Particle detectors. Physical principles of detection of elementary particles

"Real" particle detectors, like those at the Large Hadron Collider, cost millions of dollars and weigh hundreds of tons, but we'll try to make do with a much more modest budget.

We will need:

dry ice (about 80 rubles per kilogram, it is advisable to buy a foam plastic thermal container for another 300 rubles - otherwise everything you bought will evaporate too quickly). A lot of dry ice is not needed, a kilogram is enough;
isopropyl alcohol (costs 370 rubles per 0.5 liter, sold in radio equipment stores);
a piece of felt (sewing shop, about 150 rubles);
glue to stick the felt to the bottom of the container (“Moment”, 150 rubles);
a transparent container, such as a plastic aquarium with a lid (we bought a hard plastic food container for 1.5 thousand rubles);
stand for dry ice, it can be a photographic cuvette (found in the editorial kitchen);
flashlight.

So let's get started. First you need to glue a piece of felt to the bottom of the container and wait a few hours for the glue to dry. After that, the felt must be soaked in isopropyl alcohol (make sure that alcohol does not get into your eyes!). It is desirable that the felt is completely saturated with alcohol, the remainder of which must then be drained. Then pour dry ice on the bottom of the cuvette, close the container with a lid and place it in dry ice with the lid down. Now you need to wait for the air inside the chamber to be saturated with alcohol vapor.

The principle of operation of the cloud chamber (aka "fog chamber") is that even a very weak impact causes the saturated vapor of alcohol to condense. As a result, even the impact of cosmic particles causes the vapor to condense, and chains of microscopic droplets - tracks - are formed in the chamber.

You can watch the experiment on our video:

A few notes from experience: you should not buy too much dry ice - it will evaporate completely in less than a day even in their thermal container, and you are unlikely to find an industrial refrigerator. It is necessary that the lid of the transparent container be black, for example, you can close it from below with black glass. Tracks will be better seen on a black background. You need to look exactly at the bottom of the container, where a characteristic fog is formed, similar to drizzling rain. It is in this fog that particle tracks appear.

What tracks can be seen:

These are not cosmic particles. Short and thick tracks are traces of alpha particles emitted by atoms of the radioactive gas radon, which continuously seeps from the bowels of the Earth (and accumulates in unventilated rooms).

Long narrow tracks are left by muons, the heavy (and short-lived) relatives of electrons. They are born in multitudes upper layers atmosphere, when high-energy particles collide with atoms and give rise to whole showers of particles, mostly consisting of muons.

Curved trajectories are a sign of electrons or their antiparticles, positrons. They are also generated by cosmic rays, collide with air molecules and can move in zigzags.

If you saw tracks bifurcating, then you are lucky: you have witnessed the decay of one particle into two.

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On April 29, late in the evening (rescheduled for now), NASA launches the Cern detector into orbit elementary particles AMS-02. This detector was built for 10 years, its older "brothers" are already working with might and main at the Large Hadron Collider, that is, underground, and this one will fly into space! :)

Here is the cern press release, here is the live stream of the launch starting at 21:30 CET, cern twitter will also send reports. The launch and all subsequent work can be tracked on the experiment's website. In the meantime, I will briefly talk about the device and scientific tasks.

AMS-02 is a real elementary particle detector with (almost) all its attributes. Its size is 4 meters, weight is 8.5 tons. Of course, it cannot be compared with such a colossus as ATLAS, but for launching into space (and installation on the ISS) this is not enough.

If underground detectors register particles born during a man-made collision of protons and other particles, then AMS-02 will register cosmic rays - particles of very high energies that come to us from deep space, dispersed on "natural accelerators". Cosmic rays, of course, have been studied for a long time, almost a century, but many mysteries are still associated with them.

The most important task of the new detector is to measure the composition of cosmic rays with ultra-high accuracy. What is the proportion of antimatter in cosmic rays? How does it change with energy? Are there some new heavy stable particles (dark matter particles) in small quantities that cannot be born at colliders, but which the Universe was able to give rise to? Or maybe some subtle features in the energy spectrum of ordinary particles will indicate that they were produced by the decay of hitherto unknown superheavy particles?

AMS-02 will study these issues by registering the passage of cosmic ray particles through the detector material and measuring their momentum, velocity, energy release, and charge. The "window" of optimal sensitivity of the detector in terms of particle energy is from about 1 GeV to several TeV. This window covers the predictions of many models and also overlaps with the sensitivity windows of the detectors at the LHC. But unlike the Large Hadron Collider, here the universe itself acts as an accelerator, and this can have far-reaching consequences.

