In 1912, a Scottish scientist named Charles Wilson invented the instrument needed to independently record the tracks of charged particles. The invention of the camera gave Wilson the opportunity in 1927 to be awarded the highest honor in the field of physics, the Nobel Prize.

Device structure

Fog camera, or otherwise called a cloud chamber, is considered to be a small container with a lid made of a material such as glass; at the very bottom of the chamber there is a piston.

The device is filled due to saturated vapor intake ether, alcohol, or ordinary water, they are first cleaned of dust and put into it: this is necessary so that the particles, while flying, are not retained by condensation centers located in water molecules.

After filling the chamber with vapor, the piston is lowered, then, due to the occurrence of adiabatic expansion, rapid cooling of the vapor occurs, which becomes supersaturated. Charged particles, passing through the entire chamber capacity, leave behind a trail of ion chains. The steam, in turn, condenses on the ions, leaving tracks - traces of particles.

Operating principle of the device

Due to the fact that in the space under study periodically occurs supersaturation with vapors of various condensation centers(ions accompanying the trail of a rapidly moving particle), small drops of liquid appear on them. The volume of these drops increases over time, and at the same time it becomes possible to record them; this is done by photographing them.

The source of the material being studied is either within the chamber or directly outside it. In the case when it is located outside the chamber, particles may fly into the small transparent window located on it. The sensitivity of the device in relation to the time interval can vary from 0.01 fractions of a second to 2 - 3 seconds, this time is necessary for the desired supersaturation of ion condensation.

Followed immediately clean the working volume of the chamber, this is done to restore her sensitivity. The Wilson chamber operates only in cyclic mode. One complete cycle is approximately equal to a minute.

Moving the fog chamber into a magnetic field can cause its personal capabilities to increase several times. This is due to the fact that such a medium is capable of bending the flight path of charged particles, which in turn determines their momentum along with the sign of the charge.

The most famous applications of the device

Using a cloud chamber in 1932, an experimental physicist from the United States named Carl David Anderson was able to determine the positron content of cosmic rays.

The first who came up with the idea of ​​placing a fog chamber in the region of the strongest magnetic field were Soviet physicists D.V. Skobeltsin and P.L. Kapitsa, which they did with great success in 1927, 15 years after its creation famous device. Soviet researchers determined, along with impulses, the signs of charges and such quantitative characteristics of particles as mass and speed, which became a sensational breakthrough in Soviet physics on a global scale.

Device Conversion

In 1948, a breakthrough occurred in the field of physics camera improvement Wilson, the author of a similar development was the English physicist Patrick Blackett, who received the Nobel Prize for his invention. A scientist has created a controlled version of a fog chamber. He installed special counters in the device that are recorded by the camera itself; they themselves “launch” the camera to observe actions of this kind.

The new improved Wilson chamber, operating in a similar mode, becomes more active, and there is a noticeable increase in its efficiency.

The controllability of the fog chamber, created by Blackett, helps ensure high speed in the area of ​​expansion of the gaseous medium, as a result of which it becomes possible for the camera to monitor the signal from external counters and further respond to it.

Wilson lived to see the transformation of his brainchild, he called the experiment successful and recognized the importance of using the version of the device presented by Patrick Blackett.

Device value

The Wilson chamber became a unique device for the first half of the 20th century, raising the prestige of physics throughout the scientific world. It allowed physicists to track traces of charged particles and present this discovery to the public.

pros

• The cloud chamber was the first instrument in the world that could trace the tracks of charged particles.
• This device is successfully used in a magnetic field.
• The cloud chamber played an important role in studying the structure of a huge number of substances (rubidium and so on).
• Using a fog chamber, physicists were able to study nuclear radiation and cosmic rays.

Minuses

• Taking into account the increase in pressure in the chamber, at the same time the time period required to measure the insensitivity of the device also increases; physicists call it dead time.
• The operation of a cloud chamber requires a pressure of 0.1 to 2 atmospheres; if higher pressure appears, then the operation of the device becomes difficult, which is directly related to fogging of the chamber glass; it must be constantly cleaned.
• The camera does not allow full automation of data.

Wilson chamber.

The Wilson chamber (Fig. 38.1) was invented by the Scottish physicist Charles Wilson in 1910–1912. and was one of the first instruments for recording charged particles. The operation of the camera is based on the property of condensation of water droplets on ions formed along the track (trace) of the particle. The advent of the cloud chamber not only made it possible to see the tracks of particles, but also made it possible to “recognize” these particles (charge, energy), and also provided a lot of new material, which served as the basis for some important discoveries.

