Having discovered the "- rays", Roentgen by careful experiments found out the conditions of their formation. He found that these rays originate in the place of the tube where the flying electrons that make up the cathode beam are delayed, hitting the wall of the tube. Based on this circumstance, Roentgen designed and built a special tube suitable for obtaining X-rays. In its essential features, the design of the X-ray tube has survived to this day.
In fig. 302 depicts a modern X-ray tube. The cathode is a thick, incandescent tungsten filament, which emits an intense stream of electrons (see Ref. II, § 100), which are accelerated by the applied electric voltage. The cathode is equipped with a tantalum cap that focuses the electrons, since the electrons are emitted perpendicular to the cathode surface. The target is a plate made of tungsten, platinum or other heavy metal, pressed into the anode (mirror of the anode), which is made of red copper to remove heat. Striking the surface of the target, the electrons are trapped and give off X-rays. The voltage between the cathode and anode reaches several tens of thousands of volts. In order for the electrons to reach the target without hindrance, the X-ray tube is evacuated to a high vacuum. The anode is usually cooled with water.
Rice. 302. Modern X-ray tube; filament heating circuit not shown
Acting on gases, X-rays are capable of causing their ionization (see Vol. II, § 92). Thus, placing a charged electroscope near the X-ray tube, we find that it quickly discharges if the tube is activated (Fig. 303). The reason for the loss of charge by the electroscope is that the surrounding air is ionized by the action of X-rays and becomes a conductor. The ionizing effect of X-rays is also used to detect and register them.
Rice. 303. Ionizing effect of X-rays: 1 - X-ray tube, 2 - electroscope. The experiment is successful with both positively and negatively charged electroscope. Ions of both signs are created in the air under the action of X-rays.
An X-ray tube device is an electrovacuum device, which necessarily has a source of radiation (cathode) and a target for inhibition (anode). Also in the device there is a generator - a device located in an incandescent transformer, which contributes to the supply of a strong voltage to the cathode through the negative high-voltage conductor.
The rays appear due to the fact that the cathode-spiral at high voltage incandescence and ejects a stream of electrons, which are retained on the plate of the anode made of tungsten. The anode promotes the conversion of energy into heat, as a result of which the anode heats up to a temperature above 2000 ° C. This is the reason for the decrease in power, the increase in the duration of the exposure.
The device is housed in a special lead case. The apron is filled with special oil. The structure of the cover includes high-voltage conductors and an exit window through which the accumulated radiation is removed. A modern electric vacuum device is designed in such a way that a person receives a minimum portion of rays.
The structure of an electrovacuum device
The X-ray tube diagram looks like this:
- standard flask;
- neck of the anode;
- moving anode disk;
- focus spot of the anode;
- filament coil of the cathode;
- cathode focus system.
Today, electric vacuum devices are equipped with two foci, large and small, and electrons are distributed to them. For this, a collimation device is built into the window, which must be in constant motion so that the X-ray tube is not damaged. For this purpose, an anode movement system is arranged from below.
Some information about RT
Electrovacuum device 0.2BDM7-50 is used in a dental X-ray device, 5D 2RT 1.6 BDM 13-90 is used for functioning with a ground point. The operation of the device should be at a voltage of no more than 110 kW, and the monoblock must be filled with special oil. For close focus operation, RT 1BTV4-100 is used. Apparatus 1.7BDM18-100 is used for RT operation in a mobile device. 2-20BD14-15 and 2-20BD14-150 are applicable for diagnostic purposes. For the operation of the X-ray tube 2.5-30BD29-150 there is a device "Proskan". 4BPM8-250 is used in medicine for research and diagnostics.
The principle of operation of the device
A RT is a device that functions like a diode, but is capable of performing its tasks in a space charge mode.
The principle of operation is quite simple: the emission is produced as a result of increased voltage. It is because of this that the RT should be located in the lead apron. Thanks to the latter, unnecessary things do not happen. As a result, an extremely harmless radiant flux is output. Further, non-hazardous rays are limited using a stationary or moving collimator. Although it is not a part of the apron, it is impossible to do an X-ray without it, since harmful radiation will leak.
