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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!"
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.
Carl Sagan quotes get you an automatic +5 on all posts.
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.
"Have you ever thought about just turning off the TV, sitting down with your kids, and hitting them?"
go to one of the nuclear plants, go to the core.
If the readings are off the scale, it works!!
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.
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.
Here are homebrew fallout meter designs (link to PDF at the bottom of the page): http://en.wikipedia.org/wiki/Kearny_Fallout_Meter
More info: http://www.oism.org/nwss/s73p921.htm
I realize that does not help you calibrate your Geiger counter, but it does offer another measurement for validation of your current calibration. It also is a great thing to share with other people who do not have Geiger counters.
I would not use a smoke detector to test a Geiger Counter. The Americium 241 is an alpha decay and chances are your Geiger counter won't be sensitive to alpha particles.
If you want to verify that a Geiger counter works, hold it beside some salt substitute (Potassium Chloride). If it starts clicking more than background, it works. Old orange Fiestaware pottery works as well.
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.
[Fuck Beta]
o0t!
Well, perhaps an Ionization type detector, but probably not other types, like Optical.
It must have been something you assimilated. . . .
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. 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:
http://www.nukalert.jp/
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.
HTH.
--ES--
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 $$$$).
Uranium Ore from Amazon?
Testing the efficacy of radiological test equipment is a solved problem as of 60 years ago. What is the issue?
If you can put aside your overt hatred of anything "US" in origin, viable counters with NRC approval (of known efficacy bearing Federal approval) are a few mouse clicks away - far less keystrokes involved than posting this "request for information" on an Internet blog.
Since you posted this on ./, one can assume you are wanting to do this on the cheap, with Linux, or without "anything US", etc. If so, you are limited to Russian units from E-bay or surplus Yugoslav detectors from 1991- which are most likely inaccurate due to calibration drift, based on chemical dosimetry, or a valve-based unit weighing more than the average Walmart patron.
You need to pick your poison.
Do the research, pony up the money or suffer buyers remorse.
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 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.
Try contacting someone at your local university in physics or chemistry. They will have access to common radioactive test sources (like Na-22) which they use in standardized amounts for calibration. They may be willing to help and can give you the exact information you need. Or maybe you could buy a small Na-22 sample from a laboratory supply shop online? I don't know. I don't think you need a license and it will come in a small 'safe' plastic disk. Then you can read online how to use it as a reference source.
But, for him to be karma whoring, there'd also have to be a significant number of slashdotters who don't know (which is quite the unlikely case). So, either he didn't know; or he over-estimated the number of slashdotters who don't know.
Arse:
1. the buttocks
2. the anus
3. a stupid person; fool
Elbow:
1. The joint or bend of the arm between the forearm and the upper arm.
2. The bony outer projection of this joint.
Well its about ask likely as a slashdotter not knowing what a Geiger counter is.
I worked at the regional radiation service for a while, one of my dutys was to calibrate counters.
It was actually trivially easy, we had a few calibrated sealed sources (it's nearly 20 years ago so don't ask me which particular isotopes) and took readings at 20cm from the source. This was fed into a program (that I wrote) which calculated the expected rate from the source based on the time & the half life and produced a calibration certificate.
There were some other checks too, electrical safety, light permiablity of scintillation tubes (nothing more funny than testing one that's faulty - point it at any light source and you get a massive reading, scares people shitless!)
Best place to start to find out if this is a service that's offered is to go to the nearest hospital with a medical physics department (there's normally one with a major radiotherapy department) they should be able to point you in the right direction.
The Geiger counter I have, and recommend:
http://www.gammascout.com/
"The DOE's position is that there is NO safe dose of radiation (linear, no cut-off model) ."
http://en.wikipedia.org/wiki/Orders_of_magnitude_(radiation)
Nice comparison of radiation doses:
http://xkcd.com/radiation/
Buy some bananas.
Be careful. Quite a lot of modern smoke detectors use LED light beams to detect smoke. These will not trigger much of a reading from your Geiger Counter
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.
"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.
Try other countries. If the USA is backlogged, then try another country. Try Europe. Try Taiwan. Try Russia. Expand.
Test discs that are marked to what they should read on a counter. If you just want to verify it works an old school watch with a glow in the dark dial or hands will set it off.
The easiest way to test a geiger counter is to get a big bag of potassium chloride (found in water treatment salts, low or no sodium salts for diet purposes) . The radiation from the naturally present potassium-40 should be easily detectable with a decent counter.
No need to search for uranium glass or thorium containing lantern mantles.A sufficiently sensitive counter should also click randomly due to background radiation.
send it to a geiger counter calibration house and get it calibrated.
really, this shit isnt hard.
http://xkcd.com/radiation/
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.
