Methods and technical means of recording radiation. Ionization methods of dosimetry. Gas discharge meters. Geiger-Muller counter: operating principle and purpose

In 1908, German physicist Hans Geiger worked in chemical laboratories owned by Ernst Rutherford. There they were also asked to test a charged particle counter, which was an ionized chamber. The chamber was an electric capacitor, which was filled with gas under high pressure. Pierre Curie also used this device in practice, studying electricity in gases. Geiger's idea - to detect the radiation of ions - was associated with their influence on the level of ionization of volatile gases.

In 1928, the German scientist Walter Müller, working with and under Geiger, created several counters that registered ionizing particles. The devices were needed for further radiation research. Physics, being a science of experiments, could not exist without measuring structures. Only a few radiations were discovered: γ, β, α. Geiger's task was to measure all types of radiation with sensitive instruments.

The Geiger-Muller counter is a simple and cheap radioactive sensor. It is not a precise instrument that captures individual particles. The technique measures the total saturation of ionizing radiation. Physicists use it with other sensors to achieve accurate calculations when conducting experiments.

A little about ionizing radiation

We could go straight to the description of the detector, but its operation will seem incomprehensible if you know little about ionizing radiation. When radiation occurs, an endothermic effect on the substance occurs. Energy contributes to this. For example, ultraviolet or radio waves do not belong to such radiation, but hard ultraviolet light does. Here the limit of influence is determined. The type is called photonic, and the photons themselves are γ-quanta.

Ernst Rutherford divided the processes of energy emission into 3 types, using an installation with a magnetic field:

• γ - photon;
• α is the nucleus of a helium atom;
• β is a high energy electron.

You can protect yourself from α particles with paper. β penetrate deeper. Penetration ability γ is the highest. Neutrons, which scientists learned later, are dangerous particles. They act at a distance of several tens of meters. Having electrical neutrality, they do not react with molecules of different substances.

However, neutrons easily reach the center of the atom, causing its destruction, which results in the formation of radioactive isotopes. As isotopes decay, they create ionizing radiation. From a person, animal, plant or inorganic object that has received radiation, radiation emanates for several days.

Design and principle of operation of a Geiger counter

The device consists of a metal or glass tube into which a noble gas (argon-neon mixture or pure substances) is pumped. There is no air in the tube. The gas is added under pressure and contains an admixture of alcohol and halogen. There is a wire stretched throughout the tube. An iron cylinder is located parallel to it.

The wire is called the anode and the tube is called the cathode. Together they are electrodes. A high voltage is supplied to the electrodes, which in itself does not cause discharge phenomena. The indicator will remain in this state until an ionization center appears in its gaseous environment. A minus is connected from the power source to the tube, and a plus is connected to the wire, directed through a high-level resistance. We are talking about a constant supply of tens of hundreds of volts.

When a particle enters the tube, noble gas atoms collide with it. Upon contact, energy is released that removes electrons from the gas atoms. Then secondary electrons are formed, which also collide, generating a mass of new ions and electrons. The speed of electrons towards the anode is affected by the electric field. During this process, an electric current is generated.

During a collision, the energy of the particles is lost, and the supply of ionized gas atoms comes to an end. When charged particles enter a gas-discharge Geiger counter, the resistance of the tube drops, immediately reducing the voltage at the midpoint of the fission. Then the resistance increases again - this entails a restoration of voltage. The momentum becomes negative. The device shows pulses, and we can count them, at the same time estimating the number of particles.

Types of Geiger counters

By design, Geiger counters come in two types: flat and classic.

Classical

Made from thin corrugated metal. Due to corrugation, the tube acquires rigidity and resistance to external influences, which prevents its deformation. The ends of the tube are equipped with glass or plastic insulators, which contain caps for output to the devices.

Varnish is applied to the surface of the tube (except for the leads). The classic counter is considered a universal measuring detector for all known types of radiation. Especially for γ and β.

Flat

Sensitive meters for recording soft beta radiation have a different design. Due to the small number of beta particles, their body has a flat shape. There is a mica window that weakly blocks β. BETA-2 sensor is the name of one of these devices. The properties of other flat counters depend on the material.

