Geiger - Mueller (GM) counter It consists of a chamber filled with an inert gas at low pressure with two electrodes. The negative electrode is the cylindrical metallic body of the chamber and the positive electrode is a coaxial thin wire inside the chamber. An ionizing particle which enters the chamber through a window, ionizes the gas. The number of ions collected versus the voltage between the electrodes is shown in fig. g3. In the flat region called the plateau, an ion produces an ‘avalanche’ , by cascade mechanism. This gives rise to pulse in the outer circuit, which is recorded by a counter. The discharge is quenched rapidly in time ~ 10 ^-4s which enables the counter to detect ~ 1000 particles per second.