Geiger counter which measures. Geiger-Muller counter: history of creation, principles of operation and purpose
1.4 Geiger-Muller counter
IN in a 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. With increasing voltage, the critical area expands. The concentration of excited molecules in it increases, and hence the number of emitted photons. Under the action of photons from the cathode and gas molecules,
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 the primary ionization. Thus, an increase in voltage U leads to the propagation of a gas discharge throughout the volume of the meter. At a certain voltage U p. Called the threshold, the gas discharge covers the entire volume of the meter. At a voltage U p, the Geiger-Muller region begins.
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. CountersGeiger-Muller - the most common detectors (sensors) of ionizing radiation.
Geiger - Muller counter -gas-discharge device for detecting and studying various kinds 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 recorded by a Geiger-Muller counter for secondary ionizing particles - photoelectrons, Compton electrons, electron-positron pairs; neutrons are recorded by recoil nuclei and products of nuclear reactions arising in the gas of the counter. The meter operates at voltages corresponding to an independent
corona discharge (section V, Fig. 21).
Figure: 21. 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 almost one hundred percent probability of detecting a charged particle, since for
for the discharge to occur, one electron-ion pair is sufficient.
Structurally, the Geiger counter is designed as a proportional counter, i.e. is a capacitor (usually cylindrical) with a highly inhomogeneous electric field. A positive potential (anode) is applied to the inner electrode (a thin metal thread), and a negative potential (cathode) to the outer one. The electrodes are enclosed in a hermetically sealed tank filled with any gas up to a pressure of 13-26 kN / m 2 (100-200 mm pm. Art.). A voltage of several hundred volts is applied to the meter 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 across the electrodes, it operates in such a mode when the appearance of one electron in the volume of the detector is enough to develop a powerful avalanche-like process due to secondary ionization (gas amplification) , which is capable of ionizing the entire area near the filament-anode. In this case, the current pulse reaches its limiting value (saturates) and does not depend on the primary ionization. Evolving 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 the Geiger counter, an independent gas discharge flares up (ignites), visible (if the balloon is transparent) even with a simple gas. In this case, the gas gain can reach 1010, and the pulse magnitude is 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 near 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 "cover" of positive ions is formed. A positive space charge increases the effective anode diameter and thereby decreases the field strength, so the discharge is interrupted. As the layer of positive ions moves away from the filament, its shielding effect is weakened 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, as a result of which neutral atoms of an inert gas are formed in an excited state. Excited atoms at
close enough to 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 filament
and the potential difference between the filament and the cathode drops rapidly, with the result that the discharge is terminated. After that, the meter sensitivity is restored after10-1 -10-3 sec (time of discharge of capacity C through resistance R). This is the time required for the slow positive ions that filled the space near the filament-anode after the flight of the particle and the passage of the electron avalanche to go to the cathode,
and the detector sensitivity was restored. Such a long dead time is inconvenient for many applications.
For practical use of a non-self-extinguishing Geiger counter, various methods of terminating the discharge are used:
a) The use of electronic circuits for quenching the discharge in gas. An electronic circuit adapted for this, at the right time, issues a “counter-signal” to the meter, which stops the self-discharge and “withstands” the counter for a while until the charged particles are completely neutralized. The characteristics of such a meter with a discharge extinguishing circuit are close to the characteristics of self-extinguishing meters and sometimes exceed them.
b) Quenching due to the selection of the values \u200b\u200bof the load resistance and the equivalent capacitance, as well as the voltage value on the meter.
IN depending on the discharge extinguishing mechanism, two groups of meters 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 discharge quenching circuits are used, which reduce the resolving time to the time of collection of positive ions at the cathode (10-4 sec).
Nowadays, non-self-extinguishing meters, in which discharge suppression is provided by resistance R, have been supplanted by self-extinguishing meters, which are more stable. In them, thanks to a special gas filling (an inert gas with an admixture of complex molecules, for example, alcohol vapor, and a small
impurity of halogens - chlorine, bromine, iodine), the discharge breaks off by itself even at low resistances R. Self-extinguishing meter dead time ~ 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 by the ionizing particle in the counter will go out by itself. Subsequently, it turned out that spontaneous extinction of the discharge in the counter also occurs when the vapor of other organic compounds with complex polyatomic compounds is added to argon. These substances are usually called extinguishing, and Geiger-Muller counters, in which these substances are used, are called counters - self-extinguishing type. The 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, quenching gas, about 10%. The components of the working mixture must satisfy the obligatory condition that the ionization potential of the quenching gas must be lower than the first excitation potential of the main gas.
Comment. Xenon wire detectors are often used to register X-rays. An example is the first domestic scanning digital medical fluorograph MCRU SIBERIA. 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 being determined, the following detectors can be used: - a flow-through proportional detector with windows 1, 2, 6 microns thick, a non-flowing neon detector with windows 25 and 50 microns thick, - a non-flowing krypton detector with a window 100 microns thick, - a xenon detector with a window 200 microns and a scintillation detector with a 300 micron window.
