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 the 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 with a sheet of paper, therefore, the danger is mainly when it gets inside the body. Beta radiation- this is a flux of electrons arising from beta decay; an aluminum plate several millimeters thick is sufficient to protect against beta particles with energies up to 1 MeV. Gamma radiation has a much greater penetrating power, since it consists of high-energy photons that do not have a charge; heavy elements (lead, etc.) with a layer of several centimeters are effective for protection. The penetrating power of all types of ionizing radiation depends on energy.
Geiger-Muller 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 anode accelerates electrons to energies at which impact ionization begins. An avalanche of ions arises, leading to the multiplication of primary carriers. With a sufficiently high field strength, 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 to 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 counter SBM-20 (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 dose of radiation, there are several factors to consider that may be important. Even in the complete absence of sources of ionizing radiation, the Geiger counter will produce 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 9 volt - 400 volt voltage converter.
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 for the manufacture of 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 this will be enough for the Geiger counter to work 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, the current does not flow through the resistors R2 - R4 (in the photo there are five resistors 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, as a result of which a current pulse arises. 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 that penetrates into rooms, water, etc. in various regions of the country and its sources.
List of radioelements
| Designation | Type of | Denomination | Quantity | Note | Shop | 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 |
Regardless of whether we wish it or not, the term "radiation" has been wedged into our consciousness and existence 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 it is practically impossible to detect it without special equipment.
From the history of the study of radiation
In 1895, X-rays were discovered. A year later, the phenomenon of uranium radioactivity was discovered, also associated with the discovery and use of X-rays. Researchers had to face a completely new, hitherto unprecedented natural phenomenon.
It should be noted that the phenomenon of radiation had already been encountered several years before, but the phenomenon had not received due attention. And this despite the fact that even the famous Nikola Tesla, as well as the workers in Edison's 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. As a result, a precedent was set 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 scheme for the operation of a "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 using 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 will require 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 flux of high energy photons 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. In the future, 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. Neutrons are considered the most dangerous, especially at a distance of many tens of meters in airspace. Due to their electrical indifference, they do not interact with any electron shell of 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 made 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 axes of the tubes, and metal cylinders are located coaxially with them. Both tubes and wires are electrodes: tubes are cathodes and wires are anodes.
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. As a rule, it can go up to several hundred volts.
During the flight 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 again 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 unsatisfactorily ionized by gamma photons. However, gamma photons can knock out many secondary electrons from the cathodes if properly selected. Most Geiger-Müller beta 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 a rather decent energy, a few MeV, alpha particles have a very strong interaction with molecules moving along the way and soon losing their energy potential. Conventional counters respond well to α-radiation, but only at a distance of a few centimeters.
To make an objective assessment of the level of ionizing radiation, dosimeters on meters with general use are often equipped with two sequentially operating meters. One may be more sensitive to α-β radiation and the other to γ radiation. Sometimes bars or plates made of alloys containing cadmium impurities are placed among the counters. When neutrons hit such bars, γ-radiation 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 the 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 the actual Geiger-Müller counters as radiation sensors. Typically, household dosimeters have one or two tubes or end counters.
Gas-discharge counter Geiger-Muller (G-M). Fig. 1 is a glass cylinder (cylinder) filled with an inert gas (with
impurities of halogens) under pressure slightly below atmospheric. A thin metal cylinder inside the balloon serves as the cathode K; the anode A is a thin conductor passing through the center of the cylinder. A voltage is applied between the anode and cathode U V = 200-1000 V. Anode and cathode are connected to the electronic circuit of the radiometric device.
Fig. 1 Cylindrical Geiger-Muller counter.
1 - anode thread 2 - tubular cathode
U v - high voltage source
R n - load resistance
WITH V - separation and storage tank
R - scaler with indication
ξ - a source of radiation.
With the help of the counter Г-М it is possible to register all particles of radiation (except for easily absorbed α-particles); so that β-particles are not absorbed by the body of the counter, there are slots in it, covered with a thin film.
Let us explain the features of the operation of the G-M counter.
β-particles interact directly with gas molecules of the counter, while neutrons and γ-photons (uncharged particles) interact weakly with gas molecules. In this case, the mechanism of the formation of ions is different.
we will conduct a dosimetric measurement of the environment near points K and A, the data obtained will be entered in table. 1.
To take measurements, you must:
1. Connect the dosimeter to the power supply (9v).
2. Close the detector window with a slide (screen) on the back of the dosimeter.
3. Set the switchMODE(mode) to position γ ("P").
4. Install the switchRANGE(range) to positionx1 (P n = 0.1-50 μSv / hour).
5. Set the power switch of the dosimeter to the positionON(On).
6. If in position х1 a sound signal is heard and the numeric rows of the display are completely filled, then it is necessary to switch to the range х10 (Р n = 50-500 μSv / hour).
7. After the end of the totalization of pulses, the dosimeter display will show the dose equivalent to the powerP Hγ μSv / hour; after 4-5 sec. the readings will be reset.
8. The dosimeter is again ready for radiation measurements. A new sampling cycle starts automatically.
Table 1.
The resulting value in the workspace (AB) is determined by the formula
=
, μSv / hour (6)
- the readings of the dosimeter give the values of the background radiation at the point;
The amount of radiation at each point of measurement obeys the laws of fluctuations. Therefore, in order to obtain the most probable value of the measured quantity, it is necessary to make a series of measurements;
- for β - radiation dosimetry, measurements should be carried out near the surface of the investigated bodies.
