In 1895, the German physicist Roentgen, conducting experiments on the passage of current between two electrodes in a vacuum, discovered that a screen covered with a luminescent substance (barium salt) glows, although the discharge tube is covered with a black cardboard screen - this is how radiation was discovered that penetrates through opaque obstacles, called X-ray X-rays. It was found that X-rays, which are invisible to humans, are absorbed in opaque objects the stronger, the higher the atomic number (density) of the obstacle, so X-rays easily pass through the soft tissues of the human body, but are retained by the bones of the skeleton. High-power X-ray sources have been designed to show through metal parts and find internal defects in them.

The German physicist Laue suggested that X-rays are the same electromagnetic radiation as the rays of visible light, but with a shorter wavelength and all the laws of optics are applicable to them, including the possible diffraction. In optics of visible light, diffraction at the elementary level can be represented as the reflection of light from a system of grooves - a diffraction grating, occurring only at certain angles, while the angle of reflection of the rays is related to the angle of incidence, the distance between the strokes of the diffraction grating and the wavelength of the incident radiation. For diffraction, the distance between the strokes must be approximately equal to the wavelength of the incident light.

Laue suggested that X-rays have a wavelength close to the distance between individual atoms in crystals, i.e. the atoms in the crystal create an X-ray diffraction grating. X-rays directed at the surface of the crystal were reflected on the photographic plate, as predicted by theory.

Any changes in the position of atoms affect the diffraction pattern, and by studying X-ray diffraction, one can find out the arrangement of atoms in a crystal and the change in this arrangement under any physical, chemical and mechanical effects on the crystal.

Now X-ray analysis is used in many fields of science and technology, with its help they learned the arrangement of atoms in existing materials and created new materials with given structure and properties. Recent advances in this area (nanomaterials, amorphous metals, composite materials) create a field of activity for the next scientific generations.

The appearance and properties of X-ray radiation

The X-ray source is an X-ray tube, which has two electrodes - a cathode and an anode. When the cathode is heated, electron emission occurs, the electrons emitted from the cathode are accelerated by the electric field and hit the surface of the anode. An X-ray tube is distinguished from a conventional radio tube (diode) mainly by a higher accelerating voltage (more than 1 kV).

When an electron flies out of the cathode, the electric field makes it fly towards the anode, while its speed increases continuously, the electron carries a magnetic field, the strength of which increases with the electron's speed. Reaching the anode surface, the electron is sharply decelerated, and an electromagnetic pulse with wavelengths in a certain interval (bremsstrahlung) arises. The distribution of the radiation intensity over wavelengths depends on the material of the anode of the X-ray tube and the applied voltage; in this case, from the side of short waves, this curve starts from a certain threshold minimum wavelength, depending on the applied voltage. The collection of rays with all possible wavelengths forms a continuous spectrum, and the wavelength corresponding to the maximum intensity is 1.5 times the minimum wavelength.

With increasing voltage, the X-ray spectrum changes dramatically due to the interaction of atoms with high-energy electrons and quanta of primary X-rays. An atom contains internal electron shells (energy levels), the number of which depends on the atomic number (denoted by the letters K, L, M, etc.) Electrons and primary X-rays knock electrons from one energy level to another. A metastable state arises, and a jump of electrons in the opposite direction is required for the transition to a stable state. This jump is accompanied by the release of a quantum of energy and the appearance of X-ray radiation. Unlike continuous-spectrum X-rays, this radiation has a very narrow wavelength range and high intensity (characteristic radiation) ( cm... rice.). The number of atoms that determine the intensity of the characteristic radiation is very large, for example, for an X-ray tube with a copper anode at a voltage of 1 kV and a current of 15 mA for 1 s, the characteristic radiation gives 10 14 –10 15 atoms. This value is calculated as the ratio of the total power of X-ray radiation to the energy of an X-ray quantum from the K-shell (K-series of characteristic X-ray radiation). In this case, the total power of X-ray radiation is only 0.1% of the consumed power, the rest is lost, mainly due to the transition to heat.

Due to its high intensity and narrow wavelength range, characteristic X-ray radiation is the main type of radiation used in scientific research and process control. Simultaneously with the beams of the K-series, beams of the L and M-series are generated, having significantly longer wavelengths, but their use is limited. The K-series has two components with close wavelengths a and b, while the intensity of the b-component is 5 times less than that of a. In turn, the a-component is characterized by two very close wavelengths, the intensity of one of which is 2 times greater than the other. To obtain radiation with a single wavelength (monochromatic radiation), special methods have been developed that use the dependence of the absorption and diffraction of X-rays on the wavelength. An increase in the atomic number of an element is associated with a change in the characteristics of the electron shells, while the higher the atomic number of the X-ray tube anode material, the shorter the K-series wavelength. The most widely used tubes are with anodes of elements with atomic numbers from 24 to 42 (Cr, Fe, Co, Cu, Mo) and wavelengths from 2.29 to 0.712 A (0.229 - 0.712 nm).

In addition to the X-ray tube, X-ray sources can be radioactive isotopes, some can directly emit X-rays, while others emit electrons and a-particles that generate X-rays when metal targets are bombarded. The intensity of X-ray radiation from radioactive sources is usually much less than that of an X-ray tube (with the exception of radioactive cobalt, which is used in flaw detection and gives radiation of a very short wavelength - g-radiation), they are small-sized and do not require electricity. Synchrotron X-ray radiation is obtained in electron accelerators, the wavelength of this radiation is much higher than that obtained in X-ray tubes (soft X-ray radiation), its intensity is several orders of magnitude higher than the radiation intensity of X-ray tubes. There are also natural sources of X-rays. Radioactive impurities are found in many minerals, X-rays from space objects, including stars, have been recorded.

Interaction of X-rays with crystals

In the X-ray study of materials with a crystal structure, interference patterns resulting from the scattering of X-rays by electrons belonging to the atoms of the crystal lattice are analyzed. Atoms are considered to be motionless, their thermal vibrations are not taken into account, and all electrons of the same atom are considered to be concentrated at one point - a site of the crystal lattice.