Geiger counter parameters and operating modes

To calculate the sensitivity of the counter, estimate the ratio of the number of microroentgens from the sample to the number of signals from this radiation. The device does not measure the energy of the particle, so it does not give an absolutely accurate estimate. Devices are calibrated using samples from isotope sources.

You also need to look at the following parameters:

Work area, entrance window area

The characteristics of the indicator area through which microparticles pass depends on its size. The wider the area, the larger number particles will be caught.

Operating voltage

The voltage should correspond to the average specifications. The operating characteristic itself is the flat part of the dependence of the number of fixed pulses on voltage. Its second name is plateau. At this point, the device reaches peak activity and is called the upper limit of measurement. Value - 400 Volts.

Working width

Working width is the difference between the output voltage on the plane and the spark discharge voltage. The value is 100 Volts.

Incline

The value is measured as a percentage of the number of pulses per 1 volt. It shows the measurement error (statistical) in the pulse count. The value is 0.15%.

Temperature

Temperature is important because the meter often has to be used in difficult conditions. For example, in reactors. Counters general use: from -50 to +70 C Celsius.

Work resource

The resource is characterized by the total number of all pulses recorded until the moment when the device readings become incorrect. If the device contains organics for self-extinguishing, the number of pulses will be one billion. It is appropriate to calculate the resource only in a state of operating voltage. When storing the device, the flow rate stops.

Recovery time

This is the amount of time it takes a device to conduct electricity after reacting to an ionizing particle. There is an upper limit on the pulse frequency that limits the measurement range. The value is 10 microseconds.

Due to the recovery time (also called dead time), the device can fail at a decisive moment. To prevent overshoot, manufacturers install lead screens.

Does the counter have a background?

The background is measured in a thick-walled lead chamber. The usual value is no more than 2 pulses per minute.

Who uses radiation dosimeters and where?

Many modifications of Geiger-Muller counters are produced on an industrial scale. Their production began during the USSR and continues now, but in the Russian Federation.

The device is used:

• at nuclear industry facilities;
• in scientific institutes;
• in medicine;
• in everyday life.

After the accident at the Chernobyl nuclear power plant, ordinary citizens also bought dosimeters. All devices have a Geiger counter. Such dosimeters are equipped with one or two tubes.

Is it possible to make a Geiger counter with your own hands?

Making a meter yourself is difficult. You need a radiation sensor, but not everyone can buy it. The counter circuit itself has long been known - in physics textbooks, for example, it is also printed. However, only a true “left-hander” will be able to reproduce the device at home.

Talented self-taught craftsmen have learned to make a substitute for the counter, which is also capable of measuring gamma and beta radiation using a fluorescent lamp and an incandescent lamp. They also use transformers from broken equipment, a Geiger tube, a timer, a capacitor, various boards, resistors.

Conclusion

When diagnosing radiation, you need to take into account the meter's own background. Even with lead protection of decent thickness, the registration speed is not reset. This phenomenon has an explanation: the cause of the activity is cosmic radiation penetrating through the layers of lead. Muons fly over the Earth's surface every minute, which are registered by the counter with a probability of 100%.

There is another source of background - radiation accumulated by the device itself. Therefore, in relation to the Geiger counter, it is also appropriate to talk about wear. The more radiation the device has accumulated, the lower the reliability of its data.

Geiger counter— a gas-discharge device for counting the number of ionizing particles passing 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, invented at the very beginning of our century for the needs of nascent nuclear physics, there is, oddly enough, no full-fledged replacement.

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

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

Geiger counter connection diagram

A high voltage U is applied to the electrodes (see figure), 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 trail of ions and electrons generated by an ionizing particle arriving from outside. Primary electrons, accelerating in an 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, significantly increasing its conductivity. A discharge occurs in the gas environment of the meter, visible (if the container is transparent) even with the naked eye.

The reverse process is the restoration of the gaseous medium to its original condition in so-called halogen meters it happens by itself. Halogens (usually chlorine or bromine), contained in small quantities in a gaseous environment, come into play and promote intense recombination of charges. But this process is quite slow. The time required to restore the radiation sensitivity of a Geiger counter and what actually determines its performance - “dead” time - is its main passport characteristic.

