4.3.1.3 Geiger Müller counters

A Geiger-Müller counter consists of a gas-filled tube equipped with a metal cylinder (the cathode) and a thin conductive wire (the anode) mounted along the tube axis. Normally argon, with an admixture of halogen vapor, is used as gas filling. GM counters are 2 to 30 cm long and 1 to 4 cm in diameter, and they operate with applied voltage of several hundred volts.

GM counters make use of the progressive growth of ionization in a strong electric field between the anode and the cathode. An incident photon interacts with the cathode and releases an electron that may be directed into the GM tube. The growth of ionization between the anode and the cathode amplifies the signal and generates an electric current between the electrodes. This results in a voltage pulse at the anode output of the GM counter. The multiplication coefficient of the gas ionizing chain reaction is of the order of 1066, and the output pulse is not proportional to the absorbed gamma ray energy. The detection efficiency of GM counters is very low (less than 2%) and the dead time is of the order 10^-4 s. If a light material is used to produce a thin entrance window, GM can detect beta particles as well.