Figure 38.1.

The operating principle of a cloud chamber is quite simple. It is known that if the partial pressure of water vapor exceeds its saturation pressure at a given temperature, fog and dew may form. Oversaturation indicator S is the ratio of partial pressure to saturation pressure at a given temperature. For spontaneous condensation of steam in clean air, high supersaturation rates are required ( S~ 10), but if there are foreign particles in the air that can serve as condensation centers, then the formation of microdroplets can begin at lower values S.

Particles produced during radioactive decay have sufficient energy to ionize a large number of gas molecules that make up the medium. The ions formed during the passage of particles effectively attract water molecules due to the asymmetry of the charge distribution in these molecules. Thus, a particle released during radioactive decay, flying through a supersaturated medium, should leave behind a trail of water droplets. It can be seen and photographed on a photographic plate in a cloud chamber.

A cloud chamber is a cylinder filled with alcohol and water vapor. The chamber has a piston, when quickly lowered due to adiabatic expansion, the temperature drops and the vapors acquire the ability to easily condense (supersaturation index 1< S< 10). Влетающие через отверстие в камере частицы вызывают ионизацию молекул среды, то есть появление туманного следа – трека частицы. Вследствие того, что частицы обладают разными энергиями, размерами и зарядами, треки от различных частиц выглядят по-разному. Например, трек электрона выглядит тоньше и прерывистей, чем трек, полученный при пролете значительно более массивной альфа-частицы.

The background radiation always present in the atmosphere remains invisible. Natural sources of radiation include cosmic rays, radioactive decay of rock elements, or even radioactive decay of elements in living organisms. The instrument, the cloud cloud chamber, is a relatively simple device that makes it possible to observe and record the passage of ionizing radiation. Essentially, the device allows for indirect observation of radiation emission within the confines of the environment. The design received its name cloud cloud chamber in honor of its inventor, Scottish physicist Charles Thomson Rhys Wilson.

Research at the beginning of the 20th century, carried out with the participation of the Cloud Chamber, culminated in the discovery of elementary particles:

• Positron
• Neutron
• Muon
• Kaon (K-meson)

There are different types of cloud cameras. A diffusion-type device is easier to make at home than other types. The diffusion type design contains a sealed container, the upper area of ​​which is heated and the lower area is cooled.

Wilson's device in its original design. A very simple design, but how many wonderful discoveries have been made thanks to this device

The cloud inside the container is formed from alcohol vapor (methanol, etc.). The heated upper area of ​​the chamber creates conditions for the evaporation of alcohol.

The resulting steam cools, falls down and condenses, ending up in the cold bottom area of ​​the container.

The volume of space between the top and bottom of the container is filled with a cloud of supersaturated vapor. When an energetic charged particle (radiation) passes through steam, that particle inevitably leaves an ionization trail.

Molecules of alcohol and water have the properties of polar elements, so they are attracted to ionized particles.

When alcohol and water molecules come close to ions in the region of supersaturated steam, droplet condensate is formed. The condensate path remains visible to the radiation source.

How to make a cloud chamber with your own hands

Making a cloud homemade camera requires the following materials and accessories:

1. Transparent glass (plastic) container with a lid.
2. Isopropyl alcohol (medicinal grade 99% alcohol).
3. Dry ice and ice tray.
4. Absorbent material.
5. Thick black paper.
6. Flashlight with high brightness.
7. Small medical heating pad.

An ordinary empty glass jar may well be a good container. Isopropyl alcohol is available from most pharmacies in the form of an analogue - medical alcohol.

Wilson's device diagram: 1 - cylindrical container; 2 — water tray; 3 — brass plunger; 4 — laboratory clamp; 5 - from the calibrator; 6 - from the pump; 7 - wooden block; 8 — mobile base; 9 - power supply; 10 - spherical vacuum container

The main thing is that medical alcohol is at least 99% density. Methanol can also be used for home projects, but be aware that this substance has a high level of toxicity.

The absorbent material can be successfully replaced by a sponge or a piece of felt. An LED flashlight is suitable for illumination.

Even the use of the flashlight function is not excluded. By the way, a phone camera is useful for photographing traces of radiation.

