What does a geiger counter measure. Geiger-Muller counter: history of creation, principles of operation and purpose

1.4 Geiger-Muller counter

IN proportional counter, the gas discharge develops only in a part of the gas volume. First, primary ionization is formed in it, and then an avalanche of electrons. The rest of the volume is not covered by the gas discharge. As the voltage increases, the critical region expands. It increases the concentration of excited molecules, and hence the number of emitted photons. Under the influence of photons from the cathode and gas molecules escapes

more and more photoelectrons. The latter, in turn, give rise to new avalanches of electrons in the volume of the counter, not occupied by the gas discharge from primary ionization. Thus, an increase in voltage U leads to the propagation of a gas discharge over the volume of the counter. At some voltage U p . Called threshold, the gas discharge covers the entire volume of the counter. At voltage U p begins the Geiger-Muller region.

A Geiger counter (or a Geiger-Muller counter) is a gas-filled counter of charged elementary particles, the electrical signal from which is amplified due to the secondary ionization of the gas volume of the counter and does not depend on the energy left by the particle in this volume. Invented in 1908 by H. Geiger and E. Rutherford, later improved by Geiger and W. Muller. Counters Geiger-Muller - the most common detectors (sensors) of ionizing radiation.

Geiger - Muller counter - a gas-discharge device for detecting and studying various types of radioactive and other ionizing radiation: α- and β-particles, γ-quanta, light and X-ray quanta, high-energy particles in cosmic rays and at accelerators. Gamma quanta are registered by a Geiger-Muller counter by secondary ionizing particles - photoelectrons, Compton electrons, electron-positron pairs; neutrons are registered by recoil nuclei and products of nuclear reactions arising in the gas of the counter. The meter operates at voltages corresponding to self-sustaining

corona discharge (section V, Fig. 21).

Rice. 21. The scheme of switching on the Geiger counter

The potential difference is applied (V) between the walls and the central electrode through the resistance R shunted by the capacitor

C1.

This counter has an almost 100% probability of detecting a charged particle, since for

A single electron-ion pair is sufficient for the discharge to occur.

Structurally, the Geiger counter is also arranged as a proportional counter, i.e. is a capacitor (usually cylindrical), with a highly non-uniform electric field. A positive potential (anode) is applied to the inner electrode (a thin metal thread), and a negative potential (cathode) is applied to the outer one. The electrodes are enclosed in a hermetically sealed tank filled with some gas up to a pressure of 13-26 kN/m 2 (100-200 mm pm st .). A voltage of several hundred V is applied to the counter electrodes. The + sign is applied to the thread through the resistance R.

Functionally, the Geiger counter also repeats the proportional counter, but differs from the latter in that, due to the higher potential difference on the electrodes, it operates in such a mode when it is enough for the appearance of one electron in the detector volume to develop a powerful avalanche-like process due to secondary ionization (gas amplification) , which is capable of ionizing the entire region near the anode filament. In this case, the current pulse reaches the limiting value (saturates) and does not depend on the primary ionization. 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 essence, when a particle enters a Geiger counter, an independent gas discharge flares up (ignites) in it, visible (if the container is transparent) even with a simple gas. In this case, the gas amplification factor can reach 1010, and the magnitude of the pulse can reach tens of volts.

A corona discharge flash occurs and current flows through the meter.

The distribution of the electric field in the counter is such that the discharge develops only in the vicinity of the anode of the counter at a distance of several filament diameters. Electrons quickly accumulate on the filament (no more than 10-6 sec), around which a "sheath" of positive ions is formed. A positive space charge increases the effective anode diameter and thereby reduces the field strength, so the discharge is interrupted. As the layer of positive ions moves away from the filament, its screening effect weakens and the field strength near the anode becomes sufficient for the formation of a new discharge flash. Positive ions, approaching the cathode, knock out electrons from the latter, resulting in the formation of neutral atoms of an inert gas in an excited state. Excited atoms at

sufficiently approaching the cathode, electrons are knocked out of its surface, which become the founders of new avalanches. Without external influence, such a counter would be in a long intermittent discharge.

