Editing Geiger–Müller tube (section)

== Quenching and dead time ==
[[File:Dead time of geiger muller tube.png|thumb|Dead time and recovery time in a Geiger–Müller tube.<ref name="knoll"/> The tube can produce no further pulses during the dead time, and only produces pulses of lesser height until the recovery time has elapsed.]]
The ideal G–M tube should produce a single pulse for every single ionizing event due to radiation. It should not give spurious pulses, and should recover quickly to the passive state, ready for the next radiation event. However, when positive argon ions reach the cathode and become neutral atoms by gaining electrons, the atoms can be elevated to enhanced energy levels. These atoms then return to their ground state by emitting photons which in turn produce further ionization and thereby spurious secondary discharges. If nothing were done to counteract this, ionization would be prolonged and could even escalate. The prolonged avalanche would increase the "dead time" when new events cannot be detected, and could become continuous and damage the tube. Some form of quenching of the ionization is therefore essential to reduce the dead time and protect the tube, and a number of quenching techniques are used.

===Gas quenching===
Self-quenching or internal-quenching tubes stop the discharge without external assistance, originally by means of the addition of a small amount of a polyatomic organic vapor originally such as butane or ethanol, but for modern tubes is a halogen such as bromine or chlorine.<ref name="knoll"/>

If a poor gas quencher is introduced to the tube, the positive argon ions, during their motion toward the cathode, would have multiple collisions with the quencher gas molecules and transfer their charge and some energy to them. Thus, neutral argon atoms would be produced and the quencher gas ions in their turn would reach the cathode, gain electrons therefrom, and move into excited states which would decay by photon emission, producing tube discharge. However, effective quencher molecules, when excited, lose their energy not by photon emission, but by dissociation into neutral quencher molecules. No spurious pulses are thus produced.<ref name="knoll"/>

Even with chemical quenching, for a short time after a discharge pulse there is a period during which the tube is rendered insensitive and is thus temporarily unable to detect the arrival of any new ionizing particle (the so-called ''dead time''; typically 50–100 microseconds). This causes a loss of counts at sufficiently high count rates and limits the G–M tube to an effective (accurate) count rate of approximately 10<sup>3</sup> counts per second even with external quenching. While a G-M tube is technically capable of reading higher count rates before it truly saturates, the level of uncertainty involved and the risk of saturation makes it extremely dangerous to rely upon higher count rate readings when attempting to calculate an equivalent radiation dose rate from the count rate. A consequence of this is that [[ion chamber]] instruments are usually preferred for higher count rates, however a modern external quenching technique can extend this upper limit considerably.<ref name="knoll"/>

===External quenching===
External quenching, sometimes called "active quenching" or "electronic quenching", uses simplistic high speed control electronics to rapidly remove and re-apply the high voltage between the electrodes for a fixed time after each discharge peak in order to increase the maximum count rate and lifetime of the tube. Although this can be used instead of a quench gas, it is much more commonly used in conjunction with a quench gas.<ref name="knoll"/>

The "time-to-first-count method" is a sophisticated modern implementation of external quenching that allows for dramatically increased maximum count rates via the use of statistical signal processing techniques and much more complex control electronics. Due to uncertainty in the count rate introduced by the simplistic implementation of external quenching, the count rate of a Geiger tube becomes extremely unreliable above approximately 10<sup>3</sup> counts per second. With the time-to-first-count method, effective count rates of 10<sup>5</sup> counts per second are achievable, two orders of magnitude larger than the normal effective limit. The time-to-first-count method is significantly more complicated to implement than traditional external quenching methods, and as a result of this it has not seen widespread use.<ref name="knoll"/>