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Testing Geiger Counters 277 277

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):
    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.
    • by mustPushCart (1871520) on Monday May 23, 2011 @02:43AM (#36214648)

      They dont just want one to detect radiation, they want one to accurately tell them how much radiation is actually present. They should also know exactly how much radiation is harmful over what period of time. A faulty counter or even poor knowledge of radiation can be just as harmful by underestimating the amount of radiation than by overestimating the amount of radiation and displacing your life / spreading panic among everyone else.

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

        by Kalidor (94097) on Monday May 23, 2011 @02:56AM (#36214720) Homepage

        Or rather, they should at least give you what various safety levels are. One of the big misconceptions is that we know what is a dangerous level of radiation: in fact all we know is what is too much radiation. Back in the 50's and 60's a group of scientist were asked to provide safety information on radiation and they came up with a scale using the points of zero and you aren't gonna see the end of the week. They then drew a linear line between these points because they had little to go on, and presented it as a best guess and further research was needed to prove it's truly linear, exponential, logarithmic, or what-have-you. Since then the linear graph has become kind of dogma and various groups have picked various points across it to set their safety thresholds.

        You'll find that you have a set threshold in most Asian nations that is quite low, due to close experience and some might say paranoia in relation to the deployment of nuclear arms.
        Roughly double these guidelines, and you get what is considered safe in many European countries.
        Roughly double them once more, and now you are heading toward the Americas.

        • by fuzzyfuzzyfungus (1223518) on Monday May 23, 2011 @06:40AM (#36215614) Journal
          Probably the bigger problem is that(outside of relatively few, exotic, situations where the ionizing radiation is sufficiently intense to have you puking out your guts by lunchtime) the bigger question is often whether you are merely being exposed to radiation(modest uptick in longterm cancer risk, risk stops accruing when you leave, basically only x, gamma, and the more can-do beta rays need apply) or radioactive particles(alpha emitters get to come to the party, many radioactive materials are also chemically toxic, or chemically very similar to biologically active compounds and can persist for years in the body after exposure.)
    • by SomePgmr (2021234) on Monday May 23, 2011 @03:16AM (#36214830) Homepage
      If it's just to see if the thing is working, I've heard a lantern mantle works. Also old radium watches. I'm sure someone can confirm or deny.
    • Wait, what? (Score:4, Funny)

      by MrEricSir (398214) on Monday May 23, 2011 @04:09AM (#36215060) Homepage

      Vaseline is radioactive?! So you're saying I shouldn't be using it to, you know, wax my carrot?

    • by igny (716218) on Monday May 23, 2011 @05:53AM (#36215448) Homepage Journal
      Or you can just go to Area 51 (in Japan Hiroshima or Nagasaki would probably work), test your GC there, compare with values on Wikipedia, update the values on Wikipedia, and you GC is now up to an international standard.
    • 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 squizzar (1031726) on Monday May 23, 2011 @06:47AM (#36215648)

        That's quite a lot of activity. Which when you have people shouting 'One single piece of ionizing radiation can cause mutations that lead to cancer, we should ban everything nuclear' makes me think that we should all be dead already. It's almost like it's not as bad as those people make out...

      • How about using a salt substitute [wikipedia.org], which is almost pure KCl? That works a lot better. Much higher activity, and the amount of K present is easy to calculate.

      • by arisvega (1414195) on Monday May 23, 2011 @07:45AM (#36215838)

        one milligram of potassium-40 should give you 263 decays/second, and so on.

        You have a good idea there but all these are rough estimates. The actual counts would depend on the distance from the source (in effect the solid angle), the instrument's sensitivity, the source's geometry and other stuff I may be forgetting.

        I have done instrument calibration (as a student, not as a lab manager, so I am not an expert by far) on both Geiger-Müller counters and solid state detectors. Calibration was mandatory before the actual measurements took place. Even in laboratory conditions, with experts running around and helping out, point sources and 0.5mm accuracy on distances and the like, let's just say that they turned out to be not the easiest instruments to use, and people where ending up being off (but not way off) in their estimates for the actual activity of the radioactive sources.

