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Hot Stuff in the Kitchen

Many objects one encounters in everyday life: bananas, some rocks, and the human body are slightly radioactive. The radioactivity comes from the presence of naturally-radioactive isotopes such as potassium-40, carbon-14, thorium-232, uranium-238, and the daughter products of the heavy isotopes. Except for unusual cases, such as well-sealed and poorly-ventilated buildings which allow radon gas produced by thorium and uranium in the soil and bedrock to accumulate, this radiation is harmless.

Granite rock is an igneous mineral composed of grains (the origin of its name) of varied chemical composition. Some of these components are naturally radioactive. Alkali feldspar, a component of many varieties of granite, contains potassium-40. Granite often contains between 10 and 20 parts per million of uranium, and some kinds of granite contain up to 56 parts per million of thorium. Both uranium and thorium produce radioactive radon gas as decay products, but this isn't a problem in areas with good ventilation.

The main kitchen at Fourmilab has granite countertops, so I thought it would be interesting to quantify this effect. First, I used a QuartaRAD RADEX RD1706 Geiger-Müller counter to measure the background radiation in my office downstairs.

Radiation monitor: 0.12 μSv/h

The value (averaged over several measurement periods), 0.12 microsieverts (μSv) per hour, is in the range I usually see. Background radiation varies slightly over the day (I know not why), and this was at near the low point of the cycle.

Next, I placed the detector on the granite countertop and left it there to average the radiation. Here's the result.

Radiation monitor: 0.28 μSv/h

Radiation from the granite increased the reading to 0.28 μSv/h, more than twice that of the background radiation measured elsewhere. Is this something to worry about? Not remotely—here is a fact sheet [PDF] from the Health Physics Society about the question, debunking an alarmist story in the New York Times.

Maybe next time I'll put a bunch of bananas on top of the detector.