Methods of observation and registration of elementary. Methods for observing and registering charged particles
Sources of elementary particles
To study elementary particles, their sources are required. Before the creation of accelerators, natural radioactive elements and cosmic rays were used as such sources. In cosmic rays, there are elementary particles of various energies up to those that cannot be obtained artificially today. The disadvantage of cosmic rays as a source of high-energy particles is that there are very few such particles. The appearance of a high-energy particle in the field of view of the device is random.
Particle accelerators produce fluxes of elementary particles with equally high energy. There are various types of accelerators: betatron, cyclotron, linear accelerator.
Located near Geneva, the European Organization for Nuclear Research (CERN *) is the owner of the largest particle accelerator to date, built in a ring tunnel underground at a depth of 100 m. The total length of the tunnel is 27 km. (the ring is approximately 8.6 km in diameter). The Super Collider was supposed to be launched in accordance with the program in 2007. About 4,000 tons of metal will be cooled to a temperature of only 2 ° above absolute zero. As a result, a current of 1.8 million amperes will flow through the superconducting cables with almost no loss.
Particle accelerators are such grandiose structures that they are called pyramids of the 20th century.
* The abbreviation CERN comes from fr. Conseil Européen pour la Recherche Nucléaire (European Council for Nuclear Research). In Russian, the abbreviation CERN is usually used.
Methods for registration of elementary particles
1. Scintillation counters
Initially, for the registration of elementary particles, fluorescent screens were used - screens covered with a special substance, a phosphor capable of converting the energy they absorb into light radiation (luminescence). When an elementary particle hits such a screen, it produces a weak flash, so weak that it can be observed only in complete darkness. It was necessary to have a fair amount of patience and attention so that, sitting in complete darkness, for hours counting the number of flashes noticed.
In a modern scintillation counter, flash counting is done automatically. The counter consists of a scintillator, a photomultiplier and electronic devices for amplifying and counting pulses.
The scintillator converts the particle energy into visible light quanta.
The light quanta enter the photomultiplier tube, which converts them into current pulses.
The pulses are amplified by the circuitry and automatically counted.
2. Chemical methods
Chemical methods are based on the fact that nuclear radiation is a catalyst for some chemical reactions, that is, they accelerate or create the possibility of their occurrence.
3. Calorimetric methods
In calorimetric methods, the amount of heat that is released when radiation is absorbed by a substance is recorded. One gram of radium, for example, emits about 585 Joules per hour. heat.
4. Methods based on the application of the Cherenkov effect
Nothing in nature can travel faster than light. But when we say that, we mean the movement of light in a vacuum. In matter, light propagates at a speed where with Is the speed of light in vacuum, and n Is the refractive index of the substance. Consequently, light moves more slowly in matter than in a vacuum. An elementary particle, moving in a substance, can exceed the speed of light in this substance, without exceeding the speed of light in a vacuum. In this case, radiation arises, which Cherenkov discovered in his time. Cherenkov radiation is recorded by photomultipliers in the same way as in the scintillation method. The method allows registering only fast, that is, having high energies, elementary particles.
The following methods not only allow you to register an elementary particle, but also to see its trace.
5. Wilson chamber
Invented by Charles Wilson in 1912, and in 1927 he received the Nobel Prize for it. The Wilson chamber is a very complex engineering structure. We provide only a simplified diagram.
The working volume of the Wilson chamber is filled with gas and contains water or alcohol vapor. With the rapid downward movement of the piston, the gas cools sharply and the steam becomes supersaturated. When a particle flies through this space, creating ions on its way, then droplets of condensed vapor are formed on these ions. A trail of the particle trajectory (track) appears in the chamber in the form of a narrow strip of fog droplets. In strong side lighting, the track can be seen and photographed.
6. Bubble chamber(invented by Glaser in 1952)
The bubble chamber acts in a similar way to the Wilson chamber. Only as a working medium, not supercooled steam is used, but an overheated liquid (propane, liquid hydrogen, nitrogen, ether, xenon, freon ...). Superheated liquid, like supercooled steam, is in an unstable state. A particle flying through such a liquid forms ions, on which bubbles immediately form. The liquid bubble chamber is more efficient than the Wilson gas chamber. After all, it is important for physicists not only to observe the track of a passing particle. It is important that a particle collides with another particle within the observation area. The picture of particle interaction is much more informative. Flying through a denser liquid with a high concentration of protons and electrons, a particle is much more likely to experience a collision.
