Geiger–Müller tube

Principle of operation

The tube consists of a chamber filled with a gas mixture at a low pressure of about 0.1 atmosphere. The chamber contains two electrodes, between which there is a potential difference of several hundred volts. The walls of the tube are either metal or have their inside surface coated with a conductor to form the cathode, while the anode is a wire in the center of the chamber.

When ionizing radiation strikes the tube, some molecules of the gas are ionized, either directly by the incident radiation or indirectly by means of secondary electrons produced in the walls of the tube. This creates positively charged ions and electrons, known as ion pairs, in the fill gas. The strong electric field created by the tube's electrodes accelerates the positive ions towards the cathode and the electrons towards the anode. Close to the anode in the "avalanche region" the electrons gain sufficient energy to ionize additional gas molecules and create a large number of electron avalanches which spread along the anode and effectively throughout the avalanche region. This is the "gas multiplication" effect which gives the tube its key characteristic of being able to produce a significant output pulse from a single ionising event.

If there were to be only one avalanche per original ionising 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 creation of multiple avalanches is due to the production of UV photons in the original avalanche, which are not affected by the electric field and move laterally to the axis of the anode to instigate further ionising events by collision with gas molecules. These collisions produce further avalanches, which in turn produce more photons, and thereby more avalanches in a chain reaction which spreads laterally through the fill gas, and envelops the anode wire. The accompanying diagram shows this graphically. The speed of propagation of the avalanches is typically 2–4 cm per microsecond, so that for common sizes of tubes the complete ionisation of the gas around the anode takes just a few microseconds. This short, intense pulse of current can be measured as a count event in the form of a voltage pulse developed across an external electrical resistor. This can be in the order of volts, thus making further electronic processing simple.

The discharge is terminated by the collective effect of the positive ions created by the avalanches. These ions have lower mobility than the free electrons due to their higher mass and remain in the area of the anode wire. This creates a "space charge" which counteracts the electric field which is necessary for continued avalanche generation. For a particular tube geometry and operating voltage this termination always occurs when a certain number of avalanches have been created, therefore the pulses from the tube are always of the same magnitude regardless of the energy of the initiating particle. Consequently, there is no radiation energy information in the pulses which means the Geiger–Muller tube cannot be used to generate spectral information about the incident radiation.

Pressure of the fill gas is important in the generation of avalanches. Too low a pressure and the efficiency of interaction with incident radiation is reduced. Too high a pressure, and the “mean free path” for collisions between accelerated electrons and the fill gas is too small, and the electrons cannot gather enough energy between each collision to cause ionisation of the gas. The energy gained by electrons is proportional to the ratio “e/p”, where “e” is the electric field strength at that point in the gas, and “p” is the gas pressure.

Types of tube

Broadly, there are two main types of geiger tube construction.

End window type

For alpha particles, low energy beta particles, and low energy X-rays, the usual form is a cylindrical end-window tube. This type has a window at one end covered in a thin material through which low-penetrating radiation can easily pass. Mica is a commonly used material due to its low mass per unit area. The other end houses the electrical connection to the anode.

Pancake tube

The pancake tube is a variant of the end window tube, but which is designed for use for beta and gamma contamination monitoring. It has roughly the same sensitivity to particles as the end window type, but has a flat annular shape so the largest window area can be utilised with a minimum of gas space. Like the cylindrical end window tube, mica is a commonly used window material due to its low mass per unit area. The anode is normally multi-wired in concentric circles so it extends fully throughout the gas space.

Windowless type

This general type is distinct from the dedicated end window type, but has two main sub-types, which use different radiation interaction mechanisms to obtain a count.

Thick walled

Used for high energy gamma detection, this type generally has an overall wall thickness of about 1-2 mm of chrome steel. Because most high energy gamma photons will pass through the low density fill gas without interacting, the tube uses the interaction of photons on the molecules of the wall material to produce high energy secondary electrons within the wall. Some of these electrons are produced close enough to the inner wall of the tube to escape into the fill gas. As soon as this happens the electron drifts to the anode and an electron avalanche occurs as though the free electron had been created within the gas. The avalanche is a secondary effect of a process that starts within the tube wall; the avalanche is not the effect of radiation directly on the gas itself.

Thin walled

Thin walled tubes are used for:

Neutron detectors

G–M tubes will not 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 the tube contains boron trifluoride or helium-3 as the fill gas. The neutrons interact with the boron nuclei, producing alpha particles, or directly with the helium-3 nuclei producing hydrogen and tritium ions and electrons. These charged particles then trigger the normal avalanche process.

Gas mixtures

The components of the gas fill mixture are an inert gas such as helium, argon or neon which is ionised 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. 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. 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 the organic vapor is gradually destroyed by the discharge process (giving organic-quenched tubes a useful life of around 10⁹ events), while the 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.

Geiger plateau

The Geiger plateau is the voltage range in which the GM tube operates in its correct mode. 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 ion current shown in this article. In the "Geiger region" the gradient flattens; this is the Geiger plateau.

Depending on the characteristics of the specific tube (manufacturer, size, gas type, etc.) the voltage range of the plateau will vary. In this region, the potential difference in the counter is strong enough to allow the creation of multiple avalanches. A lower voltage is not sufficient to cause a complete discharge along the anode, and individual Townsend avalanches are the result, and the tube tries to act as a proportional counter. If the applied voltage is higher than the plateau, a continuous glow discharge is formed and the tube cannot detect radiation.

