Editing Geiger counter (section)

==Principle of operation==
[[File:Geiger-Muller-counter-en.png|thumb|upright=1.2|Diagram of a Geiger counter using an "end window" tube for low-penetration radiation. A loudspeaker is also used for indication.]]
{{Main|Geiger–Müller tube}}
A Geiger counter consists of a Geiger–Müller tube (the sensing element which detects the radiation) and the processing electronics, which display the result.

The Geiger–Müller tube is filled with an inert gas such as [[helium]], [[neon]], or [[argon]] at low pressure, to which a high voltage is applied. The tube briefly conducts electrical charge when [[particle physics|high energy particle]]s or [[gamma rays|gamma radiation]] make the gas conductive by ionization.  The ionization is considerably amplified within the tube by the [[Townsend discharge]] effect to produce an easily measured detection pulse, which is fed to the processing and display electronics. This large pulse from the tube makes the Geiger counter relatively cheap to manufacture, as the subsequent electronics are greatly simplified.<ref name="knoll" /> The electronics also generate the high voltage, typically 400–900 volts, that has to be applied to the Geiger–Müller tube to enable its operation. This voltage must be carefully selected, as too high a voltage will allow for continuous discharge, damaging the instrument and invalidating the results. Conversely, too low a voltage will result in an electric field that is too weak to generate a current pulse.<ref>{{cite web |last1=Siegel |first1=Peter |last2=Eskandari |first2=Sephir |title=Introduction to Geiger Counters |url=https://www.cpp.edu/~pbsiegel/bio431/texnotes/chapter4.pdf |archive-url=https://web.archive.org/web/20170221172657/https://www.cpp.edu/~pbsiegel/bio431/texnotes/chapter4.pdf |archive-date=2017-02-21 |url-status=live}}</ref> The correct voltage is usually specified by the manufacturer. To help quickly terminate each discharge in the tube a small amount of halogen gas or organic material known as a [[Geiger–Müller tube#Gas quenching|quenching mixture]] is added to the fill gas.
===Readout===
There are two types of detected radiation readout: [[Counts per minute|counts]] and [[Absorbed dose|radiation dose]].

* The counts display is the simplest, and shows the number of ionizing events detected, displayed either as a count rate, such as "counts per minute" or "counts per second", or as a total number of counts over a set time period (an integrated total). The counts readout is normally used when alpha or beta particles are being detected.
* More complex to achieve is a display of radiation dose rate, displayed in units such as the [[sievert]], which is normally used for measuring gamma or X-ray dose rates. A Geiger–Müller tube can detect the presence of radiation, but not its [[Electronvolt|energy]], which influences the radiation's ionizing effect. Consequently, instruments measuring dose rate require the use of an [[Geiger–Müller tube#Photon energy compensation|energy compensated]] Geiger–Müller tube, so that the dose displayed relates to the counts detected.<ref name="knoll">Glenn F Knoll. ''Radiation Detection and Measurement'', third edition 2000. John Wiley and Sons, {{ISBN|0-471-07338-5}}</ref> The electronics will apply known factors to make this conversion, which is specific to each instrument and is determined by design and calibration.

The readout can be analog or digital, and modern instruments offer serial communications with a host computer or network.

There is usually an option to produce audible [[Clicking noise|clicks]] representing the number of ionization events detected. This is the distinctive sound associated with handheld or portable Geiger counters. The purpose of this is to allow the user to concentrate on manipulation of the instrument while retaining auditory feedback on the radiation rate.

===Limitations===
There are two main limitations of the Geiger counter:
# Because the output pulse from a Geiger–Müller tube is always of the same magnitude (regardless of the energy of the incident radiation), the tube cannot differentiate between radiation types or measure radiation energy, which prevents it from correctly measuring [[dose rate]].<ref name="knoll" />
# The tube is less accurate at high radiation rates, because each ionization event is followed by a "dead time", an insensitive period during which any further incident radiation does not result in a count. Typically, the dead time will reduce indicated count rates above about 10<sup>4</sup> to 10<sup>5</sup> counts per second, depending on the characteristic of the tube being used. <ref name="knoll" /> While some counters have circuitry which can compensate for this, for measuring very high dose rates, [[ion chamber]] instruments are preferred for high radiation rates.