7. Emulsion chamber
It was first used by Soviet physicists Mysovsky and Zhdanov. The photographic emulsion is made on the basis of gelatin. Moving in dense gelatin, the elementary particle is subject to frequent collisions. Due to this, the path of the particle in the emulsion is often very short and, after the development of the photographic emulsion, it is studied under a microscope.
8. Spark chamber (inventor of Cranschau)
In the cell BUT a system of mesh electrodes is located. These electrodes are supplied with high voltage from the power supply. B... When an elementary particle flies through the camera IN, it creates an ionized trail. A spark slips along this trail, which makes the particle track visible.
9. Streamer camera
The streamer chamber is similar to the spark chamber, only the distance between the electrodes is greater (up to half a meter). The voltage is applied to the electrodes for a very short time in such a way that a real spark would not have time to develop. Only the rudiments of a spark - streamers - have time to emerge.
10. Geiger counter
The Geiger counter is, as a rule, a cylindrical cathode, along the axis of which the wire is stretched - the anode. The system is filled with a gas mixture.
When passing through the counter, a charged particle ionizes the gas. The generated electrons, moving to the positive electrode - filaments, falling into the region of a strong electric field, are accelerated and, in turn, ionize gas molecules, which leads to a corona discharge. The signal amplitude reaches several volts and is easily recorded.
The Geiger counter registers the fact of the passage of a particle through the counter, but does not allow measuring the particle energy.
- Grade 12.
- Explain to students the device and principle of operation of installations for registration and study of elementary particles.
- What is an "atom"?
- What are its dimensions?
- What model of the atom did Thomson suggest?
- What model of the atom did Rutherford suggest?
- Why was Rutherford's model called the "Planetary model of the structure of the atom"?
- What is the structure of the atomic nucleus?
- Methods for observation and registration of elementary particles.
- Atom - "indivisible" (Democritus).
- Molecule
- substance
- microworld
- macrocosm
- megaworld
- Classical physics
- The quantum physics
- Problem!
- Problem!
- We begin with you to study the physics of the atomic nucleus, consider their various transformations and nuclear (radioactive) radiation. This area of knowledge is of great scientific and practical importance.
- Various applications in science, medicine, technology, agriculture have received radioactive varieties of atomic nuclei.
- Today we will consider devices and methods of registration that allow detecting microparticles, studying their collisions and transformations, that is, they give all the information about the microworld, and on the basis of this, about the measures of protection against radiation.
- They give us information about the behavior and characteristics of particles: the sign and magnitude of the electric charge, the mass of these particles, its speed, energy, etc. With the help of recording devices, scientists were able to gain knowledge about the "microworld".
- The recording device is a complex macroscopic system that can be in an unstable state. With a small perturbation caused by a passing particle, the system begins to transition to a new, more stable state. This process allows registering a particle.
- Many different methods of particle detection are currently used.
- Geiger counter
- Wilson chamber
- Bubble chamber
- Photographic
- emulsions
- Scintillation
- method
- Methods for observation and registration of elementary particles
- Spark chamber
- Depending on the goals of the experiment and the conditions in which it is carried out, certain recording devices are used that differ from each other in basic characteristics.
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- Use F - 12th grade, § 33, A.E. Maron, G.Ya. Myakishev, EG Dubitskaya
- serves to count the number of radioactive particles (mainly electrons).
- It is a glass tube filled with gas (argon) with two electrodes inside (cathode and anode). When a particle flies through, impact gas ionization and an electric current pulse arises.
- Device:
- Purpose:
- Advantages:-one. compactness -2. efficiency -3. speed -4. high accuracy (10OOO particles / s).
- Cathode.
- Glass tube
- Where it is used: - registration of radioactive contamination on the ground, indoors, clothing, food, etc. - at storage facilities for radioactive materials or with operating nuclear reactors - when searching for deposits of radioactive ore (U - uranium, Th - thorium).