In addition, the apron helps protect against high voltages that are created between the anode and cathode. The charge passes through a cable that comes from a step-up transformer box with a generator. X-ray radiation is formed with huge expenditures of energy, mainly aimed at heating the elements located inside the X-ray tube. In the smallest fractions of a second, energy is concentrated on the focus, then it is located throughout the entire focal spot.
It takes a longer time to transfer energy to non-conductive oil, which is located in the PT apron. At the same time, energy, like hot radiation, is transferred to the apron made of metal. And, finally, energy is released from the walls as a convention or ventilation. During this heat exchange, the X-ray tube heats up to a certain limit - an extreme temperature, which should in no case go beyond the required parameters. Otherwise, the X-ray tube will be destroyed. The temperature regime of the focus and its spot is subject to control by setting a certain time regime and voltage supplied from the generator under a minimum, limited filling factor. The latter is calculated using the developed table of load characteristics.
The anode temperature regime is determined by correct exposure. This is done so that the time of the ratio of the energy drop is observed.
Cooling time is controlled by devices with native software using a special scheme for simulating accumulated heat. If there is no such function, then the control is carried out using a planned schedule, which was compiled by the working personnel, based on the change in the heating and cooling waves of the anode. The temperature regime of the apron is also controlled by alternating heating and cooling. In this case, it should be performed at long intervals in time: half a day for cooling and heating. The temperature in the casing is regulated by 3 devices:
- external temperature switch;
- internal temperature switch;
- microswitch.
The jetting material filters the useful rays. RT serves them:
- glass;
- oil;
- plastic.
But this filtering is, of course, not enough to limit the low energy of soft rays. The latter harm the human body, and the image is not transmitted. For this reason, the device is equipped with additional filters on harmless rays. Assessing the benefits and harms of X-rays is difficult. X-ray equipment should only be operated by a trained, qualified person. These devices are not intended to be operated manually or as a substitute for automatic cooling time control. However, without them, one cannot speak of the complete safety of the apparatus. IN routine work such devices are not used. It should be noted that RT itself does not have these devices for creating boundaries. temperature regime... Based on this, it is necessary to control the cycle of energy that comes from the generator. This will help not to harm the patient. Filament calibration at one level is carried out using additional programming system containing the necessary information.
Introduction.
X-rays were discovered by accident in 1895 by the famous German physicist Wilhelm Roentgen. He studied cathode rays in a low-pressure gas discharge tube with a high voltage between its electrodes. Despite the fact that the tube was in a black box, Roentgen noticed that a fluorescent screen, which was accidentally nearby, glowed whenever the tube was operating.
Roentgen determined that the gas discharge tube is the source of a new type of invisible radiation with great penetrating power. The scientist could not determine if this radiation was a stream of particles or waves, and he decided to give it the name X-rays. Subsequently, they were called X-rays.
It is now known that X-rays are a form of electromagnetic radiation that has a shorter wavelength than ultraviolet electromagnetic waves. The longest-wavelength X-ray radiation is blocked by short-wavelength ultraviolet radiation, and the short-wavelength by long-wavelength γ-radiation.
X-ray wavelength ranges from 70 nm up to 10 -5 nm... The shorter the wavelength of the X-rays, the greater the energy of their photons and the greater the penetrating power. X-rays with a relatively long wavelength (more than 10 nm) are called soft... Wavelength 1 - 10 nm characterizes tough X-rays.
X-ray excitation theory.
X-rays are electromagnetic radiation, which arises either during the deceleration of a freely moving charged particle, or during electronic transitions in the inner shells of the atom.
In the normal state, a many-electron atom is a positively charged nucleus surrounded by a system of electron shells from the innermost one with the principal quantum number n = 1 to the outer one with n corresponding to this element (maximum value n = 7 corresponds to the end of the table of the periodic table). The shells are denoted letters K, L, M, N, O, P, Q in accordance with the growth of n, starting from unity. Each shell contains a certain number of electrons in accordance with the Pauli principle. It should be noted that the concept of “shell” is more consistent with the energy concept (which can be replaced by the term “energy level”) than the coordinate one. In accordance with quantum mechanics, electrons in an atom are, as it were, “smeared” over the volume with a maximum probability of localization on the shell.
