Purpose of counters

A Geiger-Muller counter is a two-electrode device designed to determine the intensity of ionizing radiation or, in other words, to count ionizing particles arising during nuclear reactions: helium ions (- particles), electrons (- particles), X-ray quanta (- particles) and neutrons. Particles spread with very high speed[up to 2 . 10 7 m/s for ions (energy up to 10 MeV) and about the speed of light for electrons (energy 0.2 - 2 MeV)], due to which they penetrate inside the counter. The role of the counter is to generate a short (fractions of a millisecond) voltage pulse (units - tens of volts) when a particle enters the volume of the device.

In comparison with other detectors (sensors) of ionizing radiation (ionization chamber, proportional counter), the Geiger-Muller counter has a high threshold sensitivity - it allows you to control the natural radioactive background of the earth (1 particle per cm 2 in 10 - 100 seconds). The upper limit of measurement is relatively low - up to 10 4 particles per cm 2 per second or up to 10 Sieverts per hour (Sv/h). A special feature of the counter is the ability to generate identical output voltage pulses regardless of the type of particles, their energy and the number of ionizations produced by the particle in the sensor volume.

The operation of a Geiger counter is based on a non-self-sustaining pulsed gas discharge between metal electrodes, which is initiated by one or more electrons resulting from the ionization of a gas -, -, or - particle. Meters usually use a cylindrical electrode design, and the diameter of the inner cylinder (anode) is much smaller (2 or more orders of magnitude) than the outer one (cathode), which is of fundamental importance. The characteristic diameter of the anode is 0.1 mm.

Particles enter the counter through a vacuum shell and a cathode in a “cylindrical” design (Fig. 2, A) or through a special flat thin window in the “end” version of the design (Fig. 2 ,b). The latter option is used to register particles that have low penetrating ability (retained, for example, by a sheet of paper), but are very biologically dangerous if the source of the particles enters the body. Detectors with mica windows are also used to count particles of relatively low energy (“soft” beta radiation).

Rice. 2. Schematic designs of a cylindrical ( A) and end ( b) Geiger counters. Designations: 1 - vacuum shell (glass); 2 - anode; 3 - cathode; 4 - window (mica, cellophane)

In the cylindrical version of the counter, designed to register high-energy particles or soft X-rays, a thin-walled vacuum shell is used, and the cathode is made of thin foil or in the form of a thin film of metal (copper, aluminum) deposited on the inner surface of the shell. In a number of designs, a thin-walled metal cathode (with stiffeners) is an element of the vacuum shell. Hard X-ray radiation (particles) has increased penetrating power. Therefore, it is recorded by detectors with fairly thick walls of a vacuum shell and a massive cathode. In neutron counters, the cathode is coated with a thin layer of cadmium or boron, in which neutron radiation is converted into radioactive radiation through nuclear reactions.

The volume of the device is usually filled with argon or neon with a small (up to 1%) admixture of argon at a pressure close to atmospheric (10 -50 kPa). To eliminate undesirable after-discharge phenomena, an admixture of bromine or alcohol vapor (up to 1%) is introduced into the gas filling.

The ability of a Geiger counter to register particles regardless of their type and energy (to generate one voltage pulse regardless of the number of electrons generated by the particle) is determined by the fact that, due to the very small diameter of the anode, almost all the voltage applied to the electrodes is concentrated in a narrow near-anode layer. Outside the layer there is a “particle trapping region” in which they ionize gas molecules. The electrons torn off by the particle from the molecules are accelerated towards the anode, but the gas is weakly ionized due to the low electric field strength. Ionization sharply increases after electrons enter the near-anode layer with high field strength, where electron avalanches (one or several) develop with a very high degree of electron multiplication (up to 10 7). However, the current resulting from this does not yet reach a value corresponding to the formation of the sensor signal.