Such meters are designated as halogen self-extinguishing meters. Very different low voltage nutrition, good parameters output signal and sufficiently high speed, they turned out to be in demand as ionizing radiation sensors 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 depends very much on its design. Thus, the input window of a counter sensitive to a- and soft b-radiation must be quite thin; For this purpose, mica with a thickness of 3...10 microns is usually used. The cylinder of the counter, which 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 counter window is made of quartz glass.

Dependence of counting speed on supply voltage in a Geiger counter

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

Each particle detected by the counter forms a short pulse in its output circuit. The number of pulses appearing per unit time—the counting rate of a Geiger counter—depends on the level of ionizing radiation and the voltage on its electrodes. A standard graph of the counting rate versus supply voltage Upit is shown in the figure above. Here Uns is the counting start voltage; Ung and Uvg are the lower and upper boundaries of the working section, the so-called plateau, on which the counting speed is almost independent of the counter supply voltage. The operating voltage Uр is usually selected in the middle of this section. It corresponds to Np - the counting 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 in nature and therefore the radiation sensitivity of the counter is often shown in terms of pulse/μR (pulses per micro-roentgen; this dimension follows from the ratio of the counting rate - pulse/s - to the radiation level - μR/s).

In cases where it is not indicated, the radiation sensitivity of the counter has to be determined by 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 background radiation, and internal - the radiation of radionuclides found in the counter structure 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 a Geiger counter

Another essential characteristic of a 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. “Riding with rigidity” will obviously affect the accuracy of the measurements taken.

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

The table provides information about self-extinguishing halogen Geiger counters of domestic production, most suitable for household appliances radiation control.

• 1 — operating voltage, V;
• 2 — plateau — region of low dependence of counting speed on supply voltage, V;
• 3 — counter’s own background, imp/s, no more;
• 4 — radiation sensitivity of the counter, imp/μR (* — for cobalt-60);
• 5 — output pulse amplitude, 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.

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 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. This is, in general terms, the principle of operation of a Geiger counter.

The reverse process, as a result of which the gaseous medium returns to initial state, happens 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 the most various 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 of beryllium, and ultraviolet is made of 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, are quite sensitive 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 used for professional or household use. Using them, 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.

Whether we like it or not, radiation has firmly entered our lives and is not going to go away. We need to learn to live with this phenomenon, which is both useful and dangerous. Radiation manifests itself as invisible and imperceptible radiation, and without special devices it is impossible to detect them.

A little history of radiation

X-rays were discovered in 1895. A year later, the radioactivity of uranium was discovered, also in connection with X-rays. Scientists realized that they were faced with completely new, hitherto unseen natural phenomena. It is interesting that the phenomenon of radiation was noticed several years earlier, but it was not given any importance, although Nikola Tesla and other workers of the Edison laboratory also received burns from X-rays. Damage to health was attributed to anything, but not to rays, which living things had never encountered in such doses. At the very beginning of the 20th century, articles began to appear about the harmful effects of radiation on animals. This, too, was not given any importance until the sensational story with the “radium girls” - workers of a factory that produced luminous watches. They just wet the brushes with the tip of their tongue. The terrible fate of some of them was not even published, for ethical reasons, and remained a test only for the strong nerves of doctors.

In 1939, physicist Lise Meitner, who, together with Otto Hahn and Fritz Strassmann, belongs to the people who were the first in the world to divide the uranium nucleus, inadvertently blurted out about the possibility of a chain reaction, and from that moment a chain reaction of ideas about creating a bomb began, namely a bomb, and not at all “peaceful atom”, for which the bloodthirsty politicians of the 20th century, of course, would not have given a penny. Those who were “in the know” already knew what this would lead to and the atomic arms race began.

How did the Geiger-Müller counter appear?

The German physicist Hans Geiger, who worked in the laboratory of Ernst Rutherford, in 1908 proposed the principle of operation of a “charged particle” counter as further development already known ionization chamber, which was electric capacitor filled with gas at low pressure. It was used by Pierre Curie in 1895 to study the electrical properties of gases. Geiger had the idea to use it to detect ionizing radiation precisely because these radiations had a direct effect on the degree of ionization of the gas.

