X-rays are produced as they propagate. What is x-rays and how is it used in medicine
X-ray radiation, from the point of view of physics, is electromagnetic radiation, the wavelength of which varies in the range from 0.001 to 50 nanometers. It was discovered in 1895 by the German physicist W.K. Roentgen.
By nature, these rays are related to solar ultraviolet. Radio waves are the longest in the spectrum. They are followed by infrared light, which our eyes do not perceive, but we feel it as heat. Next come the rays from red to purple. Then - ultraviolet (A, B and C). And right behind it are x-rays and gamma rays.
X-ray can be obtained in two ways: by deceleration in the matter of charged particles passing through it and by the transition of electrons from the upper layers to the internal ones when energy is released.
Unlike visible light, these rays are very long, so they are able to penetrate opaque materials without being reflected, refracted, or accumulated in them.
Bremsstrahlung is easier to obtain. Charged particles emit electromagnetic radiation when braking. The greater the acceleration of these particles and, consequently, the sharper the deceleration, the more X-rays are produced, and the wavelength becomes shorter. In most cases, in practice, they resort to the generation of rays in the process of deceleration of electrons in solids. This allows you to control the source of this radiation, avoiding the danger of radiation exposure, because when the source is turned off, the X-ray emission completely disappears.
The most common source of such radiation - The radiation emitted by it is inhomogeneous. It contains both soft (long-wave) and hard (short-wave) radiation. The soft one is characterized by the fact that it is completely absorbed by the human body, therefore such X-ray radiation does twice as much harm as the hard one. With excessive electromagnetic radiation in the tissues of the human body, ionization can damage cells and DNA.
The tube is with two electrodes - a negative cathode and a positive anode. When the cathode is heated, electrons evaporate from it, then they are accelerated in an electric field. Colliding with the solid matter of the anodes, they begin deceleration, which is accompanied by the emission of electromagnetic radiation.
X-ray radiation, the properties of which are widely used in medicine, is based on obtaining a shadow image of the object under study on a sensitive screen. If the diagnosed organ is illuminated with a beam of rays parallel to each other, then the projection of shadows from this organ will be transmitted without distortion (proportionally). In practice, the radiation source is more like a point source, so it is located at a distance from the person and from the screen.
To receive a person is placed between the x-ray tube and the screen or film, acting as radiation receivers. As a result of irradiation, bone and other dense tissues appear in the image as clear shadows, look more contrast against the background of less expressive areas that transmit tissues with less absorption. On x-rays, a person becomes "translucent".
As X-rays propagate, they can be scattered and absorbed. Before absorption, the rays can travel hundreds of meters in the air. In dense matter, they are absorbed much faster. Human biological tissues are heterogeneous, so their absorption of rays depends on the density of the tissue of the organs. absorbs rays faster than soft tissues, because it contains substances that have large atomic numbers. Photons (individual particles of rays) are absorbed by different tissues of the human body in different ways, which makes it possible to obtain a contrast image using x-rays.
In 1895, the German physicist W. Roentgen discovered a new, previously unknown type of electromagnetic radiation, which was named X-ray in honor of its discoverer. W. Roentgen became the author of his discovery at the age of 50, holding the post of rector of the University of Würzburg and having a reputation as one of the best experimenters of his time. One of the first to find a technical application for Roentgen's discovery was the American Edison. He created a handy demonstration apparatus and already in May 1896 organized an X-ray exhibition in New York, where visitors could look at their own hand on a luminous screen. After Edison's assistant died from the severe burns he received from constant demonstrations, the inventor stopped further experiments with X-rays.
X-ray radiation began to be used in medicine due to its high penetrating power. Initially, X-rays were used to examine bone fractures and locate foreign bodies in the human body. Currently, there are several methods based on X-rays. But these methods have their drawbacks: radiation can cause deep damage to the skin. Appearing ulcers often turned into cancer. In many cases, fingers or hands had to be amputated. Fluoroscopy(synonymous with translucence) is one of the main methods of X-ray examination, which consists in obtaining a planar positive image of the object under study on a translucent (fluorescent) screen. During fluoroscopy, the subject is between a translucent screen and an x-ray tube. On modern X-ray translucent screens, the image appears at the moment the X-ray tube is turned on and disappears immediately after it is turned off. Fluoroscopy makes it possible to study the function of the organ - heart pulsation, respiratory movements of the ribs, lungs, diaphragm, peristalsis of the digestive tract, etc. Fluoroscopy is used in the treatment of diseases of the stomach, gastrointestinal tract, duodenum, diseases of the liver, gallbladder and biliary tract. At the same time, the medical probe and manipulators are inserted without tissue damage, and the actions during the operation are controlled by fluoroscopy and are visible on the monitor.
Radiography - method of X-ray diagnostics with the registration of a fixed image on a photosensitive material - special. photographic film (X-ray film) or photographic paper with subsequent photo processing; With digital radiography, the image is fixed in the computer's memory. It is performed on X-ray diagnostic devices - stationary, installed in specially equipped X-ray rooms, or mobile and portable - at the patient's bedside or in the operating room. On radiographs, the elements of the structures of various organs are displayed much more clearly than on a fluorescent screen. Radiography is performed in order to detect and prevent various diseases, its main goal is to help doctors of various specialties correctly and quickly make a diagnosis. An x-ray image captures the state of an organ or tissue only at the time of exposure. However, a single radiograph captures only anatomical changes at a certain moment, it gives the statics of the process; through a series of radiographs taken at certain intervals, it is possible to study the dynamics of the process, that is, functional changes. Tomography. The word tomography can be translated from Greek as slice image. This means that the purpose of tomography is to obtain a layered image of the internal structure of the object of study. Computed tomography is characterized by high resolution, which makes it possible to distinguish subtle changes in soft tissues. CT allows to detect such pathological processes that cannot be detected by other methods. In addition, the use of CT makes it possible to reduce the dose of X-ray radiation received by patients during the diagnostic process.
Fluorography- a diagnostic method that allows you to get an image of organs and tissues, was developed at the end of the 20th century, a year after X-rays were discovered. In the pictures you can see sclerosis, fibrosis, foreign objects, neoplasms, inflammations that have a developed degree, the presence of gases and infiltrate in the cavities, abscesses, cysts, and so on. Most often, a chest x-ray is performed, which allows to detect tuberculosis, a malignant tumor in the lungs or chest, and other pathologies.
