The use of x-rays. Analysis of imperfections in the crystal structure. Exposure of X-rays to pregnant women
FEDERAL AGENCY FOR EDUCATION OF THE RUSSIAN FEDERATION
STATE EDUCATIONAL INSTITUTION
HIGHER PROFESSIONAL EDUCATION
MOSCOW STATE INSTITUTE OF STEEL AND ALLOYS
(UNIVERSITY OF TECHNOLOGY)
NOVOTROITSKY BRANCH
Department of OEND
COURSE WORK
Discipline: Physics
Topic: X-RAY
Student: Nedorezova N.A.
Group: EiU-2004-25, No. З.К.: 04Н036
Checked by: Ozhegova S.M.
Introduction
Chapter 1
1.1 Biography of Roentgen Wilhelm Conrad
1.2 Discovery of X-rays
Chapter 2
2.1 X-ray sources
2.2 Properties of X-rays
2.3 Registration of X-rays
2.4 Use of X-rays
Chapter 3
3.1 Analysis of crystal structure imperfections
3.2 Spectrum analysis
Conclusion
List of sources used
Applications
Introduction
A rare person has not gone through an x-ray room. Pictures taken in x-rays are familiar to everyone. In 1995, this discovery was 100 years old. It is hard to imagine what great interest it aroused a century ago. In the hands of a man turned out to be an apparatus with which it was possible to see the invisible.
This invisible radiation, capable of penetrating, albeit to varying degrees, into all substances, which is electromagnetic radiation with a wavelength of about 10 -8 cm, was called X-ray radiation, in honor of Wilhelm Roentgen, who discovered it.
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 less 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, in chemistry to analyze compounds, and in physics to study the structure of crystals.
Roentgen's discovery was followed by experiments by other researchers who discovered many new properties and possibilities for using this radiation. A major contribution was made by M. Laue, W. Friedrich, and P. Knipping, who in 1912 demonstrated the diffraction of X-rays as they pass 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. Braggi, who received the Nobel Prize in 1915 for developing the fundamentals of X-ray diffraction analysis.
The purpose of this course work is to study the phenomenon of x-ray radiation, the history of discovery, properties and identify the scope of its application.
Chapter 1
1.1 Biography of Roentgen Wilhelm Conrad
Wilhelm Conrad Roentgen was born on March 17, 1845 in the border region of Germany with Holland, in the city of Lenepe. He received his technical education in Zurich at the same Higher Technical School (Polytechnic) where Einstein later studied. Passion for physics forced him after leaving school in 1866 to continue physical education.
In 1868 he defended his dissertation for the degree of Doctor of Philosophy, he worked as an assistant at the Department of Physics, first in Zurich, then in Giessen, and then in Strasbourg (1874-1879) with Kundt. Here Roentgen went through a good experimental school and became a first-class experimenter. Roentgen performed part of the important research with his student, one of the founders of Soviet physics, A.F. Ioffe.
Scientific research relates to electromagnetism, crystal physics, optics, molecular physics.
In 1895, he discovered radiation with a wavelength shorter than the wavelength of ultraviolet rays (X-rays), later called x-rays, and investigated their properties: the ability to reflect, absorb, ionize air, etc. He proposed the correct design of the tube for obtaining X-rays - an inclined platinum anticathode and a concave cathode: he was the first to take photographs using X-rays. He discovered in 1885 the magnetic field of a dielectric moving in an electric field (the so-called "roentgen current"). His experience clearly showed that the magnetic field is created by moving charges, and was important for the creation of X. Lorentz's electronic theory. A significant number of Roentgen's works are devoted to the study properties of liquids, gases, crystals, electromagnetic phenomena, discovered the relationship between electrical and optical phenomena in crystals.For the discovery of the rays that bear his name, Roentgen in 1901 was the first among physicists to be awarded the Nobel Prize.
From 1900 until the last days of his life (he died on February 10, 1923) he worked at the University of Munich.
1.2 Discovery of X-rays
End of the 19th century was marked by increased interest in the phenomena of the passage of electricity through gases. Even Faraday seriously studied these phenomena, described various forms of discharge, discovered a dark space in a luminous column of rarefied gas. Faraday dark space separates the bluish, cathode glow from the pinkish, anode glow.
A further increase in the rarefaction of the gas significantly changes the nature of the glow. The mathematician Plücker (1801-1868) discovered in 1859, at sufficiently strong rarefaction, a weakly bluish beam of rays emanating from the cathode, reaching the anode and causing the glass of the tube to glow. Plücker's student Gittorf (1824-1914) in 1869 continued his teacher's research and showed that a distinct shadow appears on the fluorescent surface of the tube if a solid body is placed between the cathode and this surface.
Goldstein (1850-1931), studying the properties of rays, called them cathode rays (1876). Three years later, William Crookes (1832-1919) proved the material nature of cathode rays and called them "radiant matter" - a substance in a special fourth state. His evidence was convincing and clear. Experiments with the "Crookes tube" were demonstrated later in all physical classrooms . The deflection of the cathode beam by a magnetic field in a Crookes tube has become a classic school demonstration.
However, experiments on the electrical deflection of cathode rays were not so convincing. Hertz did not detect such a deviation and came to the conclusion that the cathode ray is an oscillatory process in the ether. Hertz's student F. Lenard, experimenting with cathode rays, showed in 1893 that they pass through a window covered with aluminum foil and cause a glow in the space behind the window. Hertz devoted his last article, published in 1892, to the phenomenon of the passage of cathode rays through thin metal bodies. It began with the words:
"Cathode rays differ from light in a significant way in terms of their ability to penetrate solids." Describing the results of experiments on the passage of cathode rays through gold, silver, platinum, aluminum, etc. leaves, Hertz notes that he did not observe any special differences in the phenomena The rays do not pass through the leaves in a straight line, but are scattered by diffraction.The nature of the cathode rays was still unclear.
It was with such tubes of Crookes, Lenard and others that the Würzburg professor Wilhelm Konrad Roentgen experimented at the end of 1895. Once, after the end of the experiment, he closed the tube with a black cardboard cover, turned off the light, but did not turn off the inductor that fed the tube, he noticed a glow of the screen from barium cyanogen located near the tube. Struck by this circumstance, Roentgen began to experiment with the screen. In his first report "On a new kind of rays", dated December 28, 1895, he wrote about these first experiments: "A piece of paper coated with barium platinum-cyanide, when approaching a tube, closed with a thin black cardboard cover that fits snugly enough to it, with each discharge it flashes with a bright light: it begins to fluoresce. Fluorescence is visible with sufficient darkening and does not depend on whether we bring the paper with the side coated with barium synerogen or not coated with barium synerogen. The fluorescence is noticeable even at a distance of two meters from the tube.”
Careful examination showed Roentgen "that black cardboard, transparent neither to the visible and ultraviolet rays of the sun, nor to the rays of an electric arc, is permeated with some kind of fluorescent agent." Roentgen investigated the penetrating power of this "agent", which he called for brevity "X-rays", for various substances. He found that the rays freely pass through paper, wood, ebonite, thin layers of metal, but are strongly delayed by lead.