Subdetectors and subsystems AMS-02 ().

Just like classic ground (more precisely, underground) detectors, it contains several separate detection systems at once, measuring different characteristics particles. Only unlike them, AMS-02 does not peer "in", but "looks out"; it looks more like one segment of an advanced modern detector.

The device is briefly described on the site of the experiment. There are also track detectors that restore the trajectory, Cherenkov detectors that measure the speed of particles, electromagnetic calorimeters that measure the energy of particles, and other systems. Two different magnets will separate different charges at once (I lied). Will separate charges permanent magnet 0.125 Tesla neodymium alloy. And in addition, AMS-02 has something that underground detectors do not have - GPS sensors and a star tracking system :)

All this was built for 10 years, the cost is about 1.5 gigadollars. The AMS collaboration includes 56 institutions from 16 countries.

The main thing is that now this thing has successfully flown away. Tomorrow evening we will follow the launch!

Tens of thousands of elementary particles from space fly through our body every second - muons, electrons, neutrinos and so on. We do not feel and do not see them, but this does not mean that they do not exist. It doesn't mean they can't be fixed. We offer readers N+1 assemble a device with your own hands that will allow you to "see" this continuous cosmic rain.

"Real" particle detectors, like those at the Large Hadron Collider, cost millions of dollars and weigh hundreds of tons, but we'll try to make do with a much more modest budget.

We will need:

dry ice (about 80 rubles per kilogram, it is advisable to buy a foam plastic thermal container for another 300 rubles - otherwise everything you bought will evaporate too quickly). A lot of dry ice is not needed, a kilogram is enough;
isopropyl alcohol (costs 370 rubles per 0.5 liter, sold in radio equipment stores);
a piece of felt (sewing shop, about 150 rubles);
glue to stick the felt to the bottom of the container (“Moment”, 150 rubles);
a transparent container, such as a plastic aquarium with a lid (we bought a hard plastic food container for 1.5 thousand rubles);
stand for dry ice, it can be a photographic cuvette (found in the editorial kitchen);
flashlight.

You can watch the experiment on our video:

What tracks can be seen:

Symmetry Magazine

Long narrow tracks are left by muons, the heavy (and short-lived) relatives of electrons. They are produced in abundance in the upper atmosphere when high-energy particles collide with atoms and create entire showers of particles, mostly muons.

As in any physical experiment, when studying elementary particles, it is required first put experiment and then register his results. The accelerator is engaged in setting up the experiment (collision of particles), and the results of collisions are studied using elementary particle detectors.

In order to reconstruct the picture of the collision, it is necessary not only to find out which particles were born, but also to measure their characteristics with great accuracy, primarily the trajectory, momentum, and energy. All this is measured using different types of detectors, which surround the place of particle collision in concentric layers.

Elementary particle detectors can be divided into two groups: track detectors, which measure the trajectory of the particles, and calorimeters that measure their energies. Track detectors try to follow the movement of particles without introducing any distortion. Calorimeters, on the other hand, must completely absorb a particle in order to measure its energy. As a result, a standard layout of a modern detector arises: inside there are several layers of track detectors, and outside - several layers of calorimeters, as well as special muon detectors. General form a typical modern detector is shown in fig. one.

The structure and principle of operation of the main components of modern detectors are briefly described below. The emphasis is on some of the most general principles detection. For specific detectors operating at the Large Hadron Collider, see Detectors at the LHC.

Track detectors

Track detectors reconstruct the trajectory of the particle. They are usually located in the region of the magnetic field, and then the particle's momentum can be determined from the curvature of the particle's trajectory.

The work of track detectors is based on the fact that a passing charged particle creates an ionization trail - that is, it knocks out electrons from atoms in its path. In this case, the ionization intensity depends both on the type of particle and on the material of the detector. Free electrons are collected by electronics, the signal from which reports the coordinates of the particles.

Vertex detector

summit(microvertex, pixel) detector- This is a multilayer semiconductor detector, consisting of separate thin plates with electronics deposited directly on them. This is the innermost layer of detectors: it usually begins immediately outside the vacuum tube (sometimes the first layer is mounted directly on the outer wall of the vacuum tube) and occupies the first few centimeters in the radial direction. Silicon is usually chosen as a semiconductor material because of its high radiation resistance (the inner layers of the detector are exposed to huge doses of hard radiation).