Setting up research instruments at home

The process of assembling the equipment begins with a piece of sponge, which is placed in the lower part of the jar. It is recommended to carefully adjust the material to the diameter of the jar so that the sponge rests against the walls and does not fall out if the jar is turned over.

Adding a small amount of plasticine or resin to the bottom of the jar will ensure that the sponge or felt is attached. Do not use adhesive tape or glue, as alcohol fumes will easily dissolve such materials.

Homemade device: 1 - dark room; 2 - glass container; 3 - medical heating pad; 4 - dry ice; 5 — flashlight beam; 6 — tray for dry ice; 7 - spongy material; 8 - alcohol vapor

The next step is to cut out a circle from thick black paper, similar to the shape of the circle in the inner area of ​​the lid that closes the jar. Use a cut out paper circle to cover the inside of the lid.

The paper insert is needed to eliminate the reflection effect. In addition, paper also works to some extent as an absorber.

To ensure guaranteed fastening, it is also wise to attach the paper insert using plasticine or resin. The lid modified in this way can be placed on the neck of the jar.

However, first there is (a can of) isopropyl alcohol. Filling is done taking into account the complete saturation of the sponge (or felt), but without obvious excess liquid.

The easiest way to achieve an accurate level is to pour alcohol until the liquid completely covers the sponge material. Then drain off the excess.

Technological process with camera

You will need a place where there are conditions for creating complete darkness (for example, a spacious closet or a bathroom without windows). You need to place the dry ice in a pre-prepared tray.

Turn the glass jar (cloud homemade cloud chamber) upside down and place it on ice. Maintain in this position for about 10 minutes.

These are the fascinating pictures that appear inside the cloud chamber. Radiation is not only capable of killing all living things. She can also draw really well

After cooling for ten minutes, take a medical heating pad, fill it with hot water and place it on the top of a homemade Wilson cloud chamber (i.e. put it on the bottom of the jar).

The heating pad activates the process of alcohol evaporation. As a result, a cloud of alcohol-saturated vapor is formed. It's time to completely darken the room (or closet) where the research is being done.

All you have to do is turn on the flashlight and direct a beam of light through the walls of the created cloud chamber. Against the background of the alcohol cloud, traces of ionizing radiation will be clearly visible inside the can.

They can be easily photographed. And if you take a series of images, you can subsequently perform an appropriate analysis of the radiation level based on them.

About process safety

Despite the fact that isopropyl alcohol is considered safe compared to methanol, this substance causes toxicity when consumed internally. Alcohol is also a highly flammable substance.

These properties of isopropyl alcohol should be kept in mind. When performing research, it is recommended to keep the substance away from heat sources or open flames.

Dry ice in the process of sublimation is a colorful phenomenon. However, if such a process takes place in a sealed container, the container may explode due to the formation of high pressure

Dry ice also has dangerous properties. This, in some way, is capable of causing frostbite with direct prolonged contact. It is recommended to wear gloves when handling dry ice.

Also, do not store dry ice in an airtight container. The process of sublimation of solid dry ice into gas is accompanied by an increase in pressure. If this happens in a closed, sealed container, the vessel may rupture.

Practical exercises with a cloud chamber

If there is a radioactive source, you can place it next to the cloud chamber to see the clear radiation effect.

Researching radiation levels at home is an interesting and educational process. You can see a lot of interesting phenomena that cannot be seen in the usual way

Some everyday products and materials are radioactive. For example:

• Brazilian nut,
• bananas,
• cat litter,
• uranium glass.

A DIY cloud chamber allows you to explore radiation protection techniques. You can place all sorts of materials between a radioactive source and a homemade cloud chamber, thereby determining their resistance to radiation.

You can, for example, study the effect of a magnetic field by creating one within the boundaries of the cloud camera.

Positively charged and negatively charged particles form curved tracks in opposite directions when exposed to a field.

Cloud and bubble chambers

The bubble chamber is actually a related design from the group of radiation detectors. The operation of the device is based on the same principles that the Cloud Cloud Chamber uses.

Design of the bubble chamber: 1 - water buffer; 2—fluorocarbon C3F8; 3 — hydraulic fluid (propylene glycol); 4 — acoustic sensors; 5 - bellows; 6 - video cameras; 7 - pressure vessel

The only difference is that superheated liquid is used to operate the bubble chamber, rather than supersaturated steam. The device has a cylinder that is filled with liquid heated to a temperature just above its boiling point.

The most common substance is liquid hydrogen. Typically a magnetic field is applied to the bubble chamber.