A further increase in the current to the operating value is due to the fact that in avalanches, simultaneously with ionization, ultraviolet photons with an energy of about 15 eV are generated, sufficient to ionize impurity molecules in the gas filling (for example, the ionization potential of bromine molecules is 12.8 V). Electrons resulting from the photoionization of molecules outside the layer are accelerated towards the anode, but avalanches do not develop here due to the low field strength and the process has little effect on the development of the discharge. In the layer the situation is different: the resulting photoelectrons, due to the high voltage, initiate intense avalanches in which new photons are generated. Their number exceeds the initial one and the process in the layer according to the “photons - electron avalanches - photons” scheme quickly (several microseconds) increases (enters the “trigger mode”). In this case, the discharge from the site of the first avalanches initiated by the particle propagates along the anode (“transverse ignition”), the anode current increases sharply and the leading edge of the sensor signal is formed.

The trailing edge of the signal (current decrease) is due to two reasons: a decrease in the anode potential due to the voltage drop from the current across the resistor (at the leading edge the potential is maintained by the interelectrode capacitance) and a decrease in the electric field strength in the layer under the influence of the space charge of ions after electrons leave the anode (charge increases the potentials of the points, as a result of which the voltage drop across the layer decreases, and in the particle trapping area increases). Both reasons reduce the intensity of avalanche development and the process according to the “avalanche - photons - avalanche” scheme fades, and the current through the sensor decreases. After the end of the current pulse, the anode potential increases to the initial level (with some delay due to the charging of the interelectrode capacitance through the anode resistor), the potential distribution in the gap between the electrodes returns to its original form as a result of the departure of ions to the cathode and the counter restores the ability to register the arrival of new particles.

Dozens of types of ionizing radiation detectors are produced. Several systems are used to designate them. For example, STS-2, STS-4 - self-extinguishing end counters, or MS-4 - counter with a copper cathode (B - with tungsten, G - with graphite), or SAT-7 - end particle counter, SBM-10 - counter - metal particles, SNM-42 - metal neutron counter, SRM-1 - counter for x-rays, etc.

Geiger-Muller counter

D To determine the level of radiation, a special device is used -. And for such household devices and most professional radiation monitoring devices, the sensing element is used Geiger counter . This part of the radiometer allows you to accurately determine the level of radiation.

The history of the Geiger counter

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

IN the period of development and establishment of nuclear physics in the former Soviet Union, corresponding devices were also created that were widely used in the armed forces, at nuclear power plants, and in special radiation control groups of civil defense. Beginning in the seventies of the last century, such dosimeters included a counter based on Geiger principles, namely SBM-20 . This counter is exactly like its other analogue STS-5 , is widely used in currently, and is also part of modern means radiation monitoring .

Fig.1. Gas discharge counter STS-5.

Fig.2. Gas discharge meter SBM-20.

Operating principle of a Geiger–Müller 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 environment under the influence of a highly charged radioactive particle or a quantum of electromagnetic oscillations. To dwell in more detail on the mechanism of operation of the counter, 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 recording 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 specifically to simplify the process of registering a charged particle. Inside the container there are two electrodes (cathode and anode) to which high voltage is applied. direct current through a special load resistor.

Fig.3. Device and circuit diagram for switching on a Geiger counter.

P When the counter is activated in an inert gas environment, no discharge occurs 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 having a charge of sufficiently high energy knocks out a certain number of electrons from the immediate environment, i.e. from the housing elements or physically the electrodes themselves. Such electrons, once in an inert gas environment, under the influence of high voltage between the cathode and the 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 a 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 makes it possible to register the passage of a particle or quantum through the recording chamber.

T This mechanism makes it possible to register one particle, however, in an environment where ionizing radiation is quite intense, a rapid return of the registration chamber to initial position, to be able to determine new radioactive particle . This is achieved by two different ways. The first of them is to stop supplying voltage to the electrodes for a short period of time; in this case, the ionization of the inert gas abruptly stops, and turning on the test chamber again 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 this structure of the test chamber, resistances sometimes of several tens of megaohms are used as a load resistor. This makes it possible to sharply reduce the potential difference at the ends of the cathode and anode during the discharge, which stops the current-conducting process and the chamber returns to its original state. It is worth noting that a voltage on the electrodes of less than 300 volts automatically stops maintaining the discharge.