To derive the basic equations of X-ray diffraction in a crystal, the interference of rays scattered by atoms located along a straight line in the crystal lattice is considered. A plane wave of monochromatic X-ray radiation is incident on these atoms at an angle whose cosine is equal to a 0. The laws of interference of rays scattered by atoms are similar to those existing for a diffraction grating that scatters light in the visible wavelength range. In order for the amplitudes of all oscillations to add up at a large distance from the atomic row, it is necessary and sufficient that the difference in the paths of the rays coming from each pair of neighboring atoms contain an integer number of wavelengths. At a distance between atoms a this condition has the form:

a(a – a 0) = h l,

where a is the cosine of the angle between the atomic row and the deflected ray, h - integer. In all directions that do not satisfy this equation, the rays do not propagate. Thus, the scattered rays form a system of coaxial cones, the common axis of which is the atomic row. The traces of the cones on the plane parallel to the atomic row are hyperbolas, and on the plane perpendicular to the row there are circles.

When the rays are incident at a constant angle, polychromatic (white) radiation is decomposed into a spectrum of rays deflected at fixed angles. Thus, the atomic series is an X-ray spectrograph.

Generalization to a two-dimensional (planar) atomic lattice, and then to a three-dimensional bulk (spatial) crystal lattice, gives two more similar equations, which include the angles of incidence and reflection of X-ray radiation and the distance between atoms in three directions. These equations are called Laue equations and are the basis of X-ray structural analysis.

The amplitudes of the rays reflected from the parallel atomic planes add up, and since the number of atoms is very large, the reflected radiation can be detected experimentally. The reflection condition is described by the Wolfe - Bragg equation 2d sinq = nl, where d is the distance between adjacent atomic planes, q is the grazing angle between the direction of the incident beam and these planes in the crystal, l is the X-ray wavelength, n is an integer called the order of reflection. The angle q is the angle of incidence with respect to the atomic planes, which do not necessarily coincide in direction with the surface of the sample under study.

Several methods of X-ray structural analysis have been developed, using both radiation with a continuous spectrum and monochromatic radiation. In this case, the object under study can be stationary or rotating, it can consist of one crystal (single crystal) or many (polycrystal), diffracted radiation can be recorded using a flat or cylindrical X-ray film or an X-ray detector moving around the circumference, however, in all cases during the experiment and the interpretation of the results uses the Wolfe - Bragg equation.

X-ray analysis in science and technology

With the discovery of X-ray diffraction, researchers have at their disposal a method that allows, without a microscope, to study the arrangement of individual atoms and changes in this arrangement under external influences.

The main application of X-rays in basic science is structural analysis, i.e. establishing the spatial arrangement of individual atoms in a crystal. For this, single crystals are grown and X-ray analysis is carried out, studying both the location and the intensity of the reflections. Now the structures of not only metals, but also complex organic substances, in which the unit cells contain thousands of atoms, have been determined.

In mineralogy, the structures of thousands of minerals have been determined by the method of retgenoanalysis and express methods have been developed for the analysis of mineral raw materials.

Metals have a relatively simple crystal structure and the X-ray method allows one to study its changes during various technological treatments and create the physical foundations of new technologies.

The position of the lines on the X-ray diffraction patterns determines the phase composition of the alloys, according to their width - the number, size and shape of crystals, according to the intensity distribution in the diffraction cone - the orientation of the crystals (texture).

These techniques are used to study the processes during plastic deformation, including crushing of crystals, the occurrence of internal stresses and imperfections in the crystal structure (dislocations). When deformed materials are heated, stress relief and crystal growth (recrystallization) are studied.

X-ray analysis of alloys determines the composition and concentration of solid solutions. When a solid solution appears, the interatomic distances change and, consequently, the distances between the atomic planes. These changes are small, therefore, special precision methods have been developed for measuring the periods of the crystal lattice with an accuracy two orders of magnitude higher than the measurement accuracy for conventional X-ray research methods. The combination of precision measurements of the crystal lattice periods and phase analysis makes it possible to plot the boundaries of the phase regions in the phase diagram. The X-ray method can also detect intermediate states between solid solutions and chemical compounds - ordered solid solutions in which impurity atoms are not randomly arranged, as in solid solutions, and at the same time, not with three-dimensional order, as in chemical compounds. There are additional lines on the X-ray diffraction patterns of ordered solid solutions; the interpretation of the X-ray diffraction patterns shows that impurity atoms occupy certain places in the crystal lattice, for example, at the vertices of a cube.

When quenching an alloy that does not undergo phase transformations, a supersaturated solid solution may appear, and upon further heating or even holding at room temperature, the solid solution decomposes with the release of particles of a chemical compound. This is the aging effect and it manifests itself on radiographs as a change in the position and width of the lines. Aging research is especially important for non-ferrous alloys, for example, aging transforms a soft, hardened aluminum alloy into a tough structural material, duralumin.

X-ray studies of heat treatment of steel are of the greatest technological importance. During quenching (rapid cooling) of steel, a diffusionless austenite - martensite phase transition occurs, which leads to a change in the structure from cubic to tetragonal, i.e. the unit cell takes the form of a rectangular prism. On radiographs, this manifests itself as an expansion of the lines and the separation of some lines into two. The reasons for this effect are not only a change in the crystal structure, but also the appearance of large internal stresses due to the thermodynamic nonequilibrium of the martensitic structure and abrupt cooling. During tempering (heating of hardened steel), the lines in the X-ray diffraction patterns narrow, this is due to the return to the equilibrium structure.

In recent years, X-ray studies of the processing of materials with concentrated energy fluxes (laser beams, shock waves, neutrons, electron pulses) have acquired great importance; they demanded new techniques and gave new X-ray effects. For example, under the action of laser beams on metals, heating and cooling occur so rapidly that in the metal, upon cooling, crystals have time to grow only to a size of several unit cells (nanocrystals) or do not have time to appear at all. After cooling, such a metal looks like an ordinary metal, but does not give clear lines on the X-ray diffraction pattern, and the reflected X-rays are distributed over the entire range of grazing angles.

