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Geiger counter: device and household variations. Geiger counter: device and household variations Why use a geiger counter

Regardless of whether we want it or not, but the term "radiation" for a long time wedged into our consciousness and being, and no one can hide from the fact of its presence. People have to learn to live with this somewhat negative phenomenon. The phenomenon of radiation can manifest itself with the help of invisible and imperceptible radiations, and it is almost impossible to reveal it without special equipment.

From the history of the study of radiation

In 1895 X-rays were discovered. A year later, the phenomenon of uranium radioactivity was discovered, also associated with the discovery and use of X-rays. The researchers had to face a completely new, hitherto unseen natural phenomenon.

It should be noted that the phenomenon of radiation had already been encountered several years before, but the phenomenon was not given due attention. And this despite the fact that even the famous Nikola Tesla, as well as the working staff in the Edison laboratory, were burned with X-rays. The deterioration of health was explained by everything they could, but not by radiation.

Later, with the beginning of the 20th century, articles appeared on the harmful effects of radiation on experimental animals. This also went unnoticed until one notorious incident in which "radium girls" - workers in a factory that produced luminous watches - suffered.

The factory management told the girls about the harmlessness of radium, and they took lethal doses of radiation: they licked the tips of brushes with radium paint, for fun they painted their nails and even teeth with a luminous substance. Five girls who suffered from such work managed to file a lawsuit against the factory. As a result, a precedent was set in relation to the rights of some workers who got occupational diseases and sued their employers.

The history of the appearance of the Geiger-Muller counter

The German physicist Hans Geiger, who worked in one of Rutherford's laboratories, developed and proposed in 1908 circuit diagram operation of the "charged particle" counter. It was a modification of the already familiar then ionization chamber, which was presented in the form of an electric capacitor filled with gas at low pressure. The camera was used by Pierre Curie when he studied the electrical properties of gases. Geiger came up with the idea of using it to detect ionizing radiation precisely because this radiation had a direct effect on the level of ionization of gases.

At the end of the 1920s, Walter Müller, under the leadership of Geiger, created some types of radiation counters, with which it was possible to register a wide variety of ionizing particles. Work on the creation of counters was very necessary, because without them it was impossible to study radioactive materials. Geiger and Muller had to purposefully work on the creation of such counters that would be sensitive to any of the varieties of radiation of the α, β and γ types identified at that time.

Geiger-Muller counters have proven to be simple, reliable, cheap, and also practical radiation sensors. This despite the fact that they were not the most accurate instruments for studying radiation or certain particles. But they were very well suited as instruments for general measurements of the saturation of ionizing radiation. In combination with other instruments, they are still used by practical physicists for more accurate measurements in the process of experimentation.

What is ionizing radiation?

For a better understanding of the operation of Geiger-Muller counters, it would not hurt to get acquainted with ionizing radiation as such. It can include everything that causes the ionization of substances that are in a natural state. This will require the presence of some kind of energy. In particular, ultraviolet light or radio waves are not classified as ionizing radiation. The demarcation can begin with the so-called "hard ultraviolet", also called "soft X-ray". This type of flow is called photon radiation. A stream of high-energy photons are gamma quanta.

For the first time, the division of ionizing radiation into three types was done by Ernst Rutherford. Everything was done on research equipment that involved a magnetic field in empty space. This was later named:

α - nuclei of helium atoms;
β - high energy electrons;
γ - gamma quanta (photons).

Later, neutrons were discovered. So, it turned out that alpha particles can easily be retained even with ordinary paper, beta particles have a slightly higher penetrating power, and gamma rays have the highest. Neutrons are considered the most dangerous, especially at a distance of many tens of meters in airspace. Due to their electrical indifference, they do not interact with any electron shell of the molecules in the substance.

However, when entering atomic nuclei with a high potential lead to their instability and decay, after which radioactive isotopes are formed. And those, further in the process of decay, themselves form the entirety of ionizing radiation.

Geiger-Muller counter devices and operating principles

Gas-discharge Geiger-Muller counters are mainly made as hermetic tubes, glass or metal, from which all the air has been evacuated. It is replaced by an added inert gas (neon or argon or a mixture thereof) at low pressure, with halogen or alcohol impurities. Thin wires are stretched along the axes of the tubes, and metal cylinders are located coaxially with them. Both tubes and wires are electrodes: tubes are cathodes, and wires are anodes.

Minuses from constant voltage sources are connected to the cathodes, and pluses from sources with constant voltage are connected to the anodes - using a large constant resistance. From an electrical point of view, a voltage divider comes out. and in the middle of it the voltage level is almost the same as the voltage at the source. As a rule, it can reach up to several hundred volts.

As the ionizing particles fly through the tubes, the atoms in the inert gas, which are already in a high-intensity electric field, collide with these particles. The energy that was given away by the particles during the collision is considerable, it is enough for the electrons to break away from the atoms of the gas. The resulting secondary order electrons themselves are able to form further collisions, after which a whole electronic and ionic cascade emerges.

When exposed to an electric field, electrons are accelerated towards the anodes, and positively charged gas ions - towards the cathodes of the tubes. As a result, an electric current is generated. Since the energy of the particles had already been used up for collisions, in whole or in part (the particles flew through the tube), the ionized gas atoms began to run out.

