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Testing Geiger Counters 277

Posted by samzenpus
from the rads-are-bad dept.
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!"
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Testing Geiger Counters

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  • Geiger Counter (Score:2, Informative)

    by Palmsie (1550787) on Monday May 23, 2011 @02:34AM (#36214596)

    In case you didn't know what it was (like me):
    Wikipedia:
    A Geiger counter, also called a Geiger-Müller counter, is a type of particle detector that measures ionizing radiation. They detect the emission of nuclear radiation: alpha particles, beta particles or gamma rays. A Geiger counter detects radiation by ionization produced in a low-pressure gas in a Geiger-Müller tube. Each particle detected produces a pulse of current, but the Geiger counter cannot distinguish the energy of the source particles. Geiger counters are popular instruments used for measurements in health physics, industry, geology and other fields, because they can be made with simple electronic circuits.

  • Vaseline glass. (Score:5, Informative)

    by Kenja (541830) on Monday May 23, 2011 @02:35AM (#36214602)
    A common way to test a Geiger counter is to use a small sample of Vaseline glass such as a bead. The glass contains a small amount of uranium oxide which should be detectable.
  • Re:Vaseline glass. (Score:5, Informative)

    by Kenja (541830) on Monday May 23, 2011 @02:47AM (#36214676)
    As I said, Vaseline glass is often used as a calibration source. The CPM for many sources and quantities is well documented. True, if you get a random piece off eBay for a few dollars you may not know what its reading is supposed to be, but it should be consistant between different devices.
  • by fahrbot-bot (874524) on Monday May 23, 2011 @02:50AM (#36214696)

    I know I can point it at a smoke detector and it should go off ...

    Well, perhaps an Ionization [wikipedia.org] type detector, but probably not other types, like Optical.

  • by Cyberax (705495) on Monday May 23, 2011 @02:54AM (#36214708)

    Geiger counters are not really useful for food testing. They generally won't detect alpha radiation which is the most harmful type. Besides, elevated concentration of caesium or strontium can be easily mimicked by elevated levels of natural K-40.

    They really need to stop worrying about food testing. Or get a professional radiometer (which will cost $$$$).

  • by goodmanj (234846) on Monday May 23, 2011 @03:09AM (#36214788)

    "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 [spectrumtechniques.com], 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.

  • Re:DIY (Score:5, Informative)

    by maxwell demon (590494) on Monday May 23, 2011 @03:16AM (#36214826) Journal

    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 [youtube.com]

    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.

  • Re:Vaseline glass. (Score:3, Informative)

    by Lefty2446 (232351) on Monday May 23, 2011 @04:43AM (#36215188) Homepage

    After rereading this a couple of times I really have to ask for confirmation :

    So :
    * "by convention", 1 mSv/year is considered "safe"
    * the location you live in outputs about 3 times that value (natural source)

    But, because we "know" where it comes from, and because it's "natural" radiation, it doesn't count as being harmfull and the safety limit is upped to those 3 mSv/year PLUS the "by convention" 1 mSv/year ??

    ??? What kind of logic is that ???

    Shouldn't they just put the limit to 3 mSv/year for all people living in that area ? (it's kind of non-practical to remove all background radiation) + pay extra attention to potential effects due to already having 3 times the 'conventional' limit to live with ?

    So nobody who lives in that area qualifies for X-rays, scans etc?

    I know the following is XKCD but it's still quite informative:
    http://blog.xkcd.com/2011/03/19/radiation-chart/ [xkcd.com]

  • by Teun (17872) on Monday May 23, 2011 @04:55AM (#36215242) Homepage
    You are very wrong in assuming radiation is less inside a building, especially granite has a high natural radiation and it might be incorporated in the concrete.

    Certain kinds of plaster board are made from material recovered at cement furnaces and it too has a quite high radiation level.

    Radiation measurements are part of my job, I'm certified for it and I can tell you making a useful measurement of foodstuff requires expensive gear and a lot of time.

    A simple way of checking the counter is to point it downwards to a non-polluted part of the ground, record the reading in counts/sec, this is called the background radiation.

    Background radiation is as low as 4-8 counts at sea and around 30-40 in an area with clay or granite. Going up in the mountains might expose you to ~100 counts/sec from cosmic radiation. Now point it at the object you want to check, when the reading is less than 3x the background it can be considered non-polluted. That doesn't mean it's safe but at least there's less worry.

    The biggest problem is these meters will not show you all radiation, usually only Gamma and Beta radiation while Alpha can be just as dangerous. Some sorts of radiation have a hard time passing through even a thin layer of moisture, that includes the skin of vegetables.

