The essential elements of a modern Coolidge X-ray vacuum tube which is widely used for commercial and medical purposes are shown in figure. Electrons are produced thermionically from a tungsten filamentary cathode F which is heated to incandescence either by a storage battery or by a low-voltage alternating current from a step down transformer T2. These electrons are focused on the target T with the help of a cylindrical shield S which surrounds F and is maintained at a negative potential. The electrons are accelerated to very high speeds(upto 10% of velocity of light) by the d.c. potential difference (of about 50 kV - 100 kV) applied between F and the anode (also called anti cathode). This high d.c. potential is obtained from a step-up transformer T1 whose output is converted into direct current by full-wave rectifier and a suitable filter.
The target T usually employed in X-ray tubes is a massive block of tungsten or in many cases, amolybdenum plug embedded in the face of a solid copper anode. The face of the copper anode is sloped at about 45º to the electron beam. Being very good conductor of heat, copper helps to conduct heat efficiently to the external cooling fins or the water-cooling system. Under the terrific bombardment of the target by so many electrons, most metals will melt. That is why metals like tungsten, platinum and molybdenum etc. are used which have high melting points and also have a high atomic weight(which is essential for abundant production of X-rays).
When the electrons strike the tungsten target, they give up their kinetic energy and there by produce X-rays.