        Furthermore, there are different 'types' of 'radiation'- alpha, beta, gamma, neutrons. Depending on your counter/instrument, you could measure on or more of these; also there is math to be done afterwards; some instruments (like G/M) measure just 'counts' (the event when a particle 'hits' the detector), others can give more detail. And you need to do some math afterwards, unless the instrument itself does it for you.

        My punchline; its use is not trivial, it needs some training (not impossible, though). In times of need such as these I would assume that the few instruments that find their way to Japan would be put to much much better use if they are delivered to experts.

      • 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.

  • Don't get one. (Score:4, Insightful)

    by Anonymous Coward on Monday May 23, 2011 @02:38AM (#36214622)

    99% of the general population can't operate one. Measuring radiation is not like measuring signal strength of an electromagnetic field. People forget that it's radioactivate _matter_ emitting radioactivity, something akin as if you had tiny mobile towers all over the place. There is a large difference between a weak emitter stuck to your geiger counter and a powerful source a lot further away, but radioactivity-wise at a specific point they are indistinguishable. There is a large difference between different kinds of radioactivity aswell.

    Geiger counters are useless for someone without at least a basic education in nuclear physics.

    • by jamesh (87723) on Monday May 23, 2011 @02:47AM (#36214678)

      Geiger counters are useless for someone without at least a basic education in nuclear physics.

      I wouldn't go that far. If you have two cans of beans in front of you and pointing the geiger counter at one gives you the same reading as background and pointing it at the other makes the thing go crazy then I think it's pretty clear which is the safer[1] one to eat.

      Likewise, with a geiger counter it should be easy to tell the difference between a lettuce still full of radioactive fallout and one that's at least been rinsed off :)

      [1] that's 'safer', which doesn't necessarily imply safe...

      • by DrXym (126579) on Monday May 23, 2011 @03:04AM (#36214754)

        Geiger counters are useless for someone without at least a basic education in nuclear physics.

        I wouldn't go that far. If you have two cans of beans in front of you and pointing the geiger counter at one gives you the same reading as background and pointing it at the other makes the thing go crazy then I think it's pretty clear which is the safer[1] one to eat.

        Likewise, with a geiger counter it should be easy to tell the difference between a lettuce still full of radioactive fallout and one that's at least been rinsed off :)

        [1] that's 'safer', which doesn't necessarily imply safe...

        In the former case what is the likelihood of that? Has the food even been canned in an affected region? If so, choose one which wasn't. I assume even if it was then there would be restrictions and possibly an outright ban on contaminated food. In the latter, if the remedy is simply rinsing the food, then rinse the food again. It wouldn't be a bad idea to rinse cans too since it would be dust that is the issue. Also buy foods which are unlikely to be affected by the outbreak, e.g. imported meat & fish & vegetables.

        Buying a geiger counter seems like an absurd overreaction.

      • Re:Don't get one. (Score:5, Insightful)

        by the_raptor (652941) on Monday May 23, 2011 @03:16AM (#36214828)

        Unless you are picking lettuce from the grounds of the Fukushima nuclear power plant that scenario isn't going to happen (and anyway you should ALWAYS wash produce, the pesticides etc that get used aren't exactly great for you either).

        From what I have read the fallout is at such low levels that it is within the bounds of variation in background radiation (ie mostly the levels are below what you would get from living in a high altitude area like Denver, Colorado). Unless you are in the immediate vicinity of the leaking reactors you aren't going to get a dose that has immediate effects, and just ignoring the whole situation will cause less damage then regularly eating junk food.

        People in general are fucking terrible at risk assessment, and that is before you use the word "radiation".

        There are several different types of radiation detectors with varying degrees of accuracy, and the type you can scrounge around and get now are probably not worth the money even if you put in the time and effort to calibrate and understand it.

      • by Registered Coward v2 (447531) on Monday May 23, 2011 @05:27AM (#36215342)

        Geiger counters are useless for someone without at least a basic education in nuclear physics.

        I wouldn't go that far. If you have two cans of beans in front of you and pointing the geiger counter at one gives you the same reading as background and pointing it at the other makes the thing go crazy then I think it's pretty clear which is the safer[1] one to eat.

        Likewise, with a geiger counter it should be easy to tell the difference between a lettuce still full of radioactive fallout and one that's at least been rinsed off :)

        [1] that's 'safer', which doesn't necessarily imply safe...