He then describes the sensational experience:
“If you hold your hand between the discharge tube and the screen, you can see the dark shadows of the bones in the faint outlines of the shadow of the hand itself.” This was the first X-ray examination of the human body.
These shots made a huge impression; the discovery had not yet been completed, and X-ray diagnostics had already begun its journey. “My laboratory was flooded with doctors bringing in patients who suspected that they had needles in various parts of the body,” wrote the English physicist Schuster.
Already after the first experiments, Roentgen firmly established that X-rays differ from cathode ones, they do not carry a charge and are not deflected by a magnetic field, but they are excited by cathode rays. "X-rays are not identical with cathode rays, but they are excited by them in the glass walls of the discharge tube ”, wrote Roentgen.
He also established that they are excited not only in glass, but also in metals.
Mentioning the Hertz-Lenard hypothesis that cathode rays “are a phenomenon occurring in the ether,” Roentgen points out that “we can say something similar about our rays.” However, he failed to detect the wave properties of the rays, they "behave differently than hitherto known ultraviolet, visible, infrared rays." In their chemical and luminescent actions, they, according to Roentgen, are similar to ultraviolet rays. In the first message, he expressed the assumption left later that they can be longitudinal waves in the ether.
Roentgen's discovery aroused great interest in the scientific world. His experiments were repeated in almost all laboratories in the world. In Moscow they were repeated by P.N. Lebedev. In St. Petersburg, the inventor of radio A.S. Popov experimented with X-rays, demonstrated them at public lectures, receiving various X-rays. In Cambridge D.D. Thomson immediately applied the ionizing effect of X-rays to study the passage of electricity through gases. His research led to the discovery of the electron.
Chapter 2
X-ray radiation - electromagnetic ionizing radiation, occupying the spectral region between gamma and ultraviolet radiation within wavelengths from 10 -4 to 10 3 (from 10 -12 to 10 -5 cm).R. l. with wavelength λ< 2 условно называются жёсткими, с λ >2 - soft.
2.1 X-ray sources
The most common source of X-rays is the X-ray tube.
X-ray tubes are used in X-ray structural analysis
The main characteristics of X-ray tubes are the maximum permissible accelerating voltage (1-500 kV), electronic current (0.01 mA - 1A), specific power dissipated by the anode (10-10 4 W / mm 2), total power consumption (0.002 W - 60 kW) and focus sizes (1 µm - 10 mm). The efficiency of the x-ray tube is 0.1-3%.
Some radioactive isotopes can also serve as sources of X-rays.
Synchrotrons and electron storage rings with energies of several GeV can serve as sources of soft X-rays with λ on the order of tens and hundreds. In intensity, the X-ray radiation of synchrotrons exceeds the radiation of an X-ray tube in the specified region of the spectrum by 2-3 orders of magnitude.
Natural sources of X-rays - the Sun and other space objects.
2.2 Properties of X-rays
Depending on the mechanism of origin of X-rays, their spectra can be continuous (bremsstrahlung) or line (characteristic). A continuous X-ray spectrum is emitted by fast charged particles as a result of their deceleration when interacting with target atoms; this spectrum reaches a significant intensity only when the target is bombarded with electrons. The intensity of bremsstrahlung X-rays is distributed over all frequencies up to the high-frequency boundary 0 , at which the photon energy h 0 (h is Planck's constant
Line radiation occurs after the ionization of an atom with the ejection of an electron from one of its inner shells. Such ionization can be the result of an atom colliding with a fast particle, such as an electron (primary x-rays), or the absorption of a photon by an atom (fluorescent x-rays). The ionized atom finds itself in the initial quantum state at one of the high energy levels and after 10 -16 -10 -15 seconds passes into the final state with a lower energy. In this case, an atom can emit an excess of energy in the form of a photon of a certain frequency. The frequencies of the lines of the spectrum of such radiation are characteristic of the atoms of each element, therefore the line X-ray spectrum is called characteristic. The dependence of the line frequency of this spectrum on the atomic number Z is determined by the Moseley law.
Moseley's law, the law relating the frequency of the spectral lines of the characteristic X-ray emission of a chemical element with its serial number. G. Moseley experimentally installed
where R is the Rydberg constant
Moseley's law was irrefutable proof of the correct placement of elements in the periodic table of elements
In accordance with Moseley's law, X-ray characteristic spectra do not exhibit the periodic patterns inherent in optical spectra. This indicates that the inner electron shells of atoms of all elements that appear in the characteristic X-ray spectra have a similar structure.
Later experiments revealed some deviations from the linear dependence for the transition groups of elements, associated with a change in the order of filling of the outer electron shells, as well as for heavy atoms, appearing as a result of relativistic effects (conditionally explained by the fact that the speeds of the inner ones are comparable to the speed of light).
Depending on a number of factors - on the number of nucleons in the nucleus (isotonic shift), the state of the outer electron shells (chemical shift), etc. - the position of the spectral lines on the Moseley diagram may change somewhat. The study of these shifts allows one to obtain detailed information about the atom.
Bremsstrahlung X-rays emitted by very thin targets are completely polarized near 0; as 0 decreases, the degree of polarization decreases. Characteristic radiation, as a rule, is not polarized.
When X-rays interact with matter, the photoelectric effect can occur.
When X-rays pass through a layer of substance with thickness x, their initial intensity I 0 decreases to the value I = I 0 e - μ x where μ is the attenuation coefficient. The attenuation of I occurs due to two processes: the absorption of X-ray photons by matter and the change in their direction upon scattering. In the long-wavelength region of the spectrum, the absorption of X-rays predominates, in the short-wavelength region, their scattering. The degree of absorption increases rapidly with increasing Z and λ. For example, hard X-rays freely penetrate through a layer of air ~ 10 cm; an aluminum plate 3 cm thick attenuates X-rays with λ = 0.027 by half; soft x-rays are significantly absorbed in air and their use and study is possible only in a vacuum or in a weakly absorbing gas (for example, He). When X-rays are absorbed, the atoms of a substance are ionized.
The effect of X-rays on living organisms can be beneficial or harmful, depending on the ionization they cause in the tissues. Since the absorption of X-rays depends on λ, their intensity cannot serve as a measure of the biological effect of X-rays. X-ray measurements are used to measure the effect of X-rays on matter.
Scattering of X-rays in the region of large Z and λ occurs mainly without a change in λ and is called coherent scattering, while in the region of small Z and λ, as a rule, it increases (incoherent scattering). There are 2 types of incoherent X-ray scattering - Compton and Raman. In Compton scattering, which has the character of inelastic corpuscular scattering, a recoil electron flies out of the atomic shell due to the energy partially lost by the X-ray photon. In this case, the energy of the photon decreases and its direction changes; the change in λ depends on the scattering angle. During Raman scattering of a high-energy X-ray photon by a light atom, a small part of its energy is spent on ionization of the atom and the direction of the photon's motion changes. The change of such photons does not depend on the scattering angle.