Self-extinguishing meters allow high counting speeds without special electronic circuits
quenching the discharge, so they are widely used. Self-extinguishing meters with organic extinguishing impurities have a limited service life (108-1010 pulses). When one of the halogens is used as a quenching impurity (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 the pulses due to the adhesion of primary electrons to the halogen molecule. "Pulling" the leading edge of the pulse in halogen counters makes them inapplicable in coincidence circuits.
The main characteristics of the meter are: counting characteristic - the dependence of the counting rate on the value of the operating voltage; counter efficiency - the ratio of the number of counted particles to the number of all particles entering the working volume of the counter, expressed as a percentage; resolving time -
the minimum time interval between impulses at which they are recorded separately and the service life of the counters.
Figure: 22. Diagram of the occurrence of dead time in the counterGeiger-Muller.(Pulse shape during discharge in a Geiger-Müller counter).
The length of time required to restore the radiation sensitivity of the Geiger counter and actually determining its response rate - "dead" time - is its important passport characteristic.
If a discharge caused by a nuclear particle began in a Geiger-Muller counter at time t 0, then the voltage across the counter drops sharply. The counter for a certain time, which is called the dead time τ m, is not able to regulate other particles. From the moment t 1, i.e. after the dead time, a self-discharge is possible in the meter again. However, at first the pulse amplitude is still small. Only after the space charge reaches the cathode surface are pulses of normal amplitude generated in the counter. The time interval τ s between the moment t 0, when an independent discharge appeared in the meter, and the moment of restoration of the operating voltage t 3 is called the recovery time. In order for the recording device 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 a self-sustained discharge t 0 and the moment of formation of the amplitude U p of the pulse t 2 is called the resolving time τ p of the Geiger-Muller counter. The resolving time τ p is slightly larger than the dead time.
If every second a large number of particles (several thousand or more) enter the counter, then the resolving time τ p will be comparable in magnitude with the average time interval between pulses, therefore a significant number of pulses are not counted. Let m be the observed counter counting rate. Then the fraction of time during which the counting device is insensitive is equal to m τ. Consequently, the number of pulses lost per unit of time is equal to nm τ p, where n is the counting rate that would be observed if the resolving time had a negligible value. therefore
n - m \u003d nmτ p |
||||||
−m τ |
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The correction to the count rate that is given by this equation is called the dead time correction.
Self-extinguishing halogen meters are characterized by the lowest supply voltage, excellent output signal parameters and a fairly high response speed, they have proved to be especially convenient 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 time - the counting rate of a Geiger counter - depends on the level of ionizing radiation and the voltage at its electrodes. A typical graph of count rate versus supply voltage V is shown in Fig. 23. Here V zzh - voltage of the beginning of counting; V 1 and V 2 - the lower and upper boundaries of the working area, the so-called plateau, at which the count rate is almost independent of the supply voltage of the meter. 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.
Figure: 23. Dependence of the count rate on the supply voltage in the Geiger counter (Counting characteristic)
The dependence of the count 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 imp / μR (pulses per microroentgen; this dimension follows from the ratio of the count rate - imp / s - to the radiation level - μR / s). IN
in cases where it is not indicated (not uncommon, unfortunately), to judge the radiation sensitivity
the counter has another very important parameter - its own background. This is the name of the count rate, which is caused by two components: external - the natural radiation background, and internal - radiation of radionuclides trapped in the very design of the counter, as well as spontaneous electron emission from 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 a 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 the "hard move." To what extent this dependence is important, the graph in the figure shows. The "stiffness stroke" will obviously affect the accuracy of the measurements.
At its core, a Geiger counter is very simple. A gas mixture consisting mainly of readily ionizable neon and argon is introduced into a well-evacuated sealed cylinder with two electrodes. The balloon can be glass, metal, etc. Usually counters perceive radiation with their entire surface, but there are those that have a special "window" in the balloon for this.
Geiger counters are capable of responding to a variety of types of ionizing radiation - α, β, γ, ultraviolet, X-ray, neutron. But the real spectral sensitivity of the counter largely depends 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 cylinder 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.
Figure: 24. Dependence of the count rate on the energy of gamma quanta ("move with rigidity") in a Geiger counter
Boron is introduced into the neutron counter, interacting with which the neutron flux is converted into easily detectable α - particles. Photonic radiation - ultraviolet, X-ray, γ - radiation - Geiger counters perceive indirectly - through the photoelectric effect, Compton effect, the effect of pair production; in each case, the radiation interacting with the cathode substance is converted into an electron flow.
Figure: 25. Radiometric installation based on the Geiger-Muller counter.
The fact that the Geiger counter is an avalanche device has its drawbacks - the reaction of such a device cannot be used to judge the root cause of its excitation. 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. Sami
particles, their energies completely disappear in the twin avalanches they generate.
The quality of a Geiger-Muller counter is usually judged by the type of its counting characteristics. 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 V slave counter is usually chosen in the middle of its counting area.