In 1928, Walter Müller, under the leadership of Geiger, created several types of radiation counters designed to register various ionizing particles. The creation of counters was a very urgent need, without which it was impossible to continue the study of radioactive materials, since physics, as an experimental science, is unthinkable without measuring instruments. Geiger and Müller purposefully worked to create counters that were sensitive to each of the types of radiation that had been discovered: α, β and γ (neutrons were discovered only in 1932).

The Geiger-Muller counter proved to be a simple, reliable, cheap and practical radiation detector. Although it is not the most accurate instrument for studying specific types of particles or radiation, it is extremely suitable as an instrument for the general measurement of the intensity of ionizing radiation. And in combination with other detectors, it is used by physicists for precise measurements during experiments.

Ionizing radiation

To better understand the operation of a Geiger-Muller counter, it is helpful to have an understanding of ionizing radiation in general. By definition, these include anything that can cause ionization of a substance in its normal state. This requires a certain amount of energy. For example, radio waves or even ultraviolet light are not ionizing radiation. The border begins with “hard ultraviolet”, also known as “soft x-ray”. This type is a photon type of radiation. High-energy photons are usually called gamma quanta.

Ernst Rutherford was the first to divide ionizing radiation into three types. This was done on an experimental setup using magnetic field in a vacuum. It later turned out that this is:

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

Later neutrons were discovered. Alpha particles are easily blocked even by ordinary paper, beta particles have a slightly greater penetrating power, and gamma rays have the highest. Neutrons are the most dangerous (at a distance of up to many tens of meters in the air!). Due to their electrical neutrality, they do not interact with the electron shells of the molecules of the substance. But once they get into the atomic nucleus, the probability of which is quite high, they lead to its instability and decay, with the formation, as a rule, of radioactive isotopes. And those, in turn, decaying, themselves form the entire “bouquet” of ionizing radiation. The worst thing is that an irradiated object or living organism itself becomes a source of radiation for many hours and days.

The design of a Geiger-Muller counter and its operating principle

A Geiger-Muller gas-discharge counter is usually made in the form of a sealed tube, glass or metal, from which the air is evacuated, and instead an inert gas (neon or argon or a mixture of both) is added under low pressure, with an admixture of halogens or alcohol. A thin wire is stretched along the axis of the tube, and a metal cylinder is located coaxially with it. Both the tube and the wire are electrodes: the tube is the cathode, and the wire is the anode. The minus from the source is connected to the cathode DC voltage, and to the anode - through a large constant resistance- plus from a constant voltage source. Electrically, a voltage divider is obtained, at the middle point of which (the junction of the resistance and the anode of the meter) the voltage is almost equal to the voltage at the source. This is usually several hundred volts.

When an ionizing particle flies through the tube, the atoms of the inert gas, already in a high-intensity electric field, experience collisions with this particle. The energy given off by the particle during a collision is enough to separate electrons from gas atoms. The resulting secondary electrons are themselves capable of forming new collisions and, thus, a whole avalanche of electrons and ions is obtained. Under the influence of an electric field, electrons are accelerated towards the anode, and positively charged gas ions are accelerated towards the cathode of the tube. Thus, an electric current arises. But since the energy of the particle has already been spent on collisions, fully or partially (the particle flew through the tube), the supply of ionized gas atoms also ends, which is desirable and is ensured by some additional measures, which we will talk about when analyzing the parameters of the counters.

When a charged particle enters a Geiger-Muller counter, due to the resulting current, the resistance of the tube drops, and with it the voltage at the midpoint of the voltage divider, which was discussed above. Then the resistance of the tube, due to an increase in its resistance, is restored, and the voltage again becomes the same. Thus, we get a negative voltage pulse. By counting the impulses, we can estimate the number of passing particles. The electric field strength is especially high near the anode due to its small size, which makes the counter more sensitive.

Geiger-Muller counter designs

Modern Geiger-Muller counters are available in two main versions: “classic” and flat. The classic counter is made of a thin-walled metal tube with corrugation. The corrugated surface of the meter makes the tube rigid, resistant to external atmospheric pressure and does not allow it to wrinkle under its influence. At the ends of the tube there are sealing insulators made of glass or thermosetting plastic. They also contain terminal caps for connecting to the device circuit. The tube is marked and coated with a durable insulating varnish, not counting, of course, its terminals. The polarity of the terminals is also indicated. This is a universal counter for all types of ionizing radiation, especially beta and gamma.

