X-ray tube

Last revised by Arlene Campos on 11 Jan 2024

An x-ray tube functions as a specific energy converter, receiving electrical energy and converting it into two other forms of energy: x-radiation (1%) and heat (99%). Heat is considered the undesirable product of this conversion process; therefore x-radiation is created by taking the energy from the electrons and converting it into photons. This very specific energy conversion takes place in the x-ray tube.

NB: This article is about the modern vacuum tube. For the first twenty years after Roentgen's discovery, all tubes were of the gas type.

The x-ray tube contains two principal elements:

  1. filament (also acts as cathode): boils off electrons by thermionic emission

  2. target (also acts as anode): electrons strike to produce x-rays

Additional components include:

  • expansion bellows (provide space for oil to expand)

  • tube envelope (evacuated)

  • tube housing

  • cooling dielectric oil

  • rotor

  • induction stator

  • tube window: usually made from beryllium, not glass

The filament/cathode and target/anode are contained in the envelope, which provides vacuum, support and electrical insulation. The envelope is most often made from glass (Pyrex or borosilicate glass) 8, although some tubes contain envelopes formed from ceramic or even metal 4. A glass envelope is more prone to failure because tungsten filament vapourises and coats on the surface of glass, altering the electric potential of tube, resulting in the formation of electric arc, causing tube failure. On the other hand, a metal envelope maintains the electric potential across the tube, making it less likely to fail. Thus, most high output x-ray tube uses metal envelopes 6.

An X-ray tube can have two filaments at the cathode where a smaller filament produces a smaller focal spot, thus images with greater detail. However, the image will be easily blurred by movement artefacts. Meanwhile, a larger filament produces high intensity X-rays with quick exposures, thus less influenced by movement artefacts 7.

For some demanding applications, such as dual energy CT, rotating envelope tubes (RET) are used. Unlike conventional x-ray tubes, in rotating envelope tubes, not only the anode, but the entire vacuum tube rotates, furthermore, the anode is in direct contact with the liquid coolant, resulting in improved heat conduction and increased performance 4

The energy used for this process is provided from a generator, connected by an electrical circuit connected to the x-ray tube. The generator also needs to convert the mains supplied alternating current (AC), into direct current (DC), as needed by the x-ray tube. The reason for this is to ensure a constant unidirectional flow of electrons from the negatively charged cathode to the positively charged anode.

The quality and the quantity of the x-radiation are controlled by adjusting the electrical parameters (kV – tube voltage (potential difference applied across the tube), mA – tube current (flows through the tube) and exposure time, usually a fraction of a second.

To summarize, x-rays are produced in a standard way: by heating a filament, which releases electrons by thermionic emission, accelerating electrons with a high voltage and allowing them to collide with the focal spot on the target/anode. X-rays are produced via two interactions in the anode.

Bremsstrahlung x-rays (German for "braking") - electrons lose kinetic energy as they pass through atoms in the anode because they are attracted to the positively charges nuclei. The closer to the nucleus the electron passes, the more kinetic energy it loses and it is deflected to continue moving in another direction at lower energy, or stopped altogether. This is where maximum kinetic energy is transferred to the production of an x-ray that is emitted from the anode.

If electrons possess an energy that is equivalent to, or greater than, the binding energy of the orbiting electrons in target atoms, these electrons are likely to be ejected from the atom. This most often occurs in the inner electron shell (K-shell). The ejected electron is known as a photoelectron. The vacancy left in the K-shell must be filled in order for the atom to remain stable (law of conservation of energy) so outer shell electrons drop down to fill the shell. This process of electron transfer between shells produces x-rays that are "characteristic" of the binding energies of that particular atom/material, hence the name.

Wilhelm Roentgen discovered x-rays using a Crookes tube in 1895. Until the invention of the Coolidge tube in 1913, all x-ray tubes were based on the Crookes or cold cathode gas tube technology. From the late 1910s onwards there was a rapid replacement of gas tubes by the far more effective Coolidge tubes.

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