X-ray radiation and its use in medicine. Brief characteristics of X-ray radiation. Areas where X-rays are used

Nature of X-rays

Bremsstrahlung X-ray radiation, its spectral properties.

Characteristic X-ray radiation (for reference).

Interaction of X-ray radiation with matter.

Physical basis of the use of x-ray radiation in medicine.

X-ray radiation(X - rays) were discovered by K. Roentgen, who in 1895 became the first Nobel laureate in physics.

1. Nature of X-rays

X-ray radiation – electromagnetic waves with a length from 80 to 10–5 nm. Long-wave X-ray radiation is overlapped by short-wave UV radiation, and short-wave X-ray radiation is overlapped by long-wave -radiation.

X-rays are produced in X-ray tubes. Fig.1.

K – cathode

1 – electron beam

2 – X-ray radiation

Rice. 1. X-ray tube device.

The tube is a glass flask (with a possibly high vacuum: the pressure in it is about 10–6 mm Hg) with two electrodes: anode A and cathode K, to which a high voltage U (several thousand volts) is applied. The cathode is a source of electrons (due to the phenomenon of thermionic emission). The anode is a metal rod that has an inclined surface in order to direct the resulting X-ray radiation at an angle to the axis of the tube. It is made of a highly thermally conductive material to dissipate the heat generated by electron bombardment. At the beveled end there is a plate of refractory metal (for example, tungsten).

The strong heating of the anode is due to the fact that the majority of electrons in the cathode beam, upon reaching the anode, experience numerous collisions with atoms of the substance and transfer great energy to them.

Under the influence high voltage electrons emitted by the hot cathode filament are accelerated to high energies. The kinetic energy of the electron is mv 2 /2. It is equal to the energy that it acquires while moving in the electrostatic field of the tube:

mv 2 /2 = eU (1)

where m, e are the mass and charge of the electron, U is the accelerating voltage.

The processes leading to the appearance of bremsstrahlung X-ray radiation are caused by intense deceleration of electrons in the anode substance by the electrostatic field of the atomic nucleus and atomic electrons.

The mechanism of occurrence can be presented as follows. Moving electrons are a certain current that forms its own magnetic field. Slowing down of electrons is a decrease in current strength and, accordingly, a change in the magnetic field induction, which will cause the appearance of an alternating electric field, i.e. appearance of an electromagnetic wave.

Thus, when a charged particle flies into matter, it is decelerated, loses its energy and speed, and emits electromagnetic waves.

1. Spectral properties of bremsstrahlung X-ray radiation.

So, in the case of electron deceleration in the anode substance, Bremsstrahlung X-ray radiation.

The spectrum of bremsstrahlung X-rays is continuous. The reason for this is the following.

When electrons are decelerated, part of the energy goes to heating the anode (E 1 = Q), the other part to create an x-ray photon (E 2 = hv), otherwise, eU = hv + Q. The relationship between these parts is random.

Thus, a continuous spectrum of X-ray bremsstrahlung is formed due to the deceleration of many electrons, each of which emits one X-ray quantum hv (h) of a strictly defined value. The magnitude of this quantum different for different electrons. Dependence of the X-ray energy flux on the wavelength , i.e. The X-ray spectrum is shown in Fig. 2.

Fig.2. Bremsstrahlung X-ray spectrum: a) at different voltages U in the tube; b) at different temperatures T of the cathode.

Short-wave (hard) radiation has greater penetrating power than long-wave (soft) radiation. Soft radiation is more strongly absorbed by matter.

On the short wavelength side, the spectrum ends abruptly at a certain wavelength  m i n . Such short-wave bremsstrahlung occurs when the energy acquired by an electron in the accelerating field is completely converted into photon energy (Q = 0):

eU = hv max = hc/ min ,  min = hc/(eU), (2)

 min (nm) = 1.23/UkV

The spectral composition of the radiation depends on the voltage on the X-ray tube; with increasing voltage, the value  m i n shifts towards short wavelengths (Fig. 2a).

When the temperature T of the cathode changes, the emission of electrons increases. Consequently, the current I in the tube increases, but the spectral composition of the radiation does not change (Fig. 2b).

The energy flow Ф  bremsstrahlung is directly proportional to the square of the voltage U between the anode and the cathode, the current strength I in the tube and the atomic number Z of the anode substance:

Ф = kZU 2 I. (3)

where k = 10 –9 W/(V 2 A).

