Structure and operating principle of a Geiger–Müller counter
IN Recently, attention to radiation safety on the part of ordinary citizens in our country has been increasingly increasing. And this is connected not only with the tragic events at the Chernobyl nuclear power plant and its further consequences, but also with various types of incidents that periodically happen in one place or another on the planet. In this regard, at the end of the last century, devices began to appear dosimetric monitoring of radiation for household purposes. And such devices have saved many people not only their health, but sometimes their lives, and this applies not only to the territories adjacent to the exclusion zone. Therefore, radiation safety issues are relevant anywhere in our country to this day.
IN All household and almost all professional modern dosimeters are equipped with . In another way, it can be called the sensitive element of the dosimeter. This device was invented in 1908 by the German physicist Hans Geiger, and twenty years later, this development was improved by another physicist Walter Muller, and it is the principle of this device that is used to this day.
N Some modern dosimeters have four counters at once, which makes it possible to increase the measurement accuracy and sensitivity of the device, as well as reduce the measurement time. Most Geiger–Muller counters are capable of detecting gamma radiation, high-energy beta radiation, and X-rays. However, there are special developments for determining high-energy alpha particles. To configure the dosimeter to detect only gamma radiation, the most dangerous of the three types of radiation, the sensitive chamber is covered with a special casing made of lead or other steel, which makes it possible to cut off the penetration of beta particles into the counter.
IN In modern dosimeters for household and professional use, sensors such as SBM-20, SBM-20-1, SBM-20U, SBM-21, SBM-21-1 are widely used. They differ overall dimensions cameras and other parameters, the line of 20 sensors is characterized by the following dimensions: length 110 mm, diameter 11 mm, and for the 21st model, length 20-22 mm with a diameter of 6 mm. It is important to understand that the larger the camera, the greater the number of radioactive elements that will fly through it, and the greater sensitivity and accuracy it has. So, for the 20th series of sensors, dimensions are 8-10 times larger than for the 21st, and we will have a difference in sensitivity in approximately the same proportions.
TO The design of a Geiger counter can be schematically described as follows. A sensor consisting of a cylindrical container into which an inert gas (for example, argon, neon, or mixtures thereof) is pumped under minimal pressure to facilitate the occurrence of an electrical discharge between the cathode and anode. The cathode, most often, is the entire metal body of the sensitive sensor, and the anode is a small wire placed on insulators. Sometimes the cathode is additionally wrapped in a protective casing made of stainless steel or lead; this is done to configure the counter to detect only gamma rays.
D la household use, at present, end sensors are most often used (for example, Beta-1, Beta-2). Such counters are designed in such a way that they are capable of detecting and registering even alpha particles. Such a counter is a flat cylinder with electrodes located inside and an input (working) window made of mica film only 12 microns thick. This design makes it possible to detect (at close range) high-energy alpha particles and low-energy beta particles. In this case, the area of the working window of the Beta-1 and Beta 1-1 counters is 7 sq.cm. The area of the mica working window for the Beta-2 device is 2 times larger than that of the Beta-1, it can be used to determine, etc.
E If we talk about the principle of operation of the Geiger counter chamber, it can be briefly described as follows. When activated, a high voltage (about 350 - 475 volts) is applied to the cathode and anode through a load resistor, but no discharge occurs between them due to the inert gas serving as a dielectric. When it enters the chamber, its energy is sufficient to knock out a free electron from the material of the chamber body or cathode; this electron, like an avalanche, begins to knock out free electrons from the surrounding inert gas and its ionization occurs, which ultimately leads to a discharge between the electrodes. The circuit is closed, and this fact can be registered using the device’s microcircuit, which is the fact of detection of either a gamma quantum or x-ray radiation. The camera then resets, allowing the next particle to be detected.
H To stop the discharge process in the chamber and prepare the chamber for recording the next particle, there are two ways, one of them is based on the fact that the voltage supply to the electrodes is stopped for a very short period of time, which stops the process of gas ionization. The second method is based on adding another substance to the inert gas, for example, iodine, alcohol and other substances, and they lead to a decrease in the voltage on the electrodes, which also stops the process of further ionization and the camera becomes able to detect the next radioactive element. At this method A high-capacity load resistor is used.
P the number of discharges in the meter chamber and one can judge the level of radiation in the measured area or from a specific object.
Whether we like it or not, radiation has firmly entered our lives and is not going to go away. We need to learn to live with this phenomenon, which is both useful and dangerous. Radiation manifests itself as invisible and imperceptible radiation, and without special devices it is impossible to detect them.
A little history of radiation
X-rays were discovered in 1895. A year later, the radioactivity of uranium was discovered, also in connection with X-rays. Scientists realized that they were faced with completely new, hitherto unseen natural phenomena. It is interesting that the phenomenon of radiation was noticed several years earlier, but no importance was attached to it, although Nikola Tesla and other workers of the Edison laboratory also received burns from X-rays. Damage to health was attributed to anything, but not to rays, which living things had never encountered in such doses. At the very beginning of the 20th century, articles began to appear about the harmful effects of radiation on animals. This, too, was not given any importance until the sensational story with the “radium girls” - workers of a factory that produced luminous clock. They just wet the brushes with the tip of their tongue. The terrible fate of some of them was not even published, for ethical reasons, and remained a test only for the strong nerves of doctors.
