X-ray radiation

X-ray radiation (synonym x-rays) is electromagnetic radiation with a wide range of wavelengths (8·10-6 to 10-12 cm). X-ray radiation occurs at braking of charged particles, usually electrons in an electric field of atoms. The resulting quantum x-ray radiation have different energy and form a continuous spectrum. The maximum energy of quanta in this spectrum equal to the energy of incident electrons. In the x-ray tube (see), the maximum energy of quanta of x-ray radiation, expressed in kiloelectron-volts, numerically equal to the value attached to the tube voltage, expressed in kilovolts. When passing through a substance x-rays interact with the electrons in its atoms. For x-ray quanta with energies up to 100 Kev most common type of interaction is the photoelectric effect. As a result of such interaction between the quantum energy is spent on the ejection of electrons from the atomic shell and messages to him kinetic energy. With increasing energy of the quantum x-ray radiation probability of the photoelectric effect is reduced and becomes predominant process of quantum dispersion free electron - so-called Compton-effect. As a result of this interaction also occurs secondary electron and, in addition, he takes a quantum of energy less than the energy of the primary Quant. If the quantum energy of x-ray radiation exceeds one million electron volts, may be so-called effect of education pairs, which are formed an electron and a positron (see Atom). Therefore, when passing through a substance is reduced energy x-ray radiation, i.e., the decrease its intensity. Since this is likely to happen absorption of quanta low energy, it is the enrichment of x-ray quanta higher energy. This property of x-ray radiation is used to increase the average energy of quanta, i.e. to increase its rigidity. Achieved an increase rigidity of the x-ray radiation by using special filters (see X-ray filters). X-rays are used for x-ray diagnostics (see x-Ray) and radiotherapy (see). Cm. also ionizing Radiation.

X-ray radiation (synonym: x-rays, x-ray) - quantum of electromagnetic radiation with the wave length from 250 to 0.025 And (or quanta anergy 5·10-2 to 5·102 Kev). In 1895 openly C. K. x-rays. Adjacent to x-ray spectral region of the electromagnetic radiation, quantum of energy of which exceeds 500 Kev, called gamma rays (see); radiation quanta of energy which is below the values of 0.05 Kev, is ultraviolet radiation (see).
Thus, presenting a relatively small part of the vast spectrum of electromagnetic radiation, which includes radio waves and visible light, x-rays, like any electromagnetic radiation travels at the speed of light (in the emptiness around 300 thousand km/s) and is characterized by a wavelength (R is the distance the radiation is distributed during one period of oscillation). X-rays also has some other wave properties (refraction, interference, diffraction), but you can watch them much more difficult than the more long-wave radiation of visible light, radio waves.

spectra of x-ray radiation
Spectra of x-ray radiation: A1 - continuous braking range at 310 sq; and - continuous braking range at 250 kV, A1 - range, filter 1 mm Cu, A2 - range, filtered 2 mm Cu, b - K-series line of tungsten.

