Devices for detecting charged particles are called detectors. There are two main types of detectors:

1) discrete(counting and determining the energy of particles): Geiger counter, ionization chamber, etc.;

2) track(making it possible to observe and photograph traces of particles in the working volume of the detector): cloud chamber, bubble chamber, thick-layer photographic emulsions, etc.

1. Gas-discharge Geiger counter. To register electrons and \(~\gamma\)-quanta (photons) of high energy, a Geiger-Muller counter is used. It consists of a glass tube (Fig. 22.4), the cathode K, a thin metal cylinder, is adjacent to the inner walls; Anode A is a thin metal wire stretched along the axis of the counter. The tube is filled with gas, usually argon. The counter is included in the recording circuit. A negative potential is applied to the body, and a positive potential is applied to the thread. A resistor R is connected in series with the counter, from which the signal is supplied to the recording device.

The counter operates based on impact ionization. Let a particle hit the counter and create at least one pair along its path: “ion + electron”. Electrons, moving towards the anode (filament), enter a field with increasing intensity (voltage between A and K ~ 1600 V), their speed rapidly increases, and on their way they create an ion avalanche (impact ionization occurs). Once on the thread, electrons reduce its potential, as a result of which current flows through resistor R. A voltage pulse appears at its ends, which enters the recording device.

A voltage drop occurs across the resistor, the anode potential decreases, and the field strength inside the meter decreases, as a result of which it decreases kinetic energy electrons. The discharge stops. Thus, the resistor plays the role of resistance, automatically extinguishing the avalanche discharge. Positive ions flow to the cathode within \(~t \approx 10^(-4)\) s after the start of the discharge.

A Geiger counter can detect 10 4 particles per second. It is used mainly for recording electrons and \(~\gamma\) quanta. However, \(~\gamma\) quanta are not directly detected due to their low ionizing ability. To detect them, the inner wall of the tube is coated with a material from which electrons are knocked out by \(~\gamma\) quanta. When registering electrons, the counter efficiency is 100%, and when registering \(~\gamma\) quanta - only about 1%.

Registration of heavy \(~\alpha\)-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 chamber uses the ability of high-energy particles to ionize gas atoms. The Wilson chamber (Fig. 22.5) is a cylindrical vessel with a piston 1. The upper part of the cylinder is made of transparent material; large number water or alcohol, for which the bottom of the vessel is covered with a layer wet velvet or cloth 2. A mixture forms inside the chamber saturated vapors and air. When quickly lowering piston 1 the mixture expands adiabatically, which is accompanied by a decrease in its temperature. Due to cooling, the steam becomes oversaturated.

If the air is cleared of dust particles, then condensation of steam into liquid is difficult due to the absence of condensation centers. However condensation centers ions can also serve. Therefore, if a charged particle flies through the chamber (entered through window 3), ionizing molecules along its path, then vapor condensation occurs on the chain of ions and the trajectory of the particle inside the chamber becomes visible thanks to the settled small droplets of liquid. The chain of liquid droplets formed forms a particle track. The thermal movement of molecules quickly blurs the particle track, and the particle trajectories are clearly visible for only about 0.1 s, which, however, is sufficient for photography.

The appearance of the track in a photograph often allows one to judge nature particles and size her energy. Thus, \(~\alpha\) particles leave a relatively thick continuous trace, protons leave a thinner one, and electrons leave a dotted one (Fig. 22.6). The emerging splitting of the track - a "fork" - indicates an ongoing reaction.

To prepare the chamber for action and clear it of remaining ions, an electric field is created inside it, attracting ions to the electrodes, where they are neutralized.

Soviet physicists P. L. Kapitsa and D. V. Skobeltsyn proposed placing the camera in a magnetic field, under the influence of which the trajectories of particles are bent in one direction or another depending on the sign of the charge. The radius of curvature of the trajectory and the intensity of the tracks determine the energy and mass of the particle (specific charge).