4. Carrying out measurements. A.1. Determination of the equivalent dose rate of the natural background radiation.
To determine the γ-background of the environment, we select (with respect to any objects (bodies)) two points A, K located at a distance of ~ 1 meter from each other, and, without touching the bodies,
Neutrons, interacting with the atoms of the cathode, generate charged microparticles (nuclear fragments). Gamma radiation
interacts mainly with the substance (atoms) of the cathode, generating photon radiation, which further ionizes the gas molecules.
As soon as ions appear in the volume of the counter, then under the action of the anode-cathodic electric field, the movement of charges will begin.
Near the anode, the lines of electric field strength sharply thicken (a consequence of the small diameter of the anode filament), the field strength increases sharply. Electrons, approaching the filament, get great acceleration, there is impact ionization of neutral gas molecules , a self-contained corona discharge propagates along the filament.
Due to the energy of this discharge, the energy of the initial impulse of the particles is sharply increased (up to 10 8 once). With the propagation of a corona discharge, some of the charges will slowly drain through a large resistance R n ~10 6 Ohm (Fig. 1). In the detector circuit on resistanceR n there will be current pulses proportional to the initial particle flux. The resulting current pulse is transmitted to the storage tank C V (C ~ 10 3 picofarad), then amplified and recorded by the R.
Having a lot of resistanceR n in the detector circuit leads to the accumulation of negative charges at the anode. The electric field strength of the anode will decrease and at some point impact ionization will be interrupted, and the discharge will decay.
Halogens in the gas of the meter play an important role in extinguishing the resulting gas discharge. The ionization potential of halogens is lower than that of inert gases; therefore, halogen atoms more actively “absorb” photons, which cause a self-sustained discharge, converting this energy into dissipation energy, thereby extinguishing an independent discharge.
After impact ionization (and corona discharge) is interrupted, the process of restoring the gas to its original (working) state begins. During this time, the counter does not work, i.e. does not register passing particles. This gap
time is called "dead time" (recovery time). For counter Г-Мdead time = Δt~10 -4 seconds.
The GM counter reacts to the hit of each charged particle, not distinguishing them in energy, but if the power falls
of radiation is unchanged, the pulse counting rate turns out to be proportional to the radiation power, and the counter can be graduated in units of radiation doses.
The quality of the gas-discharge self-extinguishing detector is determined by the dependence of the average pulse frequencyNper unit of time from voltageU on its electrodes at a constant radiation intensity. This functional dependence is called the counting characteristic of the detector (Fig. 2).
As follows from Figure 2, forU < U 1 the applied voltage is insufficient for the occurrence of a gas discharge when a charged particle or gamma quantum enters the detector. Starting with voltage U V > U 2 impact ionization occurs in the counter, a corona discharge propagates along the cathode, and the counter records the flight of almost every particle. With growth U V beforeU 3 (see Fig. 2) the number of recorded pulses slightly increases, which is associated with a slight increase in the degree of ionization of the counter gas. A good GM meter has a graph plot from U 2 beforeU R almost independent ofU V , i.e. runs parallel to the axisU V , the average pulse frequency is almost independentU V .
Rice. 2. Counting characteristic of a gas-discharge self-extinguishing detector.
3. Relative error of instruments when measuring P n : δР n = ± 30%.
Let us explain how the counter pulse is converted into readings of the radiation dose rate.
It is proved that at a constant radiation power, the pulse count rate is proportional to the radiation power (measured dose). The measurement of the radiation dose rate is based on this principle.
As soon as a pulse appears in the counter, this signal is transmitted to the recalculation unit, where it is filtered by duration, amplitude, summed up and the result is transmitted to the counter display in units of the dose rate.
Correspondence between the count rate and the measured power, i.e. the dosimeter is calibrated (at the factory) according to a known radiation source C s 137 .
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, both useful and dangerous, phenomenon. Radiation manifests itself as invisible and imperceptible radiation, and it is impossible to detect them without special devices.
A little about the history of radiation
X-rays were discovered in 1895. A year later, the radioactivity of uranium was discovered, also in connection with X-rays. Scientists realized that they were faced with completely new, hitherto unseen natural phenomena. Interestingly, the phenomenon of radiation was noticed several years earlier, but they did not attach importance to it, although Nikola Tesla and other employees of the Edison laboratory received X-ray burns. The harm to health was attributed to anything, but not to the rays, with which the living has never encountered in such doses. At the very beginning of the 20th century, articles began to appear on the harmful effects of radiation on animals. This, too, did not attach importance to the sensational story with the "radium girls" - workers of the factory that produced luminous clocks. They just moistened 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 Lisa Meitner, who, together with Otto Hahn and Fritz Strassmann, refers to the people who divided the uranium nucleus for the first time in the world, inadvertently blurted out about the possibility of a chain reaction, and from that moment a chain reaction of ideas about creating a bomb, namely a bomb, and not "Peaceful atom", to which the bloodthirsty politicians of the XX century, of course, would not give a penny. Those who were "in the know" already knew what this would lead to, and the atomic arms race began.
