How does a Geiger counter detect radiation?

A Geiger counter detects radiation by measuring ionising particles that pass through a gas-filled tube.

A Geiger counter, also known as a Geiger-Müller counter, is a device used to detect and measure ionising radiation. It consists of a gas-filled tube, typically filled with a noble gas like argon or neon, and a wire anode in the centre. The tube is sealed and has a thin window that allows radiation to enter. The gas inside the tube is at low pressure, which allows ionising particles to pass through it.

When ionising radiation enters the tube, it ionises the gas atoms by knocking off their electrons. This creates positive ions and free electrons. The free electrons are attracted to the anode, and as they move towards it, they gain energy. If they gain enough energy, they can ionise more gas atoms, creating an avalanche of ionisation known as a Townsend avalanche. This avalanche results in a large number of electrons reaching the anode, creating a pulse of current that can be measured.

The strength of the current pulse is independent of the energy of the radiation that triggered it. This means that a Geiger counter can measure the count rate of radiation (how many particles are detected per unit time) but not the energy of the radiation. To measure the energy, other types of detectors are needed.

The Geiger counter is a popular device for detecting radiation because it is relatively cheap, robust, and easy to use. It can detect alpha, beta, and gamma radiation, although it is most sensitive to beta particles and least sensitive to gamma rays. The thin window at the end of the tube allows alpha particles to enter, but it must be handled with care to avoid damage.

In summary, a Geiger counter detects radiation by measuring the current pulse created when ionising radiation triggers a Townsend avalanche in a gas-filled tube. It is a simple and effective tool for measuring the presence and intensity of radiation, but not its energy.