Geiger-Muller counter is a relatively simple measurement tool. In stores, these dosimeters are not cheap (from 5,000 rubles), but if you have the sensor itself, then you can make this meter with minimal costs. To increase sensitivity, the design presented here contains three STS-5 sensors at once. This is useful for measuring natural sources With low level radiation - soil, stones, water.
The principle of operation of a Geiger-Muller counter is that a high voltage (usually 400 V) is applied to the detector flask. It does not conduct electricity, but for a short period when particle radiation arrives, a current pulse passes through it. Level ionizing radiation proportional to the number of pulses detected over a constant time interval.
The Geiger-Muller counter (detector) itself consists of two electrodes, and the ionizing particle creates a spark gap between them. To reduce the amount of current that flows, a high-resistance resistor is placed in series with the tube. Indicated as R1 in the diagram. Typically it is selected in the range of 1-10 megaohm, valid values indicated in the documentation for the Geiger counter.
Eat different ways receiving data from the detector, in the circuit shown here, a resistor is connected in series between the tube and ground, and the change in voltage across the resistor is measured using the detector. This resistor is designated as R2 in the diagram. Usually it is in the range of 10-220 kilo-ohms. Similar to diodes, the Geiger-Muller counter has its own polarity and if connected in the opposite direction it will not work correctly.
Electrical circuit of a Geiger-Muller counter
Here the MC34063 chip is a DC/DC converter which is used to obtain the required high voltage from the low voltage of the battery. Its main advantage over the simple m/s NE555 or similar generators is that it can control the output voltage and adjust the parameters to make it stable (R3, R4, R5, C3). Op amp elements IC1A, R8, R9 are used as a comparator to filter out noise and generate a binary signal (low = no pulse, high = pulse passes).
Attention! The device uses high voltage and can lead to unpleasant consequences when touching some current-carrying structural elements. Don't touch printed circuit board or sensor tube when power is turned on.
Launching and setting up the meter
The voltage at C4 must be in an acceptable range for Geiger operation. Typically around 400V - be careful when measuring! If the voltage is out of range, then elements C1 (converter frequency direct current), and C3, R3, R4, R5 (converter voltage feedback) can be adjusted.
Geiger counter (Geiger-Muller) is a gas-discharge device for automatically counting the number of ionizing particles that have entered it. It is a gas-filled capacitor, which breaks through when an ionizing particle passes through a volume of gas. The counter was invented in 1908 by Hans Geiger and improved by Müller. It is the most common detector (sensor) of ionizing radiation. Until now, it, invented at the very beginning of the last century for the needs of nascent nuclear physics, has, oddly enough, no full-fledged replacement.
Additional electronic circuit provides the meter with power (usually at least 300 V), provides, if necessary, discharge suppression and counts the number of discharges through the counter.
Geiger counters are divided into non-self-quenching and self-quenching (not requiring an external discharge termination circuit).
The sensitivity of the meter is determined by the composition of the gas, its volume, as well as the material and thickness of its walls.
Most often, meters are used in devices with an operating voltage of about 400 V, such as:
1. “SBM-20” (slightly thicker than a pencil in size).
2. “SBM-21” (both with steel housings, suitable for measuring beta and gamma radiation).
3. “SI-8B” (with a mica window in the body, suitable for measuring beta radiation).
A cylindrical Geiger-Muller counter consists of a metal tube or metallized inside glass tube, and a thin metal thread stretched along the axis of the cylinder. The thread serves as the anode, the tube as the cathode. The tube is filled with rarefied gas; in most cases, noble gases are used - argon and neon. A voltage of about 400 V is created between the cathode and anode. For most meters there is a so-called plateau, which lies from approximately 360 to 460 V, in this range small voltage fluctuations do not affect the counting speed.
The counter operates based on impact ionization. Gamma quanta emitted by a radioactive isotope, hitting the walls of the counter, knock electrons out of it. Electrons moving through the gas and colliding with gas atoms knock electrons out of the atoms and create positive ions and free electrons. The electric field between the cathode and anode accelerates the electrons to energies at which impact ionization begins. An avalanche of ions occurs, and the current through the counter increases sharply. In this case, a voltage pulse is formed across the resistance, which is fed to the recording device. In order for the counter to register the next particle that hits it, the avalanche discharge must be extinguished. This happens automatically. At the moment the current pulse appears, a large voltage drop occurs across the resistance, so the voltage between the anode and cathode decreases sharply - so much so that the discharge stops and the meter is ready for use again.
