What methods exist for detecting charged particles. Methods for recording elementary particles

The study of the structure of the atomic nucleus is inextricably linked with the consideration of the phenomena of spontaneous or forced decay of the atomic nucleus and nuclear particles. By examining fragments of a collapsed atomic nucleus and tracing the fate of these fragments, we are able to draw conclusions about the structure of the nucleus and nuclear forces.

It is quite natural that at first the phenomena of spontaneous decay of nuclei, i.e., radioactive phenomena, were studied in detail. In parallel with this, research began cosmic rays- radiation, which has exceptional penetrating power and comes to us from outer space. When interacting with matter, cosmic radiation particles play the role of projectile particles. For a long time, cosmic ray research was in the most important way study of interconvertibility elementary particles and even to some extent by the method of studying the atomic nucleus. Currently, studies of the destruction of the atomic nucleus by bombardment by streams of particles created in accelerators are acquiring primary importance.

The experimental methods discussed now are equally applicable to the study of cosmic rays and particles resulting from nuclear bombing certain targets.

Trail cameras.

The first device that made it possible to see the trace (track) of a particle was a cloud chamber. If a fast particle flies through a chamber containing supersaturated water vapor, creating ions along its path, then such a particle leaves a trail, very similar to the “tail” that sometimes remains in the sky after an airplane. This trail is created by condensed steam. The ions marking the path of the particle are centers of vapor condensation - this is the reason for the appearance of a clearly visible trace. The trace of a particle can be observed directly and photographed.

To regulate the state of steam in the chamber, the volume of the chamber is changed by moving the piston. Rapid adiabatic expansion of the vapor leads to a state of supersaturation.

If the trail camera is placed in a magnetic field, then from the curvature of the trajectory one can determine either the particle speed at in a certain respect or, conversely, at a known speed (cf. formulas on page 406).

The Wilson chamber already belongs to history. Because the chamber is filled with gas, collisions are rare. The camera “cleansing” time is very long: photos can be taken only after 20 seconds. Finally, the trace lives for a time of the order of a second, which can lead to displacement of the pictures.

In 1950, the bubble chamber was proposed, which plays a major role in particle physics. The substance of the chamber is superheated liquid. A charged particle forms ions, and bubbles are created near the ions, which make the trace visible. This camera can take 10 photos per second. The biggest drawback of the camera is the inability to control how it turns on. Therefore, thousands of photographs are often needed to select one that captures the phenomenon under study.

Spark chambers based on a different principle are of great importance. If a high voltage is applied to a parallel-plate capacitor, a spark will jump between the plates. If there are ions in the gap, the spark will jump at a lower voltage. Thus, an ionizing particle flying between the plates creates a spark.

In the spark chamber, the particle itself switches on the high voltage between the plates of the capacitor for a millionth of a second. However, the advantages regarding the possibility of inclusion in right moment weakened by shortcomings: only particles forming an angle of no more than 45° with the plates are visible, the trace is very short and not all secondary phenomena have time to manifest themselves.

Recently, Soviet researchers proposed a new type of trail camera (the so-called streamer camera), which has already found wide application. The block diagram of such a camera is shown in Fig. 237. A particle falling between the plates, which, unlike the spark chamber, are located at a large distance from each other, is detected by a counter. Electronic logic device

distinguishes primary events and selects the one that interests the experimenter. At this moment, the high voltage on short time fed to the plates. The ions formed along the path of the particle form dashes (streamers), which are photographed. The path of the particle is outlined by these dashes.

If the photograph is taken along the direction of the dashes, then the particle path looks like a dotted line.

The success of the streamer chamber depends on the correct correlation of the formation of an electron avalanche from the primary ion with the parameters of the high voltage pulse. In a mixture of 90% neon and 10% helium with a distance between the plates of 30 cm, good results are obtained with a voltage of 600,000 V and a pulse time. In this case, the pulse should be applied no later than s after the primary ionization event. This type of wake chamber is a complex, expensive setup that is as far removed from a cloud chamber as modern particle accelerators are from a electron tube.

Ionization counters and ionization chambers.