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.
try this:
http://cgi.ebay.com/CDV700-GEIGER-COUNTER-Radiation-Detector-NEW-OS-/350465234219
or
http://cgi.ebay.com/CDV700-GEIGER-COUNTER-Radiation-Detector-NEW-OS-/350465234219
It runs on D batteries.
There are lots of caveats as most of these meters are for detecting contamination rather than dose rate.
The CDV -700 has a pretty thick window so not super sensitive. They are all pretty old so you would be wise to check it carefully before using.
To reliably detect small amounts of radiation contaminating food you will need to spend a fair bit of money on something more sensitive that most of the survivalist supply stores:
http://www.ludlums.com/index.php?page=shop.product_details&flypage=flypage_ludlum.tpl&product_id=300&category_id=115&keyword=3_with&option=com_virtuemart&Itemid=95
(they will also do calibration)
Eberline made very good instruments but I can't find them on the web as making them currently. They made also very good equivalent products. You may find a good used one of these.
Proper NIST traceable calibration may be worth your time the meters are generally calibrated for dose rate or energy from a standard Cesium-137 source Cobalt-60 is commonly used and they are both major fission products so they are good choices for the stated application):
One vendor near NY I found on web: http://biomedphysics.com/survey-meter-calibration-and-repair
The Reed College Reactor Facility might also do it. This will likely be the cheapest method (website quotes $50 per probe +shipping): http://reactor.reed.edu/metercal.html
As others have mentioned most smoke detectors use Americium which is an alpha emitter. You need a very thin window and large surface area probe to detect this reliably. These don't make great test sources.
If you can find an older colman style lantern mantle made of thorium (the newer "safer" ones do NOT have thorium) they make great test sources and will set off most Geiger counters and are really useful as you should check to make sure it's working with every use. The probes are delicate, the batteries die etc so if it's important check every time. Keep them in a plastic bag so they don't contaminate your detector!
Good Luck!
http://unitednuclear.com/index.php?main_page=index&cPath=2_5
I'd go on a Vegan diet but the delivery time from Vega is too long. --brownkitty
So this doesn't specifically address your question about geiger counter testing, but if you're interested in the radiation levels in japan, and an attempt to crowdsource radiation collection, check out http://www.safecast.org/ Also, I recall reading somewhere that they were working with some group to create a DIY geiger counter. Might include some testing info as well.
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
See that "Preview" button?
Test it compared to a known source. The Geiger counter should come with a test source of known activity, which will allow you to calibrate it. Aside from that, go online and find acceptable levels of radioactivity in foods to determine where your threshold should be. Young children and the elderly should have their acceptable thresholds adjusted, but you should be safe in staying with the average.
http://www.amazon.com/Images-SI-Inc-Uranium-Ore/dp/B000796XXM
my associative arrays can kick your hash - TCL
Guess what, I live in Japan too. Geiger counters are easy to get.
go to Akihabara station, go out the exit with Yodobashi camera, and turn left. Walk across to where the McDonalds is, and go down the stairs to the place that sells electronics stuff (Mainly used PCs and unlocked mobile phones). They have new Geiger counters - as do many places in Akihabara.
Then again, you don't really need it, since the government, scientific agencies, educational institutions, and even other hobbiests are measuring and posting the numbers online all the time.
There is also the issue of knowing what you are measuring and what it means. Unless she lives reasonably near to Fukushima, the levels in Tokyo and below are at average.
Potassium 40 is a beta emitter. Buy some kg of a potassium compound (KCl, KNO3, etc. Plenty of fertilizers are made up of potassium salts), estimate the total amount of K present in your sample, and from this chart you can easily estimate the total activity of your sample. If you are lazy, the 0.0117% of your potassium is radioactive. Estimating the amount of radiations emitted in a second and how to estimate the expected number of counts hitting your Geiger detector is a trivial exercise, that you should be capable of performing, since you want to succesfully calibrate and use a Geiger counter.
You can purchase an exempt quantity Cs-137 check source from several vendors, Isotope Products, Inc is one. Probably run around a hundred bucks. Exempt quantity is the amount below which a license is not required for possession. for Cs-137 it's 10 uCi, which is strong enough to get a decent reading on a GM instrument. What you really should do is send the instrument to a calibration facility like Ludlum Instruments in TX and have it calibrated to a NIST traceable source. Once you have a calibrated instrument, you need to know what you're looking for. If you're testing food, you need a thin window GM, capable of reading in counts per minute and you want to see 100 cpm above background, which is a common release limit in the nuclear industry based on a 10% instrument efficiency. If your geiger counter doesn't have a thin window detector, it's probably not sensitive enough to detect minimally contaminated foods. I'm sorry to say that most of the detectors available to the public are useless in detecting the levels of contamination you'll be looking for. I think your best bet is to wash your food thoroughly and trust that the government of Japan is monitoring the levels carefully. Try not to panic. The stress of worry is much more harmful than any radiation exposure you're likely to receive.
