X-ray tube

x-ray tube
Fig. 1. Therapeutic, x-ray tube with a massive tungsten anode: 1 - cathode; 2 anode.

X-ray tube is vacuum device intended for obtaining x-ray radiation. X-ray radiation occurs at deceleration of the accelerated electrons on the screen of a cathode (anode), made of heavy metal (e.g. tungsten). Getting electron acceleration and braking is carried out in the x-ray tube, which is vakuumirovannoj glass bottle, in which soldered metal electrodes: cathode (see) for electrons and anode (see) - for their braking (Fig. 1). For acceleration of electrons to the electrodes is brought high voltage.

X-ray
Wilhelm Conrad Roentgen
(Wilhelm Conrad Röntgen)

The first x-ray tube, which C. K. the X-ray has done its opening was ion. X-ray tube of this type (brittle and difficult to manage) is now completely superseded by more sophisticated electronic tubes. In them the electrons are obtained by incandescent cathode. Adjusting current in the circuit of the intensity of x-ray tube, and thus the temperature of the cathode, you can change the amount emitted by the cathode electrons. At low voltage not all emitted by the cathode electrons participate in creating the anode current and the cathode is formed so-called electron cloud. When the voltage electron cloud disperses and, starting from a certain voltage (voltage saturation), all electrons reach the anode. Through the tube when it flows maximum current (saturation). The voltage on the x-ray tube are usually higher voltage saturation, so it is possible to separately regulate the voltage and current R. t. It means that the rigidity of the radiation is determined by the voltage can be controlled independently of intensity, which is due to the anode current.
Anode x-ray tube is usually done in the form of a massive copper case, facing the cathode beveled end to facing the x-ray emission was perpendicular to the tube axis. In the thick of the anode soldered tungsten plate 2- (mirror of the anode).
The cathode of an electron x-ray tube contains refractory filament, usually made of tungsten, which is made in the form of cylindrical or flat spiral and surrounded by a metal Cup for focusing of the electron beam on the mirror of the anode (the focus x-ray tube). In dvukhfaznykh x-ray tube cathode contains two filament.
When operating x-ray tube anode large amount of heat. To protect the anode from overheating and to increase the intensity of x-ray tube used anode cooling device: air radiator, oil, water cooling, cooling by radiation. As a material of the shell x-ray tube is usually used glass, which allows you to attach to the electrodes is high enough voltage, skip x-rays without any noticeable weakening (for Bucky tray-rays do beryllium window), fairly durable and impermeable to gases (vacuum x-ray tube 10-6 - 10-7 mm RT. Art.). Diagnostic x-ray tube operate with maximum voltage of 150 kV, therapeutic - up to 400 square


Fig. 2. Ruled focus diagnostic x-ray tube; 1 - mirror anode; 2 - real focus; 3 - anode; 4 - the Central beam; 5 - optical focus; 6 - axis of the tube; 7 - cathode.

Fig. 4. The focus tube with rotating disc anode: 1 - real focus; 2 - scan valid focus; 3 - immediate focus; 4 - axis of the tube; 5 - cathode; 6 - optical focus; 7 - anode.

The x-ray image sharpness due to the amount of focus. The main requirement to diagnostic x-ray tubes - high power with a small focus. Modern x-ray tube are ruled focus 10-40 mm2, but has practical value, not the actual amount of focus, and its visible projection in the direction of the beam, i.e. the size of the effective optical focus (Fig. 2). When the angle of inclination of the anode 19 degrees the effective area of focus in 3 times less valid, which allows to increase the power x-ray tube in half. Further capacity increase achieved in the tube with rotating anode (Fig. 3 and 4).
At present produce x-ray tubes of various purposes, characterized as constructive and power, cooling methods, radiation protection and high voltage. Designation of the x-ray tube is a combination of letters and numbers. The first figure is the power of the tube in kilowatts; the second character determines the kind of protection (P - protection from radiation, B - protection against radiation and high voltage, no letters indicates the lack of protection); the third sign defines the purpose of x-ray tube (D - diagnosis, T - therapy); the fourth specifies the method of cooling (air radiator, M-oil, air, no letters means cooling by radiation); fifth digit specifies the maximum anode voltage, kV. So, for example, 6-WFD-110 - shestilavotchny protective diagnostic tube with water cooling at 110 kV; tube 1-T-1-200-therapeutic, without protection, cooling by radiation, with a capacity of 1 ket on the stress 200 kV (conditional number 1).