The refractive index n for x-rays differs from 1 by a very small amount δ = 1-n ≈ 10 -6 -10 -5 . The phase velocity of X-rays in a medium is greater than the speed of light in a vacuum. The deviation of X-rays during the transition from one medium to another is very small (a few arc minutes). When X-rays fall from a vacuum onto the surface of a body at a very small angle, their total external reflection occurs.
2.3 Registration of X-rays
The human eye is not sensitive to x-rays. X-ray
rays are recorded using a special x-ray film containing an increased amount of Ag, Br. In the region λ<0,5 чувствительность этих плёнок быстро падает и может быть
искусственно повышена плотно прижатым к плёнке флуоресцирующим экраном. В
области λ>5, the sensitivity of ordinary positive film is quite high, and its grains are much smaller than the grains of X-ray film, which increases the resolution. At λ of the order of tens and hundreds, X-rays act only on the thinnest surface layer of the photographic emulsion; to increase the sensitivity of the film, it is sensitized with luminescent oils. In X-ray diagnostics and flaw detection, electrophotography is sometimes used to record X-rays.
X-rays of high intensity can be recorded using an ionization chamber
2.4 Use of X-rays
X-rays are most widely used in medicine for X-ray diagnostics.
X-ray structural analysis
X-ray microscopy
X-rays coming from space carry information about the chemical composition of cosmic bodies and about the physical processes taking place in space. X-ray astronomy deals with the study of cosmic x-rays
Chapter 3
One of the main tasks of X-ray diffraction analysis is the determination of the real or phase composition of a material. The X-ray diffraction method is direct and is characterized by high reliability, rapidity and relative cheapness. The method does not require a large amount of substance, the analysis can be carried out without destroying the part. The areas of application of qualitative phase analysis are very diverse both for scientific research and for control in production. You can check the composition of the raw materials of metallurgical production, synthesis products, processing, the result of phase changes during thermal and chemical-thermal treatment, analyze various coatings, thin films, etc.
Each phase, having its own crystal structure, is characterized by a certain set of discrete values of interplanar distances d/n from the maximum and below, inherent only to this phase. As follows from the Wulf-Bragg equation, each value of the interplanar distance corresponds to a line on the x-ray pattern from a polycrystalline sample at a certain angle θ (at a given value of the wavelength λ). Thus, a certain system of lines (diffraction maxima) will correspond to a certain set of interplanar distances for each phase in the X-ray diffraction pattern. The relative intensity of these lines in the X-ray pattern depends primarily on the structure of the phase. Therefore, by determining the location of the lines on the radiograph (its angle θ) and knowing the wavelength of the radiation at which the radiograph was taken, it is possible to determine the values of the interplanar distances d/n using the Wulf-Bragg formula:
/n = λ/ (2sin θ). (one)
Having determined the set of d/n for the material under study and comparing it with the previously known d/n data for pure substances, their various compounds, it is possible to establish which phase the given material comprises. It should be emphasized that it is the phases that are determined, and not the chemical composition, but the latter can sometimes be deduced if there are additional data on the elemental composition of a particular phase. The task of qualitative phase analysis is greatly facilitated if the chemical composition of the material under study is known, because then it is possible to make preliminary assumptions about the possible phases in this case.
The key to phase analysis is to accurately measure d/n and line intensity. Although this is in principle easier to achieve using a diffractometer, the photomethod for qualitative analysis has some advantages, primarily in terms of sensitivity (the ability to detect the presence of a small amount of phase in the sample), as well as the simplicity of the experimental technique.
The calculation of d/n from the X-ray pattern is carried out using the Wulf-Bragg equation.
As the value of λ in this equation, λ α cf K-series is usually used:
λ α cf = (2λ α1 + λ α2) /3 (2)
Sometimes the K α1 line is used. Determining the diffraction angles θ for all X-ray lines allows you to calculate d / n according to equation (1) and separate the β-lines (if there was no filter for (β-rays).
3.1 Analysis of crystal structure imperfections
All real single-crystal and even more so polycrystalline materials contain certain structural imperfections (point defects, dislocations, various types of interfaces, micro- and macrostresses), which have a very strong effect on all structure-sensitive properties and processes.
Structural imperfections cause distortions of the crystal lattice of different nature and, as a result, different types of changes in the diffraction pattern: a change in interatomic and interplanar distances causes a shift in diffraction maxima, microstresses and dispersity of the substructure lead to a broadening of diffraction maxima, lattice microdistortions - to a change in the intensity of these maxima, the presence dislocations causes anomalous phenomena during the passage of X-rays and, consequently, local contrast inhomogeneities on X-ray topograms, etc.
As a result, X-ray diffraction analysis is one of the most informative methods for studying structural imperfections, their type and concentration, and the nature of their distribution.
The traditional direct method of X-ray diffraction, which is implemented on stationary diffractometers, due to their design features, allows quantitative determination of stresses and strains only on small samples cut from parts or objects.
Therefore, at present, there is a transition from stationary to portable small-sized X-ray diffractometers, which provide an assessment of stresses in the material of parts or objects without destruction at the stages of their manufacture and operation.
Portable X-ray diffractometers of the DRP * 1 series make it possible to control residual and effective stresses in large-sized parts, products and structures without destruction
The program in the Windows environment allows not only to determine the stresses using the "sin 2 ψ" method in real time, but also to monitor the change in the phase composition and texture. The linear coordinate detector provides simultaneous registration at diffraction angles 2θ = 43°. small-sized X-ray tubes of the "Fox" type with high luminosity and low power (5 W) ensure the radiological safety of the device, in which at a distance of 25 cm from the irradiated area, the radiation level is equal to the natural background level. Devices of the DRP series are used in determining stresses at various stages of metal forming, cutting, grinding, heat treatment, welding, surface hardening in order to optimize these technological operations. Control over the drop in the level of induced residual compressive stresses in especially critical products and structures during their operation makes it possible to take the product out of service before its destruction, preventing possible accidents and catastrophes.
3.2 Spectrum analysis
Along with the determination of the atomic crystal structure and phase composition of the material, for its complete characterization, it is obligatory to determine its chemical composition.
Increasingly, various so-called instrumental methods of spectral analysis are used in practice for these purposes. Each of them has its own advantages and applications.
One of the important requirements in many cases is that the method used ensures the safety of the analyzed object; It is these methods of analysis that are discussed in this section. The next criterion according to which the methods of analysis described in this section were chosen is their locality.
The method of fluorescence X-ray spectral analysis is based on the penetration of rather hard X-ray radiation (from an X-ray tube) into the analyzed object, penetrating into a layer with a thickness of the order of several micrometers. The characteristic X-ray radiation arising in this case in the object makes it possible to obtain averaged data on its chemical composition.
