X-rays are produced when high-speed electrons are decelerated rapidly on hitting a metal
Medical Imaging - OCR A-Level Physics
- X-rays are produced when high-speed electronsSubatomic particles with charge −1.6 × 10⁻¹⁹ C and mass 9.11 × 10⁻³¹ kg; produce X-rays when decelerated. are decelerated rapidly on hitting a metal target (usually tungstenA high-density metal (Z = 74) used as the target in X-ray tubes because of its high melting point and efficiency.).
- An X-ray tube contains a heated cathode (electron source) and a metal anode, with a high p.d. (25-250 kV) across them in a vacuum.
- Electrons are accelerated from cathode to anode. Their kinetic energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). an object possesses due to its motion. on impact: $E_{k} = eV$.
- When electrons decelerate in the target, they emit a continuous spectrumThe broad range of X-ray wavelengths produced by bremsstrahlung (braking radiation) as electrons decelerate. of X-rays (Bremsstrahlung / braking radiation). The maximum photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. energyThe capacity to do work. Measured in joules (J). equals eV: $E_{max} =$ eV, giving a minimum $wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). \lambda_{min}$ = hc/eV.
- Characteristic X-raysX-rays of specific wavelengths produced when electrons knock out inner-shell electrons and outer electrons fill the gaps. are also produced when incoming electrons knock out inner-shell electrons; outer electrons drop down, emitting photons of specific energies (line spectrum superimposed on the continuous spectrum).
- Less than 1% of the electron's kinetic energyThe energy an object possesses due to its motion. is converted to X-rays; the rest becomes thermal energyThe energy stored in a system due to the random motion of its particles; it is related to temperature. in the target, which is why the anode must be cooled (often by rotation).