The entire mechanism described makes it possible to register a huge number of radioactive particles in a short period of time.

Types of radioactive radiation

H to understand what exactly is being recorded Geiger–Muller counters , it is worth dwelling on what types of it exist. It’s worth mentioning right away that gas-discharge counters, which are part of most modern dosimeters, are only capable of recording the number of radioactive charged particles or quanta, but cannot determine either their energy characteristics or the type of radiation. For this purpose, dosimeters are made more multifunctional and targeted, and in order to compare them correctly, their capabilities should be more accurately understood.

P According to modern concepts 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 includes gamma particle flux or x-ray radiation . Their main feature is the ability to propagate in the form of a wave over very long distances, while they quite easily pass through various objects and can easily penetrate a wide variety of materials. For example, if a person needs to hide from a stream of gamma rays due to a nuclear explosion, then by taking refuge in the basement of a house or bomb shelter, provided that it is relatively hermetically sealed, he can only protect himself from this type of radiation by 50 percent.

Fig.4. X-ray and gamma radiation quanta.

T This type of radiation is pulsed in nature and is characterized by propagation in 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-rays have a frequency thousands of times lower than gamma rays. That's why Gamma rays are significantly 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, releasing a colossal amount of energy. In this case, beta particles represent a stream of electrons, and alpha particles are significantly larger and more stable formations, consisting of two neutrons and two protons bound to each other. In fact, the nucleus of a helium atom has this structure, so it can be argued that the flow of alpha particles is a flow of helium nuclei.

Accepted next classification , alpha particles have the least penetrating ability; in order to protect themselves from them, thick cardboard is enough for a person; beta particles have greater penetrating ability; in order for a person to protect himself from the flow 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 propagate over considerable distances, fading as they move away from the epicenter or source, and obeying the laws of propagation of electromagnetic waves.

Fig.5. Radioactive particles of alpha and beta type.

TO The amount of energy that all three types of radiation possess is also different, and the flux of alpha particles has the greatest 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 ability of various types of radiation is inversely proportional to their penetrating ability.

D For the human body, the most dangerous type of radioactive radiation is considered gamma quanta , due to the high penetrating power, and then in decreasing order, beta particles and alpha particles. Therefore, it is quite difficult to determine alpha particles, even if it is impossible to tell 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 detect 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, a conventional Geiger–Müller counter is ineffective.

ABOUT The situation is similar to gamma radiation; there is a possibility that they will pass through the container without starting the ionization reaction. To do this, a special screen (made of dense steel or lead) is installed in the counters, which makes it possible to reduce the energy of gamma rays and thus activate the discharge in the counter chamber.

Basic characteristics and differences of Geiger–Müller counters

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

The most common Geiger–Müller 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 small sizes, 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 combinations thereof (for example, combinations of gamma and beta rays, or the entire spectrum of alpha, beta and gamma). This becomes possible thanks to the specially designed design of the meter housing, as well as the material from which it is made.

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

E one more important indicator, which is indicated in the specifications for the dosimeter, is noise magnitude (measured in pulses per second). In other words, this indicator can be called the value of its own background. It can be determined in a laboratory setting by placing the device in a well-protected room or chamber, usually with thick lead walls, and recording the level of radiation that the device itself emits. It is clear that if such a level is sufficiently significant, 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 micro-roentgen (imp/μR). This parameter, or rather its use, directly depends on the source of ionizing radiation to which the counter is tuned and against which further measurements will be carried out. Often, tuning is done using sources that include radioactive materials such 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 through 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 of 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.

WITH The next criterion to focus on is range of recorded energies . Any radioactive particle or quantum of radiation has a different energy characteristic. Therefore, dosimeters are designed to measure not only a specific type of radiation, but also their corresponding energy characteristic. This indicator is measured in megaelectronvolts or kiloelectronvolts (MeV, KeV). For example, if the 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 detected, or only high-energy alpha particles will be able to break through the material of the Geiger-Müller counter housing and knock out the electron.

