If U is expressed in kilovolts and the ratio between other quantities is taken into account, then the formula looks like: l k \u003d 1.24 / U (nm) or l k \u003d 1.24 / U (Å) (1Å \u003d 10 -10 m).

From the graphs above, it can be established that the wavelength l m, which accounts for the maximum radiation energy, is in constant relation to the limiting wavelength l k:

.

The wavelength characterizes the energy of a photon, on which the penetrating power of radiation depends when it interacts with matter.

Short-wavelength X-rays usually have a high penetrating power and are called hard, while long-wavelength X-rays are called soft. As can be seen from the above formula, the wavelength at which the maximum radiation energy falls is inversely proportional to the voltage between the anode and cathode of the tube. Increasing the voltage at the anode of the x-ray tube, change the spectral composition of the radiation and increase its hardness.

When the filament voltage changes (the filament temperature of the cathode changes), the number of electrons emitted by the cathode per unit time changes, or, accordingly, the current strength in the tube anode circuit. In this case, the radiation power changes in proportion to the first power of the current. The spectral composition of the radiation will not change.

The total flux (power) of radiation, the distribution of energy over wavelengths, and also the boundary of the spectrum on the side of short wavelengths depend on the following three factors: the voltage U that accelerates the electrons and is applied between the anode and cathode of the tube; the number of electrons involved in the formation of radiation, i.e. tube filament current; atomic number Z of the anode material, in which the electron deceleration occurs.

The bremsstrahlung flux is calculated by the formula: , where ,

Z-serial number of an atom of a substance (atomic number).

By increasing the voltage on the x-ray tube, one can notice the appearance of separate lines (line spectrum) against the background of continuous bremsstrahlung radiation, which corresponds to the characteristic x-ray radiation. It arises during the transition of electrons between the inner shells of atoms in a substance (shells K, L, M). The line character of the characteristic radiation spectrum arises due to the fact that accelerated electrons penetrate deep into the atoms and knock out electrons from their inner layers outside the atom. Electrons (Fig. 2) from the upper layers pass to free places, as a result of which X-ray photons are emitted with a frequency corresponding to the difference in the transition energy levels. The lines in the spectrum of characteristic radiation are combined into series corresponding to transitions of electrons with a higher level at the level of K, L, M.

The external action, as a result of which the electron is knocked out of the inner layers, must be strong enough. In contrast to optical spectra, the characteristic x-ray spectra of different atoms are of the same type. The uniformity of these spectra is due to the fact that the inner layers of different atoms are the same and differ only energetically, because the force effect from the side of the nucleus increases as the ordinal number of the element increases. This leads to the fact that the characteristic spectra shift towards higher frequencies with increasing nuclear charge. This relationship is known as Moseley's law: , where A and B are constants; Z-order number of the element.

There is another difference between X-ray and optical spectra. The characteristic spectrum of an atom does not depend on the chemical compound in which the atom is included. So, for example, the X-ray spectrum of the oxygen atom is the same for O, O 2 , H 2 O, while the optical spectra of these compounds are significantly different. This feature of the x-ray spectra of atoms served as the basis for the name "characteristic".

Characteristic radiation occurs whenever there are free places in the inner layers of an atom, regardless of the reasons that caused it. For example, it accompanies one of the types of radioactive decay, which consists in the capture of an electron from the inner layer by the nucleus.

2. The device of x-ray tubes and protozoa

x-ray machine.

The most common source of X-ray radiation is an X-ray tube - a two-electrode vacuum device (Fig. 3). It is a glass container (p = 10 -6 - 10 -7 mm Hg) with two electrodes - anode A and cathode K, between which a high voltage is created. The heated cathode (K) emits electrons. Anode A is often referred to as the anticathode. It has an inclined surface in order to direct the resulting X-ray radiation at an angle to the axis of the tube. The anode is made of a metal with good thermal conductivity (copper) to remove the heat generated by the impact of electrons. At the beveled end of the anode there is a plate Z made of refractory metal (tungsten) with a high atomic number, called the anode mirror. In some cases, the anode is specially cooled with water or oil. For diagnostic tubes, the pinpointness of the X-ray source is important, which can be achieved by focusing the electrons in one place of the anode. Therefore, constructively, two opposite tasks have to be taken into account: on the one hand, electrons must fall on one place of the anode, on the other hand, in order to prevent overheating, it is desirable to distribute electrons over different parts of the anode. For this reason, some X-ray tubes are manufactured with a rotating anode.