    All in all, buying a Geiger counter is most likely a total waste of money and certainly a source of misinterpretation.

  • Re:Vaseline glass. (Score:5, Informative)

    by umghhh (965931) on Monday May 23, 2011 @05:14AM (#36215302)
    The question about what is a safe level is a really tricky one especially as it involves statistics i.e. it is usually not: if you cross this line you will be shot but rather if you cross this line you may shorten your life by this much or become a customer of cancer clinic in course of your life but that you can also without additional exposure. There are different people and different effects depending also which part of the body got hit the most. On top of it if certain radioactive materials get into your body the effects depend on what these were and which part of the body is affected the most. Plutonium is highly toxic by itself and getting already small amounts into your system will probably kill you or made you uncomfortable (as with kidneys' failure uncomfortable for instance). On top of it you have empirical data from people working with extreme levels of radiations and these are not straight forward either - members of army units that cleaned up the shit in Chernobyl are mostly dead by now and those living are mostly sick yet President Carter took part in similarly adventures operation in Canada at Chalk River Laboratories in 1952 and he is still well.

    I guess the bottom line is that you just need to ensure thatbloody thing is working in the first place i.e. shows something and then compare the results - assuming majority of the food stuffs are safe then measure those and see whether there is change.

  • by kf6auf (719514) on Monday May 23, 2011 @05:44AM (#36215416)

    Look up Americium in a Table of Isotopes; there are a decent number gamma rays that it emits at 60keV or 73keV depending on the isotope (Am-241 or Am-243) after it alpha decays. That said, smoke detectors vary a lot depending on the amount of Americium inside and you're always better off with a long half-life calibration source.

  • Don't. (Score:5, Informative)

    by Alex Belits (437) * on Monday May 23, 2011 @05:54AM (#36215450) Homepage

    geiger counters for testing food and other materials

    Geiger counters are absolutely useless for testing anything other than minerals, background radiation and things like ventilation ducts (surprisingly a major collector of everything radioactive). After Chernobyl disaster I made, used and later calibrated a simple Geiger-counter-based ionizing radiation meter, and it was useful to determine how contaminated the areas around my city (Gomel) were. The result was exactly the same as what was confirmed later -- some short-lived contamination within the city (easily attributable to I-131 due to distinctive half-life around a week), mostly clean to the southwest, more contamination (longer-lived, counter was useless for determining its nature but later I have learned that it was Sr-90, Cs-134 and Cs-137) to the northeast.

    However to test anything that even resembles food, you need a gamma spectrometer, complete with a test chamber made of lead bricks. I happened to participate in those measurements much later, and I am certain, Japanese environmental/food safety authorities are already using something similar now. You have absolutely no chance to get anything close to it on your own, so just don't.

  • by pepax (748182) on Monday May 23, 2011 @06:01AM (#36215480)
    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.
  • by the_raptor (652941) on Monday May 23, 2011 @06:26AM (#36215574)

    '"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.'

    When I was in high school 12 years ago the radiation sources the science department had were from the 80's and barely registered above background using the Geiger counter we had. I bought in some Americium based smoke detectors from home and those where emitting massively more amounts of radiation.

    It is my understanding that the alpha from those sources would be stopped by the plastic housing and a few centimetres of air, not by the ionization chamber housing itself.

  • by Proudrooster (580120) on Monday May 23, 2011 @07:00AM (#36215694) Homepage
    First off, a smoke detector is not a good source for testing a geiger counter. The high voltage gas canister inside is usually tuned for Cesium and Americium (the source in a smoke detector) usually gives a false high reading.

    To test properly, you need a known source. The better counters come with a source, usually taped to the side of the unit, but you can get sources off of Ebay.
    http://cgi.ebay.com/Radioactive-Mantle-Geiger-Counter-Detector-Test-Source-/160587370187?pt=LH_DefaultDomain_0&hash=item2563c0cecb [ebay.com]

    I don't have much time this morning, so here is an excerpt from my radiation monitor manual for how it works and what it detects. Good luck.