Thus, with a sufficiently large R (108 -1010 ohm), a negative charge accumulates on the thread

And the potential difference between the filament and the cathode drops rapidly, causing the discharge to terminate. After that, the sensitivity of the counter is restored after 10-1 -10-3 sec (discharging time of capacitance C through resistance R). It is this time that is required for the slow positive ions, which filled the space near the anode filament after the passage of the particle and the passage of the electron avalanche, to go to the cathode,

And restored the sensitivity of the detector. Such a long dead time is inconvenient for many applications.

For the practical use of a non-self-extinguishing Geiger counter, various methods are used to terminate the discharge:

a) The use of electronic circuits for extinguishing a discharge in a gas. An electronic circuit adapted for this, at the right time, issues a “counter signal” to the counter, which stops the independent discharge and “holds” the counter for a while until the charged particles that have arisen are completely neutralized. The characteristics of such a counter with a discharge suppression circuit are close to those of self-extinguishing counters and sometimes exceed them.

b) Quenching by selecting the values ​​of the load resistance and equivalent capacitance, as well as the voltage on the meter.

IN Depending on the discharge quenching mechanism, two groups of counters are distinguished: non-self-extinguishing and self-extinguishing. In non-self-extinguishing meters, the "dead" time is too long(10-2 sec), for him

reduction, electronic circuits for quenching the discharge are used, which reduce the resolution time to the time of collection of positive ions at the cathode (10-4 sec).

Now non-self-extinguishing counters, in which the discharges are quenched by the resistance R , are replaced by self-extinguishing counters, which are also more stable. In them, thanks to a special gas filling (an inert gas with an admixture of complex molecules, such as alcohol vapor, and a small

an admixture of halogens - chlorine, bromine, iodine) the discharge breaks by itself even at low resistances R. Dead time of self-extinguishing counter ~10-4 sec.

IN 1937 Trost drew attention to the fact that if a counter filled with argon,

add a small amount (a few percent) of ethyl alcohol vapor (C2 H5 OH), then the discharge caused in the counter by an ionizing particle will go out by itself. Subsequently, it turned out that spontaneous extinction of the discharge in the counter also occurs when vapors of other organic compounds containing complex polyatomic compounds are added to argon. These substances are usually called quenching, and Geiger-Muller counters, in which these substances are used, are called self-extinguishing type counters. A self-extinguishing meter is filled with a mixture of two (or more) gases. One gas, the main one, is about 90% in the mixture, the other, the quenching gas, is about 10%. The components of the working mixture must satisfy the mandatory condition that the ionization potential of the quenching gas must be below the first excitation potential of the main gas.

Comment. Xenon wire detectors are often used to detect x-rays. An example is the first domestic scanning digital medical fluorograph MTsRU SIBIR. Another application of X-ray counters is an X-ray fluorescence wave-dispersive spectrometer (for example, Venus 200), designed to determine various elements in substances and materials. Depending on the element to be determined, the following detectors can be used: - flow proportional detector with windows 1, 2, 6 microns thick, non-flow neon detector with windows 25 and 50 microns thick, - non-flow krypton detector with a window 100 microns thick, - xenon detector with a window 200 microns and a scintillation detector with a window of 300 microns.

Self-extinguishing counters allow high counting rates without special electronic circuits

discharge quenching, so they are widely used. Self-extinguishing counters with organic quenching impurities have a limited operating life (108 -1010 pulses). When one of the halogens is used as a quenching impurity (the less active Br2 is most often used), the service life becomes practically unlimited due to the fact that diatomic halogen molecules are formed again after dissociation into atoms (during the discharge process). The disadvantages of halogen counters include the complexity of their manufacturing technology due to the chemical activity of halogens and the long rise time of the leading edge of pulses due to the attachment of primary electrons to the halogen molecule. The "pulling" of the leading edge of the pulse in halogen counters makes them inapplicable in coincidence circuits.

The main characteristics of the counter are: counting characteristic - the dependence of the counting rate on the magnitude of the operating voltage; counter efficiency - expressed as a percentage of the ratio of the number of counted particles to the number of all particles falling into the working volume of the counter; resolving time -

the minimum time interval between pulses at which they are recorded separately and the service life of the counters.

Rice. 22. Scheme of dead time occurrence in the counterGeiger-Muller.(Pulse shape during discharge in a Geiger-Muller counter).

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.