Plutonium should be available in every corner drugstore by now....if not....talk to some Libyens....they may want to trade with some used pinball machine parts.
As a resident of Japan, it strikes me as far more productive to donate the money I would use to buy a Geiger counter to disaster relief aimed at helping those more directly affected by the quake and tsunami. The government has been strictly monitoring levels in food and has been quite quick to prevent shipments of any food which might present even a small risk.
Vaseline is radioactive?! So you're saying I shouldn't be using it to, you know, wax my carrot?
There's no -1 for "I don't get it."
Which is why radiation badges were invented. Doesn't require much intervention or thought. Might not be perfect for this application, but they are relatively cheap and plentiful. Probably set for higher doses than required, but it's an idea.
Use this chart and a bushel of bananas.
Well, I might have a way, but it only works on a semi spherical planet in a vacuum.
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.
http://michaelsmith.id.au
The results you are going to get testing foodstuffs with a Gieger-Muller are going to be poor, almost certainly the levels of radiation in food are so low that the statistical fluctuations in the background count rate will render your results useless. You are definitely going to have to invest in some lead shielding to remove some of the background count to determine anything useful from your data. To do this properly I would recommend a more sensitive detector such as HPGe or an inorganic scintillator to actually determine the radioactive source content for proper analysis of the risks. I regret that someone is profiting horribly from the sale of GM tubes to people who are naturally scared for their health but have no idea how to use them and most importantly how to interpret the data they produce. Alpha particle emitters in foodstuffs are the most dangerous (tissue weighting factor x20 compared to gamma) but they will not be detected with a GM tube unless you place the source inside the tube itself. GM tubes with a thin window are quite good at detecting beta-particle emitters which are also harmful but it will be difficult to tell what source is actually emitting the radiation and the energy of the beta particles. The efficiency of a GM tube isn't that great with gamma radiation because it's so penetrating and gas naturally isn't very dense. As mentioned already calibration with a Cs-137 gamma source and a Sr-90 beta source by a should be carried out for a GM tube regularly, the efficiency of the detector can be calculated using some simple geometry calculations.
http://www.ehow.com/how_6786889_calibrate-geiger-counter.html
and im guessing you have something like this to send there.
http://www.coleparmer.com/catalog/product_view.asp?sku=0899050
... on getting proper calibration equipment.
Recoup the expenditure by playing on the paranoia of others that they should be suspicious of the calibration of their own Geiger counters.
Red/Orange FiestaWare from the 1930s
http://www.orau.org/ptp/collection/consumer%20products/fiesta.htm
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...
I'm a consultant - I convert gibberish into cash-flow.
I have a couple of Geiger counters, including one I made and calibrated in college. I noticed right after the Fukushima accident that on ebay, Geiger counter bids and prices went way up fast, but there are plenty of them there.
Clocks and watches (and other things) were painted with a glow-in-the-dark paint containing Radium, phosphorus, etc. many years ago, and you can still find them here and there.
By far best advice: purchase a calibrated source!
Try using it in Japan! Oh your already there! Great!
Granite is a natural source of radiation, like most natural stones. However, some granites have been reported to have higher radioactivity thereby raising some concerns about their safety.
Some granites contain around 10 to 20 parts per million of uranium.
Eschew Geiger, using a bubble detector is very straightforward and they are pre calibrated. As Neutrons pass through it they displace bubbles. They come in several "strengths" for good dosage metering over time. Shake to reset.
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.
Contrary to the popular belief, there indeed is no God.
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.
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.
It's a Geiger-Muller counter, not a Geiger counter.
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).
"Give a man fire, and he'll be warm for a day; set a man on fire, and he'll be warm for the rest of his life
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).
... 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).
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
Thanks for all the excellent input and we will be reviewing it throughly.
> "My girlfriend's family lives in Japan and is very interested in obtaining geiger counters for testing food and other materials.
For this very purpose you would choose specialized food-tester equipment, like this:
http://www.gammatech.hu/img/products/ih111l.jpg
and this
http://www.gammatech.hu/dl.php?lang=hun&file=ih111.pdf&cat=datasheets
Used by the hungarian military for field-mobile food and drink testing, hence the "beautiful" dark green casing with the "stylish" lead door hinge. The device measures beta and gamma rays simultaneously, according to stored patterns, to understand short-lived and long-lived isotopic composition. Mostly Iodine-131 and Caesium-137 contamination are important, but all are measured. In 10 seconds, the device's small LCD screen returns four possible simple answers on a sample: can consume now, wait 3 days, wait 1 week or discard immediately, as defined by European Union and NATO regulations.
