X-ray generator is an x-ray tube. A modern electronic tube is designed according to a single principle and has the following device. The base is a glass bulb in the form of a ball or a cylinder, into the end sections of which electrodes are soldered: anode and cathode. A vacuum is created in the tube, which facilitates the escape of electrons from the cathode and their fastest movement.

Cathode is a spiral made of tungsten (refractory) filament, which is fixed on molybdenum rods and placed in a metal cap that directs the electron flow in the form of a narrow beam towards the anode.
Anode made of copper (it gives off heat faster and cools relatively easily), has massive dimensions. The end facing the cathode is cut obliquely at an angle of 45-70 °. In the central part of the beveled anode, there is a tungsten plate on which the anode focus is located - an area of ​​10-15 mm2, where X-rays are mainly formed.

X-ray formation process... The filament of an X-ray tube - a tungsten coil of the cathode, when a low voltage current (4-15 V, 3-5A) is applied to it, heats up, forming free electrons around the filament. The inclusion of a high voltage current creates a potential difference at the poles of the X-ray tube, as a result of which free electrons rush to the anode at high speed in the form of a stream of electrons - cathode rays, which, once at the focus of the anode, are sharply decelerated, as a result of which part of the kinetic energy of electrons is converted into energy electromagnetic oscillations with a very short wavelength. This will be X-rays (braking rays).

At the request of the doctor and technique you can adjust both the quantity of X-rays (intensity) and their quality (hardness). By increasing the degree of heating of the tungsten filament of the cathode, an increase in the number of electrons can be achieved, which determines the intensity of X-rays. An increase in the voltage applied to the poles of the tube leads to an increase in the flight speed of the electrons, which is the basis of the penetrating quality of the beams.

It was already noted above that x-ray tube focus- this is the area on the anode where electrons enter and where they are generated. The size of the focus affects the quality of the X-ray image: the smaller the focus, the sharper and more structured the pattern, and vice versa, the larger it is, the more blurry the image of the object under study becomes.

Practice has proven the sharper focus, the faster the tube becomes unusable - the tungsten plate of the anode melts. Therefore, in modern devices, tubes are designed with several focuses: small and large, or linear in the form of a narrow strip with an anode bevel angle correction of 71 °, which makes it possible to obtain optimal image sharpness at the highest electrical load on the anode.

Successful X-ray tube design is a generator with a rotating anode, which allows you to focus on small dimensions and thereby lengthen the service life of the apparatus.

From the stream cathode rays only about 1% of the energy is converted into X-rays, the rest of the energy is converted into heat, which leads to overheating of the anode. For the purpose of cooling the anode, various methods are used: water cooling, air-heating, oil cooling under pressure and combined methods.

X-ray tube is placed in a special leaded case or a casing with a hole for X-ray radiation out of the tube anode. Filters made of various metals are installed on the way out of the X-ray radiation from the tube, which filter out soft rays and make the radiation of the X-ray apparatus more uniform.

In many designs X-ray machines transformer oil is poured into the case, which flows around the X-ray tube from all sides. All this: a metal case, oil, filters shield the office staff and patients from the effects of X-rays.

Having discovered the "- rays", Roentgen by careful experiments found out the conditions of their formation. He found that these rays arise in the place of the tube, where the flying electrons that make up the cathode beam are delayed, hitting the wall of the tube. Based on this circumstance, Roentgen designed and built a special tube suitable for obtaining X-rays. In its essential features, the design of the X-ray tube has survived to this day.

In fig. 302 depicts a modern X-ray tube. The cathode is a thick incandescent tungsten filament, which emits an intense stream of electrons (see Ref. II, § 100), which are accelerated by the applied electric voltage. The cathode is equipped with a tantalum cap that focuses the electrons, since the electrons are emitted perpendicular to the cathode surface. The target is a plate made of tungsten, platinum or other heavy metal pressed into the anode (mirror of the anode), which is made of red copper to remove heat. Striking the surface of the target, the electrons are trapped and give off X-rays. The voltage between the cathode and anode reaches several tens of thousands of volts. In order for the electrons to reach the target without hindrance, the X-ray tube is evacuated to a high vacuum. The anode is usually cooled with water.

