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Testing Geiger Counters
thesandbender writes "My girlfriend's family lives in Japan and is very interested in obtaining geiger counters for testing food and other materials. Geiger counters are now impossible to get in Japan and are on long back order from most providers in the U.S. which makes me suspicious of anything we can get our hands on. My question is, what's the best way to test/verify a geiger counter. I know I can point it at a smoke detector and it should go off but I'm not sure what I should see on the gauge. We'd even take it to any reasonable local facilities for testing (NYC area). Any input would be greatly appreciated!"
"Point it at a smoke detector" won't work: the americium in smoke detectors emits alpha radiation, which can't penetrate the walls of the detector. There's no sense messing around here: if you want to do it, do it right. You will need a little bit of money and the ability to do math.
Buy a calibrated radiation source: you can buy them here, among other places. They're relatively cheap -- tens of dollars. Cs137 is very easy to get, but you also might want to get some Sr90, which is a pure beta emitter. These sealed disks contain such a tiny amount of radioactive material that the risk to health from them is negligible, and they can be mailed and used without a license. However, I do not know mailing them internationally is legal or wise.
(The same company will also sell you a lead container to hold your sources in, but I'll tell you from personal experience that quite a few gamma rays will go right through the container.)
Put the source in front of the detector, a short distance away. If your detector is working, it should start clicking/beeping like crazy. Calculate the count rate. By working out the geometry, looking up the properties of your source, and converting curies to counts per second (hey, nobody said this would be easy), you can work out the "efficiency" of the detector. Move the source farther from the detector: the counts should fall off as an inverse square law.
Now that the detector is calibrated, you can use that efficiency factor to calculate the radioactivity of an *unknown* source.
Important note: while these sources are generally considered safe, the radiation they emit will be *many* orders of magnitude more than any contamination in Japanese food products. You can look at this fact in two ways: either this shows that concerns about food safety are overblown, or suggests that the best way to protect yourself from unnecessary radiation is to not do this experiment.
If you don't have access to or don't want to buy calibrated radiation sources, you can buy yourself some "No Salt" salt substitute, which is food-grade potassium chloride. The naturally radioactive potassium-40 in it is easily detectable with a good Geiger counter: you can look up the natural abundance of 40K and do a little chemistry to figure out the number of curies in a carefully measured gram of KCl, and use it as a calibration standard.
If Geiger counters are hard to buy, you can make one. Here's an absolutely brilliant video on how to:
http://www.youtube.com/watch?v=G6Q7VfWdgEg
The basic idea, and brilliance, is simple. Get a plastic scintillator and hook it up to a CCD camera. Use a time exposure to record the flashes of light, and you have a cheap and easy Geiger counter.
That's a radiation detector, but it's not a Geiger counter.
Of course, what the poster wants most probably is just a radiation detector (and the Geiger counter is just the one radiation detector he knows of), so your advice isn't wrong; it's just wrong to call that a Geiger counter.
The Tao of math: The numbers you can count are not the real numbers.
Use common kitchen salt (NaCl). It contains a small amount of potassium chloride (KCl). The amount of KCl in the salt you buy should be listed on the packaging. 0.012% of the KCl present will contain a naturally occurring radioactive isotope of potassium, potassium-40 (half-life of 1.3 billion years). So, if you weigh the amount of salt you test with your Geiger counter, you should be able to figure out how much potassium-40 you have. The specific activity of potassium-40 is 0.0000071 Curie/gram. One Curie is 3.7×10^10 decays per second, so one gram of potassium-40 should give you 263000 decays per second, one milligram of potassium-40 should give you 263 decays/second, and so on. By comparing your measurement results to what you would expect, you can tell how well your Geiger counter is performing. Be ready to measure for at least several minutes, though.