STATIONARY ANODE X-RAY TUBE

STATIONARY ANODE X-RAY TUBE

In earlier periods, gas tubes were used to produce X-rays. But, these tubes suffered with disadvantages. Hence, Coolidge proposed a prototype X-ray tube based on the thermionic emission principle. On the basis of Coolidge tube, several X-ray tubes have been designed. The stationary anode X-ray tube is one of the modern X-ray tubes in which the anodeis stationary. The stationary anode X-ray tube consists of a cathode and an anode which are kept in a evacuated glass envelope . The cathode consists of a tungsten filament in the form of a coil placed in a shallowfocusing cup. The filament is heated by passing an electric current through it from a low voltage supply. The anode is made of copper block in which a small tungsten plate is embedded. The tungsten plate serves as a target. The target is positioned on line focus principle, in order to increase the ratio of the actual focal area to the effective focal area. The anode angle is usually 15–20o. A high voltage supply is applied between the cathode and anode to accelerate the electrons. A vacuum of the order of 10–5 mm Hg is maintained in the tube. When the filament is heated to white light, it emits electrons. The focusing cup (made up of nickel) produces an electric field (negative) that focuses the electron to the focal area. The focusing cup also protects the adjacent parts of the tube wall from damage by electron bombardment. If the anode is made positive with respective to the filament, these electrons will be attracted to the anode. This will constitute an electron current around the circuit in the anticlock wise direction. The tube current is measured by the milliammeter (mA). Since the space between anode and cathode is a high vacuum, the electrons do not collide with gas molecules in crossing the gap and so acquire very high velocities. The electrons which are accelerated by the applied voltage possess high kinetic energy. When they suddenly stopped in the target, X-rays are emitted in all directions. About onehalf of these are absorbed in the target itself. The remaining portion emerges as a useful primary X-ray beam. During the production of X-rays, large amount of heat is produced in the target. The tube is also provided with suitable cooling system to remove the heat very quickly. Stationary anode tubes have a small target area that limits the heat dissipation and this limit the X-ray output, but they are small in size and weight. Dental X-ray units (intraoral and ortho-pantomography), portable X-ray units, and portable fluoroscopy systems use stationary anode X-ray tubes.