Methods for observing and recording elementary particles - Knowledge Hypermarket. Methods for observing and recording elementary particles Methods for observing and recording charged elementary particles

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. A charged particle (electron, alpha 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 accelerates the electrons to energies at which impact ionization begins. Operating principle 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.

Features In order for the counter to register the next particle that hits it, the avalanche discharge must be extinguished. This happens automatically. The counter records almost all the electrons that enter it; As for γ-quanta, it registers approximately only one γ-quantum out of a hundred. Registration of heavy particles (for example, α-particles) is difficult, since it is difficult to make a sufficiently thin “window” in the counter that is transparent to these particles.

Cloud chamber 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 microcosm, i.e. the world elementary particles and the systems consisting of them.

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

Features The length of the track can determine the energy of the particle, and the number of droplets per unit length of the track can be used to estimate 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. A cloud chamber can be placed in a uniform magnetic field. A magnetic field acts on a moving charged particle with a certain force. This force bends the particle's trajectory. 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.

Operating principle In the initial state, the liquid in the chamber is under high pressure, which prevents it from boiling, despite the fact that the temperature of the liquid is 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 this particular time cause the appearance of tracks consisting of vapor bubbles. The liquids used are mainly liquid hydrogen and propane.

Features The operating cycle of the vial 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.

Method of thick-layer photographic emulsions 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 method was developed by Soviet physicists L.V. Mysovsky, A.P. Zhdanov and others.

Principle of operation Photo 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 developed, metallic silver is reduced in these crystals 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.

Features Due to the high density of the photographic emulsion, the tracks are very short (on the order of cm for alpha particles emitted by radioactive elements), but when photographed they can be enlarged. The advantage of photographic emulsions is that the exposure time can be as long as desired. This allows you to register rare phenomena. It is also important that due to the high stopping power of photoemulsions, the number of observed interesting reactions between particles and nuclei.

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) Cloud 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 vapors of 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 a possible version of decoding 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 is less than the 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.

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.

4) Method of thick film emulsions

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.

Until now, in phenomena, each such particle behaves as a single whole. Elementary particles can transform into each other. Currently, four types of interactions between elementary particles are known: strong, electromagnetic, weak and gravitational (in descending order of intensity). Strong interaction. This type of interaction is otherwise called nuclear, since it provides communication...

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In laboratory experiments and astronomical observations. These constituent elements of cosmic microphysics have their own specifics, which we now move on to discuss. 4. Cosmic rays The development of elementary particle physics is closely connected with the study of cosmic radiation - radiation coming to Earth almost isotropically from all directions of outer space. Intensity measurements...

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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. Cosmic rays contain elementary particles of very different energies, including 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 streams of elementary particles that have equally high energy. There are different types of accelerators: betatron, cyclotron, linear accelerator.

Located near Geneva, the European Organization for Nuclear Research (CERN*) has the largest particle accelerator to date, built in a circular 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 4000 tons of metal will be cooled to a temperature of just 2° above absolute zero. As a result, a current of 1.8 million amperes will flow through the superconducting cables with almost no losses.

Particle accelerators are such grandiose structures that they are called pyramids of the 20th century.

* The abbreviation CERN comes from the French. Conseil Européen pour la Recherche Nucléaire (European Council for Nuclear Research). In Russian the abbreviation CERN is usually used.

Methods for recording elementary particles

1. Scintillation counters

Initially, luminescent screens were used to register elementary particles - screens coated with a special substance, a phosphor, capable of converting the energy they absorb into light radiation (luminesce). When an elementary particle hits such a screen, it gives a weak flash, so weak that it can only be observed in complete darkness. It was necessary to have a fair amount of patience and attention in order to sit in complete darkness and count for hours the number of flashes noticed.

In a modern scintillation counter, flashes are counted automatically. The counter consists of a scintillator, a photomultiplier and electronic devices for amplification and counting of pulses.

The scintillator converts the particle's energy into visible light quanta.

Light quanta enter a photomultiplier tube, which converts them into current pulses.

The pulses are amplified by an electrical circuit and automatically counted.

2. Chemical methods

Chemical methods are based on the fact that nuclear radiation is a catalyst for certain 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, releases approximately 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 travels at a speed where With is the speed of light in vacuum, and n– refractive index of the substance. Consequently, light moves slower in matter than in 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 occurs, which was discovered by Cherenkov in his time. Cherenkov radiation is detected by photomultipliers in the same way as in the scintillation method. The method allows you to register only fast, that is, high-energy, elementary particles.

The following methods not only allow you to register an elementary particle, but also see its trace.

5. Wilson chamber

Invented by Charles Wilson in 1912, and in 1927 he received a Nobel Prize. A cloud chamber is a very complex engineering structure. We present only a simplified diagram.

The working volume of the cloud chamber is filled with gas and contains water or alcohol vapor. When the piston moves down quickly, the gas cools sharply and the steam becomes supersaturated. When a particle flies through this space, creating ions along its path, then droplets of condensed vapor are formed on these ions. A trace 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 Glaeser in 1952)

The bubble chamber operates similarly to a cloud chamber. Only the working fluid is not supercooled steam, but superheated liquid (propane, liquid hydrogen, nitrogen, ether, xenon, freon...). A superheated liquid, like supercooled steam, is in an unstable state. A particle flying through such a liquid forms ions, on which bubbles immediately form. A liquid bubble chamber is more efficient than a gas cloud chamber. It is important for physicists not only to observe the track of a flying particle. It is important that within the observation region the particle collides with another particle. The picture of particle interaction is much more informative. By flying through a denser fluid, which has a high concentration of protons and electrons, the particle has a much greater chance of experiencing a collision.