3. Bubble chamber. Currently in scientific research a bubble chamber is used. The working volume in the bubble chamber is filled with liquid under high pressure, protecting 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 remains in an unstable state for a short time. If a charged particle flies through such a liquid, then along its trajectory the liquid will boil, 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, i.e. is made visible. The liquids used are mainly liquid hydrogen and propane C 3 H 3 . The operating cycle time is about 0.1 s.

Advantage The bubble chamber in front of the cloud chamber is due to the higher density of the working substance, as a result of which the particle loses more energy than in a gas. The particle paths turn out to be shorter, and even high-energy particles get stuck in the chamber. This makes it possible to determine much more accurately the direction of motion of the particle and its energy, and to observe a series of successive transformations of the particle and the reactions it causes.

4. Thick film emulsion method developed by L.V. Mysovsky and A.P. Zhdanov.

It is based on the use of blackening of a photographic layer under the influence of fast charged particles passing through the photographic emulsion. Such a particle causes the decomposition of silver bromide molecules into Ag + and Br - ions and blackening of the photographic emulsion along the trajectory of movement, forming a latent image. When developed, metallic silver is reduced in these crystals and a particle track is formed. The length and thickness of the track is used to judge the energy and mass of the particle.

To study the tracks of particles that have very high energy and produce long tracks, a large number of plates are stacked.

A significant advantage of the photoemulsion method, in addition to ease of use, is that it gives permanent trace particles, which can then be carefully studied. This led to the widespread use of this method in the study of new elementary particles. By this method, with the addition of boron or lithium compounds to the emulsion, traces of neutrons can be studied, which, as a result of reactions with boron and lithium nuclei, create \(~\alpha\) particles that cause blackening in the layer of nuclear emulsion. Based on the traces of \(~\alpha\)-particles, conclusions are drawn about the speed and energies of the neutrons that caused the appearance of \(~\alpha\)-particles.

Literature

Aksenovich L. A. Physics in high school: Theory. Assignments. Tests: Textbook. allowance for institutions providing general education. environment, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsiya i vyhavanne, 2004. - P. 618-621.

Experimental methods and tools for particle research

Competition "I'm going to class"

G.G. Emelina,
school named after Hero of Russia I.V. Sarychev,
Korablino, Ryazan region.

Experimental methods and tools for particle research

Open lesson. 9th grade

Although the proposed topic, in accordance with the program, is studied in the 9th grade, the material will also be of interest for lessons in the 11th grade. – Ed.

Educational goals of the lesson: to familiarize students with devices for recording elementary particles, to reveal the principles of their operation, to teach them to determine and compare the speed, energy, mass, charge of elementary particles and their ratio by tracks.

Lesson outline

Carrying out homework, the guys remembered and found examples of unstable systems (see pictures) and ways to remove them from an unstable state.

I am conducting a frontal survey:

How to obtain supersaturated steam? (Answer: Increase the volume of the vessel sharply. In this case, the temperature will drop and the steam will become supersaturated.

What will happen to supersaturated steam if a particle appears in it? (Answer: It will be the center of condensation, and dew will form on it.)

How does a magnetic field affect the motion of a charged particle? (Answer: In a field, the speed of a particle changes in direction, but not in magnitude.)

What is the name of the force with which a magnetic field acts on a charged particle? Where is it headed? (Answer: This is the Lorentz force; it is directed towards the center of the circle.)

When explaining new material I use reference summary: big poster hanging with it at the board, copies for each student (they will take them home with them, put them in a notebook and return them to the teacher at the next lesson). I’m talking about a scintillation counter and a Geiger counter, trying to save time on working with photographs of tracks. I rely on children's knowledge of voltage in a circuit in a series connection. Sample text: “The simplest means of recording radiation was a screen covered with a luminescent substance (from the Latin lumen - light). This substance glows when a charged particle hits it, if the energy of this particle is sufficient to excite the atoms of the substance. In the place where the particle hits, a flash occurs - scintillation (from the Latin scintillatio - sparkling, sparkling). Such counters are called scintillation counters. The operation of all other devices is based on the ionization of atoms of matter by flying particles.