An important characteristic of the meter is its efficiency. Not all Gamma photons that hit the counter will give secondary electrons and will be registered, since acts of interaction of gamma rays with matter are relatively rare, and some of the secondary electrons are absorbed in the walls of the device without reaching the gas volume.
The efficiency of the counter depends on the thickness of the counter walls, their material and the energy of gamma radiation. The most efficient are counters whose walls are made of a material with a high atomic number Z, since this increases the formation of secondary electrons.
Note. Atomic number, Z Ї is serial number chemical element in periodic table elements by D.I. Mendeleev. The atomic number is equal to the number of protons in the atomic nucleus, which, in turn, is equal to the number of electrons in the electron shell of the corresponding neutral atom. The nuclear charge is equal to Ze, where e is the positive elementary electric charge, equal in absolute value to the charge of the electron.
In addition, the walls of the meter must be thick enough. The thickness of the counter wall is selected from the condition that it is equal to the mean free path of secondary electrons in the wall material. At large thickness walls, secondary electrons will not pass into the working volume of the counter and a current pulse will not occur. SG has its drawbacks; by the reaction of such a device one cannot judge the root cause of its excitation. The output pulses generated by the SG under the influence of alpha particles, electrons, and gamma quanta are no different.
Let's give some passport data, using the SBM 20 meter as an example.
· Rated operating voltage Ї 400 V.
· The length of the plateau of the counting characteristic Ї is not less than 100 V.
· The change in the sensitivity of the counter during the entire resource does not exceed.
· Own background Ї no more than 1 pulse/sec.
· Pulse amplitude Ї not less than 50 V.
· Range of recorded powers Ї (0.001…10) micror/sec.
· Radiation sensitivity Ї 460 pulses/sec.
Rice. 1.1 Ї Dependence of counting speed on supply voltage
Rice. 1.2 Ї Dependence of counting rate on radiation level
In 1908, German physicist Hans Geiger worked in chemical laboratories owned by Ernst Rutherford. There they were also asked to test a charged particle counter, which was an ionized chamber. The chamber was an electric capacitor, which was filled with gas under high pressure. Pierre Curie also used this device in practice, studying electricity in gases. Geiger's idea - to detect the radiation of ions - was associated with their influence on the level of ionization of volatile gases.
In 1928, the German scientist Walter Müller, working with and under Geiger, created several counters that registered ionizing particles. The devices were needed for further radiation research. Physics, being a science of experiments, could not exist without measuring structures. Only a few radiations were discovered: γ, β, α. Geiger's task was to measure all types of radiation with sensitive instruments.
The Geiger-Muller counter is a simple and cheap radioactive sensor. It is not a precise instrument that captures individual particles. The technique measures the total saturation of ionizing radiation. Physicists use it with other sensors to achieve accurate calculations when conducting experiments.
A little about ionizing radiation
We could go straight to the description of the detector, but its operation will seem incomprehensible if you know little about ionizing radiation. When radiation occurs, an endothermic effect on the substance occurs. Energy contributes to this. For example, ultraviolet or radio waves do not belong to such radiation, but hard ultraviolet light does. Here the limit of influence is determined. The type is called photonic, and the photons themselves are γ-quanta.
Ernst Rutherford divided the processes of energy emission into 3 types, using an installation with a magnetic field:
- γ - photon;
- α is the nucleus of a helium atom;
- β is a high energy electron.
You can protect yourself from α particles with paper. β penetrate deeper. Penetration ability γ is the highest. Neutrons, which scientists learned later, are dangerous particles. They act at a distance of several tens of meters. Having electrical neutrality, they do not react with molecules of different substances.
However, neutrons easily reach the center of the atom, causing its destruction, which results in the formation of radioactive isotopes. As isotopes decay, they create ionizing radiation. From a person, animal, plant or inorganic object that has received radiation, radiation emanates for several days.
Design and principle of operation of a Geiger counter
The device consists of a metal or glass tube into which a noble gas (argon-neon mixture or substances in pure form). There is no air in the tube. The gas is added under pressure and contains an admixture of alcohol and halogen. There is a wire stretched throughout the tube. An iron cylinder is located parallel to it.
