What is Anode?

Component of X-Ray Tube:

(ANODE)

• The positive terminal in x-ray tube is called the anode.
• Anode of x-ray tubes are of two types.

        1. Stationary Anode 
        2. Rotating Anode

Stationary Anode: 

• The anode of a stationary anode x-ray tube consist of a small plate of a tungsten, 2 or 3 mm thick, that is embedded in a large mass of copper.

  

  Fig.1   Stationary Anode X-Ray Tube 
• The tungsten plate is square or rectangular in shape with each dimension usually being greater than 1 cm.
• The anode angle is usually 15⁰ to 20⁰.

Tungsten: Tungsten is chosen as the target material for several reasons.

-It has a high atomic number (74). Which makes a more efficient for the production of x-ray.- In addition, because of its, high melting point, it is able to withstand the high temperature produced.- Mostly metals melt between 300 and 1500⁰c, whereas tungsten melts at 3370⁰c.- Tungsten is a reasonably good material for the absorption of heat and for the rapid dissipation of the heat away from the target area.-The rather small tungsten target must be bonded to the much larger copper portion of the anode to facilitate heat dissipation in spite of its good thermal characteristics, tungsten cannot withstand the heat of repeated exposures.

Fig.2    Stationary Anode Tube

-The size of the tungsten target is larger then the area bombarded by the electron stream. ---This is necessary because of the low melting point of copper (1070⁰c). A single x-ray exposure may raise the temperature of the bombarded area of the tungsten target by 1000⁰c or more.

-All the metals expand when heated, but they expand at different rates.

Rotating Anode:

• The anode of a rotating anode tube consists of a large disc of tungsten, which theoretically rotates at a speed of about 3600 revolution per minute (rpm) when an exposure in being made.
• In practice, the anode never reaches a speed of 3600 rpm because of mechanical factors such as slipping between the rotor and bearing. Therefore, to calculate the ability of a tube to withstand high loads, a peed of 3000 rpm is usually assumed.

  

  Fig.3 Rotating Anode X-Ray Tube

• Functioning rotating anode will never drop below  3000 rpm, and will usually be greater than 3000 rpm, if 60 hertz (HZ) current is used.
• The tungsten disc has a beveled edge.
• The angle of the bevel may very from 6 to 20⁰. The bevel is used to take advantage of the line focus principle.
• The purpose of the rotating anode is to spread the heat produced during an exposure over a large area of the anode.
• The filament and focusing cup of the x-ray tube produce an electron beam that covers an area of the anode 7-mm high and 2-mm wide, the area of the anode bombarded by electrons is represented by a 14-mm rectangle. We recognise that a 2-mm focal spot is a useful way to illustrate a point.
• If the bevel of the target is 16.5⁰.
• If the anode were stationary, the entire heat load would be delivered to this one small 14-mm area of the target.
• If the target is made to rotate at a speed 3600 rpm. however, the electron will bombard a constantly changing area of the target.
•  To make the anode rotate, some mechanical problems must be overcome, be cause the anode is contained within the vacuum of the tube. The power to effect rotating is provided by a magnetic field produced by stator coil that surround the neck of the x-ray tube outside the envelope.
• The magnetic field produced by the stator coils induces a current in the copper rotor of the induction motor, and this induced current provides the power to rotate the anode assembly
• the development of rotating anode x-ray tubes, the life of the tube was quite short because of the lack of durable bearing on which the anode assembly could rotate. because of the friction produced it was necessary to lubricate the bearing, but commonly available lubricate the bearing, but commonly available lubricants could not be used.
• Lubricants such as oil vaporise when heated and destroy the vacuum in the tube, dry lubricants such as a graphite would wear as a powder and destroy the vacuum. The problem was solved by the use of metallic lubricants (especially silver). Which are suitable for use in a high vacuum.
• In modern rotating anode tubes, bearing wear has become a negligible factor in overall tube life.
• Heat dissipation in a rotating anode tube presents an additional problem.
• Heat generated in a solid tungsten disc is dissipated by radiating through the vacuum to the wall of the tube, and then into the surrounding oil and tube housing.
• The rotating anode tube, absorption of heat by the anode assembly is undesirable because heat absorbed by the bearing of the anode assembly would cause them to expand and bind. Because of this problem the stem, which connects the tungsten target to the remainder of the anode assembly, is made of molybdenum.
• Molybdenum has a high melting point (2600⁰) and is a poor heat conductor.
• Thus, the molybdenum stem provides a partial heat barrier between the tungsten dic and the bearing of the anode assembly. 

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