Geiger-Muller counter: principle of operation and purpose. Geiger counter: device and household variations Geiger counter is used for registration

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), which are 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. With the lowest supply voltage, excellent output signal parameters and sufficiently high speed, they have proved to be particularly suitable for use as ionizing radiation sensors in household radiation monitoring devices.

Geiger counters are capable of responding 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 responds 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 counters 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 detect β-particles that have a 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 register high-energy particles or soft X-rays, a thin-walled vacuum shell is used, and the cathode is made of thin foil or in the form of a thin 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) with a very high degree of electron multiplication (up to 10 7) develop. 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 the photoionization of molecules outside the layer are accelerated towards the anode, but avalanches do not develop here due to the low field strength and the process has little effect on the development of the discharge. In the layer, the situation is different: the resulting photoelectrons, due to the high 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 a certain 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 one (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)

It follows from physical considerations that as the dose rate increases, the sensor readings 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

Radiation dose rate control 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

Schematically, the device of the Geiger-Muller gas-discharge counter is shown in fig. 5.4. The counter is made in the form of a metal cylinder serving as a cathode To, mm diameter. anode BUT a thin steel wire with a diameter of mm is used, stretched along the axis of the cylinder and insulated from the cathode with insulating plugs P. The cylinder is filled with argon at reduced pressure ( 100 mm Hg) with the addition of a small amount ( 0,5 %) vapors of ethyl alcohol or halogens.

On fig. 5.4 shows the connection diagram of the counter to study its current-voltage characteristics. A constant voltage is supplied to the electrodes from an EMF source e. The amount of current passing through the gas is measured by the voltage drop across the measuring resistance R.

Assume that the gas is exposed to radiation of constant intensity (ionizer). As a result of the action of the ionizer, the gas acquires some electrical conductivity and a current will flow in the circuit, the dependence of which on the applied voltage is shown in
rice. 5.5.

At low voltages, the current passing through the device is small. It is possible to register only the total current caused by the passage of a large number of particles. Devices that operate in this mode are called ionization chambers. This mode corresponds to areas I and II.

Location on I the current increases in proportion to the voltage, i.e. Ohm's law is satisfied. In this area, simultaneously with the ionization process, the reverse process takes place - recombination (connection of positive ions and electrons with the formation of neutral particles).

With a further increase in voltage, the increase in current strength slows down and completely stops (section II). Saturation current occurs. The saturation current is the maximum value of the current when all ions and electrons created by the external ionizer per unit of time reach the electrodes in the same time. The value of the saturation current is determined by the power of the ionizer. The saturation current is a measure of the ionizing action of the ionizer: if the action of the ionizer is stopped, the discharge will also stop.

With a further increase in voltage, the current increases rather slowly (section III). At high voltages, electrons generated under the action of an external ionizer, strongly accelerated by an electric field, collide with neutral gas molecules and ionize them. As a result, secondary electrons and positive ions are formed. Secondary electrons, having accelerated in an electric field, can again ionize gas molecules. The total number of electrons and ions will increase like an avalanche as the electrons move towards the anode (this process is called impact ionization). Counters operating in this area ( III), are called proportional.

The number of electrons reaching the anode divided by the number of primary electrons is called gas amplification factor. The gas amplification factor increases rapidly with increasing voltage and, at high voltages, begins to depend on the number of primary electrons. At the same time, the counter switches from the proportional mode to the mode limited proportionality(plot IV). There are no counters operating in this area.

At an even higher voltage, the appearance of at least one pair of ions leads to the onset of a self-sustained discharge (the voltage at which a self-sustained discharge occurs is called breakdown voltage). The current ceases to depend on the number of initially formed ions and the energy of registered particles. The counter starts working in the Geiger mode (section V). The device that works in this area is called Geiger-Muller counter. The independence of the current strength from the energy of ionizing particles makes Geiger-Muller counters convenient for registration b-particles having a continuous spectrum.

A further increase in voltage leads to the appearance continuous gas discharge. The current in this case increases sharply (section VI), and the meter may fail.

Thus, the Geiger-Muller counter works on the principle of internal gas amplification. When a high voltage is applied to the counter, the field near the thin filament (anode) is extremely inhomogeneous. Due to the large potential gradient, a charged particle that enters the counter is accelerated by the field to an energy of more than 30 eV. At such an energy of the particle, the mechanism of impact ionization begins to operate, due to which the electrons are multiplied in number to an avalanche. As a result, a negative pulse is formed on the anode load resistance. An electron avalanche can arise from a single electron caught between the cathode and anode.

