Air filter and rainwater measurements from forum participants
Originally posted: 5/20/2011 10:30am
Since there have been a few recent discussions where forum participants have taken Geiger counter measurements of radioactivity in rainwater and air filters, I thought I would collect these threads here for ease of reference -- and for quieting the fears some still have that the fallout from Japan is anything to worry about in the U.S.
If you take your own measurements of rain or air filters, please post your data to the forum and I will add you to this list!
Any radioactivity detected by Geiger counters in air filters or rainwater must have a natural source, since the radioactive isotopes we in BRAWM have detected from Japan are a small fraction of the total natural background. The elevated radioactivities that people are observing are very consistent with the decay products of naturally-occurring Radon gas (primarily the longer-lived isotope Radon-222). The count rates can be many times the background level at first, but they disappear in a few hours.
To prove that the radioactivities result from the decay of radon progeny and not radioisotopes from Japan, a simple experiment can be done. The count rate of the sample must be measured consistently over the course of time. In about 40 minutes, the excess count rate should be about half of what it was at the start of the test. In about 4 hours, the count rate should have returned to background levels. This half-life of approximately 40 minutes is a result of the decay chain of Pb-214 (27 minutes) to Bi-214 (20 minutes), both radon decay products and strong gamma and beta emitters. Nothing with a half-life this short has made it across the ocean from Japan, and nothing with a half-life this short could still exist in any appreciable quantity even a couple days after March 11.
First, a gold star goes to James, who posted on 5/11 that he had measured a high count rate when testing his home air filter: "House return air filter high total CPM" One week later, James took 10 minute average count rates for 4 hours and posted his data. BRAWM members analyzed it and confirmed that the rate of decay was consistent with the decay of radon progeny in the dust: "RETURN of the Return Air Filter"
Anonymous took Geiger counter readings of their home air filters and found elevated count rates, although they appeared to decrease by about 50% soon after.
BC has made two measurements:
- BC tested dust that was stuck on their vehicle after a dust storm but did not find any increase in radioactivity.
- BC also tested mud from their garden after a rainstorm. Our analysis is that the sample did not have an elevated level of activity.
Another forum participant took rainwater samples from their car and measured them with a Geiger counter. They measured count rates that were about 2 to 5 times higher than background, and they dropped down to background in 2 hours:
The counts drop to normal background levels within 2 hours. The sample is not detected if it is further than 2-3 cm from sensor window. When I pour water from the tap onto the car (any car) the readings are nbl [normal background levels]. When dry wiped the sample is nbl [normal background levels].
The decay to background levels happens in 2 hours -- not quite as long as in James' case (4 hours) — but there is a wide spread in what this person measured as background levels, and so it is still very consistent with the decay of radon daughters.
A forum participant found that a blogger in Missouri has been taking measurements of rainwater collected on their car and have been finding elevated levels of radioactivity with a radiation counter since late March. In another blog post and video, this person (unwittingly) confirms the radon progeny explanation:
The fallout sample maxed out at 0.686 mR/hr, approximately 62 times normal background radiation as measured in Saint Louis, Missouri. Data indicates that the radioactivity in this sample will dissipate completely within 3 hours.
Someone asked about a website in LA that is taking measurements of rain water and air filters. It appears that one-time readings are being taken — but not readings over the course of a few hours — and they have been interpreting the results as due to fallout from Japan, which cannot be the case.
Added 7/27/2011:
I (Mark, from BRAWM) have made some handheld detector measurements myself:
- Measurement of radiation from potassium chloride (KCl): I measured KCl both to show how radioactive naturally-occurring potassium is, as well as to demonstrate how one can determine what kind of radiation is coming from a sample.
- Measurement of radioactivity of radon decay products in rainwater: I measured a swipe of a metal surface during a rainstorm to show that significant levels of radioactivity can come from radon decay products. These measurements are not Fukushima fallout; they are the radiation we live with every day.
Mark [BRAWM Team Member]
