Editing Geiger counter (section)

==Types and applications==
[[File:Geiger counter 2.jpg|thumb|Geiger counter with pancake type probe]]
[[File:Geiger counter in use.jpg|thumb|Laboratory use of a Geiger counter with end-window probe to measure beta radiation]]
The intended detection application of a Geiger counter dictates the tube design used. Consequently, there are a great many designs, but they can be generally categorized as "end-window", windowless "thin-walled", "thick-walled", and sometimes hybrids of these types.

===Particle detection===
The first historical uses of the Geiger principle were to detect α- and β-particles, and the instrument is still used for this purpose today. For α-particles and low energy β-particles, the "end-window" type of a Geiger–Müller tube has to be used, as these particles have a limited range and are easily [[Stopping power (particle radiation)|stopped]] by a solid material. Therefore, the tube requires a window which is thin enough to allow as many as possible of these particles through to the fill gas. The window is usually made of [[mica]] with a density of about 1.5–2.0&nbsp;mg/cm<sup>2</sup>.<ref name="cent" />

α-particles have the shortest range, and to detect these the window should ideally be within 10&nbsp;mm of the radiation source due to α-particle [[attenuation]].<ref name="cent" /> However, the Geiger–Müller tube produces a pulse output which is the same magnitude for all detected radiation, so a Geiger counter with an end window tube cannot distinguish between α- and β-particles.<ref name="knoll" /> A skilled operator can use varying distance from a radiation source to differentiate between α- and high energy β-particles.

The "pancake" Geiger–Müller tube is a variant of the end-window probe, but designed with a larger detection area to make checking quicker. However, the pressure of the atmosphere against the low pressure of the fill gas limits the window size due to the limited strength of the window membrane.

Some β-particles can also be detected by a thin-walled "windowless" Geiger–Müller tube, which has no end-window, but allows high energy β-particles to pass through the tube walls. Although the tube walls have a greater stopping power than a thin end-window, they still allow these more energetic particles to reach the fill gas.<ref name="cent" />

End-window Geiger counters are still used as a general purpose, portable, [[radioactive contamination]] measurement and detection instrument, owing to their relatively low cost, robustness and relatively high detection efficiency; particularly with high energy β-particles.<ref name="knoll" /><ref>{{Cite news|url=https://xn--messgert-test-hfb.de/geigerzaehler/|title=G-M detector function and measuring methods|access-date=2017-03-07}}</ref> However, for discrimination between α- and β-particles or provision of particle energy information, [[scintillation counter]]s or [[proportional counter]]s should be used.<ref name = "ukhse"/> Those instrument types are manufactured with much larger detector areas, which means that checking for surface contamination is quicker than with a Geiger counter.

===Gamma and X-ray detection===
Geiger counters are widely used to detect [[gamma radiation]] and [[X-rays]], collectively known as [[photons]], and for this the windowless tube is used. However, detection efficiency is low compared to alpha and beta particles.
The article on the [[Geiger–Müller tube]] carries a more detailed account of the techniques used to detect photon radiation. For high energy photons, the tube relies on the interaction of the radiation with the tube wall, usually a material with a high [[atomic number]] such as [[stainless steel]] of 1–2&nbsp;mm thickness, to produce free electrons within the tube wall, due to the [[photoelectric effect]]. If these migrate out of the tube wall, they enter and ionize the fill gas.<ref name="knoll" />

This effect increases the detection efficiency because the low-pressure gas in the tube has poorer interaction with higher energy photons than a steel tube. However, as photon energies decrease to low levels, there is greater gas interaction, and the contribution of direct gas interaction increases. At very low energies (less than 25 [[kiloelectronvolt|keV]]), direct gas ionisation dominates, and a steel tube attenuates the incident photons. Consequently, at these energies, a typical tube design is a long tube with a thin wall which has a larger gas volume, to give an increased chance of direct interaction of a particle with the fill gas.<ref name="cent" />