Characteristics of the Geiger-Muller counter

Efficiency counter is the ratio of the number of registered particles to the total number of particles passing through it. The efficiency of the counter to electrons can reach 99,9 %. Registration g-rays are carried through fast electrons, formed during absorption or scattering g-quanta in the counter. Meter efficiency to g-quantum is usually on the order of %.

An important characteristic of the counter is background. background name the readings of the device in the absence of the studied radiation sources. The background of the counter is due to: cosmic radiation; the presence of radioactive substances in the environment, including in the materials from which the meter is made; spontaneous discharges in the counter (false impulses). Usually, for Geiger-Muller counters of various designs, the background fluctuates within the limits of pulses/min. Special methods can reduce the background by an order of magnitude.

The Geiger-Muller counter can register only one particle. To register the next particle, it is necessary to first extinguish the self-sustained discharge. Therefore, an important characteristic of the counter is dead time t– time of inactivity of the counter, during which the gas discharge is extinguished. Typically, the dead time is of the order of s.

The gas discharge in the counter can be extinguished in two ways:

1) by introducing a complex organic compound into the gas. Many complex molecules are opaque to ultraviolet and do not allow the corresponding quanta to reach the cathode. The energy released by ions at the cathode, in the presence of such substances, is spent not on pulling out electrons from the cathode, but on the dissociation of molecules. The occurrence of an independent discharge under such conditions becomes impossible;

2) using resistance. This method is explained by the fact that as the discharge current flows through the resistance, a large voltage drop occurs on it. As a result, only a part of the applied voltage falls on the interelectrode gap, which is insufficient to maintain the discharge.

Dead time depends on many factors: voltage value on the counter; gas composition - filler; extinguishing method; service life; temperature, etc. Therefore, it is difficult to calculate.

One of the simplest methods for experimental determination of dead time is two source method.

Nuclear transformations and interactions of radiation with matter are of a statistical nature. Therefore, there is a certain probability that two or more particles will hit the counter during the dead time t, which will be registered as one particle. Let us assume that the efficiency of the counter is equal to 100 %. Let be the average velocity of hitting the particle counter. n is the average count rate (the number of particles registered per unit time). During t particles will be registered. Total dead time t will be , and the number of uncounted particles will be equal to . We will assume that the number of particles that have entered the counter will be equal to the sum of registered and uncounted particles.

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 is used that characterizes the number of decays per unit of time, 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.

Invented back in 1908 by the German physicist Hans Wilhelm Geiger, a device that can determine is widely used today. The reason for this is the high sensitivity of the device, its ability to register a variety of radiation. Ease of operation and low cost make it possible to buy a Geiger counter for any person who decides to independently measure the level of radiation at any time and in any place. What is this device and how does it work?

The principle of operation of the Geiger counter

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

The reverse process, as a result of which the gas medium returns to its original state, occurs by itself. Under the influence of halogens (usually bromine or chlorine is used), an intense recombination of charges occurs in this medium. This process is much slower, 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 the Geiger counter is quite simple, it is able to respond to ionizing radiation of various types. This is α-, β-, γ-, as well as X-ray, neutron and Everything depends on the design of the device. Thus, the entrance window of a Geiger counter capable of registering α- and soft β-radiation is made of mica with a thickness of 3 to 10 microns. For detection, it is made from beryllium, and ultraviolet - from quartz.

Where is the Geiger counter used?

The principle of operation of the Geiger counter is the basis for the operation of most modern dosimeters. These small, relatively low-cost devices are quite sensitive and can display results in readable units. Their ease of use makes it possible to operate these devices even for those who have a very remote understanding of dosimetry.

According to their capabilities and measurement accuracy, dosimeters are professional and household. With their help, it is possible to timely and effectively determine the existing source of ionized radiation both in open areas and indoors.

These devices, which use the principle of operation of the Geiger counter in their work, can give a timely signal of danger using both visual and sound or vibration signals. So, you can always check food, clothes, examine furniture, equipment, building materials, etc. for the absence of radiation harmful to the human body.

Geiger counter

Geiger counter SI-8B (USSR) with a mica window for measuring soft β-radiation. The window is transparent, under it you can see a spiral wire electrode, the other electrode is the body of the device.

An additional electronic circuit provides the counter with power (usually not less than 300 volts), provides, if necessary, suppression of the discharge and counts the number of discharges through the counter.

Geiger counters are divided into non-self-extinguishing and self-extinguishing (not requiring an external discharge termination circuit).

The sensitivity of the counter is determined by the composition of the gas, its volume, as well as the material and thickness of its walls.