In a tube of any design, electrons accelerated by the voltage between the anode and the cathode fall on the anode mirror and penetrate deep into the substance, interact with atoms and are decelerated by the field of atoms. This produces bremsstrahlung X-rays. Simultaneously with the bremsstrahlung, a small amount (several percent) of characteristic radiation is formed. Only 1-2% of the electrons that hit the anode cause bremsstrahlung, and the rest cause a thermal effect. For the concentration of electrons, the cathode has a guide cap. The part of the tungsten mirror on which the main electron flow falls is called the focus of the tube. The width of the radiation beam depends on its area (focus sharpness).

To power the tube, two sources are required: a high voltage source for the anode circuit and a low voltage source (6-8 V) to power the filament circuit. Both sources must be independently regulated. By changing the anode voltage, the hardness of the X-ray radiation is regulated, and by changing the incandescence, the current of the output circuit and, accordingly, the radiation power.

Schematic diagram of the simplest X-ray machine is shown in Fig.4. The circuit has two high voltage transformers Tr.1 and Tr.2 for powering the filament. The high voltage on the tube is regulated by an autotransformer Tr.3 connected to the primary winding of the transformer Tr.1. Switch K regulates the number of turns of the autotransformer winding. In this regard, the voltage of the secondary winding of the transformer, supplied to the anode of the tube, also changes, i.e. hardness is adjustable.

The filament current of the tube is regulated by a rheostat R, included in the primary circuit of the transformer Tr.2. The anode circuit current is measured with a milliammeter. The voltage applied to the electrodes of the tube is measured with a kV kilovoltmeter, or the voltage in the anode circuit can be judged by the position of the switch K. The filament current, regulated by the rheostat, is measured with an ammeter A. In the scheme under consideration, the x-ray tube simultaneously rectifies a high alternating voltage.

It is easy to see that such a tube radiates only in one half-cycle of alternating current. Therefore, its power will be small. In order to increase the radiated power, many devices use high-voltage full-wave X-ray rectifiers. For this purpose, 4 special kenotrons are used, which are connected in a bridge circuit. An x-ray tube is included in one diagonal of the bridge.

3. Interaction of X-ray radiation with matter

(coherent scattering, incoherent scattering, photoelectric effect).

When X-rays fall on a body, it is reflected from it in a small amount, but mostly passes deep into. In the mass of the body, radiation is partially absorbed, partially scattered, and partially passes through. Passing through the body, X-ray photons interact mainly with the electrons of the atoms and molecules of the substance. Registration and use of X-ray radiation, as well as its impact on biological objects, is determined by the primary processes of interaction of an X-ray photon with electrons. Three main processes take place depending on the ratio of photon energy E and ionization energy AI.

a) coherent scattering.

Scattering of long-wavelength X-rays occurs mainly without changing the wavelength, and it is called coherent. The interaction of a photon with the electrons of the inner shells, tightly bound to the nucleus, only changes its direction, without changing its energy, and hence the wavelength (Fig. 5).

Coherent scattering occurs if the photon energy is less than the ionization energy: E = hn<А И. Так как энергия фотона и энергия атома не изменяется, то когерентное рассеяние не вызывает биологического действия. Однако при создании защиты от рентгеновского излучения следует учитывать возможность изменения направления первичного пучка.

b) Incoherent scattering (Compton effect).

In 1922, A. Compton, observing the scattering of hard X-rays, discovered a decrease in the penetrating power of the scattered beam compared to the incident beam. The scattering of X-rays with changing wavelength is called the Compton effect. It occurs when a photon of any energy interacts with the electrons of the outer shells of atoms weakly bound to the nucleus (Fig. 6). An electron is detached from an atom (such electrons are called recoil electrons). The energy of the photon decreases (the wavelength increases accordingly), and the direction of its movement also changes. The Compton effect occurs if the X-ray photon energy is greater than the ionization energy: , . In this case, recoil electrons with kinetic energy E K appear. Atoms and molecules become ions. If E K is significant, then electrons can ionize neighboring atoms by collision, forming new (secondary) electrons.

in) Photoelectric effect.