To determine the elemental composition of a substance, one can use the analysis of the characteristic X-ray spectrum of a sample placed on the anode of an X-ray tube and subjected to electron bombardment - the emission method, or the analysis of the spectrum of secondary (fluorescent) X-ray radiation of a sample subjected to irradiation with hard X-rays from an X-ray tube or other source - fluorescent method.
The disadvantage of the emission method is, firstly, the need to place the sample on the anode of the X-ray tube, followed by evacuation with vacuum pumps; obviously, this method is unsuitable for fusible and volatile substances. The second drawback is related to the fact that even refractory objects are damaged by electron bombardment. The fluorescent method is free from these shortcomings and therefore has a much wider application. The advantage of the fluorescent method is also the absence of bremsstrahlung, which improves the sensitivity of the analysis. Comparison of the measured wavelengths with tables of spectral lines of chemical elements is the basis of a qualitative analysis, and the relative intensities of the spectral lines of different elements that form the sample substance form the basis of a quantitative analysis. From a consideration of the mechanism of excitation of characteristic X-ray radiation, it is clear that the radiations of one or another series (K or L, M, etc.) arise simultaneously, and the ratio of line intensities within the series is always constant. Therefore, the presence of this or that element is established not by individual lines, but by a series of lines as a whole (except for the weakest ones, taking into account the content of this element). For relatively light elements, the analysis of the K-series lines is used, for heavy elements, the L-series lines; under different conditions (depending on the equipment used and on the analyzed elements), different regions of the characteristic spectrum may be most convenient.
The main features of X-ray spectral analysis are as follows.
Simplicity of X-ray characteristic spectra even for heavy elements (compared to optical spectra), which simplifies the analysis (small number of lines; similarity in their mutual arrangement; with an increase in the serial number, a regular shift of the spectrum to the short-wavelength region occurs; comparative simplicity of quantitative analysis).
Independence of wavelengths from the state of atoms of the analyzed element (free or in a chemical compound). This is due to the fact that the occurrence of characteristic X-ray radiation is associated with the excitation of internal electronic levels, which in most cases practically do not change with the degree of ionization of atoms.
The possibility of separation in the analysis of rare earth and some other elements that have small differences in the spectra in the optical range due to the similarity of the electronic structure of the outer shells and differ very little in their chemical properties.
X-ray fluorescence spectroscopy is "non-destructive", so it has an advantage over conventional optical spectroscopy when analyzing thin samples - thin metal sheet, foil, etc.
X-ray fluorescence spectrometers, among them multichannel spectrometers or quantometers, providing express quantitative analysis of elements (from Na or Mg to U) with an error of less than 1% of the determined value, a sensitivity threshold of 10 -3 ... 10 -4% .
x-ray beam
Methods for determining the spectral composition of x-rays
Spectrometers are divided into two types: crystal-diffraction and crystalless.
The decomposition of X-rays into a spectrum using a natural diffraction grating - a crystal - is essentially similar to obtaining a spectrum of ordinary light rays using an artificial diffraction grating in the form of periodic strokes on glass. The condition for the formation of a diffraction maximum can be written as the condition of "reflection" from a system of parallel atomic planes separated by a distance d hkl .
When conducting a qualitative analysis, one can judge the presence of an element in a sample by one line - usually the most intense line of the spectral series suitable for a given analyzer crystal. The resolution of crystal diffraction spectrometers is sufficient to separate the characteristic lines even of elements adjacent in position in the periodic table. However, it is also necessary to take into account the imposition of different lines of different elements, as well as the imposition of reflections of different orders. This circumstance should be taken into account when choosing analytical lines. At the same time, it is necessary to use the possibilities of improving the resolution of the device.
Conclusion
Thus, x-rays are invisible electromagnetic radiation with a wavelength of 10 5 - 10 2 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.
Having considered the positive aspects of V. Roentgen's discovery, it is necessary to note its harmful biological effect. It turned out that X-rays can cause something like a severe sunburn (erythema), accompanied, however, by deeper and more permanent damage to the skin. Appearing ulcers often turn into cancer. In many cases, fingers or hands had to be amputated. There were also deaths.
It has been found that skin damage 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. Effects due to X-rays and other ionizing radiations (such as gamma rays emitted by radioactive materials) include:
) temporary changes in the composition of the blood after a relatively small excess exposure;
) irreversible changes in the composition of the blood (hemolytic anemia) after prolonged excessive exposure;
) an increase in the incidence of cancer (including leukemia);
) faster aging and early death;
) the occurrence of cataracts.
The biological impact of X-rays on the human body is determined by the level of radiation dose, as well as by which particular organ of the body was exposed to radiation.
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 books.
To avoid the harmful effects of X-rays, control methods are used:
) availability of adequate equipment,
) monitoring compliance with safety regulations,
) correct use of the equipment.
List of sources used
1) Blokhin M.A., Physics of X-rays, 2nd ed., M., 1957;
) Blokhin M.A., Methods of X-ray spectral studies, M., 1959;
) X-rays. Sat. ed. M.A. Blokhin, trans. with him. and English, M., 1960;
) Kharaja F., General course of X-ray engineering, 3rd ed., M. - L., 1966;
) Mirkin L.I., Handbook of X-ray diffraction analysis of polycrystals, M., 1961;
) Weinstein E.E., Kakhana M.M., Reference tables on X-ray spectroscopy, M., 1953.
) X-ray and electron-optical analysis. Gorelik S.S., Skakov Yu.A., Rastorguev L.N.: Proc. Allowance for universities. - 4th ed. Add. And a reworker. - M.: "MISiS", 2002. - 360 p.
Applications
Attachment 1
General view of X-ray tubes
Appendix 2
Scheme of X-ray tube for structural analysis
Scheme of an X-ray tube for structural analysis: 1 - metal anode glass (usually grounded); 2 - windows made of beryllium for x-ray output; 3 - thermionic cathode; 4 - glass bulb, isolating the anode part of the tube from the cathode; 5 - cathode terminals, to which the filament voltage is applied, as well as high (relative to the anode) voltage; 6 - electrostatic system for focusing electrons; 7 - anode (anticathode); 8 - branch pipes for input and output of running water cooling the anode glass.
Annex 3
Moseley diagram
Moseley diagram for K-, L- and M-series of characteristic X-rays. The abscissa shows the serial number of the element Z, the ordinate - ( With is the speed of light).
Appendix 4
Ionization chamber.
Fig.1. Section of a cylindrical ionization chamber: 1 - cylindrical body of the chamber, which serves as a negative electrode; 2 - cylindrical rod serving as a positive electrode; 3 - insulators.
Rice. 2. Scheme of switching on the current ionization chamber: V - voltage on the electrodes of the chamber; G is a galvanometer that measures the ionization current.
Rice. 3. Current-voltage characteristic of the ionization chamber.
Rice. 4. Scheme of switching on the pulsed ionization chamber: C - capacitance of the collecting electrode; R is resistance.