X-ray therapy- This is a modern method with which the treatment of certain pathologies of the joints is performed. The main directions of treatment of orthopedic diseases by this method are: Chronic. Inflammatory processes of the joints (arthritis, polyarthritis); Degenerative (osteoarthritis, osteochondrosis, deforming spondylosis). The purpose of radiotherapy is the inhibition of the vital activity of cells of pathologically altered tissues or their complete destruction. In non-tumor diseases, X-ray therapy is aimed at suppressing the inflammatory reaction, inhibiting proliferative processes, reducing pain sensitivity and secretory activity of the glands. It should be borne in mind that the sex glands, hematopoietic organs, leukocytes, and malignant tumor cells are most sensitive to X-rays. The radiation dose in each case is determined individually.
For the discovery of X-rays, Roentgen was awarded the first Nobel Prize in Physics in 1901, and the Nobel Committee emphasized the practical importance of his discovery.
Thus, X-rays are invisible electromagnetic radiation with a wavelength of 105 - 102 nm. X-rays can penetrate some materials that are opaque to visible light. They are emitted during the deceleration of fast electrons in matter (continuous spectrum) and during transitions of electrons from the outer electron shells of the atom to the inner ones (linear spectrum). Sources of X-ray radiation are: X-ray tube, some radioactive isotopes, accelerators and accumulators of electrons (synchrotron radiation). Receivers - film, luminescent screens, nuclear radiation detectors. X-rays are used in X-ray diffraction analysis, medicine, flaw detection, X-ray spectral analysis, etc.
X-RAY RADIATION
invisible radiation capable of penetrating, albeit to varying degrees, all substances. It is electromagnetic radiation with a wavelength of about 10-8 cm. Like visible light, X-rays cause blackening of photographic film. This property is of great importance for medicine, industry and scientific research. Passing through the object under study and then falling on the film, X-ray radiation depicts its internal structure on it. Since the penetrating power of X-ray radiation is different for different materials, parts of the object that are less transparent to it give brighter areas in the photograph than those through which the radiation penetrates well. Thus, bone tissues are less transparent to x-rays than the tissues that make up the skin and internal organs. Therefore, on the radiograph, the bones will be indicated as lighter areas and the fracture site, which is more transparent for radiation, can be quite easily detected. X-ray imaging is also used in dentistry to detect caries and abscesses in the roots of teeth, as well as in industry to detect cracks in castings, plastics and rubbers. X-rays are used in chemistry to analyze compounds and in physics to study the structure of crystals. An X-ray beam passing through a chemical compound causes a characteristic secondary radiation, the spectroscopic analysis of which allows the chemist to determine the composition of the compound. When falling on a crystalline substance, an X-ray beam is scattered by the atoms of the crystal, giving a clear, regular pattern of spots and stripes on a photographic plate, which makes it possible to establish the internal structure of the crystal. The use of x-rays in cancer treatment is based on the fact that it kills cancer cells. However, it can also have an undesirable effect on normal cells. Therefore, extreme caution must be exercised in this use of X-rays. X-ray radiation was discovered by the German physicist W. Roentgen (1845-1923). His name is immortalized in some other physical terms associated with this radiation: the international unit of the dose of ionizing radiation is called the roentgen; a picture taken with an x-ray machine is called a radiograph; The field of radiological medicine that uses x-rays to diagnose and treat diseases is called radiology. Roentgen discovered radiation in 1895 while a professor of physics at the University of Würzburg. While conducting experiments with cathode rays (electron flows in discharge tubes), he noticed that a screen located near the vacuum tube, covered with crystalline barium cyanoplatinite, glows brightly, although the tube itself is covered with black cardboard. Roentgen further established that the penetrating power of the unknown rays he discovered, which he called X-rays, depended on the composition of the absorbing material. He also imaged the bones of his own hand by placing it between a cathode ray discharge tube and a screen coated with barium cyanoplatinite. Roentgen's discovery was followed by experiments by other researchers who discovered many new properties and applications of this radiation. A great contribution was made by M. Laue, W. Friedrich and P. Knipping, who demonstrated in 1912 the diffraction of X-rays when it passes through a crystal; W. Coolidge, who in 1913 invented a high-vacuum X-ray tube with a heated cathode; G. Moseley, who established in 1913 the relationship between the wavelength of radiation and the atomic number of an element; G. and L. Braggy, who received the Nobel Prize in 1915 for developing the fundamentals of X-ray diffraction analysis.
OBTAINING X-RAY RADIATION
X-ray radiation occurs when electrons moving at high speeds interact with matter. When electrons collide with atoms of any substance, they quickly lose their kinetic energy. In this case, most of it is converted into heat, and a small fraction, usually less than 1%, is converted into X-ray energy. This energy is released in the form of quanta - particles called photons that have energy but have zero rest mass. X-ray photons differ in their energy, which is inversely proportional to their wavelength. With the conventional method of obtaining x-rays, a wide range of wavelengths is obtained, which is called the x-ray spectrum. The spectrum contains pronounced components, as shown in Fig. 1. A wide "continuum" is called a continuous spectrum or white radiation. The sharp peaks superimposed on it are called characteristic x-ray emission lines. Although the entire spectrum is the result of collisions of electrons with matter, the mechanisms for the appearance of its wide part and lines are different. A substance consists of a large number of atoms, each of which has a nucleus surrounded by electron shells, and each electron in the shell of an atom of a given element occupies a certain discrete energy level. Usually these shells, or energy levels, are denoted by the symbols K, L, M, etc., starting from the shell closest to the nucleus. When an incident electron of sufficiently high energy collides with one of the electrons bound to the atom, it knocks that electron out of its shell. The empty space is occupied by another electron from the shell, which corresponds to a higher energy. This latter gives off excess energy by emitting an X-ray photon. Since the shell electrons have discrete energy values, the resulting X-ray photons also have a discrete spectrum. This corresponds to sharp peaks for certain wavelengths, the specific values of which depend on the target element. The characteristic lines form K-, L- and M-series, depending on which shell (K, L or M) the electron was removed from. The relationship between the wavelength of X-rays and the atomic number is called Moseley's law (Fig. 2).
If an electron collides with a relatively heavy nucleus, then it slows down, and its kinetic energy is released in the form of an X-ray photon of approximately the same energy. If he flies past the nucleus, he will lose only part of his energy, and the rest will be transferred to other atoms that fall in his way. Each act of energy loss leads to the emission of a photon with some energy. A continuous X-ray spectrum appears, the upper limit of which corresponds to the energy of the fastest electron. This is the mechanism for the formation of a continuous spectrum, and the maximum energy (or minimum wavelength) that fixes the boundary of the continuous spectrum is proportional to the accelerating voltage, which determines the speed of the incident electrons. The spectral lines characterize the material of the bombarded target, while the continuous spectrum is determined by the energy of the electron beam and practically does not depend on the target material. X-rays can be obtained not only by electron bombardment, but also by irradiating the target with X-rays from another source. In this case, however, most of the energy of the incident beam goes into the characteristic X-ray spectrum, and a very small fraction of it falls into the continuous spectrum. Obviously, the incident X-ray beam must contain photons whose energy is sufficient to excite the characteristic lines of the bombarded element. The high percentage of energy per characteristic spectrum makes this method of X-ray excitation convenient for scientific research.