    How the Radiation Monitor Works The Radiation Monitor senses ionizing radiation by means of a Geiger-Mueller (GM) tube. The tube is fully enclosed inside the instrument. When ionizing radiation or a particle strikes the tube, it is sensed electronically and monitored by its own display, a computer, or by a flashing count light. When the switch is in the AUDIO position, the instrument will also beep with each ionizing event. It is calibrated for Cesium-137, but also serves as an excellent indicator of relative intensities for other sources of ionizing radiation. Gamma radiation is measured in milli-Roentgens per hour. Alpha and beta are measured in counts/minute (CPM). About 5 to 25 counts at random intervals (depending on location and altitude) can be expected every minute from naturally occurring background radiation. The end of the GM tube has a thin mica window. This mica window is protected by the screen at the end of the sensor. It allows alpha particles to reach the GM tube and be detected. The mica window will also sense low energy beta particles and gamma radiation that cannot penetrate the plastic case or the side of the tube. Note: Some very low energy radiation cannot be detected through the mica window. The Radiation Monitor does not detect neutron, microwave, radio frequency (RF), laser, infrared, or ultraviolet radiation. It is calibrated for Cesium-137, and is most accurate for it and other isotopes of similar energies. Some isotopes it will detect relatively well are cobalt-60, technicium-99m, phosphorus-32, and strontium-90. Some types of radiation are very difficult or impossible for this GM tube to detect. Beta emissions from tritium are too weak to detect using the Radiation Monitor. Americium-241, used in some smoke detectors, can overexcite the GM tube and give an indication of a higher level of radiation than is actually there.
  • by vlm (69642) on Monday May 23, 2011 @07:25AM (#36215780)

    "Point it at a smoke detector" won't work: the americium in smoke detectors emits alpha radiation

    Most of your post is sooo good but this tiny part is soooo far off. I can tell you're quoting some simplified "book learning" not having done experiments in a lab with some Am241 and even a cheapy scintillator MCA. Trust me, you get a nice assortment of gammas from Am-241.

    http://wiki.4hv.org/index.php/Americium-241 [4hv.org]

    Go to google image search, enter "am 241 gamma spectrum" and see pretty graphs of Am241 gamma emission.

    Also, never forget that there is no such thing as a chemically pure (or especially isotope pure) substance. The instant that miraculously 100% pure Am-241 target was refined, 237Np started building up, and 237Np leads inevitably to 233Pa, 233Pa leads inevitably to 232U while emitting a nice strong beta, etc. So, in any "real world" sample you'll have a whole vegetable soup of pretty much ... everything.

    Now ratios, yeah, you're gonna see exactly one zillion alphas for every geiger detectable gamma. That in no way excludes the detection of gammas and betas from a chunk of 241Am.

    Radiology is a very analog science... the digital 1s and 0s types have a rough time in radiology.

  • by Anonymous Coward on Monday May 23, 2011 @08:53AM (#36216254)

    This is probably below detection limits -- not because of the sensitivity of typical Geiger counters isn't theoretically able to detect that, but because background sources from all around you will contribute too, and 0.012% 40K in whatever KCl is in table salt is nothing by comparison to those. The signal will be swamped. With a real Geiger counter set at high sensitivity, you'll hear a steady peck-peck-peck of radioactive particles from the environment. With a high enough sensitivity, it will be a steady buzz. If I take a typical granite or clay sample, which contain many times more K than in table salt (up to several weight percent), it can sometimes still be difficult to differentiate them from background with any Geiger counter I've used, like this old classic [wikipedia.org]. 40K has a fairly slow decay rate too (a half-life of ~1.250 billion years), but if you get pure KCl, it is probably detectable.

    Many Geiger counters come with their own radiation source for calibration purposes, although obviously they wear out with time due to decay. I'd say if you don't get a calibration source with your Geiger counter purchase, then it probably isn't a very good one, because you have no way of verifying that it is working.

    Also, be aware that while Geiger counters detect gamma rays, they don't detect beta rays unless they have the right window over the tube, and it is unusual for them to detect alpha radiation at all unless they've been geared up with a special probe. Furthermore, it is very unlikely that you could reliably test for potential food contamination of any significance with an ordinary Geiger counter without careful laboratory calibration of both the instrument and the materials you are testing (i.e. food samples that are "known to be non-radioactive"). A Geiger counter tells you "it's hot" to varying degrees. It probably won't be able to tell the difference between an ordinary uncontaminated banana (plenty of K) and a contaminated cucumber, or maybe even the difference between cucumbers grown in different soil nutrients or with a little bit of clay on them. There is going to be a lot of natural variation between foods. We're talking about vanishingly small amounts of radioactive material, and trying to differentiate those from background isn't easy. To do anything useful you probably need an instrument tuned to the particular gamma ray frequencies of the isotopes that are greatest biological concern, such as isotopes of iodine, cesium, and strontium.

    Unless you know you're buying and eating food from within the contaminated region (for some strange reason), the best bet is probably to trust the Japanese food inspectors and to wash food thoroughly. Self-measurement of food for potential contamination by radioactive materials is likely to be an exercise in frustration with a regular Geiger counter.

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