If a discharge caused by a nuclear particle begins in the Geiger-Muller counter at the time t 0, then the voltage on the counter drops sharply. The counter for a certain time, which is called dead time τ m , is not able to regulate other particles. From the moment t 1 , i.e. after the dead time has elapsed, the meter again may have a self-discharge. However, at the beginning the pulse amplitude is still small. Only after the space charge reaches the cathode surface, pulses of normal amplitude are formed in the counter. The time interval τ s between the moment t 0, when an independent discharge occurred in the counter, and the moment of restoration of the operating voltage t 3 is called the recovery time. In order for the recording device to be able to count the pulse, it is necessary that its amplitude exceed a certain value U p . The time interval between the moment of occurrence of an independent discharge t 0 and the moment of formation of the amplitude U p of the pulse t 2 is called the resolution time τ p of the Geiger-Muller counter. The resolving time τ p is slightly longer than the dead time.

If a large number of particles enter the counter every second (several thousand or more), then the resolving time τ p will be comparable in magnitude to the average time interval between pulses, so a significant number of pulses are not counted. Let m be the observed count rate of the counter. Then the fraction of time during which the counting unit is insensitive is equal to m τ . Therefore, the number of pulses lost per unit of time is equal to nm τ p , where n is the count rate that would be observed if the resolution time had a negligible value. That's why

	n – m = nmτ p
				−m τ

The count rate correction that is given by this equation is called the settling dead time correction.

Halogen self-extinguishing meters are distinguished by the lowest supply voltage, excellent output signal parameters and sufficiently high speed, they have proven to be especially suitable for use as ionizing radiation sensors in household radiation monitoring devices.

Each particle detected by the counter causes a short pulse to appear in its output circuit. The number of pulses that occur per unit of time - the count rate of the Geiger counter - depends on the level of ionizing radiation and the voltage on its electrodes. A typical plot of counting rate versus supply voltage V is shown in Fig. 23. Here V zazh - the voltage of the beginning of the account; V 1 and V 2 are the lower and upper limits of the working area, the so-called plateau, on which the counting rate is almost independent of the counter supply voltage. The operating voltage V slave is usually chosen in the middle of this section. It corresponds to N p - the count rate in this mode.

Rice. 23. Dependence of the counting rate on the supply voltage in the Geiger counter (Counting characteristic)

The dependence of the counting rate on the level of radiation exposure of the counter is its most important characteristic. The graph of this dependence is almost linear and therefore often the radiation sensitivity of the counter is expressed in terms of pulses / μR (pulses per microroentgen; this dimension follows from the ratio of the count rate - pulses / s - to the radiation level - μR / s). IN

in cases where it is not indicated (not rare, unfortunately), to judge the radiation sensitivity

The counter is accounted for by its other very important parameter - its own background. This is the name of the counting rate, the cause of which is two components: external - the natural radiation background, and internal - the radiation of radionuclides trapped in the counter design itself, as well as the spontaneous electron emission of its cathode. (“background” in dosimetry has almost the same meaning as “noise” in radio electronics; in both cases, we are talking about fundamentally unavoidable effects on equipment.)

Another important characteristic of the Geiger counter is the dependence of its radiation sensitivity on the energy ("hardness") of ionizing particles. In professional jargon, the graph of this dependence is called a "stroke with rigidity." To what extent this dependence is important, the graph in the figure shows. "Travel with rigidity" will obviously affect the accuracy of the measurements taken.

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.

Geiger counters are able to respond to a variety of types of ionizing radiation - α, β, γ, 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 α- and soft β-radiation must be very thin; for this, mica with a thickness of 3 ... 10 microns is usually used. The balloon of the counter, which reacts to hard β - and γ -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.

Rice. Fig. 24. Dependence of the counting rate on the energy of gamma quanta ("movement with rigidity") in a Geiger counter

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

Rice. 25. Radiometric installation based on the Geiger-Muller counter.

The fact that the Geiger counter is an avalanche device also has its disadvantages - one cannot judge the root cause of its excitation by the reaction of such a device. The output pulses generated by the Geiger counter under the action of α-particles, electrons, γ-quanta (in a counter that reacts to all these types of radiation) do not differ in any way. themselves

particles, their energies completely disappear in the twin avalanches they generate.

The quality of a Geiger-Muller counter is usually judged by the form of its counting characteristic. For "good" counters, the length of the counting part is 100-300 V with a plateau slope of no more than 3 - 5% per 100 V. The operating voltage of the counter V slave is usually chosen in the middle of its counting area.

