RADD 2501 Rotating Anode X-ray Tube

RADD 2501 Rotating Anode X-ray Tube - The Rotating Anode...

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The Rotating Anode X-Ray Tube The rotating Anode tube assemble is a complex piece of electro-mechanical engineering comprising of around 350 parts taking 150 assembly operations and can cost as much as £20,000. General Principles The anode target disc (1) rotates on a highly specialised ball bearing system (2) The target is subjected to a focussed stream of electrons (3) emanating from the cathode (4) and accelerated by a high potential difference between the target disc and the cathode, when the electron beam hits the anode it produces the x-ray beam (5)
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The Cathode The cathode of the rotating anode x-ray tube provides a controlled source of electrons for the generation of the x-ray beam. The electrons are produced by heating a tungsten spiral wire or filament, the temperature of which controls the quantity of electrons released. This determines the tube current (mA) and thus becomes the control means for selecting mA. At high temperatures the tungsten starts to vaporise. The filament is supported in and one end connected to a highly polished nickel focussing cup which along with carefully designed shape provides electrostatic focussing of the electron beam onto the anode. The Filament The filament is an accurately constructed tungsten wire coil of precise pitch and length, it is crystallised during construction by heating to a high temperature in the absence of oxygen, this crystallised structure gives the filament dimensional stability expanding and contracting only a minute amount during the heating cycle from 25 0 C to 2600 0 C. The dimensions of the wire and it site in the focussing cup determine the large and small foci of the tube. The Rotor and Bearing System The anode disc needs to be rotate at high speed and this is achieved by attaching the stem to a large copper rotor, which forms the armature of a motor. The target disc, or rotor, is mounted on a shaft extending from a rotor body, which can spin on internal bearings on the rotor shaft. This rotor shaft extends through the end of the insert to the outside of the insert vacuum for connection to the anode wire, and also is the mounting point for the insert inside the housing. The rotor bearing are special as they need to operate in a vacuum, conduct a high voltage and reach high temperatures (500 o C). Using steel bearings lubricated by silver powder solves the problem.
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The Anode The anode disc is between 55mm and 100mm in diameter and 7 mm thick machined to high tolerance to prevent in balance and wobble. The disc will experience rotational speeds up to 10,000 RPM and temperatures of 2000 o C The disc has a tungsten rhenium target area as tungsten has a high melting point 3370 o C and atomic number of 74. The addition of a small quantity (5- 10%) of rhenium prevents crazing of the anode surface, the tungsten is faced onto a molybdenum disc as molybdenum whilst not having as high a melting point as tungsten has twice the heat capacity. The target disc is mounted on a molybdenum stem
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RADD 2501 Rotating Anode X-ray Tube - The Rotating Anode...

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