The Gieger Muller Tube
The first transducer we’re looking at as part of our series on transducers is the GM Tube. You might not know what a GM Tube is, or even what they look like, but you almost certainly know what one sounds like.
A slow-clicking sound in the background suddenly rises to a crescendo when the sensor is pointed at something highly radioactive. The GM Tube is a transducer that converts ionising radiation to electrical pulses. They have been used in some of the most hostile environments on this planet. When connected to a speaker (another transducer I might add) we get an instrument that can be used to measure radiation rates: The Gieger Counter.
The GM tube is a fascinating little device, relatively simple and practical to make yet extremely effective. The first tube was developed in 1928 by Hans Gieger and Walther Muller, hence the name GM. Gieger had developed the theoretical principles of operation in 1908, hence why he got to put his name first, and the final instrument is often named after him.
Ionising radiation can create ions, the clue’s in the name. These ions are created by throwing off an electron from the electron shell of an atom. The tube works by capturing these electrons in a strong electric field formed by the outer sheath of the tube and a rod down the center. A single electron can’t create enough of a pulse to be detectable by practical means (at least in 1928) and here’s where the clever bit is. The tube is filled with low-pressure gas; As the electrons accelerate through the electric field they, themselves, become ionising* and so, when colliding with the gas create more free electrons, and those can create more free electrons and so on. This effect is aptly named the electron avalanche.
This large group of electrons, many millions to billions of times more numerous than the initial one creates a large detectable pulse which, when connected to a speaker, creates the distinct click. There are several additional effects and techniques that need to be used to ensure that the GM tube can continue to operate.
The astute among you will have realised that for every electron released, a positively charged ion is also created, which will make its way towards the negatively charged sheath of the tube, they do this considerably slower than the electrons however, due to their higher mass. When they make contact with the sheath, they neutralise which allows the tube to continue to function without the gas becoming entirely ionised over time. There is a limit to how quickly this process can take effect and so a tube can become ‘saturated’ in very high energy environments.
So what are they used for? The size of the pulse is largely the same whatever radiation enters the tube, be it alpha, beta, or gamma. Alpha particles aren’t dangerous to humans if they remain outside the body, our skin protects us from them. Alpha and beta will also be blocked by the sheath of the tube, but gamma won’t be, this means that the detector is very directional for alpha and beta sources, but not for gamma. For both these reasons, it can be a disadvantage not being able to tell what kind of radiation is triggering the device. It also means its use as a scientific instrument can be limited.
Due to saturation, in highly radioactive environments the Gieger tube has a limited working life meaning it also has limited use in these scenarios. More sophisticated tube-based detectors, such as ion tubes (only measuring the voltage caused by the initial ions) and proportional detectors which can discriminate in radiation type have become more and more common, but rely on more complex electronics raising their cost. Solid state detectors, easily incorporated into modern electrical devices are also highly useful as dosimeters and will function longer in extremely hostile environments.
However, because they are simple, robust, and low cost the GM Tube still has extensive use cases, especially as a survey tool and a basic detector, in most scenarios, its limitations are not important, simply knowing there is a radiation source present is key.
The audible clicks when connected with a speaker mean that someone operating or making use of it can be working on something else, or concentrating on navigating their environment, confident that if they do encounter a source of radiation, the Gieger counter will give them audible warning. It also allows a quick way to check for contamination and flagging of personnel or vehicles that may need decontamination after exiting potential hot zones.
Here at Flintmore, we haven’t had the opportunity to work with a GM tube or any other type of ionising radiation detector. However, we find them to be an extremely fascinating type of transducer. They are an excellent example of how interesting physics can be used to measure and quantify phenomena from the world around us and then put to use.
- Beta radiation, or beta particles, one of the types of ionising radiation are simply electrons moving at high speed.
Further Reading
Generally I find articles on Gieger Counters are either slightly over simplistic, contain misleading information or are behind paywalls. Despite it’s reputation, the wikipedia resources available are generally of a high standard and are well maintained, if not always optimised for accessibility. I have found this with many differen’t transducer types:
https://en.wikipedia.org/wiki/Geiger%E2%80%93M%C3%BCller_tube
https://en.wikipedia.org/wiki/Geiger_counter
For an accessible high quality articles check out:
https://www.explainthatstuff.com/how-geiger-counters-work.html
Which even has some information on building your own!
