X-RAY TUBE DESIGN

X-RAY TUBE DESIGN

In an X-ray tube, the anode and cathode are closed in an air tight envelope, and the envelope and its inner part are called tube inserts. In most X-ray tubes, this envelope is called Pyrex Glass. ), But in some modern X-ray tubes, it is also made of metal or ceramic.     
 Functions of X-ray tube      

(i)                  It provides insulation and support to the anode and cathode.       

(ii)                It maintains vacuum in the x-ray tube.

CATHODE- the cathode is made up of tungsten wire in the form of helical filament, surrounded by a focusing cup. The filament circuit supplies a potential difference of 10v and a selectable filament current of 3-6 amperes. Electrical resistance to electron flow heats the filament to very high temperatures releasing surface electron through thermionic emission process. The rate of emission depends on the temperature. a trace of thorium in the filament not only increase the efficiency but also prolong the filament life.

the focusing cup controls the width of electron distribution and directs the electron towards the target. Usually the focusing cup is at the same potential (non biased) as the filament. In some tubes the cup is kept at more negative voltage (biased) then the filament. The later will facilitate to have a lesser focal spot size

TARGET – the anode is the target electrode, which is maintained at a positive potential. the target material should posses the following properties

·         It should have a high melting point for with standing with high temperature.

·         It should posses a high atomic number to increase the x-ray production efficiency.

·         It should posses a high thermal conductivity to dissipate heat quickly.

·         It should have a lower vapor pressure at high temperature to prevent the evaporation of target material.

Tungsten (W) is the metal widely used as the target because of its high melting point, 3770c and high atomic no 74. However, its thermal conductivity is low as 0.3. Hence a layer of tungsten is embedded over a thick block of copper. Since the thermal conductivity of copper is 0.9 the heat will be removed very quickly to the surrounding. The anode has a tendency to crack under severe stress caused by heating .therefore, tungsten-rhenium alloy 90% tungsten +10%rhenium is always used, which makes the target tougher and less likely to crack.

Molybdenum (mo ,z=42) and rhodium (rh,z+45) are commonly used as anode materials for a mammography x-ray tubes. These targets are capable of giving characteristic x-ray, suitable for soft tissue contrast studies

FOCAL SPOT SIZE - the area of the target with which the electrons are absorbed and x-rays are generated is called focal spot or focal area. if the focal area is very small .penumbra will be minimum and the picture sharpness will be good but heat removal is difficult. On the other hand, if the focal area is large, heat will be removed quickly and the picture sharpness will be reduced. This can be compromised by carful design of tube.

Usually focal spot is defined in two ways. The real focal spot size is the place on the anode that is hit by electrons. The effective focal spot size is the length and width of the emitted x-ray beam as projected down the central axis of the x-ray tube. The effective foal spot length is always smaller than the actual focal spot

TUBE COOLING - in the x-ray tube only less than 1 percent of electrical power supplied is converted into x-rays .the remaining electrical power (over 99%) is converted into heat. this large amount of heat may melt the target and therefore heat should be removed quickly from the target. Hence efficient cooling system is requiring form the x-ray tube the different method of cooling are:

·         target are made by inserting a layer of tungsten in copper block which remove the heat from the tungsten very quickly

·         X-ray tubes usually enclosed in metal case which are filled with oil for insulation purpose. this oil surround the glass envelope as well as copper block (static oil cooling).the oil convection transfer the heat from the copper block into metal case. air convection removes the heat from the metal case

·         In some modern tubes the anode is earthed and water is allowed to circulate through the anode. Sometimes the water is additionally cooled by feron gas.