Geiger counter

Geiger counter (Счетчик Гейгера) is an electronic device for counting of ionizing particles, id est, to detect the ionizing radiation, characterized in that no pre-acceleration is used, and the energy of the detected particle causes the avalanche of electrons.

Sensor of the Geiger counter

In the simplest case, the sensor of the Geiger counter is a tube filled with the noble gas. The special electrode(s) provides the electric fild of order of Kv/mm; then the ionizing particle passing-by triggers the electric discharge. This discharge can be registered even without any amplification.

The modern Geiger counters often use the electric circuits for the handling of the avalanche, so even the avalanche of electrons in a metastable semiconductor film may be sufficient to detect the particle.

Design of the simple Geiger counter

The example of electric the circuit of the primitive counter is shown in figure.

The sensor at the top of the figure is a thin metallic tube filled with a noble gas, the Argon seems to be most convenient, at the pressure lower than atmospheric (for example, 0.1 of the atmospheric pressure). This tube is corked with plugs of glass (or ceramics) that keep electrode(s). In the picture, the left hand side plug keeps the central wire connected to the outer contact, while the right hand side of the wire is isolated. The other outher contact is connected directly to the thin wall of the tube. In order to avoid cracks (and keep the tube the hermetic), the material of the tube should have the thermal extension coefficient similar to that of the plugs; the molybdenum can be used for the tube with glass plugs; the wolfram can be used for the central wire.

In order to have high electric field (to allow the avalanche discharge) at moderate voltage at the tube, the central wire is thin, say, of order of a hundred micron in diameter. While working, this wire is kept at the positive voltage of order of 300 Volt; this is sufficient to allow an occasional electron (produced by the ionizing particle passing by) to cause an avalanche provoking the discharge.

Work of the device

The electric discharge created by a high–energy particle continues during a microsecond, until the charge stored in the capacitor \(c\) is exhausted. This current passes through the sound generator ("telephone"), making sharp "clap", click. Then, during a short time, say, some milliseconds, the discharge stops, the electrons and ions recombine and the tube is ready for detection of another ionizing particle. However, no memory to store the number of claps is provided in the simple design shown; the claps are supposed to be counted by the user such a Geiger counter. Practically, it is difficult to count these claps if they come with the mean rate several per second. At the high counting rate, the telephone produces a continuous "crashing" sound, similar to that of a wood while cracking.

Roughly, while a human can count the claps, it indicates, that there is no immediate danger for his/her life; although, in may be not a good idea, to consume a food that causes the significant increasing of the mean rate of counting. At the high counting rate, the claps make a permanent crashing sound. Such a sound indicates that someone is in a wrong, dangerous place, and should leave it as soon as possible.

At very high level of radiation, the particle come so frequently, that they maintain the continuous discharge; then the telephone remains silent. However, the current through the tube can be measured with a microampermeter switched on into a cut in the left upper wire of the scheme.

The crashing sound may indicate also some cloth contaminated with the unstable isotopes. Such indication may disappear after the cleaning of the cloth in a commercial washing-machine.

Adjusting parameters of the device

Usually, there is relatively wide interval of the voltage applied, while the counting rate is not sensitive to this voltage. Namely this interval should be used for the operation. For the most of tubes, this interval covers value 300V. Such a voltage can be obtained by the rectification of the alternating voltage 220V. The rectifier is not shown in the scheme; for the use in a country with lower voltage standard, in addition, the transformer or a c-generator may be necessary. With variable autotransformer, the working range of the Geiger sensor can be measured, and the working voltage can be adjusted.

On the base of the appropriate voltage (which is expected to be of order of 300 V), the capacitance \(c\) can be chosen. Being fully charged, the direct connection to the telephone should produce a hearable clap. The appropriate value of the capacitance \(c\) may be of order of 100 microfarad.

The resistance \(r\) should be chosen in such a way, that the capacitor \(c\) can be charged many times per second; the reasonable value may be between \(r\approx0.001\)second\( /c~\) and \(~r\approx0.01\)second\( /c\).

