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.See also:
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
A G-M 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 conducting material or a spiral wire to form the cathode, while the anode is a wire mounted axially in the center of the chamber.
When ionizing radiation strikes the tube, some molecules of the fill gas are ionized directly by the incident radiation, and if the tube cathode is an electrical conductor, such as stainless steel, indirectly by means of secondary electrons produced in the walls of the tube, which migrate into the gas. This creates positively charged ions and free electrons, known as ion pairs, in the gas. The strong electric field created by the voltage across 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" where the electric field strength rises inversely proportional to radial distance as the anode is approached, free electrons gain sufficient energy to ionize additional gas molecules by collision and create a large number of electron avalanches. These 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 original ionizing event.Glenn F Knoll. Radiation Detection and Measurement, third edition 2000. John Wiley and sons,
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 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 ionizing 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 ionization 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 move slowly from the vicinity of the anode wire. This creates a "space charge" which counteracts the electric field that is necessary for continued avalanche generation. For a particular tube geometry and operating voltage this termination always occurs when a certain number of avalanches has 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–Müller tube cannot be used to generate spectral information about the incident radiation. In practice the termination of the avalanche is improved by the use of "quenching" techniques (see later).
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 ionization 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 important 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 utilized 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 gamma radiation detection above energies of about 25 KeV, this type generally has an overall wall thickness of about 1-2mm 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 with the production of electrons that migrate to the inner surface of the tube wall, and then enter the fill gas. This effect is considerably attenuated at low energies below about 20 KeV
Thin walled Thin walled tubes are used for: High energy beta detection, where the beta enters via the side of the tube and interacts directly with the gas, but the radiation has to be energetic enough to penetrate the tube wall. Low energy beta, which would penetrate an end window, would be stopped by the tube wall. Low energy gamma and X-ray detection. The lower energy photons interact better with the fill gas so this design concentrates on increasing the volume of the fill gas by using a long thin walled tube and does not use the interaction of photons in the tube wall. The transition from thin walled to thick walled design takes place at the 300–400 keV energy levels. Above these levels thick walled designs are used, and beneath these levels the direct gas ionization effect is predominant.
Neutron detection
G–M tubes will not detect neutrons since these do not ionize 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, or the tube is wrapped in about thick cadmium foil. 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. 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 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.
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.
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.