What is Cathode?

COMPONENT OF X-RAY TUBE (Cathode)

• The negative terminal of the x-ray tube is called the cathode.
• It has got a filament (which is the source of electron for the x-ray tube).
• The cathode has two other elements. These are the connecting wires, which supply both the voltage (Average about 10 volts and Ampere average about 3-5 Ampere) that heat the filament.
• And a metallic Focusing cup.
• The number (quantity) of x-rays produced depends entirely on the number of electrons that flow from the filament to the target (Anode) of the tube.
• The x-ray tube current measured in Milliamperes (1 mA=0.001 A), refers to the number of electron flowing per second from the filament to the target.
• The filament is made of tungsten wire, about 0.2 mm in diameter, that is coiled to form a vertical spiral about 0.2 cm in diameter and 1 cm or less in length.
• When current flows through this fine tungsten wire, it becomes heated. when a metal is heated its atoms absorb thermal energy and some of the electrons in the metal acquire enough energy to allow them to move a small distance from the surface of the metal (normally, electron can move with in a metal, but cannot escape from it). Their escape is referred to as the process of thermionic emission.
• The electron cloud surrounding the filament produced by thermionic emission, has been termed the “Edison effect.”
• A pure tungsten filament must be heated to a temperature of at least (2200 degree c) to emit a useful number of electrons (Therm-ions)
• Tungsten is not as efficient an emitting material as other material (such as alloys of tungsten) used is some electron tubes.
• It is chosen for use is x-ray tubes, however, because it can be drown into a thin wire that is quite strong, has a high melting point (3370 degree c).
• It has little tendency to vaporise; thus, such a filament has a reasonably long life expectancy.
• Electron emitted from the tungsten filament form a small cloud in the immediate vicinity of the filament. This collection what is called the space charge. However in this case electrons does not move until they acquired sufficient thermal energy to overcome the force caused by space charge. Thus emission of further electrons from filament is limited by the space charge and this is called a space charge effect.
• When electrons leave the filament the loss of negative charges causes the filament to acquire a positive charge. The filament then attracts some emitted electrons back to itself. When a filament is heated to its emission temperature, a state of equilibrium is quickly reached.
• In equilibrium the number of electrons returning to the filament is equal to the number of electrons being emitted.
• As a result the number of electrons in the space charge remains constant, with the actual number depending on filament temperature.
• The high currents that can be produced by the use of thermionic emission are possible because large numbers of electrons can be accelerated from the cathode (Negative electrode) of the x-ray tube.
• The number of electrons involved is enormous.
• The unit of electric current is the ampere, which may be defined as the rate of “Flow” when 1 coulomb of electricity flows through a conductor in 1 sec.
• The coulomb is the equivalent of the amount of electric charge carried by 6.25x1018 electrons. Therefore, an x-ray tube current of 100 mA (0.01 A) may be considered as the “Flow” of 6.25x1017 electrons from the cathode to the anode in 1 sec.
• Electrons current across an x-ray tube is in one direction only (Always cathode to anode).
• FOCUSING CUP- Which surrounds the filament. When the x-ray tube is conducting, the focusing cup is maintained at the same negative potential as the filament.
• The focusing cup usually made of nickel.
• Modern x-ray tubes may be supplied with a single or more commonly, a double filament, each filament consists of a spiral of wire, and mounted side by side or one above the other, with one being longer than the other.
• It is important to understand that only one filament is used for any given x-ray exposure; the larger filament is generally used for larger exposure.
• FOCAL SPOT- The radiation is produced in very small area on the surface of the anode known as the focal spot.

• 

  A double filament contained in a focusing cup

• Two additional filament arrangements may be seen in highly specialised x-ray tubes. A tube with three filaments (triple focus) is available. Another application is a Stereoscopic Angiographic tube, in this tube focal spot widely separated (about a 4-cm separation). When two films are exposed, using a different focal spot for each film.
• This tube is useful in angiography when rapid exposure of multiple stereoscopic film pairs is desired. Interval as short as 0.1 sec between exposure can be obtained with stereoscopic tubes.
• Vaporization of the filament when it is heated acts to shorten the life of an x-ray tube, because the filament will break if it becomes too thin.
• The filament should never be heated for longer periods than necessary.
• The focal spot is the area of the tungsten target (Anode) that is bombarded by electrons from the cathode,(Most of the energy of the electrons is converted into heat, with less than 1% being converted into x-ray).
• LINE FOCUS PRINCIPLE- The problems posed by the need for a large focal spot to allow greater heat loading, and the conflicting need for a small focal area to produce good radiography details, were resolved in 1918 with the development of the line focus principle.
• The size and shape of focal spot are determined by the size and shape of the electron stream when it hits the anode. The size and shape of the filament tungsten wire coil, the construction of the focusing cup, and the position of the filament in the focusing cup.



Tungsten

• REGION FOR CHOOSING TUNGSTEN AS A FILAMENT -It is capable of stable electrons emission at high temperature. -It has high melting point. -It has mechanical strength, long life, and can be easily drown out into a fine filament. -It has high atomic number 74. -it has high thermal conductivity metal.

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