Here are some results from measurements I made last night and today using cotton make-up pads with a vacuum sweeper. I am now in a better position to understand why my past results have not agreed with results others have observed.
An executive summary of my results would be that I can agree that others have indeed observed count rates of several hundred cpm from cotton pads that have been placed over a vacuum sweeper intake, but only if the radon levels at their locations are higher than they claim.
Over some number of years I have passed air through various filters and then tried to detect activity with both GM-tube detectors and NaI(Tl) detectors, and I have observed next to nothing. I repeated this yesterday by drawing air (with a vacuum sweeper) through a cotton make-up pad for five minutes. When placed in gentle contact with the mica window of a 3-cm diameter end-window GM tube connected to a survey meter, I could not detect any counts above background. When placed in contact with a 2"-diameter by 2"-tall cylindrical NaI(Tl) detector, and counted for 10 minutes, I could not detect any counts above background.
This test (and previous tests) were conducted in my lab in our science facility. Previous measurements at this location have indicated radon levels between 1 and 2 pCi/L, so this is a rather low radon area. But people have reported easily seeing outdoor radon using this technique, and outdoor radon typically measures 1 to 2 pCi/L unless you live in a "hot spot." So my measurements have always been in disagreement with these reports.
Last night I went to a basement that I know runs between 10 and 12 pCi/L of radon and I used a sweeper to draw air through a make-up pad for 5 minutes. That pad easily showed activity with my GM-tube and survey meter. The meter has an analog readout (plus audible clicks) so it is not an actual count rate, but there is fairly long integration on the signal to the meter, and the needle hovered at roughly 200 cpm and sometimes approached 300 cpm and sometimes dropped to 100 cpm. Background with that meter is about 20 cpm.
Therefore, for the first time, I clearly saw activity with this method, but it was in a radon area five to ten times higher than what other people have been reporting.
I returned to my lab with the pad and started a 10-minute count with the NaI(Tl) system. This count was started within 10 or 11 minutes from removing the pad from the sweeper. The 214-Pb and 214-Bi gamma emissions were clearly observed, and the 10-minute count registered 4439 counts (total) in these gamma peaks using a 10-minute count. If you have not seen this type of spectrum you may want to have a look at it. You can observe this gamma spectrum using this link...
www.bluffton.edu/~edmistonm/Radon Daughters on a Cotton Pad.pdf
This morning I counted the pad again, and there was no observable activity. This indicates the pad collected 214-Pb and 214-Bi but did not collect 226-Rn. When we collect radon samples using activated charcoal, the 214-Pb and 214-Bi activity decays with an apparent 3.8-day half-life. This indicates our usual collection and detection method actually collects radon itself.
If the sample from last night really came from a basement with 10-12 pCi/L of radon (and I believe that is accurate) then it makes sense that I cannot detect these radon daughters on a make-up pads that sample my lab air. My lab levels are roughly 10 times less radon than the basement I sampled. That would bring the pad down to roughly background levels. Background in my NaI(Tl) system consistently runs between 410 and 430 counts per 10 minutes (in the combined gamma peaks for 214-Pb and 214-Bi). So I detected levels ten-times background when I sampled a room that has at least 5 times, and probably 10 times more radon than my lab. Therefore it makes sense I have not been able to detect radon with this method in my lab.
But this makes me fearful that people measuring 200 to 400 cpm in GM-tube counts of cotton pads might be living in higher radon than they think. There is an additional possibility that I will explain in a follow-up message in a few minutes.
Michael D. Edmiston, PhD.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
Bluffton, OH 45817
Office 419-358-3270
Cell 419-230-9657