AND Based on all of the above, 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 variants of 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 previously produced and modern ones, both household ones, for example: and professional radiation dosimeters, for example: , are designed for this measurement range. Such radiation has sufficient energy and high penetrating power for the Geiger counter camera to 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 Popular counters such as SBM-20 , having a sensor in the form of a cylindrical balloon tube with a coaxial wire cathode and anode. Moreover, the walls of the sensor tube serve as both 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 is about 280 pulses/s, or 70 pulses/μR (testing was carried out for cesium - 137 at 4 μR/s);
• the dosimeter's own background is about 1 pulse/s;
• The sensor is designed to register 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 at the lower limit.

Fig.6. Geiger counter device SBM-20.

U 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 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, which can be found in stores today.

WITH 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 quanta of electromagnetic radiation that move at the speed of light (about 300,000 km/s), so registering such an object seems to be a rather difficult task.

The operating efficiency of such Geiger counters is about one percent.

H To increase it, an increase in the cathode surface is required. In fact, gamma rays are recorded indirectly, thanks to the electrons they knock out, which subsequently participate in the ionization of the inert gas. To promote this phenomenon as effectively as possible, the material and thickness of the counter chamber walls, 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 recording chamber, and too small will allow high-frequency beta radiation to easily enter the chamber, and will also increase the amount of radiation noise natural to the device, which will drown out the accuracy of determining gamma quanta. Naturally, the exact proportions are selected by the manufacturers. Essentially, 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 exposure, which makes it possible to accurately determine radiation contamination and the level of negative impact on humans only by gamma radiation.

IN In domestic dosimeters, which 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. Such sensors include Beta - 1M, Beta - 2M, Beta - 5M, Gamma - 6, Beta - 6M and others.

H To understand more clearly the principle of their operation, it is worth taking a closer look at one of these counters. For example, an end counter with a sensor Beta – 2M , which has a rounded working window of about 14 square centimeters. In this case, the radiation sensitivity to cobalt-60 is about 240 pulses/μR. This type the meter has very low self-noise , 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 record 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, model SBM - 20. If in this dosimeter model you want to exclude the registration of beta particles, then to do this 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 method that most developers use when creating gamma and x-ray counters.

Registration of “soft” beta radiation.

TO As we have already mentioned, registering soft beta radiation (radiation with low energy characteristics and a relatively low frequency) is a rather difficult task. To do this, it is necessary to ensure the possibility of easier penetration into the registration chamber. For these purposes, a special thin working window is made, usually from mica or polymer film, which creates virtually no obstacles to the penetration of beta radiation of this type into the ionization chamber. In this case, the sensor body itself can act as the cathode, and the anode is a system of linear electrodes that 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 gets in the way of beta particles. In dosimeters with such counters, gamma radiation is registered as an application and, in fact, as additional opportunity. And if you want to get rid of the registration of gamma rays, then it is necessary to minimize the cathode surface.

Fig.8. Device of an end-mounted Geiger counter.

WITH It is worth noting 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 like SBT10 And SI8B , which had thin-walled mica working windows. A more modern version of this device Beta-5 has a working window area of ​​about 37 sq/cm, rectangular in shape made of mica material. For such sizes of the sensitive element, the device is able to register about 500 pulses/μR, if measured by cobalt - 60. At the same time, the particle detection efficiency is up to 80 percent. Other indicators of this device are as follows: its own 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 registering soft beta radiation seems to be a rather difficult task, then detecting an alpha particle, even one with high energy indicators, is an even more difficult task. This problem can only be solved by appropriately reducing the thickness of the working window to a thickness that will be sufficient for the passage of an alpha particle into the recording chamber of the sensor, as well as by almost completely bringing the input window closer to the source of alpha particle radiation. This distance should be 1 mm. It is clear that such a device will automatically detect any other types of radiation, and with fairly high efficiency. There is both a positive and negative side to this:

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

Negative – due to increased sensitivity, a significant amount of noise will arise, which will complicate the analysis of the received registration data.

