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 electric field and hit the anode surface. 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, the electric field forces it to fly towards the anode, while its speed continuously increases; the electron carries a magnetic field, the strength 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 manifests itself 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.
Although scientists have only discovered the effect of X-rays since the 1890s, the medical use of X-rays for this natural force has progressed rapidly. Today, for the benefit of humanity, X-ray electromagnetic radiation is used in medicine, academia and industry, as well as to generate electricity.
In addition, radiation has useful applications in areas such as Agriculture, archaeology, space, law enforcement work, geology (including mining) and many other activities, even cars are being developed using the phenomenon of nuclear fission.
Medical uses of X-rays
In healthcare settings, physicians and dentists use a variety of nuclear materials and procedures to diagnose, monitor, and treat a wide range of metabolic processes and diseases in the human body. As a result, medical procedures using beams have saved thousands of lives by detecting and treating diseases ranging from an overactive thyroid gland to bone cancer.
The most common of these medical procedures involve the use of rays that can pass through our skin. When an image is taken, our bones and other structures appear to cast shadows because they are denser than our skin, and these shadows can be detected on film or a monitor screen. The effect is similar to placing a pencil between a piece of paper and a light. The shadow of the pencil will be visible on the piece of paper. The difference is that the rays are invisible, so a recording element is needed, something like photographic film. This allows doctors and dentists to evaluate the use of X-rays when seeing broken bones or dental problems.
The use of X-ray radiation for medicinal purposes
The use of X-ray radiation in a targeted manner for therapeutic purposes is not only for detecting damage. When used specifically, it is intended to kill cancerous tissue, reduce tumor size, or reduce pain. For example, radioactive iodine (specifically iodine-131) is often used to treat thyroid cancer, a condition that affects many people.
Devices using this property also connect to computers and scan, called: computed axial tomography or computed tomography.
These instruments provide doctors with color images that show the outline and details of internal organs. It helps doctors detect and identify tumors, size abnormalities, or other physiological or functional organ problems.
In addition, hospitals and radiology centers perform millions of procedures annually. In such procedures, doctors release slightly radioactive substances into patients' bodies to look at certain internal organs, such as the pancreas, kidneys, thyroid, liver or brain, to diagnose clinical conditions.
Radiology is a branch of radiology that studies the effects of x-ray radiation on the body of animals and humans resulting from this disease, their treatment and prevention, as well as methods for diagnosing various pathologies using x-rays (x-ray diagnostics). A typical X-ray diagnostic apparatus includes a power supply (transformers), a high-voltage rectifier, a converter alternating current electrical network in a permanent one, control panel, tripod and x-ray tube.
X-rays are a type of electromagnetic oscillations that are formed in an X-ray tube during a sharp deceleration of accelerated electrons at the moment of their collision with atoms of the anode substance. Currently, the generally accepted point of view is that X-rays, by their nature, physical nature are one of the types of radiant energy, the spectrum of which also includes radio waves, infrared rays, visible light, ultra-violet rays and gamma rays from radioactive elements. X-ray radiation can be characterized as a collection of its smallest particles - quanta or photons.
Rice. 1 - mobile X-ray unit:
A - X-ray tube;
B - power supply device;
B - adjustable tripod.
Rice. 2 - X-ray machine control panel (mechanical - on the left and electronic - on the right): A - panel for adjusting exposure and hardness;
B - feed button high voltage.
Rice. 3 - block diagram of a typical X-ray machine 1 - network;
2 - autotransformer;
3 - step-up transformer;
4 - X-ray tube;
5 - anode;
6 - cathode;
7 - step-down transformer.
Mechanism of X-ray generation
X-rays are formed at the moment of collision of a stream of accelerated electrons with the anode substance. When electrons interact with a target, 99% of their kinetic energy is converted into thermal energy and only 1% - into x-ray radiation.
