Introduction

1. Appointment of counters

The device and principle of operation of the counter

Basic physical laws

1 Recovery after particle registration

2 Dosimetric characteristic

3 Sensor counting characteristic

Conclusion

Bibliography

Introduction

Geiger-Muller counters are the most common detectors (sensors) of ionizing radiation. Until now, they, invented at the very beginning of our century for the needs of nascent nuclear physics, do not, oddly enough, have any full-fledged replacement. At its core, the Geiger counter is very simple. A gas mixture consisting mainly of readily ionizable neon and argon was introduced into a well-evacuated sealed container with two electrodes. The cylinder can be glass, metal, etc. Usually, meters perceive radiation with their entire surface, but there are also those that have a special “window” in the cylinder for this.

A high voltage U is applied to the electrodes (see Fig.), which in itself does not cause any discharge phenomena. The counter will remain in this state until an ionization center appears in its gaseous medium - a trace of ions and electrons generated by an ionizing particle that has come from outside. Primary electrons, accelerating in 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 interelectrode space, which sharply increases its conductivity. In the gas environment of the counter, a discharge occurs, which is visible (if the container is transparent) even with a simple eye.

The reverse process - the return of the gas medium to its original state in the so-called halogen meters - occurs by itself. Halogens (usually chlorine or bromine), contained in a small amount in the gaseous medium, come into action, which contribute to the intensive recombination of charges. But this process is much slower. The length of time required to restore the radiation sensitivity of the Geiger counter and actually determines its speed - "dead" time - is its important passport characteristic. Such meters are called halogen self-extinguishing meters. Featuring the lowest supply voltage, excellent output signal parameters, and sufficiently high speed, they have proven to be particularly suitable for use as ionizing radiation sensors in household radiation monitoring devices.

Geiger counters are able to respond to a variety of types of ionizing radiation - a, b, g, ultraviolet, x-ray, neutron. But the real spectral sensitivity of the counter depends to a large extent on its design. Thus, the input window of a counter sensitive to a- and soft b-radiation must be very thin; for this, mica with a thickness of 3 ... 10 microns is usually used. The balloon of a counter that reacts to hard b- and g-radiation usually has the shape of a cylinder with a wall thickness of 0.05 .... 0.06 mm (it also serves as the cathode of the counter). The X-ray counter window is made of beryllium, and the ultraviolet counter is made of quartz glass.

geiger muller dosimetric radiation counter

1. Appointment of counters

The 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 from nuclear reactions: helium ions (- particles), electrons (- particles), X-ray quanta (- particles) and neutrons. Particles propagate at a 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 form a short (fraction 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 Sievert per hour (Sv / h). A feature of the counter is the ability to form the same output voltage pulses, regardless of the type of particles, their energy and the number of ionizations produced by the particle in the sensor volume.

2. Device and principle of operation of the counter

The operation of the Geiger counter is based on a non-self-sustained pulsed gas discharge between metal electrodes, which is initiated by one or more electrons that appear as a result of gas ionization -, -, or -particle. The meters usually use a cylindrical design of electrodes, 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 anode diameter is 0.1 mm.

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

Rice. 2. Schematic constructions 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 detect 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 metal film (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 a high penetrating power. Therefore, it is recorded by detectors with sufficiently thick walls of the 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 post-discharge phenomena, an admixture of bromine or alcohol vapors (up to 1%) is introduced into the gas filling.

The ability of a Geiger counter to detect particles regardless of their type and energy (to generate one voltage pulse regardless of the number of electrons formed 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 the entry of electrons into the near-anode layer with a high field strength, where electron avalanches (one or several) develop with a very high degree of electron multiplication (up to 10 7). However, the resulting current does not yet reach a value corresponding to the generation of the sensor signal.

A further increase in the current to the operating value is due to the fact that, simultaneously with ionization, ultraviolet photons are generated in avalanches with an energy of about 15 eV, sufficient to ionize impurity molecules in the gas filling (for example, the ionization potential of bromine molecules is 12.8 V). The electrons that appeared as a result of 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 intensity, initiate intense avalanches in which new photons are generated. Their number exceeds the initial one and the process in the layer according to the scheme "photons - electron avalanches - photons" rapidly (several microseconds) increases (enters the "trigger mode"). In this case, the discharge from the place of the first avalanches initiated by the particle propagates along the anode (“transverse ignition”), the anode current sharply increases and the leading edge of the sensor signal is formed.

