home biology tutor How can neutral particles be detected? Physics of the atomic nucleus. Experimental methods for registration of elementary particles. A brief excursion into the theory of the structure of matter

How can neutral particles be detected? Physics of the atomic nucleus. Experimental methods for registration of elementary particles. A brief excursion into the theory of the structure of matter

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 counter 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 a gas, usually argon. The operation of the counter is based on impact ionization. A charged particle (electron, £-particle, etc.), flying through 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 an energy at which impact ionization begins. An avalanche of ions appears, and the current through the counter increases sharply. In this case, a voltage pulse is formed on 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 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, Y-quanta are not directly registered due to their low ionizing ability. 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 the electrons that enter it; as for Y-quanta, it registers approximately only one Y-quantum out of a hundred. Registration of heavy particles (for example, L-particles) is difficult, since it is difficult to make a sufficiently thin "window" transparent for these particles in the counter.

2) cloud chamber

The action of the cloud chamber is based on the condensation of supersaturated vapor on ions with the formation of 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 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. With a sharp lowering of the piston down, i.e. During adiabatic expansion, the vapor cools and becomes supersaturated. In this state, the vapor is easily condensed. The centers of condensation are the ions formed by the particle flying at this time. So a foggy trace (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 with an electric field, the 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 possibilities of the camera to study the properties of particles are greatly expanded. In this case, the Lorentz force acts on the moving particle, which makes it possible to determine the value of the particle charge and its momentum from the curvature of the trajectory. Figure 4 shows a possible variant of deciphering the photo of the electron and positron tracks. The induction vector B of the magnetic field is directed perpendicular to the plane of the drawing beyond the drawing. The positron deviates to the left, the electron to the right.

3) bubble chamber

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

Flying in such a liquid, the 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 lower than the ambient temperature.

The liquid goes into an unstable (overheated) state. This ensures the appearance of bubbles in the path of particle motion. Hydrogen, xenon, propane and some other substances are used as a working mixture.

The advantage of a bubble chamber over a cloud chamber is due to the greater 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 makes it possible to observe a series of successive transformations of the particle and the reactions it causes.

4) Method of thick-layer photographic emulsions

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

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

Lesson plan for physics in grade 11.

Topic: Methods of observation and registration of elementary particles.

The purpose of the lesson: to acquaint students with the devices with which the physics of atomic nuclei and elementary particles developed; the necessary information about the processes in the microworld was obtained precisely thanks to these devices.

During the classes

Checking homework by frontal survey

What was the contradiction between Rutherford's model of the atom and classical physics.

Bohr's quantum postulates.

9) Task. How much has the energy of the electron in the hydrogen atom changed when the atom emitted a photon with a wavelength of 4.86 ∙10-7m?

Solution. ∆Е = h ν; ν = c/λ; ∆E = h c /λ; ∆E=4.1 ∙10-19 J.

2. Learning new material

Recording device is a macroscopic system in an unstable position. For any perturbation caused by a passing particle, the system goes into a more stable position. The transition process makes it possible to register a particle. Currently, there are many devices for registration of elementary particles. Let's consider some of them.

A) Gas-discharge Geiger counter.

This instrument is used for automatic particle counting.

Explain the device of the counter using the poster. The operation of the counter is based on impact ionization.

A Geiger counter is used to register γ - quanta and electrons, the counter notices well and counts almost all electrons and only one out of a hundred γ - quantum.

Heavy particles are not counted by the counter. There are counters that work on other principles.

B)Wilson chamber.

The counter only counts the number of flying particles. The cloud chamber, designed in 1912, has a track (trail) left after the passage of the particle, which can be observed, photographed, studied.

Scientists called the cloud chamber a window into the microcosm.

Explain the device and principle of operation of the camera according to the poster. The action of the cloud chamber is based on the condensation of supersaturated vapor, which forms tracks of water droplets on the ions. The particle energy can be determined from the track length; by the number of droplets per unit length of the track, its speed is calculated; the track thickness determines the charge of the flying particle. By placing the camera in a magnetic field, we noticed the curvature of the track, which is the greater, the greater the charge and the smaller the mass of the particle. Having determined the charge of the particle and knowing the curvature of the track, its mass is calculated.

AT)bubble chamber.

The American scientist Glaser, in 1952, created a new type of chamber to study elementary particles. It was similar to the cloud chamber, but the working body was replaced in it; supersaturated vapors were replaced by a superheated liquid. A fast-moving particle, when moving through a liquid, formed bubbles on ions (since the liquid boiled) - the chamber was called a bubble chamber.

The high density of the working substance gives the advantage of the bubble chamber over the cloud chamber.

The particle paths in the bubble chamber are short, while the interactions are stronger and some of the particles get stuck in the working substance. As a result, it becomes possible to observe transformations of particles. Tracks are the main source of information about particle properties.

G)Method of thick-layer photographic emulsions.