In its normal state, an atom does not emit or absorb energy. Radiation associated with transitions in the inner shells is possible only when one or more inner electrons are removed. Any electron belonging to the outer (with respect to the formed vacancy - “hole”) shell turns out to be excited. This leads to transitions from more high levels to the vacancy level with the emission of an X-ray quantum.
If the K-electron is knocked out, then the transitions to the formed vacancy from the higher-lying L, M, N ... levels form the shortest-wave K-series of radiation. A similar process is observed during transitions to L-level vacancies (L-series), M-level vacancies (M-series), etc. (Figure 1)
Rice. 1. Diagram of X-ray levels and transitions forming the K, L, M, N series. Kgr, Lgr, Mgr, Ngr are the series boundaries corresponding to transitions to the continuous spectrum. n is the principal quantum number.
X-ray tube device.
The most common X-ray source is an X-ray tube.
The X-ray tube is a two-electrode vacuum device (Fig. 2.1). The heated cathode 1 emits electrons 4. The anode 2, often called the anti-cathode, has an inclined surface in order to direct the emerging X-rays 3 at an angle to the axis of the tube. The anode is made of a highly thermally conductive material to dissipate the heat generated by the impact of electrons. The surface of the anode is made of refractory materials with a large atomic number in the periodic table, for example, tungsten. In some cases, the anode is specially cooled with water or oil. The potential difference between the cathode and the anode (anti-cathode) reaches several hundred kilovolts. Electrons are accelerated by an electric field in an X-ray tube. Since there is a very small number of gas molecules in the tube, electrons practically do not lose their energy on the way to the anode. They reach the anode at a very high speed. Some of the energy that does not dissipate in the form of heat is converted into energy of electromagnetic waves (X-rays). Thus, X-rays are the result of electron bombardment of the anode material.
For diagnostic tubes, the pinpoint of the X-ray source is important, which can be achieved by focusing electrons in one place of the anti-cathode. Therefore, constructively it is necessary to take into account two opposite problems: 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. One of the interesting technical solutions is an X-ray tube with a rotating anode (Fig. 2.2).
Rice. 2.1 Fig. 2.2
According to the method of excitation, X-ray radiation is divided into bremsstrahlung and characteristic.
Braking radiation.
As a result of deceleration of an electron (or other charged particle) by the electrostatic field of the atomic nucleus and atomic electrons of the anti-cathode substance, bremsstrahlung X-ray radiation occurs.
Its mechanism can be explained as follows. With moving electric charge associated magnetic field, the induction of which depends on the speed of the electron. When braking, the magnetic induction decreases and, in accordance with Maxwell's theory, an electromagnetic wave appears.
When electrons are decelerated, only part of the energy goes to create an X-ray photon, the other part is spent on heating the anode. Since the ratio between these parts is random, then when a large number of electrons are decelerated, a continuous X-ray spectrum is formed. In fig. 3 shows the dependences of the X-ray radiation flux on the wavelength λ (spectra) at different voltages in the X-ray tube: U 1< U 2 < U 3 .
The X-ray flux is calculated by the formula (1):
where U and I - voltage and current in the X-ray tube; Z is the atomic number of the anode substance; k - coefficient of proportionality.
In each of the spectra, the shortest-wavelength bremsstrahlung λ ηίη arises when the energy acquired by the electron in the accelerating field is completely converted into photon energy.
, (2)
This formula can be converted into a more practical expression:
, (4)
where is the wavelength in angstroms, U is the voltage in kV. Formula (4) corresponds to Fig. 3
Note that, based on (3), one of the most accurate methods of experimental determination of the Planck constant has been developed.
By increasing the voltage across the X-ray tube, the spectral composition of the radiation is changed, as can be seen from Fig. 3 and formula (4), and increase the rigidity.
