Weird readings of a Geiger counter in my basement

Summary
The readings of my Geiger counter (Radiascan-701, Pancake probe, should be able to detect alpha, beta and gamma) climb continuously the longer I stay in my basement. Even after going back to "normal" environment the reading only slowly decreases over several hours.
Hi everyone! I'm new to this forum, so I hope my question fits into this category.

Yesterday I wanted to check if I could measure any radiation from Radon in my basement with my Radiascan-701 Geiger counter which features a pancake-style probe that should be able to handle alpha, beta and gamma radiation just fine. To distinguish between different types of radiation the Geiger counter has a removable back cover with a copper inlay to block alpha and beta radiation and to enhance the efficiency of gamma detection. With this cover on I was not able to measure any significant increase over normal gamma background (~0.1 to 0.15 uS/h at my place, according to the Radiascan), but without the back cover the reading immediately increased to just over 0.3 uS/h. At first I was not too surprised because I live in an old house built on rocky ground, but during the next ~15 min of me checking various corners of my basement the reading constantly increased, reaching just over 0.8 uS/h before I finally left.
What surprised me however was that even in an environment where the Geiger counter showed normal background before, the reading did not drop below ~0.5 uS/h, regardless of whether the back cover was on or not. Also the frequency of clicks was much higher than before. I left the Geiger counter with the back cover open on my desk for the next few hours, regularly checking the reading it was giving, and found that after about 2 to 3 hours it was back down to just over normal background.

I am now wondering what could cause such a behaviour. Maybe you guys have an idea? Just as side information: my basement is rather wet (so the humidity could easily be around 80% or even higher) and a bit dusty, but I'm not really sure if that matters.

Regards, Erik.
 
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Radon does dissolve in water. It was not my area of study but some of the initial work at U Maine showed remarkable spikes in Radon in homes after somebody took a shower. Could the instrument have gotten wet?
 
Could the instrument have gotten wet?
Only due to the high humidity, but no drops of water or something like that. I tested this a second time this morning, paying extra attention to not touch any surface with the Geiger counter, not holding it in the hand I touched any of the walls before, removing any spider webs in the way etc. The result was the same.
 
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I still believe you got some contamination from your basement. What is most difficult to understand is why the readings returned to normal spontaneously over a few hours. Water evaporation was my best guess.....I am not expert so I don't have a good feel for relative strengths. Could radon/air have been entrapped somehow? Some short-lived daughter deposited? You could repeat the experiment several times and graph the time course of the return to normal background. Might tell something.
Of course somebody more knowledgeable may know exactly whats going on here!
 

Wrichik Basu

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Is this the first time you are measuring the radiation? If not, what were the readings previously?

If this is the first time, a wild guess would be radioactive rocks. The radiation is getting shielded as you are going upstairs.
 
Could radon/air have been entrapped somehow? Some short-lived daughter deposited? You could repeat the experiment several times and graph the time course of the return to normal background. Might tell something.
I can't really imagine how it should be trapped inside the detector. I even tried to blow air across the detector to replace any possibly contaminated air with no effect. But if I find some time I will do a more controlled experiment and write down the values, maybe this will us something.

Is this the first time you are measuring the radiation? If not, what were the readings previously?

If this is the first time, a wild guess would be radioactive rocks. The radiation is getting shielded as you are going upstairs.
It was the first time I went into my basement with the detector, yes. But if the radiation gets shielded, why does it take several hours to return to normal background?
 
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Hi, Erik !
You say 'old house on rocky ground'. If you research Radon, there are very specific geology requirements, plus the 'joker' that the gas may be moved about by ground water, via subterranean streams, faulting etc.

IIRC, a rapid rise in the water table may flush Radon to the surface.

Um, do you need a cellar 'sump pump' from time to time ??

Please, please check out your 'basement' geology, plus any 'Radon Hazard Map' info.

Also, if you open your un-vented cellar after enough time for a build-up, that Radon will spread through the rest of the house, contaminating your after-cellar readings, be gradually lost to 'exchange' air.

You may need to get a professional survey done.
IIRC, mitigation may be some mix of room ventilation, replacing existing cellar floor by one with a gas seal, and underfloor vents, like field drains...
 
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I can't really imagine how it should be trapped inside the detector. I even tried to blow air across the detector to replace any possibly contaminated air with no effect. But if I find some time I will do a more controlled experiment and write down the values, maybe this will us something.
One more stupid idea: could the contamination be carried by you somehow on your person?. As you move about it would diminish.