X-ray tubes. In order to obtain X-ray radiation due to the interaction of electrons with matter, it is necessary to have a source of electrons, means of accelerating them to high speeds, and a target capable of withstanding electron bombardment and producing X-ray radiation of the required intensity. The device that has all this is called an x-ray tube. Early explorers used "deep vacuum" tubes such as today's discharge tubes. The vacuum in them was not very high. Discharge tubes contain a small amount of gas, and when a large potential difference is applied to the electrodes of the tube, the gas atoms turn into positive and negative ions. The positive ones move towards the negative electrode (cathode) and, falling on it, knock electrons out of it, and they, in turn, move towards the positive electrode (anode) and, bombarding it, create a stream of X-ray photons. In the modern X-ray tube developed by Coolidge (Fig. 3), the source of electrons is a tungsten cathode heated to a high temperature. The electrons are accelerated to high speeds by the high potential difference between the anode (or anticathode) and the cathode. Since the electrons must reach the anode without colliding with atoms, a very high vacuum is required, for which the tube must be well evacuated. This also reduces the probability of ionization of the remaining gas atoms and the associated side currents.
The electrons are focused on the anode by a specially shaped electrode surrounding the cathode. This electrode is called the focusing electrode and together with the cathode forms the "electronic searchlight" of the tube. The anode subjected to electron bombardment must be made of a refractory material, since most of the kinetic energy of the bombarding electrons is converted into heat. In addition, it is desirable that the anode be made of a material with a high atomic number, since the x-ray yield increases with increasing atomic number. Tungsten, whose atomic number is 74, is most often chosen as the anode material. The design of X-ray tubes can be different depending on the application conditions and requirements.
X-RAY DETECTION
All methods for detecting X-rays are based on their interaction with matter. Detectors can be of two types: those that give an image, and those that do not. The former include X-ray fluorography and fluoroscopy devices, in which the X-ray beam passes through the object under study, and the transmitted radiation enters the luminescent screen or film. The image appears due to the fact that different parts of the object under study absorb radiation in different ways - depending on the thickness of the substance and its composition. In detectors with a luminescent screen, the X-ray energy is converted into a directly observable image, while in radiography it is recorded on a sensitive emulsion and can only be observed after the film has been developed. The second type of detectors includes a wide variety of devices in which the X-ray energy is converted into electrical signals that characterize the relative intensity of the radiation. These include ionization chambers, a Geiger counter, a proportional counter, a scintillation counter, and some special detectors based on cadmium sulfide and selenide. Currently, scintillation counters can be considered the most efficient detectors, which work well in a wide energy range.
see also PARTICLE DETECTORS . The detector is selected taking into account the conditions of the problem. For example, if it is necessary to accurately measure the intensity of diffracted X-ray radiation, then counters are used that allow measurements to be made with an accuracy of fractions of a percent. If it is necessary to register a lot of diffracted beams, then it is advisable to use X-ray film, although in this case it is impossible to determine the intensity with the same accuracy.
X-RAY AND GAMMA DEFECTOSCOPY
One of the most common applications of X-rays in industry is material quality control and flaw detection. The x-ray method is non-destructive, so that the material being tested, if found to meet the required requirements, can then be used for its intended purpose. Both x-ray and gamma flaw detection are based on the penetrating power of x-rays and the characteristics of its absorption in materials. Penetrating power is determined by the energy of X-ray photons, which depends on the accelerating voltage in the X-ray tube. Therefore, thick samples and samples from heavy metals, such as gold and uranium, require an X-ray source with a higher voltage for their study, and for thin samples, a source with a lower voltage is sufficient. For gamma-ray flaw detection of very large castings and large rolled products, betatrons and linear accelerators are used, accelerating particles to energies of 25 MeV and more. The absorption of X-rays in a material depends on the thickness of the absorber d and the absorption coefficient m and is determined by the formula I = I0e-md, where I is the intensity of the radiation transmitted through the absorber, I0 is the intensity of the incident radiation, and e = 2.718 is the base of natural logarithms. For a given material, at a given wavelength (or energy) of X-rays, the absorption coefficient is a constant. But the radiation of an X-ray source is not monochromatic, but contains a wide spectrum of wavelengths, as a result of which the absorption at the same thickness of the absorber depends on the wavelength (frequency) of the radiation. X-ray radiation is widely used in all industries associated with the processing of metals by pressure. It is also used to test artillery barrels, foodstuffs, plastics, to test complex devices and systems in electronic engineering. (Neutronography, which uses neutron beams instead of X-rays, is used for similar purposes.) X-rays are also used for other purposes, such as examining paintings to determine their authenticity or detecting additional layers of paint on top of the main layer.