The wire is called the anode and the tube is called the cathode. Together they are electrodes. A high voltage is 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 environment. A minus is connected from the power source to the tube, and a plus is connected to the wire, directed through a high-level resistance. We are talking about a constant supply of tens of hundreds of volts.
When a particle enters the tube, noble gas atoms collide with it. Upon contact, energy is released that removes electrons from the gas atoms. Then secondary electrons are formed, which also collide, generating a mass of new ions and electrons. The speed of electrons towards the anode is affected by the electric field. During this process, it is formed electricity.
During a collision, the energy of the particles is lost, and the supply of ionized gas atoms comes to an end. When charged particles enter gas discharge meter Geiger, the resistance of the tube drops, which immediately reduces the voltage at the midpoint of the division. Then the resistance increases again - this entails a restoration of voltage. The momentum becomes negative. The device shows pulses, and we can count them, at the same time estimating the number of particles.
Types of Geiger counters
By design, Geiger counters come in two types: flat and classic.
Classical
Made from thin corrugated metal. Due to corrugation, the tube becomes rigid and resistant to external influence, which prevents its deformation. The ends of the tube are equipped with glass or plastic insulators, which contain caps for output to the devices.
Varnish is applied to the surface of the tube (except for the leads). The classic counter is considered a universal measuring detector for everyone known species radiation. Especially for γ and β.
Flat
Sensitive meters for recording soft beta radiation have a different design. Due to the small number of beta particles, their body has flat shape. There is a mica window that weakly blocks β. BETA-2 sensor is the name of one of these devices. The properties of other flat counters depend on the material.
Geiger counter parameters and operating modes
To calculate the sensitivity of the counter, estimate the ratio of the number of microroentgens from the sample to the number of signals from this radiation. The device does not measure the energy of the particle, so it does not give an absolutely accurate estimate. Devices are calibrated using samples from isotope sources.
You also need to look at the following parameters:
Work area, entrance window area
The characteristics of the indicator area through which microparticles pass depends on its size. The wider the area, the more particles will be caught.
Operating voltage
The voltage should correspond to the average specifications. The operating characteristic itself is the flat part of the dependence of the number of fixed pulses on voltage. Its second name is plateau. At this point, the device reaches peak activity and is called the upper limit of measurement. Value - 400 Volts.
Working width
Working width is the difference between the plane output voltage and the spark discharge voltage. The value is 100 Volts.
Incline
The value is measured as a percentage of the number of pulses per 1 volt. It shows the measurement error (statistical) in the pulse count. The value is 0.15%.
Temperature
Temperature is important because the meter is often used in difficult conditions. For example, in reactors. General use meters: -50 to +70 Celsius.
Work resource
The resource is characterized total number all pulses recorded until the moment when the instrument readings become incorrect. If the device contains organics for self-extinguishing, the number of pulses will be one billion. It is appropriate to calculate the resource only in a state of operating voltage. When storing the device, the flow rate stops.
Recovery time
This is the amount of time it takes a device to conduct electricity after reacting to an ionizing particle. There is an upper limit on the pulse frequency that limits the measurement range. The value is 10 microseconds.
Due to the recovery time (also called dead time), the device can fail at a decisive moment. To prevent overshoot, manufacturers install lead screens.
Does the counter have a background?
The background is measured in a thick-walled lead chamber. The usual value is no more than 2 pulses per minute.
Who uses radiation dosimeters and where?
Many modifications of Geiger-Muller counters are produced on an industrial scale. Their production began during the USSR and continues now, but in the Russian Federation.
The device is used:
- at nuclear industry facilities;
- in scientific institutes;
- in medicine;
- at home.
After the accident at Chernobyl nuclear power plant Ordinary citizens also buy dosimeters. All devices have a Geiger counter. Such dosimeters are equipped with one or two tubes.
Is it possible to make a Geiger counter with your own hands?
Making a meter yourself is difficult. You need a radiation sensor, but not everyone can buy it. The counter circuit itself has long been known - in physics textbooks, for example, it is also printed. However, only a true “left-hander” will be able to reproduce the device at home.
Talented self-taught craftsmen have learned to make a substitute for the counter, which is also capable of measuring gamma and beta radiation using fluorescent lamp and incandescent lamps. They also use transformers from broken equipment, a Geiger tube, a timer, a capacitor, various boards, and resistors.