After neutron irradiation, additional spots (diffuse maxima) appear on the X-ray diffraction patterns. Radioactive decay also causes specific X-ray effects associated with structural changes, as well as the fact that the sample under study itself becomes a source of X-ray radiation.

X-ray radiation refers to electromagnetic waves with a wavelength of approximately 80 to 10 -5 nm. The longest-wavelength X-ray radiation is blocked by short-wavelength ultraviolet radiation, and the short-wavelength by long-wavelength γ-radiation. According to the excitation method, X-rays are divided into bremsstrahlung and characteristic.

31.1. X-RAY TUBE DEVICE. BRAKE X-RADIATION

The most common X-ray source is the X-ray tube, which is a two-electrode vacuum unit (Figure 31.1). Heated cathode 1 emits electrons 4. Anode 2, often called the anti-cathode, has an inclined surface in order to direct the resulting X-ray radiation 3 at an angle to the tube axis. The anode is made of a good heat-conducting material to remove the heat generated by the electron impact. The surface of the anode is made of refractory materials with a large atomic number in the periodic table, for example, tungsten. In some cases, the anode is specially cooled with water or oil.

For diagnostic tubes, the pinpoint of the X-ray source is important, which can be achieved by focusing electrons in one place of the anti-cathode. Therefore, constructively it is necessary to take into account two opposite problems: on the one hand, electrons must fall on one place of the anode, on the other hand, in order to prevent overheating, it is desirable to distribute electrons over different parts of the anode. One of the interesting technical solutions is the X-ray tube with a rotating anode (Fig. 31.2).

As a result of deceleration of an electron (or other charged particle) by the electrostatic field of the atomic nucleus and atomic electrons of the substance of the anti-cathode, bremsstrahlung x-ray radiation.

Its mechanism can be explained as follows. A moving electric charge is associated with a magnetic field, the induction of which depends on the speed of the electron. When braking, the magnetic

induction and, in accordance with Maxwell's theory, an electromagnetic wave appears.

When electrons are decelerated, only part of the energy goes to create an X-ray photon, the other part is spent on heating the anode. Since the relationship between these parts is random, then when a large number of electrons are decelerated, a continuous X-ray spectrum is formed. In this connection, bremsstrahlung is also called continuous. In fig. 31.3 shows the dependences of the X-ray flux on the wavelength λ (spectra) at different voltages in the X-ray tube: U 1< U 2 < U 3 .

In each of the spectra, the shortest-wavelength bremsstrahlung is λ ηίη arises when the energy acquired by an electron in an accelerating field is completely converted into the energy of a photon:

Note that on the basis of (31.2) one of the most accurate methods of experimental determination of the Planck constant has been developed.

Shortwave X-rays are usually more penetrating than longwave and are called tough and long-wave - soft.

By increasing the voltage across the X-ray tube, the spectral composition of the radiation is changed, as can be seen from Fig. 31.3 and formulas (31.3), and increase the rigidity.

If you increase the filament temperature of the cathode, then the emission of electrons and the current in the tube will increase. This will increase the number of X-ray photons emitted every second. Its spectral composition will not change. In fig. 31.4 shows the spectra of bremsstrahlung X-ray radiation at one voltage, but at different intensities of the cathode filament current: / h1< / н2 .

The X-ray flux is calculated by the formula:

where U and I - X-ray tube voltage and current; Z- serial number of the anode substance atom; k- coefficient of proportionality. Spectra obtained from different anticathodes at the same U and I H are shown in Fig. 31.5.

31.2. CHARACTERISTIC X-RAY RADIATION. ATOMIC X-RAY SPECTRA

By increasing the voltage across the X-ray tube, one can notice the appearance of a line-like line against the background of the continuous spectrum, which corresponds to

characteristic X-ray radiation(fig.31.6). It arises due to the fact that accelerated electrons penetrate deep into the atom and knock out electrons from the inner layers. Electrons from the upper levels are transferred to free places (Fig. 31.7), as a result, photons of characteristic radiation are released. As can be seen from the figure, characteristic X-ray radiation consists of series K, L, M etc., the name of which served to designate the electronic layers. Since the radiation of the K-series frees up places in the higher layers, the lines of other series are also emitted at the same time.

In contrast to optical spectra, the characteristic X-ray spectra of different atoms are of the same type. In fig. 31.8 shows the spectra of various elements. The uniformity of these spectra is due to the fact that the inner layers of different atoms are the same and differ only energetically, since the force effect from the side of the nucleus increases as the ordinal number of the element increases. This circumstance leads to the fact that the characteristic spectra shift towards higher frequencies with an increase in the nuclear charge. This pattern is seen from Fig. 31.8 and is known as Moseley's law:

where v - spectral line frequency; Z- the atomic number of the emitting element; A and V- permanent.

There is another difference between optical and X-ray spectra.

The characteristic X-ray spectrum of an atom does not depend on the chemical compound to which this atom is included. For example, the X-ray spectrum of the oxygen atom is the same for O, O 2 and H 2 O, while the optical spectra of these compounds are significantly different. This feature of the X-ray spectrum of the atom served as the basis for the name characteristic.

Characteristic radiation always occurs when there is free space in the inner layers of an atom, regardless of the reason that caused it. For example, characteristic radiation accompanies one of the types of radioactive decay (see 32.1), which consists in the capture of an electron by the nucleus from the inner layer.

31.3. INTERACTION OF X-RAY RADIATION WITH SUBSTANCE

Registration and use of X-ray radiation, as well as its effect on biological objects are determined by the primary processes of interaction of an X-ray photon with electrons of atoms and molecules of a substance.

Depending on the energy ratio hv photon and ionization energy 1 A and there are three main processes.

Coherent (classical) scattering

Scattering of long-wavelength X-rays occurs mainly without changing the wavelength, and it is called coherent. It occurs if the photon energy is less than the ionization energy: hv< A and.

Since in this case the energy of the X-ray photon and the atom does not change, then coherent scattering in itself does not cause a biological effect. However, when creating protection against X-ray radiation, one should take into account the possibility of changing the direction of the primary beam. This type of interaction is important for X-ray structural analysis (see 24.7).