An ionization device designed to work with radiation is mostly a cylindrical capacitor filled with gas; one electrode is a cylindrical plate, and the other is a thread or tip running along the axis of the cylinder (Fig. 237a). The voltage applied to the capacitor and the pressure of the gas filling the meter must be selected in a special way depending on the problem statement. In a common variation of this device, called a Geiger counter, a breakdown voltage is applied to the cylinder and filament. If through the wall or through the end of such a meter it gets into

ionizing particle, then a current pulse will flow through the capacitor, continuing until the primary electrons and the self-discharge electrons and ions they create approach the positive plate of the capacitor. This current pulse can be amplified by conventional radio engineering methods and the passage of the particle through the counter can be recorded either by a click, or by a flash of light, or, finally, by a digital counter.

Such a device can count the number of particles entering the device. For this, only one thing is necessary: the current pulse must stop by the time the next particle enters the counter. If the operating mode of the meter is selected incorrectly, the meter begins to “choke” and counts incorrectly. The resolution of the ionization counter is limited, but still quite high: up to particles per second.

You can lower the voltage and achieve a mode in which a current pulse proportional to the number of ions formed would pass through the capacitor (proportional counter). To do this, you need to work in the region of a non-self-sustaining gas discharge. Primary electrons, moving in the electric field of the capacitor, gain energy. Impact ionization begins and new ions and electrons are created. The initial ion pairs created by the particle flying into the counter are converted into ion pairs. When operating in a non-self-sustaining discharge mode, the gain will be a constant value and proportional counters will not only establish the fact of the particle passing through the counter, but also measure its ionizing ability.

The discharge in proportional counters, as well as in the Geiger counters described above, goes out when ionization stops. The difference between a Geiger counter is that in it the incoming particle acts like a trigger mechanism and the breakdown time is not related to the initial ionization.

Since proportional counters respond to the ionizing ability of a particle, the operating mode of the counter can be selected so that it detects only particles of a certain type.

If the device operates in saturation current mode (which can be achieved by reducing the voltage), then the current through it is a measure of the radiation energy absorbed in the volume of the device per unit time. In this case, the device is called an ionization chamber. The gain is equal to unity in this case. The advantage of the ionization chamber is its greater stability. The designs of ionization chambers can vary significantly. The chamber filling, wall materials, number and shape of electrodes vary depending on the purpose of the study. In addition to tiny chambers with a volume of the order of a cubic millimeter, one has to deal with chambers with a volume of up to hundreds of meters. Under the influence of a constant source of ionization, currents arise in the chambers ranging from to

Scintillation counters.

The method of counting flashes of a fluorescent substance (scintillation) as a means of counting elementary particles was first used by Rutherford for his classical studies of the structure of the atomic nucleus. The modern embodiment of this idea bears little resemblance to Rutherford’s simple device.

The particle causes a flash of light in a solid substance - phosphorus. It is very well known large number organic and inorganic substances, having the ability to convert the energy of charged particles and photons into light energy. Many phosphors have a very short afterglow duration, on the order of billionths of a second. This makes it possible to build scintillation counters with high speed accounts. For a number of phosphors, the light output is proportional to the energy of the particles. This makes it possible to construct counters for estimating particle energy.

In modern counters, phosphors are combined with photomultipliers that have conventional photocathodes that are sensitive to visible light. Electric current, created in the multiplier, is amplified and then sent to the counting device.

The most commonly used organic phosphorus: anthracene, stilbene, terphenyl, etc. All of these chemical compounds belong to a class of so-called aromatic compounds, built from hexagons of carbon atoms. To use them as scintillators, these substances must be taken in the form of single crystals. Since growing large single crystals is somewhat difficult and since crystals organic compounds are very fragile, then the use of plastic scintillators is of significant interest - this is the name given to solid solutions of organic phosphorus in transparent plastics - polystyrene or other similar high-polymer substance. Halides are used from inorganic phosphorus alkali metals, zinc sulfide, tungstates of alkaline earth metals.

Cherenkov counters.

Back in 1934, Cherenkov showed that when a fast charged particle moves in a completely pure liquid or solid dielectric, a special glow appears, which is fundamentally different from both the fluorescence glow associated with energy transitions in the atoms of the substance, and from bremsstrahlung such as the X-ray continuous spectrum . Cherenkov radiation occurs when a charged particle moves at a speed exceeding the phase speed of light propagation in a dielectric. The main feature of the radiation is that it propagates along the conical surface forward in the direction of particle motion. The cone angle is determined by the formula:

where is the angle of the generatrix of the cone with the direction of motion of the particle, V is the speed of the particle, the speed of light in the medium. Thus, for a medium with a given refractive index, there is a critical speed below which there will be no radiation. At this critical speed, the radiation will be parallel to the direction of motion of the particle. For a particle moving at a speed very close to the speed of light, a maximum radiation angle will be observed For cyclohexane

The Cherenkov radiation spectrum, as experience and theory show, is located mainly in the visible region.