Fig. 3. Tube with rotating disc anode: 1 - cathode; 2 - disc anode; 3 - protective drive; 4 - axis anode; 5 - steel cylinder - rotor motor.


Each new up before starting the unit should be checked for vacuum, not including glow. If you will see a pink glow, or spark, x-ray tube lost vacuum and unfit to work. Tube, kept vacuum, subjected training: set a current of 1-2 mA at a high voltage of about 1/3 of the nominal and within 30 - 60 minutes voltage and current gradually increase to values of long-term regime specified in the passport of x-ray tube. When operating x-ray tube must strictly adhere to the modes, specified in the passport.

X-ray tube is vacuum device used to generate x-rays by emission of electrons from the cathode, focusing
and increase in electric field of high voltage with further deceleration of the electron flow in the mirror of the anode. As a result of inhibition of flow of electrons at the anode x-ray tube produces a lot of heat and only a small amount of this energy is transformed into x-ray radiation energy (see).
With the discovery of x-rays x-rays and before the first world war for x-ray diagnostics and radiotherapy was used so-called ion gassy R. T. (Fig. 1), fragile and difficult to manage. Lilienfeld (L. Lilienfeld) offered a better R. T. with intermediate electrodes, hot cathode and water cooling (Fig. 2). However, high-vacuum electrode R. T. proposed by the American Coolidge (W. D. Coolidge), gradually superseded all others R. T. and used in different modifications to the present.
Modern x-ray tube is a high-voltage vacuum diode (with two electrodes, cathode and anode). Cathode R. T. contains refractory filament, usually from tungsten. In dvukhfaznykh diagnostic x-ray tubes, designed for different operating modes, the cathode contains two filament for each of tricks. The filament, as a rule, executed in the form of cylindrical or flat spiral (Fig. 3, 1, and 2 respectively for bar or all of focus.
Anode x-ray tube are usually made in the form of a massive copper case, facing the cathode beveled edge, into the thickness of which is soldered tungsten plate thickness is 2-2,5 mm (mirror of the anode), which is the target, which focuses the flow of electrons from the cathode, and represents, thus, the focus of x-ray optics tube. There R. T. for special purposes, for example for intracavitary radiotherapy (Fig. 4), in which the anode is the bottom of a hollow cylinder, enter in the appropriate cavity.
To increase the resolution of modern diagnostic tubes-focus x-ray tube is a priority, because the sharper focus, the sharper x-ray image.
When assessing the x-ray properties R. T. note that crucial than the size of the actual focus on the mirror of the anode, and a visible projection of the focal spot in the direction of the Central beam, i.e. the size of the effective optical focus. Reducing the size of the optical focus is achieved by reducing the angle of the cutting of the anode in relation to the Central ray.
Unlike therapeutic R. T. (Fig. 5)with round or elliptical optical focus, modern diagnostic tubes have a so-called ruled focus (Fig. 6). In the tubes with a linear focus the effective area of focus, having the form of a square, about 3 times less space actual focus, with rectangular shape. At the same x-ray properties of power x-ray tube with a linear focus approximately in 2 times more than in the P. I. with all the focus.
Further increase of capacity diagnostic R. T. achieved in the tube with rotating anode (Fig. 7 and 8). In these x-ray tubes massive tungsten anode ruled focus, stretched around the entire circumference, fixed on the axis of a rotating bearing, and the cathode tube shifted on its axis so that the focused electron beam was always on the beveled mirror surface of the anode. When rotating anode of a focused beam of electrons is adjudged to changing area of focus of the anode, the effective value of which, i.e. the optical focus, is because of this very small (1X1 mm, 2,5X2,5 mm). As the speed of rotation is large enough (anode is a continuation of the motor shaft, rotating with angular speed of 2500 rpm), power tube at shutter speeds in 0,1 s. can reach 40-50 kW.
A significant amount of heat generated at the anode working tube, requires its cooling by heat removal from the anode to the environment. This is achieved by air radiator cooling (Fig. 9), water cooling (Fig. 10 and 11) or oil cooling (Fig. 12); the oil is simultaneously isolating environment; oil cooling is usually applied in the so-called block devices (see Rentgenotechnika).
In connection with various requests of radiodiagnosis and radiotherapy currently available x-ray tubes of various purposes, characterized as constructive design, and size, power, cooling methods and protection from unnecessary radiation. The legend of various types of pipes consist of combinations of numbers and letters. The first figure is the maximum allowable power tubes (in kW); the first letter identifies the protection against radiation (R - self; B - in a protective cover; no letters means no protection); the second letter defines the purpose R. T. (D - diagnostic; T - therapy); the third letter indicates the cooling system (radiator air cooling, M - oil-In - water, no letters means cooling by radiation); the latter figure corresponds to the maximum admissible anode voltage in kV. So, for example, 3 PM-2-100 - three kW diagnostic tube oil cooling system (radiator) 100 kV to work in a protective cover (conditional room type - 2); tube - 1-T-1-200 - therapeutic without protection with cooling by radiation, with a capacity of 1 ket at 200 kV (conditional room type - 1).
Regardless of the type x-ray tube of the General principle of their work is the following. The intensity of cathode R. T. causes thermionic emission emission with the formation of the so-called cathode electron cloud. With the inclusion of high voltage on electrodes R. T. free electrons under the influence of an electric field rush to the anode, hampered by his mirror, and part of braking energy is converted into x-ray radiation.
When the voltage on the x-ray tube emission current initially increases steeply, due to the gradual decrease of the density of electron cloud. When the number of electrons produced at the cathode, becomes equal to the number of electrons, reaching the anode, further increasing the voltage does not cause increase of the current through R. so, it only increases the kinetic energy of the electrons reach the anode. Mode R. so, when using all of the electrons produced at the cathode, and further increasing the voltage does not cause increase of the anode current is called saturation. Almost saturation current i is achieved in diagnostic x-ray tubes with potential difference σ about 10-20 kV (Fig. 13). So usually R. T. for the most part work in the current mode of saturation. If necessary, increase the anode current should be increased accordingly, the filament current cathode and, raising the voltage to re-create the current mode of saturation.
In the process of industrial production of R. T. remove the gas to a residual pressure of 10-6-10-7 mm RT. Art. At this rate, vacuum the current passing through R. T. almost caused only thermionic emission emission from the cathode. However, when excessive heating of parts of the tube, or when her after a long break in work, can appear gas; this raises the effect of ionization; x-ray tube begins to pass current in both directions. Measuring instruments on the control panel detects a sharp fluctuations anode current. If such "gateway" R. I. to include under high voltage without filament cathode, it creates a sustained gas discharge, accompanied by a characteristic glow tube. This tube is to work unsuitable and must be replaced.
Each new Works. so before starting the unit should be checked for vacuum high voltage, not including heat, then be subjected to a "training". To do this anode voltage of about 1/3 of the nominal set a current of 1-2 mA. Then within 30-60 minutes voltage and current is gradually increased until the nominal values of long-term regime in accordance with the passport R. T. When operating R. so you must strictly follow the modes, specified in the passport.
Cm. also X-ray machines, X-ray radiation.

