X-rays

X-rays were discovered in 1895 by W.C. Röntgen and named by him X-rays. X-rays are obtained in special X-ray tubes, a special type of discharge tubes between the electrodes of which an electric «21 field of several tens of thousands of volts is created. X-rays arise due to the abrupt deceleration of electrons by the substance of the anode, or, as it is otherwise called, the anticathode of the X-ray tube. As was established by further research, X-rays are electromagnetic radiation, invisible rays, with a wavelength ten thousand times shorter than the wavelength of visible rays. Many substances fluoresce under the action of X-rays. The green light of fluorescence of barium platinocyanide, to which the human eye is particularly sensitive,¹ is used in the fluorescent screens made from this salt, which serve both for various physical purposes and for X-ray diagnostics. The law of absorption of X-rays in their passage through any substance is expressed by the following formula: where I is the intensity of the transmitted rays, I₀ is the intensity of the incident rays, μ is the thickness of the absorbing layer, e is the base of natural logarithms, and μ is the absorption coefficient of X-rays. Instead of the absorption coefficient [i, the mass absorption coefficient is often used -, where ρ is the density of the absorbing substance. Then the previous formula should be written as follows: where m is the mass of substance per 1 cm². The absorption coefficient of X-rays depends both on the type of X-rays, i.e., on their wavelength, and on the absorbing substance. For X-rays of a certain wavelength, the absorption coefficient by different elementary substances increases with their atomic number in the periodic system of Mendeleev. In a given substance, between the edges of the absorption bands, X-rays are absorbed more strongly the longer their wavelength. Since the absorption of X-rays is an atomic property, not depending on how the absorbing atoms combine with others, the mass absorption coefficient - of any complex substance, the percentage content of which consists of elementary substances p₁, p₂, p₃, etc., is obtained from the absorption coefficients -, -, ^ etc., of the individual substances as follows: Human and animal bones are less permeable to X-rays than muscles and cavities filled^! with air, this, in connection with the generally different weakening of X-rays in their passage through different substances, is the basis of X-ray diagnostics (see). For various cases of fluoroscopy, it is important to use rays with a certain interval of wavelengths. This follows from the fact that rays with short wavelengths, the so-called "hard" rays, obtained in tubes operating at large potential differences between their electrodes, have great penetrating power, differing little for various tissues of the body, and do not give sufficient contrasts. "Soft" rays, with longer wavelengths, obtained at small potential differences on the tube electrodes, on the contrary, can be so strongly absorbed by tissues that they give only weak outlines of the objects being examined when used for fluoroscopy. Lead strongly absorbs X-rays, therefore various protective devices for workers from the action of these rays are mainly made from this metal. The action of X-rays on the body was noticed in the very first years of work with X-ray "imi" rays. Besides erythema, burns of varying degrees, skin cancer, X-rays can cause destruction of tissue cells, and healthy cells offer greater resistance to the destructive action of these rays, while pathological ones offer less. -This property of X-rays formed the basis of X-ray therapy (see). X-rays act on the light-sensitive layer of photographic plates and films. The blackening of a photographic plate exposed to X-rays, at small values of the incident energy increases proportionally to the latter, but with further increase in energy it increases more slowly. Since the photochemical effect increases with the amount of absorbed energy, the action of hard rays on a photographic plate is weaker than that of soft rays, for equal amounts of energy of these rays incident on the plate. To increase the photochemical effect under the action of X-rays, the light-sensitive layer of X-ray plates is taken to be much thicker than that of ordinary ones, and X-ray films are made double-sided. In X-ray photography, intensifying screens are often used. An intensifying screen made of calcium tungstate is a screen that fluoresces blue light under the action of X-rays. For the duration of exposure, it is applied with its light-sensitive layer to the light-sensitive layer of the photographic plate, then the photochemical reaction in the plate proceeds not only under the action of the energy of X-rays absorbed by it, but also under the action of the blue light of the screen. X-rays have the ability to ionize the gases they illuminate. Between the number of ions