The trailing edge of the signal (a decrease in current) is due to two reasons: a decrease in the anode potential due to a 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 action of the space charge of ions after the electrons leave for the anode (charge increases the potentials of the points, as a result of which the voltage drop on the layer decreases, and on the area of ​​particle trapping increases). Both reasons reduce the intensity of avalanche development and the process according to the scheme "avalanche - photons - avalanches" 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 charge 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 escape 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 for their designation. For example, STS-2, STS-4 - face self-extinguishing counters, or MS-4 - a counter with a copper cathode (V - with tungsten, G - with graphite), or SAT-7 - face particle counter, SBM-10 - counter - metal particles, SNM-42 - metal neutron counter, CPM-1 - counter for X-ray radiation, etc.

3. Basic physical laws

.1 Recovery after particle detection

The time for ions to leave the gap after registration of a particle turns out to be relatively long - a few milliseconds, which limits the upper limit of measuring the radiation dose rate. At a high radiation intensity, the particles arrive at an interval shorter than the ion departure time, and the sensor does not register some particles. The process is illustrated by an oscillogram of the voltage at the anode of the sensor in the course of restoring its performance (Fig. 3).

Rice. 3. Oscillograms of the voltage at the anode of the Geiger counter. U o- signal amplitude in normal mode (hundreds of volts). 1 - 5 - numbers of particles

The entry of the first particle (1 in Fig. 3) into the sensor volume initiates a pulsed gas discharge, which leads to a voltage decrease by U o(normal signal amplitude). Further, the voltage increases as a result of a slow decrease in the current through the gap as the ions go to the cathode and due to the charge of the interelectrode capacitance from the voltage source through the limiting resistor. If another particle (2 in Fig. 3) enters the sensor in a short time interval after the arrival of the first one, then the discharge processes develop weakly due to the low voltage and low field strength at the anode under the action of the ion space charge. The sensor signal in this case is unacceptably small. The arrival of the second particle after a longer time interval after the first (particles 3 - 5 in Fig. 3) gives a signal of greater amplitude, since the voltage increases and the space charge decreases.

If the second particle enters the sensor after the first one after an interval less than the time interval between particles 1 and 2 in Fig. 3, then for the reasons stated above, the sensor does not generate a signal at all (“does not count” the particle). In this regard, the time interval between particles 1 and 2 is called the “dead time of the counter” (the signal amplitude of particle 2 is 10% of normal). The time interval between particles 2 and 5 in Fig. 3 is called "sensor recovery time" (particle 5 signal is 90% normal). During this time, the amplitude of the sensor signals is reduced, and they may not be registered by the electrical impulse counter.

Dead time (0.01 - 1 ms) and recovery time (0.1 - 1 ms) are important parameters of the Geiger counter. The highest recorded dose rate is the higher, the smaller the values ​​of these parameters. The main factors that determine the parameters are the gas pressure and the value of the limiting resistor. With a decrease in pressure and resistor value, the dead time and recovery time decrease, since the rate of ion escape from the gap increases and the time constant of the process of charging the interelectrode capacitance decreases.

3.2 Dosimetric characterization

The sensitivity of the Geiger counter is the ratio of the frequency of pulses generated by the sensor to the radiation dose rate, measured in microsieverts per hour (µSv/h; options: Sv/s, mSv/s, µSv/s). Typical sensitivity values: 0.1 - 1 pulses per microsievert. In the operating range, sensitivity is a proportionality factor between the meter reading (number of pulses per second) and the dose rate. Outside the range, proportionality is violated, which reflects the dosimetric characteristic of the detector - the dependence of readings on the dose rate (Fig. 4).

Rice. Dependences of the counting rate on the dose rate of radioactive radiation (dosimetric characteristics) for two counters with different gas pressures (1 - 5 kPa, 2 - 30 kPa)

From physical considerations, it follows that the sensor readings as the dose rate increases cannot exceed the value (1/), where is the dead time of the sensor (particles arriving after a time interval less than are not considered). Therefore, the working linear section of the dosimetric characteristic smoothly passes in the area of ​​intense radiation into a horizontal straight line at the level (1/).