X-RAY DIFFRACTION
X-ray diffraction provides important information about solids—their atomic structure and crystal form—as well as about liquids, amorphous bodies, and large molecules. The diffraction method is also used for accurate (with an error of less than 10-5) determination of interatomic distances, detection of stresses and defects, and for determining the orientation of single crystals. The diffraction pattern can identify unknown materials, as well as detect the presence of impurities in the sample and determine them. The importance of the X-ray diffraction method for the progress of modern physics can hardly be overestimated, since the modern understanding of the properties of matter is ultimately based on data on the arrangement of atoms in various chemical compounds, on the nature of the bonds between them, and on structural defects. The main tool for obtaining this information is the X-ray diffraction method. X-ray diffraction crystallography is essential for determining the structures of complex large molecules, such as those of deoxyribonucleic acid (DNA), the genetic material of living organisms. Immediately after the discovery of X-rays, scientific and medical interest was concentrated both on the ability of this radiation to penetrate through bodies, and on its nature. Experiments on the diffraction of X-rays on slits and diffraction gratings showed that it belongs to electromagnetic radiation and has a wavelength of the order of 10-8-10-9 cm. Even earlier, scientists, in particular W. Barlow, guessed that the regular and symmetrical shape of natural crystals is due to the ordered arrangement of atoms that form the crystal. In some cases, Barlow was able to correctly predict the structure of a crystal. The value of the predicted interatomic distances was 10-8 cm. The fact that the interatomic distances turned out to be of the order of the X-ray wavelength made it possible in principle to observe their diffraction. The result was the idea for one of the most important experiments in the history of physics. M. Laue organized an experimental test of this idea, which was carried out by his colleagues W. Friedrich and P. Knipping. In 1912, the three of them published their work on the results of X-ray diffraction. Principles of X-ray diffraction. To understand the phenomenon of X-ray diffraction, one must consider in order: firstly, the spectrum of X-rays, secondly, the nature of the crystal structure and, thirdly, the phenomenon of diffraction itself. As mentioned above, the characteristic X-ray radiation consists of a series of spectral lines of a high degree of monochromaticity, determined by the anode material. With the help of filters, you can select the most intense of them. Therefore, by choosing the anode material in an appropriate way, it is possible to obtain a source of almost monochromatic radiation with a very precisely defined wavelength value. The wavelengths of the characteristic radiation typically range from 2.285 for chromium to 0.558 for silver (the values for the various elements are known to six significant figures). The characteristic spectrum is superimposed on a continuous "white" spectrum of much lower intensity, due to the deceleration of the incident electrons in the anode. Thus, two types of radiation can be obtained from each anode: characteristic and bremsstrahlung, each of which plays an important role in its own way. Atoms in the crystal structure are located at regular intervals, forming a sequence of identical cells - a spatial lattice. Some lattices (for example, for most ordinary metals) are quite simple, while others (for example, for protein molecules) are quite complex. The crystal structure is characterized by the following: if one shifts from some given point of one cell to the corresponding point of the neighboring cell, then exactly the same atomic environment will be found. And if some atom is located at one or another point of one cell, then the same atom will be located at the equivalent point of any neighboring cell. This principle is strictly valid for a perfect, ideally ordered crystal. However, many crystals (for example, metallic solid solutions) are disordered to some extent; crystallographically equivalent places can be occupied by different atoms. In these cases, it is not the position of each atom that is determined, but only the position of an atom "statistically averaged" over a large number of particles (or cells). The phenomenon of diffraction is discussed in the article OPTICS and the reader may refer to this article before moving on. It shows that if waves (for example, sound, light, X-rays) pass through a small slit or hole, then the latter can be considered as a secondary source of waves, and the image of the slit or hole consists of alternating light and dark stripes. Further, if there is a periodic structure of holes or slots, then as a result of the amplifying and attenuating interference of rays coming from different holes, a clear diffraction pattern arises. X-ray diffraction is a collective scattering phenomenon in which the role of holes and scattering centers is played by periodically arranged atoms of the crystal structure. Mutual amplification of their images at certain angles gives a diffraction pattern similar to that which would result from the diffraction of light on a three-dimensional diffraction grating. Scattering occurs due to the interaction of the incident X-ray radiation with electrons in the crystal. Due to the fact that the wavelength of X-ray radiation is of the same order as the dimensions of the atom, the wavelength of the scattered X-ray radiation is the same as that of the incident. This process is the result of forced oscillations of electrons under the action of incident X-rays. Consider now an atom with a cloud of bound electrons (surrounding the nucleus) on which X-rays are incident. Electrons in all directions simultaneously scatter the incident and emit their own X-ray radiation of the same wavelength, although of different intensity. The intensity of the scattered radiation is related to the atomic number of the element, since the atomic number is equal to the number of orbital electrons that can participate in scattering. (This dependence of the intensity on the atomic number of the scattering element and on the direction in which the intensity is measured is characterized by the atomic scattering factor, which plays an extremely important role in the analysis of the structure of crystals.) Let us choose in the crystal structure a linear chain of atoms located at the same distance from each other, and consider their diffraction pattern. It has already been noted that the X-ray spectrum consists of a continuous part ("continuum") and a set of more intense lines characteristic of the element that is the anode material. Let's say we filtered out the continuous spectrum and got an almost monochromatic X-ray beam directed at our linear chain of atoms. The amplification condition (amplifying interference) is satisfied if the path difference of waves scattered by neighboring atoms is a multiple of the wavelength. If the beam is incident at an angle a0 to a line of atoms separated by intervals a (period), then for the diffraction angle a the path difference corresponding to the gain will be written as a(cos a - cosa0) = hl, where l is the wavelength and h is integer (Fig. 4 and 5).
To extend this approach to a three-dimensional crystal, it is only necessary to choose rows of atoms in two other directions in the crystal and solve the three equations thus obtained jointly for three crystal axes with periods a, b and c. The other two equations are
These are the three fundamental Laue equations for X-ray diffraction, with the numbers h, k and c being the Miller indices for the diffraction plane.
see also CRYSTALS AND CRYSTALLOGRAPHY. Considering any of the Laue equations, for example the first one, one can notice that since a, a0, l are constants, and h = 0, 1, 2, ..., its solution can be represented as a set of cones with a common axis a (Fig. . 5). The same is true for directions b and c. In the general case of three-dimensional scattering (diffraction), the three Laue equations must have a common solution, i.e. three diffraction cones located on each of the axes must intersect; the common line of intersection is shown in fig. 6. The joint solution of the equations leads to the Bragg-Wulf law:
l = 2(d/n)sinq, where d is the distance between the planes with indices h, k and c (period), n = 1, 2, ... are integers (diffraction order), and q is the angle formed by incident beam (as well as diffracting) with the plane of the crystal in which diffraction occurs. Analyzing the equation of the Bragg - Wolfe law for a single crystal located in the path of a monochromatic X-ray beam, we can conclude that diffraction is not easy to observe, because l and q are fixed, and sinq DIFFRACTION ANALYSIS METHODS
Laue method. The Laue method uses a continuous "white" spectrum of X-rays, which is directed to a stationary single crystal. For a specific value of the period d, the wavelength corresponding to the Bragg-Wulf condition is automatically selected from the entire spectrum. The Laue patterns obtained in this way make it possible to judge the directions of the diffracted beams and, consequently, the orientations of the crystal planes, which also makes it possible to draw important conclusions about the symmetry, orientation of the crystal, and the presence of defects in it. In this case, however, information about the spatial period d is lost. On fig. 7 shows an example of a Lauegram. The X-ray film was located on the side of the crystal opposite to that on which the X-ray beam was incident from the source.