To generate the x-rays used x-ray tube (see), in which the radiation is emitted in the interaction of fast electrons with atoms of the substance of the anode. Distinguish R. I. two types: bremsstrahlung and characteristic. Brake R. I. with a continuous spectrum, like an ordinary white light. Distribution of intensity depending on wavelength (Fig) it seems a curve with a maximum; in the direction of long waves curve falls hollow, and in the direction of short - cool and ends at a specific wavelength (0)called shortwave boundary of the continuous spectrum. The value 0 is inversely proportional to the voltage of the tube. Bremsstrahlung occurs in the interaction of fast electrons with nuclei of atoms. The intensity of the bremsstrahlung is directly proportional to the power of anode current, the square of the voltage of the tube and the atomic number (Z) of the substance of the anode.
If the energy of accelerated in the x-ray tube electrons exceeds critical for substances anode value (this energy is determined critical for this substance voltage on the tube Vкр), then there characteristic radiation. The characteristic spectrum - bar, its spectral lines form of a series of letters K, L, M, n
Series K - the most shortwave, series L - far more, series M and N are only seen in heavy elements (Vкр tungsten for K-series - 69,3 kV, for the L-series - 12,1 kV). Characteristic radiation occurs in the following way. Quick knock electrons atomic electrons from the inner shells. The atom is excited, and then returns to the basic state. Thus electrons from the external, less related shells fill the vacant in the inner shells of the place, and emitted photons characteristic radiation with an energy equal to the energy difference of the atoms in the excited and ground state. This difference (and hence the photon energy) has a certain value, characteristic for each item. This phenomenon underlies the x-ray spectral analysis of the elements. Figure visible linear spectrum of tungsten on the background of the continuous spectrum of bremsstrahlung.
The energy of accelerated in the x-ray tube electrons are converted almost entirely in heat (anode when this gets very hot), only a small proportion (about 1% at a voltage close to 100 kV) is transformed into the energy of bremsstrahlung radiation.
The application of x-ray radiation in medicine is based on the laws of absorption of x-rays by matter. Absorption R. I. completely depends on the optical properties of the substance of the absorber. Colorless and transparent leaded glass that is used to protect personnel x-ray rooms, almost completely absorbs x-rays. On the contrary, a sheet of paper, not transparent to light, does not weaken R. I.
The intensity of homogeneous (i.e. a certain wavelength) x-ray beam passing through a layer of absorbent decreases exponentially (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 (n/R) cm2/g on the thickness of the absorber in g/cm2 (here R is the density of a substance in g/cm3). The attenuation of x-ray radiation is caused by the scattering and absorption. Accordingly mass attenuation coefficient is the sum of the mass of the coefficients of absorption and scattering. Mass absorption coefficient rises sharply with increasing atomic number (Z) absorber (proportionally Z3 or Z5) and with increasing wavelength (in proportion λ3). This dependence on the wavelength of the observed within the absorption bands, on the borders of which the coefficient reveals race.
Mass dissipation factor increases with increasing atomic number of the substance. When λ = 0,the ЗÅ dissipation factor of wavelength does not depend, at λ<0,ЗÅ it decreases λ.
The decrease of the coefficients of absorption and scattering with decreasing wavelength causes the increase of the penetrating power of x-ray radiation. Mass absorption coefficient for bones [absorption mainly due to Sa3 (RO4)2] almost 70 times more than for the soft tissues, where the absorption is mainly caused by water. This explains why the x-ray so sharply distinguished shadow of bones on the background of soft tissues.
The heterogeneous distribution of the x-ray beam through any environment along with the intensity reduction is accompanied by a change in the spectral composition, changing the quality of radiation long-wave part of the spectrum are absorbed to a greater extent than short-wave radiation is becoming more homogenous. The filtering off the long-wave region of the spectrum allows for radiotherapy centers, deeply buried in the body, improve the ratio between deep and surface doses (see X-ray filters). To describe the quality of an inhomogeneous beam of x-rays used the concept of "half value layer weakening (L)" - a layer of substances that reduce the emission by half. The thickness of this layer depends on the voltage of the tube, thickness and material of the filter. To measure layers half weakening use cellophane (up to 12 Kev energy), aluminum (20-100 Kev), copper (60-300 Kev), lead and copper (>300 Kev). For x-rays generated at voltage 80-120 kV, 1 mm copper by filtering capacity equivalent to 26 mm aluminium, 1 mm lead - 50,9 mm aluminium.
The scattering and absorption of x-ray radiation due to his corpuscular properties; R. I. interacts with atoms as a stream of particles (particles - photons, each of which has a certain energy (inversely proportional to the wavelength R. I.). The energy interval of x-ray photons 0,05-500 Kev.
Absorption R. I. due to the photoelectric effect: the absorption of a photon electron shell is accompanied by the ejection of electrons. The atom is excited and, returning to the ground state, emits a characteristic radiation. Departing photoelectron takes all the energy of a photon (minus the binding energy of the electron in the atom).
The scattering of x-ray radiation is due to electron scattering medium. There are classical scattering (wavelength radiation is not changing, but changing the direction of propagation and scattering with the change in wavelength - Compton-effect (wavelength of the scattered radiation than falling). In the latter case, the photon behaves like a Bouncing ball, and the scattering of photons are, according to the figurative expression of Cantona, like a game of Billiards photons and electrons: the face of an electron, photon passes a part of his energy and dispersed, having already lower energy (respectively wavelength of the scattered radiation increases), the electron is ejected from the atom with the recoil energy (these electrons is called the Compton electrons, or recoil electrons). Energy absorption of x-ray radiation occurs during the formation of secondary electrons (Compton and photoelectrons) and transferring to them energy. Energy R. I. transferred per unit mass of substance, determines the absorbed dose R. I. Unit this dose corresponds to 1 rad 100 ergs/year due to the absorbed energy in a substance absorber runs a number of secondary processes of importance for dosimetry R. I., because they are based measurement methods R. I. (see Dosimetry).
All gases and many liquid, semiconductors and dielectrics under the influence of x-ray radiation increase the electrical conductivity. Conductivity find the best insulation materials: the paraffin, mica, rubber, amber. Changing the conductivity due to ionization of the environment, i.e. the separation of neutral molecules on the positive and negative ions (ionization produce secondary electrons). Ionization in air is used to determine the exposure dose x-ray radiation dose in the air), which is measured in roentgens (see Doses of ionizing radiation). At a dose of 1 R is the absorbed dose in the air is 0,88 happy.
Under the influence of x-ray radiation in the result of excitation of the molecules of matter (and recombination of ions) is excited in many cases, the apparent luminosity of the substance. At high intensities R. I. there is a visible glow air, paper, wax, etc., (with the exception of metals). The largest output of visible radiation give such crystalline phosphors, as Zn·CdS·Ag-phosphorus and other applicable for screens by x-ray.
Under the influence of x-ray radiation in matter can be various chemical processes: degradation halide compounds of silver (photographic effect used in radiography), decomposition of water and aqueous solutions of hydrogen peroxide, change properties of celluloid (turbidity and the allocation of camphor), paraffin (turbidity and bleaching).
In the result the complete transformation of all absorbed chemically inert substance energy R. I. into warmth. The measurement of extremely small quantities of heat requires highly sensitive methods, but is the primary method of absolute measurements R. I.
Secondary biological effects from exposure to x-rays are the basis of medical therapy (see). R. I., quanta which are 6-16 Kev (effective wavelengths from 2 to 5+), are almost completely absorbed by the skin tissue of the human body; they are called border rays, or sometimes rays Bucky tray (see Bucky tray rays). For deep x-ray therapy is used hard filtered radiation effective energy quanta from 100 to 300 Kev.
Biological action R. I. should be considered not only during the therapy, but when x-rays, as well as in all other cases, contact with R. I. requiring antiradiation protection (see).