The ionizing effect of charged particles on a photographic plate emulsion is used to study the properties of elementary particles along with a bubble chamber and a cloud chamber. A charged particle penetrates a photographic emulsion containing silver bromide crystals at high speed. Tearing off electrons, a latent image appears from some of the bromine atoms in the photographic emulsion. The particle track appears after the development of the photographic plate. The energy and mass of the particles are calculated from the length and thickness of the track.

There are many other devices and devices that register and study elementary particles.

3. Consolidation of the studied material.

1) What is a recording device?

2) The principle of operation of the Geiger counter; cloud chambers; bubble chamber, method of thick-layer photographic emulsions.

3) What are the advantages of a bubble chamber over a cloud chamber?

Let's summarize the lesson.

Homework: §98, rep, §97

Studying the effect of luminescent substances on photographic film, the French physicist Antoine Becquerel discovered an unknown radiation. He developed a photographic plate, on which in the dark for some time there was a copper cross covered with uranium salt. The photographic plate produced an image in the form of a distinct shadow of a cross. This meant that uranium salt spontaneously radiates. Becquerel was awarded the Nobel Prize in 1903 for his discovery of the phenomenon of natural radioactivity. RADIOACTIVITY is the ability of some atomic nuclei to spontaneously transform into other nuclei, while emitting various particles: Any spontaneous radioactive decay is exothermic, that is, it occurs with the release of heat.

ALPHA PARTICLE(a-particle) is the nucleus of a helium atom. Contains two protons and two neutrons. The emission of a-particles is accompanied by one of the radioactive transformations (alpha decay of nuclei) of certain chemical elements.
BETA PARTICLE –

electron emitted during beta decay. The flux of beta particles is one of the types of radioactive radiation with a penetrating power greater than that of alpha particles, but less than that of gamma radiation. GAMMA RADIATION (gamma quanta) - short-wave electromagnetic radiation with a wavelength of less than 2 × 10–10 m. Due to the short wavelength, the wave properties of gamma radiation are weak, and corpuscular properties come to the fore, and therefore its represent in the form of a stream of gamma quanta (photons). The time it takes for half of the initial number of radioactive atoms to decay is called the half-life. During this time, the activity of the radioactive substance is halved. The half-life is determined only by the type of substance and can take on different values - from several minutes to several billion years.

ISOTOPS- these are varieties of a given chemical element that differ in the mass number of their nuclei. The nuclei of isotopes of the same element contain the same number of protons, but a different number of neutrons. Having the same structure of electron shells, isotopes have almost the same chemical properties. However, the physical properties of isotopes can differ quite sharply. All three components of radioactive radiation, passing through the medium, interact with the atoms of the medium. The result of this interaction is the excitation or even ionization of the atoms of the medium, which in turn initiates the occurrence of various chemical reactions. Therefore, radioactive radiation has a chemical effect. If the cells of a living organism are exposed to radioactive radiation, then the reactions initiated by radioactive radiation can lead to the formation of substances that are harmful to the given organism and, ultimately, to the destruction of tissues. For this reason, the impact of radioactive radiation on living organisms is detrimental. Large doses of radiation can cause serious illness or even death. 3. Nuclear reactions

NUCLEAR REACTIONS are the transformations of atomic nuclei as a result of interaction with each other or with any elementary particles. For a nuclear reaction to occur, it is necessary that the colliding particles approach at a distance of about 10–15 m. Nuclear reactions obey the laws of conservation of energy, momentum, electric and baryon charges. Nuclear reactions can proceed both with the release and absorption of kinetic energy, and this energy is approximately 106 times greater than the energy absorbed or released in chemical reactions.

Discovery of the neutron by D. Chadwick in 1932

In 1932, the German physicist W. Heisenberg and the Soviet physicist D.D. Ivanenko was offered proton-neutron model of the atomic nucleus. According to this model, atomic nuclei consist of elementary particles - protons and neutrons.

Nuclear forces are very powerful, but decrease very quickly with increasing distance. They are a manifestation of the so-called strong interaction. A feature of nuclear forces is their short-range nature: they manifest themselves at distances of the order of the size of the nucleus itself. Physicists jokingly call nuclear forces "a hero with short arms." The minimum energy required for the complete splitting of the nucleus into individual nucleons is called the binding energy of the nucleus. This energy is equal to the difference between the total energy of free nucleons and the total energy of the nucleus. Thus, the total energy of free nucleons is greater than the total energy of the nucleus consisting of these nucleons.

Very accurate measurements made it possible to fix the fact that the rest mass of the nucleus is always less than the sum of the rest masses of its constituents.

slopes by a certain amount, called the mass defect. The specific binding energy characterizes the stability of nuclei. The specific binding energy is equal to the ratio of the binding energy to the mass number and characterizes the stability of the nucleus. The higher the specific binding energy, the more stable the core is. The plot of specific binding energy versus the number of nucleons in a nucleus has a weakly pronounced maximum in the range from 50 to 60. This suggests that nuclei with average mass numbers, such as iron, are the most stable. Light nuclei tend to merge, while heavy nuclei tend to separate.

