Geiger counter

Geiger counters are used for the detection and measurement of ionizing radiation, ie, belong to the radiation and particle detectors.

Depending on the design and operating voltage, the counter tube works

• As ionization chamber,
• As a proportional counter (also proportional counter )
• (also called countertube, Geiger-Müller counter or Geiger-Müller counter ) or as a Geiger- Müller counter.

The frequently encountered term Geiger counter called fachsprachlich the Geiger- Müller counter. Colloquially, though there may be also a complete radiometer be meant as a contamination detection device or a dose rate meter. The detector in such devices is often, but not always, a Geiger- Müller counter.

In principle is possible with one and the same counter tube each of these three modes. Most Geiger counters are built but optimized for certain of these applications.

Construction

The simplest counting tubes consist of a closed at both sides of the cylindrical metal tube, which is the cathode. The anode is a wire of, for example 0.1 mm diameter located in the axis of the cylinder and is passed out at one end by an insulator ( glass) of the counter tube. The pipe diameter is a few centimeters.

Such counter tubes are suitable for the detection of gamma radiation, as this passes through the metal tube. If alpha and beta radiation to be detected, the counter tube at one end should be used with a low-mass foil (eg, mica or biaxially oriented PET film ) closed ( -window ). The film must withstand the pressure difference to the outside air, but allow to enter the particles in the counter tube.

The tube is filled with a gas ( counter gas ), as described in more detail below.

Function

Between the anode and cathode, a DC voltage is applied. If ionizing radiation is incident, it produces free electrons in the gas filling, which migrate in the electric field to the anode. In the case of charged- particle radiation, the number of electrons proportional to the energy emitted by the incident particles in the gas.

The further procedure depends largely on the voltage between the anode and cathode on how the pictured curve ( characteristic) shows. At low voltage, a part of the electrons towards the anode recombined with the ions. Which occurs in the circuit current pulse represents only the electrons that have reached the anode; this proportion is different in size depending on the location of ionization in the tube and therefore gives no indication of the energy emitted by the detected particles ( recombination ).

Ionization chamber

At higher voltage - 100 volts magnitude - reach all electrons released the anode. The measurable pulse in the circuit is thus proportional to the energy, which has submitted in the counter tube radiation. The counter tube is now working as ionization chamber.

If the entire energy of a Strahlungsteilchens be detected that the particle, the range of the radiation in the gas must end in the gas, that is shorter than the dimension of the counter tube in radial direction to be. Accordingly, this relatively large counter tubes (up to about 1 m long) and gas fills up to a few bar pressure is used.

Proportional

With further increase of the voltage released by the radiation electrons are close to the anode wire so greatly accelerated due to the high electric field strength that it can cause more electrons by collisions with the gas atoms. This results electron avalanches with each n electrons ( n can be up to 1 million ); This is also called gas amplification. Since the avalanches occur only in a very small region of space near the anode, the magnitude of the measured current pulse is independent of the site of the original ionization and still proportional to the energy of the incident radiation (proportional band ). But the pulse is n times higher than in Ionisationskammerbetrieb and therefore easier to measure.

For dimensions and gas pressure, the same applies as for ionization chambers. Since the proportional band is set in a steep part of the characteristic, the operating voltage must be very accurate constant. During an ionization chamber, for example, also have parallel plate electrodes, the proportional counter, the field geometry with the thin anode wire is essential. The cylindrical shape of the cathode, however, is not decisive; Proportional counter may vary according to requirements, have other geometric shapes, and also contain a plurality of parallel anode wires.

Proportional counter offer not only the ability to measure particle energies, but are used for example in radiation protection because of the good possibility of distinguishing between alpha and beta radiation. Therefore also the hand-foot monitors for routine inspection when leaving controlled areas contain proportional counter.

From physics research, for example, the Homestake Neutrino Experiment to call, where proportional counter were used to safely distinguish very rare beta decays of a gaseous sample of other radiation can. In a more advanced form of the proportional counter is used as a multi-wire proportional chamber in high-energy physics.

Proportional counters for neutron

Also, neutron radiation can be measured with proportional counters. For the energy measurement of fast neutrons ( about 0.1 to 6 MeV) is of a few bars used as counting gas, hydrogen or methane overpressure. From the thus measured energy spectrum of recoil protons from elastic scattering can be close to the neutron spectrum.

For slow, that is, thermal neutrons, the boron trifluoride gas is (BF3 ). In both of the exothermic reaction, the core 10B ( n ) simultaneously 7Li resulting ions, alpha particles and the lithium - nucleus cause ionization. For the purpose of higher detection probability BF3 often used with enriched B-10 boron.