As soon as the charged particles entered the Geiger-Muller counter, the resistance of the tube dropped by the nascent current, and at the same time the voltage at the central mark of the separator changed, as was indicated earlier. After that, the resistance in the tube, as a result of its growth, resumes, and the voltage level returns to its previous state. As a result, negative voltage pulses are obtained. By counting the pulses, you can set the number of particles that have flown. The greatest intensity of the electric field is observed near the anode, due to its small size, as a result of which the counters become more sensitive.

Designs of Geiger-Muller counters

All modern Geiger-Muller counters have two main varieties: "classical" and flat. Classic counters are made of thin-walled corrugated metal tubes. The corrugated surfaces of the meters make the tubes rigid, they will withstand external atmospheric pressure, and will not allow them to wrinkle under any influence. At the ends of the tubes there are glass or plastic hermetic insulators. There are also taps-caps to connect to the circuit. The tubes are marked and coated with a durable insulating varnish indicating the polarity of the taps. In general, these are universal counters for any kind of ionizing radiation, especially for beta-gamma radiation.

Counters that may be sensitive to soft β radiation are manufactured differently. Due to the small ranges of β-particles, they are made flat. Mica windows weakly delay beta radiation. One such counter can be called a BETA-2 sensor. In all other counters, the determination of their properties is attributed to the materials of their manufacture.

All counters that register gamma radiation have cathodes made of such metals, in which there is a large charge number. Gases are extremely unsatisfactorily ionized by gamma photons. However, gamma photons can knock out a lot of secondary electrons from cathodes if chosen properly. Most Geiger-Muller counters for beta particles are made to have thin windows. This is done to improve the permeability of the particles, because they are just ordinary electrons that have received more energy. They have a very good and fast interaction with substances, as a result of which energy is lost.

With alpha particles, things are much worse. For example, despite a fairly decent energy, a few MeV, alpha particles have a very strong interaction with molecules moving along the way and soon losing their energy potential. Ordinary counters respond well to α-radiation, but only at a distance of a few centimeters.

To make an objective assessment of the level of ionizing radiation, dosimeters on counters with general application are often equipped with two counters operating in series. One may be more sensitive to α-β radiation, and the other to γ radiation. Sometimes bars or plates made of alloys containing cadmium impurities are placed among the counters. When neutrons hit such bars, γ-radiation occurs, which is recorded. This is done for the possible determination of neutron radiation, and simple Geiger counters have practically no sensitivity to it.

How Geiger counters are used in practice

The Soviet, and now the Russian industry produces many varieties of Geiger-Muller counters. Such devices are mainly used by people who have something to do with nuclear industry facilities, scientific or educational institutions, civil defense, and medical diagnostics.

After the Chernobyl disaster, household dosimeters, previously completely unfamiliar to the population of our country even by name, began to gain truly nationwide popularity. Many household models began to appear. All of them use their own Geiger-Muller counters as radiation sensors. Usually, one or two tubes or end counters are installed in household dosimeters.

Using a modern Geiger counter, you can measure the level of radiation building materials, land plot or apartments, as well as food. It demonstrates an almost one hundred percent probability of a charged particle, because only one electron-ion pair is enough to fix it.

The technology on the basis of which a modern dosimeter based on the Geiger-Muller counter was created makes it possible to obtain high-precision results in a very short period of time. The measurement takes no more than 60 seconds, and all information is displayed in graphical and numerical form on the screen of the dosimeter.

Instrument setup

The device has the ability to adjust the threshold value, when it is exceeded, an audible signal is emitted to warn you of the danger. Select one of the preset threshold values in the corresponding settings section. The beep can also be turned off. Before taking measurements, it is recommended to individually configure the device, select the display brightness, the parameters of the sound signal and batteries.

Measurement order

Select the "Measurement" mode, and the device will start assessing the radioactive environment. After about 60 seconds, the measurement result appears on its display, after which the next analysis cycle begins. In order to obtain an accurate result, it is recommended to carry out at least 5 measurement cycles. Increasing the number of observations gives more reliable readings.

To measure the radiation background of objects, such as building materials or food products, you need to turn on the “Measurement” mode at a distance of several meters from the object, then bring the device to the object and measure the background as close to it as possible. Compare the readings of the device with the data obtained at a distance of several meters from the object. The difference between these readings is the additional radiation background of the object under study.

If the measurement results exceed the natural background characteristic of the area in which you are, this indicates radiation contamination of the object under study. To assess the contamination of a liquid, it is recommended to measure above its open surface. To protect the device from moisture, it must be wrapped with plastic wrap, but not more than in one layer. If the dosimeter has been at a temperature below 0°C for a long time, it must be kept at room temperature for 2 hours before taking measurements.

Geiger-Muller counter

D used to determine the level of radiation special device– . And for such devices of household and most professional dosimetric control devices, as a sensitive element is used Geiger counter . This part of the radiometer allows you to accurately determine the level of radiation.