The capacitor \(C\) should be large, it is limited only by its weight and size. One may choose \(C=1000c~\); then, of order of a hundred clicks can be counted after the counter is disconnected from the electric power supply; one can charge the capacitor in the laboratory, then bring the counter to the street and measure the radiation there.

In principle, the telephone can be replaced to a discharge lamp indicator, or sensitive galvanometer (ampermeter), which should be placed into the cut in the upper conductor; then it not only indicates each particle detected (with a small shake of the arrow), but also shows the mean current through the tube. For the registration of the radiation at the level of the background radiation (or slightly higher), the counting of the sound clicks seems to be more convenient.

The modern Geiger counters may use also the avalanche in a semiconductor films; the energy of such an avalanche is small, and the post-amplification is required.

Particles that can be detected

Energy required for penetration of \(\beta\)-particles through aluminum versus the thickness, by

Some Geiger counters registers mainly beta-radiation, id est, electrons with energy of order of a MeV, but they can be used also for X-rays, gamma-rays and alfa-particles of energy high enough to allow them to penetrate into the Geiger tube through its walls (or through the especially-thin window). The efficiency of the registration of particles of each type depends on their energy and on the design of the registering element.

The Geiger counter registers particles with energy sufficient for the ionization of the working medium; usually the noble gas argon is used, and the enegy of ionization is of order of 10 eV. Practically, the particles with low energy cannot penetrate the walls of the Geiger tube, and the lower bound of the sensitivity is determined by the thiskness of the walls of the tube (or the thickness of the entry window). Such a window is usually protected with a mesh; this mesh is seen at the first glance on the sensitive element of many Geiger counters.

Practically, the lower limit of energy is of order of a MeV; it depends on the type of particles and the design of the sensor. An example of a plot of the electron energy required for penetration of electrons through the aluminum versus its thickness is shown in figure at right (figure by ). Similar dependence takes place also for other materials. In order to simplify the penetration of particles into the Geiger tube, the walls should be thin.

In principle, the Geiger counter could register also neutrons: Neutrons undergo the beta-decay, producing protons, antineutrino and fast electrons; these electrons are beta-radiation that can be detected. This could be used for registration of trapped ultra-cold neutrons. There exist traps for neutral atoms, but the application to neutrons have not yet been reported. Also, the neutrons are absorbed by nuclei, producing the secondary radiation (alpha, beta and/or gamma), detectable in the usual way.

History

Invention of the Geiger counter is usually attributed to the German physicist Johannes Wilhelm Geiger, who constructed and used it for the detection of high-energy particles since the beginning of century 20.

One of the first Geiger counters is attributed to Enrico Fermi. In 1930-1938, in Italy, Fermi worked with artificial radioactivity, made the source of neutrons of radon (gas) and Berillum pouder sealed in a glass tube. For the measurement of secondary radioactivity produced by these neutrons, he build the Geiger Counter Fermi received the Nobel Prize in 1938 for "his discovery of new radioactive elements produced by neutron irradiation".

The analysis of the efficiency of nuclear reactions on the energy of neutrons (neutrons were slowed down in a hydrogen-containing materials) and discovery of the fission of nuclei of Uranium caused by the neutrons was the key to the chain nuclear reaction and the atomic bomb.

The intensive collaboration between fascistic countries in 1938–1941 allows the hypothesis, that the Soviet bolsheviks for year 1941 already could count with the nuclear bombs in amount, sufficient to destroy all the big cities of Western Europe, but the beginning of the internal conflict within the USSR-Germany coalition (June, 1941) destroyed the Soviet nuclear arsenal and saved Europe from the total destruction. Such a hypothesis is considered in the article Ядерная доктрина (in Russian). Perhaps, the Geiger counters in the USSR are used since that time by the secret researchers. The unauthorized use of Geiger counters by the civil people in the Soviet Union was prohibited in order to avoid mass migration of the population from the contaminated regions around the factories of the Soviet nuclear industry. However, the Soviet concept of history attributes the creation of the first soviet nuclear bomb to the post-war period.

