained with no attenuating material and would be more representative of the penetrating dose component near the granite surface.
As I stated above, you need not be concerned about any negative health consequences from exposure to the radiations coming from the granite.
George Chabot, PhD, CHP