History of the Geiger counter

AT first, a device for determining the intensity of the decay of radioactive materials was born in 1908, it was invented by a German physicist Hans Geiger . Twenty years later, together with another physicist Walter Müller the device was improved, and in honor of these two scientists it was named.

AT period of development and formation nuclear physics in the former Soviet Union, corresponding devices were also created, which were widely used in the armed forces, in nuclear power plants, and in special civil defense radiation monitoring groups. Since the seventies of the last century, such dosimeters included a counter based on Geiger principles, namely SBM-20 . This counter, exactly like another one of its analogues STS-5 , is widely used in this moment, and is also part of modern means dosimetric control .

Fig.1. Gas-discharge counter STS-5.

Fig.2. Gas-discharge counter SBM-20.

The principle of operation of the Geiger-Muller counter

And The idea of registering radioactive particles proposed by Geiger is relatively simple. It is based on the principle of the appearance of electrical impulses in an inert gas medium under the action of a highly charged radioactive particle or a quantum of electromagnetic oscillations. To dwell on the mechanism of action of the counter in more detail, let us dwell a little on its design and the processes occurring in it, when a radioactive particle passes through the sensitive element of the device.

R the registering device is a sealed cylinder or container that is filled with an inert gas, it can be neon, argon, etc. Such a container can be made of metal or glass, and the gas in it is under low pressure, this is done on purpose to simplify the process of detecting a charged particle. Inside the container there are two electrodes (cathode and anode) to which high voltage direct current through a special load resistor.

Fig.3. The device and circuit for switching on the Geiger counter.

P When the counter is activated in an inert gas medium, a discharge does not occur on the electrodes due to the high resistance of the medium, however, the situation changes if a radioactive particle or a quantum of electromagnetic oscillations enters the chamber of the sensitive element of the device. In this case, a particle with a sufficiently high energy charge knocks out a certain number of electrons from the nearest environment, i.e. from the body elements or the physical electrodes themselves. Such electrons, once in an inert gas environment, under the action of a high voltage between the cathode and anode, begin to move towards the anode, ionizing the molecules of this gas along the way. As a result, they knock out secondary electrons from the gas molecules, and this process grows on a geometric scale until a breakdown occurs between the electrodes. In the discharge state, the circuit closes for a very short period of time, and this causes a current jump in the load resistor, and it is this jump that allows you to register the passage of a particle or quantum through the registration chamber.

T This mechanism makes it possible to register one particle, however, in an environment where ionizing radiation is sufficiently intense, a rapid return of the registration chamber to its original position is required in order to be able to determine new radioactive particle . This is achieved by two different ways. The first of these is to stop the voltage supply to the electrodes for a short period of time, in which case the ionization of the inert gas stops abruptly, and a new inclusion of the test chamber allows you to start recording from the very beginning. This type of counter is called non-self-extinguishing dosimeters . The second type of devices, namely self-extinguishing dosimeters, the principle of their operation is to add special additives based on various elements, for example, bromine, iodine, chlorine or alcohol. In this case, their presence automatically leads to the termination of the discharge. With such a structure of the test chamber, resistances sometimes of several tens of megaohms are used as a load resistor. This allows during the discharge to sharply reduce the potential difference at the ends of the cathode and anode, which stops the conductive process and the chamber returns to its original state. It should be noted that the voltage on the electrodes of less than 300 volts automatically stops maintaining the discharge.

The whole described mechanism allows to register a huge number of radioactive particles in a short period of time.

Types of radioactive radiation

H to understand what is registered Geiger–Muller counters , it is worth dwelling on what types of it exist. It is worth mentioning right away that gas-discharge counters, which are part of most modern dosimeters, are only able to register the number of radioactive charged particles or quanta, but cannot determine either their energy characteristics or the type of radiation. To do this, dosimeters are made more multifunctional and targeted, and in order to compare them correctly, one should more accurately understand their capabilities.

P according to modern ideas of nuclear physics, radiation can be divided into two types, the first in the form electromagnetic field , the second in the form particle flow (corpuscular radiation). The first type can be flux of gamma particles or x-rays . Their main feature is the ability to propagate in the form of a wave over very long distances, while they easily pass through various objects and can easily penetrate into the most various materials. For example, if a person needs to hide from the flow of gamma rays due to a nuclear explosion, then hiding in the basement of a house or bomb shelter, subject to its relative tightness, he can only protect himself from this type of radiation by 50 percent.

Fig.4. Quanta of x-ray and gamma radiation.

T what type of radiation is of a pulsed nature and is characterized by propagation in the environment in the form of photons or quanta, i.e. short bursts of electromagnetic radiation. Such radiation can have different energy and frequency characteristics, for example, X-ray radiation has a thousand times lower frequency than gamma rays. That's why gamma rays are much more dangerous for human body and their impact is much more destructive.

And Radiation based on the corpuscular principle is alpha and beta particles (corpuscles). They arise as a result of a nuclear reaction, in which some radioactive isotopes are converted into others with the release of an enormous amount of energy. In this case, beta particles are a stream of electrons, and alpha particles are much larger and more stable formations, consisting of two neutrons and two protons bound to each other. In fact, the nucleus of the helium atom has such a structure, so it can be argued that the flow of alpha particles is the flow of helium nuclei.