In 1939, physicist Lise Meitner, who, together with Otto Hahn and Fritz Strassmann, belongs to the people who were the first in the world to divide the uranium nucleus, inadvertently blurted out about the possibility of a chain reaction, and from that moment a chain reaction of ideas about creating a bomb began, namely a bomb, and not at all “peaceful atom”, for which the bloodthirsty politicians of the 20th century, of course, would not have given a penny. Those who were “in the know” already knew what this would lead to and the atomic arms race began.
How did the Geiger-Müller counter appear?
The German physicist Hans Geiger, who worked in the laboratory of Ernst Rutherford, in 1908 proposed the principle of operation of a “charged particle” counter as a further development of the already known ionization chamber, which was an electric capacitor filled with gas at low pressure. It was used by Pierre Curie in 1895 to study the electrical properties of gases. Geiger had the idea to use it to detect ionizing radiation precisely because these radiations had a direct effect on the degree of ionization of the gas.
In 1928, Walter Müller, under the leadership of Geiger, created several types of radiation counters designed to register various ionizing particles. The creation of counters was a very urgent need, without which it was impossible to continue the study of radioactive materials, since physics, as an experimental science, is unthinkable without measuring instruments. Geiger and Müller purposefully worked to create counters that were sensitive to each of the types of radiation that had been discovered: α, β and γ (neutrons were discovered only in 1932).
The Geiger-Muller counter proved to be a simple, reliable, cheap and practical radiation detector. Although it is not the most accurate research tool individual species particles or radiation, but is extremely suitable as an instrument for general measurement of the intensity of ionizing radiation. And in combination with other detectors, it is used by physicists for precise measurements during experiments.
Ionizing radiation
To better understand the operation of a Geiger-Muller counter, it is helpful to have an understanding of ionizing radiation in general. By definition, these include anything that can cause ionization of a substance in its normal state. This requires a certain amount of energy. For example, radio waves or even ultraviolet light are not ionizing radiation. The border begins with “hard ultraviolet”, also known as “soft x-ray”. This type is a photon type of radiation. High-energy photons are usually called gamma quanta.
Ernst Rutherford was the first to divide ionizing radiation into three types. This was done on an experimental setup using magnetic field in a vacuum. It later turned out that this is:
α - nuclei of helium atoms
β - high energy electrons
γ - gamma quanta (photons)
Later neutrons were discovered. Alpha particles are easily blocked even by ordinary paper, beta particles have a slightly greater penetrating power, and gamma rays have the highest penetrating power. Neutrons are the most dangerous (at a distance of up to many tens of meters in the air!). Due to their electrical neutrality, they do not interact with the electron shells of the molecules of the substance. But once they get into the atomic nucleus, the probability of which is quite high, they lead to its instability and decay, with the formation, as a rule, of radioactive isotopes. And those, in turn, decaying, themselves form the entire “bouquet” of ionizing radiation. The worst thing is that an irradiated object or living organism itself becomes a source of radiation for many hours and days.
The design of a Geiger-Muller counter and its operating principle
A Geiger-Muller gas-discharge counter is usually made in the form of a sealed tube, glass or metal, from which the air is evacuated, and instead an inert gas (neon or argon or a mixture of both) is added under low pressure, with an admixture of halogens or alcohol. Tensioned along the axis of the tube thin wire, and a metal cylinder is located coaxially with it. Both the tube and the wire are electrodes: the tube is the cathode, and the wire is the anode. A minus from a constant voltage source is connected to the cathode, and a plus from a constant voltage source is connected to the anode through a large constant resistance. Electrically, a voltage divider is obtained, at the middle point of which (the junction of the resistance and the anode of the meter) the voltage is almost equal to the voltage at the source. This is usually several hundred volts.
When an ionizing particle flies through the tube, the atoms of the inert gas, already in a high-intensity electric field, experience collisions with this particle. The energy given off by the particle during a collision is enough to separate electrons from gas atoms. The resulting secondary electrons are themselves capable of forming new collisions and, thus, a whole avalanche of electrons and ions is obtained. Under the influence of an electric field, electrons are accelerated towards the anode, and positively charged gas ions are accelerated towards the cathode of the tube. Thus, an electric current arises. But since the energy of the particle has already been spent on collisions, fully or partially (the particle flew through the tube), the supply of ionized gas atoms also ends, which is desirable and is ensured by some additional measures, which we will talk about when analyzing the parameters of the counters.
When a charged particle enters a Geiger-Muller counter, due to the resulting current, the resistance of the tube drops, and with it the voltage at the midpoint of the voltage divider, which was discussed above. Then the resistance of the tube, due to an increase in its resistance, is restored, and the voltage again becomes the same. Thus, we get a negative voltage pulse. By counting the impulses, we can estimate the number of passing particles. The electric field strength is especially high near the anode due to its small size, which makes the counter more sensitive.
Geiger-Muller counter designs
Modern Geiger-Muller counters are available in two main versions: “classic” and flat. The classic counter is made of thin-walled metal tube with corrugation. The corrugated surface of the meter makes the tube rigid and resistant to external atmospheric pressure and does not allow it to crumple under its influence. At the ends of the tube there are sealing insulators made of glass or thermosetting plastic. They also contain terminal caps for connecting to the device circuit. The tube is marked and coated with a durable insulating varnish, not counting, of course, its terminals. The polarity of the terminals is also indicated. This is a universal counter for all types of ionizing radiation, especially beta and gamma.
