• 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