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How to measure radon with a Geiger counter

Do I have radon gas in my house?

It's not possible to measure low levels of radon in the house by just holding a Geiger counter in the air but it is quite easy to do if you have a HVAC system (heating or air conditioning that circulates air though a filter).

Radon is a decay product of uranium. Everything in the decay chain before radon is a solid and everything after radon is a solid but randon is a gas. This means radon can move. It can get into your house and especially in winter it has the potential to reach high concentrations because there is a limited amount of fresh air coming into your house.

This is what the uranium-238 decay chain looks like:

uranium-238 decay chain, with radon marked in red
Uranium-238 decay chain (from https://nuclearsafety.gc.ca/eng/resources/fact-sheets/radon-fact-sheet.cfm ). The chain shown here is the most common (highest probability). There are some other chains of Uranium-238 decay that occur with a lower probability. All of them end in Lead-206. The gas Radon has no stable isotopes.

Radon-222 gets into your house and after a half life of 3.8 days it will suddenly "explode" and be no longer a gas. It emits an alpha particle and becomes Polonium-218, a solid. Polonium-218 is electrically charged after the alpha radiation and it will therefore attach itself easily to dust or other surfaces such as air filters and stay there. You can catch the Polonium-218 with the air filter of your heating system. All the elements between Radon-222 and Lead-210 have a rather short half-life:

So what you have to do is: Measure the background radiation around your house to know what the baseline is and then you go to your heating. Pull out the air filter and put the Geiger counter on top. Make sure the heating was running for at least two hours before you measure the filter.

The difference between baseline radiation level and the radiation level on top of your HVAC air filter is a measure for the amount of radon in your house (other factors include how much air per minute goes though the filter and how fine the filter is).

It is quite surprising how radioactive the air filter is. Here are some pictures from my house and the house of a friend:

baseline-houseA filter houseA
baseline houseB filter houseB
Pic 1: Baseline house A: 20CPM, 0.133μSv/h; Pic 2: filter house A: 76CPM, 0.506μSv/h;
Pic 3: Baseline house B: 29CPM, 0.193μSv/h; Pic 4: filter house B: 197CPM, 1.246μSv/h;

It's interesting how radioactive such a HVAC filter is and it shows that there is radon-222. At this point I don't know how to convert these measurements into useful levels. That is: I don't know at what μSv/h level produced by this measurement should you take action and install a radon mitigation system. I can however say that official tests consider the above levels as "no action required" because I had done an official radon test.

The Polonium-218 decays into other elements until it reaches within a half-life of 47min the much more stable Lead-210. Thus if you put the filter on the side without using it then you will see that it goes quickly back to near background levels:
filter after 2h of not using it
Filter house B after setting it aside for 2hours: 32CPM, 0.213μSv/h

My Geiger counter and normal background levels

I am using a Geiger counter that came as a kit. It's called GC10. It uses a Russian SBM-20 tube. A very reliable and popular tube. Geiger counters measure the amount of radiation crossing the tube per minute. This is called CPM, counts per minute. It can then be converted into μSv/h. This conversion factor from CPM to μSv/h is a function of the size of the tube. For the SBM-20 you have to divide the CPM value by 150 to get μSv/h. μSv/h (micro sievert per hour) is an official measurement for the radiation doses absorbed per body volume. The GC10 counter shows unfortunately in its display μSv and not μSv/h. This is a "typo" and it was probably done to save space in the display and align the numbers nicely. It is measuring μSv/h and the number behind μSv is the amount of μSv/h.

The Canadian government publishes a map of normal Natural Background Radiation in Canada and it changes from city to city dependent on the amount of natural radioactive elements in the soil. Basically the amount of uranium that is in the ground in a given area:

That table uses the unit μSv/y (micro sievert per year). You have to divide those numbers by 8760 to get to μSv/h (the unit that most Geiger counters show).

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