An X-ray tube consists of a glass cylinder into which 2 electrodes are soldered: a cathode and an anode. The air has been pumped out of the glass balloon: the movement of electrons from the cathode to the anode is possible only under conditions of relative vacuum (10 -7 –10 -8 mm Hg). The cathode has a filament, which is a tightly twisted tungsten spiral. When submitting electric current Electron emission occurs on the filament, in which electrons are separated from the filament and form an electron cloud near the cathode. This cloud is concentrated at the focusing cup of the cathode, which sets the direction of electron motion. The cup is a small depression in the cathode. The anode, in turn, contains a tungsten metal plate onto which electrons are focused - this is where X-rays are produced.
Rice. 4 - X-ray tube device: A - cathode; 
B - anode;
B - tungsten filament;
G - focusing cup of the cathode;
D - flow of accelerated electrons;
E - tungsten target;
F - glass flask;
Z - window made of beryllium;
And - formed x-rays;
K - aluminum filter.
There are 2 transformers connected to the electronic tube: a step-down and a step-up. A step-down transformer heats up a tungsten coil low voltage(5-15 volts), resulting in electron emissions. A step-up, or high-voltage, transformer fits directly to the cathode and anode, which are supplied with a voltage of 20–140 kilovolts. Both transformers are placed in the high-voltage block of the X-ray machine, which is filled with transformer oil, which ensures cooling of the transformers and their reliable insulation.
After an electron cloud has been formed with the help of a step-down transformer, the step-up transformer is turned on, and power is supplied to both poles of the electrical circuit. high voltage: positive impulse - to the anode, and negative - to the cathode. Negatively charged electrons are repelled from the negatively charged cathode and tend to the positively charged anode - due to this potential difference, a high speed of movement is achieved - 100 thousand km/s. At this speed, electrons bombard the tungsten plate of the anode, short-circuiting electrical circuit, resulting in the generation of x-rays and thermal energy.
X-ray radiation is divided into bremsstrahlung and characteristic. Bremsstrahlung occurs due to a sharp slowdown in the speed of electrons emitted by a tungsten helix. Characteristic radiation occurs at the moment of restructuring of the electronic shells of atoms. Both of these types are formed in the X-ray tube at the moment of collision of accelerated electrons with atoms of the anode substance. The emission spectrum of an X-ray tube is a superposition of bremsstrahlung and characteristic X-rays.
Rice. 5 - principle of formation of bremsstrahlung X-ray radiation.
Rice. 6 - principle of formation of characteristic x-ray radiation.
Basic properties of X-ray radiation
- X-rays are invisible to the eye.
- X-ray radiation has a high penetrating ability through organs and tissues of a living organism, as well as dense structures inanimate nature, do not transmit visible light rays.
- X-rays cause certain chemical compounds to glow, called fluorescence.
- Zinc and cadmium sulfides fluoresce yellow-green,
- Calcium tungstate crystals are violet-blue.
Electromagnetic vibration scale
X-rays have a specific wavelength and vibration frequency. The wavelength (λ) and oscillation frequency (ν) are related by the relation: λ ν = c, where c is the speed of light, rounded to 300,000 km per second. The energy of X-rays is determined by the formula E = h ν, where h is Planck's constant, a universal constant equal to 6.626 10 -34 J⋅s. The wavelength of the rays (λ) is related to their energy (E) by the ratio: λ = 12.4 / E.