Since the rate of particle counting on a plateau changes 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 the large number of additional corrections. When working with low-intensity sources, one should take into account the counter background caused by cosmic radiation, radioactivity of the environment and radioactive contamination of the counter material. Initially, noble gases, in particular argon and neon, were the most commonly used gases to fill the meter. Most meters have pressure in the range from 7 to 20 cm Hg, although they sometimes work at high pressures, up to 1 atm. In meters of this type, it is necessary to use special electronic circuits to extinguish the gas discharge that occurs when ionizing radiation enters the counter. Therefore, these counters are called non-self-extinguishing Geiger-Müller counters. They have a very poor resolution. The use of circuits for forced quenching of the discharge, improving
resolution, significantly complicates the experimental setup, especially in the case of using a large number of counters simultaneously.
A typical glass Geiger-Muller counter is shown in Fig. 25.
Figure: 25. Glass Geiger-Muller counter: 1 -
geometrically sealed glass tube; 2 - cathode (thin copper layer on a stainless steel tube); 3 - output of the cathode; 4 - anode (thin stretched thread).
Table. 1 provides information on self-extinguishing halogen Geiger counters
russian-made, most suitable for household radiation monitoring devices.
Designations: 1 - operating voltage, V; 2 - plateau - area of \u200b\u200blow dependence of the count rate on the supply voltage, V; 3 - counter's own background, imp / s, no more; 4 - radiation sensitivity of the counter, imp / μR (* - for cobalt-60); 5 - amplitude of the output pulse, 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. Clock with 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 s) with stiffening ribs for strength. The meter is filled with a mixture of neon and bromine vapor. Bromine extinguishes the discharge.
Meter designs are very diverse and depend on the type of radiation and its energy, as well as on the measurement technique).
A radiometric setup based on a Geiger - Muller counter is shown in Fig. 27. The voltage is supplied to the meter from a high-voltage power source. The pulses from the counter are fed into the amplifier unit, where they are amplified, and then recorded by the counter.
Geiger-Müller counters are used to register all types of radiation. They can be used for both absolute and relative measurements of radioactive radiation.
Figure: 27. Construction of Geiger-Muller counters: a - cylindrical; b
- internal filling; d - flow-through for liquids.1 - anode (collecting electrode); 2 - cathode; 3 - glass container; 4 - leads of electrodes; 5 - glass tube; 6 - insulator; 7 - mica window; 8 - valve for gas inlet.
Regardless of whether we wish it or not, the term "radiation" has been wedged into our consciousness and being for a long time, 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 radiation, and without special equipment it is almost impossible to detect it.
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. 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 had not received proper attention. And this despite the fact that even the famous Nikola Tesla, as well as the workers in the Edison laboratory, were burned with X-rays. The deterioration in health was explained by everything they could, but not by radiation.
Later, at the beginning of the 20th century, an article appeared on the harmful effects of radiation on experimental animals. This also passed unnoticed until one sensational incident, in which the "radium girls" - the workers of the factory that produced the 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. This set a precedent for the rights of some workers who received occupational diseases and sued their employers.
The history of the emergence of the Geiger-Muller counter
The German physicist Hans Geiger, who worked in one of Rutherford's laboratories, in 1908 developed and proposed a principle diagram of the "charged particle" counter. It was a modification of the then familiar 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 was studying the electrical properties of gases. Geiger came up with the idea of \u200b\u200busing it to detect ionizing radiation precisely because this radiation had a direct effect on the level of ionization of gases.
In the late 1920s, Walter Müller, under the leadership of Geiger, created some 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 investigate radioactive materials. Geiger and Müller had to purposefully work on the creation of such counters that would be sensitive to any of the types of radiation such as α, β and γ identified at that time.
Geiger-Muller counters have proven to be simple, reliable, cheap and practical radiation detectors. 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 devices, they are still used by practicing 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 become familiar with ionizing radiation as such. It can include everything that causes the ionization of substances in a natural state. This requires the presence of some kind of energy. In particular, ultraviolet light or radio waves are not counted as ionizing radiation. The delineation can begin with the so-called "hard ultraviolet", also called "soft X-ray". This type of flux is called photon radiation. The high energy photon stream is gamma quanta.
For the first time, the separation of ionizing radiation into three types was done by Ernst Rutherford. Everything was done on research equipment that used a magnetic field in empty space. Later, all this was called:
- α - nuclei of helium atoms;
- β - high energy electrons;
- γ - by gamma quanta (photons).
Later, neutrons were discovered. So, it turned out that alpha particles can be easily retained even with ordinary paper, beta particles have a slightly higher penetrating power, and gamma rays are the highest. The most dangerous are neutrons, 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 molecules in matter.
However, when hitting 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 entire completeness of ionizing radiation.
Geiger-Muller counter devices and principles of operation
Gas discharge Geiger-Müller counters are mainly manufactured as sealed tubes, glass or metal, from which all the air is pumped out. It is replaced by an added inert gas (neon or argon or their mixture) at low pressure, with halogen or alcohol impurities. Thin wires are stretched along the tube axes, and metal cylinders are located coaxially with them. Both tubes and wires are electrodes: tubes are cathodes, and wires are anodes.