Incoherent scattering (Compton effect)

In 1922 A.Kh. Compton, observing the scattering of hard X-rays, found a decrease in the penetrating power of the scattered beam in comparison with the incident one. This meant that the wavelength of the scattered X-ray radiation is greater than that of the incident one. The scattering of X-rays with a change in wavelength is called incoherent nym, and the phenomenon itself - the Compton effect. It occurs if the energy of the X-ray photon is greater than the ionization energy: hv> A and.

This phenomenon is due to the fact that when interacting with an atom, the energy hv photon is spent on the formation of a new scattered X-ray photon with energy hv ", on the separation of an electron from an atom (ionization energy A and) and the transmission of kinetic energy to the electron E to:

hv = hv "+ A and + E k.(31.6)

1 Here, the ionization energy is understood as the energy required to remove internal electrons outside the atom or molecule.

Since in many cases hv>> And and the Compton effect occurs on free electrons, then we can write approximately:

hv = hv "+ E K.(31.7)

It is essential that in this phenomenon (Fig. 31.9), along with secondary X-ray radiation (energy hv photon), recoil electrons appear (kinetic energy E to electron). The atoms or molecules then become ions.

Photo effect

In the photoeffect, X-ray radiation is absorbed by the atom, as a result of which an electron is emitted, and the atom is ionized (photoionization).

The three main interaction processes discussed above are primary, they lead to subsequent secondary, tertiary, etc. phenomena. For example, ionized atoms can emit a characteristic spectrum, excited atoms can become sources of visible light (X-ray luminescence), etc.

In fig. 31.10 shows a diagram of possible processes that occur when X-ray radiation enters a substance. Several dozen processes similar to the one shown can occur before the energy of the X-ray photon converts into the energy of molecular-thermal motion. As a result, changes in the molecular composition of the substance will occur.

The processes represented by the diagram in Fig. 31.10, underlie the phenomena observed when X-rays act on matter. Let's list some of them.

X-ray luminescence- the glow of a number of substances under X-ray irradiation. This luminescence of platinum-cyanide barium allowed Roentgen to discover the rays. This phenomenon is used to create special luminous screens for the visual observation of X-rays, sometimes to enhance the effect of X-rays on a photographic plate.

The chemical action of X-rays is known, for example the formation of hydrogen peroxide in water. A practically important example is the impact on a photographic plate, which makes it possible to fix such rays.

The ionizing effect is manifested in an increase in electrical conductivity under the influence of X-rays. This property is used

in dosimetry to quantify the effects of this type of radiation.

As a result of many processes, the primary X-ray beam is attenuated in accordance with law (29.3). Let's write it in the form:

I = I 0 e- / ", (31.8)

where μ is the linear attenuation coefficient. It can be represented as consisting of three terms corresponding to coherent scattering μ κ, incoherent μ ΗΚ, and photoelectric effect μ f:

μ = μ k + μ hk + μ f. (31.9)

The intensity of X-ray radiation is attenuated in proportion to the number of atoms of the substance through which this flow passes. If you compress the substance along the axis X, for example, in b times, increasing by b once its density, then

31.4. PHYSICAL BASIS OF APPLICATION OF X-RAY RADIATION IN MEDICINE

One of the most important medical uses of X-rays is to scan internal organs for diagnostic purposes. (X-ray diagnostics).

For diagnostics, photons with energies of the order of 60-120 keV are used. At this energy, the mass attenuation coefficient is mainly determined by the photoelectric effect. Its value is inversely proportional to the third power of the photon energy (proportional to λ 3), in which the high penetrating power of hard radiation is manifested, and is proportional to the third power of the atomic number of the absorbing substance:

A significant difference in the absorption of X-ray radiation by different tissues makes it possible to see images of the internal organs of the human body in the shadow projection.

X-ray diagnostics are used in two ways: fluoroscopy - the image is viewed on an X-ray luminescent screen, radiography - the image is fixed on photographic film.

If the examined organ and surrounding tissues attenuate X-ray radiation approximately equally, then special contrast agents are used. So, for example, filling the stomach and intestines with a mushy mass of barium sulfate, you can see their shadow image.

The brightness of the image on the screen and the exposure time on the film depend on the intensity of the X-ray radiation. If it is used for diagnostics, then the intensity cannot be high, so as not to cause undesirable biological consequences. Therefore, there are a number of technical devices that improve the image at low intensities of X-ray radiation. An example of such a device is an image converter (see 27.8). In a mass examination of the population, a variant of radiography is widely used - fluorography, in which an image from a large X-ray luminescent screen is recorded on a sensitive small-format film. When shooting, a lens of high aperture is used, the finished images are examined with a special magnifier.

An interesting and promising option for radiography is a method called X-ray tomography, and its "machine version" - CT scan.

Let's consider this question.

A typical radiograph covers a large area of ​​the body, with different organs and tissues obscuring each other. This can be avoided if periodically together (Fig. 31.11) in antiphase move the X-ray tube RT and film FP relative to the object About research. The body contains a number of inclusions opaque to X-rays, they are shown by circles in the figure. As you can see, X-rays at any position of the X-ray tube (1, 2 etc.) pass through

cut the same point of the object, which is the center, relative to which the periodic movement is performed RT and FP. This point, or rather a small opaque inclusion, is shown by a dark circle. Its shadow image moves with FP, sequentially occupying positions 1, 2 etc. The rest of the inclusions in the body (bones, seals, etc.) are created on FP some general background, as X-rays are not constantly obscured by them. By changing the position of the swing center, a layer-by-layer X-ray image of the body can be obtained. Hence the name - tomography(layer-by-layer recording).

It is possible, using a thin X-ray beam, a screen (instead of Фп), consisting of semiconductor detectors of ionizing radiation (see 32.5), and a computer, to process the shadow X-ray image during tomography. This modern version of tomography (computational or computed X-ray tomography) allows you to obtain layer-by-layer images of the body on the screen of a cathode-ray tube or on paper with details less than 2 mm with a difference in X-ray absorption of up to 0.1%. This allows, for example, to distinguish between gray and white matter of the brain and to see very small tumor formations.