The first device for detecting particles was invented by Geiger and improved by Müller. A Geiger–Muller counter (records and counts particles) is a metal cylinder filled with an inert gas (for example, argon) with a metal thread inside isolated from the walls. A negative potential is applied to the cylinder body, and a positive potential is applied to the filament, so that a voltage of about 1500 V is created between them, high, but not sufficient to ionize the gas. A charged particle flying through the gas ionizes its atoms, a discharge occurs between the walls and the filament, the circuit is closed, current flows, and a voltage drop UR = IR is created across the load resistor with resistance R, which is removed by the recording device. Since the device and the resistor are connected in series (Uist = UR + Uarrib), then with an increase in UR, the voltage Uarrib between the cylinder walls and the thread decreases, and the discharge quickly stops, and the meter is ready for operation again.

In 1912, a cloud chamber was proposed, a device that physicists called an amazing instrument.

The student gives a 2-3 minute presentation, prepared in advance, showing the importance of the cloud chamber for studying the microworld, its shortcomings and the need for improvement. I briefly introduce the structure of the camera and show it so that students keep in mind when preparing their homework that the camera can be designed in different ways (in the textbook - in the form of a cylinder with a piston). Sample text: “The chamber is a metal or plastic ring 1, tightly closed at the top and bottom with glass plates 2. The plates are attached to the body through two (upper and lower) metal rings 3 with four bolts 4 with nuts. On the side surface of the chamber there is a pipe for attaching a rubber bulb 5. A radioactive drug is placed inside the chamber. The top glass plate has a transparent conductive layer on the inner surface. Inside the camera there is a metal annular diaphragm with a series of slits. It is pressed against the corrugated diaphragm 6, which is the side wall of the chamber’s working space and serves to eliminate vortex air movements.”

The student is given a safety briefing followed by an experiment that reveals how a cloud chamber works and demonstrates that solid particles or ions can be nuclei of condensation. The glass flask is rinsed with water and placed upside down in the tripod leg. Install the backlight. The opening of the flask is closed with a rubber stopper into which a rubber bulb is inserted. First, the bulb is slowly squeezed and then quickly released - no changes are observed in the flask. The flask is opened, a burning match is brought to the neck, closed again and the experiment is repeated. Now, as the air expands, the flask is filled with a thick fog.

I explain the principle of operation of a cloud chamber using the results of the experiment. I introduce the concept of a particle track. We conclude that particles and ions can be condensation centers. Sample text: “When the pear is quickly released (the process is adiabatic, because heat exchange with environment) the mixture expands and cools, so the air in the chamber (flask) becomes supersaturated with water vapor. But the vapors do not condense, because there are no condensation centers: no dust particles, no ions. After introducing soot particles from a match flame and ions into the flask when heated, supersaturated water vapor condenses on them. The same thing happens if a charged particle flies through the chamber: it ionizes air molecules on its way, vapor condensation occurs on the chain of ions, and the trajectory of the particle inside the chamber is marked by a thread of fog droplets, i.e. becomes visible. Using a cloud chamber, you can not only see the movement of particles, but also understand the nature of their interaction with other particles.”

Another student demonstrates an experiment with a cuvette.

A homemade cuvette with a glass bottom is installed on a device with a device for horizontal projection. Drops of water are applied to the glass of the cuvette with a pipette and the ball is pushed. On its way, the ball tears off “fragments” from the droplets and leaves a “track”. Similarly, in the chamber, the particle ionizes the gas, the ions become condensation centers and also “make a track.” The same experiment gives a clear idea of ​​the behavior of particles in a magnetic field. When analyzing the experiment, we fill in the empty spaces on the second poster with the characteristics of the movement of charged particles:

The longer the track, the greater the energy (energy) of the particle and the smaller the density of the medium.

The greater the (charge) of the particle and the smaller its (velocity), the greater the track thickness.

When a charged particle moves in a magnetic field, the track turns out to be curved, and the radius of curvature of the track is greater, the greater the (mass) and (speed) of the particle and the smaller its (charge) and (induction modulus) magnetic field.

The particle moves from the end of the track with the (larger) radius of curvature to the end with the (smaller) radius of curvature. The radius of curvature decreases as you move, because due to the resistance of the medium, the speed of the particle (decreases).