Essentially, the vertex detector works in the same way as a digital camera sensor. When a charged particle flies through this plate, it leaves a trace in it - an ionization cloud several tens of microns in size. This ionization is read by the electronic element directly below the pixel. By knowing the coordinates of the intersection points of a particle with several consecutive pixel detector plates, it is possible to reconstruct the three-dimensional trajectories of the particles and trace them back inside the pipe. Through the intersection of such reconstructed trajectories at some point in space, vertex- the point at which these particles were born.

Sometimes it turns out that there are several such vertices, and one of them usually lies directly on the axis of collision of colliding beams (primary vertex), and the second one is at a distance. This usually means that protons collided at the primary vertex and immediately gave rise to several particles, but some of them managed to fly some distance before decaying into child particles.

In modern detectors, the vertex reconstruction accuracy reaches 10 microns. This makes it possible to reliably register cases when the secondary vertices are 100 microns away from the collision axis. It is precisely at such distances that various metastable hadrons fly off, which have a c- or b-quark in their composition (the so-called "enchanted" and "charming" hadrons). Therefore, the vertex detector is essential tool detector LHCb, whose main task will be to study these hadrons.

Semiconductors work on a similar principle. microstrip detectors, in which, instead of small pixels, the thinnest, but rather long strips of sensitive material are used. In them, ionization does not settle immediately, but shifts along the strip and is read at its end. The strips are designed in such a way that the speed of the charge cloud displacement along it is constant and that it does not blur. Therefore, knowing the moment when the charge arrives at the reading element, it is possible to calculate the coordinates of the point where the charged particle pierced the strip. The spatial resolution of microstrip detectors is worse than that of pixel detectors, but they can cover much more about large area, since they do not require such a large number reading elements.

Drift cameras

Drift cameras- These are gas-filled chambers that are placed outside the semiconductor track detectors, where the radiation level is relatively low and such a high accuracy of position determination is not required, as with semiconductor detectors.

A classic drift chamber is a tube filled with gas, inside which many very thin wires are stretched. It works like a vertex detector, but not on a flat plate, but in volume. All wires are under tension, and their arrangement is chosen in such a way that a uniform electric field. When a charged particle flies through a gas chamber, it leaves a spatial ionization trail. Under the influence electric field ionization (first of all, electrons) moves at a constant speed (physicists say "drifts") along the field lines towards the anode wires. Having reached the edge of the chamber, the ionization is immediately absorbed by the electronics, which transmits a signal pulse to the output. Since there are a lot of reading elements, the signals from them can be used to restore the coordinates of a passing particle, and hence the trajectory, with good accuracy.

Usually the amount of ionization that creates in gas chamber the passing particle is small. In order to increase the reliability of charge collection and registration and reduce the error in its measurement, it is necessary to amplify the signal even before it is registered by the electronics. This is done using a special network of anode and cathode wires stretched near the reading equipment. Passing near the anode wire, the electron cloud generates an avalanche on it, as a result of which the electronic signal is multiplied.

The stronger the magnetic field and the larger the dimensions of the detector itself, the stronger the particle trajectory deviates from a straight line, which means that the more reliably it is possible to measure its curvature radius and reconstruct the particle momentum from this. Therefore, to study reactions with particles of very high energies, hundreds of GeV and TeV, it is desirable to build larger detectors and use magnetic fields stronger. For purely engineering reasons, it is usually possible to increase only one of these values at the expense of the other. The two largest detectors at the LHC - ATLAS and CMS - just differ in which of these values is optimized. At the ATLAS detector larger sizes, but a smaller field, while the CMS detector has a stronger field, but in general it is more compact.

Time projection camera

A special type of drift chamber is the so-called time projection camera(VPK). In fact, the VPK is one large, several meters in size, cylindrical drift cell. In its entire volume, a uniform electric field is created along the axis of the cylinder. The entire swirling ionization trail that particles leave when flying through this chamber drifts uniformly to the ends of the cylinder, retaining its spatial shape. The trajectories are, as it were, “projected” onto the ends of the chamber, where a large array of reading elements registers the arrival of the charge. The radial and angular coordinates are determined by the sensor number, and the coordinate along the cylinder axis is determined by the time of signal arrival. Thanks to this, it is possible to restore a three-dimensional picture of the movement of particles.

Among the experiments running at the LHC, the ALICE detector uses the time-projection camera.

Roman Pots detectors

There is a special type of semiconductor pixel detectors that work directly inside the vacuum tube, in close proximity to the beam. They were first proposed in the 1970s by a research group from Rome and have since become known as Roman pots("Roman pots").