The plateau has a slight slope caused by increasing sensitivity to low energy radiation as the voltage increases. Normally when a particle ionizes gas atoms, complete ionization of the gas occurs. But for a low energy particle, it is possible that the kinetic energy in addition to the potential energy of the voltage are insufficient for the avalanche to occur and the ion recombines. As applied voltage rises, the threshold for the minimum radiation response falls, thus the counter's sensitivity rises; giving rise to the slope.

The counting rate for a given radiation source varies slightly as the applied voltage is varied and to prevent this, a regulated voltage is used. However, it is normal to operate the tube in the middle of the plateau to allow for variations in the tube supply voltage.

Quenching and dead time

The ideal G–M tube should produce a single pulse for every single ionising event due to radiation. It should not give spurious pulses, and should recover quickly to the passive state, ready for the next radiation event. However, when positive argon ions reach the cathode and become neutral atoms by gaining electrons, the atoms can be elevated to enhanced energy levels. These atoms then return to their ground state by emitting photons which in turn produce further ionisation and thereby spurious secondary discharges. If nothing were done to counteract this, ionisation would be prolonged and could even escalate. The prolonged avalanche would increase the "dead time" when new events cannot be detected, and could become continuous and damage the tube. Some form of quenching of the ionisation is therefore essential to reduce the dead time and protect the tube, and a number of quenching techniques are used.

Chemical quenching

Self-quenching or internal-quenching tubes stop the discharge without external assistance, originally by means of the addition of a small amount of a polyatomic organic vapor originally such as butane or ethanol, but for modern tubes is a halogen such as bromine or chlorine.

If a poor gas quencher is 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. Thus, neutral argon atoms would be produced and the quencher gas ions in their turn would reach the cathode, gain electrons therefrom, and move into excited states which would decay by photon emission, producing tube discharge. However, effective quencher molecules, when excited, lose their energy not by photon emission, but by dissociation into neutral quencher molecules. No spurious pulses are thus produced.

Even with chemical quenching, for a short time after a discharge pulse there is a period during which the tube is rendered insensitive and is thus temporarily unable to detect the arrival of any new ionizing particle (the so-called dead time; typically 50–100 microseconds). This causes a loss of counts at sufficiently high count rates and limits the G–M tube to an effective (accurate) count rate of approximately 10³ counts per second even with external quenching. While a G-M tube is technically capable of reading higher count rates before it truly saturates, the level of uncertainty involved and the risk of saturation makes it extremely dangerous to rely upon higher count rate readings when attempting to calculate an equivalent radiation dose rate from the count rate. A consequence of this is that ion chamber instruments are usually preferred for higher count rates, however a modern external quenching technique can extend this upper limit considerably.

External quenching

External quenching, sometimes called "active quenching" or "electronic quenching", uses simplistic high speed control electronics to rapidly remove and re-apply the high voltage between the electrodes for a fixed time after each discharge peak in order to increase the maximum count rate and lifetime of the tube. Although this can be used instead of a quench gas, it is much more commonly used in conjunction with a quench gas.

The "time-to-first-count method" is a sophisticated modern implementation of external quenching that allows for dramatically increased maximum count rates via the use of statistical signal processing techniques and much more complex control electronics. Due to uncertainty in the count rate introduced by the simplistic implementation of external quenching, the count rate of a geiger tube becomes extremely unreliable above approximately 10³ counts per second. With the time-to-first-count method, effective count rates of 10⁵ counts per second are achievable, two orders of magnitude larger than the normal effective limit. The time-to-first-count method is significantly more complicated to implement than traditional external quenching methods, and as a result of this it has not seen widespread use.

Fold-back effect

One consequence of the dead time effect is the possibility of a high count rate continually triggering the tube before the recovery time has elapsed. This may produce pulses too small for the counting electronics to detect and lead to the very undesirable situation whereby a G–M counter in a very high radiation field is falsely indicating a low level. This phenomenon is known as "fold-back". An industry rule of thumb is that the discriminator circuit receiving the output from the tube should detect down to 1/10 of the magnitude of a normal pulse to guard against this. 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–Muller 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.

Energy compensation

If a G–M tube is to be used for gamma or X-ray dosimetry measurements the energy of incident radiation, which affects the ionising effect, must be taken into account. However individual pulses from a G–M tube do not carry any energy information. A solution is to assign a radiation dose to each counting event, so the tube characteristic relates the number of counts to the intensity of incident radiation.

At low photon energy levels the response increases as low energy photons have a greater interaction with the fill gas than high energy photons. The tube therefore has an increased response for radiation which has a lower dose rate, and a correction must be applied to prevent an incorrect high reading for low energy photons. This discrepancy can be 2–3 times greater or more, and for a thick-walled tube usually peaks at about 60 keV, where radiation interactions with the gas are still large, but the shielding effect of the wall has not become dominant.

This correction is achieved by 'energy compensation' of the tube, which modifies the number of count events in accordance with the energy of the incident radiation by using an external filter collar of energy absorbing material. The collar has an increased attenuation of low energy gamma, and so compensates for the increased energy response of the naked tube at those levels. The aim is that sensitivity/energy characteristic of the tube should be matched by the absorption/energy characteristic of the filter. This results in a more uniform response over the stated range of detection energies for the tube.

Lead and tin are commonly used materials, and a simple filter effective above 150 keV can be made using a continuous collar along the length of the tube. However, at lower energy levels this attenuation can become too great, so air gaps are left in the collar to allow low energy radiation to have a greater effect. In practice, compensation filter design is an empirical compromise to produce an acceptably uniform response, and a number of different materials and geometries are used to obtain the required correction.