Conclusion
When diagnosing radiation, you need to take into account the meter’s own background. Even with lead protection decent thickness the registration speed is not reset. This phenomenon has an explanation: the cause of activity is cosmic radiation penetrating through layers of lead. Muons fly over the Earth's surface every minute, which are registered by the counter with a probability of 100%.
There is another source of background - radiation accumulated by the device itself. Therefore, in relation to the Geiger counter, it is also appropriate to talk about wear. The more radiation the device has accumulated, the lower the reliability of its data.
Whether we like it or not, radiation has firmly entered our lives and is not going to go away. We need to learn to live with this phenomenon, which is both useful and dangerous. Radiation manifests itself as invisible and imperceptible radiation, and without special devices it is impossible to detect them.
A little history of radiation
X-rays were discovered in 1895. A year later, the radioactivity of uranium was discovered, also in connection with X-rays. Scientists realized that they were faced with completely new, hitherto unseen natural phenomena. It is interesting that the phenomenon of radiation was noticed several years earlier, but no importance was attached to it, although Nikola Tesla and other workers of the Edison laboratory also received burns from X-rays. Damage to health was attributed to anything, but not to rays, which living things had never encountered in such doses. At the very beginning of the 20th century, articles began to appear about the harmful effects of radiation on animals. This, too, was not given any importance until the sensational story with the “radium girls” - workers of a factory that produced luminous watches. They just wet the brushes with the tip of their tongue. The terrible fate of some of them was not even published, for ethical reasons, and remained a test only for the strong nerves of doctors.
In 1939, physicist Lise Meitner, who, together with Otto Hahn and Fritz Strassmann, belongs to the people who were the first in the world to divide the uranium nucleus, inadvertently blurted out about the possibility of a chain reaction, and from that moment a chain reaction of ideas about creating a bomb began, namely a bomb, and not at all “peaceful atom”, for which the bloodthirsty politicians of the 20th century, of course, would not have given a penny. Those who were “in the know” already knew what this would lead to and the atomic arms race began.
How did the Geiger-Müller counter appear?
The German physicist Hans Geiger, who worked in the laboratory of Ernst Rutherford, in 1908 proposed the principle of operation of a “charged particle” counter as a further development of the already known ionization chamber, which was an electric capacitor filled with gas at low pressure. It was used by Pierre Curie in 1895 to study the electrical properties of gases. Geiger had the idea to use it to detect ionizing radiation precisely because these radiations had a direct effect on the degree of ionization of the gas.
In 1928, Walter Müller, under the leadership of Geiger, created several types of radiation counters designed to register various ionizing particles. The creation of counters was a very urgent need, without which it was impossible to continue the study of radioactive materials, since physics, as an experimental science, is unthinkable without measuring instruments. Geiger and Müller purposefully worked to create counters that were sensitive to each of the types of radiation that had been discovered: α, β and γ (neutrons were discovered only in 1932).
The Geiger-Muller counter proved to be a simple, reliable, cheap and practical radiation detector. Although he is not the most precision instrument for research individual species particles or radiation, but is extremely suitable as an instrument for general measurement of the intensity of ionizing radiation. And in combination with other detectors, it is used by physicists for precise measurements during experiments.
Ionizing radiation
To better understand the operation of a Geiger-Muller counter, it is helpful to have an understanding of ionizing radiation in general. By definition, these include anything that can cause ionization of a substance in its normal state. This requires a certain amount of energy. For example, radio waves or even ultraviolet light are not ionizing radiation. The border begins with “hard ultraviolet”, also known as “soft x-ray”. This type is a photon type of radiation. Photons 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 magnetic field in a vacuum. It later turned out that this is:
α - nuclei of helium atoms
β - high energy electrons
γ - gamma quanta (photons)
Later neutrons were discovered. Alpha particles are easily blocked even by ordinary paper, beta particles have a slightly greater penetrating power, and gamma rays have the highest penetrating power. Neutrons are the most dangerous (at a distance of up to many tens of meters in the air!). Due to their electrical neutrality, they do not interact with electronic shells molecules of matter. But once in atomic nucleus, the probability of which is quite high, lead to its instability and decay, with the formation, as a rule, of radioactive isotopes. And those, in turn, decaying, themselves form the entire “bouquet” of ionizing radiation. The worst thing is that an irradiated object or living organism itself becomes a source of radiation for many hours and days.