7. Emulsion chamber

It was first used by Soviet physicists Mysovsky and Zhdanov. Photographic emulsion is made from gelatin. Moving through dense gelatin, the elementary particle undergoes frequent collisions. Due to this, the path of the particle in the emulsion is often very short and, after developing the photographic emulsion, it is studied under a microscope.

8. Spark chamber (inventor Cranshaw)

In the cell A 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 chamber IN, it creates an ionized trail. A spark jumps 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). Voltage is applied to the electrodes for a very short time in such a way that a real spark does not have time to develop. Only the rudiments of a spark - streamers - have time to appear.

10. Geiger counter

A Geiger counter is, as a rule, a cylindrical cathode, along the axis of which a 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 resulting electrons, moving towards the positive electrode - the filament, fall into the region of strong electric field, accelerate 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.

In this article we will help you prepare for a physics lesson (9th grade). Particle research is not an ordinary topic, but a very interesting and exciting excursion into the world of molecular nuclear science. Civilization was able to achieve such a level of progress quite recently, and scientists are still arguing whether humanity needs such knowledge? After all, if people are able to repeat the process of the atomic explosion that led to the emergence of the Universe, then perhaps not only our planet, but also the entire Cosmos will collapse.

What particles are we talking about and why study them?

Partial answers to these questions are provided by a physics course. Experimental methods particle studies are a way to see what is inaccessible to humans even using the most powerful microscopes. But first things first.

An elementary particle is a collective term that refers to particles that can no longer be split into smaller pieces. In total, physicists have discovered more than 350 elementary particles. We are most used to hearing about protons, neurons, electrons, photons, and quarks. These are the so-called fundamental particles.

Characteristics of elementary particles

All the smallest particles have the same property: they can interconvert under the influence of their own influence. Some have strong electromagnetic properties, others weak gravitational ones. But all elementary particles are characterized by the following parameters:

• Weight.
• Spin is the intrinsic angular momentum.
• Electric charge.
• Life time.
• Parity.
• Magnetic moment.
• Baryon charge.
• Lepton charge.

A brief excursion into the theory of the structure of matter

Any substance consists of atoms, which in turn have a nucleus and electrons. Electrons are like planets in solar system, each move around the core along its own axis. The distance between them is very large, on an atomic scale. The nucleus consists of protons and neurons, the connection between them is so strong that they cannot be separated by any known to science way. This is the essence of experimental methods for studying particles (briefly).

It’s hard for us to imagine, but nuclear communication exceeds all forces known on earth by millions of times. We know a chemical, nuclear explosion. But what holds protons and neurons together is something else. Perhaps this is the key to unraveling the mystery of the origin of the universe. This is why it is so important to study experimental methods for studying particles.

Numerous experiments led scientists to the idea that neurons consist of even smaller units and called them quarks. What is inside them is not yet known. But quarks are inseparable units. That is, there is no way to single out one. If scientists use an experimental method of studying particles in order to isolate one quark, then no matter how many attempts they make, at least two quarks are always isolated. This once again confirms the indestructible power of nuclear potential.

What methods of particle research exist?

Let's move directly to experimental methods for studying particles (Table 1).

Methods for recording 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 influence of a flying charged particle.

Geiger counter.

Geiger counter- a particle detector, the operation of which is based on the occurrence of an independent electrical discharge in a gas when a particle enters its volume. Invented in 1908 by H. Geiger and E. Rutherford, it was later improved by Geiger and Muller.

A 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 the order of 200-1000 V is applied between the cathode and the anode. A charged particle, having entered the volume of the counter, forms a certain number of electron-ion pairs, which move to the corresponding electrodes and at a high voltage at the mean free path (on the way to the next table). ionization) gain energy exceeding the ionization energy and ionize gas molecules. An avalanche is formed, the current in the circuit increases. From the load resistance, a voltage pulse is supplied 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.

A Geiger counter records 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 chamber.

Wilson chamber- track (from English. track— trace, trajectory) particle detector. Created by Charles 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 region of cosmic rays) in 1929, the positron in 1932, detection of traces of muons, discovery of strange particles. Subsequently, the Wilson chamber was practically replaced by the bubble chamber as a faster one. A cloud chamber is a vessel filled with water or alcohol vapor that is close to saturation (see figure). Its action is based on the condensation of supersaturated vapor (water or alcohol) on ions formed by a passing particle. Supersaturated steam will be created by a sharp lowering of the piston (see figure) (the steam in the chamber expands adiabatically, as a result of which its temperature sharply increases).

Droplets of liquid deposited on the ions make the trace of the flying particle visible - the track, which makes it possible to photograph it. 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, you can estimate its speed. Placing a camera in a magnetic field makes it possible to determine from the curvature of the track the ratio of the particle's charge to its mass (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 the particle.

The first bubble chamber (1954) was a metal chamber with glass windows for lighting and photography, filled with liquid hydrogen. Subsequently, it was created and improved in all laboratories in the world equipped with charged particle accelerators. From a cone with a volume of 3 cm 3 the size of the bubble chamber reached several cubic meters. Most bubble chambers have a volume of 1 m3. For the invention of the bubble chamber, Glaser was awarded the Nobel Prize in 1960.

The operating cycle of the vial chamber is 0.1. Its advantage over a cloud chamber is the higher density of the working substance, which makes it possible to register high-energy particles.