Then I talk about the disadvantages of a cloud chamber (the main one is the short range of particles) and the need to invent a device with a denser medium - a superheated liquid (bubble chamber), photographic emulsion. Their principle of operation is the same, and I suggest the children study it on their own at home.

I am working with photographs of the tracks on p. 242 tutorials on drawing. 196. The guys work in pairs. Finish the work on the remaining drawings of the house.

Let's summarize the lesson. We conclude that using the methods considered, only charged particles can be directly observed. Neutral ones are not possible, they do not ionize the substance and, therefore, do not produce tracks. I give ratings.

Homework: § 76 (G.Ya. Myakishev, B.B. Bukhovtsev. Physics-11. - M.: Education, 1991), No. 1163 according to the problem book by A.P. Rymkevich; LR No. 6 “Study of tracks of charged particles using ready-made photographs.” Formalize and learn OK.

ABOUT THE AUTHOR. Galina Gennadievna Emelina – teacher of the 1st qualification category, teaching experience 16 years. Actively speaks at meetings of the regional methodological association of physics teachers; More than once she gave good open lessons to physicists in the region and teachers at her school. She is loved and respected by her students.

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, the study of cosmic rays began - radiation that has exceptional penetrating power and comes to us from outer space. Particles interacting with matter cosmic radiation play the role of projectile particles. For a long time, cosmic ray research was in the most important way studying the interconvertibility of elementary particles and even, to some extent, by 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 that will be discussed now are equally applicable to the study of cosmic rays and particles resulting from the nuclear bombardment of 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 that mark the path of the particle are centers of vapor condensation - this is the reason for the occurrence of well 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. The 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 speed of the particle at a known ratio 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 switching on at the right moment are weakened by disadvantages: 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, located, in contrast to the spark chamber, at a large distance from each other, is detected by the 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. Along with 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 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 sulphide, 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 of transformation and interaction 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 the further fate of 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 “coincidence 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.

Studies using conventional photographic plates with thin emulsions are of little use for the purposes nuclear physics. 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.

Lesson Objectives

• Educational: give an idea of ​​the methods for recording charged particles, reveal the features of each method, identify the main patterns, study the application of the methods.
• Developmental: develop memory, thinking, perception, attention, speech through individual preparation for the lesson; develop skills in working with additional literature and Internet resources.
• Educational: develop educational motivation, cultivate patriotism through studying the contribution of domestic scientists to world science.

Lesson progress

І . Familiarize yourself with the theoretical material.

Theoretical information

Numerous methods for recording elementary particles and radiation have been developed to study nuclear phenomena. Let's look at some of them that are most widely used.

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, Υ-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.

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, J-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 working volume of the chamber is filled with gas, which contains saturated steam. When the piston quickly moves downwards, the gas in the volume expands adiabatically and cools, while becoming 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 camera in the form of a strip of fog (Fig. 3), which can be observed and photographed. The track exists for tenths of a second. Returning the piston to its original position and removing the ions 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 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. The induction vector B of the magnetic field is directed perpendicular to the drawing plane behind 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 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 pressure saturated vapors.

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 lower than the ambient temperature.

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) 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 makes it possible to record rare events. 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.

5) Scintillation method

A scintillation counter consists of a scintillator, a photomultiplier and electronic devices for amplification and counting of pulses. The scintillator converts energy ionizing radiation into visible light quanta, the magnitude of which depends on the type of particles and scintillator material. Visible light quanta, hitting the photocathode, knock out electrons from it, the number of which is increased many times by the photomultiplier. As a result, a significant pulse is formed at the output of the photomultiplier, which is then amplified and counted by a recalculation unit. Thus, due to energy a-or b-particles, g-quantum or other nuclear particle, a light flash-scintillation appears in the scintillator, which is then converted into a current pulse using a photomultiplier tube (PMT) and recorded.

