Answer to Question #7240 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:


I use a Palmrad 907 dosimeter in my work as a radon abatement contractor to help identify sources of radon soil gas in the basements of homes. This device displays readings in units of counts per minute (cpm) and counts per second (cps). Is there a formula that will convert cpm or cps to Bq/L?


The instrument you describe uses a conventional pancake-type, thin-window Geiger-Mueller (GM) detector. While the detector is capable of measuring alpha radiation, it is not intended for the application you describe in which you wish to measure the concentration of 222Rn in gases seeping into homes. If the radon concentrations of infiltrating gas are sufficiently high, you would be able to detect the presence of radon, but a meaningful conversion to activity concentration from count rate would be tenuous unless you had a specified geometry and had done a reasonable calibration of the instrument for that geometry.

Only alpha radiation emitted within a rather short distance from the detector would be measured. If you consider that the range of radon alpha radiation in air is about 4 cm (4.8 mg/cm2) and that the window thickness is about 2 mg/cm2, alpha radiation would have to be emitted within a distance of about 2 cm of the window in order to penetrate the detector window.

In order to do a very crude estimation, we may consider an air volume that is cylindrical in shape and with a diameter equal to that of the active window (4.5 cm for this detector) and use the average chord length of 2 cm as that which we would allow for any alpha particle emitted by the radon. We can calculate the thickness of the cylindrical air volume as 1.8 cm (this comes from the fact that the average chord volume in the cylindrical volume of radius R and depth H is given by 2RH/(R+H)); the real average alpha particle track length in the volume would be somewhat less than the chord length. The volume of the cylinder is then about 29 cm3.

If the detector was placed in a uniform air concentration of 37 Bq/L, the total alpha emission rate would be (37 Bq/L)(1 L/1,000 cm3)(29 cm3)(60 dpm/Bq) = 64 dpm. No more than half of the emitted alpha particles would be traveling in the general direction of the detector window; therefore, the maximum possible count rate would be 32 cpm, implying a conversion factor of 1.15 Bq/L per cpm. The 32 cpm is roughly equal to what one might expect for normal background on the pancake-type GM; thus the gross count rate might approximately double for this situation.

In actuality, the expected count rate would probably be considerably less since alpha particles emitted at shallow angles with respect to the window would travel greater pathlengths through the window, and other particles emitted in the generalized 2 p direction of the window might exit the sides of cylindrical volume; instead of 32 cpm due to the radon, one might well see less than 10 cpm.

One useful calibration might involve exposing the detector in a uniform concentration of airborne radon. This would have to be done by a calibration laboratory that can generate a reasonably high concentration of radon in a closed chamber. Such a calibration might be useful if you are concerned with very high radon concentrations that are uniform over a reasonable volume. For example, the calibration might provide a decent estimate of the concentration in a basement sump (pit). It would not be very useful if you are holding the detector close to some ingress points such as cracks in a basement floor where the concentration would be very localized and provide nonuniform exposure of the detector to the contaminated gas.

The bottom line is that there is no quick and easy conversion factor to get radon concentration from the detector count rate. Because the detector is not sufficiently sensitive to provide measurable responses to generalized volume concentrations of interest (i.e., a few pCi/L), and many of the source ingress measurements made where concentrations may be much higher represent unknown source-detector geometries, acceptable calibrations may not be available, the exception noted above for the uniform volume source. For most situations the detector provides primarily a qualitative indication of where the radon might be entering a dwelling. Happy hunting.

George Chabot, PhD, CHP

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