The minuses from constant voltage sources are connected to the cathodes, and the pluses from constant voltage sources 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. Typically, it can go up to several hundred volts.
During the passage of ionizing particles through the tubes, atoms in an inert gas that are already in a high-intensity electric field collide with these particles. The energy that was given off by the particles during the collision is considerable, it is enough to detach electrons from the gas atoms. 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 hit the Geiger-Muller counter, the resistance of the tube dropped by the incipient current, and at the same time the voltage at the central mark of the separator changes, which was mentioned 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 produced. By counting the pulses, you can set the number of particles that flew. The highest 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.
Geiger-Muller counter designs
All modern Geiger-Müller counters have two main varieties: "classic" and flat. Classic meters 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 crumple under any influences. At the ends of the tubes there are glass or plastic hermetic insulators. There are also taps-caps to connect to the circuit. Tubes are marked and coated with a durable insulating varnish indicating the polarity of the taps. In general, these are universal counters for any type of ionizing radiation, especially for beta-gamma radiation.
Counters that may be sensitive to mild β-radiation are manufactured differently. Due to the small ranges of β-particles, they are made flat. Mica windows weakly trap beta radiation. One such counter is the BETA-2 sensor. In all other counters, the determination of their properties is referred to the materials of their manufacture.
All counters that register gamma radiation have cathodes made of metals with a high charge number. Gases are extremely poorly ionized by gamma photons. However, gamma photons can knock out many secondary electrons from the cathodes if properly selected. Most Geiger-Müller beta-particle counters are manufactured 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 interact with substances very good and fast, as a result of which energy is lost.
With alpha particles, things are much worse. For example, despite the rather decent energy, several MeV, alpha particles have a very strong interaction with molecules moving along the path and soon losing their energy potential. Conventional counters respond well to α-radiation, but only at a distance of several centimeters.
To make an objective assessment of the level of ionizing radiation, dosimeters on meters with general use are often equipped with two sequentially functioning meters. One may be more sensitive to α-β radiation and the other to γ \u200b\u200bradiation. Sometimes bars or plates made of alloys containing cadmium impurities are placed among the counters. When neutrons hit such bars, γ-radiation is generated, 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 Russian industry produces many varieties of Geiger-Muller counters. Such devices are mainly used by people who have something to do with nuclear facilities, scientific or educational institutions, civil defense, and medical diagnostics.
After the Chernobyl disaster occurred, household dosimeters, previously completely unfamiliar to the population of our country even by name, began to acquire truly nationwide popularity. Many household models began to appear. All of them use Geiger-Müller counters proper as radiation sensors. Typically, household dosimeters have one or two tubes or end counters.
In 1908, German physicist Hans Geiger worked in chemical laboratories owned by Ernst Rutherford. There they were asked to test the counter of charged particles, which was an ionized chamber. The chamber was an electro-condenser filled with high pressure gas. Pierre Curie used this device in practice, studying electricity in gases. Geiger's idea - to detect ion radiation - was associated with their influence on the level of ionization of volatile gases.
In 1928, the German scientist Walter Müller, who worked with Geiger and under him, created several counters that register ionizing particles. The devices were needed to further research radiation. Physics, being a science of experiments, could not exist without measuring structures. Only a few emissions 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 overall saturation of ionizing radiation. Physicists use it with other sensors to get accurate calculations when conducting experiments.
A little about ionizing radiation
You could go straight to the description of the detector, but its operation will seem incomprehensible if you know little about ionizing radiation. With 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 is quite. Here the limit of influence is defined. The species is called photonic, and the photons themselves are γ-quanta.
Ernst Rutherford divided the processes of energy emission into 3 types, using a device with a magnetic field:
- γ is a photon;
- α is the nucleus of the helium atom;
- β is a high energy electron.
Particles α can be protected with a paper web. β penetrate deeper. The penetration ability γ is the highest. Neutrons, which scientists later learned about, 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 an atom, provoke its destruction, which is why radioactive isotopes are formed. When isotopes decay, they create ionizing radiation. Radiation emanates from a person, animal, plant or inorganic object that has received radiation 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 (argon-neon mixture or pure substance) is injected. There is no air in the tube. The gas is added under pressure and contains alcohol and halogen. There is a wire stretched along the entire 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 medium. 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, atoms of a noble gas collide with it. On contact, energy is released, which tears electrons off the gas atoms. Then secondary electrons are formed, which also collide, giving rise to a mass of new ions and electrons. The speed of the electrons towards the anode is influenced by the electric field. 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 the gas discharge Geiger counter, the resistance of the tube drops, which immediately lowers the midpoint voltage. Then the resistance grows again - this entails the restoration of tension. The impulse becomes negative. The device shows impulses, and we can count them, at the same time estimating the number of particles.
Types of Geiger counters
By design, Geiger counters are of 2 types: flat and classic.
Classical
Made of 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 leading to the devices.
The tube surface is coated with varnish (except for the leads) The classic counter is considered a universal measuring detector for all known types of radiation. Especially for γ and β.