        Not necessarily - all you know is one is emitting more penetrating particles than the other - whose particles are probably already penetrating your skin as well. The other can could be loaded with alpha emitters which are blocked by the can. Once you eat them, however...

        As a result, your readings don't amount to a hill of beans when it comes to assessing safety...

      • by Lumpy (12016) on Monday May 23, 2011 @08:04AM (#36215928) Homepage

        If you put the two cans of beans in front of you, you now have two cans of contaminated beans.
        Again, more than a basic education is needed.

    • by TheDarkMaster (1292526) on Monday May 23, 2011 @07:45AM (#36215840)
      Wrong. The goal of the TFA author is simply whether he is entering a potentially dangerous area or if an object is radioactive. He does not need be an expert on the subject to know that if the geiger starts beeping, is because something in place is radioactive enough for him to have to avoid.
  • by dentext (117409) on Monday May 23, 2011 @02:44AM (#36214652)

    test G counters with a mantle for a gas lantern, like Colman. it's a strong short range source, so when you hold it an inch or 2 away, it'll be loud. anything thats that loud, worry about. less than that, don't worry about. That's what I learned in a Nuke Med R&D/mfg facility.

  • by TubeSteak (669689) on Monday May 23, 2011 @02:50AM (#36214694) Journal

    1. Your local college/university science department
    2. Local Fire Dept or the nearest Fire Dept hazmat team

    My personal experience is that geiger counters come with a sample for calibration, but apparently yours didn't.

    • Old (esp. military grade) geiger counters came with callibration samples. This doesn't appear to be the case with modern hand-held devices though - they come pre-callibrated and you are not supposed to mess with the callibration in any way.

      One significant reason might be sensitivity. The old ones wouldn't pick background radiation or anything slightly above it. You needed a source just to confirm the device works at all. Nowadays the counters can guage the background quite accurately so you don't need extra confirmation it works.

  • 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 Jane Q. Public (1010737) on Monday May 23, 2011 @03:31AM (#36214890)
      Not even then. If your radiation counter goes off near a smoke detector, throw that thing out and get a new one, ASAP!

      Yes, most ionization-type detectors contain a small amount of Americium 241. It is radioactive. BUT... it is housed in a metal can that has angled louvers so that air can pass through, but there is no line-of-sight to the radioactive material. All radiation emitted by the Americium should be (and normally is) fully contained by the can.

      The only way you should be able to measure radiation that is above background levels would be to disassemble the can.
  • DIY (Score:5, Interesting)

    by Anonymous Coward on Monday May 23, 2011 @02:53AM (#36214706)

    If Geiger counters are hard to buy, you can make one. Here's an absolutely brilliant video on how to:

    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. Suitable for checking food, as well as getting an idea for the radiation around you. It's not as immediate as a real Geiger counter, but at least you have some way of seeing what's going on around you instead of being blind. The scintillators are a little hard to get retail, but very available on eBay. Cost is cheap. About $32 for a 2x2" square (which is overkill). And a simple test here is to just buy a bunch of bananas, which are naturally radioactive, though very low level.

    The next step up is to add some electronics. The NukAlert is great here. Japanese customers can find it at:

    I have no association with nukalert.com other than as a satisfied customer. I also don't read Japanese, so I have no idea as to what it says.

    Now, to test these suckers out, you need actual radiation. You can get low level radiation devices in the States, 5 uCurie Cs-137 sources for about $80. These are used to calibrate various instruments. I would imagine that there is a way also in Japan, given how much equipment is built there. But I'm not sure if these can be imported.



    • 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.

    • by SharpFang (651121) on Monday May 23, 2011 @05:31AM (#36215352) Homepage Journal

      The basic idea is brillant. The effect, not quite so.

      Read the comments on the video and follow the links. Plastic scintillator simply does not emit enough light to be captured by a consumer-grade camera. Scientific camera, correct temperature, darkness achieved by thick black plastic - yes, it works. Best of consumer-grade night-vision cameras, 1h exposure - nothing. half-inch plywood appears "transparent" for night streetlights, but the scintillator remains dark. Sources so strong that they make alarm go off while enclosed in lead container - scintillator still not visible. This is doable but NOT with consumer-grade cameras, and as such, the whole concept of "cheap, commonly available dosimeter" falls.

