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The methods of observation and registration of the charged particles

In the beginning of XX century. approaches have been developed to study the phenomenon of atomic physics and created the devices, it possible not only to find out the main problems of the structure of atoms, but watch the transformation of chemical elements.

The difficulty of creating such devices was, that used in experiments with charged particles are ionized atoms of any elements or, for example, electrons, and the device should register a hit only one particle or make visible the trajectory of its movement.

As one of the first and simplest of the devices for registration of particles was used for screen, covered lyuminestsiruyushchikh composition. At that point of the screen, where does a particle with sufficiently high energy, there is a flash — scintillate (from the Latin "scintillation" — sparkle, flash).

The first main instrument for registration of the particles was invented in 1908 g. G. The GM. After, as this device has been improved In. Muller, he could count the number of entering particles. The action of Geiger — Muller, and based on the fact, that passing through the gas ionize the charged particles encountered on their path the atoms of gas: negatively charged particle, pushing electrons, knocks them out of atoms, a positively charged particle attracts the electrons and pulls them out of atoms.

The meter consists of a hollow metal cylinder, with a diameter of about 3 cm (rice. 37.1), with a window of thin glass or aluminum. Swap the cylinder which is insulated from the walls of metallic yarn. Cylinder (camera) is filled with a rarefied gas, for example, argon. Between the walls of the cylinder and the filament generates a voltage of the order of 1500 In, insufficient to form an independent discharge. The filament is grounded through a large resistance R. When you enter the chamber, the particles with high energy ionization of the gas atoms in the path of this particle, and between the walls and thread the discharge occurs. The discharge current creates a large voltage drop across the resistance R, and the voltage between the filament and the walls is greatly reduced. Therefore, the discharge quickly ceases. After the termination of the current voltage may be again concentrated between the walls of the chamber and thread, and the counter is prepared for this new particle. Voltage with resistance R is fed to the input of amplifying tubes, in the anode circuit which switches on the counting mechanism.

The ability of high energy particles to ionize the atoms of gas are used, and in one of the most remarkable devices of modern physics — the cloud chamber. In 1911 g. English scientist H. Wilson built the device, with the help of which you can see and photograph the trajectories of charged particles.

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The Cloud Chamber (rice. 37.2) consists of cylinder with piston; the upper part of the cylinder is made of transparent material. In the camera introduces a small amount of water or alcohol, and within it is formed a mixture of vapor and air. With the rapid lowering of the piston the mixture adiabatically expands and cools, therefore, the air in the chamber is supersaturated vapors.

If the air is cleaned from dust, then the transformation of excess vapor in the liquid is difficult due to the lack of centres of condensation. However, the centres of condensation can serve and ions. So, if the camera flies through at this time, a charged particle, ionizing on its way air molecules, the chain ion vapour condensation takes place and the trajectory of the particles inside the chamber is obtained is marked by a thread of mist, t. e. becomes visible. The thermal motion of the air quickly erodes the threads of fog, and the particle trajectories are visible clearly only about 0,1 with, what, however, enough to take pictures.

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View of the trajectory on the photograph often gives an indication of the nature of the particles and the size of its energy. So, alpha particles leave a relatively thick solid trace, protons are more subtle, while electrons dotted trail. One of the photos of the alpha particles in the cloud chamber is shown in figure. 37.3.

To prepare the camera for action and clear it from the remaining ions, inside it create an electric field, attracting ions to the electrodes, where they are neutralized.

As mentioned above, in the cloud chamber to receive the particle tracks is used, the condensation of supersaturated vapor, t. e. its transformation into a liquid. For this purpose you can use the opposite, t. e. the transformation of liquid to vapor. If the liquid be enclosed in a closed vessel with a piston and with the piston to create high blood pressure, and then a sharp movement of the piston to reduce the pressure in the liquid., it is at the proper temperature, the liquid may be in a superheated state. If using such liquid will pass charged particle, along its trajectory the liquid boils, because the liquid formed in the ions serve as centers of vaporization. In this case, the trajectory of the particle is marked with a chain of vapor bubbles, t. e. is made visible. Based on this principle the action of the bubble chamber.

In the study of particle tracks from high energy bubble chamber easier camera Wilson, since the motion in the fluid, the particle loses much more energy, than in Gaza. In many cases, this allows much more accurately determine the direction of motion of the particle and its energy. Currently there is a bubble chamber with a diameter of about 2 m. They are filled with liquid hydrogen. Particle tracks in a liquid hydrogen are very distinct.

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For registration of particles and to obtain their marks also serves as a method of thick plates. It is based on the, that flying through the emulsion particles are at grain silver bromide, so the trail left by the particles after development, the photographic plate becomes visible (rice. 37.4) and it can be studied with a microscope. That track was long enough, use of thick layers of emulsion.

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