Also, if you open your un-vented cellar after enough time for a build-up, that Radon will spread through the rest of the house, contaminating your after-cellar readings, be gradually lost to 'exchange' air.
ooh that's a great thought. My new favorite hypothesis.
 
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...my basement is rather wet...
Moisture is a safe first bet for any weird behavior of sensitive instruments, but it can be checked easily. You just need a bag, a damp towel, a hairdryer and some fresh desiccant if you have any.
And, of course some imagination 😉
 

rbelli1

Gold Member
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Heat the instrument enough that is stays above the dew point during the entire sampling duration. Does the effect diminish?

Cool the instrument significantly. Does the effect get enhanced? Don't do this if your probe has a water soluble scintillator installed.

BoB
 
You say 'old house on rocky ground'. If you research Radon, there are very specific geology requirements, plus the 'joker' that the gas may be moved about by ground water, via subterranean streams, faulting etc.
I tried to find a good radon-map for my area, the best I could find lists the radon concentration of the air near the ground as '41-100 kBq/m^3', which is the second highest category. I am not really sure what that compares to in other parts of the world because the map only showed Saxony.

Um, do you need a cellar 'sump pump' from time to time ??
No, we don't have to use a 'sump pump' at all.

Also, if you open your un-vented cellar after enough time for a build-up, that Radon will spread through the rest of the house, contaminating your after-cellar readings, be gradually lost to 'exchange' air.
I'd think this is not the reason for the readings, even outside it showed the same values, and I let the Geiger counter sit on my desk two floors higher behind several doors with all windows open (because it's freaking hot here right now).

You may need to get a professional survey done.
IIRC, mitigation may be some mix of room ventilation, replacing existing cellar floor by one with a gas seal, and underfloor vents, like field drains...
I hope this won't be necessary. I might try to get another reading, maybe with a different instrument, to find out how high the radon background actually is. I'm still curious why the reading kept rising the longer I stayed in the basement.

Still, thanks for your input!
 
One more stupid idea: could the contamination be carried by you somehow on your person?. As you move about it would diminish.
It could well be, if the radon or its decay products somehow stuck to my clothes or something. But wouldn't the reading change depending on how close I am to the detector?

ooh that's a great thought. My new favorite hypothesis.
As I've said, I think that is unlikely. The radon would have to spread across multiple floors, through several doors, and not be flushed out through the windows. And how would this explain the readings outside the house?
 
Moisture is a safe first bet for any weird behavior of sensitive instruments, but it can be checked easily. You just need a bag, a damp towel, a hairdryer and some fresh desiccant if you have any.
And, of course some imagination 😉
This may actually be an interesting experiment! Thanks for your ideas!
 
Heat the instrument enough that is stays above the dew point during the entire sampling duration. Does the effect diminish?

Cool the instrument significantly. Does the effect get enhanced? Don't do this if your probe has a water soluble scintillator installed.

BoB
I imagined that condensation should not be a problem. The Geiger counter was in a ~27 - 30°C environment before I went into the (quite a bit colder) basement. I would have to think of something to keep it at those higher temperature throughout the measurement, maybe a hair dryer?

Cooling however would be harder I think, maybe put it into the fridge before? But I'm not sure if the Mica window of the pancake probe perheps doesn't like too much moisture? Can someone with a bit more experience perheps tell me something about that? A quick google search regarding Mica windows and moisture did not really help.
 

Vanadium 50

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I am now wondering what could cause such a behaviour.
Any nuclear wars in your neighborhood lately?

Radon daughters are notorious for collecting on surfaces. They like some surfaces better than others, but it's not alwasy predictable.
 
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I tried to find a good radon-map for my area, the best I could find lists the radon concentration of the air near the ground as '41-100 kBq/m^3', which is the second highest category. I am not really sure what that compares to in other parts of the world because the map only showed Saxony.
Parts of Saxony tend to have relatively high natural background radiation. Here is a map with all of Germany.
I wonder if some radon managed to get into your device - and escaped again over a few hours.

Would be interesting to repeat the test, see if it happens again.
Condensation is probably not an issue with the current heat wave.
 
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Thinking about how to separate the Geiger counter from environmental moisture and dust.

