Geiger-Muller Counter

It is convenient for the researcher to have at her/his disposal, a device for measurement of ionizing radiation flux produced by whatever she/he is working on. Then again, it's also fun just to play with for no good reason.

There are many plans online for Geiger-Muller counters. Unfortunately, many of them seem to fall prey to one or more problems, the most common of which are:

-- A complete schematic is not given, including G-M tube, HV generator, and audio/visual output circuitry

-- The G-M tube is not properly shielded, and cannot be easily used in a realistic lab environment

-- Obscure parts are used (not counting the necessarily obscure G-M tube itself)

What I have tried to do here is take a few tried and true circuit designs, and meld them into something simple and easily constructed with reasonably readily available parts. It is not the most impressive thing, but it does work, and quite well. I'll give the schematic first, and then discuss it in further detail down the page:

The basic charging circuit produces in excess of 500V+, depending on the inductor used, leakage of various components, etcetera. The current output is limited by a 10Mohm resistor, which keeps short-circuit current under ~60uA. Thus this is a relatively safe project. The transistors used are by no means sacred; this design can be seen in several places across the web, and no two schematics use similar transistors. The 2N3904 could be replaced with a 2N2222, the 2N3906 with a 2N2907, and so on. The 2N6517 is rated at 350V collector-emitter, so a substitute NPN transistor should meet these specifications. If in doubt, download some datasheets and see what looks like it will work.

All four diodes should be rated for at least 600V. 1N4007's are common, but anything similar can be used. The 15mH choke is a small, 0.75" x .2" diameter epoxy encapsulated inductor with a ferrite core. Anything around this value should work. If all you've got is a few 4.7mH, just put two or three in series. The 0.01uF capacitors should be rated for 300V or so. Given the low currents involved, this may not be absolutely necessary.

For the 'voltage regulator', two NE-2 lamps are used; I've seen versions of this which used harder to acquire zener diodes. In my experience, all zeners do is cut down on power consumed by the supply, which here at United Neko is the least of concerns. You are most likely not going to be using this in a post-Apocalyptic world where batteries are scarce. The neons also look a bit more impressive as they gently flicker in their distinctive orange hue.

The audio stage is simply an LM386N audio amplifier chip, probably one of the most common out there. If you don't like this chip (many people do not, and it does give me hell at times), you could use an amp made from discrete transistors. Doesn't really matter, as long as it can pick up the click from the counter tube. The 68pF capacitor cuts down on noise (particularly 60 cycle hum) entering the audio stage, while easily passing the high-risetime impulse of a detection 'count.' It should be rated for at least 600V, more if available, as it also keeps HVDC from the charging supply/G-M tube anode from reaching the amplifier input, while passing AC signals. Additional shielding which is recommended is to build the entire circuit (battery as well) inside of a metal box, preferably steel, and have the metal enclosure common with circuit ground. An RCA jack mounted on the side of this enclosure allows a shielded coaxial cable of about 3 foot length to connect with the G-M tube itself. This has the advantage of further shielding the counter signal from outside spurious influence, allows the tube to be enclosed in a wand for easy manipulation by hand, and adds a measure of isolation of the anode supply wire. The speaker was obtained from a scrapped handheld FM radio, and is about 1 1/2" diameter.

The hardest part to obtain is also the key to the whole thing: the Geiger-Muller tube itself. There are a few different variants available; some are entirely composed of glass. These will work well for detecting gamma and X-rays, but will not allow you to detect alphas or much beta. They are cheap, however, and readily available on places such as Ebay. But if you wish to get the most out of your counter, I would suggest you get what is called an end-window tube. These are generally made of metal, but have a very thin window of some material on one end. This in itself gives the tube a sense of directionality; rays entering from the direction of the window will have a greater count-inducing effect, and thus you will be more easily able to tell from what direction the radioactive emission is coming from. Among the best of these end-window tubes are the ones whose windows are composed of a very thin sheet of mica. It is permeable enough to radiation that even alphas can easily penetrate and be detected. The tube I used in this design can be obtained for a reasonable price from Images Scientific Equipment, part number GMT-01, at this link.

The tube was placed inside a 1/2" inner diameter PVC pipe length to offer it a measure of protection and to provide a more hand-comfortable 'wand' shape. (NOTE: these tubes are FRAGILE, particularly the mica end-window. Don't stab it with a ball-point pen or some other sharp thing and expect it to like this.) An RCA jack was mounted in a cap on the far end, the internal anode connection going to the jack's inner conductor, the cathode/tube envelope going to the shield of the jack.

When switched on, the speaker should produce a faint, repetitive tapping sound. This is an artifact of the charging supply, and is to be ignored; when you hear radiation counts, they are quite different, having a very sharp clicking or popping sound when individually detected. When close to a source of radiation, the output may sound something like heavy rain on metal.

Even without any source of radiation, a few counts will be heard here and there. These are the result of a combination of factors, from ambient radioisotopes in the surroundings (your body included), traces of radon gas in the air (if any), and high-energy cosmic rays originating at a source which may lie light-years away.

What you choose to do with the counter is up to you. Please do not do anything illegal.

Maybe you have some Uranium ore you obtained from a rock dealer you'd like to experiment with. Maybe you're experimenting with X-rays. Whatever the case may be, always be careful. You can still have fun whilst making sure you are not excessively shredding your body's DNA into component nucleotides.

UPDATE: Error in schematic corrected.

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