(Attach a Windows PC with MultiACT software and you can download numerical measurement results from the IH-111L device for further lab office work. Calibration source is built into the device. The full suite, device and software is supposedly available for civilians, but probably only for companies, rather than individuals, but I saw it being advertised in the press after March 11.)
http://en.wikipedia.org/wiki/Banana_equivalent_dose
Found it while I was researching and quantifies what many of you have been pointing out about the effective level of contamination.
(I) really would like to go there by bicycle (almost impossible to get in by car because they have barricaded off most of the area, but from what people have said, it's pretty easy to sneak in on foot or cycle).
Why are you trying to slip past the barricades?
Do you really want to be asked this question in a Japanese lock-up?
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
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.
There are several different kind of detectors depending on what type of radiation/contamination you are looking to measure. A 'geiger counter' is, more accurately, a Geiger-Meuller detector that measures gamma radiation (photons) but does not accurately measure neutrons, alpha's or surface contamination. It works on an anode and cathode principle where inbound gamma's excite electrons into escaping their atoms (aka the photoelectric effect). In your case you may benefit better by taking a portable air sample and frisking the paper filter. Perhaps you can get ahold of some old military versions, as long as they still work. The devices I am familiar with are:
AN/PDR-27 gamma detector, most iconic device commonly referred to as a geiger counter
AN/PDR-56 alpha survey meter . Detects helium ions
AN/PDR-70 neutron detector, has outer poly sheilding to simulate the effect on human tissue
I've no idea if they still do (probably don't), but they used to generally have a test source right on the machine that put out just enough radiation that you could stick the input cylinder right up to it and get something.
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.
If you have cash to splurge get a gammaspectrometer rather than a geiger counter. It will actually give you an idea of the elements involved. Modern models dont need an internal cesium reference so you can measure that as well. You may have to set up new regions of interest (ROIs) for the exotic isotopes, they usually come set for uranium, thorium, and potassium, with a window for cosmic, plus total count which includes everything.
Watashi wa chikyubutsurigakusha desu.
I once purchased a small Thorium rock form eBay to test a home-made Geiger Counter. It is relatively inexpensive.
We all live with background radiation, which varies significantly by location. No one seems to worry about that when they pick a place to live, but everyone gets apoplectic about much smaller amounts of radiation released by reactor accidents.
Probably be a waste of your time, I had a math professor or two try to and you see how much good it did ;-)
Geiger counters are only useful at detecting if there is a background radiation hazard. They are useless for food.
You might as well point it at yourself and see if you can detect anything, and you will not even though it should be going like mad.
If you want to detect if you have eaten too much radiation, like Cesium, then have your urine tested. You will be peeing Cesium out at a known rate and your total dose can be determined. Anything else is useless.
I'm certain Japan may have that test available as a measure of tracking contamination. And yes, your urine is radioactive naturally thanks to potassium and heap-ton of other isotopes.
Every geiger I have used in my life (which have been a lot) have come with a test source (Cs-137 disc) attached to the side. Maybe this is a Canada thing or a Ludlum thing but yeah they SHOULD come with a Cs-137 calibrated disc. Else you could find some fiesta dinnerware which is slightly radioactive, I think orange is the greatest emitting.
Uranium for the Win!
I don't know about the new ones, but the old yellow Civil Defense Geiger counters have a small uranium check source under a label on the side. It has a very long half life so for all practical purposes the counts are stable even after 50 years.
Old Coleman lantern mantles are thoriated (why you do not want to breathe the dust when changing one out) and put out 1200 to 2500+ counts per minute (1.80 - 3.75 mR/hr). Even "Lite Salt" is somewhat radioactive because there is a significant potassium isotope that is radioactive. You would have to know the counts for a certain amount of salt, but it should be pretty straightforward to calculate. I'm sure Wikipedia has lots more examples.
It's important to know the difference between Geiger counters and "survey meters". Geiger counters are much more sensitive and show counts on lower ranges. Survey meters are for higher levels of radiation. It could be in Japan you need both depending on where the girlfriend's family lives, but I would bet a Geiger counter would be sufficient.
You just didn't look very well. We just bought another 10 Geiger counters for our Physics classes a few weeks ago. IMHO, the cheapest/best value are from Vernier (www.vernier.com). Any scientific supply company has these things.
By the way, they are digital now. No more big yellow boxes with a huge battery!
If you MUST get a GM detector, get wone with a thin Mylar windo, or you won't see anything but gamma radiation.
here is an intereting demo.