II. Using theoretical material and Internet resources, fill out the table

Spinthariscope

Geiger counter

Wilson chamber

Bubble Chamber

2. Device

3. Particle information

4. Particle type

5. Benefits

6. Disadvantages

7. Physical laws

8. Operating principle

9. Discoveries made using the device

III. Do the lab

Subject: “Studying tracks of charged particles using ready-made photographs”

Target: identify a charged particle by comparing its track with the track of a proton in a cloud chamber placed in a magnetic field; evaluate the error of the experiment, systematize the information obtained from the analysis of tracks in photographs, form conclusions and conclusions.

Equipment: finished photograph of two tracks of charged particles. Track I is a proton, track II is a particle that needs to be identified.

Explanations

When performing this laboratory work it should be remembered that:

• the longer the track length, the higher the energy of the particle (and the lower the density of the medium);
• the greater the charge of the particle and the lower its speed, the greater the thickness of the track;
• When a charged particle moves in a magnetic field, its track turns out to be curved, and the radius of curvature of the track is greater, the greater the mass and speed of the particle and the smaller its charge and the modulus of magnetic field induction.
• the particle moved from the end of the track with a large radius of curvature to the end with a smaller radius of curvature (the radius of curvature decreases as it moves, since the particle speed decreases due to the resistance of the medium).

Work order

1. Check out the photograph of the tracks of two charged particles. (Track I belongs to the proton, track II to the particle that needs to be identified) (see Fig. 1).
2. Measure the radii of curvature of the tracks in their initial sections (see Fig. 2).

There will be an image here:

Table particle

Relative error,

6. Additional task.

a) In what direction did the particles move?

b) The length of the particle tracks is approximately the same. What does this mean?

c) How did the thickness of the track change as the particles moved? What follows from this?

There will be a file here: /data/edu/files/y1445085758.doc (Larissa Belova: Methods for recording charged particles)

Elementary particles can be observed thanks to the traces they leave when passing through matter. The nature of the traces allows us to judge the sign of the particle’s charge, its energy, momentum, etc. Charged particles cause ionization of molecules along their path. Neutral particles do not leave traces, but they can reveal themselves at the moment of decay into charged particles or at the moment of collision with any nucleus. Consequently, neutral particles are ultimately also detected by the ionization caused by the charged particles they generate.

Instruments used to detect ionizing particles are divided into two groups. The first group includes instruments that record the passage of a particle and, in addition, make it possible in some cases to judge its energy. The second group is formed by the so-called track devices, i.e. devices that make it possible to observe traces (tracks) of particles in matter.

Recording instruments include a scintillation counter, a Cherenkov counter, an ionization chamber, a gas-discharge counter, and a semiconductor counter.

1. Scintillation counter. A charged particle flying through a substance causes not only ionization, but also excitation of atoms. Returning to their normal state, the atoms emit visible light. Substances in which charged particles cause a noticeable flash of light (scintillation) are called phosphorus. The most commonly used phosphorus are (zinc sulphide activated by silver) and (sodium iodide activated by thallium).

The scintillation counter consists of phosphorus, from which light is supplied through a special light guide to a photomultiplier. The pulses obtained at the output of the photomultiplier are counted. The pulse amplitude, proportional to the flash intensity, is also determined. This provides additional information about the detected particles. For this type of counter, the detection efficiency for charged particles is 100%.

2. Cherenkov counter. The operating principle of this counter is discussed in paragraph 3.3.3. (p. 84). The purpose of the counters is to measure the energy of particles moving in matter at a speed exceeding the phase speed of light in a given medium. In addition, counters allow you to separate particles by mass. Knowing the angle of emission of radiation, it is possible to determine the speed of the particle, which, with a known mass, is equivalent to determining its energy. If the mass of the particle is unknown, then it can be determined by independent measurement of the particle energy.

Cherenkov counters are installed on spaceships for the study of cosmic radiation.

3. Ionization chamber is an electric capacitor filled with gas, to the electrodes of which is supplied constant voltage. The detected particle, entering the space between the electrodes, ionizes the gas. The voltage on the capacitor plates is selected so that all the formed ions, on the one hand, reach the electrodes without having time to recombine, and on the other hand, do not accelerate so much as to produce secondary ionization. Consequently, ions generated directly under the action of charged particles are collected on the plates: the total ionization current is measured or the passage of single particles is recorded. In the latter case, the camera works as a counter.