Since the particle count rate on the plateau varies in proportion to the intensity of irradiation with nuclear particles, Geiger-Muller counters are successfully used for relative measurements of the activity of radioactive sources. Absolute measurements are difficult due to taking into account a large number of additional corrections. When working with low-intensity sources, one should take into account the background of the counter due to cosmic radiation, radioactivity of the environment, and radioactive contamination of the counter material. Initially, noble gases, in particular, argon and neon, were most often used as filling gases in the counter. Most meters have a pressure in the range of 7 to 20 cm Hg, although they sometimes operate at high pressures, up to 1 atm. In counters of this type, it is necessary to use special electronic circuits to extinguish the gas discharge that has arisen when ionizing radiation enters the counter. Therefore, such counters are called Geiger-Muller counters of the non-self-extinguishing type. They have very poor resolution. The use of circuits for forced quenching of the discharge, improving

resolution significantly complicates the experimental setup, especially if a large number of counters are used simultaneously.

A typical glass Geiger-Muller counter is shown in Fig. 25.

Rice. 25. Glass Geiger-Muller counter: 1 -

geometrically sealed glass tube; 2 – cathode (a thin layer of copper on a stainless steel tube); 3 - output of the cathode; 4 - anode (thin stretched thread).

In Table. 1 provides information about self-extinguishing halogen Geiger counters

Russian-made, most suitable for household radiation monitoring devices.

Designations: 1 - operating voltage, V; 2 - plateau - area of ​​low dependence of the count rate on the supply voltage, V; 3 - own counter background, imp/s, no more; 4 - radiation sensitivity of the counter, pulse/μR (* - for cobalt-60); 5 - output pulse amplitude, V, not less; 6 - dimensions, mm - diameter x length (length x width x

height); 7.1 - hard β - and γ - radiation; 7.2 - the same and soft β - radiation; 7.3 - the same and α - radiation; 7.4 - γ - radiation.

Fig.26. Watches with a built-in Geiger-Muller counter.

The Geiger-Muller counter, type STS-6, counts β and γ particles and belongs to self-extinguishing counters. It is a stainless steel cylinder with a wall thickness of 50 mg/(cm2) with stiffeners for strength. The counter is filled with a mixture of neon and bromine vapors. Bromine extinguishes the discharge.

The designs of counters are very diverse and depend on the type of radiation and its energy, as well as on the measurement technique).

The radiometric setup based on the Geiger-Muller counter is shown in Fig. 27. Voltage is supplied to the meter from a high-voltage power source. The pulses from the counter are fed into the amplifier block, where they are amplified, and then registered by the counting device.

Geiger-Muller counters are used to register all types of radiation. They can be used for both absolute and relative measurements of radioactive emissions.

Rice. 27. Design of Geiger-Muller counters: a - cylindrical; b

– internal filling; g - flowing for liquids. 1 – anode (collecting electrode); 2 - cathode; 3 - glass bottle; 4 - electrode leads; 5 - glass tube; 6 - insulator; 7 - mica window; 8 - gas inlet valve.

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

From the history of the study of radiation

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

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

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

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

The history of the appearance of the Geiger-Muller counter

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

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

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

What is ionizing radiation?

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

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

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

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

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

Geiger-Muller counter devices and operating principles

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

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

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

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

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

Designs of Geiger-Muller counters

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

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

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

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

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

How Geiger counters are used in practice

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

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

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

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

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

A little about ionizing radiation

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

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

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

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

However, neutrons easily fall into the center of the atom, provoke its destruction, due to which radioactive isotopes are formed. Decaying, isotopes create ionizing radiation. From a person, animal, plant or inorganic object that has received radiation, radiation emanates for several days.

The device and principle of operation of the Geiger counter

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

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

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

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

Types of Geiger counters

By design, Geiger counters come in 2 types: flat and classic.

Classical

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

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

Flat

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

Parameters and operating modes of the Geiger counter

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

You also need to look at the following parameters:

Working area, entrance window area

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

Working voltage

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

Working width

Working width - the difference between the output voltage to the plane and the voltage of the spark discharge. The value is 100 volts.