TO In addition, a too thin mica working window, although it increases the capabilities of the counter, is, however, 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 SBT10 and SI8B counters, which we mentioned above, with a working window area of ​​​​about 30 sq/cm, the thickness of the mica layer is 13 - 17 microns, and with the required thickness for recording alpha particles of 4-5 microns, the input the window can be made only no more than 0.2 sq/cm, we're talking about about the SBT9 counter.

ABOUT however, the large thickness of the registration working window can be compensated by proximity to a radioactive object, and vice versa, with a relatively small thickness of the mica window, it becomes possible to register an alpha particle at an already 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's continue, with the thickness of the input window up to 10 microns, it is possible to register alpha particles at a distance of up to 13 mm, if we make a mica window up to 5 microns thick, then alpha radiation will be registered at a distance of 24 mm, etc. Another important parameter that directly affects the ability to detect alpha particles is their energy indicator. If the energy of an alpha particle is more than 5 MeV, then the registration distance for the thickness of the working window of any type will correspondingly increase, 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, which makes it possible to increase the sensitivity of the alpha counter, 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 recording chamber without causing ionization. This 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 recording chamber, but there is a fairly 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 amount of alpha radiation in this case is calculated as the difference between the total radiation and a separate calculation indicator for beta radiation.

For example , it is worth proposing the characteristics of the modern Beta-1 counter, which allows you to register alpha, beta, and gamma radiation. These are the indicators:

• area of ​​the working area 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, radiation sensitivity is achieved on the order of 144 pulses/μR);
• radiation measurement efficiency for alpha particles - 20% (for plutonium - 239), beta particles - 45% (for thallium -204), and gamma quanta - 60% (for composition strontium - 90, yttrium - 90);
• the dosimeter's own background is about 0.6 pulses/s;
• The sensor is designed to register 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 at the lower limit, and alpha particles with an energy of 5 MeV or more.

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

TO Of course, there is also a fairly wide range of meters that are intended for more specific and professional use. Such devices have a number of additional settings and options (electrical, mechanical, radiometric, climatic, etc.), which include many special terms and capabilities. However, we will not concentrate on them. After all, for understanding basic principles actions Geiger–Muller counters , the models described above are quite sufficient.

IN It is also important to mention that there are special subclasses Geiger counters , which are specially designed to determine various types other radiation. For example, to determine the amount of ultraviolet radiation, to register 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.

Uncontrolled ionizing radiation in any form is dangerous. Therefore, there is a need for its registration, monitoring and accounting. The ionization method of recording II is one of the dosimetry methods that allows you to be aware of the real radiation situation.

What is the ionization method for detecting radiation?

This method is based on recording ionization effects. The electric field prevents the ions from recombining and directs their movement to the appropriate electrodes. Thanks to this, it becomes possible to measure the charge of ions formed under the influence of ionizing radiation.

Detectors and their features

The following are used as detectors in the ionization method:

• ionization chambers;
• Geiger-Muller counters;
• proportional counters;
• semiconductor detectors;
• and etc.

All detectors, with the exception of semiconductor ones, are cylinders filled with gas, into which two electrodes are mounted with a direct current voltage applied to them. The electrodes collect ions formed when ionizing radiation passes through a gaseous medium. Negative ions move to the anode, and positive ions move to the cathode, forming an ionization current. Based on its value, one can estimate the number of registered particles and determine the intensity of radiation.

Operating principle of a Geiger-Muller counter

The operation of the counter is based on impact ionization. Electrons moving in the gas (knocked out by radiation when they hit the walls of the counter) collide with its atoms, knocking out electrons from them, resulting in the creation of free electrons and positive ions. The electric field existing between the cathode and anode imparts acceleration to free electrons sufficient to initiate impact ionization. As a result of this reaction, it appears a large number of ions with a sharp increase in current through the counter and a voltage pulse, which is recorded by a recording device. Then the avalanche discharge is extinguished. Only after this can the next particle be detected.