X-ray radiation differs from other types of electromagnetic oscillations in wavelength (see table) and quantum energy. The shorter the wavelength, the higher its frequency, energy and penetrating power. The X-ray wavelength is in the range
. By changing the wavelength of X-ray radiation, its penetrating ability can be adjusted. X-rays have a very short wavelength, but a high vibration frequency, so they are invisible by the human eye. Due to their enormous energy, quanta have great penetrating power, which is one of the main properties that ensure the use of X-ray radiation in medicine and other sciences.Characteristics of X-ray radiation
Intensity- a quantitative characteristic of X-ray radiation, which is expressed by the number of rays emitted by the tube per unit time. The intensity of X-ray radiation is measured in milliamps. Comparing it with the intensity of visible light from ordinary lamp incandescent, we can draw an analogy: for example, a 20-watt lamp will shine with one intensity, or strength, and a 200-watt lamp will shine with another, while the quality of the light itself (its spectrum) is the same. The intensity of an X-ray is essentially the amount of it. Each electron creates one or more quanta of radiation at the anode, therefore, the number of X-rays when exposing an object is regulated by changing the number of electrons tending to the anode and the number of interactions of electrons with atoms of the tungsten target, which can be done in two ways:
- By changing the degree of heating of the cathode spiral using a step-down transformer (the number of electrons generated during emission will depend on how hot the tungsten spiral is, and the number of radiation quanta will depend on the number of electrons);
- By changing the magnitude of the high voltage supplied by a step-up transformer to the poles of the tube - the cathode and the anode (the higher the voltage is applied to the poles of the tube, the more kinetic energy the electrons receive, which, due to their energy, can interact with several atoms of the anode substance in turn - see. rice. 5; electrons with low energy will be able to enter into fewer interactions).
The X-ray intensity (anode current) multiplied by the exposure time (tube operating time) corresponds to the X-ray exposure, which is measured in mAs (milliamperes per second). Exposure is a parameter that, like intensity, characterizes the number of rays emitted by the X-ray tube. The only difference is that the exposure also takes into account the operating time of the tube (for example, if the tube works for 0.01 seconds, then the number of rays will be one, and if 0.02 seconds, then the number of rays will be different - twice more). The radiation exposure is set by the radiologist on the control panel of the X-ray machine, depending on the type of examination, the size of the object being examined and the diagnostic task.
Rigidity- qualitative characteristics of x-ray radiation. It is measured by the magnitude of the high voltage on the tube - in kilovolts. Determines the penetrating power of x-rays. It is regulated by the high voltage supplied to the X-ray tube by a step-up transformer. The higher the potential difference is created across the electrodes of the tube, the more force the electrons are repelled from the cathode and rush to the anode and the stronger their collision with the anode. The stronger their collision, the shorter the wavelength of the resulting X-ray radiation and the higher the penetrating ability of this wave (or the hardness of the radiation, which, like the intensity, is regulated on the control panel by the voltage parameter on the tube - kilovoltage).
λ - wavelength; 
Rice. 7 - Dependence of wavelength on wave energy:
E - wave energy 
Rice. 8 - The relationship between the voltage on the X-ray tube and the wavelength of the resulting X-ray radiation:
Classification of X-ray tubes
- By purpose
- Diagnostic
- Therapeutic
- For structural analysis
- For translucent
- By design
- By focus
- Single-focus (one spiral on the cathode, and one focal spot on the anode)
- Bifocal (there are two spirals of different sizes on the cathode, and two focal spots on the anode)
- By anode type
- Stationary (fixed)
- Rotating
X-rays are used not only for x-ray diagnostic purposes, but also for therapeutic purposes. As noted above, the ability of X-ray radiation to suppress the growth of tumor cells allows its use in radiation therapy oncological diseases. In addition to the medical field of application, X-ray radiation has found wide application in engineering, materials science, crystallography, chemistry and biochemistry: for example, it is possible to identify structural defects in various products (rails, welds, etc.) using X-ray radiation. This type of research is called flaw detection. And at airports, train stations and other crowded places, X-ray television introscopes are actively used to scan hand luggage and baggage for security purposes.
Depending on the type of anode, X-ray tubes vary in design. Due to the fact that 99% of the kinetic energy of electrons is converted into thermal energy, during operation of the tube, significant heating of the anode occurs - the sensitive tungsten target often burns out. The anode is cooled in modern X-ray tubes by rotating it. The rotating anode has the shape of a disk, which distributes heat evenly over its entire surface, preventing local overheating of the tungsten target.
The design of X-ray tubes also differs in terms of focus. The focal spot is the area of the anode where the working X-ray beam is generated. Divided into real focal spot and effective focal spot ( rice. 12). Because the anode is angled, the effective focal spot is smaller than the actual one. Different focal spot sizes are used depending on the size of the image area. The larger the image area, the wider the focal spot must be to cover the entire area of the image. However, a smaller focal spot produces better image clarity. Therefore, when producing small images, a short filament is used and electrons are directed to a small target area of the anode, creating a smaller focal spot.