Annex 5
Scintillation counter.
Scheme of a scintillation counter: light quanta (photons) "knock out" electrons from the photocathode; moving from dynode to dynode, the electron avalanche multiplies.
Appendix 6
Geiger-Muller counter.
Rice. 1. Scheme of a glass Geiger-Muller counter: 1 - hermetically sealed glass tube; 2 - cathode (a thin layer of copper on a stainless steel tube); 3 - output of the cathode; 4 - anode (thin stretched thread).
Rice. 2. Scheme of switching on the Geiger-Muller counter.
Rice. 3. The counting characteristic of the Geiger-Muller counter.
Appendix 7
proportional counter.
Scheme of a proportional counter: a - electron drift region; b - area of gas amplification.
Appendix 8
Semiconductor detectors
Semiconductor detectors; the sensitive area is highlighted by hatching; n - region of a semiconductor with electronic conductivity, p - with hole, i - with intrinsic conduction; a - silicon surface-barrier detector; b - drift germanium-lithium planar detector; c - germanium-lithium coaxial detector.
LECTURE
X-RAY RADIATION
The nature of X-rays
Bremsstrahlung X-ray, its spectral properties.
Characteristic x-ray radiation (for review).
Interaction of X-ray radiation with matter.
Physical basis for the use of X-rays in medicine.
X-rays (X - rays) were discovered by K. Roentgen, who in 1895 became the first Nobel laureate in physics.
The nature of X-rays
x-ray radiation - electromagnetic waves with a length of 80 to 10 -5 nm. Long-wave X-ray radiation is covered by short-wave UV radiation, and short-wave radiation by long-wave radiation.
X-rays are produced in x-ray tubes. fig.1.
K - cathode
1 - electron beam
2 - X-ray radiation
Rice. 1. X-ray tube device.
The tube is a glass flask (with a possibly high vacuum: the pressure in it is about 10–6 mm Hg) with two electrodes: anode A and cathode K, to which a high voltage U (several thousand volts) is applied. The cathode is a source of electrons (due to the phenomenon of thermionic emission). The anode is a metal rod that has an inclined surface in order to direct the resulting X-ray radiation at an angle to the axis of the tube. It is made of a highly heat-conducting material to remove the heat generated during electron bombardment. On the beveled end there is a plate made of refractory metal (for example, tungsten).
The strong heating of the anode is due to the fact that the main number of electrons in the cathode beam, having hit the anode, experience numerous collisions with the atoms of the substance and transfer a large amount of energy to them.
Under the action of high voltage, the electrons emitted by the hot cathode filament are accelerated to high energies. The kinetic energy of an electron is equal to mv 2 /2. It is equal to the energy that it acquires by moving in the electrostatic field of the tube:
mv 2 /2 = eU(1)
where m, e are the electron mass and charge, U is the accelerating voltage.
The processes leading to the appearance of bremsstrahlung X-rays are due to the intense deceleration of electrons in the anode material by the electrostatic field of the atomic nucleus and atomic electrons.
The origin mechanism can be represented as follows. Moving electrons are some kind of current that forms its own magnetic field. Electron deceleration is a decrease in the current strength and, accordingly, a change in the magnetic field induction, which will cause the appearance of an alternating electric field, i.e. appearance of an electromagnetic wave.
Thus, when a charged particle flies into matter, it slows down, loses its energy and speed, and emits electromagnetic waves.
Spectral properties of X-ray bremsstrahlung .
So, in the case of electron deceleration in the anode material, bremsstrahlung radiation.
The bremsstrahlung spectrum is continuous. The reason for this is as follows.
When electrons decelerate, each of them has part of the energy used to heat the anode (E 1 \u003d Q), the other part to create an X-ray photon (E 2 \u003d hv), otherwise, eU \u003d hv + Q. The ratio between these parts is random.
Thus, the continuous spectrum of X-ray bremsstrahlung is formed due to the deceleration of many electrons, each of which emits one X-ray quantum hv (h) of a strictly defined value. The value of this quantum different for different electrons. Dependence of the X-ray energy flux on the wavelength , i.e. the X-ray spectrum is shown in Fig.2.
Fig.2. Bremsstrahlung spectrum: a) at different voltages U in the tube; b) at different temperatures T of the cathode.
Short-wave (hard) radiation has a greater penetrating power than long-wave (soft) radiation. Soft radiation is more strongly absorbed by matter.
From the side of short wavelengths, the spectrum ends abruptly at a certain wavelength m i n . Such short-wavelength bremsstrahlung occurs when the energy acquired by an electron in an accelerating field is completely converted into photon energy (Q = 0):
eU = hv max = hc/ min , min = hc/(eU), (2)
min (nm) = 1.23/UkV
The spectral composition of the radiation depends on the voltage on the X-ray tube; with increasing voltage, the value of m i n shifts towards short wavelengths (Fig. 2a).
When the temperature T of the cathode incandescence changes, the electron emission increases. Consequently, the current I in the tube increases, but the spectral composition of the radiation does not change (Fig. 2b).
The energy flux Ф of bremsstrahlung is directly proportional to the square of the voltage U between the anode and the cathode, the current strength I in the tube and the atomic number Z of the anode substance:
Ф = kZU 2 I. (3)
where k \u003d 10 -9 W / (V 2 A).
Characteristic X-rays (for familiarization).
Increasing the voltage on the X-ray tube leads to the fact that against the background of a continuous spectrum, a line appears, which corresponds to the characteristic X-ray radiation. This radiation is specific to the anode material.
The mechanism of its occurrence is as follows. At a high voltage, accelerated electrons (with high energy) penetrate deep into the atom and knock electrons out of its inner layers. Electrons from upper levels pass to free places, as a result of which photons of characteristic radiation are emitted.
The spectra of characteristic X-ray radiation differ from optical spectra.
- Uniformity.
The uniformity of the characteristic spectra is due to the fact that the internal electron layers of different atoms are the same and differ only energetically due to the force action from the nuclei, which increases with increasing elemental number. Therefore, the characteristic spectra shift towards higher frequencies with increasing nuclear charge. This was experimentally confirmed by an employee of Roentgen - Moseley, who measured X-ray transition frequencies for 33 elements. They made the law.
MOSELY'S LAW – the square root of the frequency of the characteristic radiation is a linear function of the ordinal number of the element:
= A (Z - B), (4)
where v is the frequency of the spectral line, Z is the atomic number of the emitting element. A, B are constants.
The importance of Moseley's law lies in the fact that this dependence can be used to accurately determine the atomic number of the element under study from the measured frequency of the X-ray line. This played a big role in the placement of the elements in the periodic table.
Independence from a chemical compound.
The characteristic X-ray spectra of an atom do not depend on the chemical compound in which the atom of the element enters. For example, the X-ray spectrum of an oxygen atom is the same for O 2, H 2 O, while the optical spectra of these compounds differ. This feature of the x-ray spectrum of the atom was the basis for the name " characteristic radiation".