How the Geiger-Muller counter appeared
The German physicist Hans Geiger, who worked in the laboratory of Ernst Rutherford, in 1908 proposed the principle of the counter of "charged particles" as a further development of the already well-known ionization chamber, which was an electric condenser filled with gas at low pressure. It was used by Pierre Curie since 1895 to study the electrical properties of gases. Geiger had the idea to use it to detect ionizing radiation precisely because these radiation had a direct effect on the degree of ionization of the gas.
In 1928, Walter Müller, under the direction of Geiger, creates several types of radiation counters designed to register various ionizing particles. The creation of counters was a very urgent need, without which it was impossible to continue the study of radioactive materials, since physics, as an experimental science, is unthinkable without measuring instruments. Geiger and Müller purposefully worked on the creation of counters that are sensitive to each of the types of radiation open to that: α, β and γ (neutrons were discovered only in 1932).
The Geiger-Muller counter has proven to be a simple, reliable, cheap and practical radiation detector. Although it is not the most accurate instrument for studying particular types of particles or radiation, it is extremely suitable as a device for general measurement of the intensity of ionizing radiation. And in combination with other detectors, it is used by physicists for the most accurate measurements in experiments.
Ionizing radiation
To better understand the operation of a Geiger-Müller counter, it is useful to have an understanding of ionizing radiation in general. By definition, these include what 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 considered ionizing radiation. The border begins with "hard ultraviolet light", aka "soft X-ray". This type is a photonic type of radiation. High energy photons are commonly called gamma quanta.
For the first time, Ernst Rutherford divided ionizing radiation into three types. This was done on an experimental setup using a magnetic field in a vacuum. Later it turned out that it is:
α - nuclei of helium atoms
β - high energy electrons
γ - gamma quanta (photons)
Later, neutrons were discovered. Alpha particles are easily trapped even by ordinary paper, beta particles have a slightly higher penetrating power, and gamma rays have the highest. The most dangerous are neutrons (at a distance of up to many tens of meters in the air!). Due to their electrical neutrality, they do not interact with the electron shells of the substance molecules. But once in the atomic nucleus, the probability of which is high enough, lead to its instability and decay, with the formation, as a rule, of radioactive isotopes. And already those, in turn, decaying, themselves form the whole "bouquet" of ionizing radiation. Worst of all, an irradiated object or living organism itself becomes a source of radiation for many hours and days.
The device of the Geiger-Muller counter and the principle of its operation
A gas-discharge Geiger-Muller counter, as a rule, is made in the form of a sealed tube, glass or metal, from which air is evacuated, and instead of it an inert gas (neon or argon or their mixture) is added under low pressure, with an admixture of halogens or alcohol. A thin wire is stretched along the axis of the tube, and a metal cylinder is located coaxially with it. Both the tube and the wire are electrodes: the tube is the cathode and the wire is the anode. A minus from a constant voltage source is connected to the cathode, and a plus from a constant voltage source is connected to the anode through a large constant resistance. Electrically, a voltage divider is obtained, at the midpoint of which (the junction of the resistance and the anode of the meter) the voltage is practically equal to the voltage at the source. This is usually a few hundred volts.
When an ionizing particle flies through the tube, atoms of an inert gas, already in an electric field of great strength, experience collisions with this particle. The energy given off by the particle during the collision is enough to detach electrons from the gas atoms. The resulting secondary electrons themselves are capable of forming new collisions and, thus, a whole avalanche of electrons and ions is obtained. Under the action of an electric field, electrons are accelerated towards the anode, and positively charged gas ions - towards the cathode of the tube. Thus, an electric current is generated. But since the energy of the particle has already been spent on collisions, in whole or in part (the particle flew through the tube), the supply of ionized gas atoms also ends, which is desirable and is provided by some additional measures, which we will talk about when analyzing the parameters of the counters.
When a charged particle enters the 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 is restored due to the increase in its resistance, and the voltage again becomes the same. Thus, we get a negative voltage pulse. By counting the momenta, we can estimate the number of particles passing by. The electric field strength is especially high near the anode due to its small size, which makes the meter more sensitive.
Geiger-Muller counter designs
Modern Geiger-Müller counters are available in two basic 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, 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 conclusions-caps for connecting to the instrument circuit. The tube is marked and covered with a durable insulating varnish, not counting, of course, its leads. The polarity of the terminals is also indicated. It is a versatile counter for all types of ionizing radiation, especially beta and gamma.
Counters sensitive to soft β-radiation are made differently. Due to the small range of β-particles, they have to be made flat, with a mica window, which weakly delays beta radiation, one of the options for such a counter is a radiation sensor BETA-2... All other properties of meters are determined by the materials from which they are made.
Counters designed for recording gamma radiation contain a cathode made of metals with a high charge number, or are coated with such metals. The gas is extremely poorly ionized by gamma photons. But on the other hand, gamma photons are capable of knocking out many secondary electrons from the cathode, if it is chosen in a suitable way. Geiger-Müller counters for beta particles are made with thin windows for better particle permeability, since they are ordinary electrons, just received a lot of 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 the very decent energy, of the order of several MeV, alpha particles interact very strongly with molecules on the way, and quickly lose energy. If a substance is compared to a forest, and an electron to a bullet, then alpha particles will have to be compared to a tank breaking through a forest. However, an ordinary counter responds well to α-radiation, but only at a distance of up to several centimeters.