Roman Pots detectors have been designed to detect particles deviated by very small angles during a collision. Conventional detectors located outside the vacuum tube are unsuitable here simply because a particle emitted at a very small angle can fly for many kilometers inside the vacuum tube, turning along with the main beam and not escaping. In order to register such particles, it is necessary to place small detectors inside the vacuum tube across the beam axis, but without touching the beam itself.

To do this, at a certain section of the accelerating ring, usually at a distance of hundreds of meters from the point of collision of colliding beams, a special section of a vacuum tube with transverse "sleeves" is inserted. Small, several centimeters in size, pixel detectors are placed in them on mobile platforms. When the beam is just injected, it is still unstable and has large transverse vibrations. The detectors at this time are hidden inside the sleeves in order to avoid damage from a direct beam hit. After the beam stabilizes, the platforms move out of their arms and move the sensitive matrices of the Roman Pots detectors in close proximity to the beam, at a distance of 1-2 millimeters. At the end of the next accelerator cycle, before discarding the old beam and injecting a new one, the detectors are drawn back into their arms and wait for the next session of operation.

The pixel detectors used in Roman Pots differ from conventional vertex detectors in that they maximize the portion of the wafer surface occupied by the sensing elements. In particular, on the edge of the plate, which is closest to the beam, there is practically no insensitive "dead" zone ( "edgeless"-technology).

One of the experiments at the Large Hadron Collider, TOTEM, will just use several of these detectors. Several more similar projects are under development. The vertex detector of the LHCb experiment also carries some elements of this technology.

You can read more about these detectors in the CERN Courier article Roman pots for the LHC or in the technical documentation of the TOTEM experiment.

Calorimeters

Calorimeters measure the energy of elementary particles. To do this, put on the path of the particles thick layer dense substance (usually heavy metal - lead, iron, brass). A particle in it collides with electrons or atomic nuclei and as a result generates a stream of secondary particles - shower. The energy of the initial particle is distributed among all shower particles, so that the energy of each individual particle in this shower becomes small. As a result, the shower gets stuck in the thickness of the substance, its particles are absorbed and annihilated, and some, quite definite, fraction of the energy is released in the form of light. This flash of light is collected at the ends of the calorimeter by photomultipliers, which convert it into an electrical impulse. In addition, the shower energy can be measured by collecting ionization with sensitive plates.

Electrons and photons, passing through matter, collide mainly with electron shells atoms and generate an electromagnetic shower - a stream of a large number of electrons, positrons and photons. Such showers develop rapidly at shallow depths and are usually absorbed in a layer of matter several tens of centimeters thick. High-energy hadrons (protons, neutrons, pi-mesons and K-mesons) lose energy mainly due to collisions with nuclei. In this case, a hadron shower is generated, which penetrates much deeper into the thickness of matter than an electromagnetic one, and besides, it is wider. Therefore, in order to completely absorb a hadronic shower from a particle of very high energy, one or two meters of matter are required.

The difference between the characteristics of electromagnetic and hadron showers is used to the maximum in modern detectors. Calorimeters are often made two-layer: inside are located electromagnetic calorimeters, in which predominantly electromagnetic showers are absorbed, and outside - hadron calorimeters, which are "reached" only by hadron showers. Thus, calorimeters not only measure energy, but also determine the "type of energy" - whether it is of electromagnetic or hadronic origin. This is very important for correct understanding occurred at the center of the proton collision detector.

To register a shower by optical means, the material of the calorimeter must have scintillation properties. AT scintillator photons of one wavelength are absorbed very efficiently, leading to the excitation of the molecules of the substance, and this excitation is removed by emitting photons of lower energy. For the emitted photons, the scintillator is already transparent, and therefore they can reach the edge of the calorimetric cell. Calorimeters use standard, long-studied scintillators, for which it is well known what part of the energy of the initial particle is converted into an optical flash.

To effectively absorb showers, it is necessary to use as dense a substance as possible. There are two ways to reconcile this requirement with the requirements for scintillators. First, one can choose very heavy scintillators and fill the calorimeter with them. Secondly, it is possible to make a "puff" of alternating plates of a heavy substance and a light scintillator. There are also more exotic versions of the calorimeter design, for example, "spaghetti" calorimeters, in which many thin quartz fibers are embedded in a massive absorber matrix. A shower, developing along such a calorimeter, creates Cherenkov light in the quartz, which is output through the fibers to the end of the calorimeter.