Counters sensitive to soft β-radiation are made differently. Due to the short range of beta particles, they have to be made flat, with a mica window that weakly blocks beta radiation; one of the options for such a counter is a radiation sensor BETA-2. All other properties of the meters are determined by the materials from which they are made.
Counters designed to record gamma radiation contain a cathode made of metals with a high charge number, or are coated with such metals. Gas is extremely poorly ionized by gamma photons. But gamma photons are capable of knocking out many secondary electrons from the cathode if it is chosen appropriately. Geiger-Muller counters for beta particles are made with thin windows for better permeability of particles, since they are ordinary electrons that have just received more energy. They interact with matter very well and quickly lose this energy.
In the case of alpha particles the situation is even worse. So, despite a very decent energy, on the order of several MeV, alpha particles interact very strongly with molecules in their path and quickly lose energy. If matter is compared to a forest, and an electron is compared to a bullet, then alpha particles will have to be compared to a tank crashing through a forest. However, a conventional counter responds well to α-radiation, but only at a distance of up to several centimeters.
For an objective assessment of the level of ionizing radiation dosimeters on the counters general use often equipped with two counters operating in parallel. One is more sensitive to α and β radiation, and the second to γ rays. This scheme of using two counters is implemented in a dosimeter RADEX RD1008 and in a dosimeter-radiometer RADEKS MKS-1009, in which the counter is installed BETA-2 And BETA-2M. Sometimes a bar or plate of an alloy containing an admixture of cadmium is placed between the counters. When neutrons hit such a bar, γ-radiation is generated, which is recorded. This is done to be able to determine neutron radiation to which simple counters Geiger is practically insensitive. Another method is to coat the housing (cathode) with impurities that can impart sensitivity to neutrons.
Halogens (chlorine, bromine) are added to the gas to quickly extinguish the discharge. Alcohol vapor also serves the same purpose, although alcohol in this case is short-lived (this is generally a feature of alcohol) and the “sobered up” meter constantly begins to “ring”, that is, it cannot work in the intended mode. This happens somewhere after 1e9 pulses (a billion) have been detected, which is not that much. Meters with halogens are much more durable.
Parameters and operating modes of Geiger counters
Sensitivity of Geiger counters.
The sensitivity of the counter is estimated by the ratio of the number of microroentgens from the reference source to the number of pulses caused by this radiation. Since Geiger counters are not designed to measure particle energy, accurate estimation is difficult. The counters are calibrated using reference isotope sources. It should be noted that this parameter can vary greatly for different types of counters; below are the parameters of the most common Geiger-Muller counters:
Geiger-Muller counter Beta-2- 160 ÷ 240 imp/µR
Geiger-Muller counter Beta-1- 96 ÷ 144 imp/µR
Geiger-Muller counter SBM-20- 60 ÷ 75 imp/µR
Geiger-Muller counter SBM-21- 6.5 ÷ 9.5 imp/µR
Geiger-Muller counter SBM-10- 9.6 ÷ 10.8 imp/μR
Entrance window area or work area
The area of the radiation sensor through which radioactive particles fly. This characteristic is directly related to the dimensions of the sensor. The larger the area, the more particles the Geiger-Muller counter will catch. Typically this parameter is indicated in square centimeters.
Geiger-Muller counter Beta-2- 13.8 cm 2
Geiger-Muller counter Beta-1- 7 cm 2
This voltage corresponds approximately to the middle performance characteristics. The operating characteristic is the flat part of the dependence of the number of recorded pulses on the voltage, which is why it is also called the “plateau”. At this point the highest operating speed is achieved (upper measurement limit). Typical value is 400 V.
Width of the counter operating characteristic.
This is the difference between the spark breakdown voltage and the output voltage on the flat part of the characteristic. Typical value is 100 V.
Slope of the meter operating characteristic.
The slope is measured as a percentage of pulses per volt. It characterizes the statistical error of measurements (counting the number of pulses). Typical value is 0.15%.
Permissible operating temperature of the meter.
For general purpose meters -50 ... +70 degrees Celsius. This is a very important parameter if the meter operates in chambers, channels, and other places of complex equipment: accelerators, reactors, etc.
Working resource of the counter.
The total number of pulses that the meter registers before its readings begin to become incorrect. For devices with organic additives, self-quenching is usually 1e9 (ten to the ninth power, or one billion). The resource is counted only if operating voltage is applied to the meter. If the counter is simply stored, this resource is not consumed.
Counter dead time.
This is the time (recovery time) during which the counter conducts current after being triggered by a passing particle. The existence of such a time means that there is an upper limit to the pulse frequency and this limits the measurement range. A typical value is 1e-4 s, which is ten microseconds.
It should be noted that due to dead time, the sensor may be “off scale” and remain silent at the most dangerous moment (for example, a spontaneous chain reaction in production). Such cases have happened, and to combat them, lead screens are used to cover part of the sensors of emergency alarm systems.
Custom counter background.
Measured in thick-walled lead chambers to assess the quality of meters. Typical value is 1 ... 2 pulses per minute.
Practical application of Geiger counters
Soviet and now Russian industry produces many types of Geiger-Muller counters. Here are some common brands: STS-6, SBM-20, SI-1G, SI21G, SI22G, SI34G, meters of the Gamma series, end counters of the series Beta"and there are many more. All of them are used for monitoring and measuring radiation: at facilities nuclear industry, in scientific and educational institutions, in civil defense, medicine, and even in everyday life. After the Chernobyl accident, household dosimeters, previously unknown to the population even by name, have become very popular. Many brands of household dosimeters have appeared. All of them use a Geiger-Muller counter as a radiation sensor. In household dosimeters, one to two tubes or end counters are installed.