For an objective assessment of the level of ionizing radiation dosimeters on meters for general use, they are often equipped with two parallel meters. One is more sensitive to α and β radiation, and the second to γ-rays. Such a scheme for using two counters is implemented in the dosimeter RADEX RD1008 and in the dosimeter-radiometer RADEX MKS-1009 in which the counter is installed BETA-2 and BETA-2M... Sometimes a bar or plate of an alloy containing an admixture of cadmium is placed between the counters. When neutrons hit such a block, γ-radiation is generated, which is recorded. This is done to be able to determine neutron radiation, to which simple Geiger counters are practically insensitive. Another way is to cover the body (cathode) with impurities capable of imparting sensitivity to neutrons.
Halogens (chlorine, bromine) are mixed with the gas for fast self-extinguishing of the discharge. Alcohol vapors serve the same purpose, although alcohol in this case is short-lived (this is generally a feature of alcohol) and the "sober" counter constantly starts "ringing", that is, it cannot work as intended. This happens sometime after the registration of 1e9 pulses (billion) which is not so much. Halogen meters are much more durable.
Parameters and operating modes of Geiger counters
Sensitivity of Geiger counters.
The counter sensitivity is estimated by the ratio of the number of micro-roentgen from a reference source to the number of pulses caused by this radiation. Since Geiger counters are not designed to measure particle energy, accurate assessment is difficult. Counters are calibrated against exemplary isotope sources. It should be noted that this parameter may differ greatly for different types of counters, below are the parameters of the most common Geiger-Muller counters:
Geiger-Muller counter Beta 2- 160 ÷ 240 imp / mkR
Geiger-Muller counter Beta 1- 96 ÷ 144 imp / microR
Geiger-Muller counter SBM-20- 60 ÷ 75 imp / mkR
Geiger-Muller counter SBM-21- 6.5 ÷ 9.5 imp / microR
Geiger-Muller counter SBM-10- 9.6 ÷ 10.8 imp / microR
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 will be captured by the Geiger-Muller counter. Usually this parameter is indicated in square centimeters.
Geiger-Muller counter Beta 2- 13.8 cm 2
Geiger-Muller counter Beta 1- 7 cm 2
This voltage corresponds to approximately the middle of the operating characteristic. The operating characteristic is the flat part of the voltage dependence of the number of recorded pulses, therefore it is also called the "plateau". At this point, the highest operating speed is reached (upper measurement limit). Typical value 400 V.
The width of the working characteristic of the counter.
This is the difference between the voltage of the spark breakdown and the voltage at the flat end of the characteristic. Typical value 100 V.
Counter slope.
The slope is measured as a percentage of pulses per volt. It characterizes the statistical measurement error (counting the number of pulses). Typical value is 0.15%.
Permissible operating temperature of the meter.
For meters of general use -50 ... +70 degrees Celsius. This is a very important parameter if the counter works in chambers, channels, and other places of complex equipment: accelerators, reactors, etc.
Counter working resource.
The total number of pulses that the meter registers until the moment when its readings begin to become incorrect. For appliances with organic additives, self-extinguishing is usually 1e9 (ten to the ninth power, or one billion). The resource is counted only if the operating voltage is applied to the meter. If the counter is simply stored, this resource is not consumed.
Dead time counter.
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 for the pulse frequency, and this limits the measurement range. Typical value is 1e-4 s, which is ten microseconds.
It should be noted that due to the dead time, the sensor may turn out to be "off scale" and be silent at the most dangerous moment (for example, a spontaneous chain reaction in production). There have been such cases, and lead screens are used to combat them, covering some of the sensors of alarm systems.
Own counter background.
Measured in thick-walled lead chambers to assess meter quality. 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, "Gamma" series meters, end-face meters " Beta”And there are many others. All of them are used for monitoring and measuring radiation: at nuclear facilities, in scientific and educational institutions, in civil defense, medicine, and even everyday life. After the Chernobyl accident, household dosimeters, previously unknown to the population even by name, became very popular. Many brands of household dosimeters have appeared. They all use the Geiger-Muller counter as a radiation sensor. In household dosimeters, one to two tubes or end counters are installed.
UNITS OF MEASURING RADIATION VALUES
For a long time, the unit of measurement P (roentgen) was widespread. However, with the transition to the SI system, other units appear. X-ray is a unit of exposure dose, "amount of radiation", which is expressed by the number of formed ions in dry air. At a dose of 1 R in 1 cm3 of air, 2.082e9 ion pairs are formed (which corresponds to 1 unit of the CGSE charge). In the SI system, the 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 in return for the outdated unit is glad. The absorbed dose rate is measured in grays per second. The exposure dose rate (DER), previously measured in roentgens per second, is now measured in amperes per kilogram. The equivalent dose of radiation at which the absorbed dose is 1 Gy (gray) and the radiation quality factor is 1 is called Sievert. Rem (the biological equivalent of an X-ray) is one hundredth of a sievert, and is now considered obsolete. Nevertheless, even today all obsolete units are very actively used.
Dose and power are considered to be the main concepts in radiation measurements. Dose is the number of elementary charges in the process of ionization of a substance, and power is the rate at which a dose is formed per unit time. And in what units it is expressed is a matter of taste and convenience.