Purchase a coleman lantern mantel. These are considered to be "safe" for general use.
Last time I checked, they have a good ammount of uranium a strong beta emitter (if I recall). Not sure why they put it in. I think it may blow brightly when hot.
Get a reading of background radiation.
Hold the mantle up to the mylar coverd part of the detector. It just scream (if it has a clicker).
Put a sheet of notebook paper between the detector and the source. The reading should drop. Repeat untill the reading is back to background.
If you can find an old Plutonium oe Amarecium based smoke detector, you can do this for alpha radiation also.
Forget using a GM detector for checking food. For the most part, GM detector efficiecy is far less than 5%, even for alpha and beta radiation with a thin window. In general, if you can detect ANY contamination with one of these devices, you have a big problem.
In general, unless you want to make the above kind of demo, don't waste your money.
And yes, I have worked as a Nuclear Physicist (before I becam esenile and retired), and calibrated these detectors.
I do nuclear research for a living in a lab I own, check my site for some info. Potassium isn't so hot, it's barely out of a low background in my lab on a very nice 6" NaI:Tl gamma spectrometer, and since you eat it daily, that's a good thing indeed. Our geiger counters can usually not see a jar of KNO3 out of the cosmic background -- partly because the sensing mechanism in a geiger tube doesn't see gammas all that well -- you want a scintillator based detector for that.
.25 microcurie)-- and those are considered "safe" to have in the lab, outside of a lead pig.
Here's some threads on detectors: http://www.coultersmithing.com/forums/viewforum.php?f=11
Most nuclear waste products and fuel produce either beta (electrons) or alpha (He nuclei) which geiger counters see quite well, especially the thin window types. That's what you want, and many are available or the parts for them, on places like ebay. The tiny ones based on Russian tubes the size of a pencil aren't that great, lousy quantum efficiency, but that's not hardly a blanket condemnation of them, since even a relatively numb sensor will still tell you "you're going to die" a long time before you're in trouble. The ones to really avoid are the ion chamber based old CDV models for civil defense -- they won't read jack on a box of rocks (U ore) so hot they'll burn you.
It's not hard for any EE type hacker to make a counter if you can find the tubes surplus, which is how we do it. Plenty of good Russian new-old-stock out there cheap, and all it needs is an appropriate power supply in the few hundred volt range, a series resistor in the 1 meg region, and a coupling cap in the hundred pf region to be able to drive cmos logic directly -- the rest is left to the student. We use inexpensive CCFL inverters and fast diodes and caps to do the HV supply -- the older ones are quite good for this, the new ones with dimming aren't what's wanted. You can control the HV output by regulating the LV input to the CCFL with a simple 3 terminal regulator - they run all the way down to about 1 volt - even the 12 volt input types, and the output scales with the input.
It's true that a decent geiger counter or the better "gamma scout" are hard to find right now, the demand way outstripped the supply on the event in Japan. There are other issues with the TSA and others buying up all the good detectors for neutrons (rare He isotope based) and so forth - but there's a lot of used stuff around that's still quite good if you look. Just avoid the old civil defense stuff and dosimeter pens -- they really stink. I've had one of those pens in a box of U ore for months and it's barely started to read at all.
A typical 2" diameter window pancake geiger tube will read about 100 cpm here on cosmic background for reference. A smoke detector source right on it will be in the 10k cpm range, as will a little bit of yellowcake in a test sample. A CS-137 cal sample will read quite high 14k cpm (it is rated at
Yes, there is plenty of controversy about what constitutes "safe" and there are even people who believe in hormesis -- that a little is good for you. Whatever your personal religion -- don't get alpha or beta sources inside you (eat them) because then you are going to absorb 100% of the radiation they produce, and most often they sequester in some bad place for that. I would point out that moderate amounts of radiation mainly increase risk of cancer in the out years -- so if you're already old, maybe not such a worry as if you're pretty young. I might get cancer from the radiation I've been exposed to - but it will have to reach down into my grave to get me 30 years out.
Why guess when you can know? Measure!
As mentioned earlier in the thread "safe" limits for ionising radiation don't exist. One single particle could be the one that hits a critical bit of DNA and gives you lethal cancer, or a billion particles could pass right through you and cause no damage. The "safety" limits set by government bodies are based on poor science and are complete bullshit.
Oh and also ALL food has radioactive particles. It did before atmospheric nuclear testing, and it especially does after all the atmospheric testing that went on. The fact is that unless a piece of food is INCREDIBLY contaminated it is no more a risk then the thousands of risky things we humans do every day and don't even think about.