With decreasing dead time, the dosimetric characteristic of the sensor changes into a horizontal straight line at a higher level at a higher radiation power, and the upper limit of measurement increases. This situation is observed when the gas pressure decreases (Fig. 4). However, at the same time, the sensitivity of the sensor decreases (the number of particles crossing the gas-discharge gap without collisions with molecules increases). Therefore, when the pressure decreases, the dosimetric characteristic goes down. Mathematically, the characteristic is described by the following relationship:

where N- counting rate (sensor readings - number of pulses per second); - counter sensitivity (pulses per second per microsievert); R- radiation dose rate; - sensor dead time (in seconds).

3.3 Sensor response

The control of the radiation dose rate most often has to be carried out outdoors or in the field, where the sensor is powered by batteries or other galvanic sources. Their tension decreases as they work. At the same time, the gas-discharge processes in the sensor depend on the voltage to a very strong extent. Therefore, the dependence of the Geiger counter readings on voltage at a constant radiation dose rate is one of the most important characteristics of the sensor. The dependence is called the counting characteristic of the sensor (Fig. 5).

On one of the presented dependences (curve 2), characteristic points are marked A-D. At low voltage (to the left of the point BUT) electrons generated in the sensor when an ionizing particle enters, initiate electron avalanches, but their intensity is insufficient to generate a current pulse of the required amplitude, and the counter readings are zero. Dot BUT corresponds to the "voltage of the beginning of the count". With an increase in voltage in the section A - B the counter readings increase, since the probability of electrons from the region of particle trapping to the near-anode layer with a high field strength increases. At a low voltage, the electrons recombine with ions during their movement to the layer (they can first “stick” to bromine impurity molecules with the formation of negative ions). At the point AT the voltage is sufficient to quickly move almost all electrons into the layer, and the recombination intensity is close to zero. The sensor generates signals of normal amplitude.

On the working section of the counting characteristic B - C(“characteristic plateau”) the counter readings slightly increase with increasing voltage, which is of great practical importance and is an advantage of the Geiger counter. Its quality is higher, the longer the plateau (100 -400 V) and the lower the slope of the horizontal section of the counting characteristic.

Rice. 5. Dependences of the counting rate on voltage (counting characteristic) at various values ​​of gas pressure and bromine impurity content: 1 - 8 kPa, 0.5%; 2 - 16 kPa, 0.5%; 3 - 16 kPa, 0.1% for a radiation dose rate of 5 µSv/h. A, B, C, D- characteristic points of curve 2

The steepness (or slope) of a plateau S characterized by a percentage change in meter readings per unit voltage:

, (2)

where NB and N C - meter reading at the beginning and end of the plateau; U B and U C- voltage values ​​at the beginning and end of the plateau. Typical slope values ​​are 0.01 - 0.05%/V.

The relative stability of readings on the plateau of the counting characteristic is provided by a specific type of discharge that occurs in the sensor with the arrival of an ionizing particle. Increasing the voltage intensifies the development of electron avalanches, but this only leads to an acceleration of the discharge propagation along the anode, and the counter's ability to generate one signal per particle is hardly disturbed.

A slight increase in the counting rate with increasing voltage at the plateau of the counting characteristic is associated with the emission of electrons from the cathode under the action of the discharge. The emission is due to the so-called -processes, which are understood as the pulling out of electrons by ions, excited atoms and photons. The coefficient is conditionally considered equal to the number of electrons per ion (excited atoms and photons are assumed). The characteristic values ​​of the coefficient are 0.1 - 0.01 (10 - 100 ions pull out an electron, depending on the type of gas and cathode material). At such values ​​of the coefficient, the Geiger counter does not function, since the electrons leaving the cathode are registered as ionizing particles (false signals are registered).

The normal functioning of the meter is ensured by the introduction of bromine or alcohol vapor into the gas filling (“quenching impurities”), which sharply reduces the coefficient (below 10 -4). In this case, the number of false signals also sharply decreases, but remains noticeable (for example, a few percent). As the voltage increases, the discharge processes intensify; the number of ions, excited atoms and photons increases and, accordingly, the number of false signals increases. This explains the slight increase in the sensor readings on the plateau of the counting characteristic (increase in slope) and the end of the plateau (transition to a steep section C- D). With an increase in the impurity content, the coefficient decreases to a greater extent, which reduces the slope of the plateau and increases its length (curves 2 and 3 in Fig. 5).