Even the smallest dose is dangerous in the sense of long-term consequences for the body. The hazard calculation is quite simple. For example, your dosimeter reads 300 milliroentgens per hour. If you stay in this place for a day, you will receive a dose of 24 * 0.3 = 7.2 roentgens. This is dangerous and you need to get out of here as soon as possible. In general, having found even weak radiation, one must move away from it and check it even at a distance. If she “follows you”, you can be “congratulated”, you have been hit by neutrons. And not every dosimeter can react to them.
For radiation sources, a value is used that characterizes the number of decays per unit of time, it is called activity and is also measured in many different units: curie, becquerel, rutherford and some others. The amount of activity, measured twice with sufficient time spacing, if it decreases, allows you to calculate the time, according to the law of radioactive decay, when the source becomes sufficiently safe.
Introduction
1. Purpose of counters
The device and principle of operation of the counter
Basic physical laws
1 Recovery after particle registration
2 Dosimetric characteristics
3 Counting characteristic of the sensor
Conclusion
Bibliography
Introduction
Geiger-Muller counters are the most common detectors (sensors) for ionizing radiation. Until now, invented at the very beginning of our century for the needs of the nascent nuclear physics, there is, oddly enough, no complete replacement. 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 for which a special "window" is provided in the balloon for this.
A high voltage U is applied to the electrodes (see Fig.), Which in itself does not cause any discharge phenomena. The counter will remain in this state until an ionization center appears in its gaseous medium - a trace of ions and electrons generated by an ionizing particle that comes from outside. Primary electrons, accelerating in an electric field, ionize other molecules of the gaseous medium "along the way", generating more and more electrons and ions. 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 the gas environment of the meter, a discharge occurs that is visible (if the container is transparent) even with the naked eye.
The reverse process - the return of the gaseous medium to its original state in the so-called halogen counters - occurs by itself. Halogens (usually chlorine or bromine), which are contained in a small amount in the gaseous medium, come into play, which contribute to intensive recombination of charges. But this process is much slower. The length of time required to restore the radiation sensitivity of the Geiger counter and actually determining its response rate - "dead" time - is its important passport characteristic. Such meters are called self-extinguishing halogen. Distinguished by the lowest supply voltage, excellent output signal parameters and sufficiently high response speed, they have proved to be especially convenient for use as ionizing radiation sensors in household radiation monitoring devices.
Geiger counters are capable of responding to a variety of types of ionizing radiation - a, b, g, ultraviolet, X-ray, neutron. But the real spectral sensitivity of the counter largely depends on its design. Thus, the entrance window of a counter sensitive to a- and soft b-radiation should be very thin; for this, mica with a thickness of 3 ... 10 microns is usually used. The balloon of a counter, which reacts to hard b- and g-radiation, usually has the shape of a cylinder with a wall thickness of 0.05 ... 0.06 mm (it also serves as the cathode of the counter). The X-ray counter window is made of beryllium, and the ultraviolet counter is made of quartz glass.
Geiger counter Müller dosimetric radiation
1. Purpose of counters
The Geiger-Muller counter is a two-electrode device designed to determine the intensity of ionizing radiation or, in other words, to count the ionizing particles arising from nuclear reactions: helium ions (- particles), electrons (- particles), X-ray quanta (- particles) and neutrons. Particles propagate at a very high speed [up to 2. 10 7 m / s for ions (energy up to 10 MeV) and about the speed of light for electrons (energy 0.2 - 2 MeV)], due to which they penetrate into the counter. The role of the counter is to form a short (fractions of a millisecond) voltage pulse (units - tens of volts) when a particle enters the volume of the device.
In comparison with other detectors (sensors) of ionizing radiation (ionization chamber, proportional counter), the Geiger-Muller counter has a high threshold sensitivity - it allows you to control the natural radioactive background of the earth (1 particle per cm 2 in 10 - 100 seconds). The upper limit of measurement is relatively low - up to 10 4 particles per cm 2 per second or up to 10 Sievert per hour (Sv / h). A special feature of the counter is the ability to form the same output voltage pulses regardless of the kind of particles, their energy and the number of ionizations produced by the particle in the volume of the sensor.
2. The device and principle of operation of the counter
The operation of the Geiger counter is based on a non-self-sustaining pulsed gas discharge between metal electrodes, which is initiated by one or more electrons resulting from the ionization of the gas -, -, or -particle. In counters, a cylindrical design of electrodes is usually used, and the diameter of the inner cylinder (anode) is much smaller (2 or more orders of magnitude) than that of the outer (cathode), which is of fundamental importance. The characteristic diameter of the anode is 0.1 mm.
The particles enter the counter through the vacuum shell and the cathode in the "cylindrical" design (Fig. 2, a) or through a special flat thin window in the "end" version of the design (Fig. 2 , b)... The latter option is used to register β-particles that have low penetrating power (they are retained, for example, by a sheet of paper), but are very biologically dangerous when the source of particles gets inside the body. Mica-window detectors are also used to count relatively low-energy β-particles (“soft” beta radiation).