Putting it in a zip-lock plastic bag ought to do the trick. A typical bag won't affect beta too much, problem is, it's thick enough to block all or nearly all of the alpha. A single layer of cling wrap will block a lot of alpha too, but at only about 0.5 mil thick (~13 microns) won't stop it all.

It would be interesting to run a couple of experiments using both materials to see what differences are observed between having the counter completely sealed versus open enough to allow ambient humidity and airborne dust a path to the probe, and whether count rate is affected by the number of layers of cling film that are used.

If moisture alone is directly affecting the measure circuit then sealing it from the environment should prevent it from happening. If the lingering high count rate is still observed once returning from the basement, and (clean your hands first) removing the counter from the bag has a dramatic effect toward slowing the rate, chances are the bag's outer surfaces become contaminated with radioactive particles.
 

Baluncore

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Have you considered that there might be a fault with the detector, maybe the HV power supply? Is the battery voltage or instrument calibration temperature dependent?
There needs to be a control reference to calibrate the instrument before and after your expeditions to the basement.
 

gleem

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Another possibility for a transient increase in count rate is an atmospheric low pressure front passing by. The lower atmospheric pressure will draw more gases and vapors from the ground. You might see if you can correlate your readings with a barometer reading. If you don't have a barometer then a reported reading for your area on for example weather underground would probably be good enough since you are only interested in changes.

In case you are not aware the Sv reading on the meter is only accurate and useful for X-rays of the calibration source. Alpha and Beta data should be recorded in counts per minute. A GM counter is only useful for detecting alphas or betas and possibly relative activities. Assessing actual amounts (Bq/L) of these particles cannot be determined with a GM counter alone.

Checking your detector with your check source before use is standard operating procedure for using a GM detector.
 
Erik said:"Hi everyone! I'm new to this forum, so I hope my question fits into this category.

Yesterday I wanted to check if I could measure any radiation from Radon in my basement with my Radiascan-701 Geiger counter which features a pancake-style probe that should be able to handle alpha, beta and gamma radiation just fine. To distinguish between different types of radiation the Geiger counter has a removable back cover with a copper inlay to block alpha and beta radiation and to enhance the efficiency of gamma detection. With this cover on I was not able to measure any significant increase over normal gamma background (~0.1 to 0.15 uS/h at my place "

Hi Erik,
First of all, let me welcome you Practical Nucleonics.
I'm George Dowell and want to say the report you gave has all the right information in it to perhaps draw some conclusions, but for sure fuel some further very meaningful tests and experiments.

The instrument you own should be capable of doing just about any
nuclear radiation detection task, plus some really important measurement tasks.
First lets talk about radon, specifically Radon-222 the kind that is part of the Uranium-238 decay chain, and the most common type of radon gas.

Attached is the Decay Series Factsheet, published by Argonne National Laboratory.

This easy to read 4 page document contains all the important information and decay series charts to get you going. It will become a much used and valuable tool as you advance your studies and use your Geiger Counter.

Go ahead and read the first page, then go to the chart that shows the progression of decay of U-238 from Uranium all the way down to stable lead-208. Notice too that there are separate
charts for Uranium-235, and Thorium-232 we'll get to those either later or in a new thread (whatever the Mentors suggest).

Back to the U-238 decay chart, follow it down to find Radon-222, notice the easy to read chart tells you the important facts about this isotope in graphical form, namely half-life = 3.8 day, decay mode = alpha particle, and the daughter element is Polonium-218. If you want to start adding nots the the chart after you print it, you can add the Rn-222 alpha particle energy(s) are as follows:
Alpha Emission Products: Rn-222
Fraction Energy(MeV)
0.000785 4.986000
0.999200 5.489700
This means that 99.92% of the time the alpha particle realeased will be of 5.4897 MeV and only a very few will be of 4.986 MeV. To keep it simple and easy to remember, mark 5.49MeV next to the alpha symbol for future reference. This is a discreet energy.

Next, look box indicating the daughter, Po-218, it's vital statistics are shown too, but the one of most importance is its half-life (T/2). This and the next few daughter's vital statistic are the key to the scientific discovery, a very real thing, that your investigation has uncovered already, and the answer to your posted question.

Congratulations on posting an intelligent, well recorded and well thought out question.

Hint- the next tool you will need is a blank piece of typing paper.

Ask any questions, it was a pleasure meeting you.

George Dowell
 

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