Counter tube


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Radiation detection device with counter tube

Counter tubes are used to detect and measure ionizing radiation , so they belong to the radiation and particle detectors .

The counter tube works depending on the design and operating voltage

  • as ionization chamber ,
  • as proportional counter tube (also proportional counter )
  • or as a Geiger-Müller counter tube (also called a trigger counter tube , Geiger-Müller counter or Geiger-Müller indicator ).

The term Geiger counter , which is often encountered , technically refers to the Geiger-Müller counter tube. Colloquially, however, it can also mean a complete radiation measuring device, such as a contamination detection device or a dose rate measuring device . The detector in such devices is often, but not always, a Geiger-Müller counter tube.

In principle, each of the three operating modes mentioned is possible with one and the same counter tube. Most counting tubes, however, are optimized for one of these applications.

construction

Basic sketch of a counter tube, here with a thin end window for low-energy and particle radiation

The simplest counter tubes consist of a cylindrical metal tube closed on both sides, which represents the cathode . The anode , a wire of e.g. B. 0.1 mm diameter, is located in the axis of the cylinder and is led out of the counter tube at one end through an insulator (glass). The pipe diameter is a few centimeters.

Such counter tubes are suitable for the detection of gamma radiation because it penetrates the metal tube. If alpha and beta radiation are also to be detected, the counter tube may only be closed at one end with a low-mass film (e.g. mica or biaxially oriented PET film ) ( window counter tube ). The film must withstand the pressure difference to the outside air, but allow the particles to get into the counter tube.

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

function

Schematic characteristics of a counter tube. The observed pulse height for incident radiation of uniform particle energy is plotted vertically.

A DC voltage is applied between the anode and cathode. When ionizing radiation is incident, it generates free electrons in the gas filling that migrate to the anode in the electric field. In the case of charged particle radiation, the number of electrons is proportional to the energy given off by the incident particle in the gas.

The further process depends essentially on the voltage between anode and cathode, as the curve shown ( characteristic ) shows. At low voltage, some of the electrons recombine with the ions on their way to the anode. The current pulse occurring in the circuit corresponds only to the electrons that have reached the anode; this proportion varies in size depending on the location of the ionization in the pipe and therefore does not provide any information about the energy given off by the detected particle. This area of ​​applied voltage is called the recombination area.

Ionization chamber

At higher voltages - of the order of 100 volts - all released electrons reach the anode. The pulse that can be measured in the circuit is therefore proportional to the energy that the radiation emitted in the counter tube. The counter tube now works as an ionization chamber and is used, for example, as a scattered radiation meter .

If the entire energy of a radiation particle is to be recorded, the particle path must end in the gas, i.e. the range of the radiation in the gas must be shorter than the dimensions of the counter tube in the direction of the beam. Accordingly, relatively large counter tubes (up to about 1 m long) and gas fillings up to a few bar overpressure are used for this.

Proportional counter tube

If the voltage is increased further, the electrons released by the radiation are accelerated so strongly due to the high electric field strength close to the anode wire that they can trigger further electrons by colliding with the gas atoms. There are electron avalanches with n electrons each ( n can be up to 1 million); this is also called gas amplification . Since the avalanches only occur in a very small area near the anode, the size of the measured current pulse is independent of the location of the original ionization and is still proportional to the energy of the incident radiation. This is why this operating voltage range is called the proportional range . Compared to ionization chamber operation, the impulse is n times greater and therefore easier to measure.

The same applies to dimensions and gas pressure as to ionization chambers. Since the proportional range lies in a steep part of the characteristic, the operating voltage must be very precisely constant. While an ionization chamber z. B. can also have parallel plate electrodes, the field geometry with the thin anode wire is essential for the proportional counter tube. The cylindrical shape of the cathode, however, is not decisive; Proportional meters can also have other shapes, depending on the geometric requirements, and also contain several parallel anode wires.

Proportional counter tubes not only offer the possibility of measuring particle energies, but are also B. used in radiation protection because of the ability to differentiate between alpha and beta radiation. The hand-foot monitors for routine checks when leaving control areas therefore also contain proportional counters.