Cherenkov radiation is a phenomenon similar to the formation of a bow wave from a ship moving through water; in this case, the speed of the ship is greater than the speed of the waves on the surface of the water.

Rice. 2376 illustrates the origin of the radiation. A charged particle moves along the axial line and along the path, the electromagnetic field following the particle temporarily polarizes the medium at points along the particle's trajectory.

All these points become sources of spherical waves. There is one single angle at which these spherical waves will be in phase and form a single front.

Let's consider two points on the path of a charged particle (Fig. 237c). They created spherical waves, one at a time, the other at a time. Obviously, there is time that the particle took to travel between these two points. In order for these two waves to propagate at some angle 9 in the same phase, it is necessary that the travel time of the first ray be greater than the travel time of the second ray by a time The path traveled by the particle in time is equal to The wave will cover the distance in the same time From here we get the above formula:

Cherenkov radiation is used in lately very widely as a way to register elementary particles. Counters based on this phenomenon are called Cherenkov counters. The luminous substance is combined, as in scintillation counters, with photomultipliers and amplifiers

photoelectric current. There are many designs of Cherenkov counters.

Cherenkov counters have many advantages. These include fast speed calculations and the ability to determine the charges of particles moving at a speed very close to the speed of light (we did not say that the light output depends sharply on the charge of the particle). Only with the help of Cherenkov counters can such important problems as direct determination of the speed of a charged particle, determination of the direction in which an ultrafast particle moves, etc. be solved.

Placement of counters.

In order to study various processes transformations and interactions of elementary particles, it is necessary to be able not only to note the appearance of a particle in a given place, but also to trace future fate the same particle. Such problems are solved using special arrangements of counters with a generalized counting circuit. For example, you can electrical diagrams connect two or more counters in such a way that counting occurs only if the discharge in all counters begins at exactly the same time. This can serve as proof that the same particle has passed through all the counters. This switching on of counters is called “matching switching”.

Method of thick-layer photographic emulsions.

As is known, the photosensitive layer of photographic plates is a gelatin film into which silver bromide microcrystals are introduced. The basis of the photographic process is the ionization of these crystals, which results in the reduction of silver bromide. This process occurs not only under the influence of light, but also under the influence of charged particles. If a charged particle flies through the emulsion, a hidden trace will appear in the emulsion, which can be seen after the photographic plate is developed. Traces in photographic emulsion tell many details about the particle that caused them. Highly ionizing particles leave a greasy residue. Since ionization depends on the charge and speed of the particles, the appearance of the trace alone speaks volumes. Valuable information is provided by the distance (track) of a particle in a photographic emulsion; By measuring the length of the trace, the energy of the particle can be determined.

Research using conventional photographic plates with thin emulsions is of little use for nuclear physics purposes. Such plates would record only those particles that move strictly along the plate. Mysovsky and Zhdanov, as well as a few years later by Powell in England, introduced photographic plates with an emulsion thickness close to (for ordinary plates the layer thickness is a hundred times less). The photo method is valuable for its clarity, the ability to observe a complex picture of the transformation that occurs when a particle is destroyed.

In Fig. 238 shows a typical photograph obtained by this Method. Nuclear transformations occurred at the points.

In the latest version of this method, large-volume emulsion chambers are used as the medium in which particle tracks are recorded.

Methods for analyzing observations.

With the help of the described instruments, the researcher has the opportunity to determine all the most important constants of an elementary particle: speed and energy, electric charge, mass; all these parameters can be determined with fairly high accuracy. In the presence of a particle flow, it is also possible to determine the value of the spin of an elementary particle and its magnetic moment. This is done using the same experiment of beam splitting in a magnetic field, which was described on page 171.

It should be remembered that only charged particles are directly observed. All data on neutral particles and photons is obtained indirectly by studying the nature of the action of these invisible particles on charged ones. The data obtained about invisible particles, however, have a high degree of reliability.

An essential role in the study of all kinds of transformations of elementary particles is played by the application of the laws of conservation of momentum and energy. Since we are dealing with fast particles, when applying the law of conservation of energy, it is necessary to take into account the possible change in mass.

