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Geiger–Müller tube

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Operation of Geiger tube of "end window" type

A Geiger–Müller tube (or GM tube) is the sensing element of a Geiger counter instrument that can detect a single particle of ionizing radiation, and typically produce an audible click for each. It was named for Hans Geiger who invented the device in 1908,[1] and Walther Müller who collaborated with Geiger in developing it further in 1928.[2][3] It is a type of gaseous ionization detector.

The Geiger counter is sometimes used as a hardware random number generator.

Description and operation

A Geiger–Müller tube consists of a tube filled with a low-pressure (~0.1 atm) inert gas such as helium, neon or argon (usually neon), in some cases in a Penning mixture, and an organic vapor or a halogen gas. The tube contains electrodes, between which there is a potential difference of several hundred volts, but no current flowing. The walls of the tube are either entirely metal or have their inside surface coated with a conductor to form the cathode while the anode is a wire passing up the center of the tube.

When ionizing radiation passes through the tube, some of the gas molecules are ionized, creating positively charged ions, and electrons. The strong electric field created by the tube's electrodes accelerates the ions towards the cathode and the electrons towards the anode. The ion pairs gain sufficient energy to ionize further gas molecules through collisions on the way, creating an avalanche of charged particles.

This results in a short, intense pulse of current which passes (or cascades) from the negative electrode to the positive electrode and is measured or counted.

Most detectors include an audio amplifier that produce an audible click on discharge. The number of pulses per second measures the intensity of the radiation field. Some Geiger counters display an exposure rate (e.g. mR/h), but this does not relate easily to a dose rate as the instrument does not discriminate between radiation of different energies.

Geiger plateau

The Geiger plateau is the voltage range in which the Geiger–Müller counter operates. If a GM tube is exposed to a steady radiation source and the applied voltage increased from zero, at first the count rate increases rapidly; at a certain voltage the rate of increase flattens out (only changing a few per cent for every 100 volts increase).

Depending on the characteristics of the specific tube (manufacturer, size, gas type etc.) the exact voltage range may vary. In this plateau region, the potential difference in the counter is strong enough to ionize all the gas inside the tube, upon triggering by the incoming ionizing radiation (alpha, beta or gamma radiation). Below the plateau the voltage is not high enough to cause complete discharge; a limited Townsend avalanche is the result, and the tube acts as a proportional counter, where the output pulse size depends on the initial ionization created by the radiation. Higher voltages give a pulse size independent of the initial ionization energy. If the applied voltage is too high, a continuous glow discharge is formed and the tube cannot detect radiation.

The plateau has a slight incline caused by increased sensitivity to low energy radiation, due to the increased voltage on the device. Normally when a particle enters the tube and ionizes one of the gas atoms, complete ionization of the gas occurs. Once a low energy particle enters the counter, it is possible that the kinetic energy in addition to the potential energy of the voltage are insufficient for the additional ionization to occur and thus the ion recombines. At higher voltages, the threshold for the minimum radiation level drops, thus the counter's sensitivity rises. The counting rate for a given radiation source varies slightly as the applied voltage is varied; for standardization of the response of the instrument, a regulated voltage is used to maintain stable counting characteristics. [4]

GM tubes

For alpha, beta and X-ray detection the usual form of GM tube is an end-window tube. This type is so-named because the tube has a window at one end through which ionizing radiation can easily penetrate. The other end normally has the electrical connectors. There are two types of end-window tubes: the glass-mantle type and the mica window type. The glass window type will not detect alpha radiation since it is unable to penetrate the glass, but is usually cheaper and will usually detect beta radiation and X-rays. The mica window type will detect alpha radiation but is more fragile.

Most tubes will detect gamma radiation, and usually beta radiation above about 2.5 MeV. The types with no end-window are used for Gamma and X-ray detection. Geiger–Müller tubes will not normally detect neutrons since these do not ionise the gas. However, neutron-sensitive tubes can be produced which either have the inside of the tube coated with boron or contain boron trifluoride or helium-3 gas. The neutrons interact with the boron nuclei, producing alpha particles or with the helium-3 nuclei producing hydrogen and tritium ions and electrons. These charged particles then trigger the normal avalanche process.