Rice. 9 - X-ray tube with a stationary anode.
Rice. 10 - X-ray tube with a rotating anode.
Rice. 11 - X-ray tube device with a rotating anode.
Rice. 12 is a diagram of the formation of a real and effective focal spot.
X-ray radiation (synonym X-rays) is with a wide range of wavelengths (from 8·10 -6 to 10 -12 cm). X-ray radiation occurs when charged particles, most often electrons, are decelerated electric field atoms of matter. The quanta formed in this case have different energies and form a continuous spectrum. The maximum energy of quanta in such a spectrum is equal to the energy of incident electrons. In (cm.) the maximum energy of X-ray quanta, expressed in kiloelectron-volts, is numerically equal to the magnitude of the voltage applied to the tube, expressed in kilovolts. When X-rays pass through a substance, they interact with the electrons of its atoms. For X-ray quanta with energies up to 100 keV, the most characteristic type of interaction is the photoelectric effect. As a result of such interaction, the energy of the quantum is completely spent on tearing the electron out of the atomic shell and imparting kinetic energy to it. As the energy of an X-ray quantum increases, the probability of the photoelectric effect decreases and the process of scattering of quantums by free electrons - the so-called Compton effect - becomes predominant. As a result of such interaction, a secondary electron is also formed and, in addition, a quantum is emitted with an energy lower than the energy of the primary quantum. If the energy of the X-ray quantum exceeds one megaelectron-volt, the so-called pairing effect can occur, in which an electron and a positron are formed (see). Consequently, when passing through a substance, the energy of X-ray radiation decreases, i.e., its intensity decreases. Since absorption of low-energy quanta occurs with a greater probability, the X-ray radiation is enriched with higher-energy quanta. This property of X-ray radiation is used to increase the average energy of quanta, i.e., to increase its hardness. An increase in the hardness of X-ray radiation is achieved using special filters (see). X-ray radiation is used for x-ray diagnostics (see) and (see). See also Ionizing radiation.
X-ray radiation (synonym: x-rays, x-rays) is quantum electromagnetic radiation with a wavelength from 250 to 0.025 A (or energy quanta from 5·10 -2 to 5·10 2 keV). In 1895 it was discovered by V.K. Roentgen. The spectral region of electromagnetic radiation adjacent to X-ray radiation, whose energy quanta exceed 500 keV, is called gamma radiation (see); radiation whose energy quanta are below 0.05 kev constitutes ultraviolet radiation (see).
Thus, representing a relatively small part of the vast spectrum of electromagnetic radiation, which includes both radio waves and visible light, X-ray radiation, like any electromagnetic radiation, propagates at the speed of light (in a vacuum of about 300 thousand km/sec) and is characterized by a wavelength λ ( the distance over which radiation travels in one oscillation period). X-ray radiation also has a number of other wave properties (refraction, interference, diffraction), but they are much more difficult to observe than longer wavelength radiation: visible light, radio waves.
X-ray spectra: a1 - continuous bremsstrahlung spectrum at 310 kV; a - continuous brake spectrum at 250 kV, a1 - spectrum filtered with 1 mm Cu, a2 - spectrum filtered with 2 mm Cu, b - K-series tungsten lines.
To generate X-ray radiation, X-ray tubes (see) are used, in which radiation occurs when fast electrons interact with atoms of the anode substance. There are two types of X-ray radiation: bremsstrahlung and characteristic. Bremsstrahlung X-rays have a continuous spectrum, similar to ordinary white light. The intensity distribution depending on the wavelength (Fig.) is represented by a curve with a maximum; to the side long waves the curve falls flatly, and towards short wavelengths it falls steeply and ends at a certain wavelength (λ0), called the short-wavelength boundary of the continuous spectrum. The value of λ0 is inversely proportional to the voltage on the tube. Bremsstrahlung occurs when fast electrons interact with atomic nuclei. The intensity of bremsstrahlung is directly proportional to the strength of the anode current, the square of the voltage across the tube and the atomic number (Z) of the anode substance.