UNITS OF MEASUREMENT OF RADIATION QUANTITIES
For a long time, the unit of measurement P (roentgen) was common. However, when moving to the SI system, other units appear. An x-ray is a unit of exposure dose, a "quantity of radiation", which is expressed as the number of ions produced in dry air. With a dose of 1 R in 1 cm3 of air, 2.082e9 pairs of ions are formed (which corresponds to 1 unit of charge of the SGSE). In the SI system, exposure dose is expressed in coulombs per kilogram, and with x-rays this is related to the equation:
1 C/kg = 3876 R
The absorbed dose of radiation is measured in joules per kilogram and is called Gray. This is a replacement for the outdated rad unit. The absorbed dose rate is measured in grays per second. Exposure dose rate (EDR), formerly measured in roentgens per second, is now measured in amperes per kilogram. The equivalent radiation dose at which the absorbed dose is 1 Gy (gray) and the radiation quality factor is 1 is called Sievert. The rem (biological equivalent of an x-ray) is a hundredth of a sievert, now considered obsolete. Nevertheless, even today all outdated units are very actively used.
The main concepts in radiation measurements are dose and power. Dose is the number of elementary charges in the process of ionization of a substance, and power is the rate of dose formation per unit time. And in what units this is expressed is a matter of taste and convenience.
Even a minimal dose is dangerous in terms of long-term consequences for the body. The calculation of danger is quite simple. For example, your dosimeter shows 300 milliroentgen per hour. If you stay in this place for a day, you will receive a dose of 24 * 0.3 = 7.2 roentgens. This is dangerous and you need to leave here as soon as possible. In general, if you detect even weak radiation, you need to move away from it and check it even from a distance. If she “follows you”, you can be “congratulated”, you have been hit by neutrons. But not every dosimeter can respond to them.
For radiation sources, a quantity characterizing the number of decays per unit of time is used; it is called activity and is also measured by many different units: curie, becquerel, rutherford and some others. The amount of activity, measured twice with a sufficient separation in time, if it decreases, makes it possible to calculate the time, according to the law of radioactive decay, when the source becomes sufficiently safe.
Using a modern Geiger counter, you can measure the radiation level of building materials, a plot of land or an apartment, as well as food. It demonstrates almost one hundred percent probability of a charged particle, because only one electron-ion pair is enough to detect it.
The technology on which a modern dosimeter based on a Geiger-Muller counter is created allows you to obtain results high precision in a very short period of time. The measurement takes no more than 60 seconds, and all information is displayed graphically and numerical form on the dosimeter screen.
Device setup
The device has the ability to set a threshold value; when it is exceeded, a sound signal is issued to warn you of danger. Select one of the specified threshold values in the corresponding settings section. The beep can also be turned off. Before taking measurements, it is recommended to individually configure the device, select the brightness of the display, the parameters of the sound signal and batteries.
Measurement procedure
Select the “Measurement” mode, and the device begins to assess the radioactive situation. After approximately 60 seconds, the measurement result appears on its display, after which the next analysis cycle begins. In order to receive exact result, it is recommended to carry out at least 5 measurement cycles. An increase in the number of observations provides more reliable readings.
To measure the background radiation of objects, such as building materials or food products, you need to turn on the “Measurement” mode at a distance of several meters from the object, then bring the device to the object and measure the background as close to it as possible. Compare the readings of the device with data obtained at a distance of several meters from the object. The difference between these readings is the additional radiation background of the object being studied.
If the measurement results exceed the natural background characteristic of the area in which you are located, this indicates radiation contamination of the object being studied. To assess fluid contamination, it is recommended to take measurements above its open surface. To protect the device from moisture, it must be wrapped in plastic film, but not more than one layer. If the dosimeter has been at a temperature below 0°C for a long time, before taking measurements it must be kept at room temperature within 2 hours.
Due to environmental consequences 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 sufficient today hot topic. Besides natural sources ionizing radiation, every year there are more and more places contaminated with radiation subsequently human activity. Thus, in order to preserve your health and the health of your loved ones, you need to know the degree of contamination of a particular area or objects and food. A dosimeter can help with this - a device for measuring the effective dose or power of ionizing radiation over a certain period of time.
Before you start making (or buying) of this device it is necessary to have an idea of the nature of the parameter being measured. Ionizing radiation (radiation) is a stream of photons elementary particles or fragments of atomic fission capable of ionizing matter. 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, so they pose a danger mainly when they enter the body. Beta radiation- this is a flow of electrons arising during beta decay; an aluminum plate several millimeters thick is sufficient to protect against beta particles with an energy of up to 1 MeV. Gamma radiation has a much greater penetrating ability, since it consists of high-energy photons that do not have a charge; heavy elements (lead, etc.) in a layer of several centimeters are effective for protection. The penetrating ability of all types of ionizing radiation depends on energy.