In the study and practical use of atomic phenomena, one of the critical roles X-rays play. Thanks to their research, many discoveries were made and methods for analyzing substances were developed, used in a variety of fields. Here we will look at one type of X-rays - characteristic X-rays.

Nature and properties of X-rays

X-ray radiation is a high-frequency change in the state of the electromagnetic field, propagating in space at a speed of about 300,000 km/s, that is, electromagnetic waves. On the scale of the range of electromagnetic radiation, x-rays are located in the wavelength region from approximately 10 -8 to 5∙10 -12 meters, which is several orders of magnitude shorter than optical waves. This corresponds to frequencies from 3∙10 16 to 6∙10 19 Hz and energies from 10 eV to 250 keV, or 1.6∙10 -18 to 4∙10 -14 J. It should be noted that the boundaries of the frequency ranges of electromagnetic radiation are quite arbitrary due to their overlap.

Is the interaction of accelerated charged particles (high energy electrons) with electric and magnetic fields and with atoms of matter.

X-ray photons are characterized by high energies and high penetrating and ionizing powers, especially for hard X-rays with wavelengths less than 1 nanometer (10 -9 m).

X-rays interact with matter, ionizing its atoms, in the processes of photoelectric effect (photoabsorption) and incoherent (Compton) scattering. In photoabsorption, an X-ray photon, absorbed by an electron of an atom, transfers energy to it. If its value exceeds the binding energy of an electron in an atom, then it leaves the atom. Compton scattering is characteristic of harder (energetic) X-ray photons. Part of the energy of the absorbed photon is spent on ionization; in this case, at a certain angle to the direction of the primary photon, a secondary one is emitted, with a lower frequency.

Types of X-ray radiation. Bremsstrahlung

To produce beams, glass vacuum cylinders with electrodes located inside are used. The potential difference across the electrodes needs to be very high - up to hundreds of kilovolts. Thermionic emission occurs on the tungsten cathode, heated by current, that is, electrons are emitted from it, which, accelerated by the potential difference, bombard the anode. As a result of their interaction with the atoms of the anode (sometimes called the anticathode), X-ray photons are born.

Depending on what process leads to the creation of a photon, types of X-ray radiation are distinguished: bremsstrahlung and characteristic.

Electrons can, when meeting the anode, be slowed down, that is, lose energy in the electric fields of its atoms. This energy is emitted in the form of X-ray photons. This type of radiation is called bremsstrahlung.

It is clear that the braking conditions will differ for individual electrons. This means that different amounts of their kinetic energy are converted into x-rays. As a result, bremsstrahlung includes photons of different frequencies and, accordingly, wavelengths. Therefore, its spectrum is continuous (continuous). Sometimes for this reason it is also called “white” X-rays.

The energy of a bremsstrahlung photon cannot exceed the kinetic energy of the electron generating it, so the maximum frequency (and shortest wavelength) of bremsstrahlung radiation corresponds to the highest value of the kinetic energy of electrons incident on the anode. The latter depends on the potential difference applied to the electrodes.

There is another type of X-ray radiation, the source of which is a different process. This radiation is called characteristic radiation, and we will dwell on it in more detail.

How does characteristic X-ray radiation arise?

Having reached the anti-cathode, a fast electron can penetrate inside the atom and knock out an electron from one of the lower orbitals, that is, transfer to it energy sufficient to overcome the potential barrier. However, if there are higher energy levels in the atom occupied by electrons, the vacated space will not remain empty.

It must be remembered that the electronic structure of the atom, like any energy system, tends to minimize energy. The vacancy formed as a result of knocking out is filled with an electron from one of the higher levels. His energy is higher, and, taking up more low level, it emits the excess in the form of a quantum of characteristic x-ray radiation.

The electronic structure of an atom is a discrete set of possible energy states of electrons. Therefore, X-ray photons emitted during the replacement of electron vacancies can also only have strictly defined energy values, reflecting the difference in levels. As a result, the characteristic X-ray radiation has a spectrum that is not continuous, but line-shaped. This spectrum makes it possible to characterize the substance of the anode - hence the name of these rays. It is thanks to the spectral differences that it is clear what is meant by bremsstrahlung and characteristic X-ray radiation.