That time, in the USSR, there was very few Geiger counters; there was no control over the contamination. As result, in 1940s–1950s, the most of territory of the USSR become contaminated with unstable isotopes and not appropriate for agriculture. The Soviet government tried to solve this problem by using of new, still not–contaminated lands; it was called "podniatie tseliny" (transliteration from Russian "поднятие целины"). The contamination was so wide, that neither the successes of the "podniatie tseliny", nor the modernization and intensification of agriculture could supply the USSR with sufficient amount of food. The soviet genseks (Nikita Khruschev and then Leonid Brezhnev) had to accept the huge import of grain from the USA and Canada. That time, the USSR become completely dependent on the import of food and export of natural resources (mainly, gas and oil).

The soviet citizen were not allowed to have private Geiger counters (the measurements of the levels of radiation was qualified as "espionage"). Such prohibition lasted until the Chernobyl disaster in 1986, when the sabotage of the official dosimetric groups became evident. That catastrophe shown that there is no hope for the true information from the government about the levels of ionizing radiation, and for the safety, every citizen should have own Geiger counter; the mass production of self-made Geiger counters was initiated. (Similar delay with the information about the contamination occurred in 2011 in Japan: the map of contamination lated for at least two month since the Fukushima disaster, showing that even in century 21, the human society is not yet ready for the civilized use of the nuclear energy.)

The modern design

In the modern design of the Geiger counters, the number of particles that come during some time interval is counted automatically and, perhaps, is transferred to a computer. In principle the Geiger counter may work continuously, allowing the continuous update of the database making the counting rate available in the network.

Up to year 2011, even in Japan (which seems to be a world-wide leader in the technology and electronics), the automatic update of the data from the Geiger counters does not seem to be a common practice. In particular, the data about contamination of the central part of the Honshu island and that for the Sendai prefecture do not seem to be available, even after the Fukushima disaster in 2011 March. Such a data would help to localize the main source of contamination: from the preliminary analysis, the main contamination did not seem to come from the Fukushima nuclear plant (as it is usually attributed).

The external design of the Geiger counters advances much faster than the automatization. Various pictures of the design can be found in Wikipedia .

Geiger tubes at the market

Pavel Alpeyev indicates that the Geiger tube (or Geiger-Muller tube) may cost of order of a hundred dollars or even less, and the cost of tube forms of order of 40 percent of cost of the device . The disability of the official structures to provide the quick and reliable information of the nuclear contamination (sabotage of the meteorological stations, the confidential character of data of the nuclear companies, the secrecy of the governmental data) force every citizen to own a Geiger counter. In spring of 2011, this was recognized in Japan.

After the Fukushima disaster, to the summer of 2011, the Geiger counters from the storage were sold out, and the market price happen to be several times higher than the estimate above. In order to justify the high price, the distributors claim that the Geiger counters are a "high–technology products". The advances in electronics for the automatic counting of clicks seems to become a factor that prevents the manufacturing and distribution of cheap, simple and reliable Geiger counters (see the scheme above) in sufficient amount.

References

http://anekdot.ru/last/j.html Japanese company Panasonic begins to produce the mobile phones with the Geiger counter incorporated. 2011 June 4. (In Russian)

http://mdn.mainichi.jp/mdnnews/news/20110909p2a00m0na011000c.html Cheap radiation detectors give inaccurate results, consumer watchdog reports. 2011.09.09. When used to measure atmospheric radiation the detectors displayed results as much as four times higher than devices known to meet international standards, .. The NCAC purchased the nine radiation detectors, all priced at less than 100,000 yen and all believed to have been made in China, on Internet shopping sites... When exposed to radioactive cesium 137, the detectors displayed results far lower than actual radiation levels, in some cases delivering figures more than 30 percent off the genuine emissions. "These cheap devices are not accurate, and cannot be used to check radiation in food or drink," an NCAC official stated. The nine models tested were: AK2011, BS2011+, DoseRAE2 PRM-1200, DP802i, FJ2000, JB4020, RAY2000A, SW83 and SW83a. Of these, the Tokyo Metropolitan Government has bought 70 DoseRAE2 PRM-1200 detectors.