Geiger-Müller tube

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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, and Walther Müller who collaborated with Geiger in developing it further in 1928. It is a type of a gaseous ionization detector.

It consists of a tube filled with an inert gas such as helium, neon or argon, in some cases in a Penning mixture, and an organic vapor or a halogen. The tube contains electrodes, between which there is an electrical voltage of several hundred volts, but no current flowing. The walls of the tube are either metal or the inside coated with metal or graphite to form the cathode whilst the anode is a wire passing up the centre of the tube.

When ionizing radiation passes through the tube, it ionizes some of the gas molecules, creating positively charged ions, and electrons. The strong electric field created by the tube's electrodes accelerate the ions towards the cathode and the electrons towards the anode. The ion pairs gain sufficient energy to ionise 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. To prevent the current from flowing continuously there are several techniques to stop, or quench the discharge. Quenching is important because a single particle entering the tube is counted by a single discharge, and so it will be unable to detect another particle until the discharge has been stopped, and because the tube is damaged by prolonged discharges.

External quenching uses external electronics to remove the high voltage between the electrodes. Self-quenching or internal-quenching tubes stop the discharge without external assistance, and contain a small amount of a polyatomic organic vapor such as butane or ethanol; or alternatively a halogen such as bromine or chlorine. Ions will collide with quench gas molecules, and give up energy to them by causing them to dissociate.

The halogen tubes were invented by Sidney H. Liebson in 1947, and are now the most common form, since the discharge mechanism takes advantage of the metastable state of the inert gas atom to ionize the halogen molecule and produces a more efficient discharge which permits it to operate at much lower voltages, typically 400-600 volts instead of 900-1200 volts. It also has a longer life because the halogen ions can recombine whilst the organic vapor cannot and is gradually destroyed by the discharge process (giving the latter a life of around 10⁸ events).

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 (mRh), but this does not relate easily to a dose rate as the instrument does not discriminate between radiation at different energy levels.

The usual form of 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. 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.

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