Those who die from cancer caused by this leak will be out weighed a thousand times by those who die in car accidents, easily preventable infectious disease, and diet related disease. Let alone minor stuff like carcinogenic fumes released from the plastic that most Japanese homes are packed full of.
Spending hundreds of dollars (probably closer to a thousand in the current panic) on a radiation detector and then continuing to drive a car is the sign of an irrational person with no capacity for risk assessment.
========
CINC, 4th Penguin Legion
If you want a cheap radiation source, you can buy 2% thoriated tungsten rods 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 for building a high voltage power supply from a hacked-up flash unit from a disposable camera and here is the detector circuit 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 but we also have detailed instructions for making your own 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.
Nostalgia's not what it used to be.
ever since three mile island
you gotta go out of state, but not sure where.
You have a girlfriend, and you post on slashdot???
I am a radiation expert in Oak Ridge. I have counted food routinely, even samples collected after Chernobyl. Merely measuring the radiation from food isn't sufficient to estimate how much exposure a person would get from eating the food. I realize the easiest path might be to not eat things that were measureably higher than background, but that may not be valid as (1) some foods _naturally_ have more radioactivity than others and (2) the background fluctuates at all scales. If I count my spinach and then the background, it may be higher or lower, randomly. That randomness may hide levels that you don't wish to eat. Or it could be fine and appear high because the background ran low for that period. Not to mention that Geiger counters only measure gamma radiation and beta radiation strong enough to make it into the probe. It cannot see low energy beta radiation or alpha radiation at all. I am sorry that I have no do-it-youself replacement solution but I would not attempt to judge food armed solely with a Geiger Counter.
A lot of people here seem pretty critical of others who are worrying. Worrying may be psychological and from ignorance, but I don't think it's irrational. Here's my perspective:
-I have a two year old daughter. I used to think about love as being willing to take a bullet for someone. But it's more like I would trade a 100% chance of my death to eliminate a 10% chance of her death.
-I don't have a strong science background. The last science courses I took were AP chem and AP physics, ten and eleven years ago. If we studied nuclear decay I don't even remember it.
-I don't trust the government. I do have a strong background in finance - I've been interested in it and studied it since junior high school (majored in it but actually knew almost all of the undergrad material already - learned much since). In financial matters, I have seen times where US government experts (treasury and fed officials) outright lie, quite a few times where they misled, and many times when they have seriously downplayed or exaggerated risk to the population and press. I don't know how it is in other fields, but in the field I know well and can really understand, they say what they want to say and motives can be difficult to determine.
-I am no expert on Japanese government but the little I do know suggests they are the same.
-My time researching this stuff is worth about 2500 yen/hr to me. I spent about 12-13 hours learning about radiation after the hydrogen blasts, and another 3 hours tonight. That's about 37.5k-40k yen (US$360-400). $500-1000 on test equipment that would settle things I could probably go for; however, even the Ludlum device linked earlier (around $700) looks insufficient (says surface contamination only) and for the $10k+ devices mentioned I'd rather send my wife and daughter overseas.
-In my 15-16 hours of research I haven't found good information about the dangers of radioactive particle ingestion. Just a lot of people talking about how small it is. I saw enough earlier to feel comfortable moving back to my home in Kanagawa but as mentioned earlier elevated levels have been discovered here.
I do really appreciate the posts with information and explanations.
Still researching, G
Just don't worry about this at all.
Go to the IAEA web site. Or that very interesting XKCD web graphic, showing how completely harmless all this stuff is. Like a chest x ray. Or a flight to Paraguay. Trust the Government and Industry to protect you when things are dangerous. Turn on the lights, pay your bills, and don't ask questions.
Besides, if you detect radiation in your food, what are you going to do? Stop eating? Move to another country? Where? Where would you go that is unaffected. Just relax and accept the new "normal". :)
These are my friends, See how they glisten. See this one shine, how he smiles in the light.
For the past 6 months I've been putting together a rad experiment unrelated to Fukishima. I've been all over eBay looking for samples and geiger counters.
Many units on eBay are old Civil Defense units. These are all bright yellow and 30 to 50 years old, and come in several varieties. Of the varieties, the ONLY ONE which is worth getting is the CDV-700 model. All others are unusable for your purposes.