Sometimes the excess energy is not emitted by the atom, but is spent on knocking out the third electron. This process - the so-called Auger effect - is more likely to occur when the electron binding energy does not exceed 1 keV. The energy of the released Auger electron depends on the structure of the energy levels of the atom, therefore the spectra of such electrons are also discrete in nature.

General view of the characteristic spectrum

Narrow characteristic lines are present in the X-ray spectral picture along with a continuous bremsstrahlung spectrum. If we imagine the spectrum as a graph of intensity versus wavelength (frequency), we will see sharp peaks at the locations of the lines. Their position depends on the anode material. These maxima are present at any potential difference - if there are X-rays, there are always peaks too. As the voltage on the tube electrodes increases, the intensity of both continuous and characteristic X-ray radiation increases, but the location of the peaks and the ratio of their intensities does not change.

The peaks in the X-ray spectra have the same appearance regardless of the material of the anticathode irradiated with electrons, but in various materials located at different frequencies, united in series based on the proximity of frequency values. Between the series themselves, the difference in frequencies is much more significant. The type of maxima does not depend in any way on whether the anode material is a pure chemical element or a complex substance. In the latter case, the characteristic X-ray spectra of its constituent elements are simply superimposed on each other.

As the atomic number of a chemical element increases, all lines of its X-ray spectrum shift toward higher frequencies. The spectrum retains its appearance.

Moseley's Law

The phenomenon of spectral shift of characteristic lines was experimentally discovered by the English physicist Henry Moseley in 1913. This allowed him to connect the frequencies of the spectrum maxima with the serial numbers of chemical elements. Thus, the wavelength of characteristic X-ray radiation, as it turned out, can be clearly correlated with a specific element. In general, Moseley's law can be written as follows: √f = (Z - S n)/n√R, where f is the frequency, Z is the serial number of the element, S n is the screening constant, n is the principal quantum number and R is the constant Rydberg. This dependence is linear and on the Moseley diagram looks like a series of straight lines for each value of n.

The n values ​​correspond to individual series of characteristic X-ray emission peaks. Moseley's law makes it possible to determine the serial number of a chemical element irradiated by hard electrons based on the measured wavelengths (they are uniquely related to the frequencies) of the maxima of the X-ray spectrum.

The structure of the electronic shells of chemical elements is identical. This is indicated by the monotonicity of the shift change in the characteristic spectrum of X-ray radiation. The frequency shift reflects not structural, but energy differences between electron shells, unique to each element.

The role of Moseley's law in atomic physics

There are slight deviations from the strict linear relationship expressed by Moseley's law. They are associated, firstly, with the peculiarities of the order of filling the electron shells of some elements, and, secondly, with the relativistic effects of the movement of electrons of heavy atoms. In addition, when the number of neutrons in the nucleus changes (the so-called isotopic shift), the position of the lines may change slightly. This effect made it possible to study the atomic structure in detail.

The significance of Moseley's law is extremely great. Its consistent application to the elements of Mendeleev's periodic system established a pattern of increasing the ordinal number corresponding to each small shift in the characteristic maxima. This helped to clarify the question of the physical meaning of the ordinal number of elements. The Z value is not just a number: it is positive electric charge nucleus, which is the sum of unit positive charges of the particles included in its composition. The correct placement of elements in the table and the presence of empty positions in it (they still existed then) received powerful confirmation. The validity of the periodic law was proven.

Moseley's law, in addition, became the basis on which a whole direction of experimental research arose - X-ray spectrometry.

The structure of the electron shells of an atom

Let us briefly recall how the electron structure is structured. It consists of shells designated by the letters K, L, M, N, O, P, Q or numbers from 1 to 7. Electrons within the shell are characterized by the same principal quantum number n, which determines possible values energy. In the outer shells, the electron energy is higher, and the ionization potential for the outer electrons is correspondingly lower.

The shell includes one or more sublevels: s, p, d, f, g, h, i. In each shell, the number of sublevels increases by one compared to the previous one. The number of electrons in each sublevel and in each shell cannot exceed a certain value. They are characterized, in addition to the main quantum number, by the same value of the orbital electron cloud that determines the shape. Sublevels are designated by the shell to which they belong, for example, 2s, 4d, and so on.

The sublevel contains which are specified, in addition to the main and orbital ones, by another quantum number - magnetic, which determines the projection of the orbital momentum of the electron onto the direction of the magnetic field. One orbital can have no more than two electrons, differing in the value of the fourth quantum number - spin.