- Geiger counter.
- 1882 him physicist Wilhelm Geiger.
- Various types of Geiger counters.
- serves for observing and photographing traces from the flight of particles (tracks).
- Purpose:
- The inner volume of the chamber is filled with vapors of alcohol or water in a supersaturated state: when the piston is lowered, the pressure inside the chamber decreases and the temperature decreases, as a result of the adiabatic process, a supersaturated vapor is formed. On the trail of the particle's flight, droplets of moisture condense and a track is formed - a visible trace.
- Glass plate
- The English physicist Wilson invented the device in 1912 for observing and photographing traces of charged particles. He was awarded the Nobel Prize in 1927.
- Soviet physicists P.L. Kapitsa and D.V. Skobeltsin proposed to place the Wilson chamber in a uniform magnetic field.
- When the camera is placed in a magnetic field, the track can be used to determine: energy, speed, mass and charge of a particle. By the length and thickness of the track, by its curvature in a magnetic field determine characteristics of a passing radioactive particle... For example, 1. an alpha particle gives a solid thick track, 2. a proton, a thin track, 3. an electron, a dotted track.
- Various types of Wilson chambers and photographs of particle tracks.
- A variant of the Wilson chamber.
- When the piston drops sharply, the fluid under high pressure goes into an overheated state. When the particle moves rapidly along the track, vapor bubbles are formed, that is, the liquid boils, a track is visible.
- Advantages over the Wilson chamber: - 1. high density of the medium, hence short tracks - 2. particles get stuck in the chamber and further observation of particles can be carried out -3. faster performance.
- 1952 year. D. Glazer.
- Various types of bubble chamber and particle track photography.
- 20s L.V. Mysovsky, A.P. Zhdanov.
- - serves for particle registration - allows you to register rare phenomena due to long exposure time... The photographic emulsion contains a large amount of microcrystals of silver bromide. The flying particles ionize the surface of the emulsions. Crystals of AgBr (silver bromide) disintegrate under the action of charged particles and, upon development, a trace from the flight of a particle - a track - is revealed. The length and thickness of the track can be used to determine the energy and mass of the particles.
- the method has the following advantages:
- 1. It can register the trajectories of all particles flying through the photographic plate during the observation time.
- 2. The photographic plate is always ready for use (the emulsion does not require procedures that would bring it into working condition).
- 3. The emulsion has a high inhibitory power due to its high density.
- 4. It gives a non-vanishing trail of a particle, which can then be carefully studied.
- Disadvantages of the method: 1. duration and 2. complexity of chemical processing of photographic plates and 3. most importantly, it takes a lot of time to examine each plate in a strong microscope.
- This method (Rutherford) uses crystals for registration. The device consists of a scintillator, a photomultiplier tube and an electronic system.
- Scintillation method
- Impact ionization method
- Vapor condensation on ions
- Thick layer emulsion method
- Particles hitting a specially coated screen cause flashes that can be observed with a microscope.
- Gas Discharge Geiger Counter
- Wilson chamber and bubble chamber
- Ionizes the surface of photographic emulsions
- Let's repeat:
- 1. What topic of the lesson did we study today?
- 2 What goal did we set before studying the topic?
- 3. Have we achieved our goal?
- 4. What is the meaning of the motto that we took for our lesson?
- 5. Do you understand the topic of the lesson, why did we get to know it?
- 1. We check together your work on the table, evaluate together, give a grade, taking into account your work in the lesson.
- 1. Internet resources.
- 2. F -12 cells, A.E. Myakishev, G.Ya. Myakishev, E.G. Dubitskaya.
Report:
Methods for registration of elementary particles
1) Gas-discharge Geiger counter
The Geiger counter is one of the most important devices for automatic particle counting.
The meter consists of a glass tube covered from the inside with a metal layer (cathode) and a thin metal thread running along the axis of the tube (anode).
