Cheap Geiger counter / Dosimeter - what to look for?

[TinkeringSteve]

Probably the first question is: What do you want to do with it?

That's already difficult
Can devices like this tell me differences in radiation of food items (that do not come out of Chernobyl), *as well* as something like "did this reported reactor incident produce anything  of concern over here", or even "this spot in the woods of godknowswhere is dangerous when staying in it for an hour or less"?
Or are these things more narrow than that...
I have zero experience with this topic, and was just delighted to see that item, randomly, with a below 3 digit price tag.
But maybe that's not really grounds for joy, I have no idea

(this is just an example, maybe there's something cooler on aliexpress with more precise specs or so)
https://www.amazon.de/gp/product/B0BHJ3RJC2

[Someone]

Can devices ... below 3 digit price tag.

No

[David Hess]

Any Geiger counter with an exposed window for detection of alpha and beta particle is sufficient for those applications, but will be too sensitive to detect dangerous levels of ambient radiation like a survey meter will.

For detecting remote nuclear events, air is drawn through a filter media, like cheesecloth, for a couple hours and then the filter media is held up to the Geiger counter's window for detection.

[DavidKo]

Solution is to have the window with shutter in front of it the counting tube. In case that the radiation is too strong, you close the shutter. Look for the army surplus items, they would be cheap and robustly build. In our country we have this (without Sr source). Communist device which uses western counter tube Philips 18504  .

[Kleinstein]

The radiation levels from food, even if relatively contaminated from Chernobyl of fucushima are still pretty low.  A Geiger counter would not be useful there.  This would need more like a scitillator counter and good shielding and experience in how to look at the results.

A geiger counter can be OK to decect things like radiation leaks at an X-ray machine or electron microscope. It may help to find some minarals that are a bit more radioactive. But even than without the knowledge to make sense of the counts it makes pretty little sense.

[DavidKo]

It will be hard to calibrate such a device - you need radioactive material with known spectrum for calibration. In university we had used Americium and Strontium, but such a materials are under control of agencies for nuclear safety. If you will be able to get it, than you will have lot of fun with photomultiplier tube used for the signal amplification. I have doubts that someone can get something more modern for "home" use. In case that you can measure energy of each pulse, than with calibration you can find what you are looking at (from the histogram of detected energies).

IT-65 is able to catch decay from lead (it is always slightly radioactive due to the isotope which is in nature), so it can be somehow useful, in case the intensity is too high than the shutter in the front of detector tube should can be closed. When is something slightly radioactive it starts "ticking". What you need is counter tube with widow like mentioned Philips tube, which is able to detect Alpha particles. I suspect that eastern block was not able to produce such a tubes and thus they were imported from west. Commonly available Geiger counter tubes on ebay are useless. They are made from metal and the sensitivity is poor even for bigger ones.

[DavidKo]

I forgot to mention one thing. It is not so much important the principle of radiation detection. What is important is the size of the detector - one click means one decayed atom and it is only about if one can catch its particle. So for weak signals helps to get closer to the subject or get bigger chamber to catch more particles. The signal itself cannot be amplified, you get ionization/scintillation from one decayed atom or not. The spectrum itself can be get by integration over the time from scintillation detector. At high radiation levels one can integrate with RC and by damping of analog meter, for low radiation levels the counting over time is necessary. The natural background is going against the integration time, it creates false signal and need to be subtracted (here can help the scintillation detector since it gives the information about energy of detected particles and one can get energy spectrum of radiation, but since WW2 there is a lot of artificial atoms in nature).

One detector which make sense mentioning is the fog chamber. It is very sensitive, does not block alpha radiation and it is "easy" to scale in 2D. With perpendicular magnetic field it is possible to estimate energy of  charged particles.

[Kleinstein]

Making a geiger counter tube is not that complicated. This is more low tech that even the Soviets could handle. A problem can now be that the old Soviet time tubes may no longer work that well, as there could be some gas leaking over the long time (30+ years). Some types may have a limites shelf life and quality control was often not good to start with - selling factory rejects is not a Chinese invention.

Getting an energy calibration for the Szintillatoris not that difficult.  The PMT gain is what needs calibration and a check from time to time, the Scintillators are pretty fixed gain though the geometry factors are a bit hard to calculate. Low level sources are available as normal chemicals with natural radiactive isotopes (e.g. potassium salt with a weak, but strong enough gamma radiation at some 1.6 MeV as the dominant signal at relatively high energy, which is handy for a 1st test). There are other "natural sources to check", but many have multiple lines and may need a good starting point.

With the high sensitivity (something like 100-1000 times that of a geiger tube) one does not need a strong source. The good sensitivity and energy resolution is needed to measure food samples. Even than it takes hours, good shielding and careful analysis (e.g. variable background) to get a good result.

[iMo]

The ebay's soviet metal type GM tubes work fine even with "low radiation sources".
Try it with a WWII wrist watch or a WWII dial from a meter (from an airplane for example) and pulses ratio against the background will easily be 100..1000. I can confirm herewith..
It cannot detect alpha. I did with the Philips too (with the front window made of mica or something like that) and the Philips is more sensitive.