Counters sensitive to soft β-radiation are made differently. Due to the short range of beta particles, they have to be made flat, with a mica window that weakly blocks beta radiation; one of the options for such a counter is a radiation sensor BETA-2. All other properties of the meters are determined by the materials from which they are made.

Counters designed to detect gamma radiation contain a cathode made of metals with a high charge number, or are coated with such metals. Gas is extremely poorly ionized by gamma photons. But gamma photons are capable of knocking out many secondary electrons from the cathode if it is chosen appropriately. Geiger-Muller counters for beta particles are made with thin windows to better transmit the particles, since they are ordinary electrons that have just received more energy. They interact with matter very well and quickly lose this energy.

In the case of alpha particles the situation is even worse. So, despite a very decent energy, on the order of several MeV, alpha particles interact very strongly with molecules in their path and quickly lose energy. If matter is compared to a forest, and an electron is compared to a bullet, then alpha particles will have to be compared to a tank crashing through a forest. However, a conventional counter responds well to α-radiation, but only at a distance of up to several centimeters.

For an objective assessment of the level of ionizing radiation dosimeters General purpose meters are often equipped with two counters operating in parallel. One is more sensitive to α and β radiation, and the second to γ ​​rays. This scheme of using two counters is implemented in a dosimeter RADEX RD1008 and in a dosimeter-radiometer RADEKS MKS-1009, in which the counter is installed BETA-2 And BETA-2M. Sometimes a bar or plate of an alloy containing an admixture of cadmium is placed between the counters. When neutrons hit such a bar, γ-radiation is generated, which is recorded. This is done to be able to detect neutron radiation, to which simple Geiger counters are practically insensitive. Another method is to coat the housing (cathode) with impurities that can impart sensitivity to neutrons.

Halogens (chlorine, bromine) are added to the gas to quickly extinguish the discharge. Alcohol vapor also serves the same purpose, although alcohol in this case is short-lived (this is generally a feature of alcohol) and the “sobered up” meter constantly begins to “ring”, that is, it cannot work in the intended mode. This happens somewhere after 1e9 pulses (a billion) have been detected, which is not that much. Meters with halogens are much more durable.

Parameters and operating modes of Geiger counters

Sensitivity of Geiger counters.

The sensitivity of the counter is estimated by the ratio of the number of microroentgens from the reference source to the number of pulses caused by this radiation. Since Geiger counters are not designed to measure particle energy, accurate estimation is difficult. The counters are calibrated using reference isotope sources. It should be noted that this parameter at different types counters can vary greatly, below are the parameters of the most common Geiger-Müller counters:

Geiger-Muller counter Beta-2- 160 ÷ 240 imp/µR

Geiger-Muller counter Beta-1- 96 ÷ 144 imp/µR

Geiger-Muller counter SBM-20- 60 ÷ 75 imp/µR

Geiger-Muller counter SBM-21- 6.5 ÷ 9.5 imp/µR

Geiger-Muller counter SBM-10- 9.6 ÷ 10.8 imp/μR

Entrance window area or work area

The area of ​​the radiation sensor through which radioactive particles fly. This characteristic is directly related to the dimensions of the sensor. The larger the area, the more particles the Geiger-Muller counter will catch. Typically this parameter is indicated in square centimeters.

Geiger-Muller counter Beta-2- 13.8 cm 2

Geiger-Muller counter Beta-1- 7 cm 2

This voltage corresponds to approximately the middle of the operating characteristic. The operating characteristic is the flat part of the dependence of the number of recorded pulses on the voltage, which is why it is also called the “plateau”. At this point the highest operating speed is achieved (upper measurement limit). Typical value is 400 V.

Width of the counter operating characteristic.

This is the difference between the spark breakdown voltage and the output voltage on the flat part of the characteristic. Typical value is 100 V.

Slope of the meter operating characteristic.

The slope is measured as a percentage of pulses per volt. It characterizes the statistical error of measurements (counting the number of pulses). Typical value is 0.15%.