The following classification has been adopted , alpha particles have the least penetrating ability to protect themselves from them, thick cardboard is enough for a person, beta particles have a greater penetrating ability, so that a person can protect himself from a stream of such radiation, he will need metal protection several millimeters thick (for example, aluminum sheet). There is practically no protection from gamma quanta, and they spread over considerable distances, fading as they move away from the epicenter or source, and obeying the laws of electromagnetic wave propagation.

Fig.5. Radioactive particles alpha and beta type.

To The amounts of energy possessed by all these three types of radiation are also different, and the alpha particle flux has the largest of them. For example, the energy possessed by alpha particles is seven thousand times greater than the energy of beta particles , i.e. The penetrating power of various types of radiation is inversely proportional to their penetrating power.

D For the human body, the most dangerous type of radioactive radiation are considered gamma quanta , due to high penetrating power, and then descending, beta particles and alpha particles. Therefore, it is quite difficult to determine alpha particles, if it is impossible to say with a conventional counter. Geiger - Muller, since almost any object is an obstacle for them, not to mention a glass or metal container. It is possible to determine beta particles with such a counter, but only if their energy is sufficient to pass through the material of the counter container.

For low-energy beta particles, the conventional Geiger–Muller counter is inefficient.

O In a similar situation with gamma radiation, there is a possibility that they will pass through the container without triggering an ionization reaction. To do this, a special screen (made of dense steel or lead) is installed in the meters, which allows you to reduce the energy of gamma rays and thus activate the discharge in the counter chamber.

Basic characteristics and differences of Geiger-Muller counters

FROM It is also worth highlighting some of the basic characteristics and differences of various dosimeters equipped with Geiger-Muller gas-discharge counters. To do this, you should compare some of them.

The most common Geiger-Muller counters are equipped with cylindrical or end sensors. Cylindrical are similar to an oblong cylinder in the form of a tube with a small radius. The end ionization chamber has a round or rectangular shape of small size, but with a significant end working surface. Sometimes there are varieties of end chambers with an elongated cylindrical tube with a small entrance window on the end side. Various configurations of counters, namely the cameras themselves, are able to register different types radiation, or their combinations (for example, combinations of gamma and beta rays, or the entire spectrum of alpha, beta and gamma). This becomes possible due to the specially designed design of the meter case, as well as the material from which it is made.

E Another important component for the intended use of meters is the area of the input sensitive element and the working area . In other words, this is the sector through which radioactive particles of interest to us will enter and be registered. The larger this area, the more the counter will be able to capture particles, and the stronger its sensitivity to radiation will be. The passport data indicate the area working surface, usually in square centimeters.

E Another important indicator, which is indicated in the characteristics of the dosimeter, is noise level (measured in pulses per second). In other words, this indicator can be called the intrinsic background value. It can be determined in the laboratory, for this the device is placed in a well-protected room or chamber, usually with thick lead walls, and the level of radiation emitted by the device itself is recorded. It is clear that if such a level is significant enough, then these induced noises will directly affect the measurement errors.

Each professional and radiation has such a characteristic as radiation sensitivity, also measured in pulses per second (imp/s), or in pulses per microroentgen (imp/µR). Such a parameter, or rather its use, directly depends on the source of ionizing radiation, to which the counter is tuned, and on which further measurement will be carried out. Often tuning is done by sources, including such radioactive materials as radium - 226, cobalt - 60, cesium - 137, carbon - 14 and others.

E Another indicator by which it is worth comparing dosimeters is ion radiation detection efficiency or radioactive particles. The existence of this criterion is due to the fact that not all radioactive particles passing through the sensitive element of the dosimeter will be registered. This can happen in the case when the gamma radiation quantum did not cause ionization in the counter chamber, or the number of particles that passed and caused ionization and discharge is so large that the device does not adequately count them, and for some other reasons. To accurately determine this characteristic a specific dosimeter, it is tested using some radioactive sources, for example, plutonium-239 (for alpha particles), or thallium - 204, strontium - 90, yttrium - 90 (beta emitter), as well as other radioactive materials.

FROM The next criterion to consider is registered energy range . Any radioactive particle or radiation quantum has a different energy characteristic. Therefore, dosimeters are designed to measure not only a specific type of radiation, but also their respective energy characteristics. Such an indicator is measured in megaelectronvolts or kiloelectronvolts, (MeV, KeV). For example, if beta particles do not have sufficient energy, then they will not be able to knock out an electron in the counter chamber, and therefore will not be registered, or, only high-energy alpha particles will be able to break through the material of the body of the Geiger-Muller counter and knock out an electron.

And Based on the foregoing, modern manufacturers of radiation dosimeters produce a wide range of devices for various purposes and specific industries. Therefore, it is worth considering specific types of Geiger counters.

Various options Geiger-Muller counters

P The first version of dosimeters are devices designed to register and detect gamma photons and high-frequency (hard) beta radiation. Almost all of the previously produced and modern, both household, for example:, and professional radiation dosimeters, for example, are designed for this measurement range. Such radiation has sufficient energy and high penetrating power so that the Geiger counter camera can register them. Such particles and photons easily penetrate the walls of the counter and cause the ionization process, and this is easily recorded by the corresponding electronic filling of the dosimeter.