Flat
Sensitive meters for fixing soft beta radiation have a different design. Due to the small amount of beta particles, their body is flat. There is a mica window that weakly retains β. BETA-2 sensor is the name of one of such devices. The properties of other flat meters depend on the material.
Geiger counter parameters and operating modes
To calculate the counter sensitivity, estimate the ratio of the number of micro-roentgen from the sample to the number of signals from this radiation. The device does not measure the particle energy, therefore it does not give an absolutely accurate estimate. The devices are calibrated using isotope source samples.
You also need to look at the following parameters:
Working area, entrance window area
The characteristic of the area of \u200b\u200bthe indicator 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 average specifications. The very characteristic of work 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 its peak activity and is called the upper measurement limit. The value is 400 volts.
Working width
The working width is the difference between the flat point 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 volt. It shows the measurement error (statistical) in pulse counting. The value is 0.15%.
Temperature
Temperature is important as the meter often has to be used in difficult conditions. For example, in reactors. General use meters: -50 to +70 C Celsius.
Work 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 contains organic matter for self-extinguishing, the number of pulses will be one billion. The resource is appropriate to calculate 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 responding to an ionizing particle. There is an upper limit for the pulse rate, which limits the measurement interval. The value is 10 microseconds.
Due to the recovery time (also called dead time), the device can fail at a crucial moment. To prevent overshooting, 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 Soviet era and continues now, but in the Russian Federation.
The device is used:
- at nuclear facilities;
- in scientific institutes;
- in medicine;
- at home.
After the accident at the Chernobyl nuclear power plant, ordinary citizens also buy dosimeters. All devices are equipped with a Geiger counter. Such dosimeters are equipped with one or two tubes.
Can you make a Geiger counter yourself?
It is difficult to make a counter yourself. 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 true "left-handed" will be able to reproduce the device at home.
Talented self-taught craftsmen have learned to make a substitute counter, which is also capable of measuring gamma and beta radiation using a fluorescent lamp and an incandescent lamp. They also use transformers from broken equipment, a Geiger tube, a timer, a capacitor, various boards, resistors.
Conclusion
When diagnosing radiation, you need to take into account the meter's own background. Even with a decently thick lead shield, the registration speed is not reset. There is an explanation for this phenomenon: the cause of the activity is cosmic radiation penetrating through the lead. Every minute muons sweep over the Earth's surface, which are registered by the counter with a probability of 100%.
There is one more background source - 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) that uses radioactive substances as a component or basis of their products, the study of the basics of radiation safety and radiation dosimetry is becoming a rather urgent topic today. In addition to natural sources of ionizing radiation, every year more and more places appear that are subsequently contaminated with radiation by 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. This can be helped by a dosimeter - 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, you must have an idea of \u200b\u200bthe nature of the measured parameter. Ionizing radiation (radiation) is a stream of photons, elementary particles or fragments of fission of atoms, capable of ionizing matter. It is divided into several types. Alpha radiation is a stream of alpha particles - helium-4 nuclei, alpha particles generated during radioactive decay can be easily stopped by a sheet of paper, therefore, the danger is mainly when it gets inside the body. Beta radiation - this is the flux of electrons arising from beta decay; an aluminum plate several millimeters thick is enough to protect against beta particles with energies up to 1 MeV. Gamma radiationhas a much greater penetrating ability, 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 energy.
Geiger-Müller counters are mainly used to register ionizing radiation. 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 a 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 the gas and colliding with gas atoms, knock out electrons from atoms and create positive ions and free electrons. The electric field between the cathode and the anode accelerates electrons to energies at which impact ionization begins. An avalanche of ions arises, leading to the multiplication of primary carriers. When the field strength is sufficiently high, the energy of these ions becomes sufficient to generate secondary avalanches capable of sustaining a self-sustained discharge, as a result of which the current through the counter rises sharply.
Not all Geiger counters can record all types of ionizing radiation. They are mainly sensitive to one radiation - alpha, beta or gamma radiation, but often they can also register other radiation to some extent. So, for example, the SI-8B Geiger counter is designed to register soft beta radiation (yes, depending on the particle energy, the radiation can be divided into soft and hard), but this sensor is also to some extent sensitive to alpha radiation and gamma radiation.
However, nevertheless approaching the construction 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 400 volts.
The main characteristics of the Geiger-Muller SBM-20 counter (table from the reference book):
This counter has relatively low accuracy rates for measuring ionizing radiation, but sufficient to determine the excess of the radiation dose permissible for a person. 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, the dosimeters are equipped with beta radiation filters, in this case the dosimeter registers only gamma radiation, but rather accurately.
When measuring the 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 own background of the counter. 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" impulses per unit of 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 counter itself) and does not contain programmable elements (microcontrollers), which will make it possible 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. Such is the classic version. The circuit consists of a voltage converter 9 volts - 400 volts.