Rice. 2. Schematic designs of cylindrical ( a) and end ( b) Geiger counters. Designations: 1 - vacuum shell (glass); 2 - anode; 3 - cathode; 4 - window (mica, cellophane)
In the cylindrical version of the counter, designed to register high-energy particles or soft X-ray radiation, a thin-walled vacuum shell is used, and the cathode is made of a thin foil or in the form of a thin metal film (copper, aluminum) deposited on the inner surface of the shell. In a number of designs, a thin-walled metal cathode (with stiffening ribs) is an element of the vacuum shell. Hard X-rays (-particles) have an increased penetrating power. Therefore, it is recorded by detectors with sufficiently thick vacuum shell walls and a massive cathode. In neutron counters, the cathode is covered with a thin layer of cadmium or boron, in which neutron radiation is converted into radioactive radiation through nuclear reactions.
The volume of the device is usually filled with argon or neon with a small (up to 1%) impurity of argon at a pressure close to atmospheric (10 -50 kPa). To eliminate undesirable post-discharge phenomena, an admixture of bromine or alcohol vapors (up to 1%) is introduced into the gas filling.
The ability of the Geiger counter to register particles regardless of their kind and energy (to generate one voltage pulse regardless of the number of electrons formed by the particle) is determined by the fact that, due to the very small diameter of the anode, almost all voltage applied to the electrodes is concentrated in a narrow anode layer. Outside the layer there is a “particle capture area” in which they ionize gas molecules. The electrons torn away by the particle from the molecules are accelerated towards the anode, but the gas is ionized weakly due to the low electric field strength. Ionization increases sharply after the entry of electrons into the anode layer with a high field strength, where electron avalanches (one or several) develop with a very high degree of electron multiplication (up to 10 7). However, the resulting current has not yet reached a value corresponding to the generation of the sensor signal.
A further increase in the current to the working value is due to the fact that in avalanches, simultaneously with ionization, ultraviolet photons are generated with an energy of about 15 eV, sufficient to ionize impurity molecules in a gas filling (for example, the ionization potential of bromine molecules is 12.8 V). The electrons that appear as a result of photoionization of molecules outside the layer are accelerated towards the anode, but avalanches do not develop here due to the low field strength and the process has little effect on the development of the discharge. In the layer, the situation is different: the generated photoelectrons, due to the high intensity, initiate intense avalanches, in which new photons are generated. Their number exceeds the initial one and the process in the layer according to the scheme "photons - electron avalanches - photons" rapidly (several microseconds) increases (enters the "trigger mode"). In this case, the discharge from the place of the first avalanches initiated by the particle propagates along the anode ("transverse ignition"), the anode current sharply increases and the leading edge of the sensor signal is formed.
The trailing edge of the signal (decrease in current) is due to two reasons: a decrease in the anode potential due to a voltage drop from the current across the resistor (at the leading edge, the potential is maintained by the interelectrode capacitance) and a decrease in the electric field strength in the layer under the action of the space charge of ions after electrons leave the anode (charge increases the potentials of the points, as a result of which the voltage drop on the layer decreases, and on the area of particle capture increases). Both reasons reduce the intensity of avalanche development and the process according to the "avalanche - photons - avalanche" scheme dies out, and the current through the sensor decreases. After the end of the current pulse, the anode potential increases to the initial level (with a certain delay due to the charge of the interelectrode capacitance through the anode resistor), the potential distribution in the gap between the electrodes returns to its original form as a result of the escape of ions to the cathode, and the counter restores the ability to register the arrival of new particles.
Dozens of types of ionizing radiation detectors are produced. Several systems are used in their designation. For example, STS-2, STS-4 - self-extinguishing end counters, or MS-4 - a counter with a copper cathode (V - with tungsten, G - with graphite), or SAT-7 - end-particle counter, SBM-10 - counter - metal particles, SNM-42 - metal neutron counter, SRM-1 - X-ray counter, etc.
3. Basic physical laws
.1 Recovery after particle registration
The time for ions to leave the gap after registration of a particle turns out to be relatively long - a few milliseconds, which limits the upper limit of measuring the radiation dose rate. At high radiation intensity, the particles arrive at an interval shorter than the ion escape time, and the sensor does not register some particles. The process is illustrated by the oscillogram of the voltage at the anode of the sensor during the restoration of its operability (Fig. 3).
Rice. 3. Oscillograms of the voltage at the anode of the Geiger counter. U o- signal amplitude in normal mode (hundreds of volts). 1 - 5 - particle numbers
The arrival of the first particle (1 in Fig. 3) into the volume of the sensor initiates a pulsed gas discharge, which leads to a decrease in voltage by the value U o(normal signal amplitude). Further, the voltage increases as a result of a slow decrease in the current through the gap as the ions leave to the cathode and due to the charge of the interelectrode capacitance from the voltage source through the limiting resistor. If, after a short time interval after the arrival of the first, another particle enters the sensor (2 in Fig. 3), then the discharge processes develop weakly due to the low voltage and low field strength at the anode under the action of the space charge of ions. In this case, the sensor signal turns out to be inadmissibly small. The arrival of the second particle after a longer time interval after the first (particles 3 - 5 in Fig. 3) gives a signal of higher amplitude, since the voltage increases and the space charge decreases.
If the second particle enters the sensor after the first at an interval less than the time interval between particles 1 and 2 in Fig. 3, then for the reasons stated above, the sensor does not generate a signal at all (it “does not count” the particle). In this regard, the time interval between particles 1 and 2 is called the “counter dead time” (the signal amplitude of particle 2 is 10% of the normal one). The time interval between particles 2 and 5 in Fig. 3 is called the “sensor recovery time” (particle 5 signal is 90% normal). During this time, the amplitude of the sensor signals is reduced, and they may not be registered by the electric pulse counter.