If the energy of a photon hn is sufficient to detach an electron, then when interacting with an atom, the photon is absorbed, and the electron is detached from it. This phenomenon is called the photoelectric effect. The atom is ionized (photoinization). In this case, the electron acquires kinetic energy and, if the latter is significant, then it can ionize neighboring atoms by collision, forming new (secondary) electrons. If the photon energy is insufficient for ionization, then the photoelectric effect can manifest itself in the excitation of an atom or molecule. In some substances, this leads to the subsequent emission of photons in the visible radiation region (X-ray luminescence), and in tissues, to the activation of molecules and photochemical reactions.

The photoelectric effect is typical for photons with an energy of the order of 0.5-1 MeV.

The three main interaction processes discussed above are primary, they lead to subsequent secondary, tertiary, etc. phenomena. When X-ray radiation enters a substance, a number of processes can occur before the energy of an X-ray photon is converted into the energy of thermal motion.

As a result of the above processes, the primary X-ray flux is weakened. This process obeys Bouguer's law. We write it in the form: Ф =Ф 0 e - mх, where m is a linear attenuation coefficient, depending on the nature of the substance (mainly on density and atomic number) and on the radiation wavelength (photon energy). It can be represented as consisting of three terms corresponding to coherent scattering, incoherent scattering, and the photoelectric effect: .

Since the linear absorption coefficient depends on the density of the substance, it is preferable to use the mass attenuation coefficient, which is equal to the ratio of the linear attenuation coefficient to the density of the absorber and does not depend on the density of the substance. The dependence of the X-ray flux (intensity) on the thickness of the absorbing filter is shown in Fig. 7 for H 2 O, Al, and Cu. Calculations show that a layer of water 36 mm thick, aluminum 15 mm and copper 1.6 mm reduce the X-ray intensity by 2 times. This thickness is called the half layer thickness d. If a substance attenuates X-ray radiation by half, then , then , or , ; ; . Knowing the thickness of the half layer, you can always determine m. Dimension .

4. The use of x-rays in medicine

(fluoroscopy, radiography, X-ray tomography, fluorography, radiotherapy).

One of the most common applications of X-rays in medicine is the transillumination of internal organs for diagnostic purposes - X-ray diagnostics.

For diagnostics, photons with an energy of 60-120 keV are used. In this case, the mass absorption coefficient is determined mainly by the photoelectric effect. Its value is proportional to l 3 (in which the large penetrating power of hard radiation is manifested) and proportional to the third power of the number of atoms of the substance - absorber: , where K is the coefficient of proportionality.

The human body consists of tissues and organs that have different absorbing capacity in relation to X-rays. Therefore, when it is illuminated with X-rays, a non-uniform shadow image is obtained on the screen, which gives a picture of the location of internal organs and tissues. The densest radiation-absorbing tissues (heart, large vessels, bones) are seen as dark, while the less absorbing tissues (lungs) are seen as light.

In many cases, it is possible to judge their normal or pathological state. X-ray diagnostics uses two main methods: fluoroscopy (transmission) and radiography (image). If the organ under study and the tissues surrounding it approximately equally absorb the X-ray flux, then special contrast agents are used. So, for example, on the eve of an X-ray examination of the stomach or intestines, a mushy mass of barium sulfate is given, in which case one can see their shadow image. In fluoroscopy and radiography, an x-ray image is a summary image of the entire thickness of the object through which the x-rays pass. The most clearly defined are those details that are closer to the screen or film, and the distant ones become fuzzy and blurry. If in some organ there is a pathologically altered area, for example, the destruction of lung tissue inside an extensive focus of inflammation, then in some cases this area on the x-ray in the amount of shadows can be “lost”. To make it visible, a special method is used - tomography (layered recording), which allows you to take pictures of individual layers of the area under study. This kind of layer-by-layer tomograms is obtained using a special apparatus called a tomograph, in which the x-ray tube (RT) and film (Fp) are periodically, jointly, in antiphase moved relative to the study area. In this case, X-rays at any position of the RT will pass through the same point of the object (changed area), which is the center relative to which the RT and FP periodically move. The shadow image of the area will be captured on film. By changing the position of the “swing center”, it is possible to obtain layered images of the object. Using a thin beam of X-rays, a special screen (instead of Fp) consisting of semiconductor detectors of ionizing radiation, it is possible to process the image during tomography using a computer. This modern variant of tomography is called computed tomography. Tomography is widely used in the study of the lungs, kidneys, gallbladder, stomach, bones, etc.