Geiger-Muller counters are mainly used to detect ionizing radiation. This simple and effective device usually consists of a metal or glass cylinder metallized from the inside and a thin metal thread stretched along the axis of this cylinder; the cylinder itself is filled with rarefied gas. The operating principle 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 the atoms and create positive ions and free electrons. Electric field between the cathode and anode accelerates electrons to energies at which impact ionization begins. An avalanche of ions occurs, leading to the multiplication of primary carriers. At a sufficiently high field strength, the energy of these ions becomes sufficient to generate secondary avalanches that can sustain a self-discharge, causing the current through the counter to increase sharply.
Not all Geiger counters can detect all types of ionizing radiation. They are primarily sensitive to one type of radiation—alpha, beta, or gamma radiation—but can often also detect other radiation to some extent. For example, the SI-8B Geiger counter is designed to register soft beta radiation (yes, depending on the energy of the particles, radiation can be divided into soft and hard), but this sensor is also somewhat sensitive to alpha radiation and gamma radiation. radiation.
However, approaching the design of the article, our task is to make as simple as possible, naturally portable, a Geiger counter, or rather a dosimeter. To make this device, I only managed to get hold of SBM-20. This Geiger counter is designed to detect hard beta and gamma radiation. Like most other meters, SBM-20 operates at a voltage of 400 volts.
Main characteristics of the Geiger-Muller counter SBM-20 (table from the reference book):
This counter has relatively low accuracy in measuring ionizing radiation, but is sufficient to determine if the dose of radiation exceeded the permissible dose 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 gamma radiation measurement, dosimeters are equipped with beta radiation filters; in this case, the dosimeter registers only gamma radiation, but quite accurately.
When measuring radiation dose, there are several factors to consider that may be important. Even in the complete absence of sources of ionizing radiation, the Geiger counter will produce a certain number of pulses. This is the so-called counter background. This also includes several factors: radioactive contamination of the materials of the counter itself, spontaneous emission of electrons from the cathode of the counter and cosmic radiation. All this gives a certain number of “extra” impulses per unit of time.
So, the diagram of a simple dosimeter based on the SBM-20 Geiger counter:
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 allow you to assemble the circuit in a short time without special labor. However, such a dosimeter does not contain a scale, and the radiation dose must be determined by ear by the number of clicks. Like this classic version. The circuit consists of a voltage converter 9 volts - 400 volts.
The NE555 chip contains a multivibrator whose operating frequency is approximately 14 kHz. To increase the operating frequency, you can reduce the value of resistor R1 to approximately 2.7 kOhm. This will be useful if the choke you have chosen (or maybe the one you have made) makes a squeaking sound - as the operating frequency increases, the squeaking noise will disappear. Inductor L1 is required with a rating of 1000 - 4000 µH. The fastest way to find a suitable inductor 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 n-channel field-effect transistor with a drain-source voltage of at least 400 volts, and preferably more. Such a converter will produce only a few milliamps of current at a voltage of 400 volts, but this will be enough to operate a Geiger counter several times. After turning off the power from the circuit, the charged capacitor C3 will operate for about another 20-30 seconds, given its small capacitance. 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, current does not flow through resistors R2 – R4 (there are five resistors on the breadboard in the photo, but their total resistance corresponds to the circuit). As soon as the corresponding particle hits the Geiger counter, the gas ionizes inside the sensor 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 are used as a power source rechargeable batteries from old phones (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, it clicks occasionally. Now how to use it. The simplest option is that it clicks a little - everything is good, clicks often or even continuously - bad. Another option is to roughly count the number of pulses per minute and convert the number of clicks to microR/h. To do this, you need to take the sensitivity value of the Geiger counter from the reference book. However, different sources always give slightly different figures. Ideally, it is necessary to carry out laboratory measurements for the selected Geiger counter with reference radiation sources. So for SBM-20, the sensitivity value varies from 60 to 78 pulses/μR according to different sources and reference books. So, we calculated 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 is closer to reality, and in the end 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 safe radiation levels is approximately 20 - 25 µR/h. Acceptable level is up to approximately 50 μR/h. IN different countries numbers may vary.
P.S. I was prompted to consider this topic by an article about the concentration of radon gas penetrating into rooms, water, etc. V different regions country and its sources.
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| IC1 | Programmable timer and oscillator | NE555 | 1 | To notepad | ||
| T1 | MOSFET transistor | IRF710 | 1 | To notepad | ||
| VD1 | Rectifier diode | 1N4007 | 1 | To notepad | ||
| VD2 | Protection diode | 1V5KE400CA | 1 | To notepad | ||
| C1, C2 | Capacitor | 10 nF | 2 | To notepad | ||
| C3 | Electrolytic capacitor | 2.7 µF | 1 | To notepad | ||
| C4 | Capacitor | 100 nF | 1 | 400V |
Geiger-Muller counter
D To determine the level of radiation, a special device is used -. And for such devices, household and most professional radiation monitoring devices, the sensing element is used 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 decay rate of radioactive materials, was born in 1908, it was invented by the German physicist Hans Geiger . Twenty years later, together with another physicist Walter Müller the device was improved, and was named in honor of these two scientists.
IN the period of development and formation of nuclear physics in the former Soviet Union, corresponding devices were also created that were widely used in the armed forces, nuclear power plants, and in special radiation monitoring groups civil defense. Beginning in the seventies of the last century, such dosimeters included a counter based on Geiger principles, namely SBM-20 . This counter is exactly like its other analogue STS-5 , is widely used to this day, and is also part of modern means radiation monitoring .
Fig.1. Gas discharge counter STS-5.