Let's assume that in the photograph there is a trace of particles in the form of a “fork”. The first particle turned into two particles: the second and the third. Then the following relations must be satisfied. Firstly, the momentum of the first particle must be equal to the vector sum of the momenta of the resulting particles:

where is the mass difference

The entire experience of nuclear physics shows that the laws of conservation are strictly satisfied in any transformation of elementary particles. This allows us to use these laws to determine the properties of a neutral particle that does not leave a trace in a photographic emulsion and does not ionize gas. If two diverging tracks are observed on a photographic plate, then it is clear to the researcher: at the point from which these tracks diverge, a transformation of a neutral particle has occurred. By determining the momenta, energies and masses of the resulting particles, one can draw confident conclusions about the value of the parameters of the neutral particle. This is how the neutron was discovered, and in this way we judge neutrinos and neutral mesons, which will be discussed below.

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Methods for detecting charged particles

Today it seems almost unbelievable how many discoveries in the physics of the atomic nucleus have been made using natural sources of radioactive radiation with energies of only a few MeV and simple detecting devices. The atomic nucleus was discovered, its dimensions were obtained, a nuclear reaction was observed for the first time, the phenomenon of radioactivity was discovered, the neutron and proton were discovered, the existence of neutrinos was predicted, etc. Main particle detector for a long time there was a plate with a layer of zinc sulphide applied to it. The particles were registered by eye by the flashes of light they produced in the zinc sulfide.

Over time, experimental setups became more and more complex. The technology of particle acceleration and detection and nuclear electronics were developed. Advances in nuclear and elementary particle physics are all in to a greater extent determined by progress in these areas. Nobel Prizes in physics are often awarded for work in the field of physical experimental techniques.

Detectors serve both to register the very fact of the presence of a particle and to determine its energy and momentum, the trajectory of the particle and other characteristics. To register particles, detectors are often used that are maximally sensitive to the detection of a particular particle and do not sense the large background created by other particles.

Usually in nuclear and particle physics experiments it is necessary to isolate “necessary” events from a gigantic background of “unnecessary” events, maybe one in a billion. To do this, various combinations of counters and registration methods are used.

Detection of charged particles is based on the phenomenon of ionization or excitation of atoms, which they cause in the detector substance. This is the basis for the work of such detectors as a cloud chamber, bubble chamber, spark chamber, photographic emulsions, gas scintillation and semiconductor detectors.

1. Geiger counter

A Geiger counter is, as a rule, a cylindrical cathode, along the axis of which a wire is stretched - the anode. System is full gas mixture. When passing through the counter, a charged particle ionizes the gas. The resulting electrons, moving towards the positive electrode - the filament, entering 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. A Geiger counter records the fact that a particle passes through the counter, but does not measure the energy of the particle.

2. Cloud chamber

A cloud chamber is a track detector of elementary charged particles, in which the track (trace) of a particle is formed by a chain of small droplets of liquid along the trajectory of its movement. Invented by Charles Wilson in 1912 (Nobel Prize 1927).

The operating principle of a cloud chamber is based on the condensation of supersaturated vapor and the formation of visible drops of liquid on ions along the trail of a charged particle flying through the chamber. To create supersaturated steam, rapid adiabatic expansion of the gas occurs using a mechanical piston. After photographing the track, the gas in the chamber is compressed again, and the droplets on the ions evaporate. The electric field in the chamber serves to “clean” the chamber of ions formed during the previous ionization of the gas. In a cloud chamber, tracks of charged particles become visible due to the condensation of supersaturated vapor on gas ions formed by the charged particle. Drops of liquid form on the ions, which grow to a size sufficient for observation (10 –3 -10 –4 cm) and photography in good lighting. The working medium is most often a mixture of water and alcohol vapor under a pressure of 0.1-2 atmospheres (water vapor condenses mainly on negative ions, alcohol vapor on positive ones). Supersaturation is achieved by rapidly reducing pressure due to expansion of the working volume. The capabilities of a cloud chamber increase significantly when placed in a magnetic field. Based on the trajectory of a charged particle curved by a magnetic field, the sign of its charge and momentum are determined. Using a cloud chamber in 1932, K. Anderson discovered a positron in cosmic rays.

3. Bubble chamber

Bubble Chamber– a track detector of elementary charged particles, in which the track (trace) of a particle is formed by a chain of vapor bubbles along the trajectory of its movement. Invented by A. Glaser in 1952 (Nobel Prize 1960).

