We all know the clicking of Geiger counters from movies, and we all know that the faster the counter clicks, the worse things are. But how exactly do they work?
There are three types of radiation, alpha, beta, and gamma. They all come from unstable molecules breaking down and emitting energy or energetic particles. Alpha particles are basically helium nuclei – two protons and two neutrons stuck together. They are stopped by skin or paper and don’t do much damage. Beta particles are fast-moving electrons or positrons (which have a positive charge but are the size of electrons). They can penetrate tissue, but only shallowly. Gamma radiation is high-energy gamma rays that are emitted during most changes in an atom’s nucleus. These are the nastiest type of radiation, and can go right through living tissue and penetrate deep into lead. These gamma rays crash into atoms in the atmosphere and rip the electrons off them, leaving behind loose electrons and positively-charged ions.
Geiger counters have different designs, but they all need gas and electricity. A Geiger counter is generally a tube filled with a certain gas. Usually this is a noble gas, which doesn’t tend to react with the molecules around it. The gas is in contact with a charged surface. Sometimes the charged surface is a wire running down the center of the tube. Sometimes metal plates in the tube are held at different voltages.
One end of the tube is covered by a lightweight material, like plastic or ceramic. This allows charged particles, or radioactive particles, and gamma rays to get in. Gamma rays ionize the gas. When they do so, the electrons head for the positively charged part of the counter while the positively-charged particles head for the negatively charged part of the counter. Electric current flows between the negative and positive parts of the counter, and that current can be measured. The measured current is sent through a loudspeaker, and we hear clicks.
The Geiger counter can measure all kinds of radiation, and can be set to different sensitivity levels – so it isn’t going crazy every time someone points it at a banana. Depending on the sensitivity level of the Geiger counter, a lot of clicks might not be terribly bad. It’s just not good.