On the NE555 microcircuit, a multivibrator is made, the operating frequency of which is approximately 14 kHz. To increase the operating frequency, you can reduce the value of the resistor R1 to about 2.7 kOhm. This will be useful if the choke you have chosen (and maybe made) will emit a squeak - with an increase in the frequency of operation, the squeak will disappear. Choke L1 is required with a nominal value of 1000 - 4000 μH. The quickest way to find a suitable choke is in a burnt 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 be used with any other field-effect n-channel with a drain-source voltage of at least 400 volts, and preferably more. Such a converter will give only a few milliamperes of current at a voltage of 400 volts, but for the operation of a Geiger counter this will be enough for the head several times. After disconnecting power from the circuit on a charged capacitor C3, the circuit will work for about 20-30 seconds, given its small capacity. The VD2 suppressor limits the voltage to 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 the 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 hits the Geiger counter inside the sensor, the gas is ionized and its resistance sharply decreases, resulting in a current pulse. Capacitor C4 cuts off the constant part and passes only a current pulse to the speaker. We hear a click.
In my case, two rechargeable 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 good, clicks often or generally continuously - bad. Another option is to roughly count the number of pulses per minute and convert the number of clicks to μR / h. For this, it is necessary 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 imp / μR according to different sources and reference books. So, we counted the number of pulses in one minute, then we multiply this number by 60 to approximate the number of pulses 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 microR / 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 radiation level is approximately 20-25 μR / h. The permissible level is up to about 50 μR / h. The numbers may vary from country to 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 radioelements
| Designation | A type | Denomination | amount | Note | Score | My notebook |
|---|---|---|---|---|---|---|
| IC1 | Programmable timer and oscillator | NE555 | 1 | Into notepad | ||
| T1 | MOSFET transistor | IRF710 | 1 | Into notepad | ||
| VD1 | Rectifier diode | 1N4007 | 1 | Into notepad | ||
| VD2 | Protective diode | 1V5KE400CA | 1 | Into notepad | ||
| C1, C2 | Capacitor | 10 nF | 2 | Into notepad | ||
| C3 | Electrolytic capacitor | 2.7 uF | 1 | Into 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 household devices and most professional dosimetric control devices, it is used as a sensitive element geiger counter ... This part of the radiometer allows you to accurately determine the level of radiation.
The history of the Geiger counter
IN the 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 During the period of development and formation of nuclear physics in the former Soviet Union, appropriate devices were also created, which were widely used in the armed forces, at nuclear power plants, and in special groups of radiation control of civil defense. The composition of such dosimeters, starting from the seventies of the last century, included a counter based on Geiger principles, namely SBM-20 ... This counter, exactly like its other counterpart 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 registration of 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 in more detail on the mechanism of action of the counter, 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 recording device is a sealed balloon or container 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 registering a charged particle. Inside the container there are two electrodes (cathode and anode) to which high DC voltage is supplied through a special load resistor.
Fig. 3. Geiger counter device and circuit.
P when the meter is activated in an inert gas, 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 charge of sufficiently high energy knocks out a certain number of electrons from the nearest environment, i.e. from body elements or physically the electrodes themselves. Such electrons, being 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 state of discharge, the circuit is closed for a very short period of time, and this causes a jump in the current in the load resistor, and it is this jump that makes it possible to register the passage of a particle or quantum through the registration chamber.
T this mechanism makes it possible to register a single particle, however, in a medium where ionizing radiation is sufficiently intense, a quick return of the registration chamber to its original position is required to be able to determine new radioactive particle ... This is accomplished in two different ways. The first of them consists in stopping the voltage supply to the electrodes for a short period of time, in this case the ionization of the inert gas stops abruptly, and a new activation of the test chamber allows starting the registration 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 to the inert gas medium based on various elements, 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 several tens of megohms 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 less than 300 volts automatically stops maintaining the discharge.
The entire described mechanism allows registering a huge amount of radioactive particles in a short period of time.
Types of radioactive radiation
H to understand what exactly is being registered geiger - Muller counters , it is worth dwelling on what types of it exist. It should be noted right away that gas-discharge counters, which are part of most modern dosimeters, are only capable of registering the amount of radioactive charged particles or quanta, but cannot determine either their energy characteristics or the type of radiation. For this, dosimeters are made more versatile and targeted, and in order to compare them correctly, one should more accurately understand their capabilities.
P on modern concepts 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 includes gamma particle flux or x-ray ... Their main feature is the ability to propagate in the form of a wave over very long distances, while they quite easily pass through various objects and can easily penetrate into a 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, provided that it is relatively tight, he will be able to protect himself from this type of radiation only 50 percent.
Fig. 4. X-ray and gamma-ray quanta.
T this type of radiation is pulsed in 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-rays have a frequency thousands of times lower than gamma rays. therefore gamma rays are significantly 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 a colossal 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 connected to each other. In fact, such a structure has the nucleus of a helium atom, so it can be argued that the flow of alpha particles is a flow of helium nuclei.
The following classification is adopted , alpha particles have the least penetrating ability, in order 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 the flow of such radiation, he will need metal protection several millimeters thick (for example, aluminum sheet). There is practically no protection from gamma quanta, and they propagate over considerable distances, attenuating with distance from the epicenter or source, and obeying the laws of propagation of electromagnetic waves.
Fig. 5. Alpha and beta type radioactive particles.