Debye-Scherrer method (for polycrystalline samples). Unlike the previous method, monochromatic radiation (l = const) is used here, and the angle q is varied. This is achieved by using a polycrystalline sample consisting of numerous small crystallites of random orientation, among which there are those that satisfy the Bragg-Wulf condition. The diffracted beams form cones, the axis of which is directed along the X-ray beam. For imaging, a narrow strip of X-ray film is usually used in a cylindrical cassette, and X-rays are propagated along the diameter through holes in the film. The debyegram obtained in this way (Fig. 8) contains exact information about the period d, i.e. about the structure of the crystal, but does not give the information that the Lauegram contains. Therefore, both methods complement each other. Let us consider some applications of the Debye-Scherrer method.
Identification of chemical elements and compounds. From the angle q determined from the Debyegram, one can calculate the interplanar distance d characteristic of a given element or compound. At present, many tables of d values have been compiled, which make it possible to identify not only one or another chemical element or compound, but also various phase states of the same substance, which does not always give a chemical analysis. It is also possible to determine the content of the second component in substitutional alloys with high accuracy from the dependence of the period d on the concentration.
Stress analysis. From the measured difference in interplanar spacings for different directions in crystals, knowing the elastic modulus of the material, it is possible to calculate small stresses in it with high accuracy.
Studies of preferential orientation in crystals. If small crystallites in a polycrystalline sample are not completely randomly oriented, then the rings on the Debyegram will have different intensities. In the presence of a pronounced preferred orientation, the intensity maxima are concentrated in individual spots in the image, which becomes similar to the image for a single crystal. For example, during deep cold rolling, a metal sheet acquires a texture - a pronounced orientation of crystallites. According to the debaygram, one can judge the nature of the cold working of the material.
Study of grain sizes. If the grain size of the polycrystal is more than 10-3 cm, then the lines on the Debyegram will consist of separate spots, since in this case the number of crystallites is not enough to cover the entire range of values of the angles q. If the crystallite size is less than 10-5 cm, then the diffraction lines become wider. Their width is inversely proportional to the size of the crystallites. Broadening occurs for the same reason that a decrease in the number of slits reduces the resolution of a diffraction grating. X-ray radiation makes it possible to determine grain sizes in the range of 10-7-10-6 cm.
Methods for single crystals. In order for diffraction by a crystal to provide information not only about the spatial period, but also about the orientation of each set of diffracting planes, methods of a rotating single crystal are used. A monochromatic X-ray beam is incident on the crystal. The crystal rotates around the main axis, for which the Laue equations are satisfied. In this case, the angle q, which is included in the Bragg-Wulf formula, changes. The diffraction maxima are located at the intersection of the Laue diffraction cones with the cylindrical surface of the film (Fig. 9). The result is a diffraction pattern of the type shown in Fig. 10. However, complications are possible due to the overlap of different diffraction orders at one point. The method can be significantly improved if, simultaneously with the rotation of the crystal, the film is also moved in a certain way.
Studies of liquids and gases. It is known that liquids, gases and amorphous bodies do not have the correct crystal structure. But here, too, there is a chemical bond between the atoms in the molecules, due to which the distance between them remains almost constant, although the molecules themselves are randomly oriented in space. Such materials also give a diffraction pattern with a relatively small number of smeared maxima. The processing of such a picture by modern methods makes it possible to obtain information about the structure of even such non-crystalline materials.
SPECTROCHEMICAL X-RAY ANALYSIS
A few years after the discovery of X-rays, Ch. Barkla (1877-1944) discovered that when a high-energy X-ray flux acts on a substance, secondary fluorescent X-ray radiation is generated, which is characteristic of the element under study. Shortly thereafter, G. Moseley, in a series of his experiments, measured the wavelengths of the primary characteristic X-ray radiation obtained by electron bombardment of various elements, and deduced the relationship between the wavelength and the atomic number. These experiments, and Bragg's invention of the X-ray spectrometer, laid the foundation for spectrochemical X-ray analysis. The possibilities of X-rays for chemical analysis were immediately recognized. Spectrographs were created with registration on a photographic plate, in which the sample under study served as the anode of an X-ray tube. Unfortunately, this technique turned out to be very laborious, and therefore was used only when the usual methods of chemical analysis were inapplicable. An outstanding example of innovative research in the field of analytical X-ray spectroscopy was the discovery in 1923 by G. Hevesy and D. Coster of a new element, hafnium. The development of high-power X-ray tubes for radiography and sensitive detectors for radiochemical measurements during World War II largely contributed to the rapid growth of X-ray spectrography in the following years. This method has become widespread due to the speed, convenience, non-destructive nature of the analysis and the possibility of full or partial automation. It is applicable in the problems of quantitative and qualitative analysis of all elements with an atomic number greater than 11 (sodium). And although X-ray spectrochemical analysis is usually used to determine the critical components in a sample (from 0.1-100%), in some cases it is suitable for concentrations of 0.005% and even lower.
X-ray spectrometer. A modern X-ray spectrometer consists of three main systems (Fig. 11): excitation systems, i.e. x-ray tube with an anode made of tungsten or other refractory material and a power supply; analysis systems, i.e. an analyzer crystal with two multi-slit collimators, as well as a spectrogoniometer for fine adjustment; and registration systems with a Geiger or proportional or scintillation counter, as well as a rectifier, amplifier, counters and a chart recorder or other recording device.
X-ray fluorescent analysis. The analyzed sample is located in the path of the exciting x-rays. The region of the sample to be examined is usually isolated by a mask with a hole of the desired diameter, and the radiation passes through a collimator that forms a parallel beam. Behind the analyzer crystal, a slit collimator emits diffracted radiation for the detector. Usually, the maximum angle q is limited to 80–85°, so that only X-rays whose wavelength l is related to the interplanar distance d by the inequality l can diffract on the analyzer crystal. X-ray microanalysis. The flat analyzer crystal spectrometer described above can be adapted for microanalysis. This is achieved by constricting either the primary x-ray beam or the secondary beam emitted by the sample. However, a decrease in the effective size of the sample or the radiation aperture leads to a decrease in the intensity of the recorded diffracted radiation. An improvement to this method can be achieved by using a curved crystal spectrometer, which makes it possible to register a cone of divergent radiation, and not only radiation parallel to the axis of the collimator. With such a spectrometer, particles smaller than 25 µm can be identified. An even greater reduction in the size of the analyzed sample is achieved in the X-ray electron probe microanalyzer invented by R. Kasten. Here, the characteristic X-ray emission of the sample is excited by a highly focused electron beam, which is then analyzed by a bent-crystal spectrometer. Using such a device, it is possible to detect amounts of a substance of the order of 10–14 g in a sample with a diameter of 1 μm. Installations with electron beam scanning of the sample have also been developed, with the help of which it is possible to obtain a two-dimensional pattern of the distribution over the sample of the element whose characteristic radiation is tuned to the spectrometer.