Fig.2. Gas discharge meter SBM-20.
Operating principle of a Geiger–Müller counter
AND The idea of registering radioactive particles proposed by Geiger is relatively simple. It is based on the principle of the appearance of electrical impulses in an inert gas environment under the influence of a highly charged radioactive particle or a quantum of electromagnetic oscillations. To dwell in more detail on the mechanism of operation 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 cylinder or container that is filled with an inert gas, it can be neon, argon, etc. Such a container can be made of metal or glass, and the gas in it is under low pressure; this is done specifically 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. Device and circuit diagram for switching on a Geiger counter.
P When the counter is activated in an inert gas environment, 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 having a charge of sufficiently high energy knocks out a certain number of electrons from the immediate environment, i.e. from the housing elements or physically the electrodes themselves. Such electrons, once in an inert gas environment, under the influence of high voltage between the cathode and the 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 gas molecules, and this process grows on a geometric scale until a breakdown occurs between the electrodes. In a discharge state, the circuit closes for a very short period of time, and this causes a current jump in the load resistor, and it is this jump that makes it possible to register the passage of a particle or quantum through the recording chamber.
T This mechanism makes it possible to register one particle, however, in an environment where ionizing radiation is quite intense, a rapid return of the recording chamber to its original position is required to be able to determine new radioactive particle . This is achieved by two different ways. The first of them is to stop supplying voltage to the electrodes for a short period of time, in this case the ionization of the inert gas abruptly stops, and turning on the test chamber again allows you to start recording from the very beginning. This type of counter is called non-self-extinguishing dosimeters . The second type of device, namely self-extinguishing dosimeters, their operating principle is to add special additives based on various elements, for example, bromine, iodine, chlorine or alcohol, to the inert gas environment. In this case, their presence automatically leads to the termination of the discharge. With this structure of the test chamber, resistances sometimes of several tens of megaohms are used as a load resistor. This makes it possible to sharply reduce the potential difference at the ends of the cathode and anode during the discharge, which stops the current-conducting process and the chamber returns to its original state. It is worth noting that a voltage on the electrodes of less than 300 volts automatically stops maintaining the discharge.
The entire described mechanism allows you to register great amount radioactive particles in a short period of time.
Types of radioactive radiation
H to understand what exactly is being recorded Geiger–Muller counters , it is worth dwelling on what types of it exist. It’s worth mentioning right away that gas-discharge counters, which are part of most modern dosimeters, are only capable of recording the number of radioactive charged particles or quanta, but cannot determine either their energy characteristics or the type of radiation. For this purpose, dosimeters are made more multifunctional and targeted, and in order to compare them correctly, their capabilities should be more accurately understood.
P According to 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 radiation . 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 the most various materials. For example, if a person needs to hide from a stream of gamma rays, due to nuclear explosion, then by taking refuge in the basement of a house or bomb shelter, provided that it is relatively airtight, he will be able to protect himself from this type of radiation by only 50 percent.
Fig.4. X-ray and gamma radiation quanta.
T This type of radiation is pulsed in nature and is characterized by propagation in environment in the form of photons or quanta, i.e. short bursts of electromagnetic radiation. Such radiation can have different energy and frequency characteristics; for example, X-ray radiation has a frequency thousands of times lower than gamma rays. That's why 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, releasing a colossal amount of energy. In this case, beta particles represent a stream of electrons, and alpha particles are significantly larger and more stable formations, consisting of two neutrons and two protons bound to each other. In fact, the nucleus of a helium atom has this structure, so it can be argued that the flow of alpha particles is a flow of helium nuclei.
The following classification is accepted , alpha particles have the least penetrating power; in order to protect themselves from them, thick cardboard is enough for a person; beta particles have a greater penetrating power; in order for a person to 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, fading as they move away from the epicenter or source, and obeying the laws of propagation of electromagnetic waves.
Fig.5. Radioactive particles of alpha and beta type.
TO The amount of energy that all three types of radiation possess is also different, and the flux of alpha particles has the greatest of them. For example, The energy possessed by alpha particles is seven thousand times greater than the energy of beta particles , i.e. penetrating power various types radiation is in the back proportional dependence on 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 in decreasing order, beta particles and alpha particles. Therefore, it is quite difficult to determine alpha particles, even if it is impossible to tell with a conventional counter Geiger-Muller, since almost any object is an obstacle for them, not to mention a glass or metal container. It is possible to detect beta particles with such a counter, but only if their energy is sufficient to pass through the material of the counter container.
For low-energy beta particles, a conventional Geiger–Müller counter is ineffective.
ABOUT The situation is similar to gamma radiation; there is a possibility that they will pass through the container without starting the ionization reaction. To do 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 rays and thus activate the discharge in the counter chamber.
Basic characteristics and differences of Geiger–Müller counters
WITH It is also worth highlighting some basic characteristics and differences between various dosimeters equipped gas-discharge Geiger-Muller counters. To do this, you should compare some of them.
The most common Geiger–Müller counters are equipped with cylindrical or end sensors. Cylindrical are similar to an oblong cylinder in the form of a tube with a small radius. The end ionization chamber has a round or rectangular shape small sizes, but with a significant end working surface. Sometimes there are varieties of end chambers with an elongated cylindrical tube with a small entrance window with 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 thanks to the specially designed design of the meter housing, as well as the material from which it is made.
