Internal Components of X-ray Tube (Anode)

Friday, February 13, 2015

The anode is the positive side of the x-ray tube. There are two types of anodes –stationary and rotating anodes. Stationary anode x-ray tube are used in dental x-ray imaging systems, portable imaging machine, and other special purpose machine in which high tube current and voltage are not required. General purpose x-ray tubes use the rotating anode because they must be capable of producing high-intensity x-ray beams in a short time.

X-ray Tube

3 functions of Anode in X-ray Tube:

It is an electrical conductor for x-ray tube
The electrons received that emitted from the cathode and conducts them through the tube to the connecting cables and back to the high voltage generator.
It provides mechanical support for the tungsten target

Anode Qualification

Anode is the positive side of the x-ray tube, electricity is conducted and radiates heat and contains the target. The anode must be a good thermal dissipator. Because when the projectile electrons from the cathode interact with the projectile electrons from the cathode interact with the anode, more than 99% of their kinetic energy is converted into heat. This heat must be dissipated quickly. Copper, molybdenum, and graphite are the most common anode materials. Adequate heat dissipation is the major engineering hurdle in designing higher capacity x-ray tubes.

Target of X-ray Tube

The target is the area of the anode struck by the electrons from the cathode. In stationary anode tubes, alloying the tungsten (usually with rhenium) gives it added mechanical strength to withstand the tress of high speed rotation. High capacity x-ray tubes have molybdenum or graphite layered under the tungsten target. Both molybdenum and graphite have lwer mass density than tungsten, making the anode easier to rotate.

Why tungsten is the material of choice for Target?

Rotating Anode

The rotating anode x-ray tube allows the electron beam to interact with a much larger target area, and therefore the heating of the anode is not confined to one small spot as in stationary anode tube. The rotating anode tube provides nearly 1000 times more area to interact with the electron beam than a stationary anode tube. Higher tube currents and shorter exposure times are possible with the rotating anode. Heat capacity can be further improved by increasing the speed of anode rotation. The speed of rotation on most rotating anodes is 3400 rpm (revolutions per minute). The anodes of high capacity tubes rotate at 3400 rmp and 10,000 rpm.

Induction Motor

An electromagnetic induction motor is used to turn the anode. An induction motor consist of two principal parts separated from each other by the glass or metal enclosure.
Stator – It is the part outside the enclosure and consist of a series of electromagnets equally spaced around the neck of the tube.Rotor – a mechanism that is located inside the glass enclosure and a shaft made of bars of copper and soft iron fabricated in one mass.
The induction motor works through electromagnetic induction, similar to a transformer and based on Lenz’s law of induced currents.

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