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Radioactive Rain

Dr. Andy Johnson, Black Hills State University, Spearfish, SD

What happens to the radiation level when it rains? Check out this surprising graph, showing data collected using a Vernier Radiation Monitor and LabQuest 2 on September 21–25, 2016 in Spearfish, South Dakota. The Radiation Monitor was sealed in a plastic bag and laid on the ground. Radiation was counted in one-minute intervals. The graph initially shows normal background radiation. For the first day (1440 minutes), the radiation counts varied randomly over a range of about 10 CPM to 35 CPM with an average of 22 CPM, which is typical for the area around Black Hills State University (BHSU). Then, it rained. The spikes in readings are due to radon daughters being washed down to the ground during multiple episodes of rainfall.

the apparent length increases as the out-of-plane angle increases
Ground level radioactivity (10 minute average)

222Rn is a decay product of trace amounts of 238U in soil. Because it is a noble gas, radon escapes the soil and enters the atmosphere. With a half-life of almost four days, 222Rn mixes into the lower atmosphere where it transforms into 218Po (half-life 3 minutes) and then to 214Pb and 214Bi, with half-lives around 30 and 20 minutes, respectively. These unstable atoms adhere to dust and water droplets. Precipitation carries them to the ground where they emit additional radiation and turn into 210Pb, which has a half-life of 22 years—comparatively almost stable and not detected in trace quantities.

The upshot is that rain and snow are temporarily radioactive! This is completely normal.

Dr. Andy Johnson of the Inquiry into Radioactivity project at BHSU uses these data to support radiation literacy. Rain data like these help students realize that they are living on a radioactive planet. And it is handy that some radon daughters—214Pb and 214Bi—have half-lives that are of the right scale, so that these radon daughters are detectable and don’t last very long.

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