D for registration of this type of radiation are excellent popular counters type SBM-20 , having a sensor in the form of a cylindrical tube-cylinder with a coaxially wired cathode and anode. Moreover, the walls of the sensor tube serve simultaneously as a cathode and a housing, and are made of stainless steel. This counter has the following characteristics:

the area of the working area of the sensitive element is 8 square centimeters;
radiation sensitivity to gamma radiation of the order of 280 pulses / s, or 70 pulses / μR (testing was carried out for cesium - 137 at 4 μR / s);
the intrinsic background of the dosimeter is about 1 imp/s;
The sensor is designed to detect gamma radiation with an energy in the range from 0.05 MeV to 3 MeV, and beta particles with an energy of 0.3 MeV along the lower boundary.

Fig.6. Geiger counter device SBM-20.

At There were various modifications of this counter, for example, SBM-20-1 or SBM-20U , which have similar characteristics, but differ in the fundamental design of the contact elements and the measuring circuit. Other modifications of this Geiger-Muller counter, and these are SBM-10, SI29BG, SBM-19, SBM-21, SI24BG, have similar parameters as well, many of them are found in household radiation dosimeters that can be found in stores today.

FROM The next group of radiation dosimeters is designed to register gamma photons and x-rays . If we talk about the accuracy of such devices, it should be understood that photon and gamma radiation are electromagnetic radiation quanta that move at the speed of light (about 300,000 km / s), so registering such an object is a rather difficult task.

The efficiency of such Geiger counters is about one percent.

H To increase it, an increase in the cathode surface is required. In fact, gamma quanta are registered in an indirect way, thanks to the electrons knocked out by them, which are involved in the consequence in the ionization of an inert gas. In order to promote this phenomenon as efficiently as possible, the material and wall thickness of the counter chamber, as well as the dimensions, thickness and material of the cathode, are specially selected. Here, a large thickness and density of the material can reduce the sensitivity of the registration chamber, and too small will allow high-frequency beta radiation to easily enter the camera, and also increase the amount of radiation noise natural for the device, which will drown out the accuracy of determining gamma quanta. Naturally, the exact proportions are selected by manufacturers. In fact, on this principle, dosimeters are manufactured based on Geiger-Muller counters for direct determination of gamma radiation on the ground, while such a device excludes the possibility of determining any other types of radiation and radioactive impact, which allows you to accurately determine the radiation contamination and the level of negative impact on a person only by gamma radiation.

AT domestic dosimeters that are equipped with cylindrical sensors, the following types are installed: SI22G, SI21G, SI34G, Gamma 1-1, Gamma - 4, Gamma - 5, Gamma - 7ts, Gamma - 8, Gamma - 11 and many others. Moreover, in some types, a special filter is installed on the input, end, sensitive window, which specifically serves to cut off alpha and beta particles, and additionally increases the cathode area, for more efficient determination of gamma quanta. These sensors include Beta - 1M, Beta - 2M, Beta - 5M, Gamma - 6, Beta - 6M and others.

H To understand more clearly the principle of their action, it is worth considering in more detail one of these counters. For example, an end counter with a sensor Beta - 2M , which has a rounded shape of the working window, which is about 14 square centimeters. In this case, the radiation sensitivity to cobalt - 60 is about 240 pulses / μR. This type of meter has very low self-noise performance. , which is no more than 1 pulse per second. This is possible due to the thick-walled lead chamber, which, in turn, is designed to detect photon radiation with energies in the range from 0.05 MeV to 3 MeV.

Fig.7. End gamma counter Beta-2M.

To determine gamma radiation, it is quite possible to use counters for gamma-beta pulses, which are designed to register hard (high-frequency and high-energy) beta particles and gamma quanta. For example, the SBM model is 20. If you want to exclude the registration of beta particles in this dosimeter model, then it is enough to install a lead screen, or a shield made of any other metal material (a lead screen is more effective). This is the most common way that most designers use when creating counters for gamma and x-rays.

Registration of "soft" beta radiation.

To As we mentioned earlier, registration of soft beta radiation (radiation with low energy characteristics and relatively low frequency) is a rather difficult task. To do this, it is required to provide the possibility of their easier penetration into the registration chamber. For these purposes, a special thin working window is made, usually from mica or a polymer film, which practically does not create obstacles for the penetration of this type of beta radiation into the ionization chamber. In this case, the sensor body itself can act as a cathode, and the anode is a system of linear electrodes, which are evenly distributed and mounted on insulators. The registration window is made in the end version, and in this case only a thin mica film appears on the path of beta particles. In dosimeters with such counters, gamma radiation is registered as an application and, in fact, as an additional feature. And if you want to get rid of the registration of gamma quanta, then you need to minimize the surface of the cathode.

Fig.8. Geiger counter device.