The principle of operation is based on the boiling of superheated liquid along the track of a charged particle. The bubble chamber is a vessel filled with a transparent superheated liquid. With a rapid decrease in pressure, a chain of vapor bubbles is formed along the track of the ionizing particle, which are illuminated by an external source and photographed. After photographing the trace, the pressure in the chamber increases, the gas bubbles collapse and the camera is ready for use again. Liquid hydrogen is used as the working fluid in the chamber, which simultaneously serves as a hydrogen target for studying the interaction of particles with protons.

The cloud chamber and bubble chamber have the great advantage that all the charged particles produced in each reaction can be directly observed. To determine the type of particle and its momentum, cloud chambers and bubble chambers are placed in a magnetic field. The bubble chamber has a higher density of detector material compared to a cloud chamber and therefore the paths of charged particles are completely contained in the volume of the detector. Deciphering photographs from bubble chambers presents a separate, labor-intensive problem.

4. Nuclear emulsions

Similarly, as happens in ordinary photography, a charged particle along its path disrupts the structure of the crystal lattice of silver halide grains, making them capable of development. Nuclear emulsion is a unique means for recording rare events. Stacks of nuclear emulsions make it possible to detect particles of very high energies. With their help, it is possible to determine the coordinates of the track of a charged particle with an accuracy of ~1 micron. Nuclear emulsions are widely used to detect cosmic particles on sounding balloons and spacecraft.
Photographic emulsions as particle detectors are somewhat similar to cloud chambers and bubble chambers. They were first used by the English physicist S. Powell to study cosmic rays. A photographic emulsion is a layer of gelatin with silver bromide grains dispersed in it. Under the influence of light, latent image centers are formed in the grains of silver bromide, which contribute to the reduction of silver bromide to metallic silver when developed with a conventional photographic developer. The physical mechanism for the formation of these centers is the formation of metallic silver atoms due to the photoelectric effect. Ionization produced by charged particles gives the same result: a trail of sensitized grains appears, which, after development, can be seen under a microscope.

5. Scintillation detector

A scintillation detector uses the property of certain substances to glow (scintillate) when a charged particle passes through. The light quanta produced in the scintillator are then recorded using photomultiplier tubes.

Modern measuring installations in high-energy physics are complex systems, including tens of thousands of counters, complex electronics, and are capable of simultaneously recording dozens of particles produced in one collision.

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>> Methods for observing and recording elementary particles

Chapter 13. PHYSICS OF THE ATOMIC NUCLEUS

The expressions atomic nucleus and elementary particles have already been mentioned several times. You know that an atom consists of a nucleus and electrons. The atomic nucleus itself consists of elementary particles, neutrons and protons. The branch of physics that studies the structure and transformation of atomic nuclei is called nuclear physics. Initially, there was no division between nuclear physics and elementary particle physics. Physicists encountered the diversity of the world of elementary particles when studying nuclear processes. The separation of elementary particle physics into an independent field of study occurred around 1950. Today, there are two independent branches of physics: the content of one of them is the study of atomic nuclei, and the content of the other is the study of the nature, properties and mutual transformations of elementary particles.

§ 97 METHODS OF OBSERVATION AND REGISTRATION OF ELEMENTARY PARTICLES

First, let's get acquainted with the devices thanks to which the physics of the atomic nucleus and elementary particles arose and began to develop. These are devices for recording and studying collisions and mutual transformations of nuclei and elementary particles. They are the ones who give people necessary information about the microcosm.

The operating principle of devices for recording elementary particles. Any device that detects elementary particles or moving atomic nuclei is like a loaded gun with the hammer cocked. A small amount of force when pressing the trigger of a gun causes an effect that is not comparable to the effort expended - a shot.

A recording device is a more or less complex macroscopic system that may be in an unstable state. With a small disturbance caused by a passing particle, the process of transition of the system to a new, more stable state begins. This process makes it possible to register a particle. There are many currently in use various methods particle registration.

Depending on the purposes of the experiment and the conditions in which it is carried out, certain recording devices are used, differing from each other in their main characteristics.

Gas-discharge Geiger counter. The Geiger counter is one of the most important devices for automatic particle counting.

