How does a Geiger counter detect radiation?

The heart of the instrument is the Geiger–Müller tube, a sealed tube filled with a low-pressure inert gas such as neon, helium or argon, with a high voltage between a central wire and the tube wall. When ionising radiation, such as alpha or beta particles or gamma rays, enters the tube, it ionises some of the gas atoms. The freed electrons race towards the wire, knocking more electrons loose along the way in an avalanche that produces a measurable pulse. Each pulse is counted and often heard as a click, and the rate is shown as counts per minute. Hans Geiger developed an early detector in 1908 while working with Ernest Rutherford, and in 1928 he and Walther Müller improved it into the Geiger–Müller tube.