Above these low energy levels, there is a considerable variance in response to different photon energies of the same intensity, and a steel-walled tube employs what is known as "energy compensation" in the form of filter rings around the naked tube, which attempts to compensate for these variations over a large energy range.<ref name="cent" /> A steel-walled Geiger–Müller tube is about 1% efficient over a wide range of energies.<ref name="cent">’’Geiger Muller Tubes; issue 1’’ published by Centronics Ltd, UK.</ref>

===Neutron detection===
[[File:boroncounter.svg|lang=en|thumb|Geiger tube filled with BF<sub>3</sub> for detection of thermal neutrons]]
A variation of the Geiger tube can be used to exclusively measure radiation dosage from [[neutron]]s rather than from [[Gamma ray|gammas]] by the process of [[neutron capture]]. The tube, which can contain the fill gas [[boron trifluoride]] or [[helium-3]], is surrounded by a plastic moderator that reduces neutron energies prior to capture. When a capture occurs in the fill gas, the energy released is registered in the detector.
[[File:A complete Geiger counter, with the Geiger-Muller tube 70 019.jpg|thumb|A modern one-piece Geiger–Müller counter, including Geiger–Müller tube type 70 019 (at the top)]]

===Gamma measurement—personnel protection and process control===
While "Geiger counter" is practically synonymous with the hand-held variety, the Geiger principle is in wide use in installed "area gamma" alarms for personnel protection, as well as in  process measurement and interlock applications. The processing electronics of such installations have a higher degree of sophistication and reliability than those of hand-held meters.

===Physical design===
[[File:Geiger tube si8b.jpg|thumb|right|Pancake G-M tube used for alpha and beta detection; the delicate mica window is usually protected by a mesh when fitted in an instrument.]]
For hand-held units there are two fundamental physical configurations: the "integral" unit with both detector and electronics in the same unit, and the "two-piece" design which has a separate detector probe and an electronics module connected by a short cable.

In the 1930s a mica window was added to the cylindrical design allowing low-penetration radiation to pass through with ease.<ref name="Korff, SNTM 2012">Korff, SNTM (2012) 20: 271. {{doi|10.1007}} / s00048-012-0080-y</ref>

The integral unit allows single-handed operation, so the operator can use the other hand for personal security in challenging monitoring positions, but the two piece design allows easier manipulation of the detector, and is commonly used for alpha and beta surface contamination monitoring where careful manipulation of the probe is required or the weight of the electronics module would make operation unwieldy. A number of different sized detectors are available to suit particular situations, such as placing the probe in small apertures or confined spaces.

Gamma and X-Ray detectors generally use an "integral" design so the Geiger–Müller tube is conveniently within the electronics enclosure. This can easily be achieved because the casing usually has little attenuation, and is employed in ambient gamma measurements where distance from the source of radiation is not a significant factor. However, to facilitate more localised measurements such as "surface dose", the position of the tube in the enclosure is sometimes indicated by targets on the enclosure so an accurate measurement can be made with the tube at the correct orientation and a known distance from the surface.

There is a particular type of gamma instrument known as a "hot spot" detector which has the detector tube on the end of a long pole or flexible conduit. These are used to measure  high radiation gamma locations whilst protecting the operator by means of distance shielding.

Particle detection of alpha and beta can be used in both integral and two-piece designs. A pancake probe (for alpha/beta) is generally used to increase the area of detection in two-piece instruments whilst being relatively light weight. In integral instruments using an end window tube there is a window in the body of the casing to prevent shielding of particles.  There are also hybrid instruments which have a separate probe for particle detection and a gamma detection tube within the electronics module. The detectors are switchable by the operator, depending the radiation type that is being measured.

===Guidance on application use===
In the [[United Kingdom]] the [[National Radiological Protection Board]] issued a user guidance note on selecting the best portable  instrument type for the radiation measurement application concerned.<ref name="ukhse">[http://files.site-fusion.co.uk/webfusion117640/file/instrumentsirp7.pdf.pdf] {{Webarchive|url=https://web.archive.org/web/20180730235921/http://files.site-fusion.co.uk/webfusion117640/file/instrumentsirp7.pdf.pdf|date=2018-07-30}} Selection, use and maintenance of portable monitoring instruments. UK HSE</ref> This covers all radiation protection instrument technologies and includes a guide to the use of G-M detectors.