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 2.GQ Geiger Muller Counter
 My bathroom tiles are radioactive!
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5 Posts

Posted - 10/08/2020 :  16:26:16  Show Profile  Reply with Quote
I got my GMC 300E Plus today, and measured my entire house. The house was made in 1940 and most of the house is a 1990s extension. Everything in the house is about 0.1uSv/h, except the bathroom. The floor tiles are an old fashioned pale orange color and there is a horizontal accent strip of tile that is blodd orange colored the Geiger counter takes a few minutes to stabilize at 3.9uSv/h. Is this dangerous? Should I have them removed?

Edited by - luke.simon on 10/10/2020 16:12:12
Reply #1


491 Posts

Posted - 10/09/2020 :  01:23:10  Show Profile  Reply with Quote
I don't think you are anywhere near a danger level!

First, let's take the number at face value: 3.4ÁSv/h = 29.8 mSv/a (Milli-Sievert per year). In my experience as a father, teenage girls may easily spend an hour per day in the bathroom, in the morning plus the evening ;-) Assuming this applies to you too, then your exposure is 29.8 / 24 * 2 = 2.5 mSv/a. For the rest of mankind, it will be less.

The "Occupational Radiation Limits", which refers to people working in fields with typically higher exposure to radiation compared to the average person, like medical people applying X-rays, workers in nuclear power plants, people in aviation, people in mining, set the following limits for yearly exposure:

Germany: 20 mSv/a,
USA:50 mSv/a.

(Why you folks in the US are hardier than we in Germany is something, which only regulatory people can understand).

In any case, you are well below this limit. Let alone that the limits are meant to already be BELOW any danger level.


Second, is your measurement even meaningful? Since you are using an GMC-300E+ device, your dose rates mean you get a background of CPM=15 (which is rather low) everywhere, and CPM=523 in the bathroom (clearly elevated well beyond background).

But of what nature is the radiation? The "calibration factor" in the counter by which you convert from counts to ÁSv is 0.0065 ÁSv/h/CPM. However, it has never been published how this had been measured! If it has been measured at all? See my discussion in the GeigerLog manual in chapter "Appendix G ľ Calibration".

If it was done properly, it would have to be made with a pure gamma source. As a consequence, the factor is valid ONLY for a pure gamma source, AND only for an energy spectrum similar to the source used for calibration, be it Cs137, Co60, or Ra226.

It is NOT, repeat: N-O-T, valid for beta emission! And while Geiger MŘller tube sensitivity to gammas is closer to the 1% level, that for betas is closer to >10% levels, but all dependent on energy level. If you have a noticeable amount of betas of sufficient energy in your tile radiation, your conversion from counts to ÁSv will produce levels too high!

So, what do you have? Unfortunately, a Geiger counter cannot discriminate between gamma and beta. But betas even of high energy are easily absorbed by several(!) mm of metal, be it aluminum, copper, steel, while gammas with typical energies of natural radiation are barely affected. See a discussion of absorption in my "Potty Training"

Put the counter directly on the tiles, and measure count rate. Then cover the tile-fronting side of the counter which such a metal plate, and measure again. Repeat at a distance of 10cm, 30cm, 1m. Caveat: if the opposing and other walls also has these tiles, you may have to use a metal box to shield all sides of the counter!

My guess is you have mainly beta radiation, but only an experiment will tell.

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Reply #2


5 Posts

Posted - 10/09/2020 :  10:18:13  Show Profile  Reply with Quote
Thanks. I wrapped the meter in several layers of aluminum foil, then mewsured again and got 2.13uSv/h. Apparently this is on the low end of the range for uranium oxide glazed ceramic. I am amazed that my granite countertops do not measure above background levels. I guess granite varies in radioactivity?

Edited by - luke.simon on 10/10/2020 16:11:04
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Reply #3


491 Posts

Posted - 10/10/2020 :  00:46:34  Show Profile  Reply with Quote
Several layers of household alu foil is only a fraction of a mm, perhaps 0.1mm. This is still very thin, but already you see a decrease from 3.4 to 2.13, or a reduction of almost 40%!

This suggests that a good part of the radiation is beta. If so, it is no longer legitimate to express the radiative strength in ÁSv, because this is based on a calibration factor which has excluded beta in the way it was determined! Assuming the std calib factor of 0.0065 ÁSv/h/CPM, CPM=328 ist still valid.

How certain are you that the tiles have 'uranium oxide glazed ceramic'? The decay chain from uranium is rich in gamma and beta; the beta energies ranging from 0.05 MeV up to 2.2MeV (see Wiki Decay chain). I have no idea how the relative shares are, but a alu thickness of <1mm won't catch the fast ones. Add more layers, and you will see a further reduction of counts.

Of interest is still how this changes with the distance between counter and wall?

I don't know about the variability of granite radioactivity. I also have slab of granite, and with a GMC-300E+ with its backsite removed and the tube as close as possible to the stone, I get a count increas from CPM=20 up to only CPM=28.

Granite may have 'hotspots', though even those will probably be pretty cold.

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Reply #4


19 Posts

Posted - 11/01/2020 :  15:01:38  Show Profile  Reply with Quote
For what is worth - a few self explanatory images:

Granite blocks in my fireplace.

Gamma spectroscopy of the Fireplace using NaI(Tl) detector.

Typical natural uranium spectrum.

Resolution compensation for NaI(Tl) detector makes the spectrum more "expressive".

As for the tiles - in the '40 and '50 in US uranium glaze was extremely popular - I am sure everyone is familiar with the Fiestaware brand famous for the red uranium glaze. I am pretty sure this is what you have in your tiles.

And when we are speaking about bathrooms - once I left a Geiger counter I was carrying in the bathroom and went to take a shower. As I was existing the shower I noticed that the background was over twice the usual reading of 0.15uSv - it was showing 0.35uSv. Next time I took a shower - I checked and the same thing - background radiation doubled - then I realized it is because of the Radon released by the hot water which has lower gas solubility than cold water - I am not on a public water supply but have my own well.

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