CIE A Level Physics (9702) 2019-2021

Revision Notes

27.4.1 The X-ray Tube

The X-ray Tube

  • An X-ray tube is a vacuum tube that converts an electrical input into X-rays. This is commonly used in the field of radiography
  • It consists of a cathode (negatively charged electrode) and rotating anode (positively charged electrode) inside a vacuum chamber
  • At the cathode, the electrons are boiled off by a hot filament
  • The beam of electrons is targeted at the tungsten anode by a cup-shaped cathode
  • There is a voltage of up to 200 kV between the two electrodes
    • When the electron arrives at the anode, its kinetic energy is 200 keV
  • When the electrons hit the anode at high speed, they lose some of their kinetic energy
    • This is emitted as X-ray photons
  • Only about 1% of the kinetic energy is converted to X-rays
    • The rest is converted to heat. To avoid overheating, the tungsten anode is spun at 3000 rpm and maybe water-cooled

Soft & Hard X-rays

  • X-rays can be classified into soft X-rays and hard X-rays
  • The hardness of an X-ray is defined as:

The measure of the penetrating strength of a beam

  • The greater the hardness, the greater the penetrating strength of the X-ray
  • Hard X-rays are shorter in wavelength and more energetic than softer rays
  • Hardness can be controlled by:
    • Changing the voltage across the anode
    • The higher the anode voltage, the greater the X-ray hardness

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