Dead time (0.01 - 1 ms) and recovery time (0.1 - 1 ms) are important parameters of the Geiger counter. The lower the values of these parameters, the higher the recorded dose rate. The main factors determining the parameters are the gas pressure and the size of the limiting resistor. With decreasing pressure and resistor value, the dead time and recovery time decrease, since the rate of ion escape from the gap increases and the time constant of the process of charging the interelectrode capacitance decreases.
3.2 Dosimetric characteristics
The Geiger counter sensitivity is the ratio of the frequency of the pulses generated by the sensor to the radiation dose rate measured in microsieverts per hour (μSv / h; options: Sv / s, mSv / s, μSv / s). Typical sensitivity values: 0.1 - 1 impulses per microsievert. In the operating range, the sensitivity is the coefficient of proportionality between the meter readings (number of pulses per second) and the dose rate. Outside the range, the proportionality is violated, which reflects the dosimetric characteristic of the detector - the dependence of the readings on the dose rate (Fig. 4).
Rice. Dependences of the counting rate on the dose rate of radioactive radiation (dosimetric characteristics) for two meters with different gas pressures (1 - 5 kPa, 2 - 30 kPa)
From physical considerations, it follows that the sensor readings as the dose rate increases cannot exceed the value (1 /), where is the sensor dead time (particles arriving at a shorter time interval are not counted). Therefore, the working linear section of the dosimetric characteristic smoothly passes in the area of intense radiation into a horizontal straight line at the level (1 /).
With a decrease in the dead time, the dosimetric characteristic of the sensor turns into a horizontal straight line at a higher level at a higher radiation power, and the upper measurement limit increases. This situation is observed with decreasing gas pressure (Fig. 4). However, at the same time, the sensitivity of the sensor decreases (the number of particles crossing the gas-discharge gap without collisions with molecules increases). Therefore, with decreasing pressure, the dosimetric characteristic goes down. Mathematically, the characteristic is described by the following relationship:
where N- count rate (sensor readings - number of pulses per second); - counter sensitivity (pulses per second per microsievert); R- radiation dose rate; - sensor dead time (in seconds).
3.3 Counting characteristic of the sensor
Monitoring of the radiation dose rate most often has to be done outdoors or in the field, where the sensor is powered from batteries or other galvanic sources. Their stress decreases as they work. At the same time, the gas-discharge processes in the sensor depend on the voltage to a very strong degree. Therefore, the dependence of the Geiger counter readings on voltage at a constant radiation dose rate is one of the most important characteristics of the sensor. The dependence is called the counting characteristic of the sensor (Fig. 5).
On one of the presented dependences (curve 2), characteristic points are marked A - D... At low voltage (to the left of the point A) the electrons generated in the sensor when an ionizing particle hits, initiate electron avalanches, but their intensity is insufficient to form a current pulse of the required amplitude, and the counter readings are zero. Point A corresponds to the "voltage of the beginning of the count". With an increase in voltage on the site A - B the counter readings increase, since the probability of the arrival of electrons from the area of particle capture into the anode layer with a high field strength increases. At a low voltage, electrons recombine with ions during their movement to the layer (they can preliminarily “stick” to bromine impurity molecules with the formation of negative ions). At the point V the voltage is sufficient for the rapid movement of almost all electrons into the layer, and the recombination intensity is close to zero. The sensor generates signals of normal amplitude.
On the working area of the counting characteristic B - C("Characteristic plateau") the counter readings slightly increase with increasing voltage, which is of practical importance and is an advantage of the Geiger counter. Its quality is the higher, the greater the length of the plateau (100 -400 V) and the less the steepness of the horizontal section of the counting characteristic.
Rice. 5. Dependences of the count rate on voltage (counting characteristic) at various values of gas pressure and bromine impurity content: 1 - 8 kPa, 0.5%; 2 - 16 kPa, 0.5%; 3 - 16 kPa, 0.1% for a radiation dose rate of 5 μSv / h. A, B, C, D- characteristic points of curve 2
The steepness (or slope) of the plateau S characterized by the percentage change in the meter readings per voltage unit:
, (2)
where N B and N C - meter reading at the beginning and end of the plateau; U B and U C- voltage values at the beginning and end of the plateau. Typical slope values are 0.01 - 0.05% / V.
The relative stability of the readings on the plateau of the counting characteristic is provided by a specific type of discharge that occurs in the sensor with the arrival of an ionizing particle. An increase in voltage intensifies the development of electron avalanches, but this only leads to an acceleration of the propagation of the discharge along the anode, and the ability of the counter to generate one signal per particle is almost not disturbed.
A slight increase in the counting rate with increasing voltage on the plateau of the counting characteristic is associated with the emission of electrons from the cathode under the action of the discharge. Emission is caused by the so-called -processes, which are understood to mean the extraction of electrons by ions, excited atoms and photons. The coefficient is conventionally considered equal to the number of electrons per ion (excited atoms and photons are implied). The characteristic values of the coefficient are 0.1 - 0.01 (10 - 100 ions pull out an electron, depending on the kind of gas and material of the cathode). At such values of the coefficient, the Geiger counter does not function, since the electrons leaving the cathode are registered as ionizing particles (“false” signals are recorded).
