(or roentgen rays), the electromagnetic ionizing radiation that occupies the spectral region between gamma and ultraviolet radiation. The wavelengths of X rays range from 10^-4 to 10³ angstroms (A), that is, from 10^-12 to 10^-5 cm. X rays with shorter wavelengths are called hard X rays, and X rays with longer wavelengths are known as soft X rays; in the USSR, the wavelength dividing hard X rays from soft X rays is usually taken as 2 A.

X rays were discovered in 1895 by W. K. Roentgen; it was he who gave them the name “X rays,” a term still used in many countries. Between 1895 and 1897, Roentgen investigated the properties of X rays and constructed the first X-ray tubes. He found that hard X rays pass through various materials and through soft tissues of the human body, a property of X rays that soon found application in medicine. The discovery of X rays attracted the attention of scientists throughout the world, and as early as 1896 more than 1,000 studies were published dealing with the investigation and application of X rays. The electromagnetic nature of X rays was suggested by G. Stokes and experimentally confirmed by C. Barkla, who discovered the polarization of X rays. In 1912, the German physicists M. von Laue, W. Friedrich, and P. Knipping found that X rays can be diffracted by the atomic lattice of a crystal (see DIFFRACTION OF X RAYS). In 1913, G. V. Vul’f and, independently, W. L. Bragg discovered a simple relation between the glancing angle at which an incident X ray is reflected by an atomic plane of a crystal, the wavelength of the X ray, and the interpla-nar spacing (see BRAGG-VULT CONDITION). The investigations of these scientists provided the basis for X-ray diffraction analysis. The use of X-ray spectra to determine the elemental composition of materials began in the 1920’s. The 1930’s saw the application of X-ray spectra to the investigation of the electronic energy structure of matter. The Physicotechnical Institute, which was founded by A. F. Ioffe, has played an important role in the development of the investigation and application of X rays in the USSR.

Sources. The most widely used source of X rays is the X-ray tube. Certain radioactive isotopes can also be used as X-ray sources. Some isotopes emit X rays directly. In the case of other isotopes, the nuclear radiations—electrons or alpha particles—of the isotopes bombard a metallic target that emits X rays. The intensity of the X-radiation of isotopic sources is several orders of magnitude less than that of an X-ray tube, but the size, weight, and cost of isotopic sources are much smaller than is the case for units with X-ray tubes.

Synchrotrons and electron accumulators with energies of a few gigaelectron volts can be used as sources of soft X rays with wavelengths of the order of tens or hundreds of angstroms. In this region of the spectrum, the intensity of X-radiation produced by synchrotrons is two or three orders of magnitude greater than that of X-radiation produced by X-ray tubes.

The sun and other cosmic bodies are natural sources of X rays.

Properties. X rays can exhibit a continuous spectrum or a line spectrum, depending on the mechanism by which they are produced: bremsstrahlung has a continuous spectrum, and characteristic X rays have a line spectrum.

When fast charged particles interact with the atoms of a target, the deceleration of the particles results in the emission of a continuous X-ray spectrum; the radiation produced in this manner is called bremsstrahlung. The continuous spectrum is of significant intensity only when the target is bombarded with electrons. The intensity of the bremsstrahlung is distributed over all frequencies up to a maximum frequency v₀, at which the photon energy hv₀ is equal to the energy eV of the bombarding electrons; here, h is Planck’s constant, e is the electronic charge, and V is the potential difference of the acceleration field traversed by the electron. To this frequency there corresponds the minimum wavelength λ₀ = hc/eV, where c is the speed of light.

Line emission occurs when an atom is ionized with the ejection of an electron from one of its inner shells. In the case of primary X rays, such ionization results from the collision of the atom with a fast particle such as an electron; in the case of X-ray fluorescence radiation, the ionization results from the absorption of a photon by the atom. The ionized atom is left in its initial quantum state at a high energy level; in 10^-16-l0 sec the atom moves to its final state of lower energy. In the process, the atom can emit its excess energy in the form of a photon of a certain frequency. The frequencies of the spectral lines of such radiation are characteristic of the atoms of each element; for this reason, X-ray line spectra are also called characteristic spectra. The frequency ν of a line of a characteristic spectrum is dependent on the atomic number Z. This relation is expressed by Moseley’s law √v = AZ + B, where A and B are constants for each line of the spectrum.

The bremsstrahlung emitted by very thin targets is completely polarized near v₀; the degree of polarization decreases with decreasing v. Characteristic radiation is generally not polarized.

When X rays interact with a substance, the photoelectric effect, which accompanies the absorption and scattering of the X rays, can occur. The photoelectric effect is observed when an atom, on absorbing an X-ray photon, ejects one of its inner electrons. The atom can then undergo either a radiative transition, wherein a photon of the characteristic radiation is emitted, or a nonradiative transition, wherein a second electron, known as an Auger electron, is ejected. When X rays act on nonmetallic crystals, such as rock salt, ions with an additional positive charge appear at some points of the atomic lattice, and the excess electrons are found near these ions. Such imperfections of the crystal structure are called X-ray excitons; they are color centers and disappear only when the temperature is considerably increased.