MEDICAL X-RAY DIAGNOSIS
The development of x-ray technology has significantly reduced the exposure time and improved the quality of images, allowing even soft tissues to be studied.
Fluorography. This diagnostic method consists in photographing a shadow image from a translucent screen. The patient is placed between an x-ray source and a flat screen of phosphor (usually cesium iodide), which glows when exposed to x-rays. Biological tissues of varying degrees of density create shadows of X-ray radiation with varying degrees of intensity. A radiologist examines a shadow image on a fluorescent screen and makes a diagnosis. In the past, a radiologist relied on vision to analyze an image. Now there are various systems that amplify the image, display it on a television screen or record data in the computer's memory.
Radiography. The recording of an x-ray image directly on photographic film is called radiography. In this case, the organ under study is located between the X-ray source and the film, which captures information about the state of the organ at a given time. Repeated radiography makes it possible to judge its further evolution. Radiography allows you to very accurately examine the integrity of bone tissue, which consists mainly of calcium and is opaque to x-rays, as well as muscle tissue ruptures. With its help, better than a stethoscope or listening, the condition of the lungs is analyzed in case of inflammation, tuberculosis, or the presence of fluid. With the help of radiography, the size and shape of the heart, as well as the dynamics of its changes in patients suffering from heart disease, are determined.
contrast agents. Parts of the body and cavities of individual organs that are transparent to X-ray radiation become visible if they are filled with a contrast agent that is harmless to the body, but allows one to visualize the shape of internal organs and check their functioning. The patient either takes contrast agents orally (such as barium salts in the study of the gastrointestinal tract), or they are administered intravenously (such as iodine-containing solutions in the study of the kidneys and urinary tract). In recent years, however, these methods have been supplanted by diagnostic methods based on the use of radioactive atoms and ultrasound.
CT scan. In the 1970s, a new method of X-ray diagnostics was developed, based on a complete photograph of the body or its parts. Images of thin layers ("slices") are processed by a computer, and the final image is displayed on the monitor screen. This method is called computed x-ray tomography. It is widely used in modern medicine for diagnosing infiltrates, tumors and other brain disorders, as well as for diagnosing diseases of soft tissues inside the body. This technique does not require the introduction of foreign contrast agents and is therefore faster and more effective than traditional techniques.
BIOLOGICAL ACTION OF X-RAY RADIATION
The harmful biological effect of X-ray radiation was discovered shortly after its discovery by Roentgen. It turned out that the new radiation can cause something like a severe sunburn (erythema), accompanied, however, by deeper and more permanent damage to the skin. Appearing ulcers often turned into cancer. In many cases, fingers or hands had to be amputated. There were also deaths. It has been found that skin lesions can be avoided by reducing exposure time and dose, using shielding (eg lead) and remote controls. But gradually other, more long-term effects of X-ray exposure were revealed, which were then confirmed and studied in experimental animals. The effects due to the action of X-rays, as well as other ionizing radiations (such as gamma radiation emitted by radioactive materials) include: 1) temporary changes in the composition of the blood after a relatively small excess exposure; 2) irreversible changes in the composition of the blood (hemolytic anemia) after prolonged excessive exposure; 3) an increase in the incidence of cancer (including leukemia); 4) faster aging and early death; 5) the occurrence of cataracts. In addition, biological experiments on mice, rabbits and flies (Drosophila) have shown that even small doses of systematic irradiation of large populations, due to an increase in the rate of mutation, lead to harmful genetic effects. Most geneticists recognize the applicability of these data to the human body. As for the biological effect of X-ray radiation on the human body, it is determined by the level of the radiation dose, as well as by which particular organ of the body was exposed to radiation. For example, blood diseases are caused by irradiation of the hematopoietic organs, mainly the bone marrow, and genetic consequences - by irradiation of the genital organs, which can also lead to sterility. The accumulation of knowledge about the effects of X-ray radiation on the human body has led to the development of national and international standards for permissible radiation doses, published in various reference publications. In addition to X-rays, which are purposefully used by humans, there is also the so-called scattered, side radiation that occurs for various reasons, for example, due to scattering due to the imperfection of the lead protective screen, which does not completely absorb this radiation. In addition, many electrical devices that are not designed to produce X-rays nevertheless generate X-rays as a by-product. Such devices include electron microscopes, high-voltage rectifier lamps (kenotrons), as well as kinescopes of outdated color televisions. The production of modern color kinescopes in many countries is now under government control.
HAZARDOUS FACTORS OF X-RAY RADIATION
The types and degree of danger of X-ray exposure for people depend on the contingent of people exposed to radiation.
Professionals working with x-ray equipment. This category includes radiologists, dentists, as well as scientific and technical workers and personnel maintaining and using x-ray equipment. Effective measures are being taken to reduce the levels of radiation they have to deal with.
Patients. There are no strict criteria here, and the safe level of radiation that patients receive during treatment is determined by the attending physicians. Physicians are advised not to unnecessarily expose patients to x-rays. Particular caution should be exercised when examining pregnant women and children. In this case, special measures are taken.
Control methods. There are three aspects to this:
1) availability of adequate equipment, 2) enforcement of safety regulations, 3) proper use of equipment. In an x-ray examination, only the desired area should be exposed to radiation, be it dental examinations or lung examinations. Note that immediately after turning off the X-ray apparatus, both primary and secondary radiation disappear; there is also no residual radiation, which is not always known even to those who are directly connected with it in their work.
see also
ATOM STRUCTURE;
Radiology is a section of radiology that studies the effects of X-ray radiation on the body of animals and humans arising 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 that converts the alternating current of the electrical network into direct current, a control panel, a tripod and an 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 the atoms of the anode substance. At present, the point of view is generally accepted that X-rays, by their physical nature, are one of the types of radiant energy, the spectrum of which also includes radio waves, infrared rays, visible light, ultraviolet rays and gamma rays of radioactive elements. X-ray radiation can be characterized as a collection of its smallest particles - quanta or photons.
Rice. 1 - mobile x-ray machine:
A - x-ray tube;
B - power supply;
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 - high voltage supply button.
Rice. 3 is a 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 production
X-rays are formed at the moment of collision of a stream of accelerated electrons with the anode material. When electrons interact with a target, 99% of their kinetic energy is converted into thermal energy and only 1% into X-rays.
