How does a Geiger counter detect radiation, and why does it make that clicking noise?
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Portable cloud chamber. What is a cloud chamber, you ask? It is an open space, a box of sorts, filled with a gas (inert gas like argon, usually). Most commonly with geiger counters, the “box” is a tube (shaped like a toiler paper roll), but it does not have to be (can be rectangular; look up muon drift chambers which are the same idea). The walls of the tube are metal, and the very center of the tube has a thin metal wire. High voltage is applied so there is a big charge difference from wall to wire. The inert gas does not normally allow any current to flow (no ions in the gas). Like the two poles of a battery that have no wire connection between them: there is still a charge difference between poles but nothing to carry any electrons from one pole to the other.
When energy from radioactive decay passes through the gas, it strips electrons off some atoms in the gas (creating a positively charged ion). The voltage between walls and wire cause the charged ion to migrate to the wire. There is a small current that results from this; the current comes from an electron or electrons neutralizing the charge of the positive ion from the wire. Also, the stripped electrons will migrate to the walls, the opposite direction of flow of the positive ions of gas atoms.
Electronics in the geiger counter amplify the current (sort of like a volume knob on a radio, the amount of gain (amplification of current) can be adjusted). That current is linked to a speaker, which is where you get a clicking noise (each click is because of the passage of electrical current that neutralizes the charged ion in the cloud chamber). There is also a gauge of some sort that indicates the intensity of the current.
A geiger counter measures “ionizing radiation”. Not just any radiation, not visible light or infra-red, which is too low in energy to strip electrons from the inert gas atoms, but only energy that is high enough to knock an electron or two away from an atom: gamma and xray radiation that is the hazard from radioactivity. Ionizing radiation will do the same to the atoms in your body if it passes into it. That is not generally very good for the person.
The basic idea is that the geiger counter is a simple counter of electrons, with each counted electron accounting for the neutralization of a charged atom of gas in the chamber. Each click is an event where a charged ion has been neutralized. Lots of clicks real fast, means lots of ionizing radiation is passing through the cloud chamber (and thus also through you), so noisy geiger counters are telling you it is very dangerous here.
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