From physical research z. One example is the Homestake neutrino experiment, where proportional counters were used to reliably differentiate very rare beta decays of a gaseous sample from other radiation. In a further developed form, the proportional counter is used as a multi-wire proportional chamber and as a straw detector, also in high-energy physics .

Proportional counter tubes for neutrons

Neutron detection in the BF 3 counter tube

Also, neutron radiation can be measured with proportional counters. To measure the energy of fast neutrons (about 0.1 to 6  MeV ), hydrogen or methane at a pressure of a few bar is used as the counting gas . The neutron spectrum can be inferred from the energy spectrum of the recoil protons from the elastic scattering measured with it .

The gas boron trifluoride (BF 3 ) is suitable for slow neutrons , especially for thermal neutrons . The two ions that arise simultaneously in the exothermic nuclear reaction 10 B (n, ) 7 Li, the alpha particle and the lithium atomic nucleus, lead to ionization. BF 3 with boron enriched in B-10 is often used for the purpose of higher detection probability .

Instead of the BF 3 gas filling, a boron-containing layer can also be used on the inside of the counter tube. This has the advantage that as counting gas z. B. argon can be used, which gives shorter pulses. The disadvantage, however, is that the nuclear reaction leaves less ionization energy in the gas, because for kinematic reasons only one of the two ions is ever emitted into the pipe interior; this makes it more difficult to distinguish between gamma pulses.

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

The boron and helium-3 counter tubes are also operated in the proportional and not in the Geiger-Müller range (see below) in order to be able to differentiate between gamma radiation and neutron radiation, for example. An important application (mostly with a BF 3 counter tube) is the long counter .

Geiger-Müller counter tube

Disassembled Geiger-Müller counter tube for gamma radiation. Below the actual counter tube made of glass with anode wire in the middle and helical wire as cathode; In the middle, shielding plates are attached between the counter tube and the housing in order to change the sensitivity to radiation of different energies; above the outer aluminum housing, length 30 cm.

From a certain even higher voltage - in the "plateau area" of the characteristic shown above - every incoming ionizing particle causes an independent gas discharge , i.e. every secondly released electron releases at least one new electron before it reaches the anode. Also, ultraviolet radiation generated which ionizes at remote locations, so that the discharge spreads through the entire counter tube. The type of counter tube that works in this way is called the Geiger-Müller counter tube. Once initiated ( ignited ) the gas discharge "burns" regardless of the type and energy of the triggering radiation (hence the alternative designation "trigger counter tube") and only goes out when the ion cloud, which slowly moves radially outwards, has sufficiently reduced the field strength through shielding. A renewed ignition of the gas discharge when the ions hit the pipe wall is prevented by adding an extinguishing gas to the filling gas (see under gas filling).

The current pulses are therefore of uniform size and so large that they u. U. can be made audible as cracking noises directly in a loudspeaker without amplification. A single released electron is sufficient for triggering, so the detector has the best possible sensitivity. The plateau area of ​​the working voltage is therefore also called the Geiger-Müller area .

Compared to other detectors, the Geiger-Müller counter tube has a relatively long dead time of the order of 100 microseconds because of the gas discharge process . This is followed by a similarly long recovery time , during which a new impulse does not reach its full height. The dead time becomes even longer if a very high resistance, around 100 kilohms, in the high-voltage supply line is to prevent re-ignition after the pulse; therefore, the addition of extinguishing gas has generally established itself as a better method.

Geiger-Müller counter tubes are used, for example, to check for contamination and for general radiation protection purposes . Information about the type and energy of radiation can only be obtained roughly with them by making comparative measurements with different shields placed between the radiation source and the counter tube .

Gas filling

Many different gases, even air, can be used as the counter tube filling. Noble gases such as B. Argon are advantageous for achieving the shortest possible pulses because they do not form negative ions, which travel much more slowly than the electrons to the anode. For the detection of gamma radiation, argon with several bar overpressure or, because of its high atomic number , xenon is used. In ionization chambers and proportional counters, a portion of a gaseous compound is often added, such as methane or carbon dioxide . This addition reduces the temperature of the electrons through inelastic collisions and thus causes a further shortening of the current pulse, thus making the detector “faster”. It also suppresses ultraviolet radiation , which could lead to excess pulses.