Rice. 302. Modern X-ray tube; filament heating circuit not shown

Acting on gases, X-rays are capable of causing their ionization (see Vol. II, § 92). Thus, placing a charged electroscope near the X-ray tube, we find that it quickly discharges if the tube is activated (Fig. 303). The reason for the loss of charge by the electroscope is that the surrounding air is ionized by the action of X-rays and becomes a conductor. The ionizing effect of X-rays is also used to detect and register them.

Rice. 303. Ionizing effect of X-rays: 1 - X-ray tube, 2 - electroscope. The experiment is successful with both positively and negatively charged electroscope. Ions of both signs are created in the air under the action of X-rays.

X-rays, invisible radiation capable of penetrating, albeit to varying degrees, into all substances. It is electromagnetic radiation with a wavelength of the order of 10-8 cm.

Like visible light, X-rays cause blackening of photographic film. This property is important for medicine, industry and scientific research. Passing through the object under study and then falling onto the photographic film, X-ray radiation depicts its internal structure on it. Since the penetrating power of X-ray radiation is different for different materials, parts of the object that are less transparent to it give lighter areas in the photograph than those through which the radiation penetrates well. Thus, bone tissue is less transparent to X-rays than the tissue that makes up the skin and internal organs. Therefore, on the X-ray, the bones will be indicated as lighter areas and the fracture site, which is more transparent for radiation, can be quite easily detected. X-rays are also used in dentistry to detect caries and abscesses in the roots of teeth, and in industry to detect cracks in moldings, plastics and rubbers.

X-rays are used in chemistry to analyze compounds and in physics to study the structure of crystals. A beam of X-rays passing through a chemical compound causes a characteristic secondary radiation, the spectroscopic analysis of which allows the chemist to determine the composition of the compound. When falling on a crystalline substance, the X-ray beam is scattered by the atoms of the crystal, giving a clear, correct pattern of spots and stripes on the photographic plate, which makes it possible to establish the internal structure of the crystal.

The use of X-rays in cancer treatment is based on the fact that it kills cancer cells. However, it can have undesirable effects on normal cells as well. Therefore, extreme care must be taken when using X-rays in this manner.

Receiving X-rays

X-ray radiation occurs when electrons moving at high speeds interact with matter. When electrons collide with atoms of any substance, they quickly lose their kinetic energy. In this case, most of it turns into heat, and a small fraction, usually less than 1%, is converted into X-ray energy. This energy is released in the form of quanta - particles called photons that have energy but whose rest mass is zero. X-ray photons differ in their energy, which is inversely proportional to their wavelength. The conventional X-ray production method produces a wide range of wavelengths called the X-ray spectrum.

X-ray tubes. To obtain X-ray radiation due to the interaction of electrons with matter, you need to have a source of electrons, means of accelerating them to high speeds and a target that can withstand electron bombardment and produce X-rays of the required intensity. The device that contains all of this is called an X-ray tube. Early researchers used "deep evacuated" tubes of the type of modern gas-discharge tubes. The vacuum in them was not very high.

Gas discharge tubes contain a small amount of gas, and when a large potential difference is applied to the electrodes of the tube, the gas atoms are converted into positive and negative ions. Positive ones move to the negative electrode (cathode) and, falling on it, knock out electrons from it, and they, in turn, move to the positive electrode (anode) and, bombarding it, create a stream of X-ray photons.

In the modern X-ray tube developed by Coolidge (Fig. 11), the source of electrons is a tungsten cathode heated to a high temperature.

Rice. eleven.

Electrons are accelerated to high speeds by the high potential difference between the anode (or anti-cathode) and the cathode. Since the electrons must reach the anode without colliding with the atoms, a very high vacuum is required, for which the tube must be well evacuated. This also reduces the probability of ionization of the remaining gas atoms and the resulting side currents.

When bombarded with electrons, the tungsten anti-cathode emits characteristic X-rays. The cross section of the X-ray beam is smaller than the actual irradiated area. 1 - electron beam; 2 - cathode with focusing electrode; 3 - glass shell (tube); 4 - tungsten target (anti-cathode); 5 - filament of the cathode; 6 - actually irradiated area; 7 - effective focal spot; 8 - copper anode; 9 - window; 10 - scattered X-ray radiation.