FROM It should be noted that counters for determining soft beta particles were created quite a long time ago and were successfully used in the second half of the last century. Among them, the most common were sensors of the type SBT10 and SI8B , which had thin-walled mica working windows. More modern version such a device Beta 5 has a working window area of about 37 sq/cm, rectangular in shape made of mica material. For such dimensions of the sensitive element, the device is able to register about 500 pulses/µR, if measured by cobalt - 60. At the same time, the detection efficiency of particles is up to 80 percent. Other indicators of this device are as follows: self-noise is 2.2 pulses / s, the energy detection range is from 0.05 to 3 MeV, while the lower threshold for determining soft beta radiation is 0.1 MeV.

Fig.9. End beta-gamma counter Beta-5.

And Naturally, it is worth mentioning Geiger-Muller counters capable of detecting alpha particles. If the registration of soft beta radiation seems to be a rather difficult task, then it is even more difficult to detect an alpha particle, even with high energy indicators. difficult task. Such a problem can only be solved by a corresponding reduction in the thickness of the working window to a thickness that is sufficient for the passage of an alpha particle into the registration chamber of the sensor, as well as by almost complete approximation of the input window to the source of radiation of alpha particles. This distance should be 1 mm. It is clear that such a device will automatically register any other types of radiation, and, moreover, with a sufficiently high efficiency. This has both positive and negative sides:

Positive - such a device can be used for the widest range of analysis of radioactive radiation

negative - at the expense hypersensitivity, there will be a significant amount of noise that will make it difficult to analyze the received registration data.

To In addition, although the mica working window is too thin, it increases the capabilities of the counter, but to the detriment of the mechanical strength and tightness of the ionization chamber, especially since the window itself has a fairly large working surface area. For comparison, in the counters SBT10 and SI8B, which we mentioned above, with a working window area of about 30 sq/cm, the thickness of the mica layer is 13–17 µm, and with the necessary thickness for recording alpha particles of 4–5 µm the window can only be made no more than 0.2 sq / cm, we are talking about the SBT9 counter.

O However, the large thickness of the registration working window can be compensated by the proximity to the radioactive object, and vice versa, with a relatively small thickness of the mica window, it becomes possible to register an alpha particle at a greater distance than 1 -2 mm. It is worth giving an example, with a window thickness of up to 15 microns, the approach to the source of alpha radiation should be less than 2 mm, while the source of alpha particles is understood to be a plutonium-239 emitter with a radiation energy of 5 MeV. Let us continue, with an input window thickness of up to 10 µm, it is possible to register alpha particles already at a distance of up to 13 mm, if a mica window is made up to 5 µm thick, then alpha radiation will be recorded at a distance of 24 mm, etc. Another important parameter that directly affects the ability to detect alpha particles is their energy index. If the energy of the alpha particle is greater than 5 MeV, then the distance of its registration for the thickness of the working window of any type will increase accordingly, and if the energy is less, then the distance must be reduced, up to the complete impossibility of registering soft alpha radiation.

E one more important point, allowing to increase the sensitivity of the alpha counter, this is a decrease in the registration ability for gamma radiation. To do this, it is enough to minimize the geometric dimensions of the cathode, and gamma photons will pass through the registration chamber without causing ionization. Such a measure makes it possible to reduce the influence of gamma rays on ionization by thousands, and even tens of thousands of times. It is no longer possible to eliminate the influence of beta radiation on the registration chamber, but there is a rather simple way out of this situation. First, alpha and beta radiation of the total type are recorded, then a thick paper filter is installed, and a second measurement is made, which will register only beta particles. The value of alpha radiation in this case is calculated as the difference between the total radiation and a separate indicator of the calculation of beta radiation.

For example , it is worth suggesting the characteristics of a modern Beta-1 counter, which allows you to register alpha, beta, gamma radiation. Here are the metrics:

the area of the working zone of the sensitive element is 7 sq/cm;
the thickness of the mica layer is 12 microns, (the effective detection distance of alpha particles for plutonium is 239, about 9 mm, for cobalt - 60, the radiation sensitivity is about 144 pulses / microR);
radiation measurement efficiency for alpha particles - 20% (for plutonium - 239), beta particles - 45% (for thallium -204), and gamma quanta - 60% (for the composition of strontium - 90, yttrium - 90);
the dosimeter's own background is about 0.6 imp/s;
The sensor is designed to detect gamma radiation with an energy in the range from 0.05 MeV to 3 MeV, and beta particles with an energy of more than 0.1 MeV along the lower boundary, and alpha particles with an energy of 5 MeV or more.

Fig.10. End alpha-beta-gamma counter Beta-1.

To Of course, there is still a fairly wide range of counters that are designed for a narrower and more professional use. Such devices have a number of additional settings and options (electrical, mechanical, radiometric, climatic, etc.), which include many special terms and options. However, we will not focus on them. Indeed, in order to understand the basic principles of action Geiger-Muller counters , the models described above are sufficient.

AT It is also important to mention that there are special subclasses Geiger counters , which are specially designed to determine various kinds other radiation. For example, to determine the value of ultraviolet radiation, to detect and determine slow neutrons that operate on the principle of a corona discharge, and other options that are not directly related to this topic will not be considered.

