X-RAY TUBE

Typical Number in Hospital:

Cost Bands:

References: 7

The X-rays are produced by two processes. Electrons may collide with the nuclear field around an atom, losing part of their energy, producing X-ray energies up to the maximum energy of the incident electrons, or electrons are ejected from the target atom which, when replaced, release energy dependent upon the atomic structure of that atom. Thus the energy spectrum produced will contain both continuous and line spectra.

The X-ray tube as used in diagnostic X-ray apparatus consists of an evacuated envelope containing a source of focused electrons (cathode) directed on to an anode which has a very high melting point and a high atomic number. The anode becomes very hot and so for all but the lowest power units this problem is dealt with by rotating the anode at high speed (3000 to 10 000 rev/min) to distribute the heat. The target material on the anode is usually tungsten (melting point 3370[d]C , atomic number 74) but molybdenum is also used for low power (e.g. mammography) sets.

The assembly, which one sees containing the tube, also contains terminals for the very high voltages applied between anode and cathode (up to 150 kV), and an air or oil-cooling system on high-power sets. There is also the rotating mechanism for the anode and special filters, cones or collimators, and diaphragms, making up the X-ray window.

X-ray tubes have a limited life.