The design of a Geiger-Muller counter and its operating principle
A Geiger-Muller gas-discharge counter is usually made in the form of a sealed tube, glass or metal, from which the air is evacuated, and instead an inert gas (neon or argon or a mixture of both) is added under low pressure, with an admixture of halogens or alcohol. A thin wire is stretched along the axis of the tube, and a metal cylinder is located coaxially with it. Both the tube and the wire are electrodes: the tube is the cathode, and the wire is the anode. 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 middle point of which (the junction of the resistance and the anode of the meter) the voltage is almost equal to the voltage at the source. This is usually several hundred volts.
When an ionizing particle flies through the tube, the atoms of the inert gas, already in electric field high tension, experience collisions with this particle. The energy given off by the particle during a collision is enough to separate electrons from gas atoms. The resulting secondary electrons are themselves capable of forming new collisions and, thus, a whole avalanche of electrons and ions is obtained. Under the influence of an electric field, electrons are accelerated towards the anode, and positively charged gas ions are accelerated towards the cathode of the tube. Thus, an electric current arises. But since the energy of the particle has already been spent on collisions, fully or partially (the particle flew through the tube), the supply of ionized gas atoms also ends, which is desirable and is ensured by some additional measures, which we will talk about when analyzing the parameters of the counters.
When a charged particle enters a Geiger-Muller counter, due to the resulting current, the resistance of the tube drops, and with it the voltage at the midpoint of the voltage divider, which was discussed above. Then the resistance of the tube, due to an increase in its resistance, is restored, and the voltage again becomes the same. Thus, we get a negative voltage pulse. By counting the impulses, we can estimate the number of passing particles. The electric field strength is especially high near the anode due to its small size, which makes the counter more sensitive.
Geiger-Muller counter designs
Modern Geiger-Muller counters are available in two main versions: “classic” and flat. The classic counter is made of a thin-walled metal tube with corrugation. The corrugated surface of the meter makes the tube rigid and resistant to external atmospheric pressure and does not allow it to crumple under its influence. At the ends of the tube there are sealing insulators made of glass or thermosetting plastic. They also contain terminal caps for connecting to the device circuit. The tube is marked and coated with a durable insulating varnish, not counting, of course, its terminals. The polarity of the terminals is also indicated. This is a universal counter for all types of ionizing radiation, especially beta and gamma.
Counters sensitive to soft β-radiation are made differently. Due to the short range of beta particles, they have to be made flat, with a mica window that weakly blocks beta radiation; one of the options for such a counter is a radiation sensor BETA-2. All other properties of the meters are determined by the materials from which they are made.
Counters designed to record gamma radiation contain a cathode made of metals with a high charge number, or are coated with such metals. Gas is extremely poorly ionized by gamma photons. But gamma photons are capable of knocking out many secondary electrons from the cathode if it is chosen appropriately. Geiger-Muller counters for beta particles are made with thin windows to better transmit the particles, since they are ordinary electrons that have just received more energy. They interact with matter very well and quickly lose this energy.
In the case of alpha particles the situation is even worse. So, despite a very decent energy, on the order of several MeV, alpha particles interact very strongly with molecules in their path and quickly lose energy. If matter is compared to a forest, and an electron is compared to a bullet, then alpha particles will have to be compared to a tank crashing through a forest. However, a conventional counter responds well to α-radiation, but only at a distance of up to several centimeters.
For an objective assessment of the level of ionizing radiation dosimeters on the counters general use often equipped with two counters operating in parallel. One is more sensitive to α and β radiation, and the second to γ rays. This scheme of using two counters is implemented in a dosimeter RADEX RD1008 and in a dosimeter-radiometer RADEKS MKS-1009, in which the counter is installed BETA-2 And BETA-2M. Sometimes a bar or plate of an alloy containing an admixture of cadmium is placed between the counters. When neutrons hit such a bar, γ-radiation is generated, which is recorded. This is done to be able to detect neutron radiation, to which simple Geiger counters are practically insensitive. Another method is to coat the housing (cathode) with impurities that can impart sensitivity to neutrons.
Halogens (chlorine, bromine) are added to the gas to quickly extinguish the discharge. Alcohol vapor also serves the same purpose, although alcohol in this case is short-lived (this is generally a feature of alcohol) and the “sobered up” meter constantly begins to “ring”, that is, it cannot work in the intended mode. This happens somewhere after 1e9 pulses (a billion) have been detected, which is not that much. Meters with halogens are much more durable.