Let us consider in more detail how characteristic X-ray radiation arises. Since the origin of this type of electromagnetic emission is associated with phenomena occurring inside the atom, it is most convenient to describe it precisely in the approximation of electronic configurations.

Mechanism for generating characteristic X-ray radiation

So, the cause of this radiation is the formation of electron vacancies in the inner shells, caused by the penetration of high-energy electrons deep into the atom. The probability that a hard electron will interact increases with the density of the electron clouds. Therefore, collisions are most likely to occur within tightly packed inner shells, such as the lowest K-shell. Here the atom is ionized and a vacancy is formed in the 1s shell.

This vacancy is filled by an electron from the shell with higher energy, the excess of which is carried away by the X-ray photon. This electron can “fall” from the second shell L, from the third shell M, and so on. This is how a characteristic series is formed, in this example the K-series. An indication of where the electron that fills the vacancy comes from is given in the form of a Greek index in the series designation. "Alpha" means it comes from the L shell, "beta" means it comes from the M shell. Currently, there is a tendency to replace the Greek letter indices with the Latin ones adopted for designating shells.

The intensity of the alpha line in the series is always the highest - this means that the probability of filling a vacancy from a neighboring shell is the highest.

Now we can answer the question, what is the maximum energy of a quantum of characteristic X-ray radiation. It is determined by the difference in the energy values ​​of the levels between which the electron transition occurs, according to the formula E = E n 2 - E n 1, where E n 2 and E n 1 are the energies of the electronic states between which the transition occurred. The highest value of this parameter is given by K-series transitions with maximum high levels atoms of heavy elements. But the intensity of these lines (the height of the peaks) is the lowest, since they are the least probable.

If, due to insufficient voltage at the electrodes, a hard electron cannot reach the K-level, it forms a vacancy at the L-level, and a less energetic L-series with longer wavelengths is formed. Subsequent series are born in a similar way.

In addition, when a vacancy is filled as a result of an electronic transition, a new vacancy appears in the overlying shell. This creates the conditions for generating the next series. Electron vacancies move higher from level to level, and the atom emits a cascade of characteristic spectral series while remaining ionized.

Fine structure of characteristic spectra

Atomic X-ray spectra of characteristic X-ray radiation are characterized by a fine structure, which, as in optical spectra, is expressed in line splitting.

Fine structure is due to the fact that the energy level - the electron shell - is a set of closely located components - subshells. To characterize the subshells, another internal quantum number j is introduced, reflecting the interaction of the electron’s own and orbital magnetic moments.

Due to the influence of spin-orbit interaction, the energy structure of the atom becomes more complex, and as a result, the characteristic X-ray radiation has a spectrum characterized by split lines with very closely spaced elements.

Elements of fine structure are usually designated by additional digital indices.

Characteristic X-ray radiation has a feature reflected only in the fine structure of the spectrum. The transition of an electron to a lower energy level does not occur from the lower subshell of the higher level. Such an event has a negligible probability.

Use of X-rays in spectrometry

This radiation, due to its characteristics described by Moseley’s law, underlies various X-ray spectral methods for analyzing substances. When analyzing the X-ray spectrum, either diffraction of radiation on crystals (wave-dispersive method) or detectors sensitive to the energy of absorbed X-ray photons (energy-dispersive method) are used. Most electron microscopes are equipped with some kind of X-ray spectrometry attachments.

Wave-dispersive spectrometry is particularly accurate. Using special filters, the most intense peaks in the spectrum are highlighted, making it possible to obtain almost monochromatic radiation with a precisely known frequency. The anode material is selected very carefully to ensure that a monochromatic beam of the desired frequency is obtained. Its diffraction on the crystal lattice of the substance under study allows one to study the lattice structure with great accuracy. This method is also used in the study of DNA and other complex molecules.

One of the features of characteristic X-ray radiation is also taken into account in gamma spectrometry. This is a high intensity characteristic peak. Gamma spectrometers use lead shielding against external background radiation that interferes with measurements. But lead, absorbing gamma rays, experiences internal ionization, as a result of which it actively emits in the X-ray range. To absorb the intense peaks of the characteristic X-ray radiation of lead, additional cadmium shielding is used. It, in turn, is ionized and also emits X-rays. To neutralize the characteristic peaks of cadmium, a third shielding layer is used - copper, the X-ray maxima of which lie outside the operating frequency range of the gamma spectrometer.

