Geiger-Muller Tubes

By far the most common radiation detector, the Geiger-Müller tube itself is nothing more than a highly modified neon lamp.

Its method of operation is rather simple. The GM tube is a partially evacuated tube containing low pressure inert gas; typically neon, and a small amount of a quench gas; isobutane or a halogen (halogen tubes last forever while isobutane ones have a finite lifetime due to radiation breaking down the organic gas). There is a long cathode wire in the center and a the metal shell is the anode. A high positive voltage is applied to the cathode, this voltage being just under the amount required to ionize the gas in the tube. Usually it’s anywhere from 500 to 900V depending on the tube. When radiation enters the tube it ionizes the gas it transverses, and when a path is made between the cathode and the anode the tube becomes conductive for a moment before the quench gas “puts out the fire.”

There are two ways one can use a GM tube in a circuit:

The first method is to simply use the GM tube in series with an amplifier. When there is an event in the tube it shortly becomes conductive and passes a “blip” onto the amplifier, and while this reduces component count the tube is not properly grounded and it may be possible to receive a small shock from it.

The second method properly grounds the GM tube and the signal is passed onto the amplifier via a small DC blocking capacitor. When there is an event in the GM tube the HV is pulled down to ground and this signal makes it through the capacitor. This is the preferred method of using a GM tube, and in fact I don’t think anyone even uses the other method.

To run a GM tube you’ll need to generate a high voltage at a very low current. Now one could do this using discrete components, but my method is to simply use a CCFL inverter and a Greinacher voltage doubler. By powering this arrangement with an adjustable voltage regulator one can make a variable voltage power supply perfect for a GM tube. Such a circuit also draws very little current and that allows it to be run off a 9V battery for a reasonable amount of time.

As for pulse detection, I find that an LM386 audio amplifier does a good job. This amplifier can directly drive a speaker, though make sure you put a 1000µF DC blocking cap in series with such a speaker or else the amplifier will waste lots of power. The pulses that come out of the amplifier can be counted with a microcontroller.

Sensitivity of GM tubes

Geiger-Müller tubes do not equally respond to all types of radiation, and certain tubes are designed to better respond to certain types. In general there are four different types of GM tubes, each suited for different tasks.

For example, the typical thin metal-walled Russian geiger tube such as the one shown here will not respond to alpha radiation at all, and it will be able to maybe detect high energy beta radiation. It is useful only for gamma and x-radiation, and that is not saying much since GM tubes usually only respond to 3% of incident gamma rays. Aside from simply detecting the presence of gamma and x-radiation these Russian tubes are not much good for anything.

Typical GM tubes respond to only 3% of incident gamma and x-radiation. Certain tubes however, are designed to better detect gamma and x-rays by utilizing a rather thick metal wall. While this may seem counterintuitive, a thick wall actually improves detection efficiency because of the way a GM tube detects this type of radiation. Unlike a beta particle that may start an avalanche in a GM tube, electromagnetic radiation is nothing more than photons; photons that must knock an electron out of the wall  in order to be detected. Thicker wall = more atoms = more electrons to be knocked out.

Both of the aforementioned types of GM tubes are useless for detecting alpha radiation such as that emitted by the 241Am found in smoke alarms. In order to detect such a large particle the tube needs to be transparent to them, and the only thing that is both transparent to most alpha particles and is strong enough to hold back a vacuum is mica. Such mica-windowed GM tubes cost more than their all-metal counterparts, but for alpha detection and beta measurement it’s the only tube suited for the task. Be careful though, a mica window is very fragile and may pop if touched!

The last type of Geiger-Müller tube is also a mica-window one, but unlike the other GM tubes which are cylindrical this looks like a pancake. Aptly named pancake tubes, these detectors also have a mica window, though it’s much larger than that of an end window tube. Thus, they much more sensitive to low levels of radiation than any of the other types. Their downfall is the fact that pancake tubes are ridiculously fragile; simply looking at it in an ugly way may be enough to break the mica window.

Mica window GM tubes are harder to find than other types of GM tubes but occasionally they will pop up on auction sites. If you are really bent on getting your hands on one though LND makes some great quality GM tubes, even if they are a little pricey.

GM tubes have a dead time; the time it takes for the quench gas to extinguish the discharge inside the tube. Typically this dead time is in the microseconds range, and it varies for each tube. During the dead time the tube is not able to detect any incoming radiation and the dead time also puts a limit on the maximum amount of radiation a GM tube can detect. When there is too much radiation bombarding the tube it saturates and the quench gas is unable to stop the discharge. This is dangerous because a saturated GM tube will read 0 on a counter and produce no clicks, so you may be exposed to dangerous levels of radiation and not know it. Fortunately it takes a lot of radiation to saturate a GM tube, the kind of radiation one would find in an x-ray beam or next to a nuclear reactor core. Therefore unless you are 100% clueless as to what is going on around you, saturation isn’t too much a problem.

When it comes to x-rays GM tubes have a very non-linear response, so although they are great for detecting the presence of x-rays they are terrible for making measurements. Just something to keep in mind.

Below I have a video of a mica-windowed GM tube detecting the radiation from different sources. First some DU bits, second a coleman lantern mantle, third a piece of fiestaware and fourth some 241Am from a smoke alarm. Notice that the alpha source must be brought very close to the mica window for the alpha particles to reach the tube.

I’m always on the lookout to buy radioactive things, so if you have anything you’d like to get rid of feel free to contact me. ∎