Geiger-Müller CounterThis is an instrument which is used to measure and detect forms of ionising radiation (which includes alpha α particles, beta β particles and gamma γ rays.
How a Geiger Counter Works
The counter works on the "principle of gas multiplication". Ionisation in the gas is caused by the entry of photon or a particulate radiation. The ions are attracted to their appropriate electrode (i.e. cation to cathode, anion to anode) and thy gain sufficient energy to eject electrons from the gas atoms as they pass through the gas. This causes the atoms to ionise. Therefore electrons are produced continuously by this process and rapid gas multiplication takes place (especially in the central electrode because of its strong electric field strength). The effect of "gas multiplication" is that more than one million electrons are collected by the central electrode for every ion produced in the primary absorption process. Theses "electron avalanches" create electric pulses which then can be amplified electronically and counted by a meter to calculate the number of initial ionisation events. Therefore a Geiger counter is able to detect low-energy radiation because even one ionised particle produces a full pulse on the central wire.
The appropriate potential difference to be applied to a Geiger-Müller tube is determined by plotting a graph of "count rate" obtained when a radioactive source is placed near the tube versus the applied voltage.
The graph contains three distinct regions
Stucture of the Geiger Counter
A Geiger-Müller tube consists of a glass envelope containing a low-pressure gas (usually a mixture of methane with argon and neon) and two electrodes, with a cylindrical mesh being the cathode and a fine-wire anode running through the centre of the tube. A potential difference of +1,000 volts relative to the tube is maintained between the electrodes, therefore creating a strong electric field near the wire.