The accuracy of restoring the energy of a particle in a calorimeter improves with increasing energy. For particles with energies of hundreds of GeV, the error is about a percent for electromagnetic calorimeters and a few percent for hadronic ones.

Muon chambers

A characteristic feature of muons is that they lose energy very slowly as they move through matter. This is due to the fact that, on the one hand, they are very heavy, therefore they cannot effectively transfer energy to electrons in a collision, and secondly, they do not participate in strong interaction, therefore they are weakly scattered by nuclei. As a result, muons can fly many meters of matter before they stop, penetrating where no other particles can reach.

This, on the one hand, makes it impossible to measure the energy of muons using calorimeters (after all, a muon cannot be completely absorbed), but on the other hand, it makes it possible to distinguish muons from other particles well. In modern detectors muon chambers located in the outermost layers of the detector, often even outside the massive metal yoke that creates a magnetic field in the detector. Such tubes measure not the energy, but the momentum of muons, and at the same time it can be assumed with good certainty that these particles are precisely muons, and not anything else. There are several varieties of muon chambers used for different purposes.

Particle identification

A separate issue is particle identification, that is, finding out what kind of particle flew through the detector. This would not be difficult if we knew the mass of the particle, but it is precisely this that we usually do not know. On the one hand, the mass can in principle be calculated using the formulas of relativistic kinematics, knowing the energy and momentum of the particle, but, unfortunately, the errors in their measurement are usually so large that they do not allow distinguishing, for example, a pi-meson from a muon due to their proximity wt.

In this situation, there are four main methods for identifying particles:

By response in different types calorimeters and muon tubes.
By energy release in track detectors. Miscellaneous particles produce different amounts of ionization per centimeter of path, and this can be measured by signal strength from track detectors.
Via Cherenkov counters. If a particle flies through a transparent material with a refractive index n at a speed greater than the speed of light in that material (that is, greater than c/n), then it emits Cherenkov radiation in strictly defined directions. If we take airgel as the detector substance (the typical refractive index n= 1.03), then the Cherenkov radiation from particles moving at a speed of 0.99 c and 0.995 c, will differ significantly.
Via time-of-flight cameras. In them, with the help of detectors with a very high temporal resolution, the time of flight of a particle in a certain section of the chamber is measured and its speed is calculated from this.

Each of these methods has its own difficulties and errors, so particle identification is usually not guaranteed to be correct. Sometimes a program for processing "raw" data from a detector can come to the conclusion that a muon flew through the detector, although in fact it was a pion. It is impossible to get rid of such errors completely. It remains only to carefully study the detector before operation (for example, using cosmic muons), find out the percentage of cases of incorrect identification of particles, and always take it into account when processing real data.

Requirements for detectors

Modern particle detectors are sometimes referred to as the "big brothers" of digital cameras. However, it is worth remembering that the operating conditions of the camera and the detector are fundamentally different.

First of all, all elements of the detector must be very fast and very precisely synchronized with each other. At the Large Hadron Collider, at peak performance, the clumps will collide 40 million times per second. In each collision, the birth of particles will occur, which will leave their “picture” in the detector, and the detector must not “choke” on this stream of “images”. As a result, in 25 nanoseconds, it is required to collect all the ionization left by flying particles, turn it into electrical signals, and clean the detector, preparing it for the next portion of particles. In 25 nanoseconds, particles fly only 7.5 meters, which is comparable to the size of large detectors. While ionization from passing particles is gathering in the outer layers of the detector, particles from the next collision are already flying through its inner layers!

The second key requirement for the detector is radiation resistance. Elementary particles flying away from the place of collision of bunches are real radiation, and very hard. For example, the expected absorbed dose of ionizing radiation that the vertex detector will receive during operation is 300 kilogray plus a total neutron flux of 5·10 14 neutrons per cm 2 . Under these conditions, the detector should work for years and still remain serviceable. This applies not only to the materials of the detector itself, but also to the electronics with which it is stuffed. It took several years to create and test fault-tolerant electronics that will work in such harsh radiation conditions.

Another requirement for electronics - low power output. Inside multi-meter detectors there is no free space - every cubic centimeter of volume is filled with useful equipment. The cooling system inevitably takes away the working volume of the detector - after all, if a particle flies right through the cooling tube, it simply will not be registered. Therefore, the energy release from the electronics (hundreds of thousands of separate boards and wires that take information from all components of the detector) should be minimal.