Spectrometry uses both bremsstrahlung and characteristic X-rays. Thus, when analyzing substances, the absorption spectra of continuous X-rays by various substances are studied.

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 penetrating through opaque barriers, called X-rays X-rays. It was discovered that X-ray radiation, invisible to humans, is absorbed in opaque objects the more strongly, the higher the atomic number (density) of the barrier, so X-rays easily pass through the soft tissues of the human body, but are retained by the bones of the skeleton. Sources of powerful X-rays have been designed to make it possible to illuminate metal parts and find internal defects in them.

The German physicist Laue suggested that X-rays are the same electromagnetic radiation as visible light rays, but with a shorter wavelength and all the laws of optics are applicable to them, including the possibility of diffraction. In visible light optics, diffraction at an elementary level can be represented as the reflection of light from a system of lines - a diffraction grating, which occurs only at certain angles, and the angle of reflection of the rays is related to the angle of incidence, the distance between the lines of the diffraction grating and the wavelength of the incident radiation. For diffraction to occur, the distance between the lines 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 a diffraction grating for x-rays. X-rays directed at the surface of the crystal were reflected onto 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 influences on the crystal.

Nowadays, X-ray analysis is used in many fields of science and technology; with its help, the arrangement of atoms in existing materials has been determined and new materials have been created with a given structure and properties. Latest achievements in this area (nanomaterials, amorphous metals, composite materials) create a field of activity for the next scientific generations.

Occurrence and properties of X-ray radiation

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

When an electron leaves the cathode, electric field causes it to fly towards the anode, while its speed continuously increases, the electron carries a magnetic field, the intensity of which increases with increasing speed of the electron. Reaching the anode surface, the electron is sharply decelerated, and an electromagnetic pulse with wavelengths in a certain interval appears (bremsstrahlung). The distribution of radiation intensity over wavelengths depends on the anode material of the X-ray tube and the applied voltage, while on the short wave side this curve begins with a certain threshold minimum wavelength, depending on the applied voltage. The combination 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.

As the voltage increases, 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 out of one energy level to another. A metastable state arises and for the transition to a stable state a jump of electrons in the opposite direction is necessary. This jump is accompanied by the release of an energy quantum and the appearance of X-ray radiation. Unlike X-rays with a continuous spectrum, this radiation has a very narrow range of wavelengths 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, 10 14 –10 15 atoms produce characteristic radiation in 1 s. 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 X-ray characteristic radiation). The total power of X-ray radiation is only 0.1% of the power consumption, the rest is lost mainly due to conversion to heat.

Due to their high intensity and narrow wavelength range, characteristic X-rays are the main type of radiation used in scientific research and process control. Simultaneously with the K-series rays, L and M-series rays are generated, which have 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 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 one wavelength (monochromatic radiation), special methods have been developed that use the dependence of absorption and diffraction of x-rays on wavelength. An increase in the atomic number of an element is associated with a change in the characteristics of the electron shells, and the higher the atomic number of the X-ray tube anode material, the shorter the K-series wavelength. The most widely used are tubes with anodes made 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, sources of X-ray radiation can be radioactive isotopes, some can directly emit X-rays, others emit electrons and a-particles that generate X-rays when bombarding metal targets. The intensity of X-ray radiation from radioactive sources is usually much less than an X-ray tube (with the exception of radioactive cobalt, which is used in flaw detection and produces radiation of a very short wavelength - g-radiation), they are small in size and do not require electricity. Synchrotron X-rays are produced in electron accelerators; the wavelength of this radiation is significantly longer than that obtained in X-ray tubes (soft X-rays), and its intensity is several orders of magnitude higher than the radiation intensity of X-ray tubes. There are also natural sources of X-ray radiation. Radioactive impurities have been found in many minerals, X-ray radiation has been recorded space objects, including stars.

Interaction of X-rays with crystals

In X-ray studies of materials with a crystalline 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 immobile, their thermal vibrations are not taken into account, and all electrons of the same atom are considered concentrated at one point - a node of the crystal lattice.

To derive the basic equations for 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 falls 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, which scatters light radiation in the visible wavelength range. In order for the amplitudes of all vibrations 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 contains an integer number of wavelengths. When the distance between atoms A this condition looks like:

A(a – a 0) = h l,

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

When 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 a spectrograph for x-rays.