If the energy of the electrons accelerated in the X-ray tube exceeds the value critical for the anode substance (this energy is determined by the voltage Vcr critical for this substance on the tube), then characteristic radiation occurs. The characteristic spectrum is lined; its spectral lines form series, designated by the letters K, L, M, N.
The K series is the shortest wavelength, the L series is longer wavelength, the M and N series are observed only in heavy elements (Vcr of tungsten for the K-series is 69.3 kV, for the L-series - 12.1 kV). Characteristic radiation arises as follows. Fast electrons knock atomic electrons out of their inner shells. The atom is excited and then returns to the ground state. In this case, electrons from the outer, less bound shells fill the spaces vacated in the inner shells, and photons of characteristic radiation are emitted with an energy equal to the difference between the energies of the atom in the excited and ground states. This difference (and therefore the photon energy) has a certain value characteristic of each element. This phenomenon underlies X-ray spectral analysis of elements. The figure shows the line spectrum of tungsten against the background of a continuous spectrum of bremsstrahlung.
The energy of electrons accelerated in the X-ray tube is converted almost entirely into thermal energy (the anode becomes very hot), only a small part (about 1% at a voltage close to 100 kV) is converted into bremsstrahlung energy.
The use of X-rays in medicine is based on the laws of absorption of X-rays by matter. The absorption of X-ray radiation is completely independent of the optical properties of the absorber substance. Colorless and transparent lead glass, used to protect personnel in x-ray rooms, almost completely absorbs x-rays. In contrast, a sheet of paper that is not transparent to light does not attenuate x-rays.
The intensity of a homogeneous (i.e., a certain wavelength) X-ray beam passing through an absorber layer decreases according to the exponential law (e-x), where e is the base of natural logarithms (2.718), and the exponent x is equal to the product of the mass attenuation coefficient (μ /p) cm 2 /g per thickness of the absorber in g/cm 2 (here p is the density of the substance in g/cm 3). The attenuation of X-ray radiation occurs due to both scattering and absorption. Accordingly, the mass attenuation coefficient is the sum of the mass absorption and scattering coefficients. The mass absorption coefficient increases sharply with increasing atomic number (Z) of the absorber (proportional to Z3 or Z5) and with increasing wavelength (proportional to λ3). This dependence on wavelength is observed within the absorption bands, at the boundaries of which the coefficient exhibits jumps.
The mass scattering coefficient increases with increasing atomic number of the substance. At λ≥0.3Å the scattering coefficient does not depend on the wavelength, at λ<0,ЗÅ он уменьшается с уменьшением λ.
A decrease in the absorption and scattering coefficients with decreasing wavelength causes an increase in the penetrating power of X-ray radiation. The mass absorption coefficient for bone [uptake is mainly due to Ca 3 (PO 4) 2 ] is almost 70 times greater than for soft tissue, where uptake is mainly due to water. This explains why the shadow of bones stands out so sharply against the background of soft tissue on radiographs.
The propagation of a non-uniform X-ray beam through any medium, along with a decrease in intensity, is accompanied by a change in the spectral composition and a change in the quality of the radiation: the long-wave part of the spectrum is absorbed to a greater extent than the short-wave part, the radiation becomes more uniform. Filtering out the long-wave part of the spectrum allows, during X-ray therapy of lesions located deep in the human body, to improve the ratio between deep and surface doses (see X-ray filters). To characterize the quality of an inhomogeneous beam of X-rays, the concept of “half-attenuation layer (L)” is used - a layer of substance that attenuates the radiation by half. The thickness of this layer depends on the voltage on the tube, the thickness and material of the filter. To measure half-attenuation layers, cellophane (up to 12 keV energy), aluminum (20-100 keV), copper (60-300 keV), lead and copper (>300 keV) are used. For X-rays generated at voltages of 80-120 kV, 1 mm of copper is equivalent in filtering capacity to 26 mm of aluminum, 1 mm of lead is equivalent to 50.9 mm of aluminum.