When X rays pass through a layer of a substance of thickness x, their initial intensity I₀ decreases to the level I = I₀e^-μx,, where μ is the linear absorption coefficient. The attenuation of I occurs through two processes: the absorption of the X-ray photons by the substance and the change in the photons’ direction when the photons are scattered. Absorption predominates in the long-wavelength region of the spectrum, and scattering in the short-wavelength region. The degree of absorption increases rapidly with Z and the wavelength λ. For example, hard X rays freely pass through a layer of air that is—10 cm in thickness, and an aluminum plate 3 cm thick attenuates by 50 percent X rays with λ = 0.027 Å. Soft X rays are largely absorbed in air and can be used and investigated only in a vacuum or a weakly absorbing gas, such as helium. When X rays are absorbed, the atoms of the absorbing substance are ionized.

The action of X rays on living organisms can be useful or harmful, depending on the ionization induced in tissues. Since the absorption of X rays depends on λ, the intensity of the radiation cannot be used as a measure of the biological effects of X rays. The quantitative study of the action of X rays on matter is the subject of roentgenometry. The roentgen is the unit of measurement of the radiation to which a substance is exposed.

In the case of large Z and λ, X rays are for the most part scattered without a change in λ; such scattering is known as coherent scattering. In the case of small Z and λ, incoherent scattering predominates: λ generally increases. The two known types of incoherent scattering of X rays are Compton scattering and Raman scattering. Compton scattering has the character of inelastic corpuscular scattering. In this type, the X-ray photon loses some of its energy; the lost energy is expended on the ejection of a recoil electron from the shell of the atom (see COMPTON EFFECT). Not only is the energy of the photon decreased, but the photon’s direction is changed. The change in λ depends on the scattering angle. In the Raman scattering of a high-energy X-ray photon by a light atom, a small part of the photon’s energy is expended on the ionization of the atom, and the photon’s direction of motion is changed. The change in the λ of such photons is independent of the scattering angle.

The refractive index η for X rays differs from unity by a very small amount: δ = 1 - η ≈ 10^-6-10^-5. The phase velocity of X rays in a medium is greater than the speed of light in a vacuum. The deflection of X rays on passing from one medium to another is very small and amounts to a few minutes of arc. When X rays in a vacuum are incident on the surface of a body at a very small angle, total external reflection occurs.

Detection. The human eye is insensitive to X rays. X rays can be detected by means of special X-ray photographic film that contains an increased amount of AgBr. For λ < 0.5 Å, the sensitivity of such film decreases rapidly and can be artificially enhanced by a fluorescent screen tightly pressed against the film. For λ > 5 Å, the sensitivity of ordinary positive photographic film is sufficiently great; the grains of the film are much smaller than those of X-ray film, and consequently the resolution is better. When λ is of the order of tens or hundreds of angstroms, X rays act only on the very thin surface layer of the photographic emulsion; to improve the sensitivity of the film, the film is sensitized with luminescent oils. In roentgen diagnosis and in flaw detection, electrophotography—for example, xeroradiography—is sometimes used to detect X rays.

High-intensity X rays can be detected by means of an ionization chamber. In the case of X rays of medium and low intensities, a scintillation counter with a Nal(Tl) crystal can be used when λ < 3 Å, a Geiger counter and a sealed-off proportional counter when 0.5 < λ < 5 Å, a liquid-flow proportional counter when 1 < λ < 100 Å, and a semiconductor detector when λ < 120 Å. For very large λ (from 10 to 1,000 Å), X rays can be detected by open-type secondary emission multipliers with different photocathodes at the input.

Applications. X rays have found their broadest application in medicine, where they are used for roentgen diagnosis and roentgenotherapy. X-ray flaw detection is of great importance for many fields of technology; X rays are used, for example, to detect cracks in rails, welding defects, and internal defects of castings, such as cavities and slag inclusions.

Through X-ray diffraction analysis it is possible to determine the spatial arrangement of atoms in the crystal lattice of minerals and compounds and in inorganic and organic molecules. The opposite task can be accomplished on the basis of the numerous atomic structures already analyzed: the crystalline composition of a polycrystalline substance, such as alloy steel, an alloy, an ore, or lunar soil, can be determined from the X-ray diffraction pattern of the substance, that is, a phase analysis can be performed (see DEBYE-SCHERRER METHOD). The X-ray analysis of materials encompasses numerous applications of X rays for studying the properties of solids.

By means of X-ray microscopy scientists can, for example, obtain an image of a cell or microorganism and view the internal structure of the cell or microorganism. X-ray spectroscopy makes use of X-ray spectra to study the energy distribution of the density of electronic states in various substances, to investigate the nature of the chemical bond, and to find the effective charge of ions in solids and molecules. X-ray spectral analysis permits the qualitative and quantitative composition of a substance to be determined from the position and intensity of the characteristic spectral lines. It is used for rapid nondestructive inspection of the composition of materials at metallurgical, cement, and concentrating plants. When such plants are automated, X-ray spectrometers and quantometers are used as instruments for analyzing the composition of substances.

X rays from space carry information on the chemical composition of cosmic bodies and on physical processes occurring in space. X-ray astronomy deals with the investigation of cosmic X rays. Powerful X rays are used in radiation chemistry to stimulate certain reactions, polymerize materials, and crack organic substances. In art, the existence of an earlier painting hidden beneath a later painting can be detected by means of X rays. The food industry makes use of X rays to detect foreign objects that have accidentally entered food products. Criminalistics and archaeology are examples of other fields where X rays are employed.