Generalization to a two-dimensional (flat) atomic lattice, and then to a three-dimensional volumetric (spatial) crystal lattice gives two more similar equations, which include the angles of incidence and reflection of X-ray radiation and the distances between atoms in three directions. These equations are called Laue's equations and form the basis of X-ray diffraction analysis.

The amplitudes of rays reflected from parallel atomic planes add up, etc. the number of atoms is very large, the reflected radiation can be detected experimentally. The reflection condition is described by the Wulff–Bragg equation2d 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 wavelength of the x-ray radiation, n is an integer called the order of reflection. Angle q is the angle of incidence with respect specifically to atomic planes, which do not necessarily coincide in direction with the surface of the sample under study.

Several methods of X-ray diffraction analysis have been developed, using both radiation with a continuous spectrum and monochromatic radiation. The object under study can be stationary or rotating, 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, but in all cases during the experiment and interpretation of the results, the Wulff–Bragg equation is used.

X-ray analysis in science and technology

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

The main use of X-rays is in fundamental science– structural analysis, i.e. establishing the spatial arrangement of individual atoms in a crystal. To do this, single crystals are grown and X-ray analysis is performed, studying both the locations and intensities of the reflections. The structures of not only metals, but also complex organic substances, in which the unit cells contain thousands of atoms, have now been determined.

In mineralogy, the structures of thousands of minerals have been determined using X-ray analysis and express methods for analyzing mineral raw materials have been created.

Metals have a relatively simple crystal structure and the X-ray method makes it possible to study its changes during various technological treatments and create the physical basis of new technologies.

The phase composition of the alloys is determined by the location of the lines on the X-ray diffraction patterns, the number, size and shape of crystals are determined by their width, and the orientation of the crystals (texture) is determined by the intensity distribution in the diffraction cone.

Using these techniques, processes during plastic deformation are studied, including crystal fragmentation, 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 and, consequently, the distances between atomic planes change. These changes are small, so special precision methods have been developed for measuring the periods of the crystal lattice with an accuracy two orders of magnitude greater than the measurement accuracy using conventional x-ray research methods. The combination of precision measurements of crystal lattice periods and phase analysis makes it possible to construct the boundaries of 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 the impurity atoms are not randomly located, as in solid solutions, and at the same time not with three-dimensional order, as in chemical compounds. X-ray diffraction patterns of ordered solid solutions contain additional lines; 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 during further heating or even holding at room temperature the solid solution decomposes with the release of particles of a chemical compound. This is the effect of aging and it appears on x-rays as a change in the position and width of the lines. Aging research is especially important for non-ferrous alloys, for example, aging turns soft hardened Aluminium alloy in durable construction material duralumin.

X-ray studies are of greatest technological importance heat treatment become. When quenching (rapid cooling) of steel, a diffusion-free austenite-martensite phase transition occurs, which leads to a change in structure from cubic to tetragonal, i.e. the unit cell takes the shape of a rectangular prism. On radiographs this appears as widening of the lines and the division of some lines into two. The reasons for this effect are not only a change in the crystal structure, but also the occurrence of large internal stresses due to the thermodynamic nonequilibrium of the martensitic structure and sudden cooling. When tempering (heating the hardened steel), the lines on the x-ray diffraction patterns narrow, this is associated with a return to the equilibrium structure.

IN last years X-ray studies of the processing of materials with concentrated energy flows (laser beams, shock waves, neutrons, electron pulses) acquired great importance; they required new techniques and produced new X-ray effects. For example, when laser beams act on metals, heating and cooling occur so quickly that during cooling, crystals in the metal only have time to grow to sizes of several elementary cells (nanocrystals) or do not have time to arise at all. After cooling, such a metal looks like 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 x-ray diffraction patterns. Radioactive decay also causes specific X-ray effects associated with changes in structure, as well as the fact that the sample under study itself becomes a source of X-ray radiation.

Modern medical diagnosis and treatment of certain diseases cannot be imagined without devices that use the properties of x-ray radiation. The discovery of X-rays occurred more than 100 years ago, but even now work continues on the creation of new techniques and devices to minimize the negative effects of radiation on the human body.

Who discovered X-rays and how?

