X-ray tube call the electrovacuum device intended for obtaining x-ray radiation. It is the vacuumized glass cylinder with the metal electrodes which are sealed in it.
Instruction
1. X-ray radiation arises when braking of the accelerated electrons on the screen of the anode made of heavy metal for receiving electrons the cathode is used. For acceleration of electrons the high voltage is brought to the cathode.
2. In modern x-ray tubes the electrons receive at incandescence of the cathode. The quantity of electrons can be changed, regulating current in a heat chain. At low voltage not all electrons participate in creation of anode current, at the same time at the cathode the electronic cloud which resolves at increase in tension is formed. Since a certain tension all electrons reach the anode, at the same time through a tube the maximum current flows, it is called saturation current.
3. As a rule, the anode of a x-ray tube is made in the form of a massive cover of copper in which thickness the tungsten plate which is called an anode mirror is sealed. The anode is turned to the cathode by a slanted face, at the same time the leaving x-ray radiation is perpendicular to a tube axis.
4. In the cathode there is a refractory filament, most often it is made of tungsten in the form of a flat or cylindrical spiral. Thread is surrounded with the metal glass intended for focusing of a bunch of electrons on an anode mirror. Bifocal x-ray tubes are equipped with two filaments.
5. On the anode a large amount of heat as a result of braking of a stream of electrons is generated, only the insignificant amount of energy is transformed to x-ray radiation. To protect the anode from overheating and to increase overall performance of a x-ray tube, apply oil, water or air cooling, sometimes for this purpose use emission.
6. The size of focus of a x-ray tube affects sharpness of the received image. At modern tubes the line focus makes from 10 to 40 mm, however not its valid size, but a visible projection in the direction of a bunch has practical value. In modern diagnostic tubes the area of effective focus is approximately three times less than the area of valid. Power of such tube is twice more, than at the device with round focus.
7. At x-ray tubes with the rotating anode the power is even more. The massive tungsten anode in them possesses the line focus stretched on a circle. It rotates on bearings, at the same time the cathode of a tube is displaced concerning its axis that the focused bunch of electrons always got on a slanted surface of a mirror of the anode.