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My Geiger counter measures a background radiation level in my home of 0.09–0.11 μSv/h.

When I stick it inside the dryer right after it finishes a cycle (while the clothes are still inside), it registers a radiation level of 0.16–0.18 μSv/h.

What happens during the dryer cycle that accounts for this reading? From what I understand it has something to do with trapping radon, but how exactly does this happen?

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    $\begingroup$ Is your dryer burning some form of fossil fuel? Piped gas? $\endgroup$ May 17, 2023 at 16:25
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    $\begingroup$ Was your dryer made from radioactive steel from a former nuclear power plant? ;-) $\endgroup$ May 17, 2023 at 16:37
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    $\begingroup$ Do note that at higher temperatures the detectors will get more background radiation. $\endgroup$ May 17, 2023 at 16:48
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    $\begingroup$ Just a thought, but is there a required operating temperature range for the instrument? $\endgroup$
    – Bob D
    May 17, 2023 at 16:50
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    $\begingroup$ Check near the lint trap. Many fibers hold on to dust particles and such. $\endgroup$
    – Boba Fit
    May 18, 2023 at 14:00

2 Answers 2

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Uranium and thorium in heavy rocks have a decay chain which includes a three-day isotope of radon. If a building has materials with some chemically-insignificant mixture of uranium and thorium, such as concrete or granite, then the radon can diffuse out of the material into the air. This is part of your normal background radiation, unless you have accidentally built a concrete basement with granite countertops and poor air exchange with the outdoors, in which case the radon can accumulate.

When radon does decay, the decays leave behind ionized atoms of the heavy metals polonium, lead, and bismuth. These ions neutralize by reacting with the air. Here my chemistry is weak, but my assumption is that they are most likely to oxide, and I assume further that the oxide molecules are electrically polarized, like the water molecule (the stable oxide of hydrogen) is polarized.

Polarized or polarizable objects are attracted to strong electric fields, even when the polarized object is electrically neutral. Imagine a static electric field around a positive charge. A dipole nearby will feel a torque until its negative end points towards the static positive charge. But because the field gets weaker away from the static charge, there’s now more attractive force on the negative end of the dipole than there is repulsive force on the positive end, so the dipole accelerates towards the stronger field. If you used to have a cathode-ray television, you may remember the way the positively-charged screen would attract dust much more than other nearby surfaces.

Clothes dryers are very effective at making statically charged surfaces. (Dryer sheets help.) So when radon and its temporary decay products are blown through the dryer, electrically-polarized molecules tend to be attracted to the charged surfaces. The decay chain is

isotope half-life decay mode
222-Rn 3.8 days alpha
218-Po 3.1 minutes alpha
214-Pb 27 minutes beta
214-Bi 20 minutes beta
214-Po microseconds alpha
210-Pb years irrelevant

If your Geiger counter is actually detecting radiation, it's almost certainly the half-hour lead and bismuth. Constructing a decay curve would make a neat home experiment (but challenging given what you've told us here).

True story: I was once prevented from leaving a neutron-science facility at Los Alamos after the seat of my pants set off a radiation alarm on exit. This was odd because the neutron beam had been off for weeks. It was a Saturday, so the radiation safety technician on call didn't arrive for half an hour — at which point I was clean, so the detective questions began. I had spent the day sitting on a plastic step stool. The tech looked at it, said that radon's decay products are concentrated by static electricity, and told me that I needed to get a real chair.

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    $\begingroup$ Yes, but because the bismuth is produced as the lead decays, the curve is more complicated than a simple exponential. Also since your data seem pretty noisy, you’ll need some largeish number of trials to get a good fit. But if you (or a junior aspiring scientist in your household) had an hour or two to sit next to your dryer and write down the reading once or twice a minute, I’d be very interested to see it. $\endgroup$
    – rob
    May 17, 2023 at 17:46
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    $\begingroup$ Do you think the door being open/closed would affect the readings? I could set up a GoPro inside and hope no one starts the dryer on accident :p $\endgroup$
    – Marsroverr
    May 17, 2023 at 17:54
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    $\begingroup$ Oh, that would be perfect. If you can automate the data collection, go for four half-lives. I will remind myself what the curve is supposed to look like, and we can get together again when you have some data. This sounds like fun. $\endgroup$
    – rob
    May 17, 2023 at 18:00
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    $\begingroup$ I think this answer is worth an upvote just for the anecdote at the end alone. $\endgroup$
    – march
    May 17, 2023 at 18:22
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    $\begingroup$ @march ONE TIME I GOT RADON ON MY BUTT $\endgroup$
    – rob
    May 17, 2023 at 20:06
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Branch & Decay  Nuclide  Half-life    
                 
                Rn-222   3.823 d      
(100.0 % alpha)   |                   
                Po-218   3.050 m      
(99.98 % alpha)   |                   
                Pb-214   26.80 m      
(100.0 % beta)    |                   
                Bi-214   19.90 m      
(99.98 % beta)    |                   
                Po-214   164.3 us     
(100.0 % alpha)   |                   
                Pb-210   22.30 a      
(100.0 % beta)    |                   
                Bi-210   5.012 d      
(100.0 % beta)    |                   
                Po-210   138.3 d      
    (alpha)       |                   
                Pb-206   stable

Most Radon floats around for 4 days or so then becomes house dust. It or one of the productsis attracted to the static build up in your washing machine. It may be that you live in a high radon area. Sample your house dust it is probably higher than background too.

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