Under natural conditions, X-ray fluxes are rare and are emitted only by certain radioactive isotopes. X-rays or X-rays were only discovered in 1895 by the German scientist Wilhelm Röntgen. This discovery occurred 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 reduced pressure and a fluorescent screen, which each time began to glow the moment the tube began to operate.

Interested in the strange effect, Roentgen conducted a series of studies showing that what was occurring was not visible to the eye radiation can penetrate various barriers: 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 harder through hard living tissues and non-living substances.

X-ray was not the first to study similar phenomenon. In the mid-19th century, similar possibilities were explored by the Frenchman Antoine Mason and the Englishman William Crookes. However, it was Roentgen who first invented a cathode tube and an indicator that could be used in medicine. He was the first to publish a scientific work, which earned him the title of the first Nobel laureate among physicists.

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

Properties of X-rays

X-rays are a component of the general spectrum of electromagnetic radiation. The wavelength lies between gamma and ultraviolet rays. X-rays have all the usual wave properties:

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

To artificially generate a flux of X-rays, special devices are used - X-ray tubes. X-ray radiation occurs due to the contact of fast electrons from tungsten with substances evaporating from the hot anode. Against the background of interaction, electromagnetic waves of short length appear, located in the spectrum from 100 to 0.01 nm and in the energy range of 100-0.1 MeV. If the wavelength of the rays is less than 0.2 nm, this is hard radiation; if the wavelength is greater than this 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. They are generated in the tube simultaneously. Therefore, X-ray irradiation and the characteristics of each specific X-ray tube - its radiation spectrum - depend on these indicators and represent their overlap.

Bremsstrahlung or continuous X-rays are the result of the deceleration of electrons evaporated from a tungsten filament.

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

The listed properties of X-rays allow them to be used in practice:

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

Principles of X-ray imaging

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

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

The photochemical effect of X-rays on photosensitive silver halide materials (exposure) allows for diagnostics - taking X-ray photographs. This property is also used when measuring the total dose received by laboratory assistants in X-ray rooms. Body dosimeters contain special sensitive tapes and indicators. The ionizing effect of X-ray radiation makes it possible to determine the qualitative characteristics of the resulting X-rays.

A single exposure to radiation from conventional X-rays increases the risk of cancer by only 0.001%.

Areas where X-rays are used

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

1. Safety. 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 - to detect defects and carry out chemical analysis substances.
3. Astronomy. Helps monitor cosmic bodies and phenomena using X-ray telescopes.
4. Military industry. To develop laser weapons.

The main application of X-ray radiation is in the medical field. Today, the section of medical radiology includes: radiodiagnosis, radiotherapy (x-ray therapy), radiosurgery. Medical universities graduate highly specialized specialists – radiologists.

X-Radiation - harm and benefits, effects on the body

The high penetrating power and ionizing effect of X-rays can cause changes in the structure of cell DNA, and therefore pose a danger to humans. The harm from x-rays is directly proportional to the radiation dose received. Different organs respond to radiation to varying degrees. The most susceptible include:

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

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

Consequences of X-ray irradiation:

• damage to the bone marrow and the occurrence of pathologies of the hematopoietic system - erythrocytopenia, thrombocytopenia, leukemia;
• damage to the lens, with subsequent development of cataracts;
• cellular mutations that are inherited;
• development of cancer;
• receiving radiation burns;
• development of radiation sickness.

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

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

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

Methods for diagnosing pathologies using X-rays

Radiodiagnostics includes the following techniques:

1. Fluoroscopy is a study during which an image is obtained on a fluorescent screen in real time. Along with the classic acquisition of an image of a body part in real time, today there are X-ray television transillumination 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, followed by transferring it from the screen to paper.
2. Fluorography is the cheapest method for examining organs chest, which consists of taking a reduced-scale photograph of 7x7 cm. Despite the likelihood of error, it is the only way to conduct a mass annual survey of the population. The method is not dangerous and does not require removal of the received radiation dose from the body.
3. Radiography is the production of a summary image on film or paper to clarify the shape of an organ, its position or tone. Can be used to assess peristalsis and the condition of mucous membranes. If there is a choice, then among modern X-ray devices preference should be given neither to digital devices, where the x-ray flux can be higher than that of old devices, but to low-dose X-ray devices with direct flat semiconductor detectors. They allow you to 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 sections of a selected organ. Among the many varieties modern devices CT, for a series of repeated studies, low-dose high-resolution computed tomographs are used.

