Answer to Question #7855 Submitted to "Ask the Experts"

Due to the recent articles and news reports, I was asked by a family member to measure a granite countertop prior to purchase. Using a Ludlum Geiger Mueller (GM) meter with a pancake probe I checked a countertop at the stone yard. The surface reading was 0.1 mR/hr and background was 0.01–0.02 mR/hr. I understand that granite contains quantities of naturally occurring isotopes but I was curious what the average levels are. I do not have the instrumentation to measure ppm. I was hoping to measure no more than two times background.

I have been asked by the HPS "Ask the Experts" editor to provide information for your request concerning radiation measurements on granite countertops.

First of all you are right that granite may contain small amounts of uranium and radium. Actually these radioactive elements are found in all materials that come from the ground. They are a natural part of the composition of all earthen materials. Granite, in particular, has long been known for having measurable amounts of radium and also to be a source of measurable gamma radiation.

With the ongoing publicity about granite as a radioactive material, many people across the country have attempted to make measurements to determine the levels of radiation. Unfortunately, the common GM detector is not very well suited for such measurements. In particular, the pancake GM probe that you have used with your radiation meter is not very good for measuring gamma radiation. It is best suited for medium- to high-energy beta particles.

Since your detector is best suited for medium- to high-energy beta radiation, this is the primary signal that you will detect with your GM meter from uranium and radium decay products. What this means is that the signal from granite with your pancake GM detector will be 90 to 95% beta particles. On this basis alone, your readings in mR/hr are too high by a factor of 10 to 20. Also, when you hold your detector in the air, you will measure gamma rays from the ground (which contains uranium and radium) and cosmic rays or gamma rays from outer space. These readings, known as background readings (of gamma rays), cannot be compared with the readings from the surface of granite, which are mostly beta particles.

Furthermore, the scale on your meter is calibrated in exposure units of milliroentgen/hour (mR/hr) in comparison with a gamma ray signal from cesium-137. Exposure, in the language of radiation safety, has a very specific meaning related to ionization in air from x rays or gamma rays. Exposure is not defined for a beta-particle signal. Thus, the scale on your meter is not calibrated for a beta-particle signal. Therefore, the reading on your meter has no meaning in units of mR/hr. You could compare readings from one piece of granite to another as relative values, but not in units of mR/hr. Beta-particle readings can be measured in units of millirad/hour (mrad/hr), if your meter is calibrated with a known source of beta particles.

In order to measure only gamma rays (which is what your detector is actually calibrated for) you would need to block out the beta-particle signal with about ¼-inch of plastic. This will eliminate the beta component or interference with measurements in mR/hr (which should only apply to x rays or gamma rays).

Eliminating the beta signal, however, will only resolve one problem with your GM detector. The other problem is that your GM detector is calibrated only for measurements of gamma rays from cesium-137 at an energy of 662 keV. Measurements of gamma radiation at any other energy will not be accurate. The gamma energy from uranium and radium decay products are below 662 keV (radium decay products have primary gamma energies from about 300 to 600 keV) and your GM detector will likely underrespond by as much as 30 to 40%.

My conclusion is that readings with your GM detector could easily be high by a factor of 10 to 20 or more because of the large beta-particle signal and never should be reported or interpreted in units of mR/hr. If you block out the beta signal, your detector may read low by 30 to 40% because of the gamma energies below those of cesium-137.

As you can now see, there are substantial limitations on the use of any type of GM detector for evaluating granite. Thus, I would urge great caution for interpretations about your measurements that are based on readings from an open-window pancake GM detector.

The best detectors for measuring radiation or exposure levels from granite would be either a closed-window standard ion chamber or a pressurized ion chamber. Both are designed to block out the beta-particle signal and also have good energy response over the range of gamma-ray energies that come from radium decay products.

I hope you will find this information helpful. I would be happy to talk with you or exchange emails, if you would like to pursue this matter further.

Ray Johnson, MS, PE, FHPS, CHP