E another important component for intended use counters this area of the input sensitive element and working area . In other words, this is the sector through which the radioactive particles of interest to us will enter and be recorded. The larger this area, the more particles the counter will be able to capture, and the greater its sensitivity to radiation will be. The passport data indicates the working surface area, 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 a laboratory setting 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 such a level is sufficiently significant, then these induced noises will directly affect the measurement errors.
Each professional and radiation has such a characteristic as radiation sensitivity, also measured in pulses per second (imp/s), or in pulses per micro-roentgen (imp/μR). This parameter, or rather its use, directly depends on the source of ionizing radiation to which the counter is tuned and against which further measurements will be carried out. Often, tuning is done using sources that include radioactive materials such as radium - 226, cobalt - 60, cesium - 137, carbon - 14 and others.
E Another indicator by which it is worth comparing dosimeters is ion radiation detection efficiency or radioactive particles. The existence of this criterion is due to the fact that not all radioactive particles passing through the sensitive element of the dosimeter will be registered. This can happen in the case when the gamma radiation quantum did not cause ionization in the counter chamber, or the number of particles that passed through and caused ionization and discharge is so large that the device does not adequately count them, and for some other reasons. To accurately determine this characteristic of a specific 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 other radioactive materials.
WITH The next criterion to focus on 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 corresponding energy characteristic. This indicator is measured in megaelectronvolts or kiloelectronvolts (MeV, KeV). For example, if the 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 detected, or only high-energy alpha particles will be able to break through the material of the Geiger-Müller counter housing and knock out the electron.
AND based on all of the above, modern manufacturers 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 Geiger–Muller counters
P The first version of dosimeters are devices designed to register and detect gamma photons and high-frequency (hard) beta radiation. Almost all previously produced and modern ones, both household ones, for example: and professional radiation dosimeters, for example: , are designed for this measurement range. Such radiation has sufficient energy and high penetrating power for the Geiger counter camera to register them. Such particles and photons easily penetrate the walls of the counter and cause the ionization process, and this is easily recorded by the corresponding electronic stuffing dosimeter.
D Popular counters such as SBM-20 , having a sensor in the form of a cylindrical balloon tube with a coaxial wire cathode and anode. Moreover, the walls of the sensor tube serve simultaneously as a cathode and a housing, and are made of of stainless steel. This counter has the following characteristics:
- the area of the working area of the sensitive element is 8 square centimeters;
- radiation sensitivity to gamma radiation is about 280 pulses/s, or 70 pulses/μR (testing was carried out for cesium - 137 at 4 μR/s);
- the dosimeter's own background 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 limit.
Fig.6. Geiger counter device SBM-20.
U There were various modifications of this counter, for example, SBM-20-1 or SBM-20U , which have similar characteristics, but differ in the fundamental design of the contact elements and 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, which can be found in stores today.
WITH The next group of radiation dosimeters is designed to register 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 seems to be a rather difficult task.
The operating efficiency of such Geiger counters is about one percent.
H To increase it, an increase in the cathode surface is required. In fact, gamma rays are recorded indirectly, thanks to the electrons they knock out, which subsequently participate in the ionization of the inert gas. To promote this phenomenon as effectively as possible, the material and thickness of the counter chamber walls, 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 recording chamber, and too small will allow high-frequency beta radiation to easily enter the chamber, and will also increase the amount of radiation noise natural to the device, which will drown out the accuracy of determining gamma quanta. Naturally, the exact proportions are selected by the manufacturers. In fact, on this principle, dosimeters are manufactured based on Geiger–Muller counters for direct determination of gamma radiation on the ground, while such a device excludes the possibility of determining any other types of radiation and radioactive exposure, which makes it possible to accurately determine radiation contamination and the level of negative impact on humans only by gamma radiation.
IN In domestic dosimeters, which are equipped with cylindrical sensors, the following types are installed: SI22G, SI21G, SI34G, Gamma 1-1, Gamma - 4, Gamma - 5, Gamma - 7ts, Gamma - 8, Gamma - 11 and many others. Moreover, in some types, a special filter is installed on the input, end, sensitive window, which specifically serves to cut off alpha and beta particles, and additionally increases the cathode area for more efficient determination of gamma rays. Such sensors include Beta - 1M, Beta - 2M, Beta - 5M, Gamma - 6, Beta - 6M and others.
H To understand more clearly the principle of their operation, it is worth taking a closer look at one of these counters. For example, an end counter with a sensor Beta – 2M , which has a rounded working window of about 14 square centimeters. In this case, the radiation sensitivity to cobalt-60 is about 240 pulses/μR. This type of meter has very low self-noise , 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 record photon radiation with energies in the range from 0.05 MeV to 3 MeV.
Fig.7. End gamma counter Beta-2M.
To determine gamma radiation, it is quite possible to use counters for gamma-beta pulses, which are designed to register hard (high-frequency and high-energy) beta particles and gamma quanta. For example, model SBM - 20. If in this dosimeter model you want to exclude the registration of beta particles, then to do this it is enough to install a lead screen, or a shield from any other metal material(lead screen is more effective). This is the most common method used by most developers when creating gamma and x-ray counters.
Registration of “soft” beta radiation.