TO the amount of energy possessed by all these three types of radiation is also different, and the largest of them is the flux of alpha particles. For example, the energy possessed by alpha particles is seven thousand times greater than the energy of beta particles , i.e. the penetrating ability of various types of radiation is inversely proportional to their penetrating ability.
D for the human body, the most dangerous type of radioactive radiation is considered gamma quanta , due to the high penetrating power, and then decreasing, beta particles and alpha particles. Therefore, it is quite difficult to determine alpha particles, if it is impossible to tell with an ordinary 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 beta particles with low energies, the usual Geiger-Muller counter is ineffective.
ABOUT fraternal situation with gamma radiation, there is a possibility that they will pass through the container without triggering the ionization reaction. For this, a special screen (made of dense steel or lead) is installed in the counters, which makes it possible to reduce the energy of gamma quanta and thus activate the discharge in the counter chamber.
Basic characteristics and differences of Geiger - Muller counters
FROM it will also highlight some of the basic characteristics and differences of various dosimeters equipped with gas-discharge Geiger-Muller 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 elongated 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 counters, namely the cameras themselves, are able to register different types of radiation, or their combinations (for example, combinations of gamma and beta rays, or the entire spectrum of alpha, beta and gamma). This becomes possible due to the specially developed design of the meter body, as well as the material from which it is made.
E another important component for the targeted use of meters is area of \u200b\u200bthe input sensing element and working area ... In other words, this is the sector through which radioactive particles of interest will enter and register. The larger this area, the more the counter will be able to capture particles, and the stronger will be its sensitivity to radiation. The passport data indicates the area of \u200b\u200bthe working surface, usually in square centimeters.
E another important indicator that is indicated in the characteristics of the dosimeter is noise magnitude (measured in pulses per second). In other words, this indicator can be called the value of its own background. It can be determined in laboratory conditions by placing the device in a well-protected room or chamber, usually with thick lead walls, and recording the level of radiation that the device itself emits. It is clear that if this level is significant enough, then these induced noises will directly affect the measurement error.
Every professional and radiation has such a characteristic as radiation sensitivity, also measured in pulses per second (imp / s), or in pulses per micro-roentgen (imp / μR). Such a parameter, or rather its use, directly depends on the source of ionizing radiation, to which the counter is adjusted, and by which further measurement will be carried out. Often, tuning is done according to sources that include such radioactive materials as radium - 226, cobalt - 60, cesium - 137, carbon - 14 and others.
E another indicator by which dosimeters should be compared is ion radiation detection efficiency or radioactive particles. The existence of this criterion is due to the fact that not all radioactive particles passed through the sensitive element of the dosimeter will be registered. This can happen when the gamma-ray quantum did not cause ionization in the counter chamber, or the number of particles passed and caused ionization and discharge is so large that the device inadequately counts them, and for some other reason. 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 on which to stop is range of recorded energies ... Any radioactive particle or quantum of radiation has a different energy characteristic. Therefore, dosimeters are designed to measure not only a specific type of radiation, but also their respective energy characteristics. This 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 Geiger-Muller counter body and knock out the 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.
Various options for Geiger - Müller counters
P the first version of dosimeters is a device designed to register and detect gamma photons and high-frequency (hard) beta radiation. Almost all of the previously produced and modern ones, both household and professional radiation dosimeters, are designed for this measurement range, for example:. Such radiation has sufficient energy and high penetrating power for the Geiger counter chamber to register them. Such particles and photons easily penetrate the counter walls and cause the ionization process, and this is easily registered by the corresponding electronic filling of the dosimeter.
D popular counters such as SBM-20 having a sensor in the form of a cylindrical tube-balloon with coaxially located wire cathode and anode. Moreover, the walls of the sensor tube serve as both a cathode and a housing, and are made of stainless steel. This counter has the following characteristics:
- the area of \u200b\u200bthe working area of \u200b\u200bthe sensitive element is 8 square centimeters;
- radiation sensitivity to gamma radiation of about 280 imp / s, or 70 imp / μR (testing was carried out for cesium - 137 at 4 μR / s);
- the intrinsic background of the dosimeter is about 1 pulse / s;
- the sensor is designed to register 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 at the lower boundary.
Fig. 6. Geiger counter SBM-20 device.
Have there were various modifications of this counter, for example, SBM-20-1 or SBM-20U which have similar characteristics, but differ in the basic 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 for registration gamma photons and x-rays ... If we talk about the accuracy of such devices, it should be understood that photon and gamma radiation are quanta of electromagnetic radiation 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 registered indirectly, thanks to the electrons knocked out by them, which are involved in the consequence of the ionization of an inert gas. To maximize this effect, the material and thickness of the walls of the meter 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 chamber, and also increase the amount of radiation noise natural for the device, which will drown out the accuracy of the determination of gamma quanta. Naturally, the exact proportions are selected by the manufacturers. In fact, based on this principle, dosimeters are manufactured based on geiger - Muller counters for the direct determination of gamma radiation on the ground, while such a device excludes the possibility of detecting any other types of radiation and radioactive exposure, which makes it possible to accurately determine the radiation contamination and the level of negative impact on humans only by gamma radiation.