If you increase the filament temperature of the cathode, then the emission of electrons and the current in the tube will increase. This will increase the number of X-ray photons emitted every second. Its spectral composition will not change. In fig. 4.1 shows the spectra of bremsstrahlung X-ray radiation at the same voltage, but at different intensities of the cathode filament current: I n1< I н2 .
The spectra obtained from different anticathodes at the same U and I H are shown in Fig. 4.2.
Rice. 4.1 Fig. 4.2
The bremsstrahlung X-ray spectrum does not depend on the nature of the anode substance. As you know, the energy of X-ray photons determines their frequency and wavelength. Therefore, X-ray bremsstrahlung is not monochromatic. It is characterized by a variety of wavelengths that can be represented continuous (continuous) spectrum. By analogy with white light, it is also called white X-ray.
An X-ray tube is an electric vacuum device designed to produce X-rays. X-ray radiation arises during deceleration of an anti-cathode (anode) made of heavy metal (for example, tungsten) accelerated on the screen. The receipt of electrons, their acceleration and deceleration is carried out in the X-ray tube itself, which is an evacuated glass bottle, into which metal electrodes are soldered: the cathode (see) - for obtaining electrons and the anode (see) - for their deceleration (Fig. 1). A high voltage is applied to the electrodes to accelerate the electrons.
Rice. 1. Therapeutic, X-ray tube with a massive tungsten anode: 1 - cathode; 2 - anode.
Wilhelm Konrad Roentgen
(Wilhelm Conrad Röntgen)
The first X-ray tube with which VK Roentgen made his discovery was ionic. X-ray tubes of this type (fragile and difficult to control) are now completely replaced by more advanced electron tubes. In them, electrons are obtained by heating the cathode. By adjusting the current in the filament of the X-ray tube, and hence the temperature of the cathode, it is possible to change the number of electrons emitted by the cathode. At low voltage, not all electrons emitted by the cathode participate in the creation of the anode current and a so-called electron cloud forms at the cathode. With increasing voltage, the electron cloud is absorbed and, starting from a certain voltage (saturation voltage), all electrons reach the anode. In this case, the maximum current flows through the tube (saturation current). The voltage across the X-ray tube is usually higher than the saturation voltage, so it is possible to separately regulate the voltage and current of the X-ray tube. This means that the hardness of the radiation, determined by the voltage, is regulated independently of the intensity, which is due to the anode current.
The anode of an X-ray tube is usually made in the form of a massive copper sheath facing the cathode with a beveled end so that the outlet is perpendicular to the axis of the tube. A tungsten plate is soldered into the thickness of the anode in 2- (anode mirror).
The cathode of an electron X-ray tube contains a refractory filament, usually of tungsten, which is made in the form of a cylindrical or flat spiral and is surrounded by a metal cup for focusing the electron beam on the anode mirror (focus of the X-ray tube). In bifocal X-ray tubes, the cathode contains two filaments.
When the X-ray tube is in operation, a large amount of heat is generated at the anode. To protect the anode from overheating and increase the power of the X-ray tube, anode cooling devices are used: air radiator, oil, water cooling, and radiation cooling. Glass is usually used as the material for the X-ray tube shell, which allows a sufficiently high voltage to be applied to the electrodes, transmits X-rays without noticeable attenuation (beryllium windows are made to obtain buccy rays), and is sufficiently strong and impermeable to gases (vacuum in the X-ray tube is 10 -6 - 10 -7 mm Hg). Diagnostic X-ray tubes operate at maximum voltages up to 150 kV, therapeutic ones - up to 400 kV.
Rice. 6. Schematic representation line focus of the diagnostic X-ray tube: 1 - anode mirror; 2 - actual focus; 3 - anode; 4 - central ray; 5 - optical focus; 6 - tube axis; 7 - cathode.
Rice. 8. Schematic representation of the focus of a tube with a rotating disk anode: 1 - actual focus; 2 - its scan; 3 - instant focus; 4 - tube axis; 5 - cathode; 6 - optical focus; 7 - anode.