The tube is filled with gas, usually argon. The counter is based on impact ionization. A charged particle (electron, £ is a particle, etc.), flying in a gas, detaches electrons from atoms and creates positive ions and free electrons. The electric field between the anode and cathode (a high voltage is applied to them) accelerates the electrons to the energy at which impact ionization begins. An avalanche of ions arises, and the current through the counter rises sharply. In this case, a voltage pulse is generated across the load resistor R, which is fed to the recording device. In order for the counter to register the next particle that got into it, the avalanche discharge must be extinguished. This happens automatically. Since at the moment the current pulse appears, the voltage drop across the unloading resistor R is large, the voltage between the anode and cathode decreases sharply - so much so that the discharge stops.
The Geiger counter is mainly used to register electrons and Y-quanta (high-energy photons). However, due to their low ionizing ability, Y-quanta are not registered directly. To detect them, the inner wall of the tube is covered with a material from which Y-quanta knock out electrons.
The counter registers almost all electrons falling into it; as for Y-quanta, it registers approximately only one Y-quantum out of a hundred. Registration of heavy particles (for example, £ -particles) is difficult, since it is difficult to make a sufficiently thin "window" in the counter, transparent for these particles.
2) Wilson chamber
The action of the Wilson chamber is based on the condensation of supersaturated steam on ions with the formation of water droplets. These ions are created by a moving charged particle along its trajectory.
The device is a cylinder with a piston 1 (Fig. 2), covered with a flat glass cover 2. The cylinder contains saturated vapors of water or alcohol. The investigated radioactive preparation 3 is introduced into the chamber, which forms ions in the working volume of the chamber. When the piston drops sharply downwards, i.e. with adiabatic expansion, the vapor is cooled and it becomes supersaturated. In this state, the vapor condenses easily. The centers of condensation are the ions formed by the particle passing by at this time. So a foggy track (track) appears in the camera (Fig. 3), which can be observed and photographed. The track exists in tenths of a second. By returning the piston to its original position and removing the ions by an electric field, the adiabatic expansion can be performed again. Thus, experiments with the camera can be carried out many times.
If the camera is placed between the poles of an electromagnet, then the capabilities of the camera for studying the properties of particles expand significantly. In this case, the moving particle is acted upon by the Lorentz force, which makes it possible to determine the value of the particle's charge and its momentum by the curvature of the trajectory. Figure 4 shows a possible interpretation of a photograph of the tracks of an electron and a positron. The induction vector B of the magnetic field is directed perpendicular to the plane of the drawing beyond the drawing. The positron is deflected to the left, the electron is deflected to the right.
3) Bubble chamber
It differs from the Wilson chamber in that the supersaturated vapors in the working volume of the chamber are replaced by an overheated liquid, i.e. such a liquid, which is under pressure, less than the pressure of its saturated vapor.
Flying in such a liquid, a particle causes the appearance of vapor bubbles, thereby forming a track (Fig. 5).
In the initial state, the piston compresses the fluid. With a sharp drop in pressure, the boiling point of the liquid turns out to be less than the ambient temperature.
The liquid passes into an unstable (overheated) state. This ensures the appearance of bubbles on the path of the particle. Hydrogen, xenon, propane and some other substances are used as a working mixture.
The advantage of the bubble chamber over the Wilson chamber is due to the higher density of the working substance. As a result, the paths of particles turn out to be rather short, and particles of even high energies get stuck in the chamber. This allows you to observe a series of successive transformations of a particle and the reactions it causes.
4) Thick layer emulsion method
To register particles, along with Wilson chambers and bubble chambers, thick-layer photographic emulsions are used. Ionizing effect of fast charged particles on a photographic plate emulsion. The photographic emulsion contains a large number of microscopic crystals of silver bromide.
A fast charged particle, penetrating the crystal, strips electrons from individual bromine atoms. A chain of these crystals forms a latent image. When it appears in these crystals, metallic silver is reduced and a chain of silver grains forms a particle track.
The length and thickness of the track can be used to estimate the energy and mass of the particle. Due to the high density of the emulsion, the tracks are very short, but they can be enlarged when photographing. The advantage of a photographic emulsion is that the exposure time can be as long as desired. This allows rare occurrences to be recorded. It is also important that due to the high stopping power of the photoemulsion, the number of interesting reactions observed between particles and nuclei increases.