X-RAY RADIATION
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-ray radiation causes 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-ray radiation, 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. X-rays were discovered by the German physicist W. Roentgen (1845-1923). His name is also immortalized in some other physical terms associated with this radiation: X-ray is the international unit of ionizing radiation dose; a picture taken in an X-ray machine is called an X-ray; the field of radiological medicine, which uses X-rays to diagnose and treat disease, is called radiology. Roentgen discovered radiation in 1895 when he was a professor of physics at the University of Würzburg. While experimenting with cathode rays (streams of electrons in discharge tubes), he noticed that a screen located near the vacuum tube, covered with crystalline barium cyanoplatinite, glows brightly, although the tube itself is covered with black cardboard. Further Roentgen found that the penetrating ability of the unknown rays he discovered, which he called X-rays, depends on the composition of the absorbing material. He also obtained an image of the bones of his own hand, placing it between a discharge tube with cathode rays and a screen coated with barium cyanoplatinite. The discovery of Roentgen was followed by experiments by other researchers who discovered many new properties and applications of this radiation. A great contribution was made by M. Laue, W. Friedrich, and P. Knipping, who demonstrated in 1912 the diffraction of X-ray radiation when it passes through a crystal; W. Coolidge, who in 1913 invented the high-vacuum X-ray tube with a heated cathode; G. Moseley, who established in 1913 the relationship between the radiation wavelength and the atomic number of an element; G. and L. Braggi, who received the Nobel Prize in 1915 for the development of the foundations of X-ray structural analysis.
OBTAINING X-RAY RADIATION
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 method of producing X-rays produces a wide range of wavelengths called the X-ray spectrum. The spectrum contains pronounced components, as shown in Fig. 1. The broad "continuum" is called continuous spectrum or white radiation. The sharp peaks superimposed on it are called characteristic X-ray emission lines. Although the entire spectrum is the result of collisions of electrons with matter, the mechanisms for the emergence of its wide part and lines are different. The substance consists of a large number of atoms, each of which has a nucleus surrounded by electron shells, and each electron in the shell of an atom of a given element occupies a certain discrete energy level. Usually these shells, or energy levels, are denoted by the symbols K, L, M, etc., starting from the shell closest to the core. When an incident electron with a sufficiently high energy collides with one of the electrons bound to the atom, it knocks that electron out of its shell. The empty place is occupied by another electron from the shell, which corresponds to a large energy. This latter gives up excess energy by emitting an X-ray photon. Since the electrons of the shells have discrete energy values, the emerging X-ray photons also have a discrete spectrum. This corresponds to sharp peaks for certain wavelengths, the specific values ​​of which depend on the target element. The characteristic lines form the K-, L- and M-series, depending on which shell (K, L or M) the electron was removed from. The relationship between the wavelength of X-rays and the atomic number is called Moseley's law (Fig. 2).