The sharpness of the X-ray image is determined by the magnitude of the focus. The main requirement for diagnostic X-ray tubes is high power at low focus. Modern X-ray tubes have a linear focus 10–40 mm 2 in size, but it is not the actual value of the focus that is of practical importance, but its visible projection in the direction of the beam, ie, the dimensions of the effective optical focus (Fig. 2). At an anode inclination angle of 19 °, the area of the effective focus is 3 times less than the actual one, which makes it possible to double the power of the X-ray tube. A further increase in power was achieved in tubes with a rotating anode (Figs. 3 and 4).
Currently, X-ray tubes are produced for various purposes, differing both structurally and power, cooling methods, protection from radiation and high voltage. The X-ray tube symbol is a combination of letters and numbers. The first number is the power of the tube in kilowatts; the second sign determines the type of protection (P - protective against radiation, B - protective against radiation and high voltage, the absence of a letter indicates the absence of protection); the third sign determines the purpose of the X-ray tube (D - diagnostics, T - therapy); the fourth - indicates the cooling method (K - air radiator, M-oil, B - air, the absence of a letter means cooling by radiation); the fifth digit indicates the maximum anode voltage in kilovolts. So, for example, 6-RDV-110 - a six-kilowatt protective diagnostic tube with water cooling for 110 kV; tube 1-T-1-200-therapeutic, without protection, cooling by radiation, with a power of 1 kW per voltage 200 kV (conditional number 1).
Rice. 3. A tube with a rotating disk anode: 1 - cathode; 2 - disk anode; 3 - protective disk; 4 - anode axis; 5 - steel cylinder - asynchronous motor rotor.
Before putting into operation, each new tube must be checked for vacuum, without turning on the glow. If a pink glow or spark appears, the X-ray tube has lost its vacuum and is unusable. The tube, which has retained a vacuum, is subjected to training: a current of 1-2 mA is set at a high voltage of the order of 1/3 of the nominal and for 30-60 minutes. voltage and current are gradually increased to the values of the continuous mode specified in the X-ray tube passport. When operating an X-ray tube, it is necessary to strictly adhere to the operating modes indicated in its passport.
An X-ray tube is an electrovacuum device used to generate X-rays by emitting electrons from the cathode, focusing and accelerating them in a high-voltage electric field, followed by decelerating the electron flow on the anode mirror. As a result of deceleration of the electron flow at the anode of the X-ray tube, a large amount of heat is released and only a small amount of this energy is transformed into the energy of X-ray radiation (see).
From the time X-rays were discovered by X-rays and until the beginning of the First World War, so-called gas-containing ionic X-ray tubes (Fig. 1), fragile and difficult to control, were used for X-ray diagnostics and X-ray therapy. Lilienfeld (L. Lilienfeld) proposed a more advanced X-ray tube with an intermediate electrode, a heated cathode and water cooling (Fig. 2). However, the high-vacuum two-electrode X-ray tube proposed by the American W. D. Coolidge gradually replaced all other X-ray tubes and is used in different modifications until now.
Rice. 1. Ion X-ray tube with air cooled and a gas regenerator.
Rice. 2. Lilienfeld X-ray tube.
A modern X-ray tube is a high-voltage vacuum diode (with two electrodes - a cathode and an anode). The X-ray tube cathode contains a refractory filament, usually tungsten. In dual focus diagnostic X-ray tubes designed for different modes work, the cathode contains two filaments for each of the focuses. The filaments are usually made in the form of a cylindrical or flat spiral (Fig. 3, 1 and 2), respectively, for a linear or circular focus.
Rice. 3. Cathodes of two-focus electron X-ray tubes: 1 - with two cylindrical filament spirals; 2 - with two flat filament spirals.
The anode of the X-ray tube is usually made in the form of a massive copper cover facing the cathode with a beveled end, into the thickness of which a tungsten plate 2-2.5 mm thick (anode mirror) is soldered, which is a target where the electron flux from the cathode is focused, and thus represents , X-ray optical focus of the tube. There are X-ray tubes for special purposes, for example, for intracavitary X-ray therapy (Fig. 4), in which the anode is the bottom of a hollow cylinder inserted into the corresponding cavity.