The electrons are focused on the anode using a specially shaped electrode that surrounds the cathode. This electrode is called focusing and together with the cathode forms the "electron spotlight" of the tube. The electron bombarded anode must be made of a refractory material, since most of the kinetic energy of the bombarding electrons is converted into heat. In addition, it is desirable that the anode be made of a material with a high atomic number, since the X-ray yield increases with increasing atomic number. Tungsten is most often chosen as the anode material, the atomic number of which is 74. The design of X-ray tubes can be different depending on the conditions of use and the requirements.

Ministry of Education and Science of the Russian Federation

federal state autonomous educational institution

higher education

"NATIONAL RESEARCH

TOMSK POLYTECHNICAL UNIVERSITY "

Laboratory work No. 1

Supervisor: professor of the departmentMMS

Kulkov Sergey Nikolaevich

Students of group 4B21:

A.I. Kondratenko

G.V. Proskurnikov

Dronov A.A.

Tomsk, 2015

Target: get to know, study, as well as gain skills in the X-ray analysis of powders.

X-ray machine device

One of the most effective methods for studying the structure of crystalline substances is X-ray diffraction.

Radiography is divided into 2 types:

1.X-ray structural analysis (RSTA);

2. X-ray phase analysis (XRF).

The first method is the most general and informative and allows you to unambiguously determine all the details of the crystal structure (coordinates of atoms, etc.). A single crystal is the object of investigation in the RStA. The second method allows one to identify a substance and determine some parameters of the crystal structure. The objects of XRD investigation are polycrystalline samples.

An X-ray machine is designed to convert electricity into X-rays. The design of an X-ray machine depends on its function, but in general it consists of a radiation source, a power supply, a control system and peripherals.

How the X-ray machine works

The apparatus is usually powered from a 126 or 220 V alternating current mains. However, modern X-ray installations operate on a substantially higher voltage direct current. In this regard, the power supply includes a transformer (or a system of transformers) and a current rectifier (sometimes a rectifier may be absent - at low power of the device). A radiation generator is an X-ray tube, one or more.

The control system is a switchgear, that is, a control panel that regulates the operation of the entire installation. In addition, the apparatus includes a tripod (tripod system) on which the radiation generator is attached. The principle of operation of the installation is as follows. Alternating current from the mains is supplied to the primary winding of the transformer. A higher voltage is removed from its secondary winding and supplied to the emitter directly (half-wave installations) or through a rectifier - a kenotron. The glow of the cathode filament of the X-ray tube regulates its operation. In this case, no more than 1% of the energy supplied to the tube goes into radiation, the rest turns into heat, first of all, the anode is heated. In order to avoid damage from overheating, either refractory materials (tungsten, molybdenum) are used, or a special cooling system is constructed (water cooling, rotating anode). Modern X-ray installations are equipped with special devices for stabilizing the current and protecting the emitter from overload. In addition, a system is installed to protect others from excessive radiation (as well as from high voltage current).

X-ray tube device

An X-ray tube is an electrovacuum device with a source of radiation of electrons (cathode) and a target in which they are decelerated (anode). The high-voltage voltage for heating the cathode is supplied through the negative high-voltage cable from the filament transformer, which is located in the generator device. The heated coil of the cathode, when a high voltage is applied to the X-ray tube, begins to eject an accelerating flux of electrons, and then they are sharply decelerated on the tungsten plate of the anode, which leads to the appearance of X-rays.

How the X-ray tube works

Figure 1 - Diagram of an X-ray tube for structural analysis: 1 - metal anode cup (usually grounded); 2 - beryllium windows for the exit of X-ray radiation; 3 - thermionic cathode; 4 - glass flask that insulates the anode part of the tube from the cathode one; 5 - cathode leads, to which the filament voltage is applied, as well as high (relative to the anode) voltage; 6 - electrostatic electron focusing system; 7 - input (anti-cathode); 8 - branch pipes for inlet and outlet of running water cooling the inlet nozzle.

