X-ray tube

a glass vacuum tube in which X rays are generated (Fig.1). Electrons are accelerated from the hot cathode filament towards the anode surface, where X-rays are produced mainly by bremsstrahlung and to a smaller extent by characteristic radiation.

The essential parts and requirements for a modern radiological X-ray tube are as follows:

1) High vacuum inside a glass or metallic container, i.e. a tube where electrons can freely be accelerated without excessive collisions with gas molecules.

2) A cathode filament made of thin tungsten wire which releases electrons to the filament vicinity when heated with electric current (see also filament circuit and saturation voltage).

3) A rotating anode consisting e.g. of a thin tungsten surface on copper plus graphite base. Many possible anode constructions with correspondingly different properties in X-ray production are in use. The stem of the rotating anode inside the tube is simultaneously the rotor, while the stator of the induction motor is situated outside the tube. The rotor is well balanced with inside bearings. The bearings must function for years in vacuum.

4) A tube shield or housing containing the vacuum tube itself surrounded by insulating oil. The leaded shield should prevent radiation leakage.

5) High voltage cables leading from the X ray generator to the cathode and anode ends of the tube. High voltage across the tube is generally achieved by applying half the voltage needed with respect to earth, e.g. +50 kV, to the anode, and the other half, -50 kV, to the cathode, for a total high voltage of 100 kV. This prolongs the tube lifetime by decreasing possibilities for voltage breakthroughs.

6) A tube port or window allowing the produced X-rays to escape from the tube housing with a minimum of attenuation.

The vast majority of the diagnostic X-ray tubes today are equipped with rotating anodes, with the exception of e.g. dental X-ray tubes which still may use a stationary anode. X-ray tubes are designed for many different purposes ranging from very demanding applications in angiography and computed tomography CT with short, but heavy-duty and repeated series of exposures to the less demanding but still specific requirements in mammography and in dental imaging. The actual design of the X-ray tube therefore differs vastly.

The flow of electrons across the X-ray tube is most often controlled by external switches (see automatic exposure control). Some X-ray tubes have their own switch which allows the X-ray production to be turned on and off rapidly, see grid controlled X ray tube.

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