For example, a CDV-715 is only useful if there is lots and lots of radiation available, as in the aftermath of a nuclear war. It's measuring range is so high that it simply cannot see radiation at small levels. Here is an example of what NOT to get:
http://cgi.ebay.com/Gieger-Geiger-Counter-Radiation-Detector-CDV-715-/120725761857?pt=BI_Security_Fire_Protection&hash=item1c1bd0ef41
Similar for the dosimeters. Their range is so high that they will be useless for your purposes. Also, the dosimeters tend to go bad after awhile (air leaks into the chamber), so it's likely that any units you get will not work anyway. Here is an example of what NOT to get:
http://cgi.ebay.com/Gieger-Geiger-Counter-Radiation-Detector-CDV-715-/120725761857?pt=BI_Security_Fire_Protection&hash=item1c1bd0ef41
A CDV-700 has about the right range, and is a very robust - if you get a working unit chances are that it will work for decades. Here is an example of what TO get (any model is OK):
http://cgi.ebay.com/Geiger-counter-radiation-detector-CDV-700-Anton-Model-6-/330566290368?pt=BI_Security_Fire_Protection&hash=item4cf7494bc0
Up until Fukishima, these were going for around $50 USD per unit. Now they are going for around $250.
I've been active on the experimenter's boards for the past several months, and many people are decrying the number of scams and bad counters that unscrupulous people are selling on eBay. People are getting counters with no tubes, which don't work, or are not as advertised. I personally ordered a CDV-700 and received a dosimeter instead - there are lots and lots of bad people trying to take advantage of the situation right now.
(The good news: If you do get a CDV-700 and it doesn't work, they're easy to fix, even for beginner electronics hobbyists.)
I'm also advising several hobbyist groups which are designing their own geiger counters. Of 6 or so separate designs by separate groups, not one of them will be useful for your purpose, for various reasons. These units will work and will detect radiation, but there is no way to assign a meaning to the measurements. They are well suited for the purpose intended, which is measuring radiation over long periods where the measurements can be compared with measurements from the same unit, but these are not useful for your purpose. They also make good hobbyist units, to show the principles of detection.
Measuring radiation, knowing the different types and what to look for isn't hard. It takes about 40 hours of research and reading and some tinkering to get a grasp on the problem. Do that first, then you will know better what to look for and what to buy.
My advice: Get together with the people in your neighborhood, purchase 1 counter and share it. Not everyone needs their own counter, and lots of times only a single measurement is needed anyway (is this area clean?). Designate one person to study up on the techniques and issues, and rely on that one person to make measurements as needed and explain the results.
It's not hard, but it takes a little dedication. Specialize and trade.
Also, CDV-700 units come with a small radioactive sample taped to the case, specifically for testing.
Hold the tube next to the black dot on the side of the case (with the shield open) and you will hear lots of clicks. That will show you that the unit is working.
Originally, the dot was manufactured so that you could roughly tell if the unit was calibrated, but over time the dots have decayed to about half their initial activity. But they still work and can still be used to test the counter.
I couldn't find a good picture link, but the dot is well marked on the unit. It's easy to find.
Pretty typical logic, Occupational Exposure limits are very tight in the US, but diagnostic exposure limits are very loose. It's kind of schizophrenic for us working in dental offices where we're limited to an exposure of about 1/10 on the job of what we get as a patient from the same machines.
Not really, unless you believe that individual patients should really really be x-rayed every year*.
*which seems like in most cases it might just be a way to pad their bill.
Putting moderation advice in your
Most all of the suggestions are utter crap. The only real way: pick up radiation calibration standards or retain the services of someone who has the same. PERIOD.
All the other suggestions won't assure you exposed to a "safe amount" or "unsafe amount". Pretty much they "add noise" to what ever answer you might get; obfuscating more than clarifying what the Geiger Counter is telling you. Just a waste of time really. Better to just use the meter completely uncalibrated instead of using a doofus technique like described.
Salt!! Jesus Christ on a Popsicle Stick! It doesn't get stupider as Wannabe Ignoramus advice than that. FYI, I used to work with radiation and radiation safety.
first there are 3 types of nuclear radiation Alpha Beta Gama Alpha particals are relativly weak and stoped by a few sheets of paper,, Beta particals are stronger requiring clothing levels of protection normally the risk from these is internal ingestion,, Gama is very energetic and needs in the range of several feet of earth to resist. food stuff is not directly harmed by radiation sealed goods like caned or bagged are fine once the containers are decontaminated even corn ears or potatoes can be used... radiation can be defended against by time, distance, or shielding.
as far as testing your meter several places come to mind first is hospitals sometimes have standard testing sources. also your emergency management has meters and testing gear usually connected with the sheriff department, FEMA, department of energy(nuclear power plants),, and homeland security.
there are some plans around for home built detectors, they look like a jar with foil leafs but i doubt the levels you will be trying to measure will will be high enough for this crude type to be useful.
your better off relying on others for detection and directing your efforts on protection mainly use the same decences you would use against smoke, dust and pollen I hope this is useful.