Parameters and operating modes of Geiger counters
Sensitivity of Geiger counters.
The sensitivity of the counter is estimated by the ratio of the number of microroentgens from the reference source to the number of pulses caused by this radiation. Since Geiger counters are not designed to measure particle energy, accurate estimation is difficult. The counters are calibrated using reference isotope sources. It should be noted that this parameter is different types counters can vary greatly, below are the parameters of the most common Geiger-Müller counters:
Geiger-Muller counter Beta-2- 160 ÷ 240 imp/µR
Geiger-Muller counter Beta-1- 96 ÷ 144 imp/µR
Geiger-Muller counter SBM-20- 60 ÷ 75 imp/µR
Geiger-Muller counter SBM-21- 6.5 ÷ 9.5 imp/µR
Geiger-Muller counter SBM-10- 9.6 ÷ 10.8 imp/μR
Entrance window area or work area
The area of the radiation sensor through which radioactive particles fly. This characteristic is directly related to the dimensions of the sensor. The larger the area, the more particles the Geiger-Muller counter will catch. Typically this parameter is indicated in square centimeters.
Geiger-Muller counter Beta-2- 13.8 cm 2
Geiger-Muller counter Beta-1- 7 cm 2
This voltage corresponds approximately to the middle performance characteristics. The operating characteristic is the flat part of the dependence of the number of recorded pulses on the voltage, which is why it is also called the “plateau”. At this point it is reached highest speed work (upper measurement limit). Typical value is 400 V.
Width of the counter operating characteristic.
This is the difference between the spark breakdown voltage and the output voltage on the flat part of the characteristic. Typical value is 100 V.
Slope of the meter operating characteristic.
The slope is measured as a percentage of pulses per volt. It characterizes the statistical error of measurements (counting the number of pulses). Typical value is 0.15%.
Permissible operating temperature of the meter.
For general purpose meters -50 ... +70 degrees Celsius. This is a very important parameter if the counter operates in cameras, channels, and other places complex equipment: accelerators, reactors, etc.
Working resource of the counter.
The total number of pulses that the meter registers before its readings begin to become incorrect. For devices with organic additives, self-quenching is usually 1e9 (ten to the ninth power, or one billion). The resource is counted only if operating voltage is applied to the meter. If the counter is simply stored, this resource is not consumed.
Counter dead time.
This is the time (recovery time) during which the counter conducts current after being triggered by a passing particle. The existence of such a time means that there is an upper limit to the pulse frequency and this limits the measurement range. A typical value is 1e-4 s, which is ten microseconds.
It should be noted that due to dead time, the sensor may be “off scale” and remain silent at the most dangerous moment (for example, a spontaneous chain reaction in production). Such cases have happened, and to combat them, lead screens are used to cover part of the sensors of emergency alarm systems.
Custom counter background.
Measured in thick-walled lead chambers to assess the quality of meters. Typical value is 1 ... 2 pulses per minute.
Practical application of Geiger counters
Soviet and now Russian industry produces many types of Geiger-Muller counters. Here are some common brands: STS-6, SBM-20, SI-1G, SI21G, SI22G, SI34G, meters of the Gamma series, end counters of the series Beta"and there are many more. All of them are used for monitoring and measuring radiation: at facilities nuclear industry, in scientific and educational institutions, in civil defense, medicine, and even in everyday life. After the Chernobyl accident, household dosimeters, previously unknown to the population even by name, have become very popular. Many brands of household dosimeters have appeared. All of them use a Geiger-Muller counter as a radiation sensor. In household dosimeters, one to two tubes or end counters are installed.
UNITS OF MEASUREMENT OF RADIATION QUANTITIES
For a long time, the unit of measurement P (roentgen) was common. However, when moving to the SI system, other units appear. An x-ray is a unit of exposure dose, a "quantity of radiation", which is expressed as the number of ions produced in dry air. With a dose of 1 R in 1 cm3 of air, 2.082e9 pairs of ions are formed (which corresponds to 1 unit of charge of the SGSE). In the SI system, exposure dose is expressed in coulombs per kilogram, and with x-rays this is related to the equation:
1 C/kg = 3876 R
The absorbed dose of radiation is measured in joules per kilogram and is called Gray. This is a replacement for the outdated rad unit. The absorbed dose rate is measured in grays per second. Exposure dose rate (EDR), formerly measured in roentgens per second, is now measured in amperes per kilogram. The equivalent radiation dose at which the absorbed dose is 1 Gy (gray) and the radiation quality factor is 1 is called Sievert. The rem (biological equivalent of an x-ray) is a hundredth of a sievert, now considered obsolete. Nevertheless, even today all outdated units are very actively used.