Examples of nuclear reactions.

Chain nuclear reactions. Thermonuclear reactions are nuclear reactions between light atomic nuclei occurring at very high temperatures (~108 K and above). In this case, the substance is in a state of fully ionized plasma. The need for high temperatures is explained by the fact that for the fusion of nuclei in a thermonuclear reaction it is necessary that they approach a very small distance and fall into the sphere of action of nuclear forces. This approach is prevented by the Coulomb repulsive forces acting between like-charged nuclei. To overcome them, the nuclei must have a very large kinetic energy. After the start of the thermonuclear reaction, all the energy spent on heating the mixture is compensated by the energy released during the reaction.
4. Nuclear power.

The use of nuclear energy is an important scientific and practical task. A device that allows a controlled nuclear reaction to take place is called a nuclear reactor. The neutron multiplication factor in the reactor is maintained equal to unity by introducing or removing control rods from the reactor. These rods are made from a substance that absorbs neutrons well - from cadmium, boron or graphite.

The main elements of a nuclear reactor are: - nuclear fuel: uranium-235, plutonium-239; – neutron moderator: heavy water or graphite; - coolant for the removal of released energy; - nuclear reaction rate regulator: a substance that absorbs neutrons (boron, graphite, cadmium).

METHODS FOR OBSERVATION AND REGISTRATION OF ELEMENTARY PARTICLES

Geiger counter

Serves to count the number of radioactive particles ( mostly electrons).

It is a glass tube filled with gas (argon) with two electrodes inside (cathode and anode).
During the passage of a particle, impact gas ionization and an electric current is generated.

Advantages:
- compactness
- efficiency
- performance
- high accuracy (10000 particles/s).

Where is used:
- registration of radioactive contamination on the ground, in premises, clothing, products, etc.
- at radioactive materials storage facilities or with operating nuclear reactors
- when searching for deposits of radioactive ore (U, Th)

cloud chamber

Serves for observation and photography traces from the passage of particles (tracks).

The internal 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, supersaturated steam.
Moisture droplets condense along the path of the passage of the particle and a track is formed - a visible trace.
When the camera is placed in a magnetic field, the track can be used to determine energy, velocity, mass and charge of the particle.

The characteristics of a flying radioactive particle are determined by the length and thickness of the track, by its curvature in a magnetic field.
For example, an alpha particle gives a continuous thick track,
proton - thin track,
electron - dotted track.

bubble chamber

Cloud chamber variant

With a sharp decrease in the piston, the liquid under high pressure passes in an overheated state. With the rapid movement of the particle along the trail, vapor bubbles are formed, i.e. the liquid boils, the track is visible.

Advantages over cloud chamber:
- high density of the medium, hence short tracks
- particles get stuck in the chamber and further observation of the particles can be carried out
- more speed.

Method of thick-layer photographic emulsions

Serves for registration of particles
- allows you to register rare phenomena due to the long exposure time.

Photo emulsion contains a large number of microcrystals silver bromide.
Incoming particles ionize the surface of photographic emulsions. AgBr crystals disintegrate under the action of charged particles, and upon development, a trace from the passage of a particle, a track, is revealed.
By track length and thickness the energy and mass of the particles can be determined.

Remember the topic "Atomic Physics" for grade 9:

Radioactivity.
radioactive transformations.
The composition of the atomic nucleus. Nuclear forces.
Communication energy. mass defect.
Fission of uranium nuclei.
Nuclear chain reaction.
Nuclear reactor.
thermonuclear reaction.

Other pages on the topic "Atomic Physics" for grades 10-11:

WHAT DO WE KNOW ABOUT PHYSICS?

Niels Bohr said in 1961: "At every stage, A. Einstein challenged science, and were it not for these challenges, the development of quantum physics would have dragged on for a long time."
___

In 1943, Niels Bohr, fleeing the invaders, was forced to leave Copenhagen. Not risking taking with him one thing very valuable to him, he dissolved it in "aqua regia" and left the flask in the laboratory. After the liberation of Denmark, returning, he isolated from the solution what he had dissolved, and by his order a new one was created. Nobel medal.
__

In 1933, in the laboratory headed by Ernest Rutherford, a powerful accelerator for those times was built. The scientist was very proud of this installation and one day, showing it to one of the visitors, he remarked: “This thing cost us a lot. With this money you can support one graduate student for a whole year! But can any graduate student do in a year so many discoveries!»

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 counter 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).

2) Cloud chamber

3) Bubble chamber

Flying in such a liquid, the 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 lower than the ambient temperature.

4) Method of thick-layer photographic emulsions

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