An X-ray tube consists of a glass container in which 2 electrodes are soldered: a cathode and an anode. Air is pumped out of the glass cylinder: the movement of electrons from the cathode to the anode is possible only under conditions of relative vacuum (10 -7 -10 -8 mm Hg). On the cathode there is a filament, which is a tightly twisted tungsten filament. When an electric current is applied to the filament, electron emission occurs, in which electrons are separated from the spiral 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 movement. Cup - a small depression in the cathode. The anode, in turn, contains a tungsten metal plate on which the electrons are focused - this is the site of the formation of x-rays.
Rice. 4 - X-ray tube device: A - cathode; 
B - anode;
B - tungsten filament;
G - focusing cup of the cathode;
D - stream of accelerated electrons;
E - tungsten target;
G - glass flask;
З - a window from beryllium;
And - formed x-rays;
K - aluminum filter.
2 transformers are connected to the electron tube: step-down and step-up. A step-down transformer heats the tungsten filament with a low voltage (5-15 volts), resulting in electron emission. A step-up, or high-voltage, transformer goes 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 provides cooling of the transformers and their reliable insulation.
After an electron cloud has formed with the help of a step-down transformer, the step-up transformer is turned on, and high-voltage voltage is applied to both poles of the electrical circuit: a positive pulse to the anode, and a negative pulse to the cathode. Negatively charged electrons are repelled from a negatively charged cathode and tend to a positively charged anode - due to such a potential difference, a high speed of movement is achieved - 100 thousand km / s. At this speed, electrons bombard the tungsten anode plate, completing an electrical circuit, resulting in X-rays and thermal energy.
X-ray radiation is subdivided into bremsstrahlung and characteristic. Bremsstrahlung occurs due to a sharp deceleration of the speed of electrons emitted by a tungsten filament. Characteristic radiation occurs at the moment of rearrangement of the electron shells of atoms. Both of these types are formed in an X-ray tube at the moment of collision of accelerated electrons with atoms of the anode material. The emission spectrum of an X-ray tube is a superposition of bremsstrahlung and characteristic X-rays.
Rice. 5 - the principle of the formation of bremsstrahlung X-rays.
Rice. 6 - the principle of formation of the characteristic x-ray radiation.
Basic properties of X-rays
- X-rays are invisible to visual perception.
- X-ray radiation has a great penetrating power through the organs and tissues of a living organism, as well as dense structures of inanimate nature, which do not transmit visible light rays.
- X-rays cause certain chemical compounds to glow, called fluorescence.
- Zinc and cadmium sulfides fluoresce yellow-green,
- Crystals of calcium tungstate - violet-blue.
Scale of electromagnetic oscillations
X-rays have a specific wavelength and frequency of oscillation. Wavelength (λ) and oscillation frequency (ν) are related by the relationship: 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 relation: 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, it is possible to control its penetrating power. X-rays have a very short wavelength, but a high frequency of oscillation, so they are invisible to the human eye. Due to their enormous energy, quanta have a high penetrating power, which is one of the main properties that ensure the use of X-rays in medicine and other sciences.X-ray characteristics
Intensity- 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-rays is measured in milliamps. Comparing it with the intensity of visible light from a conventional incandescent lamp, we can draw an analogy: for example, a 20-watt lamp will shine with one intensity, or power, and a 200-watt lamp will shine with another, while the quality of the light itself (its spectrum) is the same . The intensity of X-ray radiation is, in fact, its quantity. Each electron creates one or more radiation quanta on the anode, therefore, the amount of X-rays during exposure of the 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 incandescence 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 value of the high voltage supplied by the 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 Fig. rice. 5; electrons with low energy will be able to enter into a smaller number of interactions).
The X-ray intensity (anode current) multiplied by the exposure (tube time) corresponds to the X-ray exposure, which is measured in mAs (milliamps per second). Exposure is a parameter that, like intensity, characterizes the amount of rays emitted by an 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 sec, then the number of rays will be one, and if 0.02 sec, 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 under study and the diagnostic task.
Rigidity- qualitative characteristic of x-ray radiation. It is measured by 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 on the electrodes of the tube, the more force the electrons repel 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 power 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 the wavelength on the energy of the wave:
E - wave energy 
Rice. 8 - The ratio of the voltage on the X-ray tube and the wavelength of the resulting X-ray radiation:
Classification of x-ray tubes
- By appointment
- Diagnostic
- Therapeutic
- For structural analysis
- For transillumination
- By design
- By focus
- Single-focus (one spiral on the cathode, and one focal spot on the anode)
- Bifocal (two spirals of different sizes on the cathode, and two focal spots on the anode)
- By type of anode
- Stationary (fixed)
- Rotating
X-rays are used not only for radiodiagnostic purposes, but also for therapeutic purposes. As noted above, the ability of X-ray radiation to suppress the growth of tumor cells makes it possible to use it in radiation therapy of oncological diseases. In addition to the medical field of application, X-ray radiation has found wide application in the engineering and technical field, 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. The type of such research is called defectoscopy. And at airports, railway stations and other crowded places, X-ray television introscopes are actively used to scan hand luggage and luggage for security purposes.
Depending on the type of anode, X-ray tubes differ in design. Due to the fact that 99% of the kinetic energy of the electrons is converted into thermal energy, during the operation of the tube, the anode is significantly heated - 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 focus. Focal spot - the section of the anode on which the working X-ray beam is generated. It is subdivided into the real focal spot and the effective focal spot ( rice. 12). Due to the angle of the anode, the effective focal spot is smaller than the real 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 image area. However, a smaller focal spot produces better image clarity. Therefore, when producing small images, a short filament is used and the electrons are directed to a small area of the anode target, 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-rays play one of the most important roles in the study and practical use of atomic phenomena. Thanks to their research, many discoveries were made and methods for analyzing substances were developed, which are used in various fields. Here we will consider one of the types of X-rays - characteristic X-rays.
Nature and properties of X-rays
X-ray radiation is a high-frequency change in the state of an 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 range 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 rather conventional 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 power, 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 the photoelectric effect (photoabsorption) and incoherent (Compton) scattering. In photoabsorption, an X-ray photon, being 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 obtain rays, glass vacuum bottles with electrodes located inside are used. The potential difference across the electrodes needs to be very high - up to hundreds of kilovolts. On a tungsten cathode heated by current, thermionic emission occurs, 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 birth of a photon, there are such types of X-ray radiation as bremsstrahlung and characteristic.
Electrons can, meeting with the anode, slow down, that is, lose energy in the electric fields of its atoms. This energy is emitted in the form of X-ray photons. Such 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 the bremsstrahlung photon cannot exceed the kinetic energy of the electron that generates it, so that the maximum frequency (and the smallest wavelength) of bremsstrahlung corresponds to the largest 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 that comes from a different process. This radiation is called characteristic, and we will dwell on it in more detail.