The brightness of the image on the screen and the exposure time on the film depend on the intensity of the x-rays. When using it for diagnostics, the intensity cannot be high, so as not to cause an undesirable biological effect. Therefore, there are a number of technical devices that improve the brightness of the image at low X-ray intensities. One of these devices is an image intensifier tube.

Another example is fluorography, in which an image is obtained on a sensitive small-format film from a large X-ray luminescent screen. When shooting, a lens of large aperture is used, the finished pictures are examined on a special magnifier.

Fluorography combines a great ability to detect latent diseases (diseases of the chest, gastrointestinal tract, paranasal sinuses, etc.) with a significant throughput, and therefore is a very effective method of mass (in-line) research.

Since photographing an x-ray image during fluorography is performed using photographic optics, the image on the fluorogram is reduced in comparison with the x-ray. In this regard, the resolution of the fluorogram (i.e., the visibility of small details) is less than that of a conventional radiograph, however, it is greater than with fluoroscopy.

An apparatus has been designed - a tomofluorograph, which allows obtaining fluorograms of body parts and individual organs at a given depth - the so-called layered images (sections) - tomofluorograms.

X-ray radiation is also used for therapeutic purposes (X-ray therapy). The biological effect of radiation is to disrupt the vital activity of cells, especially rapidly developing ones. In this regard, X-ray therapy is used to influence malignant tumors. It is possible to choose a dose of radiation sufficient for the complete destruction of the tumor with relatively minor damage to the surrounding healthy tissues, which are restored due to subsequent regeneration.

X-ray radiation, from the point of view of physics, is electromagnetic radiation, the wavelength of which varies in the range from 0.001 to 50 nanometers. It was discovered in 1895 by the German physicist W.K. Roentgen.

By nature, these rays are related to solar ultraviolet. Radio waves are the longest in the spectrum. They are followed by infrared light, which our eyes do not perceive, but we feel it as heat. Next come the rays from red to purple. Then - ultraviolet (A, B and C). And right behind it are x-rays and gamma rays.

X-ray can be obtained in two ways: by deceleration in the matter of charged particles passing through it and by the transition of electrons from the upper layers to the inner layers when energy is released.

Unlike visible light, these rays are very long, so they are able to penetrate opaque materials without being reflected, refracted, or accumulated in them.

Bremsstrahlung is easier to obtain. Charged particles emit electromagnetic radiation when braking. The greater the acceleration of these particles and, consequently, the sharper the deceleration, the more X-rays are produced, and the wavelength becomes shorter. In most cases, in practice, they resort to the generation of rays in the process of deceleration of electrons in solids. This allows you to control the source of this radiation, avoiding the danger of radiation exposure, because when the source is turned off, the X-ray radiation completely disappears.

The most common source of such radiation - The radiation emitted by it is inhomogeneous. It contains both soft (long-wave) and hard (short-wave) radiation. The soft one is characterized by the fact that it is completely absorbed by the human body, therefore such X-ray radiation does twice as much harm as the hard one. With excessive electromagnetic radiation in the tissues of the human body, ionization can damage cells and DNA.

The tube is with two electrodes - a negative cathode and a positive anode. When the cathode is heated, electrons evaporate from it, then they are accelerated in an electric field. Colliding with the solid matter of the anodes, they begin deceleration, which is accompanied by the emission of electromagnetic radiation.

X-ray radiation, the properties of which are widely used in medicine, is based on obtaining a shadow image of the object under study on a sensitive screen. If the diagnosed organ is illuminated with a beam of rays parallel to each other, then the projection of shadows from this organ will be transmitted without distortion (proportionally). In practice, the radiation source is more like a point source, so it is located at a distance from the person and from the screen.

To receive a person is placed between the x-ray tube and the screen or film, acting as radiation receivers. As a result of irradiation, bone and other dense tissues appear in the image as clear shadows, look more contrast against the background of less expressive areas that transmit tissues with less absorption. On x-rays, a person becomes "translucent".

As X-rays propagate, they can be scattered and absorbed. Before absorption, the rays can travel hundreds of meters in the air. In dense matter, they are absorbed much faster. Human biological tissues are heterogeneous, so their absorption of rays depends on the density of the tissue of the organs. absorbs rays faster than soft tissues, because it contains substances that have large atomic numbers. Photons (individual particles of rays) are absorbed by different tissues of the human body in different ways, which makes it possible to obtain a contrast image using x-rays.