The Geiger counter consists of a metal cylinder — the cathode — and a thin wire stretched along its axis — the anode, enclosed in a sealed volume filled with gas (usually argon) at a pressure of about 100-260 GPA (100-260 mm Hg). A voltage of about 200-1000 V is applied between the cathode and the anode. A charged particle, once in the volume of the counter, forms a certain number of electron-ion pairs that move to the corresponding electrodes and, at a high voltage along the free path (on the way to the next collision), gain energy that exceeds the ionization energy and ionize the gas molecules. An avalanche is formed, the current in the circuit increases. From the load resistance, a voltage pulse is applied to the recording device. A sharp increase in the voltage drop on the load resistance leads to a sharp decrease in the voltage between the anode and cathode, the discharge stops, and the tube is ready to register the next particle.
The Geiger counter registers mainly electrons and gamma-quanta (the latter, however, with the help of additional material applied to the walls of the vessel, from which the gamma-quanta knock out electrons).