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User Question About Rain

Note: A new interesting graph and text from Frank added to bottom of this page showing activity of carbon impregnated filter paper after running 1000 cc of rainwater through it.

Received: from
Subject: Gamma Radiation


I hope this finds you doing well.

I have been monitoring the gamma radiation background from the NE corner of my house for some time now, actually several years, hoping to capture some outstanding cosmic rays.

However I stumbled onto something else that you may already know about or at least the scientific community may be well aware of the increase in gamma radiation during times of rainfall. I have attached a GIF file of one of the most recent examples obtained with the RM-G90. I have dozens of examples of this increase in the levels of gamma radiation during rainfall or snowfall or in some cases a modest increase in the radiation level during an increase in humidity. Humidity levels here in the west may exhibit wild swings during the course of a day.

There may be several explanations for this effect i.e. scrubbing of volcanic particles from the air or the concentration of radon etc.

About a month ago I did send about ten of my graphs to Dr. Frank at the univ. of Iowa (unsolicited). My thought was there might be some connection to his discovery of large numbers of icy house sized comets entering the earths atmosphere, however he has not responded so he may have been too busy or disregarded the information.

Frank Dement - Kennewick, Washington, USA

If reader has a more definitive explanation for increase in gamma radiation with rain and snowfall, in the Washington State area, please E-mail Bryan Boardman at or Frank at

Graph Displaying Decay Curve of Precipitation Residue


I haven't heard from anybody concerning the rain/snow problem. My efforts in trying to understand have consisted of evaporating a 1 gallon sample of rainwater to dryness on a planchet and counted it with the RM-70. It took several days to accomplish this. There was a bare indication of decay of some long T1/2 material but it was too close to background do determine anything. There should be a speedy method of method of liquid concentration but I don't know of it. The solution turned out to be the 3DECAY program (I should have known). Any alpha particles are shielded by the moisture and beta particles cannot penetrate the sides of the house so that left only the gamma emitters for the RM-G90.

I made the assumption that it was Radon or Thoron. Looking at the gamma T1/2 of the these gases and comparing the decay curves it looked as if Rn-220 was the best bet. This led to the assumption that the snow/rainfall was scrubbing the gases and decay products from the atmosphere. Thallium 208 has lots of gammas but with its short 3 minute T1/2 it could not be a very significant contributor however Lead 212 and Bismuth 212 turned out to be the main components as indicated by the 3DECAY plot (see attached file: dec05_97.gif). I am surprised that someone, at least in my area, did not point out the obvious to me. I did spend quite some time on this and I did not note the obvious until now.

Best regards,


I followed your suggestion and ran 1000 cc of rainwater through a carbon impregnated filter paper. The results were surprising to say the least. The count rate was at least fifteen fold greater than the previous gamma only count rate and using the MAKDECAY utility there was an almost perfect match of decay curves. I have attached a gif file, carbfilt.gif. From the time that I collected the sample until the filtration process was complete took about half an hour so the activity in the rainwater must have been much more than we see on the graph.

I was so pleased with the result that I did not take the time to section the graph so there is a lot of blank space on the graph.

I received an e-mail from Mark in Longmont Colorado wherein he referenced a book titled "Health Physics" by Cember who mentions "atmospheric washout," I have an earlier printing of Cember's book Introduction to Health Physics but he made no mention of washout.

Latest graph showing activity of carbon impregnated filter paper after running 1000 cc of rainwater through it.
Carbon Filter Paper
Best Regards,
Frank Dement

I was looking through your very interesting website and saw the graphs and discussions about increased radiation levels observed during rainfall. I don't know how old that discussion is... but if still interested you might find the following worth a visit:

It's a subject I became interested in after observing some surprising GM measurements during a recent trip in very stormy weather (in WA state as well).


Charles Krebs
Issaquah, WA

BTW... I would be very appreciative if you could offer some incite on a question I've wondered about. I understand why people are concerned about the "214" radon progeny, but how come there does not seem to be too much concern over Pb 210 or Po 210. Since they are longer lived, wouldn't they tend to accumulate considerably with time in a house that had fairly high levels of radon? Or is is just that their activity or quantity is so low it's not a health concern?

Charles, Thank you for the interesting links. As regards Pb 210 Po 210 you are right i.e. their activity is so low that it is hard to detect but an older house will have a build-up of these in old dust which can be detected with the RM unit.

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