Interaction of X-ray radiation with matter
The impact of X-ray radiation on objects is determined by the primary processes of X-ray interaction. photon with electrons atoms and molecules of matter.
X-ray radiation in matter absorbed or dissipates. In this case, various processes can occur, which are determined by the ratio of the X-ray photon energy hv and the ionization energy Аu (the ionization energy Аu is the energy required to remove internal electrons from the atom or molecule).
a) Coherent scattering(scattering of long-wave radiation) occurs when the relation
For photons, due to interaction with electrons, only the direction of movement changes (Fig. 3a), but the energy hv and the wavelength do not change (therefore, this scattering is called coherent). Since the energies of a photon and an atom do not change, coherent scattering does not affect biological objects, but when creating protection against X-ray radiation, one should take into account the possibility of changing the primary direction of the beam.
b) photoelectric effect happens when
In this case, two cases can be realized.
The photon is absorbed, the electron is detached from the atom (Fig. 3b). Ionization occurs. The detached electron acquires kinetic energy: E k \u003d hv - A and. If the kinetic energy is large, then the electron can ionize neighboring atoms by collision, forming new ones. secondary electrons.
The photon is absorbed, but its energy is not enough to detach the electron, and excitation of an atom or molecule(Fig. 3c). This often leads to the subsequent emission of a photon in the visible radiation region (X-ray luminescence), and in tissues - to the activation of molecules and photochemical reactions. The photoelectric effect occurs mainly on the electrons of the inner shells of atoms with high Z.
in) Incoherent scattering(Compton effect, 1922) occurs when the photon energy is much greater than the ionization energy
In this case, the electron is detached from the atom (such electrons are called recoil electrons), acquires some kinetic energy E k, the energy of the photon itself decreases (Fig. 4d):
hv=hv" + A and + E k. (5)
The resulting radiation with a changed frequency (length) is called secondary, it scatters in all directions.
Recoil electrons, if they have sufficient kinetic energy, can ionize neighboring atoms by collision. Thus, as a result of incoherent scattering, secondary scattered X-ray radiation is formed and the atoms of the substance are ionized.
These (a, b, c) processes can cause a number of subsequent ones. For example (Fig. 3d), if during the photoelectric effect electrons are detached from the atom on the inner shells, then electrons from higher levels can pass to their place, which is accompanied by secondary characteristic X-ray radiation of this substance. Photons of secondary radiation, interacting with electrons of neighboring atoms, can, in turn, cause secondary phenomena.
coherent scattering
uh 
energy and wavelength remain unchanged
photoelectric effect
photon is absorbed, e - detached from the atom - ionization
hv \u003d A and + E to
atom A is excited upon absorption of a photon, R is X-ray luminescence
incoherent scattering
hv \u003d hv "+ A and + E to
secondary processes in the photoelectric effect
Rice. 3 Mechanisms of X-ray interaction with matter
Physical basis for the use of X-rays in medicine
When X-rays fall on a body, it is slightly reflected from its surface, but mainly passes deep into, while it is partially absorbed and scattered, and partially passes through.
The law of weakening.
The X-ray flux is attenuated in matter according to the law:
F \u003d F 0 e - x (6)
where is linear attenuation factor, which essentially depends on the density of the substance. It is equal to the sum of three terms corresponding to coherent scattering 1, incoherent 2 and photoelectric effect 3:
= 1 + 2 + 3 . (7)
The contribution of each term is determined by the photon energy. Below are the ratios of these processes for soft tissues (water).
|
Energy, keV |
photoelectric effect |
Compton - effect |
enjoy mass attenuation coefficient, which does not depend on the density of the substance :
m = /. (eight)
The mass attenuation coefficient depends on the energy of the photon and on the atomic number of the absorbing substance:
m = k 3 Z 3 . (9)
The mass attenuation coefficients of bone and soft tissue (water) are different: m bone / m water = 68.
If an inhomogeneous body is placed in the path of X-rays and a fluorescent screen is placed in front of it, then this body, absorbing and attenuating the radiation, forms a shadow on the screen. By the nature of this shadow, one can judge the shape, density, structure, and in many cases the nature of bodies. Those. a significant difference in the absorption of x-ray radiation by different tissues allows you to see the image of the internal organs in the shadow projection.
If the organ under study and the surrounding tissues equally attenuate x-rays, then contrast agents are used. So, for example, filling the stomach and intestines with a mushy mass of barium sulfate (BaSO 4 ), one can see their shadow image (the ratio of the attenuation coefficients is 354).
Use in medicine.
In medicine, X-ray radiation with photon energy from 60 to 100-120 keV is used for diagnostics and 150-200 keV for therapy.
X-ray diagnostics – Recognition of diseases by transilluminating the body with X-rays.
X-ray diagnostics is used in various options, which are given below.
With fluoroscopy the x-ray tube is located behind the patient. In front of it is a fluorescent screen. There is a shadow (positive) image on the screen. In each individual case, the appropriate hardness of the radiation is selected so that it passes through soft tissues, but is sufficiently absorbed by dense ones. Otherwise, a uniform shadow is obtained. On the screen, the heart, the ribs are visible dark, the lungs are light.
When radiography the object is placed on a cassette, which contains a film with a special photographic emulsion. The X-ray tube is placed over the object. The resulting radiograph gives a negative image, i.e. the opposite in contrast to the picture observed during transillumination. In this method, there is a greater clarity of the image than in (1), therefore, details are observed that are difficult to see when transilluminated.
A promising variant of this method is X-ray tomography and "machine version" - computer tomography.
3. With fluoroscopy, On a sensitive small-format film, the image from the large screen is fixed. When viewed, the pictures are examined on a special magnifier.
X-ray therapy- the use of X-rays to destroy malignant tumors.
The biological effect of radiation is to disrupt vital activity, especially rapidly multiplying cells.
COMPUTED TOMOGRAPHY (CT)
The method of X-ray computed tomography is based on the reconstruction of an image of a certain section of the patient's body by registering a large number of X-ray projections of this section, made at different angles. Information from the sensors that register these projections enters the computer, which, according to a special program calculates distribution tightsample size in the investigated section and displays it on the display screen. The image of the section of the patient's body obtained in this way is characterized by excellent clarity and high information content. The program allows you to increase image contrast in dozens and even hundreds of times. This expands the diagnostic capabilities of the method.
Videographers (devices with digital X-ray image processing) in modern dentistry.
In dentistry, X-ray examination is the main diagnostic method. However, a number of traditional organizational and technical features of X-ray diagnostics make it not quite comfortable for both the patient and dental clinics. This is, first of all, the need for the patient to come into contact with ionizing radiation, which often creates a significant radiation load on the body, it is also the need for a photoprocess, and, consequently, the need for photoreagents, including toxic ones. This is, finally, a bulky archive, heavy folders and envelopes with x-ray films.
