Editing Geiger–Müller tube (section)

== Principle of operation ==
[[File:Detector regions.gif|thumb|300px|Plot of [[ionization|ion pair]] current against voltage for a cylindrical gaseous radiation detector with a central wire anode.]]
[[File:Spread of avalanches in G-M tube.jpg|thumb|300px|Visualization of the spread of [[Townsend avalanche]]s by means of UV photons. This mechanism allows a single ionizing event to ionize all the gas surrounding the anode by triggering multiple avalanches.]]
[[File:Geiger gamma interaction.jpg|thumb|300px|Detection of gamma in a G-M tube  with a thick-walled stainless steel cathode. Secondary electrons generated in the wall can reach the fill gas to produce avalanches. This effect is considerably attenuated at low energies below about 20 KeV <ref name="centronics"/>]]
A G-M tube consists of a chamber filled with a gas mixture at a low pressure of about 0.1 [[Atmosphere (unit)|atmosphere]]. The chamber contains two electrodes, between which there is a potential difference of several hundred [[volt]]s. The walls of the tube are either metal or have their inside surface coated with a conducting material or a spiral wire to form the [[cathode]], while the [[anode]] is a [[wire]] mounted axially in the center of the chamber.

When [[ionizing radiation]] strikes the tube, some molecules of the fill gas are ionized directly by the incident radiation, and if the tube cathode is an electrical conductor, such as stainless steel, indirectly by means of secondary electrons produced in the walls of the tube, which migrate into the gas. This creates positively charged [[ion]]s and free [[electron]]s, known as [[Ionization|ion pairs]], in the gas. The strong electric field created by the voltage across the tube's electrodes accelerates the positive ions towards the cathode and the electrons towards the anode. Close to the anode in the "avalanche region" where the electric field strength rises inversely proportional to radial distance as the anode is approached, free electrons gain sufficient energy to ionize additional gas molecules by collision and create a large number of [[Townsend discharge|electron avalanches]]. These spread along the anode and effectively throughout the avalanche region. This is the "gas multiplication" effect which gives the tube its key characteristic of being able to produce a significant output pulse from a single original ionizing event.<ref name="knoll">Glenn F Knoll. ''Radiation Detection and Measurement'', third edition 2000. John Wiley and sons,  {{ISBN|0-471-07338-5}}</ref>
   
If there were to be only one avalanche per original ionizing event, then the number of excited molecules would be in the order of 10<sup>6</sup> to 10<sup>8</sup>. However the production of ''multiple avalanches'' results in an increased multiplication factor which can produce 10<sup>9</sup> to 10<sup>10</sup> ion pairs.<ref name="knoll"/>  The creation of multiple avalanches is due to the production of UV photons in the original avalanche, which are not affected by the electric field and move laterally to the axis of the anode to instigate further ionizing events by collision with gas molecules. These collisions produce further avalanches, which in turn produce more photons, and thereby more avalanches in a chain reaction which spreads laterally through the fill gas, and envelops the anode wire. The accompanying diagram shows this graphically. The speed of propagation of the avalanches is typically 2–4&nbsp;cm per microsecond, so that for common sizes of tubes the complete ionization of the gas around the anode takes just a few microseconds.<ref name="knoll"/>
This short, intense pulse of current can be measured as a ''count event'' in the form of a voltage pulse developed across an external electrical resistor. This can be in the order of volts, thus making further electronic processing simple.

The discharge is terminated by the collective effect of the positive ions created by the avalanches. These ions have lower mobility than the free electrons due to their higher mass and move slowly from the vicinity of the anode wire. This creates a "space charge" which counteracts the electric field that is necessary for continued avalanche generation. For a particular tube geometry and operating voltage this termination always occurs when a certain number of avalanches has been created, therefore the pulses from the tube are always of the same magnitude regardless of the energy of the initiating particle. Consequently, there is no radiation energy information in the pulses<ref name="knoll"/> which means the Geiger–Müller tube cannot be used to generate spectral information about the incident radiation. In practice the termination of the avalanche is improved by the use of "quenching" techniques (see later).
 
Pressure of the fill gas is important in the generation of avalanches. Too low a pressure and the efficiency of interaction with incident radiation is reduced. Too high a pressure, and the “mean free path” for collisions between accelerated electrons and the fill gas is too small, and the electrons cannot gather enough energy between each collision to cause ionization of the gas. The energy gained by electrons is proportional to the ratio “e/p”, where “e” is the electric field strength at that point in the gas, and “p” is the gas pressure.<ref name="knoll"/>