Due to this addition, ionizing radiation travels along a spiral path, in accordance with its speed, charge and mass ratio.

Bubble chambers are usually larger than cloud chambers. This type of device is more complex to manufacture, but opens up wide possibilities for tracking more energetic elementary particles.

Video supplement to the topic of elementary particle research

WILSON CAMERA, track particle detector. Created by C. T. R. Wilson in 1912. In a cloud chamber, traces of charged particles become visible due to the condensation of supersaturated vapor on ions formed by a moving charged particle in the gas. Liquid droplets formed on the ions grow to large sizes, and with sufficiently strong lighting they can be photographed. Supersaturation is achieved by rapid (almost adiabatic) expansion of the mixture of gas and steam and is determined by the ratio of the pressure p 1 of steam to the pressure p 2 of saturated vapor at the temperature established after expansion. The amount of supersaturation required for the formation of droplets on ions depends on the nature of the vapor and the sign of the ion charge. Thus, water vapor condenses mainly on negative ions, ethyl alcohol vapor - on positive ones. In a Wilson chamber, a mixture of water and alcohol is more often used, in this case the required supersaturation p 1 / p 2 ≈1.62, which is the minimum of all possible values.

The particles under study can either be emitted by a source placed inside the chamber, or enter the chamber through a window transparent to them. The nature and properties of the particles under study can be determined from the path length and momentum of the particles. To measure the momenta of Wilson particles, the camera is placed in a magnetic field; To form secondary particles, plates of dense material are placed in the Wilson chamber, leaving gaps between them to observe traces of particles.

The Wilson chamber can be used in the so-called controlled mode, when it is activated by a trigger device that is triggered when the particle under study hits it. The total cycle time of a conventional Wilson chamber is ≥ 1 min. It consists of the time required for slow (purifying) expansion, the time required to stop the movement of the gas, and the time for the diffusion of vapor in the gas. High-power flash lamps are used as light sources when photographing particle tracks.

With the help of the Wilson camera, a number of discoveries were made in nuclear physics and elementary particle physics. The most striking of them are associated with the study of cosmic rays: the discovery of extensive air showers (1929), the positron (1932), the discovery of traces of muons, the discovery of strange particles. In the 1950s and 60s, the Wilson chamber was almost completely replaced by the bubble chamber, which was faster and therefore more suitable for use in modern charged particle accelerators.

Lit.: Das Gupta N., Ghosh S. Cloud chamber and its applications in physics. M., 1947; Wilson J. Wilson chamber. M., 1954; Principles and methods of registration of elementary particles. M., 1963.

As we have seen, radioactive radiation has ionizing and photographic effects. Both of these actions are characteristic of both fast charged particles and X-rays, which are electromagnetic waves. To find out whether radioactive radiation has a charge, it is enough to expose it to an electric or magnetic field.

Consider the following experiment. A radioactive drug 1 (for example, a grain of radium) is placed in the evacuated box (Fig. 377, a) in front of a narrow gap in the lead partition 2. Let's install photographic plate 3 on the other side of the slit. After development, we will see a black stripe on it - a shadow image of the slit. The lead partition therefore blocks radioactive rays; and they pass in the form of a narrow beam through the slit. Let us now place the box between the poles of a strong magnet (Fig. 377, b) and again install the photographic plate in position 3. Having developed the plate, we will find on it not one, but three stripes, of which the middle one corresponds to the rectilinear propagation of the beam from the preparation through the slit.

Rice. 377. Deflection of radioactive radiation by a magnetic field: a) trajectories of rays in a magnetic field (the dashed circle is the projection of the magnet poles; the field lines are directed towards us from beyond the plane of the drawing); c) a sheet of paper with thickness completely absorbs radiation, 1 - a radioactive drug, 2 - a lead screen, 3 - a photographic plate, 4 - a sheet of paper with thickness

Thus, in a magnetic field, a beam of radioactive radiation is divided into three components, two of which are deflected by the field in opposite directions, and the third does not experience deflection. The first two components are streams of oppositely charged particles. Positively charged particles are called particles or radiation. Negatively charged particles are called particles or radiation. The magnetic field deflects particles incomparably weaker than particles. The neutral component, which does not experience deviation in the magnetic field, is called radiation.

radiations differ greatly from each other in properties, in particular in their ability to penetrate matter. To study the penetrating ability of radioactive radiation, you can use the same device (Fig. 377, c). We will place screens of increasing thickness between specimen 1 and the slit, take photographs in the presence of a magnetic field, and note at what screen thickness the traces of each type of ray will disappear.