Incline

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

Temperature

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

Working resource

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

Recovery time

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

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

Does the counter have a background

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

Who and where uses radiation dosimeters?

On an industrial scale, many modifications of Geiger-Muller counters are produced. Their production began during the Soviet era and continues now, but already in the Russian Federation.

The device is used:

• at nuclear industry facilities;
• in scientific institutes;
• in medicine;
• at home.

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

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

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

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

Conclusion

When diagnosing radiation, it is necessary to take into account the own background of the meter. Even with a decent thickness of lead shielding, the registration rate is not reset. This phenomenon has an explanation: the reason for the activity is cosmic radiation penetrating through the thicknesses of lead. Muons rush over the Earth's surface every minute, which are registered by the counter with a probability of 100%.

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

In connection with the environmental consequences of human activities related to nuclear energy, as well as industry (including the military), using radioactive substances as a component or basis of their products, the study of the basics of radiation safety and radiation dosimetry is becoming a fairly relevant topic today. In addition to natural sources of ionizing radiation, every year more and more places appear contaminated with radiation as a result of human activity. Thus, in order to preserve your health and the health of your loved ones, you need to know the degree of contamination of a particular area or objects and food. A dosimeter can help with this - a device for measuring the effective dose or power of ionizing radiation over a certain period of time.

Before proceeding with the manufacture (or purchase) of this device, it is necessary to have an idea of ​​the nature of the measured parameter. Ionizing radiation (radiation) is a stream of photons, elementary particles or fission fragments of atoms capable of ionizing a substance. It is divided into several types. alpha radiation is a stream of alpha particles - helium-4 nuclei, alpha particles born during radioactive decay can be easily stopped by a sheet of paper, so it poses a danger mainly when it enters the body. beta radiation- this is the flow of electrons that arise during beta decay, to protect against beta particles with energies up to 1 MeV, an aluminum plate a few millimeters thick is enough. Gamma radiation has a much greater penetrating power, since it consists of high-energy photons that do not have a charge; heavy elements (lead, etc.) with a layer of several centimeters are effective for protection. The penetrating power of all types of ionizing radiation depends on the energy.

To register ionizing radiation, Geiger-Muller counters are mainly used. This simple and effective device is usually a metal or glass cylinder metallized from the inside and a thin metal thread stretched along the axis of this cylinder, the cylinder itself is filled with rarefied gas. The principle of operation is based on impact ionization. When ionizing radiation hits the walls of the counter, electrons are knocked out of it, electrons, moving in 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. An avalanche of ions arises, leading to the multiplication of primary carriers. At a sufficiently high field strength, the energy of these ions becomes sufficient to generate secondary avalanches capable of maintaining an independent discharge, as a result of which the current through the counter increases sharply.

Not all Geiger counters can register all types of ionizing radiation. Basically, they are sensitive to one radiation - alpha, beta or gamma radiation, but often they can also detect other radiation to some extent. So, for example, the SI-8B Geiger counter is designed to detect soft beta radiation (yes, depending on the energy of the particles, the radiation can be divided into soft and hard), but this sensor is also somewhat sensitive to alpha radiation and gamma radiation. radiation.

However, approaching nevertheless the design of the article, our task is to make the most simple, naturally portable, Geiger counter, or rather a dosimeter. For the manufacture of this device, I managed to get only SBM-20. This Geiger counter is designed to register hard beta and gamma radiation. Like most other meters, SBM-20 operates at a voltage of 400 volts.

The main characteristics of the Geiger-Muller counter SBM-20 (table from the reference book):

This counter has a relatively low accuracy of measuring ionizing radiation, but sufficient to determine the excess of the permissible dose of radiation for humans. SBM-20 is currently used in many household dosimeters. To improve performance, several tubes are often used at once. And to increase the accuracy of measuring gamma radiation, dosimeters are equipped with beta radiation filters; in this case, the dosimeter registers only gamma radiation, but rather accurately.

When measuring radiation dose, there are several factors to consider that may be important. Even in the complete absence of sources of ionizing radiation, the Geiger counter will give a certain number of pulses. This is the so-called custom counter background. This also includes several factors: radioactive contamination of the materials of the counter itself, spontaneous emission of electrons from the cathode of the counter, and cosmic radiation. All this gives a certain amount of "extra" pulses per unit time.