Although most tubes will detect gamma radiation, standard tubes are relatively inefficient, as most gamma photons will pass through the low density gas without interacting. Using the heavier noble gases krypton or xenon for the fill effects a small improvement, but dedicated gamma detectors use dense cathodes of lead or stainless steel in windowless tubes. The dense cathode then interacts with the gamma flux, producing high-energy electrons, which are then detected.

Quenching and dead time

The ideal GM tube should produce a single pulse on entry of a single ionising particle. It must not give any spurious pulses, and must recover quickly to the passive state. Unfortunately for these requirements, when positive argon ions reach the cathode and become neutral argon atoms again by obtaining electrons from it, the atoms can acquire their electrons in enhanced energy levels. These atoms then return to their ground state by emitting photons which can in turn produce further ionisation and hence cause spurious secondary pulse discharges. If nothing were done to counteract it, ionisation could even escalate, causing a so-called current "avalanche" which if prolonged could damage the tube. Some form of quenching of the ionisation is therefore essential. The disadvantage of quenching is that for a short time after a discharge pulse has occurred (the so-called dead time, which is typically a few microseconds), the tube is rendered insensitive and is thus temporarily unable to detect the arrival of any new ionising particle. This effectively causes a loss of counts at sufficiently-high count rates.

External quenching uses control electronics to temporarily remove the high voltage between the electrodes. Self-quenching or internal-quenching tubes stop the discharge without external assistance, by means of the addition of a small amount of a polyatomic organic vapor such as butane or ethanol; or alternatively a halogen such as bromine or chlorine.

If a poor diatomic gas quencher were 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. Neutral argon atoms would then be produced and the quencher gas ions would reach the cathode instead, gain electrons in excited states which would decay by photon emission, thereby producing spurious tube discharge as before. However, effective quencher molecules, when excited, do not lose their energy by photon emission but by dissociation into neutral quencher atoms. No spurious output pulses are then produced.

Halogen tube

The halogen GM tube was invented by Sidney H. Liebson in 1947,[5] The discharge mechanism takes advantage of a metastable state of the inert gas atom to more-readily ionize a halogen molecule, enabling the tube to operate at much lower voltages, typically 400–600 volts instead of 900–1200 volts. This type of GM tube is therefore by far the most common form now. It has a longer life than tubes quenched with organic compounds, because the halogen ions can recombine while the organic vapor is gradually destroyed by the discharge process (giving the latter a life of around 108 events)

See also

References

  1. ^ E. Rutherford and H. Geiger (1908) "An electrical method of counting the number of α particles from radioactive substances," Proceedings of the Royal Society (London), Series A, vol. 81, no. 546, pages 141-161.
  2. ^ H. Geiger and W. Müller (1928). "Elektronenzählrohr zur Messung schwächster Aktivitäten (Electron counting tube for measurement of weakest radioactivities)". Die Naturwissenschaften. 16 (31): 617–618. Bibcode:1928NW.....16..617G. doi:10.1007/BF01494093.
  3. ^ See also:
    1. Geiger, H. and Müller, W. (1928) "Das Elektronenzählrohr" (The electron counting tube), Physikalische Zeitschrift, 29: 839-841.
    2. Geiger, H. and Müller, W. (1929) "Technische Bemerkungen zum Elektronenzählrohr" (Technical notes on the electron counting tube), Physikalische Zeitschrift, 30: 489-493.
    3. Geiger, H. and Müller, W. (1929) "Demonstration des Elektronenzählrohrs" (Demonstration of the electron counting tube), Physikalische Zeitschrift, 30: 523 ff.
  4. ^ A Handbook of Radioactivity Measurements Procedures, 2nd edition: (Report No. 58), National Council on Radiation Protection and Measurements (NCRP) , 1985 ISBN 0-913392-71-5,pages 30-31
  5. ^ S. H. Liebson (1947) "The discharge mechanism of self-quenching Geiger–Mueller counters," Physical Review, vol. 72, no. 7, pages 602-608.
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