Methods for registration of elementary particles are based on the use of systems in a long-lived unstable state, in which a transition to a stable state occurs under the action of a passing charged particle.
Geiger counter.
Geiger counter- particle detector, the action of which is based on the occurrence of an independent electric discharge in the gas when a particle enters its volume. Invented in 1908 by H. Geiger and E. Rutherford, later improved by Geiger and Müller.
The Geiger counter consists of a metal cylinder - the cathode - and a thin wire stretched along its axis - the anode, enclosed in a sealed volume filled with gas (usually argon) under a pressure of about 100-260 GPa (100-260 mm Hg). A voltage of about 200-1000 V is applied between the cathode and the anode.A charged particle, having entered the volume of the counter, forms a certain amount of electron-ion pairs, which move to the corresponding electrodes and at a high voltage along the free path (on the way to the next table) keniya) gain energy that exceeds the energy of ionization, and ionize gas molecules. An avalanche is formed, the current in the circuit increases. From the load resistance, a voltage pulse is fed to the recording device. A sharp increase in the voltage drop across the load resistance leads to a sharp decrease in the voltage between the anode and cathode, the discharge stops, and the tube is ready to register the next particle.
The Geiger counter registers mainly electrons and γ-quanta (the latter, however, with the help of additional material applied to the walls of the vessel, from which γ-quanta knock out electrons).
Wilson's chamber.
Wilson chamber- track (from the English. track- trace, trajectory) particle detector. Created by C. Wilson in 1912. With the help of the Wilson chamber, a number of discoveries were made in nuclear physics and elementary particle physics, such as the discovery of extensive air showers (in the field of cosmic rays) in 1929, the positron in 1932, detection of traces of muons, the discovery of strange particles. Subsequently, the Wilson chamber was practically supplanted by the bubble chamber as a faster one. The Wilson chamber is a vessel filled with water or alcohol vapors close to saturation (see Fig.). Its action is based on the condensation of supersaturated vapor (water or alcohol) on the ions formed by the passing particle. Supersaturated steam will be created by a sharp lowering of the piston (see Fig.) (Steam in the chamber expands adiabatically, as a result of which the temperature drops sharply).
Liquid droplets deposited on the ions make the trail of the flown particle visible - a track, which makes it possible to photograph it. The particle energy can be determined from the track length, and its velocity can be estimated from the number of droplets per track length unit. Placing the camera in a magnetic field makes it possible to determine the ratio of the particle's charge to its mass by the curvature of the track (first proposed by Soviet physicists P. L. Kapitsa and D. V. Skobeltsyn).
Bubble chamber.
Bubble chamber- a device for recording traces (tracks) of charged particles, the action of which is based on the boiling of a superheated liquid along the trajectory of a particle.
The first bubble chamber (1954) was a metal chamber with glass windows for illumination and photography, filled with liquid hydrogen. Later it was created and improved in all laboratories of the world equipped with charged particle accelerators. From a cone with a volume of 3 cm 3, the size of the vial chamber reached several cubic meters. Most vials have a volume of 1 m 3. For the invention of the bubble chamber, Glazer was awarded the Nobel Prize in 1960.
The duration of the working cycle of the vial is 0.1. Its advantage over the Wilson chamber is in the higher density of the working substance, which makes it possible to register high-energy particles.
At the beginning of the XX century. methods for studying the phenomenon of atomic physics were developed and devices were created that made it possible not only to clarify the basic questions of the structure of atoms, but also to observe the transformations of chemical elements.
The difficulty in creating such devices lay in the fact that the charged particles used in experiments are ionized atoms of some elements or, for example, electrons, and the device must register only one particle hitting it or make its trajectory visible.
A screen covered with a luminescent composition was used as one of the first and simplest devices for registering particles. At that point on the screen, where a particle with a sufficiently high energy falls, a flash occurs - scintillation (from the Latin "scintillation" - flashing, flash).