Fig. 1. Ion x-ray tube with air cooling and gas regenerator.
Fig. 2. X-ray tube of Lilienfeld.
Fig. 3. Cathodes dvukhfaznykh e-x-ray tubes: 1 - with two cylindrical spiral filament; 2 - with two flat spiral filament.
Fig. 4. Safe x-ray tube for intracavitary radiotherapy: 1 - cathode; 2 - anode tube; 3 - the quit window of x-rays; 4 - anode cap; 5 - a water jacket; 6 - pipes of cooling.
Fig. 5. Therapeutic x-ray tube with a massive tungsten anode: 1 - cathode; 2 anode.
Fig. 6. Schematic representation of bar focus diagnostic x-ray tube: 1 - mirror anode; 2 - real focus; 3 - anode; 4 - the Central beam; 5 - optical focus; 6 - axis of the tube; 7 - cathode.
Fig. 7. Tube with rotating disc anode: 1 - cathode; 2 - disc anode; 3 - protective drive; 4 - axis anode; 5 - steel cylinder-rotor induction motor.
Fig. 8. Schematic representation of focus tube with rotating disc anode: 1 - real focus; 2 - it scan; 3 - immediate focus; 4 - axis of the tube; 5 - cathode; 6 - optical focus; 7 - anode.
Fig. 9. Tube radiator with air cooling.
Fig. 10. The anode of the tube with water cooling: 1 - the anode rod; 2 - tanks with cooling water.
Fig. 11. The anode of the tube cooled by flowing water: 1 - connecting tube water cooling.
Fig. 12. The miniature x-ray tube, oil-cooled x-ray of teeth.
Fig. 13. Anode characteristically electronic x-ray tube: S'- when the filament current 3,8 a; S when the filament current 3,4 and.