How characteristic X-rays are produced
Having reached the anticathode, a fast electron can penetrate inside the atom and knock out any 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 occupied by electrons in the atom, the vacated place will not remain empty.
It must be remembered that the electronic structure of the atom, like any energy system, seeks to minimize energy. The vacancy formed as a result of the knockout is filled with an electron from one of the higher levels. Its energy is higher, and, occupying a lower level, it radiates a surplus 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 have only strictly defined energy values, reflecting the level difference. As a result, the characteristic X-ray radiation has a spectrum not of a continuous, but of a line type. Such a spectrum makes it possible to characterize the substance of the anode - hence the name of these rays. It is precisely because of the spectral differences that it is clear what is meant by bremsstrahlung and characteristic X-rays.
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, so the spectra of such electrons are also discrete.
General view of the characteristic spectrum
Narrow characteristic lines are present in the X-ray spectral pattern along with a continuous bremsstrahlung spectrum. If we represent the spectrum as a plot 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. With increasing voltage at the electrodes of the tube, 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 shape regardless of the material of the anticathode irradiated by electrons, but for different materials they are located at different frequencies, uniting in series according to the proximity of the frequency values. Between the series themselves, the difference in frequencies is much more significant. The shape of the maxima does not depend in any way on whether the anode material represents a pure chemical element or whether it is a complex substance. In the latter case, the characteristic X-ray spectra of its constituent elements are simply superimposed on each other.
With an increase in the atomic number of a chemical element, all lines of its X-ray spectrum are shifted towards increasing frequency. The spectrum retains its form.
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 associate the frequencies of the maxima of the spectrum with the ordinal numbers of the chemical elements. Thus, the wavelength of the characteristic X-ray radiation, as it turned out, can be clearly correlated with a particular 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 element's ordinal number, S n is the screening constant, n is the principal quantum number, and R is the constant Rydberg. This relationship is linear and appears on the Moseley diagram as a series of straight lines for each value of n.
The values of n correspond to individual series of characteristic X-ray peaks. Moseley's law allows one to determine the serial number of a chemical element irradiated by hard electrons from the measured wavelengths (they are uniquely related to the frequencies) of the X-ray spectrum maxima.
The structure of the electron 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 for each element.
The role of Moseley's law in atomic physics
There are small deviations from the strict linear relationship expressed by Moseley's law. They are connected, firstly, with the peculiarities of the filling order of the electron shells in some elements, and, secondly, with the relativistic effects of the motion of electrons in heavy atoms. In addition, when the number of neutrons in the nucleus changes (the so-called isotopic shift), the position of the lines can 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 the pattern of increasing the serial number according to each small shift in the characteristic maxima. This contributed to the clarification of the question of the physical meaning of the ordinal number of elements. The Z value is not just a number: it is the positive electric charge of the nucleus, which is the sum of the unit positive charges of the particles that make up it. The correct placement of elements in the table and the presence of empty positions in it (then they still existed) received powerful confirmation. The validity of the periodic law was proved.
Moseley's law, in addition, became the basis on which a whole area of experimental research arose - X-ray spectrometry.
The structure of the electron shells of the atom
Let us briefly recall how the electron is arranged. It consists of shells, denoted 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 main quantum number n, which determines the possible energy values. In outer shells, the energy of electrons is higher, and the ionization potential for 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 labeled with the shell they belong to, such as 2s, 4d, and so on.
The sublevel contains which are set, in addition to the main and orbital, by one more quantum number - magnetic, which determines the projection of the electron's orbital momentum 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.
The mechanism of generation of characteristic X-rays
So, the cause of this radiation is the formation of electron vacancies in the inner shells, due to 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 within densely 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 a 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 the characteristic series is formed, in this example, the K-series. An indication of where the electron filling the vacancy comes from is given in the form of a Greek index when designating the series. "Alpha" means that it comes from the L-shell, "beta" - from the M-shell. At present, there is a tendency to replace the Greek letter indices with the Latin ones adopted to designate shells.
The intensity of the alpha line in the series is always the highest, which 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 the characteristic x-ray quantum. It is determined by the difference in the energy values of the levels between which the electron transition occurs, according to the formula E \u003d 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 from the highest possible levels of atoms of heavy elements. But the intensity of these lines (peak heights) is the smallest, since they are the least likely.
If, due to insufficient voltage on 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, a new vacancy appears in the overlying shell as a result of an electronic transition. This creates the conditions for generating the next series. Electronic 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 is expressed, as in optical spectra, in line splitting.
The fine structure is due to the fact that the energy level - the electron shell - is a set of closely spaced components - subshells. To characterize the subshells, one more, internal quantum number j is introduced, which reflects the interaction of the intrinsic and orbital magnetic moments of the electron.
In connection with the influence of the spin-orbit interaction, the energy structure of the atom becomes more complicated, and as a result, the characteristic X-ray radiation has a spectrum that is characterized by split lines with very closely spaced elements.
Fine structure elements are usually denoted by additional digital indices.
The characteristic X-ray radiation has a feature that is reflected only in the fine structure of the spectrum. The transition of an electron to the lowest energy level does not occur from the lower subshell of the overlying level. Such an event has a negligible probability.
The use of X-rays in spectrometry
This radiation, due to its features described by Moseley's law, underlies various X-ray spectral methods for the analysis of substances. When analyzing the X-ray spectrum, either diffraction of radiation by 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 form of X-ray spectrometry attachment.
Wave-dispersive spectrometry is characterized by especially high accuracy. With the help of special filters, the most intense peaks in the spectrum are selected, thanks to which it is possible to obtain almost monochromatic radiation with a precisely known frequency. The anode material is chosen very carefully to ensure that a monochromatic beam of the desired frequency is obtained. Its diffraction on the crystal lattice of the studied substance makes it possible to study the structure of the lattice with great accuracy. This method is also used in the study of DNA and other complex molecules.
One of the features of the characteristic X-ray radiation is also taken into account in gamma spectrometry. This is the high intensity of the characteristic peaks. Gamma spectrometers use lead shielding against external background radiation that interferes with measurements. But lead, absorbing gamma quanta, experiences internal ionization, as a result of which it actively emits in the X-ray range. Additional cadmium shielding is used to absorb the intense peaks of the characteristic x-ray radiation from lead. 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, in the analysis of substances, the absorption spectra of continuous X-rays by various substances are studied.