Anode (x-ray tube)

The anode 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 the 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 the electrons into x-rays dissipating heat as a byproduct.

Most x-ray tubes are built of tungsten. Tungsten has a high atomic (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. AlloyAn alloy containing tungsten and rhenium is also used since 5-10% of rhenium prevents crazing of the anode surface. The body of the anode is made of the materials that are light and have a good heat storage capacity, like molybdenum and graphite. Molybdenum is also often used as the surface material for the anodes used in mammography for its characteristics: intermediate atomic number (Z=42), with the produced x-rays of the energies suited for this purpose. Some of the anodes used for mammography are also made of rhodium (Z=45), producing more penetrating radiation, preferably for use in dense breast imaging.

Anodes are designed as bevelled disks attached to the large copper rotor of the electric motor, rotating them at the speeds up to 10,000 RPM, with the temperature of 2000 °C. The purpose of the rotation is to dissipate the 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 emanated from the cathode, which are then accelerated by the high potential difference between the cathode and the target disc. When the electron beam hit the target, i.e. the focal spot, it then produces the x-ray beam. The anode angle is measured from vertical to the angle of the anode and is about 15°. A smaller angle would make a smaller focal spot.

The whole anode is not included in the x-ray production. The x-rays are produced on the rather small rectangular surface – the focal spot. Some x-rays have two focal spots, chosen according to the imaging procedure.