Answer to Question #13705 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation — Radon

The following question was answered by an expert in the appropriate field:

Q

I recently read that one can wipe the dust off of an old CRT-type television and measure the radon decay products, or progeny decay in the dust with a Geiger counter. Is it that the dust in the air naturally collects radon progeny and then the dust is attracted to the static charge generated on the CRT screen OR is it that the dust that collects on the CRT screen can now attract radon progeny? In other words does the dust need to have a charge from being on the CRT screen to attract the radon progeny or does dust naturally do this and the CRT screen just acts like a great collector for the dust?

A

Radon progeny carry a positive charge and thus can attach to dust particles that plate out on the CRT screen. This is referred to as the "attached fraction" of radon progeny most prevalent in indoor air. The radon progeny can also exist as essentially single positively charged atoms, known as the "unattached fraction," that makes up a smaller portion of the total radon progeny in typical indoor air. Both the attached and the unattached fractions can plate out on anything with a static charge such as the CRT screen or even articles of clothing like fleece vests or polyester. The radon progeny can plate out on the CRT screen without being attached to dust particles. So, the short answer to your question is that it is not necessary for the dust to be on the screen for the radon to be present. It is the charge on the radon progeny that results in plate out on the CRT screen.

Your interesting question prompted me to check the radon progeny on the screen of my older CRT-model TV. I wiped the screen, which was quite dusty, with a paper towel then measured the activity on the paper. It was about 20 times the normal background activity. I wiped the screen several more times to make sure to remove all of the dust, then wiped the clean screen. The measured activity on the wipe of the clean screen was about six times background. This indicates that the screen attracts dust particles to which the radon progeny had attached but also probably attracted the unattached radon progeny. Science is fun!

I hope this is helpful.

Jan Johnson, PhD, CHP Emeritus

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