The electrons are focused on the anode by 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.
DETECTION OF X-RAY RADIATION
All X-ray detection methods are based on their interaction with matter. Detectors can be of two types: those that give an image, and those that do not. The first include devices for X-ray fluorography and fluoroscopy, in which the X-ray beam passes through the object under study, and the transmitted radiation falls on a luminescent screen or photographic film. The image appears due to the fact that different parts of the object under study absorb radiation in different ways - depending on the thickness of the substance and its composition. In detectors with a luminescent screen, the energy of X-ray radiation is converted into a directly observed image, while in X-ray diffraction it is recorded on a sensitive emulsion and can be observed only after the film has developed. The second type of detectors includes a wide variety of devices in which the energy of X-ray radiation is converted into electrical signals that characterize the relative intensity of the radiation. This includes ionization chambers, a Geiger counter, a proportional counter, a scintillation counter, and some special cadmium sulfide and selenide detectors. Currently, the most efficient detectors can be considered scintillation counters, which work well in a wide range of energies.
see also PARTICLE DETECTORS. The detector is selected taking into account the conditions of the problem. For example, if it is necessary to accurately measure the intensity of diffracted X-ray radiation, then counters are used that allow measurements to be made with an accuracy of fractions of a percent. If you need to register a lot of diffracted beams, then it is advisable to use an X-ray film, although in this case it is impossible to determine the intensity with the same accuracy.
X-RAY AND GAMMA DEFECTOSCOPY
One of the most common applications of X-rays in industry is material quality control and non-destructive testing. The X-ray method is non-destructive, so that the material being tested, if found to meet the required requirements, can then be used as intended. Both X-ray and gamma-ray flaw detection are based on the penetrating power of X-ray radiation and the characteristics of its absorption in materials. Penetration is determined by the energy of X-ray photons, which depends on the accelerating voltage in the X-ray tube. Therefore, thick samples and samples of heavy metals, such as gold and uranium, require an X-ray source with a higher voltage for their study, and for thin samples, a source with a lower voltage is sufficient. For gamma-ray flaw detection of very large castings and large rolled products, betatrons and linear accelerators are used, which accelerate particles to energies of 25 MeV and more. The absorption of X-ray radiation in a material depends on the thickness of the absorber d and the absorption coefficient m and is determined by the formula I = I0e-md, where I is the intensity of radiation transmitted through the absorber, I0 is the intensity of the incident radiation, and e = 2.718 is the base of natural logarithms. For a given material at a given wavelength (or energy) of X-ray radiation, the absorption coefficient is constant. But the radiation of an X-ray source is not monochromatic, but contains a wide spectrum of wavelengths, as a result of which the absorption at the same thickness of the absorber depends on the wavelength (frequency) of the radiation. X-rays are widely used in all metal forming industries. It is also used to inspect artillery barrels, foodstuffs, plastics, and to inspect complex devices and systems in electronic engineering. (Neutron diffraction is also used for similar purposes, in which neutron beams are used instead of X-rays.) X-rays are also used for other tasks, for example, to examine paintings in order to establish their authenticity or to detect additional layers of paint on top of the main layer.
DIFFRACTION OF X-RAY RADIATION
X-ray diffraction provides important information about solids - their atomic structure and crystal shape, as well as about liquids, amorphous bodies and large molecules. The diffraction method is also used for accurate (with an error less than 10-5) determination of interatomic distances, detection of stresses and defects, and to determine the orientation of single crystals. The diffraction pattern can identify unknown materials, as well as detect the presence of impurities in the sample and determine them. The importance of the X-ray diffraction method for the progress of modern physics can hardly be overestimated, since the modern understanding of the properties of matter is ultimately based on data on the arrangement of atoms in various chemical compounds, on the nature of the bonds between them and on structural defects. The main tool for obtaining this information is the X-ray diffraction method. X-ray diffraction crystallography is extremely important for determining the structures of complex large molecules, such as the molecules of deoxyribonucleic acid (DNA), the genetic material of living organisms. Immediately after the discovery of X-ray radiation, scientific and medical interest was focused both on the ability of this radiation to penetrate bodies and on its nature. Experiments on the diffraction of X-ray radiation by slits and diffraction gratings showed that it belongs to electromagnetic radiation and has a wavelength of the order of 10-8-10-9 cm. Even earlier, scientists, in particular W. Barlow, guessed that the correct and symmetrical shape of natural crystals is due to the ordered arrangement of the atoms that form the crystal. In some cases, Barlow was able to correctly predict the structure of the crystal. The predicted interatomic distances were 10–8 cm. The fact that the interatomic distances were of the order of the X-ray wavelength, in principle, made it possible to observe their diffraction. The result was the idea of ​​one of the most important experiments in the history of physics. M. Laue organized an experimental test of this idea, which was carried out by his colleagues W. Friedrich and P. Knipping. In 1912, the three of them published their work on the results of X-ray diffraction. Principles of X-ray diffraction. To understand the phenomenon of X-ray diffraction, it is necessary to consider in order: first, the X-ray spectrum, secondly, the nature of the crystal structure and, thirdly, the diffraction phenomenon itself. As mentioned above, the characteristic X-ray radiation consists of a series of spectral lines of a high degree of monochromaticity, determined by the anode material. Using filters, you can select the most intense ones. Therefore, by appropriately choosing the material of the anode, it is possible to obtain a source of almost monochromatic radiation with a very precisely defined wavelength value. The characteristic radiation wavelengths typically range from 2.285 for chromium to 0.558 for silver (the values ​​for the various elements are known to within six significant digits). The characteristic spectrum is superimposed on the continuous "white" spectrum of a much lower intensity, due to the deceleration of the incident electrons in the anode. Thus, two types of radiation can be obtained from each anode: characteristic and bremsstrahlung, each of which plays an important role in its own way. The atoms in the crystal structure are arranged with the correct periodicity, forming a sequence of identical cells - a spatial lattice. Some lattices (for example, for most common metals) are quite simple, while others (for example, for protein molecules) are quite complex. The following is characteristic of the crystal structure: if one moves from a given point of one cell to the corresponding point of an adjacent cell, then exactly the same atomic environment will be found. And if some atom is located at one point or another of one cell, then the same atom will be located at the equivalent point of any neighboring cell. This principle is strictly true for a perfect, perfectly ordered crystal. However, many crystals (for example, metal solid solutions) are disordered to one degree or another, i.e. crystallographically equivalent sites can be occupied by different atoms. In these cases, it is not the position of each atom that is determined, but only the position of the atom, "statistically averaged" over a large number of particles (or cells). The phenomenon of diffraction is discussed in the OPTICS article, and the reader can refer to this article before moving on. It shows that if waves (for example, sound, light, X-ray radiation) pass through a small slit or hole, then the latter can be considered as a secondary source of waves, and the image of the slit or hole consists of alternating light and dark stripes. Further, if there is a periodic structure of holes or slits, then as a result of amplifying and attenuating interference of the rays coming from different holes, a clear diffraction pattern arises. X-ray diffraction is a collective scattering phenomenon, in which the role of holes and scattering centers is played by periodically located atoms of the crystal structure. The mutual enhancement of their images at certain angles gives a diffraction pattern similar to that which would arise in the diffraction of light on a three-dimensional diffraction grating. Scattering occurs due to the interaction of the incident X-ray radiation with the electrons in the crystal. Due to the fact that the wavelength of the X-ray radiation is of the same order of magnitude as the dimensions of the atom, the wavelength of the scattered X-ray radiation is the same as that of the incident one. This process is the result of forced oscillations of electrons under the influence of incident X-ray radiation. Consider now an atom with a cloud of bound electrons (surrounding the nucleus), on which X-rays are incident. Electrons in all directions simultaneously scatter the incident and emit their own X-rays of the same wavelength, albeit of different intensities. The intensity of the scattered radiation is related to the atomic number of the element, since the atomic number is equal to the number of orbital electrons that can participate in the scattering. (This dependence of the intensity on the atomic number of the scattering element and on the direction in which the intensity is measured is characterized by the atomic scattering factor, which plays an extremely important role in the analysis of the structure of crystals.) Let us choose in the crystal structure a linear chain of atoms located at the same distance from each other, and consider their diffraction pattern. It has already been noted that the X-ray spectrum consists of a continuous part ("continuum") and a set of more intense lines characteristic of the element that is the material of the anode. Let's say we filtered out the continuous spectrum and got an almost monochromatic X-ray beam aimed at our linear chain of atoms. The amplification (amplifying interference) condition is satisfied if the path difference between the waves scattered by neighboring atoms is a multiple of the wavelength. If the beam is incident at an angle a0 to the atomic line separated by intervals a (period), then for the diffraction angle a, the path difference corresponding to the amplification is written as a (cos a - cosa0) = hl, where l is the wavelength and h is an integer (fig. 4 and 5).

These are the three fundamental Laue equations for X-ray diffraction, with the numbers h, k and c being the Miller indices for the diffraction plane.
see also CRYSTALS AND CRYSTALLOGRAPHY. Considering any of the Laue equations, for example the first, one can notice that, since a, a0, l are constants, and h = 0, 1, 2, ..., its solution can be represented as a set of cones with a common axis a (Fig. . 5). The same is true for directions b and c. In the general case of three-dimensional scattering (diffraction), the three Laue equations must have a general solution, i.e. three diffraction cones located on each of the axes must intersect; the general line of intersection is shown in fig. 6. The joint solution of the equations leads to the Bragg - Wolfe law:

Modern medical diagnostics and treatment of some diseases cannot be imagined without devices that use the properties of X-ray radiation. The discovery of X-rays happened more than 100 years ago, but even now work continues on the creation of new techniques and devices that make it possible to minimize the negative effect of radiation on the human body.