  • 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 PsychoticSpoon (1580137) on Monday May 23, 2011 @03:02AM (#36214744)
    I have a Geiger counter from the 1960's that includes a small sample of radioactive material on its side for testing and calibration. The manual states that there should be a certain number of clicks per second, and based on the half life of the material, it looks like it still works fine. Amazon [amazon.com] also sells small samples of uranium that have a specific number of clicks per second that you can use to test your equipment.
    Other than that, there is a normal level of background radiation that amounts to about 14 clicks per minute if no other material is available, but this might not be viable in your area.
    Actually, I don't really have a pressing need for my Geiger counter, and it sure sounds like you need it more than me. If you want, I'd be happy to ship it. Let me know.
  • by meerling (1487879) on Monday May 23, 2011 @03:08AM (#36214784)
    I can't say anything about calibrating, but an easy way to check it's functionality and great way to demonstrate science is as follows:

    Go outside, preferably during the day, take a reading. This is background radiation, you live in it your entire life, it varies, and the sun puts out a lot so it will be lower during the night. Don't panic, Hollywood, like usual, got the science wrong. (Think about it, how often do cars actually explode in real life. Yeah, Hollywood science is useless.)

    Great, now go inside a building, take another reading. If you've got access to a nice sturdy concrete building with a basement, or some caves, those are even better. See how much it dropped? That's because of the building (or earth and solid rock) blocking the radiation coming from the sky.

    Now keeping an eye on the changing levels is probably what someone in Japan really wants, but you might have to ask someone that's in the science department at a university to find out what the readings were before the Fukishima incident.
    Also, distance from source will effect intensity by a lot! So a chunk of radioactive material 1 meter away will read much much higher than one 10 meters away. Since the sun and other stars are so far away, the measly distance of the Earths diameter won't make much different to those, so unless there's a flare or something, only the terrestrial sources will be a big worry.

    Anyhow, this is all high school stuff, or it used to be before they started dumbing down science in schools, so it's easy to find books about it in most libraries.

    As a side note, you can NOT detect a modern unexploded nuke with a geiger counter, their cases are so heavily shielded you can use them for radiation shielding.
    Again, Hollywood is so full of it. :)
    • 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.

  • 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.

    • mportant 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.

      And here-in lies the problem. Even if you calibrate your Geiger counter the readings will only be assured for much higher levels of radiation than you are likely to encounter. Instruments that can measure very low levels accurately are extremely expensive and require professional calibration. You couldn't just point one at your food either, it would need to be prepared (and destroyed) to get a good reading.

      The good news is that I'm sure there are plenty of people with high end equipment checking food and soil for you. Even if you don't trust the government they can't just hide radiation, and there are plenty of academic institutions which are taking their own measurements.

    • 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.

    • 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 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 drolli (522659) on Monday May 23, 2011 @03:22AM (#36214854) Journal

    if you know somebody who organizes a lab course in physics, in a university you can ask if you can take the geiger counter there and compare it to their calibrated samples. Typically there is a box of sealed test samples (well locked away), which have well defined radiation doses in different gamma-ranges, so you can test the sensitivity. However, you will have to take an safety instruction to even touch the box. So if you know somebody there well, he may help you. He may even tell you how to calibrate the device correctly using that sample. Another way, which is less technically challenging and will not give you a quantitative calibration is to use one of the typical stones which radiate stronger. Refer to any standard textbook which these are in you region. Look e.g. for granite on wikipedia and follow to the original sources. However none of these means will provide you with any information about the sensitivity of the counter.

    As for your friend trying to measure food: More than a quantitative comparison "this radiates stronger than that" will not be possible. The data will be problematically low for the prescribed doses if the counter has no good integrator/long term counter and is stable. Any quantitative measurement of contamination with isotopes is completely unrealistic outside the lab and with an inexperienced operator, especially if the device has no energy resolution. A simple workaround around the latter would be insert materials with different absorption coefficients into the path and compare the measurements, but i cant tell how well that works. Moreover 100-1000Bq/kg is not much. I doubt you manage to get more than a count rate of 1-10clicks per second from a sample of acceptable size. which means that in order to get a 10Percent resolution you may have to integrate over 100seconds or more. That means that the dark count rate should be acceptably stable.