The absorption and scattering of X-ray radiation is due to its corpuscular properties; X-ray radiation interacts with atoms as a stream of corpuscles (particles) - photons, each of which has a certain energy (inversely proportional to the wavelength of X-ray radiation). The energy range of X-ray photons is 0.05-500 keV.
The absorption of X-ray radiation is due to the photoelectric effect: the absorption of a photon by the electron shell is accompanied by the ejection of an electron. The atom is excited and, returning to the ground state, emits characteristic radiation. The emitted photoelectron carries away all the energy of the photon (minus the binding energy of the electron in the atom).
X-ray scattering is caused by electrons in the scattering medium. A distinction is made between classical scattering (the wavelength of the radiation does not change, but the direction of propagation changes) and scattering with a change in wavelength - the Compton effect (the wavelength of the scattered radiation is greater than that of the incident radiation). In the latter case, the photon behaves like a moving ball, and the scattering of photons occurs, according to Comton’s figurative expression, like playing billiards with photons and electrons: colliding with an electron, the photon transfers part of its energy to it and is scattered, having less energy (accordingly, the wavelength of the scattered radiation increases), an electron flies out of the atom with recoil energy (these electrons are called Compton electrons, or recoil electrons). Absorption of X-ray energy occurs during the formation of secondary electrons (Compton and photoelectrons) and the transfer of energy to them. The energy of X-ray radiation transferred to a unit mass of a substance determines the absorbed dose of X-ray radiation. The unit of this dose 1 rad corresponds to 100 erg/g. Due to the absorbed energy, a number of secondary processes occur in the absorber substance, which are important for X-ray dosimetry, since it is on them that the methods for measuring X-ray radiation are based. (see Dosimetry).
All gases and many liquids, semiconductors and dielectrics increase electrical conductivity when exposed to X-rays. Conductivity is detected by the best insulating materials: paraffin, mica, rubber, amber. The change in conductivity is caused by ionization of the medium, i.e., the separation of neutral molecules into positive and negative ions (ionization is produced by secondary electrons). Ionization in air is used to determine X-ray exposure dose (dose in air), which is measured in roentgens (see Ionizing Radiation Doses). At a dose of 1 r, the absorbed dose in air is 0.88 rad.
Under the influence of X-ray radiation, as a result of the excitation of molecules of a substance (and during the recombination of ions), in many cases a visible glow of the substance is excited. At high intensities of X-ray radiation, a visible glow is observed in air, paper, paraffin, etc. (with the exception of metals). The highest yield of visible luminescence is provided by crystalline phosphors such as Zn·CdS·Ag-phosphorus and others used for fluoroscopy screens.
Under the influence of x-ray radiation, various chemical processes can also occur in a substance: decomposition of silver halide compounds (a photographic effect used in x-ray photography), decomposition of water and aqueous solutions of hydrogen peroxide, changes in the properties of celluloid (turbidity and release of camphor), paraffin (turbidity and bleaching) .
As a result of complete conversion, all the energy absorbed by the chemically inert substance, the x-ray radiation, is converted into heat. Measuring very small amounts of heat requires highly sensitive methods, but is the main method for absolute measurements of X-ray radiation.
Secondary biological effects from exposure to x-ray radiation are the basis of medical x-ray therapy (see). X-ray radiation, whose quanta are 6-16 keV (effective wavelengths from 2 to 5 Å), is almost completely absorbed by the skin tissue of the human body; these are called boundary rays, or sometimes Bucca's rays (see Bucca's rays). For deep X-ray therapy, hard filtered radiation with effective energy quanta from 100 to 300 keV is used.
The biological effect of X-ray radiation should be taken into account not only during X-ray therapy, but also during X-ray diagnostics, as well as in all other cases of contact with X-ray radiation that require the use of radiation protection (see).
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 power of X-rays and their absorption by human body tissues and artificial materials are the most important properties, which 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 with high photon energy and radiation frequency and 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.