IN Domestic dosimeters, which are equipped with cylindrical sensors, have the following types: 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 in order to understand more clearly the principle of their operation, 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 working window, about 14 square centimeters. In this case, the radiation sensitivity to cobalt - 60 is about 240 imp / μR. This type of meter has very low self-noise values. , 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 register photon radiation with an energy in the range from 0.05 MeV to 3 MeV.
Fig. 7. End gamma counter Beta-2M.
To determine gamma radiation, it is quite possible to use counters for gamma-beta pulses, which are designed to register hard (high-frequency and high-energy) beta particles and gamma quanta. For example, model SBM - 20. If in this model of the dosimeter you want to exclude registration of beta particles, then it is enough to install a lead shield, or a shield made of any other metal material (lead shield is more effective). This is the most common method used by most designers to build gamma and X-ray counters.
Registration of "soft" beta radiation.
TO as we have already mentioned, registration of soft beta radiation (radiation with low energy characteristics and relatively low frequency) is a rather difficult task. For this, it is required to ensure the possibility of their easier penetration into the registration chamber. For these purposes, a special thin working window is made, as a rule, of mica or polymer film, which practically does not hinder the penetration of this type of beta radiation into the ionization chamber. In this case, the sensor body itself can act as the cathode, and the anode is a system of linear electrodes, which are uniformly distributed and mounted on insulators. The registration window is made in the end version, and in this case only a thin mica film appears in the path of beta particles. In dosimeters with such counters, gamma radiation is recorded as an application and, in fact, as an additional feature. And if you want to get rid of the registration of gamma quanta, then it is necessary to minimize the cathode surface.
Fig. 8. The device of the end Geiger counter.
FROM it should be noted that counters for the determination of 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 \u200b\u200babout 37 sq / cm, rectangular made of mica material. For such a size of a sensitive element, the device is able to register about 500 imp / μR, if measured by cobalt - 60. The efficiency of particle detection is up to 80 percent. Other indicators of this device are as follows: intrinsic noise is 2.2 pulses / s, the range of energy determination is from 0.05 to 3 MeV, while the lower threshold for determining soft beta radiation is 0.1 MeV.
Fig. 9. End-face beta-gamma counter Beta-5.
AND naturally worth mentioning geiger - Muller counterscapable of registering alpha particles. If registration of soft beta radiation seems to be a rather difficult task, then fixing an alpha particle, even with high energy parameters, is an even more difficult task. Such a problem can be solved only by a corresponding decrease 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 an almost complete approach 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 a positive and a negative side:
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 appear, which will complicate the analysis of the obtained registration data.
TO in addition, a too thin mica working window, although it increases the capabilities of the counter, at the expense of the mechanical strength and tightness of the ionization chamber, especially since the window itself has a sufficiently large working surface area. For comparison, in the SBT10 and SI8B counters, which we mentioned above, with a working window area of \u200b\u200babout 30 sq / cm, the thickness of the mica layer is 13 - 17 microns, and with the required thickness for registering alpha particles of 4-5 microns, the input the window can be made only 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 as the emitter plutonium - 239 with a radiation energy of 5 MeV. Let us continue, with the entrance window thickness up to 10 μm, it is possible to register alpha particles already at a distance of up to 13 mm, if we make a mica window 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 an alpha particle is more than 5 MeV, then the distance of its registration for the thickness of the working window of any type will correspondingly increase, and if the energy is less, then the distance must also be reduced, up to the complete impossibility of registering soft alpha radiation.
E another important point to increase the sensitivity of the alpha counter is to reduce 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. This measure makes it possible to reduce the effect on the ionization of gamma quanta by a factor of thousands, and even tens of thousands of times. It is no longer possible to eliminate the influence of beta radiation on the registration camera, but there is a fairly simple way out of this situation. First, alpha and beta radiation of the total type is 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 offering the characteristics of a modern Beta-1 counter, which allows registering alpha, beta, gamma radiation. These indicators are:
- the area of \u200b\u200bthe working area of \u200b\u200bthe sensitive element is 7 sq / cm;
- the thickness of the mica layer is 12 microns, (the distance of effective detection of alpha particles for plutonium is 239, about 9 mm, for cobalt - 60, the radiation sensitivity is reached about 144 pulses / μR);
- radiation measurement efficiency for alpha particles - 20% (for plutonium - 239), beta particles - 45% (for thallium -204), and gamma quanta - 60% (for strontium composition - 90, yttrium - 90);
- the intrinsic background of the dosimeter is about 0.6 pulses / s;
- the sensor is designed to register 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 at 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 meters that are designed for 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 capabilities. However, we will not focus on them. Indeed, to understand the basic principles of action geiger - Muller counters , the models described above are quite enough.
IN it is also worth mentioning that there are special subclasses geiger counters which are specially designed to detect different types of other radiation. For example, to determine the magnitude of ultraviolet radiation, to register and determine slow neutrons that operate on the principle of a corona discharge, and other options that are not directly related to this topic and will not be considered.