The counter (Fig. 13.1) consists of a glass tube coated on 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 operates based on impact ionization. A charged particle (electron, -particle, etc.), flying through a gas, removes electrons from atoms and creates positive ions and free electrons. The electric field between the anode and cathode (high voltage is applied to them) accelerates the electrons to energies at which impact ionization begins. An avalanche of ions occurs, and the current through the counter increases 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 hits it, the avalanche discharge must be extinguished. This happens automatically. Since at the moment the current pulse appears, the voltage drop across the load resistor R is large, the voltage between the anode and cathode decreases sharply - so much so that the discharge stops.

The Geiger counter is used mainly for recording electrons and -quanta (high-energy photons).

Currently, meters have been created that operate on the same principles.

Wilson chamber. Counters only allow you to register the fact of a particle passing through them and record some of its characteristics. In a cloud chamber, created in 1912, a fast charged particle leaves a trace that can be observed directly or photographed. This device can be called a window into the microworld, that is, the world of elementary particles and systems consisting of them.

The operating principle of a cloud chamber is based on the condensation of supersaturated vapor on ions to form water droplets. These ions are created along its trajectory by a moving charged particle.

A cloud chamber is a hermetically sealed vessel filled with water or alcohol vapor close to saturation (Fig. 13.2). When the piston sharply lowers, caused by a decrease in pressure under it, the vapor in the chamber expands adiabatically. As a result, cooling occurs and the steam becomes supersaturated. This is an unstable state of steam: it condenses easily if condensation centers appear in the vessel. Centers
condensation becomes ions, which are formed in the working space of the chamber by a flying particle. If a particle enters the chamber immediately after the steam expands, then water droplets appear on its path. These droplets form visible trace flying particle - track (Fig. 13.3). The chamber is then returned to its original state and the ions are removed electric field. Depending on the size of the camera, the time to restore the operating mode varies from several seconds to tens of minutes.

The information that tracks in a cloud chamber provide is much richer than what counters can provide. From the length of the track, you can determine the energy of the particle, and from the number of droplets per unit length of the track, its speed. The longer the particle's track, the greater its energy. And the more water droplets are formed per unit length of the track, the lower its speed. Particles with a higher charge leave a thicker track.

Soviet physicists P. L. Kapitsa and D. V. Skobeltsyn proposed placing a cloud chamber in a uniform magnetic field.

A magnetic field acts on a moving charged particle with a certain force (Lorentz force). This force bends the trajectory of the particle without changing the modulus of its velocity. The greater the charge of the particle and the lower its mass, the greater the curvature of the track. From the curvature of the track, one can determine the ratio of the particle's charge to its mass. If one of these quantities is known, then the other can be calculated. For example, from the charge of a particle and the curvature of its track, one can find the mass of the particle.

Bubble chamber. In 1952, the American scientist D. Glaser proposed using superheated liquid to detect particle tracks. In such a liquid, vapor bubbles appear on the ions (centers of vaporization) formed during the movement of a fast charged particle, giving a visible track. Chambers of this type were called bubble chambers.

In the initial state, the liquid in the chamber is under high pressure, protecting it from boiling, despite the fact that the temperature of the liquid is slightly higher than the boiling point at atmospheric pressure. With a sharp decrease in pressure, the liquid becomes overheated, and for a short time it will be in an unstable state. Charged particles flying at precisely this time cause the appearance of tracks consisting of vapor bubbles (Fig. 1.4.4). And the liquids used are mainly liquid hydrogen and propane. The operating cycle of the bubble chamber is short - about 0.1 s.

The advantage of the bubble chamber over the Wilson chamber is due to the higher density of the working substance. As a result, the particle paths turn out to be quite short, and particles of even high energies get stuck in the chamber. This allows one to observe a series of successive transformations of a particle and the reactions it causes.

Cloud chamber and bubble chamber tracks are one of the main sources of information about the behavior and properties of particles.

Observing traces of elementary particles produces a strong impression and creates a feeling of direct contact with the microcosm.

Method of thick-layer photoemulsions. To detect particles, along with cloud chambers and bubble chambers, thick-layer photographic emulsions are used. The ionizing effect of fast charged particles on the emulsion of a photographic plate allowed the French physicist A. Becquerel to discover radioactivity in 1896. The photoemulsion method was developed by Soviet physicists L. V. Mysovsky, G. B. Zhdanov and others.