Additional literature:

K. Groupen. "Elementary particle detectors" // Siberian Chronograph, Novosibirsk, 1999.
Particle Detectors (PDF, 1.8 Mb).
Particle detectors // chapter from study guide B. S. Ishkhanov, I. M. Kapitonov, E. I. Kabin. “Particles and Nuclei. Experiment". M.: Publishing house of Moscow State University, 2005.
N. M. Nikityuk. Precision microapex detectors (PDF, 2.9 Mb) // ECHAYA, vol. 28, no. 1, pp. 191–242 (1997).

In ch. XXIII we got acquainted with the devices used to detect microparticles - a cloud chamber, a scintillation counter, a gas-discharge counter. Although these detectors are used in elementary particle studies, they are not always convenient. The fact is that the most interesting processes of interaction, accompanied by mutual transformations of elementary particles, occur very rarely. A particle must meet a lot of nucleons or electrons on its way for an interesting collision to occur. In practice, it must go through a path measured in tens of centimeters - meters in dense matter (on such a path, a charged particle with an energy of billions of electron volts loses only part of its energy due to ionization).

However, in a cloud chamber or a gas-discharge counter, the sensitive layer (in terms of a dense substance) is extremely thin. In connection with this, some other methods for detecting particles have been applied.

The photographic method proved to be very fruitful. In special fine-grained photographic emulsions, each charged particle crossing the emulsion leaves a trace, which, after developing the plate, is detected under a microscope in the form of a chain of black grains. By the nature of the trace left by a particle in a photographic emulsion, one can determine the nature of this particle - its charge, mass, and energy. The photographic method is convenient not only because thick materials can be used, but also because in a photographic plate, in contrast to a cloud chamber, traces of charged particles do not disappear soon after the passage of the particle. When studying rare events, records may be exposed long time; this is especially useful in cosmic ray studies. Examples of rare events captured in photographic emulsion are shown above in Fig. 414, 415; Fig. is especially interesting. 418.

Another remarkable method is based on the use of the properties of superheated liquids (see Volume I, § 299). When a very pure liquid is heated to a temperature even slightly above the boiling point, the liquid does not boil, as surface tension prevents the formation of vapor bubbles. The American physicist Donald Glaeser (b. 1926) noted in 1952 that a superheated liquid instantly boils when irradiated sufficiently intensely; the additional energy released in the traces of fast electrons created in the liquid by -radiation provides the conditions for the formation of bubbles.

Based on this phenomenon, Glaeser developed the so-called liquid bubble chamber. Liquid at high blood pressure heated to a temperature close to, but less than, the boiling point. Then the pressure, and with it the boiling point, decrease, and the liquid is superheated. A trace of vapor bubbles is formed along the trajectory of a charged particle crossing the liquid at this moment. With the right lighting, it can be captured by a camera. As a rule, bubble chambers are located between the poles of a strong electromagnet, the magnetic field bends the particle trajectories. By measuring the length of the particle track, the radius of its curvature, and the density of bubbles, it is possible to establish the characteristics of the particle. Now bubble chambers have reached a high level of perfection; work, for example, chambers filled with liquid hydrogen, with a sensitive volume of several cubic meters. Examples of photographs of traces of particles in a bubble chamber are shown in fig. 416, 417, 419, 420.

Rice. 418. Transformations of particles recorded in a stack of photographic emulsions irradiated with cosmic rays. At a point, an invisible fast neutral particle caused the splitting of one of the emulsion nuclei and formed mesons (a "star" of 21 tracks). One of the mesons, the -meson, having traveled a path around (only the beginning and end of the trace are shown in the photograph; with the magnification used in the photograph, the length of the entire trace would have been ), stopped at a point and decayed according to the scheme . -meson, the trace of which is directed downward, was captured by the nucleus at the point, causing its splitting. One of the fragments of splitting was the nucleus, which, by means of decay, turned into a nucleus, instantly disintegrating into two particles flying in opposite directions - in the picture they form a “hammer”. -meson, having stopped, turned into -muon (and neutrino) (point). The end of the -muon trace is given in the right upper corner drawing; the trace of the positron formed during the decay is visible.

Rice. 419. Formation and decay of -hyperons. In a hydrogen bubble chamber in a magnetic field and irradiated with antiprotons, the reaction . It occurred at the end point of the trail (see diagram at the top of the figure). Neutral lambda and anti-lambda hyperons, having flown a short distance without the formation of a trace, decay according to the schemes. The antiproton annihilates with the proton, forming two and two -meson-quantum on the proton; proton does not visible trace, since, due to the large mass, it does not receive sufficient energy when interacting with the -quantum