The Heel Effect of X-ray tube课件.ppt

X-RAY TUBE CHARACTERISTICS Introduction An x-ray tube consists essentially of an evacuated enclosure surrounded by the protective housing. Inside the tube is a cathode (held at a –ve voltage) and an anode (held at a +ve voltage). Protective Housing The protective housing is lined with lead to minimize leakage radiation. It also protects against accidental electric shock. It may contain oil to help cool the tube. ;Tube Enclosure Maintains a vacuum inside the tube. If air were present it would interfere with movement of electrons and x-ray production could not be accurately controlled. Generally made of glass but high power x-ray tubes are made of metal. The x-ray window is a thin area of the tube enclosure through which the useful x-ray beam exits. Cathode The –ve electrode of the tube and the source of electrons which are accelerated to the anode to produce x-rays. It consists of: Tube filaments (usually two) Focussing cups (one for each filament) Control circuitry (external to tube for filament heating, focussing and switching) ;Filament. Produces electrons by the process of thermionic emission (electrons are “boiled off” at high temperature). Material must have the following characteristics: Minimal energy (ie, temperature) required to liberate electrons High melting point to resist vapourisation Mechanically strong to resist heating/cooling Able to be manufactured into filaments. Commonly made of tungsten (W). Is heated to 2000 °C or more for efficient emission. Melts at 3370 ° C. Evaporation of W from filament can coat tube enclosure and shorten tube life. 1 – 2 % thorium (Th) added to reduce this. Produced as a wire 0.2 mm thick and coiled to form a cylinder 2 mm diameter by 10 to 15 mm long. A 10 V and 3 - 5 A current is applied to the filament to heat it in order for it to eject electrons by thermionic emission. The ejected electrons form a cloud around the filament called a space charge. A high voltage is applied between the cathode and anode which accel