The normal functioning of the meter is ensured by the introduction of bromine or alcohol vapor (“quenching impurities”) into the gas filling, which sharply reduces the coefficient (below 10 -4). In this case, the number of false signals also sharply decreases, but remains perceptible (for example, a few percent). With an increase in voltage, the discharge processes intensify, i.e. the number of ions, excited atoms and photons increases and, accordingly, the number of false signals increases. This explains a slight increase in the sensor readings on the plateau of the counting characteristic (increase in the slope) and the end of the plateau (transition to a steep section C- D). With an increase in the impurity content, the coefficient decreases to a greater extent, which decreases the slope of the plateau and increases its length (curves 2 and 3 in Fig. 5).
The physical mechanism of action of quenching impurities consists in a sharp decrease in the supply of ions, excited atoms and photons to the cathode, which can cause the emission of electrons, as well as in an increase in the work function of electrons from the cathode. The ions of the main gas (neon or argon) in the process of moving to the cathode become neutral atoms as a result of "charge exchange" in collisions with impurity molecules, since the ionization potentials of neon and argon are higher than that of bromine and alcohol (respectively: 21.5 V; 15, 7 V; 12.8 V; 11.3 V). The energy released in this case is spent on the destruction of molecules or on the formation of low-energy photons that are not capable of causing photoemission of electrons. Moreover, such photons are well absorbed by impurity molecules.
Impurity ions formed during the charge exchange reach the cathode, but they do not cause the emission of electrons. In the case of bromine, this is explained by the fact that the potential energy of the ion (12.8 eV) is insufficient to pull out two electrons from the cathode (one to neutralize the ion, and the other to start an electron avalanche), since the work function of electrons from the cathode in the presence of an impurity bromine rises to 7 eV. In the case of alcohol, when the ions are neutralized at the cathode, the released energy is usually spent on the dissociation of a complex molecule, and not on the extraction of electrons.
The long-lived (metastable) excited atoms of the main gas arising in the discharge can in principle hit the cathode and cause the emission of electrons, since their potential energy is sufficiently high (for example, 16.6 eV for neon). However, the probability of the process turns out to be very small, since atoms in collisions with impurity molecules transfer their energy to them - they are "quenched". Energy is spent on dissociation of impurity molecules or on the emission of low-energy photons, which do not cause photoemission of electrons from the cathode and are well absorbed by impurity molecules.
In approximately the same way, high-energy photons coming from the discharge, which can cause the emission of electrons from the cathode, are “extinguished”: they are absorbed by impurity molecules with subsequent energy consumption for dissociation of molecules and emission of low-energy photons.
The durability of counters with the addition of bromine is much higher (10 10 - 10 11 pulses), since it is not limited by the decomposition of the quenching impurity molecules. The decrease in the concentration of bromine is due to its relatively high chemical activity, which complicates the sensor manufacturing technology and imposes restrictions on the choice of the cathode material (for example, stainless steel is used).
The counting characteristic depends on the gas pressure: with its increase, the voltage of the start of counting increases (point A shifts to the right in Fig. 5), and the plateau level increases as a result of more efficient trapping of ionizing particles by gas molecules in the sensor (curves 1 and 2 in Fig. 5). The increase in the start voltage is explained by the fact that the conditions in the sensor correspond to the right branch of the Paschen curve.
Conclusion
The widespread use of the Geiger-Muller counter is explained by its high sensitivity, the ability to register various kinds of radiation, and the comparative simplicity and low cost of the installation. The counter was invented in 1908 by Geiger and improved by Müller.
A cylindrical Geiger-Müller counter consists of a metal tube or a glass tube metallized from the inside, and a thin metal thread stretched along the axis of the cylinder. The filament serves as the anode, the tube as the cathode. The tube is filled with a 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 approximately from 360 to 460 V, in this range small voltage fluctuations do not affect the counting rate.
The work of the counter is based on impact ionization. Γ-quanta emitted by a radioactive isotope hitting the walls of the counter and knocking out electrons from it. Electrons, moving in a 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 anode accelerates electrons to energies at which impact ionization begins. An avalanche of ions arises, and the current through the counter rises sharply. In this case, a voltage pulse is formed on the resistance R, which is fed to the recording device. In order for the counter to be able to register the next particle that got into 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 R, so the voltage between the anode and cathode decreases sharply - so much so that the discharge stops and the counter is ready for operation again.
An important characteristic of a meter is its efficiency. Not all γ-photons hitting the counter will give secondary electrons and will be registered, since the acts of interaction of γ-rays with matter are relatively rare, and some of the secondary electrons are absorbed in the walls of the device without reaching the gas volume.
The efficiency of the counter depends on the thickness of the walls of the counter, their material and the energy of γ-radiation. The most effective are counters, the walls of which are made of material with a large atomic number Z, since this increases the formation of secondary electrons. In addition, the walls of the meter must be thick enough. The counter wall thickness is selected from the condition of its equality to the mean free path of secondary electrons in the wall material. With a large wall thickness, secondary electrons will not pass into the working volume of the counter, and a current pulse will not occur. Since γ-radiation weakly interacts with matter, the efficiency of γ-counters is usually also low and amounts to only 1–2%. Another disadvantage of the Geiger-Muller counter is that it does not make it possible to identify particles and determine their energy. These disadvantages are absent in scintillation counters.
Bibliography
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