Difference between an ionization chamber and a Geiger-Muller counter.

A gas counter (Geiger counter) uses secondary ionization to create a large gas amplification of current, which occurs because the speed of moving ions created by the ionizing substance is so great that new ions are formed. They, in turn, can also ionize the gas, thereby developing the process. Thus, each particle produces 10 6 times more ions than is possible in the ionization chamber, thus allowing even low-intensity ionizing radiation to be measured.

Semiconductor detectors

The main element of semiconductor detectors is a crystal, and the operating principle differs from an ionization chamber only in that ions are created in the thickness of the crystal, and not in the gas gap.

Examples of dosimeters based on ionization registration methods

A modern device of this type is the clinical dosimeter 27012 with a set of ionization chambers, which is the standard today.

Among individual dosimeters, KID-1, KID-2, DK-02, DP-24, etc., as well as ID-0.2, which is a modern analogue of those mentioned above, have become widespread.

Invented back in 1908 by the German physicist Hans Wilhelm Geiger, a device capable of determining is widely used today. The reason for this is the high sensitivity of the device and its ability to detect a wide variety of radiation. Ease of operation and low cost allow anyone who decides to independently measure the level of radiation to buy a Geiger counter at any time and anywhere. What kind of device is this and how does it work?

Operating principle of a Geiger counter

Its design is quite simple. A gas mixture consisting of neon and argon is pumped into a sealed cylinder with two electrodes, which is easily ionized. It is supplied to the electrodes (about 400V), which in itself does not cause any discharge phenomena until the very moment when the ionization process begins in the gaseous environment of the device. The appearance of particles arriving from outside leads to the fact that primary electrons, accelerated in the corresponding field, begin to ionize other molecules of the gaseous medium. As a result, under the influence of an electric field, an avalanche-like creation of new electrons and ions occurs, which sharply increase the conductivity of the electron-ion cloud. A discharge occurs in the gas environment of the Geiger counter. The number of pulses occurring within a certain period of time is directly proportional to the number of detected particles. Such is the general outline principle of operation of a Geiger counter.

The reverse process, as a result of which the gaseous medium returns to its original state, occurs by itself. Under the influence of halogens (usually bromine or chlorine is used), intense charge recombination occurs in this environment. This process occurs much more slowly, and therefore the time required to restore the sensitivity of the Geiger counter is a very important passport characteristic of the device.

Despite the fact that the principle of operation of a Geiger counter is quite simple, it is capable of responding to ionizing radiation of a wide variety of types. These are α-, β-, γ-, as well as x-ray, neutron and everything depends on the design of the device. Thus, the input window of a Geiger counter, capable of detecting α- and soft β-radiation, is made of mica with a thickness of 3 to 10 microns. For detection it is made from beryllium, and ultraviolet is made from quartz.

Where is a Geiger counter used?

The operating principle of a Geiger counter is the basis for the operation of most modern dosimeters. These small devices, which have a relatively low cost, differ quite high sensitivity and are able to display results in easy-to-understand units of measurement. The ease of use allows these devices to be used even by those who have very little understanding of dosimetry.

Depending on their capabilities and measurement accuracy, dosimeters can be professional or household. With their help, you can timely and effectively determine the existing source of ionized radiation both in open areas and indoors.

These devices, which use the principle of a Geiger counter in their operation, can promptly provide a danger signal using both visual and audio or vibration signals. Thus, you can always check food, clothing, examine furniture, equipment, building materials, etc. to ensure the absence of radiation harmful to the human body.

The Geiger counter is the main sensor for measuring radiation. It detects gamma, alpha, beta radiation and x-rays. It has the highest sensitivity compared to other methods of detecting radiation, for example, ionization chambers. This is the main reason for its widespread use. Other sensors for measuring radiation are used very rarely. Almost all radiation monitoring devices are based on Geiger counters. They are mass produced, and there are devices of various levels: from military-grade dosimeters to Chinese consumer goods. Nowadays, purchasing any device for measuring radiation is not a problem.