TO As we have already mentioned, registering soft beta radiation (radiation with low energy characteristics and a relatively low frequency) is a rather difficult task. To do this, it is necessary to ensure the possibility of easier penetration into the registration chamber. For these purposes, a special thin working window, usually made of mica or polymer film, which creates virtually no obstacles to the penetration of beta radiation of this type 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 that are evenly distributed and mounted on insulators. The registration window is made in the end version, and in this case only a thin mica film gets in the way of beta particles. In dosimeters with such counters, gamma radiation is registered as an application and, in fact, as additional opportunity. And if you want to get rid of the registration of gamma rays, then it is necessary to minimize the cathode surface.
Fig.8. Device of an end-mounted Geiger counter.
WITH It is worth noting that counters for determining soft beta particles were created quite a long time ago and were successfully used in the second half of the last century. Among them, the most common were sensors like SBT10 And SI8B , which had thin-walled mica working windows. More modern version such a device Beta-5 has a working window area of about 37 sq/cm, rectangular in shape made of mica material. For such sizes of the sensitive element, the device is able to register about 500 pulses/μR, if measured by cobalt - 60. At the same time, the particle detection efficiency is up to 80 percent. Other indicators of this device are as follows: its own noise is 2.2 pulses/s, the energy detection range is from 0.05 to 3 MeV, while the lower threshold for determining soft beta radiation is 0.1 MeV.
Fig.9. End beta-gamma counter Beta-5.
AND Naturally, it is worth mentioning Geiger–Muller counters, capable of detecting alpha particles. If registering soft beta radiation seems to be a rather difficult task, then detecting an alpha particle, even one with high energy indicators, is an even more difficult task. This problem can only be solved by appropriately reducing the thickness of the working window to a thickness that will be sufficient for the passage of an alpha particle into the recording chamber of the sensor, as well as by almost completely bringing the input window closer to the source of alpha particle radiation. This distance should be 1 mm. It is clear that such a device will automatically detect any other types of radiation, and with fairly high efficiency. There is both a positive and negative side to this:
Positive – such a device can be used for the widest range of radioactive radiation analysis
Negative – due to increased sensitivity, a significant amount of noise will arise, which will complicate the analysis of the received registration data.
TO In addition, a too thin mica working window, although it increases the capabilities of the meter, is, however, to the detriment of mechanical strength and the tightness of the ionization chamber, especially since the window itself has sufficient large area working surface. For comparison, in the SBT10 and SI8B counters, which we mentioned above, with a working window area of about 30 sq/cm, the thickness of the mica layer is 13 - 17 microns, and with the required thickness for recording alpha particles of 4-5 microns, the input the window can be made only no more than 0.2 sq/cm, we're talking about about the SBT9 counter.
ABOUT However, the large thickness of the registration working window can be compensated by the proximity to the radioactive object, and vice versa, with a relatively small thickness of the mica window, it becomes possible to register an alpha particle at a greater distance than 1 -2 mm. It is worth giving an example: with a window thickness of up to 15 microns, the approach to the source of alpha radiation should be less than 2 mm, while the source of alpha particles is understood to be a plutonium-239 emitter with a radiation energy of 5 MeV. Let's continue, with the thickness of the input window up to 10 microns, it is possible to register alpha particles at a distance of up to 13 mm, if we make a mica window up to 5 microns thick, then alpha radiation will be registered at a distance of 24 mm, etc. Another important parameter that directly affects the ability to detect alpha particles is their energy indicator. If the energy of an alpha particle is more than 5 MeV, then the registration distance for the thickness of the working window of any type will correspondingly increase, and if the energy is less, then the distance must be reduced, up to the complete impossibility of registering soft alpha radiation.
E one more important point, which makes it possible to increase the sensitivity of the alpha counter, is a decrease in the registration ability for gamma radiation. To do this, it is enough to minimize geometric dimensions cathode, and gamma photons will pass through the recording chamber without causing ionization. This measure makes it possible to reduce the influence of gamma rays on ionization by thousands and even tens of thousands of times. It is no longer possible to eliminate the influence of beta radiation on the recording chamber, but there is a fairly simple way out of this situation. First, alpha and beta radiation of the total type are recorded, then a thick paper filter is installed, and a second measurement is made, which will register only beta particles. The amount of alpha radiation in this case is calculated as the difference between the total radiation and a separate calculation indicator for beta radiation.
For example , it is worth proposing the characteristics of the modern Beta-1 counter, which allows you to register alpha, beta, and gamma radiation. These are the indicators:
- area of the working area of the sensitive element is 7 sq/cm;
- the thickness of the mica layer is 12 microns, (the effective detection distance of alpha particles for plutonium is 239, about 9 mm. For cobalt - 60, radiation sensitivity is achieved on the order of 144 pulses/μR);
- radiation measurement efficiency for alpha particles - 20% (for plutonium - 239), beta particles - 45% (for thallium -204), and gamma quanta - 60% (for composition strontium - 90, yttrium - 90);
- the dosimeter's own background 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 limit, and alpha particles with an energy of 5 MeV or more.
Fig. 10. End-mounted alpha-beta-gamma counter Beta-1.
TO Of course, there is still a fairly wide range of counters that are designed for a narrower and professional use. Such devices have a number additional settings and options (electrical, mechanical, radiometric, climate, etc.), which include many special terms and capabilities. However, we will not concentrate on them. After all, to understand the basic principles of action Geiger–Muller counters , the models described above are quite sufficient.
IN It is also important to mention that there are special subclasses Geiger counters , which are specially designed to determine various types other radiation. For example, to determine the value ultraviolet radiation, for recording and determining 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.