Geiger–Müller tube

thumb|right| A complete Geiger counter, with the Geiger–Müller tube mounted in a cylindrical enclosure connected by a cable to the instrument

The Geiger–Müller tube or G–M tube is the sensing element of the Geiger counter instrument used for the detection of ionizing radiation. It is named after Hans Geiger, who invented the principle in 1908, and Walther Müller, who collaborated with Geiger in developing the technique further in 1928 to produce a practical tube that could detect a number of different radiation types.

It is a gaseous ionization detector and uses the Townsend avalanche phenomenon to produce an easily detectable electronic pulse from as little as a single ionizing event due to a radiation particle. It is used for the detection of gamma radiation, X-rays, and alpha and beta particles. It can also be adapted to detect neutrons. The tube operates in the "Geiger" region of ion pair generation. This is shown on the accompanying plot for gaseous detectors showing ion current against applied voltage.

While it is a robust and inexpensive detector, the G–M is unable to measure high radiation rates efficiently, has a finite life in high radiation areas and cannot measure incident radiation energy, so no spectral information can be generated and there is no discrimination between radiation types; such as between alpha and beta particles. In other words the Geiger-Müller counter provides no information about the energy or the precise timing of the detected radiation, as all ionizing events produce the same output pulse, and the detector has a relatively long dead time after each event.

Principle of operation

thumb|300px|Plot of [[ionization|ion pair current against voltage for a cylindrical gaseous radiation detector with a central wire anode.]]

thumb|300px|Visualization of the spread of [[Townsend avalanches 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

If there were to be only one avalanche per original ionizing event, then the number of excited molecules would be in the order of 10⁶ to 10⁸. However the production of multiple avalanches results in an increased multiplication factor which can produce 10⁹ to 10¹⁰ ion pairs. The neutrons interact with the boron nuclei, producing alpha particles, or directly with the helium-3 nuclei producing hydrogen and tritium ions and electrons, or with the cadmium, producing gamma rays. These energetic particles interact and produce ions that then trigger the normal avalanche process.

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. 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 10⁹ 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.

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, 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.

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.

Geiger plateau

thumb|The characteristic curve of Geiger–Müller tube response with constant radiation against varying tube voltage.

The Geiger plateau is the voltage range in which the G-M tube operates in its correct mode, where ionization occurs along the length of the anode. If a G–M tube is exposed to a steady radiation source and the applied voltage is increased from zero, it follows the plot of current shown in the "Geiger region" where the gradient flattens; this is the Geiger plateau.

Quenching and dead time

[[File:Dead time of geiger muller tube.png|thumb|Dead time and recovery time in a Geiger–Müller tube. Additionally the circuit should detect when "pulse pile-up " has occurred, where the apparent anode voltage has moved to a new DC level through the combination of high pulse count and noise. The electronic design of Geiger–Müller counters must be able to detect this situation and give an alarm; it is normally done by setting a threshold for excessive tube current.

Detection efficiency

The efficiency of detection of a G–M tube varies with the type of incident radiation. Tubes with thin end windows have very high efficiencies (can be nearly 100%) for high energy beta, though this drops off as the beta energy decreases due to attenuation by the window material. Alpha particles are also attenuated by the window. As alpha particles have a maximum range of less than 50 mm in air, the detection window should be as close as possible to the source of radiation. The attenuation of the window adds to the attenuation of air, so the window should have a density as low as 1.5 to 2.0 mg/cm² to give an acceptable level of detection efficiency. The article on stopping power explains in more detail the ranges for particles types of various energies.

The counting efficiency of photon radiation (gamma and X-rays above 25 keV) depends on the efficiency of radiation interaction in the tube wall, which increases with the atomic number of the wall material. Chromium iron is a commonly used material, which gives an efficiency of about 1% over a wide range of energies.