    If your friend does this to protect the own health, i recommend the following: don't do it. There are two possibilities: either the food in monitored professionally and marked correctly (which i believe is normally the case in Japan) or its not. If its monitored professionally then there will be no long-term contamination which is undetected. The effect of a spurious peak in one meal to ten or even hundred times of the allowed level wont kill you or have any adverse effects, and reliably i think you will be only able to detect starting from about 10-100 times of the allowed dose. If the food which is not monitored professionally *and* comes from within 50-100km around the reactor then don't eat it, if you have the choice, until the situation stabilized (that is, when any kind of containment, even by a simple plastic foil is reestablished and then after a few months, look at the ieae website). If you believe you must support the farmers there, then donate money, don't buy the food.

    An non-reading can also provide you with a false sense of safety, and that is true for all uncontrolled foods. There is no way for a layman to establish safety of a food which comes from within the problematic range around the reactor.

    My personal feeling is that *in Japan, which has high food quality in general* an inexperienced operator of a Geiger counter trying to measure his own food will have higher stress due to mis/unclear readings and the constant (lets remember, this may have to be done for 20years if you take it seriously) reminder of the danger just before eating. The adverse health effects of this and possible associated psychological effects (stress before eating) will outweigh the negative effects of getting a higher dose from time to time. If you take the 30min-1h per day which you need to check the food *seriously* for such low doses of radiation, then there are other thing you can do in this hour (go jogging, ride a bike etc.) which will help the body more to develop the immune system.

    • by vlm (69642) on Monday May 23, 2011 @07:40AM (#36215820)

      Been there, done that. Humorously, WRT the NRC exempt samples, the main problem is each source is moderately expensive (think of the cost of a nice lunch). And nuts think it would be funny, cool, scary and 'leet to steal them. So its kind of like walking up to the lab counter in the chem building and asking for a bottle of gold chloride... they probably have it in stock, and they're probably not amused at the idea of "loaning" it to you without leaving behind an ID and signatures blah blah. The main danger is usually storing the sources in bare lead, lead is not something you want in your body. The other problem is stereotypically NRC exempt sources Usually have ridiculous half lives, like 60 days or whatever. Aside from only being 1 uCi, they also decay away to just about nothing in a couple years. So, despite the original expense, you might get them to give one away to you if its old and useless enough.

      The registered sources I got to play with were generally intrinsically safe. Sure, you can play with the Ra sources... They're permanently mounted in the bottom of a 55 gallon drum full of cement with a sample tube in the center that you can drop your sample / scintillator into. And of course the sources are in the basement and they went to great effort to remove elevator access to the basement and the stairs were replaced with cheapy wooden ones that will shatter at about a 1000 pound load, and the 55 gallon drum of cement and lead weight about 2000 pounds, so... (admittedly this was long before the ADA law existed) And there was a drainage "moat" in front of the stairs that had a grate over it that I am told was designed to shatter if you shoved a heavy source over it...

      They were much more nervous about people playing with the easily stealable exempt sources than the unstealable registered sources.

  • by kombipom (1274672) on Monday May 23, 2011 @03:30AM (#36214884) Journal

    If I remember correctly the reported levels of contamination in the food and water supply in Japan were, even at their peek, in the order of a couple of 100Bq per kg. You need to put a sample in a counter or spectrometer for some time to be able to tell those levels from background. Pointing a GM tube at pieces of spinach to see if one is contaminated more than another is futile, all you are going to notice is variations in background. You can have fun finding all sorts of slightly radioactive things with a counter if you like but unless you are willing to spend >$10k on a portable gamma spectrometer which _might_ be able to distinguish tiny amounts of I-131 or Cs-137 from background you are not going to find anything in the food.