The physical mechanism of action of quenching impurities consists in a sharp decrease in the supply of ions, excited atoms and photons to the cathode that can cause electron emission, as well as in an increase in the work function of electrons from the cathode. Ions of the main gas (neon or argon) in the process of moving towards the cathode become neutral atoms as a result of "recharging" in collisions with impurity molecules, since the ionization potentials of neon and argon are greater than those of bromine and alcohol (respectively: 21.5 V; 15, 7V; 12.8V; 11.3V). The energy released in this case is spent on the destruction of molecules or on the formation of low-energy photons that are not capable of causing photoemission of electrons. Such photons, moreover, are well absorbed by impurity molecules.

The impurity ions formed during recharging enter the cathode, but do not cause electron emission. In the case of bromine, this is explained by the fact that the potential energy of the ion (12.8 eV) is insufficient to pull out two electrons from the cathode (one to neutralize the ion, and the other to start an electron avalanche), since the work function of electrons from the cathode in the presence of impurities bromine increases to 7 eV. In the case of alcohol, when ions are neutralized at the cathode, the energy released is usually spent on the dissociation of a complex molecule, and not on the ejection of electrons.

The long-lived (metastable) excited atoms of the main gas that arise in the discharge can in principle fall on the cathode and cause the emission of electrons, since their potential energy is quite high (for example, 16.6 eV for neon). However, the probability of the process turns out to be very small, since atoms, in collisions with impurity molecules, transfer their energy to them - they are “quenched”. Energy is spent on the dissociation of impurity molecules or on the emission of low-energy photons that do not cause photoemission of electrons from the cathode and are well absorbed by impurity molecules.

Approximately similarly, high-energy photons coming from the discharge, which can cause the emission of electrons from the cathode, are “quenched”: they are absorbed by impurity molecules with subsequent energy consumption for the dissociation of molecules and the emission of low-energy photons.

The durability of counters with the addition of bromine is much higher (10 10 - 10 11 pulses), since it is not limited by the decomposition of quenching impurity molecules. The decrease in the concentration of bromine is due to its relatively high chemical activity, which complicates the manufacturing technology of the sensor and imposes restrictions on the choice of cathode material (for example, stainless steel is used).

The counting characteristic depends on the gas pressure: with its increase, the counting start voltage increases (point BUT shifts to the right in Fig. 5), and the plateau level rises as a result of more efficient trapping of ionizing particles by gas molecules in the sensor (curves 1 and 2 in Fig. 5). The increase in the countdown voltage is explained by the fact that the conditions in the sensor correspond to the right branch of the Paschen curve.

Conclusion

The widespread use of the Geiger-Muller counter is explained by its high sensitivity, the ability to register various kinds of radiation, and the comparative simplicity and low cost of installation. The counter was invented in 1908 by Geiger and improved by Müller.

A cylindrical Geiger-Muller counter consists of a metal tube or a glass tube metallized from the inside, and a thin metal thread stretched along the axis of the cylinder. The filament serves as the anode, the tube serves as the cathode. The tube is filled with a rarefied gas, in most cases noble gases such as argon and neon are used. A voltage of about 400V is created between the cathode and anode. For most meters, there is a so-called plateau, which lies approximately from 360 to 460 V, in this range small voltage fluctuations do not affect the counting rate.

The operation of the counter is based on impact ionization. γ-quanta emitted by a radioactive isotope, falling on the walls of the counter, knock out electrons from it. Electrons, moving in the gas and colliding with gas atoms, knock electrons out of atoms and create positive ions and free electrons. The electric field between the cathode and the anode accelerates the electrons to energies at which impact ionization begins. There is an avalanche of ions, and the current through the counter increases sharply. In this case, a voltage pulse is formed on the resistance R, which is fed to the recording device. In order for the counter to be able to register the next particle that fell into it, the avalanche discharge must be extinguished. This happens automatically. At the moment a current pulse appears on the resistance R, a large voltage drop occurs, so the voltage between the anode and cathode decreases sharply - so much so that the discharge stops and the counter is ready for operation again.

An important characteristic of the counter is its efficiency. Not all γ-photons that hit the counter will give secondary electrons and will be registered, since the acts of interaction of γ-rays with matter are relatively rare, and some of the secondary electrons are absorbed in the walls of the device before reaching the gas volume.