Permissible operating temperature of the meter.

For general purpose meters -50 ... +70 degrees Celsius. This is a very important parameter if the meter operates in chambers, channels, and other places of complex equipment: accelerators, reactors, etc.

Working resource of the counter.

The total number of pulses that the meter registers before its readings begin to become incorrect. For devices with organic additives, self-quenching is usually 1e9 (ten to the ninth power, or one billion). The resource is counted only if operating voltage is applied to the meter. If the counter is simply stored, this resource is not consumed.

Counter dead time.

This is the time (recovery time) during which the counter conducts current after being triggered by a passing particle. The existence of such a time means that there is an upper limit to the pulse frequency and this limits the measurement range. A typical value is 1e-4 s, which is ten microseconds.

It should be noted that due to dead time, the sensor may be “off scale” and remain silent at the most dangerous moment (for example, a spontaneous chain reaction in production). Such cases have happened, and to combat them, lead screens are used to cover part of the sensors of emergency alarm systems.

Custom counter background.

Measured in thick-walled lead chambers to evaluate the quality of meters. Typical value is 1 ... 2 pulses per minute.

Practical application of Geiger counters

Soviet and now Russian industry produces many types of Geiger-Muller counters. Here are some common brands: STS-6, SBM-20, SI-1G, SI21G, SI22G, SI34G, meters of the Gamma series, end counters of the series Beta"and there are many more. All of them are used for monitoring and measuring radiation: at nuclear industry facilities, in scientific and educational institutions, in civil defense, medicine, and even in everyday life. After the Chernobyl accident, household dosimeters, previously unknown to the population even by name, have become very popular. Many brands of household dosimeters have appeared. All of them use a Geiger-Muller counter as a radiation sensor. In household dosimeters, one to two tubes or end counters are installed.

UNITS OF MEASUREMENT OF RADIATION QUANTITIES

For a long time, the unit of measurement P (roentgen) was common. However, when moving to the SI system, other units appear. An x-ray is a unit of exposure dose, a "quantity of radiation", which is expressed as the number of ions produced in dry air. At a dose of 1 R, 2.082e9 pairs of ions are formed in 1 cm3 of air (which corresponds to 1 unit of charge of the SGSE). In the SI system, exposure dose is expressed in coulombs per kilogram, and with x-rays this is related to the equation:

1 C/kg = 3876 R

The absorbed dose of radiation is measured in joules per kilogram and is called Gray. This is a replacement for the outdated rad unit. The absorbed dose rate is measured in grays per second. Exposure dose rate (EDR), formerly measured in roentgens per second, is now measured in amperes per kilogram. The equivalent radiation dose at which the absorbed dose is 1 Gy (gray) and the radiation quality factor is 1 is called Sievert. The rem (biological equivalent of an x-ray) is a hundredth of a sievert, now considered obsolete. Nevertheless, even today all outdated units are very actively used.

The main concepts in radiation measurements are dose and power. Dose is the number of elementary charges in the process of ionization of a substance, and power is the rate of dose formation per unit time. And in what units this is expressed is a matter of taste and convenience.

Even a minimal dose is dangerous in terms of long-term consequences for the body. The calculation of danger is quite simple. For example, your dosimeter shows 300 milliroentgen per hour. If you stay in this place for a day, you will receive a dose of 24 * 0.3 = 7.2 roentgens. This is dangerous and you need to leave here as soon as possible. In general, if you detect even weak radiation, you need to move away from it and check it even from a distance. If she “follows you”, you can be “congratulated”, you have been hit by neutrons. But not every dosimeter can respond to them.

For radiation sources, a quantity characterizing the number of decays per unit of time is used; it is called activity and is also measured by many different units: curie, becquerel, rutherford and some others. The amount of activity, measured twice with a sufficient separation in time, if it decreases, allows us to calculate the time, according to the law of radioactive decay, when the source becomes sufficiently safe.

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 different 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 the source high voltage(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 false positive 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 defect in the design of the meter itself, which has been verified by numerous laboratory research. 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, which 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 is 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 to self-made. 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, which have detailed instructions for setting up 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 output of a Geiger counter can be connected to the audio input of household equipment, such as 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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