Geiger counter- a gas-discharge device for counting the number of ionizing particles that have passed through it. It is a gas-filled capacitor that breaks through when an ionizing particle appears in the gas volume. Geiger counters are quite popular detectors (sensors) of ionizing radiation. Until now, they, invented at the very beginning of our century for the needs of nascent nuclear physics, do not, oddly enough, have any full-fledged replacement.

The design of the Geiger counter is quite simple. A gas mixture consisting of easily ionizable neon and argon is introduced into a sealed container with two electrodes. The material of the container can be different - glass, metal, etc.

Usually meters perceive radiation with their entire surface, but there are also those that have a special “window” in the cylinder for this. The widespread use of the Geiger-Muller counter is explained by its high sensitivity, the ability to register various radiation, and the comparative simplicity and low cost of installation.

Geiger counter wiring diagram

A high voltage U is applied to the electrodes (see Fig.), which in itself does not cause any discharge phenomena. The counter will remain in this state until an ionization center appears in its gaseous medium - a trace of ions and electrons generated by an ionizing particle that has come from outside. Primary electrons, accelerating in electric field, ionize "along the way" other molecules of the gaseous medium, generating more and more new electrons and ions. Developing like an avalanche, this process ends with the formation of an electron-ion cloud in the space between the electrodes, which significantly increases its conductivity. In the gas environment of the counter, a discharge occurs, which is visible (if the container is transparent) even with a simple eye.

The reverse process - the restoration of the gaseous medium to its original state in the so-called halogen meters - occurs by itself. Halogens (usually chlorine or bromine), which are contained in a small amount in the gaseous medium, come into play, which contribute to the intensive recombination of charges. But this process is rather slow. The time required to restore the radiation sensitivity of the Geiger counter and actually determines its speed - "dead" time - is its main passport characteristic.

Such meters are designated as halogen self-extinguishing meters. With very low supply voltage, good parameters output signal and sufficiently high speed, they turned out to be in demand as sensors of ionizing radiation in household radiation monitoring devices.

Geiger counters are capable of detecting the most different types ionizing radiation - a, b, g, ultraviolet, x-ray, neutron. But the actual spectral sensitivity of the counter is very dependent on its design. Thus, the input window of a counter sensitive to a- and soft b-radiation should be rather thin; for this, mica 3–10 µm thick is usually used. The balloon of a counter that reacts to hard b- and g-radiation usually has the shape of a cylinder with a wall thickness of 0.05 .... 0.06 mm (it also serves as the cathode of the counter). The X-ray counter window is made of beryllium, and the ultraviolet window is made of quartz glass.

The dependence of the counting rate on the supply voltage in the Geiger counter

Boron is introduced into the neutron counter, upon interaction with which the neutron flux is converted into easily detectable a-particles. Photon radiation - ultraviolet, x-ray, g-radiation - Geiger counters perceive indirectly - through the photoelectric effect, the Compton effect, the effect of pair production; in each case, the radiation interacting with the material of the cathode is converted into a stream of electrons.

Each particle detected by the counter forms a short pulse in its output circuit. The number of pulses that appear per unit time - the count rate of the Geiger counter - depends on the level of ionizing radiation and the voltage on its electrodes. The standard plot of the counting rate versus the supply voltage Upit is shown in the figure above. Here Uns is the voltage of the beginning of counting; Ung and Uvg are the lower and upper limits of the working area, the so-called plateau, on which the counting rate is almost independent of the meter supply voltage. The operating voltage Ur is usually chosen in the middle of this section. It corresponds to Nr, the count rate in this mode.

The dependence of the counting rate on the degree of radiation exposure of the counter is its main characteristic. The graph of this dependence is almost linear and therefore often the radiation sensitivity of the counter is shown in terms of pulses / μR (pulses per micro-roentgen; this dimension follows from the ratio of the count rate - pulse / s - to the radiation level - μR / s).

In those cases when it is not indicated, it is necessary to determine the radiation sensitivity of the counter according to its other extremely important parameter - its own background. This is the name of the counting rate, the factor of which is two components: external - the natural radiation background, and internal - the radiation of radionuclides trapped in the counter design itself, as well as the spontaneous electron emission of its cathode.

Dependence of the counting rate on the energy of gamma quanta ("stroke with rigidity") in the Geiger counter

Another essential characteristic of the Geiger counter is the dependence of its radiation sensitivity on the energy ("hardness") of ionizing particles. The extent to which this dependence is significant is shown by the graph in the figure. "Travel with rigidity" will obviously affect the accuracy of the measurements taken.

The fact that the Geiger counter is an avalanche device also has its drawbacks - one cannot judge the root cause of its excitation by the reaction of such a device. The output pulses generated by the Geiger counter under the influence of a-particles, electrons, g-quanta are no different. The particles themselves, their energies completely disappear in the twin avalanches they generate.

The table shows information about self-extinguishing halogen Geiger counters of domestic production, the most suitable for household radiation monitoring devices.