The efficiency of the counter depends on the thickness of the counter walls, their material, and the γ-radiation energy. The most efficient are counters whose walls are made of a material with a large atomic number Z, since this increases the formation of secondary electrons. In addition, the walls of the counter must be thick enough. The wall thickness of the counter is chosen from the condition of its equality to the mean free path of secondary electrons in the wall material. With a large wall thickness, secondary electrons will not pass into the working volume of the counter, and a current pulse will not occur. Since γ-radiation weakly interacts with matter, the efficiency of γ-counters is usually also low and amounts to only 1-2%. Another disadvantage of the Geiger-Muller counter is that it does not make it possible to identify particles and determine their energy. These shortcomings are absent in scintillation counters.

Bibliography

1 Acton D.R. Gas-discharge devices with a cold cathode. M.; L.: Energy, 1965.

2 Kaganov I.L. Ionic devices. Moscow: Energy, 1972.

3 Katsnelson B.V., Kalugin A.M., Larionov A.S. Electrovacuum electronic and gas-discharge devices: a Handbook. Moscow: Radio and communication, 1985.

4 Knol M., Eichmeicher I. Technical electronics T. 2. M .: Energy, 1971.

5 Sidorenko V.V. Ionizing Radiation Detectors: A Handbook. L .: Shipbuilding, 1989

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

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

Instrument setup

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

Measurement order

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

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

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

Geiger-Muller counter

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

History of the Geiger counter

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

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

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

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

The principle of operation of the Geiger-Muller counter

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

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

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

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

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

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

Types of radioactive radiation

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

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

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

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

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

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

Fig.5. Radioactive particles alpha and beta type.

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

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

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

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

Basic characteristics and differences of Geiger-Muller counters

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

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

E Another important component for the intended use of meters is the area of ​​the input sensitive element and the working area . In other words, this is the sector through which radioactive particles of interest to us will enter and be registered. The larger this area, the more the counter will be able to capture particles, and the stronger its sensitivity to radiation will be. The passport data k indicates the area of ​​\u200b\u200bthe working surface, as a rule, in square centimeters.

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

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

E Another indicator by which it is worth comparing dosimeters is ion radiation detection efficiency or radioactive particles. The existence of this criterion is due to the fact that not all radioactive particles passing through the sensitive element of the dosimeter will be registered. This can happen in the case when the gamma radiation quantum did not cause ionization in the counter chamber, or the number of particles that passed and caused ionization and discharge is so large that the device does not adequately count them, and for some other reasons. To accurately determine this characteristic of a particular 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 others. radioactive materials.

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

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

Different 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 of the previously produced and modern, both household, for example:, and professional radiation dosimeters, for example, are designed for this measurement range. Such radiation has sufficient energy and high penetrating power so that the Geiger counter camera can register them. Such particles and photons easily penetrate the walls of the counter and cause the ionization process, and this is easily recorded by the corresponding electronic filling of the dosimeter.

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

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

Fig.6. Geiger counter device SBM-20.

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

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

The efficiency of such Geiger counters is about one percent.

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

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

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

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

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

Registration of "soft" beta radiation.

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

Fig.8. Geiger counter device.

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

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

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

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

negative - due to the increased sensitivity, a significant amount of noise will occur, which will make it difficult to analyze the received registration data.

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

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

E Another important point that 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 registration chamber without causing ionization. Such a measure makes it possible to reduce the influence of gamma rays on ionization by thousands, and even tens of thousands of times. It is no longer possible to eliminate the influence of beta radiation on the registration chamber, but there is a rather simple way out of this situation. First, alpha and beta radiation of the total type are recorded, then a thick paper filter is installed, and a second measurement is made, which will register only beta particles. The value of alpha radiation in this case is calculated as the difference between the total radiation and a separate indicator of the calculation of beta radiation.

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

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

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

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

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

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

A bit of the 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 phenomena of nature. Interestingly, the phenomenon of radiation was noticed several years earlier, but it was not given importance, although Nikola Tesla and other workers in the Edison laboratory received burns from X-rays. Harm to health was attributed to anything, but not to rays that the living thing had never encountered in such doses. At the very beginning of the 20th century, articles about the harmful effects of radiation on animals began to appear. This, too, was not given any importance until the sensational story of the "radium girls" - workers in 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, the physicist Lisa Meitner, who, together with Otto Hahn and Fritz Strassmann, refers to people who for the first time in the world divided 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 give a penny. Those who were "in the know" already knew what this would lead to and the nuclear arms race began.