The main concepts in radiation measurements are dose and power. Dose is the number of elementary charges in the process of ionization of a substance, and power is the rate of dose formation per unit time. And in what units this is expressed is a matter of taste and convenience.
Even a minimal dose is dangerous in terms of long-term consequences for the body. The calculation of danger is quite simple. For example, your dosimeter shows 300 milliroentgen per hour. If you stay in this place for a day, you will receive a dose of 24 * 0.3 = 7.2 roentgens. This is dangerous and you need to leave here as soon as possible. In general, if you detect even weak radiation, you need to move away from it and check it even from a distance. If she “follows you”, you can be “congratulated”, you have been hit by neutrons. But not every dosimeter can respond to them.
For radiation sources, a quantity characterizing the number of decays per unit of time is used; it is called activity and is also measured by the set various units: curie, becquerel, rutherford and some others. The amount of activity, measured twice with a sufficient separation in time, if it decreases, makes it possible to calculate the time, according to the law of radioactive decay, when the source becomes sufficiently safe.
A Geiger counter is an evacuated cylinder with two electrodes, into which a gas mixture is introduced, consisting of easily ionized neon and argon with a small addition of halogen - chlorine or bromine.
A high voltage is applied to the electrodes, which in itself does not cause any discharge phenomena (see figure).
The counter will remain in this state until an ionization center appears in its gaseous medium - a trail of ions and electrons generated by an ionizing particle arriving from outside.
Primary electrons, accelerating in an electric field, ionize “along the way” other molecules of the gaseous medium, generating more and more new electrons and ions. Developing like an avalanche, this process ends with the formation of an electron-ion cloud in the interelectrode space, sharply increasing its conductivity. A discharge occurs in the gas environment of the meter, visible (if the container is transparent) even with the naked eye.
Reverse process- the return of the gaseous medium to its original state occurs under the influence of the halogen contained in it, which promotes intense recombination of charges. But this process is much slower. The period of time required to restore the radiation sensitivity of the counter and actually determines its performance - the so-called "dead" time - is an important passport characteristic of the counter.
Halogen- consumable part of the gas environment of the meter. But this part is so large that in background counting mode it would last for centuries (the halogen operating time of, for example, the SBM20 counter is at least 2 10 10 pulses).
Meters of this type are called halogen self-extinguishing meters. Differing the most low voltage power supply, excellent output signal parameters and fairly high speed, they turned out to be especially convenient for use as ionizing radiation sensors in household appliances radiation control.
Geiger counters are capable of responding to the most different types ionizing radiation - α, β, γ, ultraviolet, x-ray, neutron. But the actual spectral sensitivity of the meter depends on its design.
More often there are meters with a cylindrical cylinder made of of stainless steel thickness 0.05....0.06 mm. The cylinder in such a counter is also its cathode. The spectral sensitivity of such a thin-walled counter is limited by γ- and hard β-radiation.
Counters with a glass cylinder are sensitive only to γ-radiation (glass 1 mm thick is an almost insurmountable barrier for β-radiation). The cathode in such counters is a thin conductive layer deposited on the inner surface of the glass. A counter with a thick-walled (more than 0.2 mm) metal cylinder also almost completely loses sensitivity to β-radiation.
In Geiger counters designed to detect soft β-radiation, special windows are made of very thin mica.
The X-ray counter window is made of beryllium, and the ultraviolet counter window is made of quartz glass.
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, Compton effect, pair creation effect; in each case, the radiation interacting with the cathode substance is converted into a flow of electrons.
Each particle detected by a Geiger counter excites a short (fractions of a millisecond) current pulse in it. The number of pulses occurring per unit time - the counting rate of a Geiger counter - depends on the level of ionizing radiation and the voltage on its electrodes. A typical graph of the counting rate versus supply voltage U power is shown in Fig. a.