It turns out that the trail of particles disappears first. the particles are completely absorbed by a sheet of paper with a thickness of about (Fig. 377, c; 378, a). The particle flow gradually weakens with increasing screen thickness and is completely absorbed when the aluminum screen is several millimeters thick (Fig. 378, 6). The most penetrating is radiation. The thickness of the aluminum layer almost does not weaken the radiation intensity.

Rice. 378. Absorption of radioactive radiation by matter

Substances with a high atomic number have a significantly greater absorption effect for radiation; in this respect, radiation is similar to x-rays. Thus, lead weakens the radiation beam by approximately two times (Fig. 378, c).

The difference in the properties of radiation is clearly manifested in the so-called Wilson chamber - a device for observing the paths of fast charged particles. A cloud chamber (Fig. 379) is a glass cylinder 1 with a glass lid in which a piston 2 can move. The volume of the cylinder above the piston is filled with air saturated with water (or alcohol) vapor. When the piston is suddenly lowered, the air in the chamber cools due to rapid expansion. Water vapor becomes supersaturated, i.e., conditions are created for steam condensation on condensation nuclei (see Volume I, § 300). Air ionization products can serve as condensation nuclei. The ions polarize water molecules and attract them towards themselves, thereby facilitating condensation. Dust particles can also serve as condensation nuclei, but when working with a cloud chamber, the air in it is thoroughly cleaned.

Rice. 379. Wilson chamber (simplified diagram): 1 – glass cylinder, 2 – piston, 3 – illuminator, 4 – camera. The air above the piston is saturated with water vapor

Let the steam in the chamber be in a state of supersaturation. A fast charged particle flying through the chamber leaves a chain of ions in its path. A droplet settles on each ion, and the particle's trajectory becomes visible as a foggy trail. By illuminating the foggy traces from the side with a strong lamp 3 (Fig. 379), you can photograph them through the transparent camera cover. Such photographs are shown in Fig. 380 and 381. Using this remarkable method, we have the opportunity to observe the flight path (trace) of a single particle. Fog trails do not exist in the chamber for long, as the air heats up, receiving heat from the walls of the chamber, and the droplets evaporate. To obtain new traces, it is necessary to remove the existing ions using an electric field, compress the air with a piston, wait until the air in the chamber, heated during compression, cools, and perform a new expansion.

Rice. 380. Traces and particles in a cloud chamber. Particles are emitted by a radioactive drug placed in the lower parts of the chamber: a) particles: chambers in a magnetic field directed perpendicular to the plane of the pattern; b) particles: the magnetic field is directed towards us

Rice. 381. Photograph of tracks in a cloud chamber placed in a magnetic field and irradiated with radiation. At the top - location of the source: 1 - radioactive drug, 2 - lead screen with a slit, - radiation beam

The value of a cloud chamber as a physical instrument increases significantly if it is placed in a magnetic field, as Soviet physicists Pyotr Leonidovich Kapitsa (1894-1984) and Dmitry Vladimirovich Skobeltsyn (b. 1892) did. The magnetic field bends the trajectories of particles (Fig. 380). The direction of the bend of the trace allows one to judge the sign of the particle's charge; By measuring the radius of the trajectory, you can determine the speed of the particle if its mass and charge are known (see § 198).

The length of traces of particles in air at atmospheric pressure is about and much less than the length of traces of most particles. Particle traces are much fatter than particle traces, which indicates a lower ionizing ability of the latter.

In Fig. 381 shows a cloud chamber placed in a magnetic field and irradiated by a radiation source. The radiation beams are not deflected by the magnetic field, and their trajectories in the chamber must be straight lines emanating from the source. There are no such linear marks in the photograph. Consequently, radiation does not leave a continuous chain of ionized atoms in its path. The effect of radiation on matter comes down to the rare knocking out of electrons from atoms, to which high speed is imparted due to the energy of the quanta; these electrons then produce ionization of the atoms of the medium. The trajectories of such electrons, bent by a magnetic field, are visible in Fig. 381. Most of the electrons come from the walls of the chamber.

Let us note in conclusion that most radioactive substances emit only one kind of particle - either particles or particles. The emission of particles is often (but not always) accompanied by the emission of radiation.