How did the Geiger-Muller counter come about?

The German physicist Hans Geiger, who worked in the laboratory of Ernst Rutherford, in 1908 proposed the principle of operation of the "charged particle" counter as a further development of the already known ionization chamber, which was an electric capacitor filled with gas at low pressure. It has been used since 1895 by Pierre Curie 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 direction of Geiger, creates 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 on the creation of counters sensitive to each of the types of radiation discovered to that: α, β and γ (neutrons were discovered only in 1932).

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

ionizing radiation

To better understand the operation of the Geiger-Muller counter, it is useful to have an understanding of ionizing radiation in general. By definition, they 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 boundary begins with "hard ultraviolet", aka "soft X-ray". This type is a photon type of radiation. Photons of high energy 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 a magnetic field in a vacuum. Later it turned out that this:

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

Later, neutrons were discovered. Alpha particles are easily retained even by ordinary paper, beta particles have a slightly greater penetrating power, and gamma rays have the highest. The most dangerous neutrons (at a distance of many tens of meters in the air!). Due to their electrical neutrality, they do not interact with the electron shells of the substance molecules. But once in 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 already those, in turn, decaying, themselves form the whole "bouquet" of ionizing radiation. Worst of all, the irradiated object or living organism itself becomes a source of radiation for many hours and days.

The device of the Geiger-Muller counter and the principle of its operation

A gas-discharge Geiger-Muller counter, as a rule, is made in the form of a sealed tube, glass or metal, from which air is evacuated, and instead an inert gas (neon or argon or a mixture of them) 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. A minus from a constant voltage source is connected to the cathode, and a plus from a constant voltage source is connected to the anode through a large constant resistance. Electrically, a voltage divider is obtained, at the midpoint of which (the junction of the resistance and the anode of the counter) the voltage is almost equal to the voltage at the source. Usually it is several hundred volts.

When an ionizing particle flies through the tube, the atoms of the inert gas, already in the electric field of high intensity, experience collisions with this particle. The energy given up by the particle during the collision is enough to detach the electrons from the 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 - towards the cathode of the tube. Thus, an electric current occurs. But since the energy of the particle has already been spent on collisions, in whole or in part (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 discuss when analyzing the parameters of the counters.

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

Designs of Geiger-Muller counters

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 counter makes the tube rigid, resistant to external atmospheric pressure and does not allow it to collapse under its action. At the ends of the tube there are sealing insulators made of glass or thermosetting plastic. They also contain terminals-caps for connecting to the instrument circuit. The tube is marked and coated with a durable insulating varnish, apart from, of course, its conclusions. The polarity of the leads is also marked. This is a universal counter for all types of ionizing radiation, especially for beta and gamma.

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

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

In the case of alpha particles, the situation is even worse. So, despite a very decent energy, of the order of several MeV, alpha particles interact very strongly with molecules that are on the way, and quickly lose energy. If matter is compared with a forest, and an electron with a bullet, then alpha particles will have to be compared with a tank bursting through a forest. However, an ordinary 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 on meters for general use, they are often equipped with two counters operating in parallel. One is more sensitive to α and β radiation, and the second to γ-rays. Such a scheme for the use of two counters is implemented in the dosimeter RADEX RD1008 and in the dosimeter-radiometer RADEX MKS-1009 in which the counter is installed BETA-2 and BETA-2M. Sometimes a bar or plate made of an alloy containing an admixture of cadmium is placed between the counters. When neutrons hit such a bar, γ-radiation occurs, which is recorded. This is done to be able to detect neutron radiation, to which simple Geiger counters are practically insensitive. Another way is to cover the body (cathode) with impurities capable of imparting sensitivity to neutrons.