Not long ago there was no widespread distribution of dosimetric instruments. So, by 1986, during the Chernobyl accident, it turned out that the population simply did not have any radiation monitoring devices, which, by the way, further aggravated the consequences of the disaster. At the same time, despite the spread of amateur radio and technical creativity circles, Geiger counters were not sold in stores, so making homemade dosimeters was impossible.

How Geiger counters work

This is an electric vacuum device with an extremely simple operating principle. The radioactive radiation sensor is a metal or glass chamber with metallization, filled with a discharged inert gas. An electrode is placed in the center of the chamber. The outer walls of the chamber are connected to a high voltage source (usually 400 volts). The internal electrode is connected to the sensitive amplifier. Ionizing radiation (radiation) is a stream of particles. They literally transfer electrons from the high voltage cathode to the anode filaments. A voltage is simply induced on it, which can already be measured by connecting it to an amplifier.

The high sensitivity of the Geiger counter is due to the avalanche effect. The energy that the amplifier detects at the output is not the energy of the source of ionizing radiation. This is the energy of the high-voltage power supply of the dosimeter itself. The penetrating particle only transfers an electron (an energy charge that turns into a current that is detected by the meter). A gas mixture consisting of noble gases: argon, neon is introduced between the electrodes. It is designed to extinguish high-voltage discharges. If such a discharge occurs, it will be a false operation of the counter. The subsequent measurement circuit ignores such emissions. In addition, the high-voltage power supply must also be protected from them.

The power circuit in a Geiger counter provides an output current of several microamps at an output voltage of 400 volts. The exact value of the supply voltage is established for each brand of meter according to its technical specifications.

Geiger counter capabilities, sensitivity, recorded radiation

Using a Geiger counter, gamma and beta radiation can be detected and measured with high accuracy. Unfortunately, the type of radiation cannot be recognized directly. This is done indirectly by installing barriers between the sensor and the object or terrain being examined. Gamma rays are highly transparent and their background does not change. If the dosimeter has detected beta radiation, then installing a separating barrier, even a thin sheet of metal, will almost completely block the flow of beta particles.

The sets of personal dosimeters DP-22 and DP-24, which were common in the past, did not use Geiger counters. Instead, an ionization chamber sensor was used, so the sensitivity was very low. Modern dosimetric instruments using Geiger counters are thousands of times more sensitive. They can be used to record natural changes in solar background radiation.

A notable feature of the Geiger counter is its sensitivity, tens and hundreds of times higher than the required level. If you turn on the counter in a completely protected lead chamber, it will show a huge natural radiation background. These readings are not a design defect of the meter itself, which has been verified by numerous laboratory tests. Such data are a consequence of the natural radiation background in space. The experiment only shows how sensitive the Geiger counter is.

Especially for measuring this parameter, the technical specifications indicate the value of “sensitivity of the imp microsecond counter” (pulses per microsecond). The more of these impulses, the greater the sensitivity.

Radiation measurement with a Geiger counter, dosimeter circuit

The dosimeter circuit can be divided into two functional modules: a high-voltage power supply and a measuring circuit. High voltage power supply - analog circuit. The measuring module on digital dosimeters is always digital. This is a pulse counter that displays the corresponding value in the form of numbers on the instrument scale. To measure the radiation dose, it is necessary to count pulses per minute, 10, 15 seconds or other values. The microcontroller converts the number of pulses into a specific value on the dosimeter scale in standard radiation units. Here are the most common ones:

• X-ray (usually micro-X-ray is used);
• Sievert (microsievert - mSv);
• Gray, I'm glad
• flux density in microwatts/m2.

The sievert is the most popular unit of measurement for radiation. All norms are related to it; no additional recalculations are required. The rem is a unit for determining the effect of radiation on biological objects.