Rice. 4. Safe X-ray tube for intracavitary X-ray therapy: 1 - cathode; 2 - anode tube; 3 - X-ray exit window; 4 - anode base; 5 - water jacket; 6 - cooling pipes.
In order to increase the resolution of modern diagnostic tubes, much attention is paid to the focus of the X-ray tube, since the sharper the focus, the sharper the X-ray image.
When evaluating the X-ray optical properties of an X-ray tube, it should be borne in mind that it is not the magnitude of the actual focus on the anode mirror, but the visible projection of the focal spot in the direction of the central beam, i.e., the dimensions of the effective optical focus, that is of decisive importance. A decrease in the size of the optical focus is achieved by a decrease in the bevelling angle of the anode with respect to the central beam.
Unlike therapeutic X-ray tubes (Fig. 5), equipped with a round or elliptical optical focus, modern diagnostic tubes have a so-called linear focus (Fig. 6). In tubes with a linear focus, the area of the effective focus in the shape of a square is approximately 3 times less than the area of the actual focus in the shape of a rectangle. With the same X-ray optical properties, the power of an X-ray tube with a linear focus is approximately 2 times higher than that of an X-ray tube with a circular focus.
A further increase in the power of diagnostic X-ray tubes was achieved in tubes with a rotating anode (Figs. 7 and 8). In these X-ray tubes, a massive tungsten anode with a linear focus stretched along the entire circumference is fixed on an axis rotating in bearings, and the tube cathode is displaced relative to its axis so that the focused electron beam always hits the beveled surface of the anode mirror. When the anode rotates, a beam of focused electrons falls on a changing area of the anode focus, the effective value of which, i.e., the optical focus, is due to this very small dimensions (about 1X1 mm, 2.5X2.5 mm). Since the speed of rotation of the anode is high enough (the anode is a continuation of the axis of the motor rotating at an angular speed of 2500 rpm), the power of the tube at exposures of 0.1 sec. can reach 40-50 kW.
A significant amount of heat generated at the anode of a working tube requires its cooling by removing heat from the anode to the environment. This is achieved by air cooling (fig. 9), water cooling (fig. 10 and 11) or oil cooling (fig. 12); oil is at the same time an insulating medium; oil cooling is usually used in the so-called block apparatus (see X-ray technology).
Rice. 9. Radiator air-cooled tube.
Rice. 10. Water-cooled tube anode: 1 - anode rod; 2 - cooling water tank.
Rice. 11. Anode of a tube cooled by running water: 1 - water-cooled connecting tubes.
Rice. 12. Oil-cooled miniature X-ray tube for dental X-ray.
In connection with the diverse requests of X-ray diagnostics and X-ray therapy, X-ray tubes for various purposes are currently being produced, differing in both design and size, power, cooling methods and protection from unused radiation. Symbols different types tubes are made up of combinations of numbers and letters. The first number is the maximum permissible power of the tube (in kW); the first letter defines the protection against radiation (R - self-protective; B - in a protective casing; the absence of a letter means no protection); the second letter defines the purpose of the X-ray tube (D - diagnostics; T - therapy); the third letter indicates the cooling system (K - air radiator cooling, M - oil, B - water, the absence of the letter means cooling by radiation); the last figure corresponds to the maximum permissible anode voltage in kilovolts. So, for example, 3-BDM-2-100 is a three-kilowatt diagnostic tube with oil cooling (radiator) for 100 kV for work in a protective casing (conditional type number - 2); tube - 1-T-1-200 - therapeutic without protection with cooling by radiation, with a power of 1 kW for a voltage of 200 kV (conditional type number - 1).
Regardless of the type of X-ray tube, the general principle of their operation is as follows. The heating of the X-ray tube cathode causes thermionic emission with the formation of a so-called electron cloud at the cathode. When a high voltage is switched on at the X-ray tube electrodes, free electrons under the action of an electric field rush to the anode, are decelerated on its mirror, and part of the braking energy is converted into X-ray radiation.
