Anode (x-ray tube)

The anode (or anticathode) is the component of the x-ray tube where x-rays are produced. It is a piece of metal, shaped in the form of a bevelled disk with a diameter between 55 and 100 mm, and thickness of 7 mm, connected to the positive side of the electrical circuit. The anode converts the energy of incident electrons into x-rays dissipating heat as a byproduct.

Most x-ray tube anodes are made of tungsten (the target material). Tungsten has a high atomic number (Z=74) and a high melting point of 3370°C with a correspondingly low rate of evaporation. The high atomic number of tungsten gives more efficient bremsstrahlung production compared to lower atomic number target materials. An alloy containing tungsten and rhenium is also used because the addition of 5-10% rhenium prevents grazing of the anode surface. The body of the anode is made of materials that are light and have a good heat storage capacity, like molybdenum and graphite. Molybdenum is also often used as the target material for anodes used in mammography because it has an intermediate atomic number (Z=42) and the produced characteristic x-rays are of energies suited for this purpose. Some anodes used in mammography are also made of rhodium (Z=45), which has characteristic x-rays of slightly higher energies, which are more penetrating and preferably used in dense breast imaging.

Anodes are designed as bevelled disks attached to a large copper rotor of the electric motor, rotating them at the speeds up to 10,000 rpm, with a temperature of 2000°C. The purpose of the rotation is to dissipate heat. Most rotating anodes actually represent rather complex electromechanical systems consisting of approximately 350 pieces, taking around 150 assembly operations.

The anode disc rotates and is subjected to a focused beam of electrons emanating from the cathode, which is accelerated by a high potential difference between the cathode and the anode. When the electron beam hits the anode (at the actual focal spot), interactions of the electrons with the target material produces the x-ray beam. The anode angle is the angle between the vertical and the target surface with most x-ray tubes having an anode angle of 12-15°. A smaller angle results in a smaller effective focal spot.

The whole anode is not included in x-ray production. X-rays are produced on the rather small rectangular surface, the focal spot.