Instead of the BF3 gas filling and a boron-containing layer may be used on the inside of the counter tube. This has the advantage that as a counting gas, such as argon can be used, which results in shorter pulses. The disadvantage, however, is that the nuclear reaction less ionization in the gas leaves because kinematic reasons is only one of the two ions emitted inside the pipe; the distinction between gamma pulses is thus difficult.

The rare helium isotope helium -3 can also be used as neutron counting gas. The exothermic reaction is also here 3He ( n, p) 3H. He-3 is more expensive than boron trifluoride, but results in a higher probability of detection, since it contains no other nuclei, the cross section of the reaction is greater, and it may be a higher inflation pressure to be used. He-3 counter tubes can be operated at higher temperatures at which BF3 would decompose.

The boron and helium-3 counter tubes are (see below) operated in the proportional and not in the Geiger-Müller region to distinguish, for example, gamma radiation from neutron radiation can. An important application (usually with BF3 counter tube ) is the Long Counter.

Geiger- Müller counter

From a certain even higher voltage of each incident ionizing particles causes an independent gas discharge, that is, whether each secondary electron can dissolve before it reaches the anode, for its part, at least one new electron from. And ultraviolet radiation is produced that ionizes at remote locations, so that the discharge spreads through the entire counter tube. The once initiated ( ignited ) gas discharge (hence the term " countertube " ) "burns " regardless of the type and energy of the triggering radiation and only goes out when the slowly migrating radially outward ion cloud has reduced by shielding the field strength enough. Re- igniting the gas discharge in the ion impingement on the tube wall is prevented by the addition of a quenching gas to the filling gas (see gas filling ).

The current pulses are therefore of uniform size and so large that they can be used in a speaker crackling noises may without amplification directly be made audible. To trigger requires only a single electron is released, so the detector has the best sensitivity. This area of ​​work is called voltage plateau or Geiger-Müller region.

Compared with other detectors, the Geiger- Müller counter has a relatively long dead time of the order of 100 microseconds because of the gas discharge process. This is followed by yet followed by a similarly long recovery time, while not reaching a new pulse the full height. Even longer is the dead time when relighting should be stopped after the pulse by a very high resistance, about 100 kilo-ohms in the high voltage supply line; therefore has become generally established as a better method of extinguishing gas additive.

Be used Geiger- Müller counter tubes, for example, to check for contamination and for general radiation protection purposes. Information on radiation type and energy can be only roughly win with them by carrying out comparative measurements with different placed between the radiation source and the counter tube shields.

Gas filling

As Zählrohrfüllung can have many different gases, even serve air. Noble gases such as argon are advantageous for achieving the shortest possible pulses, because they do not form negative ions, which migrate more slowly than the electrons to the anode. For the detection of gamma radiation is used with a plurality of bar argon pressure and, due to its high atomic number, xenon. In ionization chambers and proportional counters, often a portion of a gaseous compound is added, such as methane or carbon dioxide. This addition reduced by inelastic collisions, the temperature of electrons, thus causing a further reduction of the current pulse, thus making the detector "faster". It also suppresses ultraviolet radiation, which could lead to supernumerary pulses.

For the Geiger-Müller operating the gas ethanol vapor or a halogen gas (chlorine or bromine) is added. This quenching gas ensures that after the extinction of the gas discharge no new ignition by impinging on the wall ions is done by sap his energy molecules by dissociation rather than by ionization.

Stationary -operated counting tubes are not sealed in some cases, but are operated as a flow meter with slow through-flowing gas. This avoids problems with contaminants, chemical reactions or the gas leaks little. In Geiger-Müller counters can so the addition of ethanol, which would consume Zählrohrbetrieb otherwise, are kept constant.

History

A precursor of the counter tubes was first described in 1913 by Hans Geiger. The Geiger- Müller counter goes to violinist development work back together with his colleague Walther Müller, whose results were published in 1928. It was the first well-known and widely used type of detector which responded to particles or radiation quanta with an electrical pulse. The practical use of the proportional band is in electronic ways - reinforcing the impulses stability of the high voltage - challenging and was only from the mid-20th century to a routine method.

Since the pulses of the Geiger- Müller tube are the same for all particles, it is especially suitable for counting the incident particles / quanta. The term " Geiger counter " or " Geiger- counter tube " therefore seems natural. This name was passed on to the later-developed detectors such as " proportional counter ", " scintillation ", etc., although these are not only for counting, but also for energy measurement and to distinguish between types of radiation.