I was in the same position that I considered buying a Geiger counter for testing food. I googled and found many people saying that you can't test food with a Geiger counter. The most specific reason given was that the reading from the food would be drowned out by background radiation. Being the sceptical type I didn't buy that reason just like that. I had this naive idea that I could point the counter at my food, wait a minute or 2, the point it away and compare the readings. So I actually did the maths. (If you find any mistakes in the computation, please do post them. And excuse me for not using the correct physical terms for the various kinds of radiation strength/dose/whatever.)
I based my computation on this Geiger counter, which seems to be among the most popular consumer grade devices available:
http://www.conrad.de/ce/de/product/120173/Gamma-Scout-GEIGERZAeHLER
According to the datasheet it will give you (for gamma rays) 95 pulses per minute for a radiation of 1 uSv/h.
Meaning that if I send gamma radiation of 1 uSv/h directly into the device it will give me an additional 95 "clicks" per minute in addition to background radiation.
Now for the question: Why is this not good enough to identify a contaminated orange?
We start out with the numbers from here
http://xkcd.com/radiation/
Now let's assume that our acceptability threshold for radiation from a single orange is 20uSv. In other words, eating a single orange gives us the same dosage as a chest x-ray. Such an orange can definitely be seen as contaminated. You would only be allowed to eat a few such oranges per year (just like chest x-rays).
So, if our Geiger counter can not tell this orange apart from an uncontaminated orange, it is clearly useless for testing food.
So how many ticks per minute would we expect from this orange on our Gamma Scout device?
First we have to transfrom 20uSv absorbed dose into a dose rate. In other words, we need to compute how much radiation per time unit the orange emits. Let's assume our orange can completely pass on all of its radition within 10 hours. In reality of course it would take the orange (depending on the isotope) weeks, months or years to emit all of its radiation. So I grossly overestimate the radiation from the orange here. But the point is that even this grossly overestimated dose rate can not be detected with a consumer grade Geiger counter.
So our dose rate would be 2uSv/h
Now an important thing we must not overlook is that the orange is not a raygun. It doesn't emit a concentrated beam. Rather, its emission will be spread evenly across its surface.
An orange of 8cm diameter has a surface of
4*PI*4cm*4cm = 201 square cm
According to the manual, the measurement window of the Gamma Scout has a diameter of 9,1mm, meaning a surface of about half a square centimeter. Let's be generous and round that to 1 square centimeter. This means that if we actually touch our Gamma Scout to the orange it will only see 1/200th of the emitted radiation. This means that the Gamma Scout sees a dose rate from the orange of 0,01uSv/h.
And now we're beginning to see the problem. Even without background raditation that translates to only 9-10 ticks per MINUTE. You'd be waiting 6s between each tick. This is clearly not the movie-type-buzz you would expect from holding a Geiger counter to an orange as heavily contaminated as this one. And remember that I've grossly overestimated (by several orders of magnitude) the dose rate of the orange.
Now if you do not actually touch the orange but measure from several centimeters away, the strength of the radiation decreases quadratically.
But it gets worse.
According to (sorry, web site in German)
http://www.chetan.homepage.t-online.de/sonstig/DETEKT.HTM
the normal average value
Full mouth radiographs once a year, individual radiograph on any tooth that's symptomatic, and again to document a root canal is finished.
Apocalypse Cancelled, Sorry, No Ticket Refunds
There are enough common or natural sources to
enable the testing of a geiger counter. Some of them
are already posted so I will not add to the list.
A GM tube triggers on an ionizing event and
the resulting cascade is counted and/ or integrated
and shown on a display. The cascade produces
a dead time so the higher the count rate the more
the detector is unavailable. This can be quantified with
a modest source by placing it measured distances
away and using the inverse square rule to predict the
count. Departure from recorded to predicted let you
calibrate the dead time.
Most GM tubs have a glass envelope that is thick
enough to shield from alpha particles. Beta radiation
can be shielded by a modest shield. Gamma is
much less attenuated. Gamma:Beta ratios can permit
some approximation of the material that is being measured.
Sensitivity depends on volume, large tubes can be
more sensitive, small tubes are more durable.
The nature of the cascade and quenching effectively makes
all counts equal so the energy spectrum is not available
for the most part. Scintillation counters can be better
for energy spectrum determinations but outside the lab
this is rare.
For calibration the critical data involves the volume of the
detector and efficiency. Volume is easy, efficiency
can be evaluated by source variations often made
by changes in distance. However a modest source
like vaseline glass can be calibrated and then the
unknown detector validated against it.
For the majority of folk changes are more interesting
perhaps important than absolute magnitude.
Truth is stranger than fiction, but it is because Fiction is obliged to stick to possibilities; Truth isn't. Mark Twain.
I’d much prefer to use my key-chain Geiger Counter and know the actual radiation levels not just what I'm being told!