Here:
Uns - counting start voltage;
Umin and Umax are the lower and upper boundaries of the working section, the so-called plateau, at which the counting speed is almost independent of the meter supply voltage.
The operating voltage Up is usually selected in the middle of this section.
It corresponds to N(Up) - the counting rate in this mode.
In Fig. b shows the dependence N(Upit) for the SBM20 counter located in the field of ionizing radiation, approximately 1000 times higher than the level of natural background radiation.
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 the radiation sensitivity of the counter is often expressed in terms of pulse/μR (pulses per microroentgen; this dimension follows from the ratio of the counting rate - pulse/s - to the radiation level - μR/s). In Fig. Figure 4 shows a graph of this dependence for the SBM20 counter.
In cases where it is not indicated (not uncommon, unfortunately), the radiation sensitivity of the counter has to be judged differently, which is also very important parameter- own background.
This is the name given to the counting rate, which is caused by two components: external - the natural background radiation, and internal - the radiation of radionuclides found in the counter structure itself, as well as the spontaneous electron emission of its cathode.
One more important characteristic Geiger counter is the dependence of its radiation sensitivity on the energy (hardness) of ionizing particles.
In professional jargon, the graph of this relationship is called a “strength move.” The extent to which this dependence is important is shown by the graph in Fig. 5
“Riding with rigidity” will obviously affect the accuracy of the measurements taken.
Without discussing the question of whether a household radiometer needs high measurement accuracy, we note that such industrial devices differ from amateur ones only in the correction of the Geiger counter in terms of hardness. To do this, they put a “shirt” on it - a passive filter. This filter must, firstly, “cut off” extraneous radiation (primarily (β-radiation), and, secondly, with its approximately inverse stiffness characteristic relative to the counter, compensate for the “stroke with rigidity” of the counter itself. Some of the industrial dosimeters also take into account the spontaneous activity of the Geiger counter.
The fact that the Geiger counter is an avalanche device also has its disadvantages - by the reaction of such a device one cannot judge the root cause of its excitation. The output pulses generated by a Geiger counter under the influence of α-particles, electrons, γ-quanta (in the counter reacting to all these types of radiation) are no different.
The particles themselves and their energies completely disappear in the twin avalanches they generate.
In principle, the radiation sensitivity of a Geiger counter can be adjusted by changing the supply voltage ranging from the counting start voltage to reaching a plateau: Upit € . But this regime is very unstable, and in any serious cases one cannot rely on it.
Stable sensitivity adjustment is possible only in a three-electrode Geiger counter, in which the configuration and volume of space in which avalanche flashes are possible depend on the voltage on the control electrode. In Fig. 6, a shows the connection diagram of such a counter, and in Fig. 6, b - dependence of its radiation sensitivity on the voltage at the control electrode.
Rice. 8. Turning on a three-electrode Geiger counter (a); dependence of its radiation sensitivity on the voltage at the control electrode (b)
However, three-electrode Geiger counters are not widely used. The reason is the Uynp generator. The electronics that take into account the real radiation sensitivity of a two-electrode Geiger counter turned out to be simpler than this high-voltage source.
In household dosimetric devices, the speed of the Geiger counter is not at all a limiting factor (a person must detect the source of radiation before he needs this speed). Therefore, there is no need to turn on a multi-anode Geiger counter as is usually recommended in reference books (Fig.). 
The time constant when directly combining even all ten anodes of the SBT10 meter, the most multi-sectional of the domestic ones, still remains small enough (R n Ca = 15 10 6 10 5 10 -12 = 0.75 ms) so as to have virtually no effect on the measurement result even in fields that are a thousand times higher than the level of natural background radiation.
Are there Geiger counters capable of responding to α-radiation - one of the most dangerous to humans?
Let us evaluate the ability of counters with mica windows (others may not be considered) to respond to α-radiation of the same plutonium-239 (Ea = 5.16 MeV). The range of its α-particles in the air is about 3.5 cm. Mica with a density of 2.8 g/cm 3 (it is about 2200 times denser than air) and a thickness of 10 microns (10 -3 cm) is equivalent to an air “cushion” with a thickness of 2200 10 - 3 = 2.2 cm. That is, a counter with a mica window 10 microns thick will be able to detect the radiation of plutonium-239 if it gets close to it. In any case, the “gap” between the emitter and the counter should be less than 3.5 - 2.2 = 1.3 cm.