  • by mrmeval (662166) <mrmeval@NOSpAM.gmail.com> on Monday May 23, 2011 @03:33AM (#36214904) Journal

    http://unitednuclear.com/index.php?main_page=index&cPath=2_5 [unitednuclear.com]

    All Isotopes are sold by the microcurie (uCi)
    ( 1 microcurie = 2.22 x 10E6 disintegrations / minute = 2,220,000 cpm )

    Normal sources are + or - 20% pf stated value.
    For calibrated sources, add $120.00 and source will be + or - 5% of stated value and will include a calibration certificate.
    Calibration is only available for gamma sources.

  • by Orgasmatron (8103) on Monday May 23, 2011 @03:39AM (#36214928)

    You can't calibrate it yourself. Or, if you could, you wouldn't be asking here. You need special equipment.

    Look here. http://www.radmeters4u.com/calibrate.htm [radmeters4u.com]

  • by kEnder242 (262421) on Monday May 23, 2011 @03:40AM (#36214940)
  • by Neil Boekend (1854906) on Monday May 23, 2011 @04:25AM (#36215118)
    Use this chart [xkcd.com] and a bushel of bananas.
  • by MichaelSmith (789609) on Monday May 23, 2011 @04:36AM (#36215154) Homepage Journal

    You get a caesium source and measure the radiation level with a known good instrument, and the instrument you want to calibrate. The physics department of any good university should be able to do this. Its a standard prac exercise.

  • by Registered Coward v2 (447531) on Monday May 23, 2011 @05:29AM (#36215348)

    1. Set the scale on as low as possible and watch people freak when the needle pegs ...

    2. When responding to a reported incident, set the scale as high as possible so people feel safe and don't panic since all they hear is a slow tick...tick...

  • 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.

    • Re:Don't. (Score:4, Interesting)

      by Attila the Bun (952109) on Monday May 23, 2011 @07:14AM (#36215742)

      You have absolutely no chance to get anything close to it on your own, so just don't.

      I'm afraid this is pretty much true, although the hobbyist in me doesn't like to admit it.

      The amount of radioactivity you're looking for is small compared to natural background, and small even compared to the normal variations in background. To identify any un-natural contamination you'll need a detector capable of distinguishing different isotopes, in a low-background environment, and it'll need to be regularly calibrated with standard sources. That entails a lot more gear than a Geiger-Muller tube, all of which is very very expensive, not to say tricky to operate.

      With a GM counter about the best you can do is to try to measure the radioactive decay of a sample, although most relevant isotopes have very long lifetimes which will be too hard to measure. 131-iodine is easy to detect and has a measurable half-life at 8 days, but I expect the authorities would find it before you do, and anyway most of it has gone by now.

      Alternatively you might befriend a physicist at your nearest nuclear research institute. Even then he or she will need to be quite a good friend, because the time and effort involved is significant, and this kind of expensive gear tends to be permanently in-use.

    • by vlm (69642) on Monday May 23, 2011 @07:45AM (#36215836)

      You have absolutely no chance to get anything close to it on your own, so just don't.

      We disagree

      http://tech.groups.yahoo.com/group/GammaSpectrometry [yahoo.com]

  • by adamofgreyskull (640712) on Monday May 23, 2011 @06:07AM (#36215508)
    John Iovine [makezine.com] long time designer/manufacturer of Geiger counters did an interview with Make: about a month ago and his concerns were just the same as yours. There's some interesting insights in the (admittedly short) interview, but it's well worth a read.

    We have a couple of good tube suppliers, because we’ve been in the business for a while. One of them just said to me, “We’ve got shoe makers calling us up, yesterday they were making shoes, and today they want to start making Geiger counters.”

    I know exactly what he was saying. And, really, anybody can make a working Geiger counter. You just put together a high voltage circuit and detect the pulses. But how accurate is it, and what’s its longevity? It took us years to perfect making a Geiger counter—a good Geiger counter. It’s still like a black art. As with a lot of things, you really have to be in it for a while, and see all the crazy things that can happen, and this enables you to can build some resiliency into the circuit so that it will keep functioning properly.

    These people who are just looking to make a quick buck are saying, “I can build a Geiger counter.” Yeah, you can build a Geiger counter, and it will probably work on some level, but you won’t know how to calibrate the thing, or even know that it’s in the ballpark of being accurate.