With an increase in the voltage across the X-ray tube, the emission current initially sharply increases due to a gradual decrease in the density of the electron cloud. When the number of electrons generated at the cathode becomes equal to the number of electrons reaching the anode, a further increase in voltage does not cause an increase in the current passing through the X-ray tube, but only increases the kinetic energy of the electrons reaching the anode. The mode of operation of the X-ray tube, in which all the electrons generated at the cathode are used, and a further increase in voltage does not cause an increase in the anode current, is called the saturation current. In practice, the saturation current i is achieved in diagnostic X-ray tubes at a potential difference σ of the order of 10-20 kV (Fig. 13). Therefore, usually X-ray tubes are mostly operated in the saturation current mode. If it is necessary to increase the anode current, the cathode filament current should be increased accordingly and, by raising the voltage, the saturation current mode should be created again.
Rice. 13. Anodic characteristic electron X-ray tubes: S "- at a filament current of 3.8 a; S-at a filament current of 3.4 a.
In the process of industrial production, gas is removed from X-ray tubes to a residual pressure of 10 -6 -10 -7 mm Hg. Art. At this degree of vacuum, the passage of current through the X-ray tube is practically due only to thermionic emission from the cathode. However, if the parts of the tube are overheated, as well as when it is turned on after a long break in operation, gas may appear in it; in this case, the effect of ionization occurs; the X-ray tube begins to pass current in both directions. The measuring instruments on the control panel detect sharp fluctuations in the anode current. If such a "gassing" X-ray tube is turned on at high voltage without heating the cathode, a stable gas discharge is created in it, accompanied by a characteristic glow of the tube. Such a tube is unusable and must be replaced.
Each new X-ray tube must be checked for high-voltage vacuum without turning on the glow before being put into operation, then subjected to "training". For this, at an anode voltage of the order of 1/3 of the nominal, a current of 1-2 mA is set. Then within 30-60 minutes. voltage and current are gradually increased to the nominal values of continuous operation in accordance with the X-ray tube passport. When operating an X-ray tube, it is necessary to strictly adhere to the operating modes indicated in its passport.
See also X-ray machines, X-ray radiation.
An X-ray tube is an electric vacuum device that serves as a source of X-ray radiation. Such radiation appears during deceleration of electrons that are emitted by the cathode and their impact on the anode; in this case, the energy of electrons, their speed in the space between the anode and the cathode is increased by a strong electric field, partially modified into the energy of X-ray radiation. X-ray tube radiation is the superposition of X-ray bremsstrahlung on specific radiation of the anode substance. X-ray tubes are distinguished; according to the method of obtaining an electron flow - with a cathode, which is bombarded with positive ions and with a radioactive source of electrons, a field emission cathode, a thermionic cathode; by the method of evacuation - collapsible, sealed off; by radiation time - pulse, continuous action; by the type of anode cooling - with radiation, oil, air, water cooling; by the size of the focus - microfocus, sharp focus and macrofocal; by its shape - ruled, round, annular; by the method of focusing electrons on the anode - with electromagnetic, magnetic, electrostatic focusing.
X-ray tubes are used in X-ray structural analysis, X-ray microscopy, flaw detection, X-ray diagnostics, X-ray therapy, X-ray spectral analysis, and microradiography. Sealed X-ray tubes with an electrostatic electron focusing system, a water-cooled anode, and a thermionic cathode are most widely used in all areas. The thermionic cathode of an X-ray tube is usually a straight filament or tungsten wire spiral that is heated electric shock... The working section of the anode is a metal mirror surface located perpendicularly or at a certain angle to the electron flow. To obtain a continuous X-ray spectrum of high intensity and energies, Au, W anodes are used; X-ray tubes with Ti, Cr, Fe, Cu, Mo, Co, Ni, Ag anodes are used in structural analysis. The main characteristics of the X-ray tube are the specific power dissipated by the anode (10-104 W / mm2), the maximum permissible accelerating voltage (1-500 kV), the electronic current (0.01 mA - 1A), the 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 to 3%.
