Basics of X-ray Physics
X-ray production

Key points

  • X-rays are produced by interaction of accelerated electrons with tungsten nuclei within the tube anode
  • Two types of radiation are generated: characteristic radiation and bremsstrahlung (braking) radiation
  • Changing the X-ray machine current or voltage settings alters the properties of the X-ray beam

X-rays are produced within the X-ray machine, also known as an X-ray tube. No external radioactive material is involved.

Radiographers can change the current and voltage settings on the X-ray machine in order to manipulate the properties of the X-ray beam produced. Different X-ray beam spectra are applied to different body parts.

The X-ray tube

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The X-ray tube

  • A small increase in the filament voltage (1) results in a large increase in tube current (2), which accelerates high speed electrons from the very high temperature filament negative cathode (3) within a vacuum, towards a positive tungsten target anode (4). This anode rotates to dissipate heat generated. X-rays are generated within the tungsten anode and an X-ray beam (5) is directed towards the patient.

X-rays are generated via interactions of the accelerated electrons with electrons of tungsten nuclei within the tube anode. There are two types of X-ray generated: characteristic radiation and bremsstrahlung radiation.

Characteristic X-ray generation

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Characteristic X-ray generation

  • When a high energy electron (1) collides with an inner shell electron (2) both are ejected from the tungsten atom leaving a 'hole' in the inner layer. This is filled by an outer shell electron (3) with a loss of energy emitted as an X-ray photon (4).

Bremsstrahlung/Braking X-ray generation

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Bremsstrahlung/Braking X-ray generation

  • When an electron passes near the nucleus it is slowed and its path is deflected. Energy lost is emitted as a bremsstrahlung X-ray photon.
  • Bremsstrahlung = Braking radiation
  • Approximately 80% of the population of X-rays within the X-ray beam consists of X-rays generated in this way.

The X-ray spectrum

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The X-ray spectrum

  • As a result of characteristic and bremsstrahlung radiation generation a spectrum of X-ray energy is produced within the X-ray beam.
  • This spectrum can be manipulated by changing the X-ray tube current or voltage settings, or by adding filters to select out low energy X-rays. In these ways radiographers are able to apply different spectra of X-ray beams to different body parts.