In addition, the current level of development of dentistry makes the subjective assessment of radiographs by the human eye insufficient. As it turned out, of the variety of shades of gray contained in the x-ray image, the eye perceives only 64.
Obviously, to obtain a clear and detailed image of the hard tissues of the dentoalveolar system with minimal radiation exposure, other solutions are needed. The search led to the creation of so-called radiographic systems, videographers - digital radiography systems.
Without technical details, the principle of operation of such systems is as follows. X-ray radiation enters through the object not on a photosensitive film, but on a special intraoral sensor (special electronic matrix). The corresponding signal from the matrix is transmitted to a digitizing device (analog-to-digital converter, ADC) that converts it into digital form and is connected to the computer. Special software builds an x-ray image on the computer screen and allows you to process it, save it on a hard or flexible storage medium (hard drive, floppy disks), print it as a picture as a file.
In a digital system, an x-ray image is a collection of dots having different digital grayscale values. The information display optimization provided by the program makes it possible to obtain an optimal frame in terms of brightness and contrast at a relatively low radiation dose.
In modern systems, created, for example, by Trophy (France) or Schick (USA), 4096 shades of gray are used when forming a frame, the exposure time depends on the object of study and, on average, is hundredths - tenths of a second, a decrease in radiation exposure in relation to film - up to 90% for intraoral systems, up to 70% for panoramic videographers.
When processing images, videographers allow:
Get positive and negative images, false color images, embossed images.
Increase contrast and magnify the area of interest in the image.
Assess changes in the density of dental tissues and bone structures, control the uniformity of canal filling.
In endodontics, determine the length of the canal of any curvature, and in surgery, select the size of the implant with an accuracy of 0.1 mm.
The unique Caries detector system with elements of artificial intelligence during the analysis of the image allows you to detect caries in the stain stage, root caries and hidden caries.
"F" in formula (3) refers to the entire range of radiated wavelengths and is often referred to as "Integral Energy Flux".
The discovery and merit in the study of the basic properties of X-rays rightfully belongs to the German scientist Wilhelm Conrad Roentgen. The amazing properties of X-rays discovered by him immediately received a huge response in the scientific world. Although then, back in 1895, the scientist could hardly imagine what benefit, and sometimes harm, X-rays can bring.
Let's find out in this article how this type of radiation affects human health.
What is x-ray radiation
The first question that interested the researcher was what is X-ray radiation? A number of experiments made it possible to verify that this is electromagnetic radiation with a wavelength of 10 -8 cm, which occupies an intermediate position between ultraviolet and gamma radiation.
Application of X-rays
All these aspects of the destructive effects of the mysterious X-rays do not at all exclude surprisingly extensive aspects of their application. Where is X-rays used?
- Study of the structure of molecules and crystals.
- X-ray flaw detection (in industry, detection of defects in products).
- Methods of medical research and therapy.
The most important applications of X-rays have become possible due to the very short wavelengths of the entire range of these waves and their unique properties.
Since we are interested in the impact of X-rays on people who encounter them only during a medical examination or treatment, then we will only consider this area of application of X-rays.
The use of x-rays in medicine
Despite the special significance of his discovery, Roentgen did not take out a patent for its use, making it an invaluable gift for all mankind. Already in the First World War, X-ray units began to be used, which made it possible to quickly and accurately diagnose the wounded. Now we can distinguish two main areas of application of x-rays in medicine:
- X-ray diagnostics;
- x-ray therapy.
X-ray diagnostics
X-ray diagnostics is used in various options:
Let's take a look at the difference between these methods.
All of these diagnostic methods are based on the ability of x-rays to illuminate film and on their different permeability to tissues and the bone skeleton.
X-ray therapy
The ability of X-rays to have a biological effect on tissues is used in medicine for the treatment of tumors. The ionizing effect of this radiation is most actively manifested in the effect on rapidly dividing cells, which are the cells of malignant tumors.
However, you should also be aware of the side effects that inevitably accompany radiotherapy. The fact is that cells of the hematopoietic, endocrine, and immune systems are also rapidly dividing. A negative impact on them gives rise to signs of radiation sickness.
The effect of X-ray radiation on humans
Shortly after the remarkable discovery of X-rays, it was discovered that X-rays had an effect on humans.
These data were obtained in experiments on experimental animals, however, geneticists suggest that similar effects may apply to the human body.
The study of the effects of X-ray exposure has led to the development of international standards for acceptable radiation doses.
Doses of x-ray radiation in x-ray diagnostics
After visiting the X-ray room, many patients are worried - how will the received dose of radiation affect their health?
The dose of general irradiation of the body depends on the nature of the procedure. For convenience, we will compare the received dose with natural exposure, which accompanies a person throughout his life.
- X-ray: chest - the received dose of radiation is equivalent to 10 days of background exposure; upper stomach and small intestine - 3 years.
- Computed tomography of the abdominal cavity and pelvis, as well as the whole body - 3 years.
- Mammography - 3 months.
- Radiography of the extremities is practically harmless.
- With regard to dental x-rays, the radiation dose is minimal, since the patient is exposed to a narrow beam of x-rays with a short radiation duration.
These radiation doses meet acceptable standards, but if the patient feels anxious before the X-ray, he has the right to ask for a special protective apron.
Exposure of X-rays to pregnant women
Each person has to undergo X-ray examination repeatedly. But there is a rule - this diagnostic method cannot be prescribed to pregnant women. The developing embryo is extremely vulnerable. X-rays can cause chromosome abnormalities and, as a result, the birth of children with malformations. The most vulnerable in this regard is the gestational age of up to 16 weeks. Moreover, the most dangerous for the future baby is an x-ray of the spine, pelvic and abdominal regions.
Knowing about the detrimental effect of x-rays on pregnancy, doctors avoid using it in every possible way during this crucial period in a woman's life.
However, there are side sources of X-rays:
- electron microscopes;
- color TV kinescopes, etc.
Expectant mothers should be aware of the danger posed by them.
For nursing mothers, radiodiagnosis is not dangerous.
What to do after an x-ray
To avoid even the minimal effects of X-ray exposure, some simple steps can be taken:
- after an x-ray, drink a glass of milk - it removes small doses of radiation;
- very handy taking a glass of dry wine or grape juice;
- some time after the procedure, it is useful to increase the proportion of foods with a high content of iodine (seafood).
But, no medical procedures or special measures are required to remove radiation after an x-ray!
Despite the undoubtedly serious consequences of exposure to X-rays, one should not overestimate their danger during medical examinations - they are carried out only in certain areas of the body and very quickly. The benefits of them many times exceed the risk of this procedure for the human body.