The first basic device for registering particles was invented in 1908 by G. Geiger. After this device was improved by W. Müller, he could count the number of particles falling into it. The action of the Geiger - Muller counter, and is based on the fact that charged particles flying through the gas ionize the gas atoms that meet in their path: a negatively charged particle, repelling electrons, knocks them out of atoms, and a positively charged particle attracts electrons and pulls them out of atoms.
The counter consists of a hollow metal cylinder, about 3 cm in diameter (Fig. 37.1), with a thin glass or aluminum window. A metal thread isolated from the walls passes through the smallpox of the cylinder. The cylinder (chamber) is filled with a rarefied gas such as argon. A voltage of about 1500 V is created between the walls of the cylinder and the thread, which is insufficient for the formation of a self-sustained discharge. The thread is grounded through a large resistanceR. When a particle with a high energy enters the chamber, gas atoms are ionized along the path of this particle, and a discharge occurs between the walls and the filament. The discharge current creates a large voltage drop across the resistance R, and the voltage between the filament and the walls is greatly reduced. Therefore, the discharge is quickly terminated. After stopping the current, all the voltage is again concentrated between the walls of the chamber and the filament, and the counter is prepared to register a new particle. Voltage with resistance R is fed to the input of an amplifying lamp, in the anode circuit of which a counting mechanism is connected.
The ability of high-energy particles to ionize gas atoms is also used in one of the most remarkable instruments of modern physics - the Wilson chamber. In 1911, the English scientist C. Wilson built a device with which it was possible to see and photograph the trajectories of charged particles.
The Wilson chamber (Fig. 37.2) consists of a cylinder with a piston; the top of the cylinder is made of transparent material. A small amount of water or alcohol is introduced into the chamber, and a mixture of vapors and air is formed inside the chamber. When the piston is quickly lowered, the mixture adiabatically expands and cools, so the air in the chamber is supersaturated with vapors.
If the air is cleaned of dust particles, then the conversion of excess vapor into liquid is difficult due to the absence of condensation centers. However, ions can also serve as centers of condensation. Therefore, if a charged particle flies through the chamber at this time, ionizing air molecules in its path, then vapor condensation occurs on the chain of ions and the trajectory of the particle inside the chamber is obtained by a marked thread of fog, i.e., it becomes visible. The thermal movement of the air quickly blurs the threads of the fog, and the trajectories of the particles are clearly visible only for about 0.1 s, which, however, is sufficient for photographing.
The view of the trajectory in the photograph often makes it possible to judge the nature of the particle and the magnitude of its energy. Thus, alpha particles leave a relatively thick continuous trail, protons - thinner, and electrons - a dotted trail. One of the photographs of alpha particles in the Wilson chamber is shown in Fig. 37.3.
To prepare the chamber for action and cleanse it of the remaining ions, an electric field is created inside it, attracting the ions to the electrodes, where they are neutralized.
As mentioned above, in the Wilson chamber, the condensation of a supersaturated vapor is used to obtain particle traces, i.e., its transformation into a liquid. For the same purpose, you can use the opposite phenomenon, that is, the transformation of liquid into vapor. If the liquid is enclosed in a closed vessel with a piston and using the piston to create an increased pressure, and then by a sharp movement of the piston to reduce the pressure in the liquid, then at an appropriate temperature the liquid may be in an overheated state. If a charged particle flies through such a liquid, then the liquid will boil along its trajectory, since the ions formed in the liquid serve as centers of vaporization. In this case, the trajectory of the particle is marked by a chain of vapor bubbles, that is, it is made visible. The operation of the bubble chamber is based on this principle.
When studying traces of particles with high energy, the bubble chamber is more convenient than the Wilson chamber, since when moving in a liquid, a particle loses much more energy than in a gas. In many cases, this allows a much more accurate determination of the direction of motion of the particle and its energy. Currently, there are bubble chambers with a diameter of about 2 m. They are filled with liquid hydrogen. Particle tracks in liquid hydrogen are very clear.
The method of thick-layer photographic plates is also used to register particles and obtain their traces. It is based on the fact that the particles flying through the emulsion act on the grains of silver bromide, so the trace left by the particles after the development of the photographic plate becomes visible (Fig. 37.4) and can be examined with a microscope. To keep the trail long enough, thick layers of emulsion are used.