Comparison of a gas-discharge Geiger counter with a semiconductor radiation sensor

The Geiger counter is a gas-discharge device, and the modern trend in microelectronics is to get rid of them everywhere. Dozens of versions of semiconductor radiation sensors have been developed. The level of background radiation they record is significantly higher than for Geiger counters. The sensitivity of a semiconductor sensor is worse, but it has another advantage - efficiency. Semiconductors do not require high voltage power. They are well suited for battery-powered portable dosimeters. Another advantage is the registration of alpha particles. The gas volume of the meter is significantly larger than the semiconductor sensor, but its dimensions are still acceptable even for portable equipment.

Measurement of alpha, beta and gamma radiation

Gamma radiation is the easiest to measure. This electromagnetic radiation, which is a stream of photons (light is also a stream of photons). Unlike light, it has much more high frequency and very short wavelength. This allows it to penetrate through atoms. In civil defense, gamma radiation is penetrating radiation. It penetrates through the walls of houses, cars, various structures and is retained only by a layer of earth or concrete of several meters. Registration of gamma quanta is carried out with the calibration of the dosimeter according to the natural gamma radiation of the sun. No radiation sources required. It's a completely different matter with beta and alpha radiation.

If ionizing radiation α (alpha radiation) comes from external objects, then it is almost harmless and represents a stream of nuclei of Helium atoms. The range and permeability of these particles is small - a few micrometers (maximum millimeters) - depending on the permeability of the medium. Due to this feature, it is almost not registered by a Geiger counter. At the same time, recording alpha radiation is important, since these particles are extremely dangerous when they penetrate the body with air, food, or water. Geiger counters are used to a limited extent for their detection. Special semiconductor sensors are more common.

Beta radiation is perfectly detected by a Geiger counter because a beta particle is an electron. It can fly hundreds of meters in the atmosphere, but is well absorbed by metal surfaces. In this regard, the Geiger counter must have a mica window. The metal chamber is made with a small wall thickness. The composition of the internal gas is selected in such a way as to ensure a small pressure drop. The beta radiation detector is placed on the remote probe. Such dosimeters are not very common in everyday life. These are mainly military products.

Personal dosimeter with Geiger counter

This class of devices is highly sensitive, unlike outdated models with ionization chambers. Reliable models are offered by many domestic manufacturers: Terra, MKS-05, DKR, Radex, RKS. These are all stand-alone devices with data displayed on the screen in standard units of measurement. There is a mode for displaying the accumulated radiation dose and the instantaneous background level.

A promising direction is a household dosimeter-attachment to a smartphone. Such devices are produced by foreign manufacturers. They are rich technical capabilities, there is a function for storing readings, calculating, recalculating and summing up radiation for days, weeks, months. So far, due to low production volumes, the cost of these devices is quite high.

Homemade dosimeters, why are they needed?

The Geiger counter is a specific element of the dosimeter, completely inaccessible for self-production. In addition, it is found only in dosimeters or sold separately in radio stores. If this sensor is available, all other components of the dosimeter can be assembled independently from various parts consumer electronics: TVs, motherboards, etc. About a dozen designs are now offered on amateur radio sites and forums. It is worth collecting them, since these are the most proven options that have detailed guides for setup and commissioning.

The Geiger counter switching circuit always implies the presence of a high voltage source. The typical operating voltage of the meter is 400 volts. It is obtained using a blocking generator circuit, and this is the most complex element of the dosimeter circuit. The counter output can be connected to a low-frequency amplifier and count the clicks in the speaker. Such a dosimeter is assembled in emergency cases, when there is practically no time for production. Theoretically, the Geiger counter output can be connected to the audio input household equipment, for example, a computer.

Homemade dosimeters, suitable for precise measurements, are all assembled on microcontrollers. Programming skills are not needed here, since the program is written ready-made from free access. The difficulties here are typical for home electronic production: obtaining printed circuit board, soldering of radio components, manufacturing of housing. All this is solved in a small workshop. Homemade dosimeters from Geiger counters are made in cases where:

• it is not possible to purchase a ready-made dosimeter;
• you need a device with special characteristics;
• It is necessary to study the process of constructing and setting up a dosimeter.

A homemade dosimeter is calibrated against the natural background using another dosimeter. This completes the construction process.

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