  • by MidnightBrewer (97195) on Monday May 23, 2011 @06:31AM (#36215588)

    Stop giving into the paranoia and get on with your lives. You're exposed to more radiation on a daily basis than you'll ever get from potentially contaminated food. Nuclear power plant meltdowns may be scary scary, but biological damage due to food contamination is so rare as to be statistically insignificant. If you want to protect your Japanese family, tell them to stop going to restaurants and other public places that allow smoking (which is not only harmful in all the known ways, but is also slightly radioactive).

  • by thesandbender (911391) on Monday May 23, 2011 @06:47AM (#36215650)
    1. I appreciate everyone's input about the comparative levels of radiation and I'm working with my g/f to translate the xkcd chart to Japanese to put things in perspective for her family (we'll be sending it to Mr. Munroe when we finish for him to post if he likes).
    2. We've already purchased Vaseline glass beads.
    3. I'm very interested in the detailed comments that testing is pointless b/c we couldn't get access to the equipment/environment needed to properly test and will be following those up.

    From a practical and scientific standpoint we both understand that the exposure they are subject to where they live is less than being at altitude on a flight to Japan. However her family and the country as a whole has been through a very traumatic event ... first from a force they can't predict (the earthquake) and now from one they can't really see (radioactive contamination). So why we can look at this objectively and say the exposure really doesn't amount to much unless you're near the site, they'll never be able to because of what they have been through. Realistically, if you survived a plane crash you'd probably be hesitant about getting on a plane even though the statistical chances of you being in two commercial plane crashes are practically 0. Just the way the human psyche works. Anyway, I would like to keep them from throwing money away if testing food is a complete impracticality (#3).

    Thanks for all the excellent input and we will be reviewing it throughly.
  • by thesandbender (911391) on Monday May 23, 2011 @06:49AM (#36215658)
    http://en.wikipedia.org/wiki/Banana_equivalent_dose [wikipedia.org]

    Found it while I was researching and quantifies what many of you have been pointing out about the effective level of contamination.
  • 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 WillRobinson (159226) on Monday May 23, 2011 @08:05AM (#36215940) Journal

    If you really need two, I have some. want much and if it would like them pm me. I have them on Craigslist in Dallas. I would rather help a fellow slashdotter. They were always calibrated, but the cal sticker ran out last year. Just pm me.

  • by smellsofbikes (890263) on Monday May 23, 2011 @11:40AM (#36217978) Journal

    If you want a cheap radiation source, you can buy 2% thoriated tungsten rods [tigdepot.com] for TIG welding. Find a local store and buy a 1/16" rod: individually they only cost about $8. Other people have suggested Coleman lantern mantles but the ones you can buy these days don't seem to have thorium in them anymore, because the old ones were *seriously* radioactive. If you *have* a Geiger counter you can go to an Army/Navy Surplus store and check the ones they have since a lot of old radioactive ones are still in stock.

    My homebuilt geiger counter, using a surplus Russian GM tube, can easily detect a single thoriated tungsten rod if held up close to the tube, as can my vintage Civil Defense CDV700. Both will also detect a smoke detector.

    If you want to build your own geiger counter and have a tube, here are instructions [madscientisthut.com] for building a high voltage power supply from a hacked-up flash unit from a disposable camera and here is the detector circuit [madscientisthut.com] that translates that into audible clicks. If you optoisolate that detector circuit you can feed it into an Arduino and log/display counts per second on a laptop. (It needs optoisolation because the output of the audio click board is negative with respect to power and way more than 5V, so it'll cook an Arduino, as I found out. Although an Arduino analog input can withstand -200V and still function, amazingly enough.)

    If you just want to detect ionizing radiation, you can build an ionization chamber. My company supplies DIY kits [madscientisthut.com] but we also have detailed instructions for making your own [madscientisthut.com] with a component list of like four transistors and a handful of resistors, and a tin can. They're more sensitive than a Geiger tube, although they're much slower to react, taking seconds to change their reading. One neat thing is you can build them as chambers so you can actually put a sample inside the chamber, if you want, and they detect alpha, beta, and gamma.

    And as other people have said, any sample you buy that'll allow you to characterize your radiation detector will expose you to tens to thousands of times as much radiation as anything in Japan unless you're actually inside the grounds of the power plant, so this whole project might not do what you want.

The longer the title, the less important the job.