Who and how discovered X-rays

Under natural conditions, the flux of X-rays is rare and is emitted only by some radioactive isotopes. X-rays or X-rays were discovered only in 1895 by the German scientist Wilhelm Röntgen. This discovery happened by chance, during an experiment to study the behavior of light rays in conditions approaching a vacuum. The experiment involved a cathode gas-discharge tube with a reduced pressure and a fluorescent screen, which started glowing every time the tube started to work.

Intrigued by the strange effect, Roentgen conducted a series of studies showing that the resulting radiation invisible to the eye can penetrate various obstacles: paper, wood, glass, some metals, and even through the human body. Despite the lack of understanding of the very nature of what is happening, whether such a phenomenon is caused by the generation of a stream of unknown particles or waves, the following pattern was noted - radiation easily passes through the soft tissues of the body, and much heavier through hard living tissues and inanimate substances.

Roentgen was not the first to study such a phenomenon. In the middle of the 19th century, the Frenchman Antoine Mason and the Englishman William Crookes studied similar possibilities. Nevertheless, it was Roentgen who first invented the cathode tube and indicator that could be used in medicine. He was the first to publish a scientific work that earned him the title of the first Nobel laureate among physicists.

In 1901, a fruitful collaboration began between three scientists who became the founding fathers of radiology and radiology.

X-ray properties

X-rays are part of the overall spectrum of electromagnetic radiation. The wavelength is located between gamma and ultraviolet rays. X-rays are characterized by all the usual wave properties:

• diffraction;
• refraction;
• interference;
• the speed of propagation (it is equal to the light speed).

For the artificial generation of a flux of X-rays, special devices are used - X-ray tubes. X-rays arise from the contact of fast electrons of tungsten with substances evaporating from a hot anode. Against the background of the interaction, electromagnetic waves of small length appear, which are in the spectrum from 100 to 0.01 nm and in the energy range of 100 to 0.1 MeV. If the wavelength of the rays is less than 0.2 nm, it is hard radiation, if the wavelength is greater than the specified value, they are called soft X-rays.

It is significant that the kinetic energy arising from the contact of electrons and the anode substance is 99% converted into heat energy and only 1% is X-rays.

X-ray radiation - bremsstrahlung and characteristic

X-radiation is a superposition of two types of rays - bremsstrahlung and characteristic rays. They are generated in the tube at the same time. Therefore, the X-ray irradiation and the characteristic of each specific X-ray tube - the spectrum of its radiation, depends on these indicators, and represents their overlap.

Braking or continuous X-rays are the result of the braking of electrons evaporated from the tungsten coil.

Characteristic or linear X-ray beams are formed at the moment of rearrangement of the atoms of the X-ray tube anode. The wavelength of the characteristic rays directly depends on the atomic number of the chemical element used to make the tube anode.

The listed properties of X-rays make it possible to apply them in practice:

• invisibility to ordinary eyes;
• high penetrating ability through living tissues and inanimate materials that do not transmit the rays of the visible spectrum;
• ionization effect on molecular structures.

Principles of obtaining an x-ray image

The properties of X-rays on which the imaging is based is the ability to either decompose or cause some substances to glow.

X-ray irradiation causes a fluorescent glow in cadmium and zinc sulfides - green, and in calcium tungstate - blue. This property is used in the technique of medical X-ray transillumination, and also increases the functionality of X-ray screens.

The photochemical effect of X-rays on light-sensitive silver halide materials (light exposure) allows for diagnostics - making X-ray images. This property is also used when measuring the value of the total dose that laboratory technicians receive in X-ray rooms. Body-worn dosimeters have special sensitive tapes and indicators. The ionizing effect of X-rays makes it possible to determine the qualitative characteristics of the obtained X-rays.

A single exposure from conventional x-rays increases the risk of cancer by as little as 0.001%.

Areas where X-rays are used

The use of X-rays is acceptable in the following industries:

1. Security. Stationary and portable devices for detecting dangerous and prohibited items at airports, customs or in crowded places.
2. Chemical industry, metallurgy, archeology, architecture, construction, restoration work - for the detection of defects and chemical analysis of substances.
3. Astronomy. Helps to observe space bodies and phenomena using X-ray telescopes.
4. The military industry. For the development of laser weapons.

The main application of X-rays is in the medical field. Today, the section of medical radiology includes: radiodiagnostics, radiotherapy (X-ray therapy), radiosurgery. Medical universities graduate narrow-profile specialists - radiologists.

X-Radiation - harm and benefit, effect on the body

The high penetrating power and ionizing effect of X-rays can cause a change in the structure of the cell's DNA, therefore, it is dangerous for humans. The damage from X-rays is directly proportional to the radiation dose received. Different organs respond to radiation to varying degrees. The most susceptible are:

• bone marrow and bone tissue;
• lens of the eye;
• thyroid;
• mammary and sex glands;
• lung tissue.

Uncontrolled use of X-ray radiation can cause reversible and irreversible pathologies.

Consequences of X-ray exposure:

• damage to the bone marrow and the occurrence of pathologies of the hematopoietic system - erythrocytopenia, thrombocytopenia, leukemia;
• damage to the lens, with the subsequent development of cataracts;
• hereditary cell mutations;
• the development of oncological diseases;
• getting radiation burns;
• development of radiation sickness.

Important! Unlike radioactive substances, X-rays do not accumulate in the tissues of the body, which means that it is not necessary to remove X-rays from the body. The harmful effect of X-rays ends when the medical device is turned off.

The use of X-rays in medicine is permissible not only for diagnostic (traumatology, dentistry), but also for therapeutic purposes:

• small doses of X-rays stimulate metabolism in living cells and tissues;
• certain dose limits are used for the treatment of oncological and benign neoplasms.