For the Geiger-Müller plant, ethanol vapor or a halogen gas ( chlorine or bromine ) is added to the gas . This extinguishing gas ensures that once the gas discharge has been extinguished , no new ignition takes place by ions hitting the wall, as its molecules consume energy through dissociation instead of ionization.

Stationary metering tubes are in some cases not tightly sealed, but are operated as flow meters with slowly flowing gas. This avoids problems with contamination, chemical reactions of the gas or small leaks. With Geiger-Müller counters, the addition of ethanol, which would otherwise be used up in the counter tube operation, can be kept constant.

history

Geiger counter, 1932. Science Museum London .

A forerunner of the counter tubes was first described by Hans Geiger in 1913 . The Geiger-Müller counter tube goes back to Geiger's development work together with his colleague Walther Müller at the University of Kiel , the results of which were published from 1928. It was the first known and commonly used type of detector that responded to particles or radiation quanta with an electrical impulse. The practical use of the proportional range is more demanding in electronic terms - amplification of the pulses, stability of the high voltage - and only became a routine method from the middle of the 20th century.

Since the impulses of the Geiger-Müller counter tube are the same for all particles, it is particularly suitable for counting the incident particles / quanta. The designation "Geiger counter" or "Geiger counter tube" therefore seems natural. This designation was carried over to the detectors developed later, such as “proportional counter”, “ scintillation counter ” etc., although these are not only used for counting, but also for measuring energy and for differentiating between types of radiation.

literature

  • Glenn F. Knoll: Radiation detection and measurement. 2nd edition, Wiley, New York 1989, ISBN 0-471-81504-7 .
  • Konrad Kleinknecht: Detectors for particle radiation . 4th edition, Teubner 2005, ISBN 978-3-8351-0058-9
  • Sebastian Korff: The Geiger-Müller counter tube. An analysis of the history of science using the replication method. In: NTM Journal for the History of Science, Technology and Medicine , Volume 20, Issue 4, 2012, pp. 271–308, ( doi: 10.1007 / s00048-012-0080-y ).

Web links

Wiktionary: Geiger counter  - explanations of meanings, word origins, synonyms, translations
Commons : Geiger counter  - collection of images, videos and audio files
Wikibooks: Gas-filled Radiation Detectors  - Learning and Teaching Materials

Individual evidence

  1. Knoll (see list of literature) p. 166 f.
  2. BT Cleveland et al: Measurement of the Solar Electron Neutrino Flux with the Homestake Chlorine Detector . In: Astrophysical Journal . 496, 1998, pp. 505-526. doi : 10.1086 / 305343 .
  3. C. Gerthsen: Physik , 6th edition, Springer, 1960.
  4. ^ EB Paul: Nuclear and Particle Physics , North-Holland, 1969, p. 124.
  5. Knoll (see list of literature) p. 168.
  6. Paul (see above) p. 127.
  7. Data from commercial radiation monitors as an example ( Memento from March 24, 2009 in the Internet Archive ).
  8. H. Geiger, W. Müller: Electron counter tube for measuring weakest activities. In: Die Naturwissenschaften , 16/31, pp. 617–618. (Lecture and demonstration at the Kiel meeting of the Lower Saxony Gauverein of the German Physical Society on July 7, 1928).
  9. H. Geiger, W. Müller: The electron counter . In: Physikalische Zeitschrift 29, pp. 839-841, (1928).
  10. H. Geiger, W. Müller: Technical remarks on the electron counter. In: Physikalische Zeitschrift. 30, pp. 489-493. (1929).
  11. H. Geiger, W. Müller: Demonstration of the electron counter. In: Physikalische Zeitschrift 30, p. 523 ff. (1929).
  12. ^ Bernard L. Cohen: Concepts of Nuclear Physics . New York etc .: McGraw-Hill, 1971, p. 217 ..,