Geiger counter— a gas-discharge device for counting the number of ionizing particles passing through it. It is a gas-filled capacitor that breaks through when an ionizing particle appears in the gas volume. Geiger counters are quite popular detectors (sensors) of ionizing radiation. Until now, invented at the very beginning of our century for the needs of nascent nuclear physics, there is, oddly enough, no full-fledged replacement.
The design of a Geiger counter is quite simple. A sealed container with two electrodes contains gas mixture, consisting of easily ionized neon and argon. The material of the cylinder can be different - glass, metal, etc.
Typically, counters perceive radiation over their entire surface, but there are also those that have a special “window” in the cylinder for this purpose. The widespread use of the Geiger-Muller counter is explained by its high sensitivity, the ability to detect various radiation, comparative simplicity and low cost of installation.
Geiger counter connection diagram
A high voltage U is applied to the electrodes (see figure), which in itself does not cause any discharge phenomena. The counter will remain in this state until it is in gas environment an ionization center will not arise - a trail of ions and electrons generated by an ionizing particle arriving from outside. Primary electrons, accelerating in electric field, ionize “along the way” other molecules of the gaseous medium, generating more and more electrons and ions. Developing like an avalanche, this process ends with the formation of an electron-ion cloud in the space between the electrodes, significantly increasing its conductivity. A discharge occurs in the gas environment of the meter, visible (if the container is transparent) even with the naked eye.
The reverse process - restoration of the gas environment to its original state in so-called halogen meters - occurs by itself. Halogens (usually chlorine or bromine), contained in small quantities in the gas environment, come into play and contribute to intense charge recombination. But this process is quite slow. The time required to restore the radiation sensitivity of a Geiger counter and what actually determines its performance - “dead” time - is its main passport characteristic.
Such meters are designated as halogen self-extinguishing meters. Featuring a very low supply voltage, good parameters output signal and sufficiently high speed, they turned out to be in demand as ionizing radiation sensors in household radiation monitoring devices.
Geiger counters are capable of detecting the most different types ionizing radiation - a, b, g, ultraviolet, x-ray, neutron. But the actual spectral sensitivity of the meter very much depends on its design. Thus, the input window of a counter sensitive to a- and soft b-radiation must be quite thin; For this purpose, mica with a thickness of 3...10 microns is usually used. The cylinder of the 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 window is made of quartz glass.
Dependence of counting speed on supply voltage in a Geiger counter
Boron is introduced into the neutron counter, upon interaction with which the neutron flux is converted into easily registered a-particles. Photon radiation - ultraviolet, x-ray, g-radiation - Geiger counters perceive indirectly - through the photoelectric effect, Compton effect, pair creation effect; in each case, the radiation interacting with the cathode substance is converted into a flow of electrons.
Each particle detected by the counter forms a short pulse in its output circuit. The number of pulses appearing per unit time—the counting rate of a Geiger counter—depends on the level of ionizing radiation and the voltage on its electrodes. A standard graph of the counting rate versus supply voltage Upit is shown in the figure above. Here Uns is the counting start voltage; Ung and Uvg are the lower and upper boundaries of the working section, the so-called plateau, on which the counting speed is almost independent of the counter supply voltage. The operating voltage Uр is usually selected in the middle of this section. It corresponds to Np - the counting rate in this mode.
Dependence of counting speed on degree radiation exposure the meter is its main characteristic. The graph of this dependence is almost linear in nature and therefore the radiation sensitivity of the counter is often shown in terms of pulse/μR (pulses per microroentgen; this dimension follows from the ratio of the counting rate - pulse/s - to the radiation level - μR/s).
In cases where it is not indicated, the radiation sensitivity of the counter has to be determined by its other extremely important parameter - its own background. This is the name for the counting rate, the factor of which is two components: external - the natural background radiation, and internal - the radiation of radionuclides found in the counter structure itself, as well as the spontaneous electron emission of its cathode.
Dependence of the counting rate on the energy of gamma quanta (“stroke with rigidity”) in a Geiger counter
One more essential characteristic Geiger counter is the dependence of its radiation sensitivity on the energy (“hardness”) of ionizing particles. The extent to which this dependence is significant is shown by the graph in the figure. “Riding with rigidity” will obviously affect the accuracy of the measurements taken.
The fact that the Geiger counter is an avalanche device also has its disadvantages - the reaction of such a device cannot be used to judge the root cause of its excitation. The output pulses generated by a Geiger counter under the influence of a-particles, electrons, and g-quanta are no different. The particles themselves and their energies completely disappear in the twin avalanches they generate.
The table provides information about self-extinguishing halogen Geiger counters of domestic production, most suitable for household appliances radiation control.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | |
| SBM19 | 400 | 100 | 2 | 310* | 50 | 19x195 | 1 |
| SBM20 | 400 | 100 | 1 | 78* | 50 | 11x108 | 1 |
| SBT9 | 380 | 80 | 0,17 | 40* | 40 | 12x74 | 2 |
| SBT10A | 390 | 80 | 2,2 | 333* | 5 | (83x67x37) | 2 |
| SBT11 | 390 | 80 | 0,7 | 50* | 10 | (55x29x23.5) | 3 |
| SI8B | 390 | 80 | 2 | 350-500 | 20 | 82x31 | 2 |
| SI14B | 400 | 200 | 2 | 300 | 30 | 84x26 | 2 |
| SI22G | 390 | 100 | 1,3 | 540* | 50 | 19x220 | 4 |
| SI23BG | 400 | 100 | 2 | 200-400* | — | 19x195 | 1 |
- 1 — operating voltage, V;
- 2 — plateau — region of low dependence of counting speed on supply voltage, V;
- 3 — counter’s own background, imp/s, no more;
- 4 — radiation sensitivity of the counter, imp/μR (* — for cobalt-60);
- 5 — output pulse amplitude, V, not less;
- 6 - dimensions, mm - diameter x length (length x width x height);
- 7.1 - hard b - and g - radiation;
- 7.2 - the same and soft b - radiation;
- 7.3 - the same and a - radiation;
- 7.4 - g - radiation.
Using a modern Geiger counter, you can measure radiation levels building materials, land plot or apartments, 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.
Geiger-Muller counter
D To determine the level of radiation it is used special device– . And for such household devices 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 control 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 a medium inert gas under the influence of a highly charged radioactive particle or 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 voltage is applied. direct current 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, however, 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, are exposed to 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 flashes 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. Various configurations of counters, namely the cameras themselves, are able to register different types radiation, or combinations thereof (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 sensing 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. Essentially, gamma rays are recorded indirect way, 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 register any other types of radiation, and, moreover, with sufficient 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, which 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 amount of ultraviolet radiation, to register and determine slow neutrons that operate on the principle of a corona discharge, and other options that are not directly related to this topic will not be considered.