Methods for diagnosing pathologies using X-rays

Radio diagnostics includes the following techniques:

1. Fluoroscopy is a study in which an image is obtained on a fluorescent screen in real time. Along with the classical acquisition of an image of a part of the body in real time, today there are X-ray television transmission technologies - the image is transferred from a fluorescent screen to a television monitor located in another room. Several digital methods have been developed for processing the resulting image, with its subsequent transfer from the screen to paper.
2. Fluorography is the cheapest method for examining the organs of the chest, which consists in making a reduced picture of 7x7 cm.Despite the probability of an error, it is the only way of a mass annual survey of the population. The method is not dangerous and does not require the withdrawal of the received radiation dose from the body.
3. Radiography - obtaining a summary image on film or paper to clarify the shape of an organ, its position or tone. It can be used to assess the peristalsis and condition of the mucous membranes. If there is a choice, then among modern X-ray devices, preference should be given to either digital devices, where the flux of x-rays can be higher than that of old devices, and low-dose X-ray devices with straight flat semiconductor detectors. They can reduce the load on the body by 4 times.
4. Computed X-ray tomography is a technique that uses X-rays to obtain the required number of images of slices of a selected organ. Among the many varieties of modern CT machines, low-dose high-resolution computed tomographs are used for a series of repeated examinations.

Radiotherapy

X-ray therapy is a topical treatment. Most often, the method is used to destroy cancer cells. Since the effect of exposure is comparable to surgical removal, this method of treatment is often called radiosurgery.

Today, x-ray treatment is carried out in the following ways:

1. External (proton therapy) - the beam of radiation enters the patient's body from the outside.
2. Internal (brachiotherapy) - the use of radioactive capsules by implanting them into the body, placing them closer to the cancerous tumor. The disadvantage of this method of treatment is that until the capsule is removed from the body, the patient needs to be isolated.

These methods are gentle, and their use is preferable to chemotherapy in some cases. Such popularity is due to the fact that the rays do not accumulate and do not require excretion from the body; they have a selective effect without affecting other cells and tissues.

Safe X-ray exposure rate

This indicator of the permissible annual exposure rate has its own name - a genetically significant equivalent dose (GZD). This indicator has no clear quantitative values.

1. This indicator depends on the age and the patient's desire to have children in the future.
2. Depends on which organs were examined or treated.
3. The GZD is influenced by the level of natural radioactive background in the region of human residence.

Today, the following averaged standards for the HDM are in force:

• the level of exposure from all sources, with the exception of medical ones, and without taking into account the natural background radiation - 167 mRem per year;
• the norm for an annual medical examination is not more than 100 mReg per year;
• the total safe value is 392 mRem per year.

X-ray radiation does not need to be removed from the body, and is dangerous only in case of intense and prolonged exposure. Modern medical equipment uses low-energy radiation of short duration, so its use is considered relatively harmless.

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 method of producing X-rays produces a wide range of wavelengths called the X-ray spectrum. The spectrum contains pronounced components, as shown in Fig. 1.

Rice. 1. REGULAR X-RAY SPECTRUM consists of a continuous spectrum (continuum) and characteristic lines (sharp peaks). Lines Кia and Кib appear due to interactions of accelerated electrons with electrons of the inner K-shell.

The broad "continuum" is called continuous spectrum or white radiation. The sharp peaks superimposed on it are called characteristic X-ray emission lines. Although the entire spectrum is the result of collisions of electrons with matter, the mechanisms for the emergence of its wide part and lines are different. The substance consists of a large number of atoms, each of which has a nucleus surrounded by electron shells, and each electron in the shell of an atom of a given element occupies a certain discrete energy level. Usually these shells, or energy levels, are denoted by the symbols K, L, M, etc., starting from the shell closest to the core. When an incident electron with a sufficiently high energy collides with one of the electrons bound to the atom, it knocks that electron out of its shell. The empty place is occupied by another electron from the shell, which corresponds to a large energy. This latter gives up excess energy by emitting an X-ray photon. Since the electrons of the shells have discrete energy values, the emerging X-ray photons also have a discrete spectrum. This corresponds to sharp peaks for certain wavelengths, the specific values ​​of which depend on the target element. The characteristic lines form the K-, L- and M-series, depending on which shell (K, L or M) the electron was removed from. The relationship between the wavelength of X-rays and the atomic number is called Moseley's law (Fig. 2).

Rice. 2. The WAVE LENGTH of the CHARACTERISTIC X-RAY RADIATION emitted by chemical elements depends on the atomic number of the element. The curve corresponds to Moseley's law: the higher the atomic number of an element, the shorter the wavelength of the characteristic line.

If an electron hits a relatively heavy nucleus, then it is decelerated, and its kinetic energy is released in the form of an X-ray photon of approximately the same energy. If it flies past the nucleus, it will lose only part of its energy, and the rest will be transferred to other atoms that come across in its path. Each act of energy loss leads to the emission of a photon with some energy. A continuous X-ray spectrum appears, the upper limit of which corresponds to the energy of the fastest electron. This is the mechanism for the formation of a continuous spectrum, and the maximum energy (or minimum wavelength) that fixes the boundary of the continuous spectrum is proportional to the accelerating voltage, which determines the speed of the incident electrons. The spectral lines characterize the material of the bombarded target, and the continuous spectrum is determined by the energy of the electron beam and is practically independent of the target material.

X-rays can be obtained not only by electron bombardment, but also by irradiating the target with X-rays from another source. In this case, however, most of the incident beam energy goes over into the characteristic X-ray spectrum, and a very small fraction of it falls on the continuous one. Obviously, the incident X-ray beam must contain photons, the energy of which is sufficient to excite the characteristic lines of the bombarded element. The high percentage of energy in the characteristic spectrum makes this method of X-ray excitation convenient for scientific research.

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 "deeply 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. The 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. 3), the source of electrons is a tungsten cathode heated to a high temperature. 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.

Rice. 3. COULIDGE X-RAY TUBE. 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 by 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 vary depending on the application and requirements.