Halogens (chlorine, bromine) are mixed with the gas to quickly extinguish the discharge. Alcohol vapors serve the same purpose, although alcohol in this case is short-lived (this is generally a feature of alcohol) and the “sobered up” counter constantly starts to “ring”, that is, it cannot work in the prescribed mode. This happens somewhere after the registration of 1e9 pulses (billion) which is not so much. Halogen meters 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 micro-roentgens from an exemplary source to the number of pulses caused by this radiation. Because Geiger counters are not designed to measure particle energy, an accurate estimate is difficult. The counters are calibrated against standard isotope sources. It should be noted that this parameter can vary greatly for different types of counters, below are the parameters of the most common Geiger-Muller counters:

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

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

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

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

Geiger-Muller counter SBM-10- 9.6 ÷ 10.8 imps/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. Usually 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 a flat part of the dependence of the number of recorded pulses on the voltage, so it is also called the "plateau". At this point, the highest operating speed (upper measurement limit) is reached. Typical value 400 V.

The width of the operating characteristic of the meter.

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.

The slope of the operating characteristic of the counter.

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.

The working resource of the counter.

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

Dead time of the counter.

This is the time (recovery time) during which the meter 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, i.e. ten microseconds.

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

Custom counter background.

Measured in lead chambers with thick walls to evaluate the quality of meters. Typical value 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, counters of the Gamma series, end counters of the series " Beta' and there are many others. All of them are used to control and measure radiation: at nuclear industry facilities, in scientific and educational institutions, in civil defense, medicine, and even 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 the 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. Roentgen is a unit of exposure dose, "amount of radiation", which is expressed by the number of ions formed 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 CGSE charge unit). In the SI system, exposure dose is expressed in coulombs per kilogram, and with X-rays this is related by the equation:

1 C/kg = 3876 R

The absorbed dose of radiation is measured in joules per kilogram and is called Gray. This is to replace the obsolete rad unit. The absorbed dose rate is measured in grays per second. The exposure dose rate (EDR), previously measured in roentgens per second, is now measured in amperes per kilogram. The equivalent dose of radiation at which the absorbed dose is 1 Gy (Gray) and the radiation quality factor is 1 is called Sievert. Rem (the biological equivalent of a roentgen) is a hundredth of a sievert, and is now considered obsolete. However, even today all obsolete 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 of time. And in what units it is expressed is a matter of taste and convenience.

Even the smallest dose is dangerous in terms of long-term effects on the body. The risk calculation is quite simple. For example, your dosimeter shows 300 milliroentgens 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 get out of here as soon as possible. In general, having discovered even weak radiation, one must move away from it and check it even at a distance. If she “follows you”, you can be “congratulated”, you have been hit by neutrons. And not every dosimeter can respond to them.

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

Gas-discharge Geiger-Muller counter (G-M). Fig.1 is a glass cylinder (cylinder) filled with an inert gas (with

halogen impurities) at a pressure slightly below atmospheric. A thin metal cylinder inside the balloon serves as the K cathode; the anode A is a thin conductor passing through the center of the cylinder. A voltage is applied between the anode and cathode U AT =200-1000 V. The anode and cathode are connected to the electronic circuit of the radiometric device.

Fig.1 Cylindrical Geiger-Muller counter.

1 – anode filament 2 – tubular cathode

U in – source of high voltage

R n – load resistance

FROM V – separating storage tank

R - counting device with indication

ξ is a source of radiation.

With the help of the G-M counter, it is possible to register all radiation particles (except for easily absorbed α-particles); so that β-particles are not absorbed by the counter case, it has slots covered with a thin film.

Let us explain the features of the operation of the counter G-M.

β-particles directly interact with gas molecules of the counter, while neutrons and γ-photons (uncharged particles) interact weakly with gas molecules. In this case, the mechanism of ion formation is different.

we will carry out a dosimetric measurement of the environment near points K and A, the data obtained will be entered in Table. one.

To carry out the measurement you need:

1. Connect the dosimeter to a power source (9v).

2. On the rear side of the dosimeter, close the detector window with a shutter (screen).

3. Set the switchMODE(mode) to position γ ("P").

4. Set the switchRANGE(range) to positionx1 (P n \u003d 0.1-50 μSv / h).

5. Set the power switch of the dosimeter to the positionON(On).

6. If a beep is heard in the x1 position and the numerical rows of the display are completely filled, then it is necessary to switch to the x10 range (P n \u003d 50-500 μSv / h).

7. After the completion of the summation of pulses, the dosimeter display will show a dose equivalent to the powerP Hγ µSv/h; after 4-5 sec. reset will occur.