The area of ​​the anode on which the electrons fall is called the focus. In modern X-ray tubes, there are usually two focuses: large and small. In the anode, over 95% of the electron energy is converted into thermal energy, heating the anode to 2000 ° and more. For this reason, as the duration of the exposure increases, the allowable power decreases.

The X-ray diagnostic tube is placed in a lead-lined casing, which is filled with transformer oil. The casing has holes for connecting high-voltage cables and an exit window through which the radiation beam is removed. To minimize the dose of X-ray radiation in modern X-ray devices, for example, the FMC, a colimation device is attached to the exit window. In order to exclude the appearance of damage on the anode of the X-ray tube, the latter must rotate; for this, a device for rotating the anode is placed at the bottom of the X-ray tube casing.

Soon after the discovery of V.-K. With X-rays of a new type of radiation, it began to be actively used in medicine for diagnostic purposes. Thus, a new medical specialty called X-ray diagnostics was born. The very new radiation, electromagnetic in nature, is called X-ray in Russia and Germany, and X-rays (X-Gau) in English-speaking countries.

The device and principle of operation of the X-ray tube

X-rays are generated in an X-ray tube when a high voltage is applied to it. The most common modern model of an X-ray tube is an electrical device consisting of two electrodes: a cathode, made in the form of a thin spiral, and an anode, in the form of a plate or disk, which are sealed in a vacuum glass bulb. Thus, there is an airless space between the cathode and the anode. Since the process of obtaining X-ray radiation is associated with strong heating of the electrodes, they are structurally made of a refractory metal (tungsten).

Before the high voltage is applied to the electrodes, the cathode is heated by a strong low voltage current (voltage 6-14 V, current 2.5-8 A). In this case, the cathode begins to emit free electrons, which form a so-called electron cloud around it, and the process of detachment of electrons from the cathode surface is called electron emission.

X-ray tube diagram: 1 - cathode, 2 - electron flow, 3 - focal spot of the anode, 4 - anode, 5 - motor on the anode axis

When a high voltage (of the order of tens and hundreds of kilovolts) is applied to the electrodes, the electrons detached from the cathode through the vacuum begin to rush to the anode at a tremendous speed. Meeting the anode on its way, electrons begin to hit its surface. In this case, electrons are decelerated and their high kinetic energy is converted into energy of electromagnetic waves with different frequencies, most of which is dissipated in the form of thermal radiation. A small amount of energy generated by the deceleration of electrons against the anode (about 1/1000) leaves the X-ray tube in the form of X-rays. Thus, X-ray radiation is a wave bremsstrahlung electromagnetic radiation. In this case, it is directed perpendicular to the axis of motion of the electrons in the vacuum of the X-ray tube. This becomes possible due to the special shape of the anode, which has a beveled surface at the point of contact with the electrons falling on it, called the focal spot. In addition, when a high voltage is applied to the X-ray tube, the disk-shaped anode begins to rotate at a high frequency. Therefore, at different times, the electron beam strikes different parts of its surface, which protects the anode from excessive heating, evenly distributing the heat load over its surface.

X-ray imaging

The principle of obtaining an X-ray image of the organ under study is based on inhomogeneous attenuation (absorption) of an X-ray beam when it passes through tissues of various densities and hits the inhomogeneously attenuated radiation on the receiving system (X-ray film or fluorescent screen).

All diagnostic images obtained by medical imaging methods are divided into two main groups - analog and digital. Analog images are obtained on a special X-ray film or fluorescent screens using the methods of classical X-ray diagnostics (X-ray, fluoroscopy, fluorography, linear tomography, techniques using artificial contrast).

Scheme of X-ray image formation due to non-uniform attenuation of X-ray radiation: 1 - X-ray source, 2 - patient's body, 3 - X-ray film, fluorescent screen

There are negative and positive images of the same object (chest organs). Organs and tissues with high X-ray density (bones, heart, diaphragm domes) are white in negative images, and black in positive images. When analyzing radiographs, it is also necessary to remember the presence of a summation effect. The summation effect consists in the layering of images of various organs and tissues located along the passage of the X-ray beam.