	1	2	3	4	5	6	7
SBM19	400	100	2	310*	50	19x195	1
SBM20	400	100	1	78*	50	11x108	1
SBT9	380	80	0,17	40*	40	12x74	2
SBT10A	390	80	2,2	333*	5	(83x67x37)	2
SBT11	390	80	0,7	50*	10	(55x29x23.5)	3
SI8B	390	80	2	350-500	20	82x31	2
SI14B	400	200	2	300	30	84x26	2
SI22G	390	100	1,3	540*	50	19x220	4
SI23BG	400	100	2	200-400*	—	19x195	1

1 - operating voltage, V;
2 - plateau - area of low dependence of the count rate on the supply voltage, V;
3 — own background of the counter, imp/s, no more;
4 - radiation sensitivity of the counter, pulses/μR (* - for cobalt-60);
5 - amplitude of the output pulse, V, not less;
6 — dimensions, mm — diameter x length (length x width x height);
7.1 - hard b - and g - radiation;
7.2 - the same and soft b - radiation;
7.3 - the same and a - radiation;
7.4 - g - radiation.

The structure and principle of operation of the Geiger-Muller counter

AT recent times, attention to radiation safety on the part of ordinary citizens in our country is increasingly increasing. And this is due not only to the tragic events at the Chernobyl nuclear power plant and its further consequences, but also to various kinds of incidents that periodically occur in one place or another on the planet. In this regard, at the end of the last century, devices began to appear dosimetric monitoring of radiation for household purposes. And such devices saved many people not only health, but sometimes life, and this applies not only to the territories adjacent to the exclusion zone. Therefore, the issues of radiation safety are relevant in any place of our country to this day.

AT All household and almost all modern professional dosimeters are equipped with . In another way, it can be called the sensitive element of the dosimeter. This device was invented in 1908 by the German physicist Hans Geiger, and twenty years later, another physicist Walter Müller improved this development, and it is the principle of this device that is used at the present time.

H Some modern dosimeters have four counters at once, which makes it possible to increase the accuracy of measurements and the sensitivity of the device, as well as to reduce the measurement time. Most Geiger-Muller counters are capable of detecting gamma radiation, high-energy beta radiation, and X-rays. However, there are special developments for the determination of high-energy alpha particles. To set the dosimeter to detect only gamma radiation, the most dangerous of the three types of radiation, the sensitive chamber is covered with a special casing made of lead or other steel, which makes it possible to cut off the penetration of beta particles into the counter.

AT modern dosimeters For household and professional purposes, sensors such as SBM-20, SBM-20-1, SBM-20U, SBM-21, SBM-21-1 are widely used. They differ in the overall dimensions of the camera and other parameters, for the line of 20 sensors the following dimensions are typical, length 110 mm, diameter 11 mm, and for the 21st model, length 20-22 mm with a diameter of 6 mm. It is important to understand that the larger the chamber, the large quantity radioactive elements will fly through it, and the greater the sensitivity and accuracy it has. So, for the 20th series of the sensor, the dimensions are 8-10 times larger than for the 21st, approximately in the same proportions we will have a difference in sensitivity.

To The design of a Geiger counter can be schematically described as follows. A sensor consisting of a cylindrical container filled with an inert gas (eg, argon, neon, or mixtures thereof) at minimal pressure to facilitate the initiation of an electrical discharge between the cathode and anode. The cathode, most often, is the entire metal case of the sensitive sensor, and the anode is a small wire placed on insulators. Sometimes the cathode is additionally wrapped in a protective casing made of stainless steel or lead, this is done to set the counter to detect only gamma rays.

D For domestic use, at present, end-face sensors are most often used (for example, Beta-1, Beta-2). Such counters are designed in such a way that they are able to detect and register even alpha particles. Such a counter is a flat cylinder with electrodes located inside, and an input (working) window made of a mica film with a thickness of only 12 microns. This design makes it possible to detect (at close range) high-energy alpha particles and low-energy beta particles. At the same time, the area of the working window of the Beta-1 and Beta 1-1 counters is 7 sq.cm. The area of the mica working window for the Beta-2 device is 2 times larger than that of Beta-1, it can be used to determine , etc.

E If we talk about the principle of operation of the Geiger counter chamber, then it can be briefly described as follows. When activated, a high voltage (of the order of 350 - 475 volts) is applied to the cathode and anode through a load resistor, but there is no discharge between them due to the inert gas serving as a dielectric. When it enters the chamber, its energy is sufficient to knock out a free electron from the material of the chamber body or cathode, this electron begins to knock out free electrons like an avalanche from the surrounding inert gas and its ionization occurs, which eventually leads to a discharge between the electrodes. The circuit closes, and this fact can be registered using the instrument's microchip, which is the fact of detection of either a gamma or X-ray quantum. The camera then resets, allowing the next particle to be detected.

H In order to stop the discharge process in the chamber and prepare the chamber for registration of the next particle, there are two methods, one of them is based on the fact that the voltage supply to the electrodes is stopped for a very short period of time, which stops the gas ionization process. The second method is based on adding another substance to the inert gas, for example, iodine, alcohol and other substances, while they lead to a decrease in the voltage on the electrodes, which also stops the process of further ionization and the camera becomes able to detect the next radioactive element. This method uses a high capacity load resistor.

P about the number of discharges in the counter chamber and one can judge the level of radiation in the measured area or from a specific object.