So, the scheme of a simple dosimeter based on the Geiger counter SBM-20:

I assemble the circuit on a breadboard:

The circuit does not contain scarce parts (except, of course, the meter itself) and does not contain programmable elements (microcontrollers), which will allow you to assemble the circuit in a short time without much difficulty. However, such a dosimeter does not contain a scale, and it is necessary to determine the radiation dose by ear by the number of clicks. This is the classic version. The circuit consists of a voltage converter 9 volts - 400 volts.

A multivibrator is made on the NE555 chip, the frequency of which is approximately 14 kHz. To increase the frequency of operation, you can reduce the value of the resistor R1 to about 2.7 kOhm. This will be useful if the choke you have chosen (or maybe made) will make a squeak - with an increase in the frequency of operation, the squeak will disappear. Inductor L1 is required with a rating of 1000 - 4000 μH. The fastest way to find a suitable choke is in a burned-out energy-saving light bulb. Such a choke is used in the circuit, in the photo above it is wound on a core, which is usually used to make pulse transformers. Transistor T1 can use any other field n-channel with a drain-source voltage of at least 400 volts, and preferably more. Such a converter will give only a few milliamps of current at a voltage of 400 volts, but this is enough for a Geiger counter to work several times. After turning off the power from the circuit on the charged capacitor C3, the circuit will work for about another 20-30 seconds, given its small capacitance. The suppressor VD2 limits the voltage at 400 volts. Capacitor C3 must be used for a voltage of at least 400 - 450 volts.

Any piezo speaker or speaker can be used as Ls1. In the absence of ionizing radiation, no current flows through resistors R2 - R4 (there are five resistors in the photo on the breadboard, but their total resistance corresponds to the circuit). As soon as the corresponding particle enters the Geiger counter, the gas ionization occurs inside the sensor and its resistance decreases sharply, as a result of which a current pulse occurs. Capacitor C4 cuts off the constant part and passes only a current pulse to the speaker. We hear a click.

In my case, two batteries from old phones are used as a power source (two, since the required power must be more than 5.5 volts to start the circuit due to the applied element base).

So, the circuit works, occasionally clicks. Now how to use it. The simplest option - it clicks a little - everything is fine, clicks often or even continuously - bad. Another option is to roughly count the number of pulses per minute and convert the number of clicks to microR / h. To do this, you need to take the sensitivity value of the Geiger counter from the reference book. However, different sources always have slightly different numbers. Ideally, laboratory measurements should be made for the selected Geiger counter with reference radiation sources. So for SBM-20, the sensitivity value varies from 60 to 78 pulses / μR according to various sources and reference books. So, we calculated the number of impulses in one minute, then we multiply this number by 60 to approximate the number of impulses in one hour and divide all this by the sensitivity of the sensor, that is, by 60 or 78 or whatever you get closer to reality and as a result we get the value in µR/h. For a more reliable value, it is necessary to take several measurements and calculate the arithmetic mean between them. The upper limit of the safe level of radiation is approximately 20 - 25 microR/h. The permissible level is up to about 50 μR / h. Numbers may vary by country.

P.S. I was prompted to consider this topic by an article on the concentration of radon gas penetrating into rooms, water, etc. in various regions of the country and its sources.

List of radio elements

Designation	Type	Denomination	Quantity	Note	Shop	My notepad
IC1	Programmable timer and oscillator	NE555	1			To notepad
T1	MOSFET transistor	IRF710	1			To notepad
VD1	rectifier diode	1N4007	1			To notepad
VD2	Protective diode	1V5KE400CA	1			To notepad
C1, C2	Capacitor	10 nF	2			To notepad
C3	electrolytic capacitor	2.7uF	1			To notepad
C4	Capacitor	100 nF	1	400V

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

IN 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.

IN 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 meter is activated in an inert gas medium, no discharge occurs on the electrodes due to the high resistance of the medium, but 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 a few 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.

ABOUT 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. Different configurations of the counters, 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, then 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 gamma quanta detection. 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.

IN 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 detect 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 be solved only by a corresponding reduction in the thickness of the working window to a thickness that will be 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 registering alpha particles of 4–5 µm, the input the window can only be made no more than 0.2 sq / cm, we are talking about the SBT9 counter.

ABOUT 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 features. 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.

IN 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 amount 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.