X-ray radiation (synonymous with 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, decelerate in the electric field of the atoms of a substance. The resulting quanta have different energies and form a continuous spectrum. The maximum photon energy in such a spectrum is equal to the energy of incident electrons. In (see) 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 passing through a substance, X-rays 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 an interaction, the quantum energy is completely spent on pulling out an electron from the atomic shell and imparting kinetic energy to it. With an increase in the energy of an X-ray quantum, the probability of the photoelectric effect decreases and the process of scattering of quanta on free electrons becomes predominant - the so-called Compton effect. As a result of such an interaction, a secondary electron is also formed and, in addition, a quantum flies out with an energy lower than the energy of the primary quantum. If the energy of an X-ray quantum exceeds one megaelectron-volt, a 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 low-energy quanta are more likely to be absorbed in this case, 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 rigidity. 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) - quantum electromagnetic radiation with a wavelength of 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-rays, whose energy quanta exceed 500 keV, is called gamma radiation (see); radiation, whose energy quanta are below 0.05 keV, is 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 (about 300 thousand km / s in a vacuum) and is characterized by a wavelength λ ( the distance over which the radiation propagates in one period of oscillation). X-ray radiation also has a number of other wave properties (refraction, interference, diffraction), but it is much more difficult to observe them than for longer-wavelength radiation: visible light, radio waves.
X-ray spectra: a1 - continuous bremsstrahlung spectrum at 310 kV; a - continuous bremsstrahlung spectrum at 250 kV, a1 - spectrum filtered by 1 mm Cu, a2 - spectrum filtered by 2 mm Cu, b - K-series of the tungsten line.
To generate x-rays, x-ray tubes are used (see), in which radiation occurs when fast electrons interact with atoms of the anode substance. There are two types of x-rays: bremsstrahlung and characteristic. Bremsstrahlung X-ray radiation, which has a continuous spectrum, is similar to ordinary white light. The distribution of intensity depending on the wavelength (Fig.) is represented by a curve with a maximum; in the direction of long waves, the curve falls gently, and in the direction of short waves, it steeply and breaks off 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 arises from the interaction of fast electrons with atomic nuclei. The bremsstrahlung intensity is directly proportional to the strength of the anode current, the square of the tube voltage, and the atomic number (Z) of the anode material.
If the energy of electrons accelerated in the X-ray tube exceeds the critical value for the anode substance (this energy is determined by the tube voltage Vcr, which is critical for this substance), then characteristic radiation occurs. The characteristic spectrum is line, its spectral lines form a series, denoted 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 the 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 with an energy equal to the difference between the energies of the atom in the excited and ground states are emitted. This difference (and hence the energy of the photon) has a certain value, characteristic of each element. This phenomenon underlies the 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 is strongly heated in this case), only an insignificant 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-rays is completely independent of the optical properties of the absorber material. The colorless and transparent lead glass used to protect personnel in x-ray rooms absorbs x-rays almost completely. 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, when passing through an absorber layer, decreases according to an 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 absorber thickness in g / cm 2 (here p is the density of the substance in g / cm 3). X-rays are attenuated by 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 the 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. For λ≥0,3Å the scattering coefficient does not depend on the wavelength, for λ<0,ЗÅ он уменьшается с уменьшением λ.
The decrease in the absorption and scattering coefficients with decreasing wavelength causes an increase in the penetrating power of X-rays. The mass absorption coefficient for bones [absorption is mainly due to Ca 3 (PO 4) 2 ] is almost 70 times greater than for soft tissues, where absorption is mainly due to water. This explains why the shadow of the bones stands out so sharply on the radiographs against the background of soft tissues.
The propagation of an inhomogeneous X-ray beam through any medium, along with a decrease in intensity, is accompanied by a change in the spectral composition, 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-wavelength part of the spectrum makes it possible to improve the ratio between deep and surface doses during X-ray therapy of foci located deep in the human body (see X-ray filters). To characterize the quality of an inhomogeneous X-ray beam, the concept of "half attenuation layer (L)" is used - a layer of a 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. Cellophane (up to an energy of 12 keV), aluminum (20–100 keV), copper (60–300 keV), lead, and copper (>300 keV) are used to measure half attenuation layers. 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.
Absorption and scattering of X-rays is due to its corpuscular properties; X-rays interact with atoms as a stream of corpuscles (particles) - photons, each of which has a certain energy (inversely proportional to the wavelength of X-rays). 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).
Scattering of X-ray radiation is due to the electrons of the scattering medium. There are 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 the incident one). In the latter case, the photon behaves like a moving ball, and the scattering of photons occurs, according to the figurative expression of Comnton, like a game of billiards with photons and electrons: colliding with an electron, the photon transfers part of its energy to it and scatters, having already less energy (respectively, the wavelength of the scattered radiation increases), the electron flies out of the atom with a recoil energy (these electrons are called Compton electrons, or recoil electrons). The 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-rays transferred to a unit mass of a substance determines the absorbed dose of X-rays. The unit of this dose 1 rad corresponds to 100 erg/g. Due to the absorbed energy in the substance of the absorber, a number of secondary processes occur that are important for X-ray dosimetry, since it is on them that X-ray measurement methods are based. (see Dosimetry).
All gases and many liquids, semiconductors and dielectrics, under the action of X-rays, increase electrical conductivity. Conductivity is found by the best insulating materials: paraffin, mica, rubber, amber. The change in conductivity is due to the 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 the exposure dose of X-ray radiation (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 action of X-rays, as a result of the excitation of the 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 of air, paper, paraffin, etc. is observed (metals are an exception). The highest yield of visible light is given by such crystalline phosphors as Zn·CdS·Ag-phosphorus and others used for screens in fluoroscopy.
Under the action of X-rays, various chemical processes can also take place in a substance: the decomposition of silver halides (a photographic effect used in X-rays), the decomposition of water and aqueous solutions of hydrogen peroxide, a change in the properties of celluloid (clouding and release of camphor), paraffin (clouding and bleaching) .
As a result of complete conversion, all the X-ray energy absorbed by the chemically inert substance is converted into heat. The measurement of very small amounts of heat requires highly sensitive methods, but is the main method for absolute measurements of X-rays.
Secondary biological effects from exposure to x-rays are the basis of medical radiotherapy (see). X-rays, the quanta of which are 6-16 keV (effective wavelengths from 2 to 5 Å), are almost completely absorbed by the skin integument of the tissue of the human body; they are called boundary rays, or sometimes Bucca rays (see Bucca 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 in x-ray therapy, but also in x-ray diagnostics, as well as in all other cases of contact with x-rays that require the use of radiation protection (see).
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 possibilities for using 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. Braggi, 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 range 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 is also used for similar purposes, which uses neutron beams instead of X-rays.) X-rays are also used for other purposes, such as examining paintings to determine their authenticity or to detect additional layers of paint over 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-ray radiation, 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-ray radiation 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 difference between the paths 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 individual 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
Already 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-rays appear, which are 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 most important 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 spacing d by the inequality l 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, a highly focused electron beam excites the characteristic X-ray emission of the sample, 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 for whose characteristic radiation the spectrometer is tuned.
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-rays become visible if they are filled with a contrast agent that is harmless to the body, but allows one to visualize the shape of the 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 damage 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 blood-forming organs, mainly 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 books. 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;