8. The dosimeter is again ready for radiation measurements. A new measuring cycle starts automatically.

Table 1.

The resulting value in the workspace (AB) is determined by the formula

=
, µSv/h (6)

- dosimeter readings give the values ​​of the radiation background at the point;

The amount of radiation at each measurement point obeys the laws of fluctuation. Therefore, in order to obtain the most probable value of the measured value, it is necessary to make a series of measurements;

- in the case of dosimetry of β-radiation, measurements must be carried out near the surface of the bodies under study.

4. Taking measurements. P.1. Determination of the equivalent dose rate of natural background radiation.

To determine the γ-background of the environment, we select (relative to any objects (bodies)) two points A, K, located at a distance of ~1 meter from each other, and, without touching the bodies,

Neutrons, interacting with cathode atoms, generate charged microparticles (fragments of nuclei). Gamma radiation

interacts mainly with the substance (atoms) of the cathode, generating photon radiation, which further ionizes the gas molecules.

As soon as ions appear in the volume of the counter, the movement of charges will begin under the action of the anode-cathode electric field.

Near the anode, the lines of the electric field strength sharply thicken (due to the small diameter of the anode filament), the field strength increases sharply. The electrons, approaching the filament, receive a large acceleration, there is impact ionization of neutral gas molecules , an independent corona discharge propagates along the filament.

Due to the energy of this discharge, the energy of the initial momentum of the particles increases sharply (up to 10 8 once). When a corona discharge propagates, part of the charges will slowly drain through a large resistance R n ~10 6 Ohm (Fig. 1). In the resistance detector circuitR n there will be current pulses proportional to the initial particle flux. The resulting current pulse is transferred to the storage capacitance C V (C~10 3 picofarad), further amplified and recorded by the conversion scheme R.

Having a lot of resistanceR n in the detector circuit leads to the fact that negative charges will accumulate on the anode. The electric field strength of the anode will decrease and at some point the impact ionization will be interrupted, the discharge will die out.

An important role in the suppression of the resulting gas discharge is played by halogens present in the gas of the counter. The ionization potential of halogens is lower than that of inert gases, therefore, halogen atoms more actively “absorb” photons that cause a self-sustained discharge, converting this energy into dissipation energy, thus quenching the self-sustained discharge.

After impact ionization (and corona discharge) is interrupted, the process of gas recovery to the initial (working) state begins. During this time, the counter does not work, i.e. does not register flying particles. This interval

time is called "dead time" (recovery time). For G-M counterdead time = Δt~10 -4 seconds.

The G-M counter reacts to the hit of each charged particle, without distinguishing them by energy, but if the power drops

radiation is unchanged, then the pulse count rate is proportional to the radiation power, and the counter can be calibrated in units of radiation doses.

The quality of a gas-discharge self-extinguishing detector is determined by the dependence of the average pulse frequencyNper unit time from voltageU on its electrodes at a constant radiation intensity. This functional dependence is called the counting characteristic of the detector (Fig. 2).

As shown in Figure 2, whenU < U 1 the applied voltage is insufficient for the occurrence of a gas discharge when a charged particle or gamma ray enters the detector. Starting with voltage U AT > U 2 impact ionization occurs in the counter, a corona discharge propagates along the cathode, and the counter records the passage of almost every particle. With growth U AT beforeU 3 (see Fig. 2), the number of recorded pulses slightly increases, which is associated with a certain increase in the degree of ionization of the counter gas. A good counter G-M plot plot from U 2 beforeU R almost independent ofU AT , i.e. runs parallel to the axisU AT , the average pulse frequency is almost independent ofU AT .

Rice. 2. Counting characteristic of a gas-discharge self-extinguishing detector.

3. Relative error of instruments when measuring P n : δP n = ±30%.

Let us explain how the meter pulse is converted into readings of the radiation dose rate.

It is proved that at a constant radiation power, the pulse count rate is proportional to the radiation power (measured dose). The measurement of the dose rate of radiation is based on this principle.

As soon as an impulse occurs in the meter, this signal is transmitted to the conversion unit, where it is filtered by duration, amplitude, summed up and the result is transmitted to the meter display in units of power dose.

Correspondence between the counting rate and the measured power, i.e. the dosimeter is calibrated (at the factory) according to a known radiation source C s 137 .