Editing Geiger–Müller tube (section)

==Gas mixtures==
The components of the gas mixture are vital to the operation and application of a G-M tube. The mixture is composed of an inert gas such as [[helium]], [[argon]] or [[neon]] which is ionized by incident radiation, and a "quench" gas of 5–10% of an organic vapor or a halogen gas to prevent spurious pulsing by quenching the electron avalanches.<ref name="knoll"/> This combination of gases is known as a [[Penning mixture]] and makes use of the [[Penning ionization]] effect.

The modern halogen-filled G–M tube was invented by [[Sidney H. Liebson]] in 1947 and has several advantages over the older tubes with organic mixtures.<ref>{{cite journal |first=S. H. |last=Liebson |year=1947 |title=The discharge mechanism of self-quenching Geiger–Mueller counters |journal=Physical Review |volume=72 |issue=7 |pages=602–608 |doi=10.1103/physrev.72.602|bibcode = 1947PhRv...72..602L |hdl=1903/17793 |url=http://drum.lib.umd.edu/bitstream/1903/17793/1/DP70461.pdf |hdl-access=free }}</ref> The halogen tube discharge takes advantage of a [[metastable]] state of the inert gas atom to more-readily ionize a halogen molecule than an organic vapor, enabling the tube to operate at much lower voltages, typically 400–600 volts instead of 900–1200 volts. While halogen-quenched tubes have greater plateau voltage slopes compared to organic-quenched tubes (an undesirable quality), they have a vastly longer life than tubes quenched with organic compounds. This is because an organic vapor is gradually destroyed by the discharge process,  giving organic-quenched tubes a useful life of around 10<sup>9</sup> events. However, halogen ions can recombine over time, giving halogen-quenched tubes an effectively unlimited lifetime for most uses, although they will still eventually fail at some point due to other ionization-initiated processes that limit the lifetime of all Geiger tubes. For these reasons, the halogen-quenched tube is now the most common.<ref name="knoll"/>

Neon is the most common filler gas. Chlorine is the most common quencher, though bromine is occasionally used as well. Halogens are most commonly used with neon, argon or krypton, organic quenchers with helium.<ref>{{Cite web|url=https://www.orau.org/health-physics-museum/collection/geiger-mueller-tubes/index.html|title=An Introduction to Geiger-Mueller (GM) Detectors|website=www.orau.org|access-date=2021-10-12}}</ref>

An example of a gas mixture, used primarily in proportional detectors, is P10 (90% argon, 10% methane).
Another is used in bromine-quenched tubes, typically 0.1% argon, 1-2% bromine, and the balance of neon.

Halogen quenchers are highly chemically reactive and attack the materials of the electrodes, especially at elevated temperatures, leading to tube performance degradation over time. The cathode materials can be chosen from e.g. chromium, platinum, or nickel-copper alloy,<ref>{{Cite patent|title=Bromine-quenched high temperature g-m tube with passivated cathode|gdate=1972-07-31|country=US|number=3892990|pubdate=1975-07-01|assign=Kewanee Oil Co.|inventor1-last=Mitrofanov |inventor1-first=Nicolas}}</ref> or coated with colloidal graphite, and suitably passivated. Oxygen plasma treatment can provide a passivation layer on stainless steel. Dense non-porous coating with platinum or a tungsten layer or a tungsten foil liner can provide protection here.<ref>{{Cite patent|number=4359661|title=Geiger-Mueller tube with tungsten liner|gdate=1982-11-16|invent1=Mitrofanov|inventor1-first=Nicolas|country=US|pubdate=1982-11-16|assign=[[OM Group#History|The Harshaw Chemical Co.]]}}</ref>

Pure noble gases exhibit threshold voltages increasing with increasing atomic weight. Addition of polyatomic organic quenchers increases threshold voltage, due to dissipation of large percentage of collisions energy in molecular vibrations. Argon with alcohol vapors was one of the most common fills of early tubes. As little as 1 ppm of impurities (argon, mercury, and krypton in neon) can significantly lower the threshold voltage. Admixture of chlorine or bromine provides quenching and stability to low-voltage neon-argon mixtures, with wide temperature range. Lower operating voltages lead to longer rise times of pulses, without appreciably changing the dead times.

Spurious pulses are caused mostly by secondary electrons emitted by the cathode due to positive ion bombardment. The resulting spurious pulses have the nature of a relaxation oscillator and show uniform spacing, dependent on the tube fill gas and overvoltage. At high enough overvoltages, but still below the onset of continuous corona discharges, sequences of thousands of pulses can be produced. Such spurious counts can be suppressed by coating the cathode with higher [[work function]] materials, chemical passivation, lacquer coating, etc.

The organic quenchers can decompose to smaller molecules (ethyl alcohol and ethyl acetate) or polymerize into solid deposits (typical for methane). Degradation products of organic molecules may or may not have quenching properties. Larger molecules degrade to more quenching products than small ones; tubes quenched with amyl acetate tend to have ten times higher lifetime than ethanol ones. Tubes quenched with hydrocarbons often fail due to coating of the electrodes with polymerization products, before the gas itself can be depleted; simple gas refill won't help, washing the electrodes to remove the deposits is necessary. Low ionization efficiency is sometimes deliberately sought; mixtures of low pressure hydrogen or helium with organic quenchers are used in some cosmic rays experiments, to detect heavily ionizing muons and electrons.

Argon, krypton and xenon are used to detect soft x-rays, with increasing absorption of low energy photons with decreasing atomic mass, due to direct ionization by photoelectric effect. Above 60-70 keV the direct ionization of the filler gas becomes insignificant, and secondary photoelectrons, Compton electrons or electron-positron pair production by interaction of the gamma photons with the cathode material become the dominant ionization initiation mechanisms. Tube windows can be eliminated by putting the samples directly inside the tube, or, if gaseous, mixing them with the filler gas. Vacuum-tightness requirement can be eliminated by using continuous flow of gas at atmospheric pressure.<ref>{{Cite web|url=https://apps.dtic.mil/dtic/tr/fulltext/u2/b196664.pdf|archive-url=https://web.archive.org/web/20211117083029/https://apps.dtic.mil/dtic/tr/fulltext/u2/b196664.pdf|url-status=live|archive-date=November 17, 2021|title=Geiger Counter Tubes|last=Naval Research Laboratory|date=May 25, 1949|website=dtic.mil|access-date=2019-09-09}}</ref>