Photo emulsion contains large number microscopic crystals of silver bromide. A fast charged particle, penetrating the crystal, removes electrons from individual bromine atoms. A chain of such crystals forms a latent image. When developed, metallic silver is restored in these crystals and a chain of silver grains forms a particle track (Fig. 13.5). 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 photographic emulsion, the tracks are very short (about 10 -3 cm for -particles emitted by radioactive elements), but when photographing they can be enlarged.

The advantage of photographic emulsions is that the exposure time can be as long as desired. This allows rare events to be recorded. It is also important that due to the high stopping power of photoemulsions, the number of observed interesting reactions between particles and nuclei increases.

We have not talked about all the devices that record elementary particles. Modern instruments for detecting rare and short-lived particles are very sophisticated. Hundreds of people take part in their creation.

1. Is it possible to register uncharged particles using a cloud chamber?
2. What advantages does a bubble chamber have over a Wilson chamber!

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Methods for recording elementary particles

1) Gas-discharge Geiger counter

A Geiger counter is one of the most important devices for automatic particle counting.

The counter consists of a glass tube coated on 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 operates based on impact ionization. A charged particle (electron, £-particle, etc.), flying through a gas, removes electrons from atoms and creates positive ions and free electrons. The electric field between the anode and cathode (high voltage is applied to them) accelerates the electrons to an energy at which impact ionization begins. An avalanche of ions occurs, and the current through the counter increases 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 hits it, the avalanche discharge must be extinguished. This happens automatically. Since at the moment the current pulse appears, the voltage drop across the discharge resistor R is large, the voltage between the anode and cathode decreases sharply - so much so that the discharge stops.

A Geiger counter is used mainly for recording electrons and Y-quanta (high-energy photons). However, Y-quanta are not directly recorded due to their low ionizing ability. To detect them, the inner wall of the tube is coated with a material from which Y-quanta knock out electrons.

The counter registers almost all electrons entering 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 that is transparent to these particles.

2) Wilson chamber

The action of a cloud chamber is based on the condensation of supersaturated vapor on ions to form water droplets. These ions are created along its trajectory by a moving charged particle.

The device is a cylinder with a piston 1 (Fig. 2), covered with a flat glass lid 2. The cylinder contains saturated couples water or alcohol. The radioactive drug 3 being studied is introduced into the chamber, which forms ions in the working volume of the chamber. When the piston sharply lowers down, i.e. During adiabatic expansion, the steam cools and becomes supersaturated. In this state, the steam condenses easily. The centers of condensation become ions formed by a particle flying at that time. This is how a foggy trail (track) appears in the camera (Fig. 3), which can be observed and photographed. The track exists for tenths of a second. By returning the piston to its original position and removing the ions with an electric field, adiabatic expansion can be performed again. Thus, experiments with the camera can be carried out repeatedly.

If the camera is placed between the poles of an electromagnet, then the camera’s capabilities for studying the properties of particles expand significantly. In this case, the Lorentz force acts on the moving particle, which makes it possible to determine the value of the particle’s charge and its momentum from the curvature of the trajectory. Figure 4 shows possible option deciphering photographs of electron and positron tracks. Induction vector B magnetic field directed perpendicular to the drawing plane beyond the drawing. The positron deflects to the left, and the electron to the right.

3) Bubble Chamber

It differs from a cloud chamber in that the supersaturated vapors in the working volume of the chamber are replaced by superheated liquid, i.e. a liquid that is under pressure less than its saturated vapor pressure.

Flying through 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 liquid. With a sharp decrease in pressure, the boiling point of the liquid turns out to be less temperature environment.

The liquid becomes unstable (overheated) state. This ensures the appearance of bubbles along the path of the particle. Hydrogen, xenon, propane and some other substances are used as the working mixture.

4) Thick film emulsion method

To detect particles, along with cloud chambers and bubble chambers, thick-layer photographic emulsions are used. Ionizing effect of fast charged particles on photographic plate emulsion. The photographic emulsion contains a large number of microscopic crystals of silver bromide.

A fast charged particle, penetrating the crystal, removes electrons from individual bromine atoms. A chain of such crystals forms a latent image. When metallic silver appears in these crystals, the 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 photographic emulsion, the tracks are very short, but when photographing they can be enlarged. The advantage of photographic emulsion is that the exposure time can be as long as desired. This allows rare events to be recorded. It is also important that due to the high stopping power of the photoemulsion, the number of observed interesting reactions between particles and nuclei increases.