Radiotherapy

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

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

1. External (proton therapy) – a radiation beam enters the patient’s body from the outside.
2. Internal (brachytherapy) - the use of radioactive capsules by implanting them into the body, placing them closer to the cancer 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. This popularity is due to the fact that the rays do not accumulate and do not require removal from the body; they have a selective effect, without affecting other cells and tissues.

Safe exposure limit to X-rays

This indicator of the norm of permissible annual exposure has its own name - genetically significant equivalent dose (GSD). This indicator does not have clear quantitative values.

1. This indicator depends on the patient’s age and 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 where a person lives.

Today the following average GZD standards are in effect:

• 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 higher than 100 mrem per year;
• the total safe value is 392 mrem per year.

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

a brief description of x-ray radiation

X-ray radiation is electromagnetic waves (flow of quanta, photons), the energy of which is located on the energy scale between ultraviolet radiation and gamma radiation (Fig. 2-1). X-ray photons have energies from 100 eV to 250 keV, which corresponds to radiation with a frequency from 3×10 16 Hz to 6×10 19 Hz and a wavelength of 0.005-10 nm. The electromagnetic spectra of X-rays and gamma radiation overlap to a large extent.

Rice. 2-1. Electromagnetic radiation scale

The main difference between these two types of radiation is the way they are generated. X-rays are produced with the participation of electrons (for example, when their flow is slowed down), and gamma rays are produced during the radioactive decay of the nuclei of certain elements.

X-rays can be generated when an accelerated flow of charged particles decelerates (the so-called bremsstrahlung) or when high-energy transitions occur in the electron shells of atoms (characteristic radiation). Medical devices use X-ray tubes to generate X-rays (Figure 2-2). Their main components are a cathode and a massive anode. Electrons emitted due to the difference in electrical potential between the anode and cathode are accelerated, reach the anode, and are decelerated when they collide with the material. As a result, X-ray bremsstrahlung occurs. During the collision of electrons with the anode, a second process also occurs - electrons are knocked out from the electron shells of the atoms of the anode. Their places are taken by electrons from other shells of the atom. During this process, a second type of X-ray radiation is generated - the so-called characteristic X-ray radiation, the spectrum of which largely depends on the anode material. Anodes are most often made of molybdenum or tungsten. Special devices are available to focus and filter X-rays to improve the resulting images.

Rice. 2-2. Diagram of the X-ray tube device:

The properties of X-rays that predetermine their use in medicine are penetrating ability, fluorescent and photochemical effects. The penetrating ability of X-rays and their absorption by tissues of the human body and artificial materials are the most important properties that determine their use in radiation diagnostics. The shorter the wavelength, the greater the penetrating power of x-rays.

There are “soft” X-rays with low energy and radiation frequency (according to the longest wavelength) and “hard” X-rays, which have high photon energy and radiation frequency and have a short wavelength. The wavelength of X-ray radiation (respectively its “hardness” and penetrating power) depends on the voltage applied to the X-ray tube. The higher the voltage on the tube, the greater the speed and energy of the electron flow and the shorter the wavelength of the x-rays.

When X-ray radiation penetrating through a substance interacts, qualitative and quantitative changes occur in it. The degree of absorption of X-rays by tissues varies and is determined by the density and atomic weight of the elements that make up the object. The higher the density and atomic weight of the substance that makes up the object (organ) being studied, the more X-rays are absorbed. The human body contains tissues and organs of different densities (lungs, bones, soft tissues, etc.), this explains the different absorption of X-rays. Visualization of internal organs and structures is based on artificial or natural differences in the absorption of X-rays by various organs and tissues.

To register radiation passing through a body, its ability to cause fluorescence of certain compounds and have a photochemical effect on the film is used. For this purpose, special screens for fluoroscopy and photographic films for radiography are used. In modern X-ray machines, special systems of digital electronic detectors - digital electronic panels - are used to record attenuated radiation. In this case, X-ray methods are called digital.

Due to the biological effects of X-rays, it is extremely important to protect patients during examination. This is achieved

maximum short time radiation, replacing fluoroscopy with radiography, strictly justified use of ionizing methods, protection by shielding the patient and personnel from exposure to radiation.

Brief description of X-ray radiation - concept and types. Classification and features of the category "Brief characteristics of X-ray radiation" 2017, 2018.