Principles Of Xray Generator Construction

X-rays for medical diagnostic procedures, research purposes or therapy are produced by one process that increases the kinetic energy of an electron, which can be achieved by:

  • accelerating electrons between electrodes with a high voltage or large potential difference (//for SXR and orthovoltage machines)
  • accelerating electrons by the absorption of electromagnetic radiation in the form of microwaves (for linear accelerators)

which subsequently collides with a metal target.

The electrons are suddenly decelerated upon collision with the metal target, and the energy loss associated with deceleration and direction change is 'expelled' as X-rays. Given the decelerative nature of the process, these x-rays are commonly called bremsstrahlung which is German for "braking radiation".

In addition, bombarding electrons with sufficient energy can knock an electron out of an inner shell of the target metal atoms. Then electrons from higher shells will drop down to fill the vacancy, and in the process of changing shells emitting x-ray photons with precise energies determined by the differences in electron energy levels. These x-rays are called characteristic x-rays.

There are two devices for producing X-rays. Both are based on the same principles which are executed in a slightly different way.

Physical Principle X-ray tube Linear accelerator Notes
1.a source of electrons heated filament; thermionic emission electron gun
2. a vacuum vacuum pump ion pump high-speed electrons will collide with any air particles; the linac waveguide requires a much lower vacuum
3. an energy source electrical potential difference (voltage) microwaves voltage is limited by the ability to insulate wires
4. a target tungsten 'wedge' tungsten 'transmission' tungsten (W) is usually used because of its high Z, its resistance to heat and hard wearing characteristics
5. heat dissipation cooled target; some machines use a spinning target cooled
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