formed in a given gas and the intensity of X-rays completely absorbed by it, there is a direct proportionality. "This regularity made it possible to establish, by international agreement in Stockholm (1928), a unit for measuring the intensity of X-rays. The intensity of X-rays is taken as unity if the ionization caused by their complete absorption in air between two electrodes creates a saturation current equal to one electrostatic unit. After the discovery of Laue (1913) of the interference of X-rays in their passage through a crystal or upon reflection from a crystal (Bragg), considered as a diffraction; spatial lattice for these rays, it became possible to study their spectral composition. In X-ray spectrometers, their beam falls on a crystal at various angles φ, measured from the surface of the crystal. If among the incident rays there are those whose wavelengths λ satisfy the Bragg formula: nλ = 2d.sin φ, where d is the constant of the crystal lattice (distance between atomic planes), and n is the order of the spectrum, then these rays will be reflected by the crystal at angles equal to the angles of incidence, and on a photographic plate placed in their path, a spectrum will be obtained. Observations indicate the existence of continuous and line spectra of X-rays. The total energy of the continuous spectrum is proportional to the square of the voltage at which the tube operates. The continuous spectrum sharply cuts off in the region of short wavelengths at a wavelength determined by the formula: where λ is the wavelength in units of Angstroms (10^-8 cm), and V is the operating voltage in kilovolts. It should be noted that of the electrical energy expended in the X-ray tube, only a few thousandths is converted into the energy of X-rays, while the rest is converted into heat. The line spectrum of X-rays consists of a series of separate rays of definite wavelengths, characteristic of the substance of the anticathode. These "characteristic" rays of the anticathode are grouped into series, i.e., such groups of lines whose wavelengths are determined by one formula. To cause the appearance of characteristic rays of the anticathode, preliminary excitation of its atoms is necessary, which requires the expenditure of energy. If the energy of the electrons moving in the field of the tube is sufficient to eject an electron from one of the inner orbits of the anticathode atoms, which constitutes the excitation of the atom, characteristic radiation occurs. If the energy is insufficient, characteristic rays will not appear, and the tube will give only a continuous spectrum. Since the energy of electrons in the tube is determined by the potential difference at which the tube operates, characteristic rays appear when the operating voltage is above a certain minimum value. The intensity of characteristic rays is expressed by the following formula: I = C(V-V_min)ⁿ where C is a constant, V is the operating voltage, and V_min is the minimum voltage that can cause the appearance of these characteristic rays. By using characteristic rays of various anticathodes and applying filters of various substances, it is possible to obtain monochromatic X-rays. In X-ray therapy, filters are often used to eliminate soft rays. Spreading in any substance, X-rays are not only absorbed by it but also scattered. The scattering of X-rays occurs either without changing the wavelength or with its change. The first phenomenon is no different from the analogous phenomenon for ordinary optical rays. The second, called the Compton effect, is explained by the loss of part of the energy by the scattered quanta of X-rays in their collision with electrons of the scattering substance. The electrons come into motion, and the reflected quanta give rays with a somewhat longer wavelength. When obtaining X-ray photographs, scattered X-rays reduce the sharpness of the photographic picture, therefore in medical practice special devices - diaphragms - are used to eliminate them. A quantum of X-rays can give an electron upon collision with it such energy that the electron is ejected from the atom and leaves the irradiated substance. The phenomenon of loss of electrons by the irradiated substance is called the photoelectric effect, and the electrons released in this process are called photoelectrons.

The discovery of total internal reflection and refraction of X-rays made it possible to determine their refractive index. It is less than one by several hundred-thousandths. X-rays played a major role in creating the theory of atomic structure which is now accepted in science. X-rays have found wide application not only in medicine. Many industries also use them, and X-ray structural analysis currently has enormous importance in the metalworking industry, where with its help complex questions about the dependence of material strength on methods of their processing and on conditions of production are resolved.