4. Gas discharge meter usually performed in the form of a gas-filled metal cylinder with a thin wire stretched along its axis. The cylinder serves as the cathode, the wire as the anode. In contrast to the ionization chamber, secondary ionization plays the main role in a gas-discharge counter. There are two types of gas-discharge counters: proportional counters and Geiger-Muller counters. In the first, the gas discharge is not self-sustaining, in the second, it is independent.

In proportional counters, the output pulse is proportional to the primary ionization, i.e., the energy of the particle flying into the counter. Therefore, these counters not only register the particle, but also measure its energy.

The Geiger-Muller counter in design and principle of operation does not differ significantly from the proportional counter, but it operates in the region of the current-voltage characteristic corresponding to a self-sustained discharge, i.e. in the region of high voltages, when the output pulse does not depend on primary ionization. This counter registers a particle without measuring its energy. To register individual pulses, the resulting independent discharge must be extinguished. To do this, a resistance is connected in series with the thread (anode) so that the discharge current generated in the meter causes a voltage drop across the resistance sufficient to interrupt the discharge.

5. Semiconductor counter. The main element of this counter is a semiconductor diode, which has a very small thickness of the working area (tenths of a millimeter). As a result, the counter cannot register high-energy particles. But it is highly reliable and can operate in magnetic fields, since for semiconductors the magnetoresistive effect (dependence of resistance on magnetic field strength) is very small.

To the number track devices include cloud chamber, diffusion chamber, bubble chamber and nuclear photographic emulsions.

1. Wilson chamber. This is the name of the device created by the English physicist Wilson in 1912. A path of ions laid by a flying charged particle becomes visible in a cloud chamber, because supersaturated vapor of a liquid condenses on the ions. The chamber is usually made in the form of a glass cylinder with a tightly fitting piston. The cylinder is filled with neutral gas saturated with water or alcohol vapor. With a sharp expansion of the gas, the vapor becomes supersaturated, and tracks of fog are formed along the trajectories of particles flying through the chamber, which are photographed from different angles. By appearance tracks, one can judge the type of particles flying by, their number and their energy. By placing the camera in a magnetic field, one can judge the sign of their charge by the curvature of the particle trajectories.

Wilson chamber for a long time was the only track-type device. However, it is not without its drawbacks, the main one being the small working hours, which is approximately 1% of the time spent preparing the camera for the next launch.

2. Diffusion The chamber is a type of Wilson chamber. Supersaturation is achieved by diffusion of alcohol vapor from the heated lid to the cooled bottom. A layer of supersaturated vapor appears near the bottom, in which flying charged particles create tracks. Unlike a cloud chamber, a diffusion chamber operates continuously.

3. Bubble camera. This device is also a modification of the Wilson chamber. The working substance is superheated liquid under high pressure. With a sudden release of pressure, the liquid is transferred to an unstable overheated state. A flying particle causes a sharp boiling of the liquid, and the trajectory turns out to be marked by a chain of vapor bubbles. The track, like in a cloud chamber, is photographed.

The bubble chamber operates in cycles. Its dimensions are the same as the dimensions of the cloud chamber. The liquid is much denser than vapor, which makes it possible to use the chamber to study long chains of creation and decay of high-energy particles.

4. Nuclear emulsions. When using this detection method, a charged particle passes through the emulsion, causing ionization of the atoms. After the emulsion is developed, traces of charged particles are detected in the form of a chain of silver grains. An emulsion is a denser medium than the vapor in a cloud chamber or the liquid in a bubble chamber, therefore the track length in the emulsion is shorter. (The track length in the emulsion corresponds to the track length in the cloud chamber.) The photoemulsion method is used to study ultra-high energy particles that are found in cosmic rays or produced in accelerators.

The advantages of counters and track detectors are combined in spark chambers, which combine the recording speed of counters with faster complete information about particles produced in chambers. We can say that the spark chamber is a set of counters. Information in spark chambers is provided immediately, without further processing. At the same time, particle tracks can be determined by the actions of many counters.