Rotating-Anode X-ray Tube (AQA A Level Physics)

• An X-ray tube is a device that converts an electrical input into X-rays
• It is composed of four main components:
  • A heated cathode
  • An anode
  • A metal target
  • A high voltage power supply

• The production of X-rays has many practical uses, such as in:
  • Medical imaging (radiography)
  • Security
  • Industrial imaging

The Rotating-Anode X-ray Tube

The main components of an X-ray tube are the heated cathode, anode, metal target and a high voltage supply

Heated Cathode

• At one end of the tube is the cathode (negative terminal) which is heated by an electric current
  • The heat causes electrons to be liberated from the cathode, gathering in a cloud near its surface
  • This process of thermionic emission is the source of the electrons 

Anode

• At the other end of the tube, an anode (positive terminal) is connected to the high-voltage supply
• This allows the electrons to be accelerated up to a voltage of 200 kV 
  • When the electrons arrive at the anode, they gain a kinetic energy of 200 keV (by the definition of an electronvolt)

• Only about 1% of the kinetic energy is converted to X-rays
  • The rest is converted to heat energy
  • Therefore, to avoid overheating, the anode is spun at 3000 rpm and sometimes water-cooled

Metal Target

• When the electrons hit the target at high speed, they lose some of their kinetic energy
  • This energy is re-emitted as X-ray photons

• A heat-resistant block of metal, usually tungsten, is embedded at the end of the anode, facing the cathode
  • This is the material that the electrons collide with and X-rays are generated in

High Voltage Power Supply

• The high voltage supply creates a large potential difference (> 50 kV) between the cathode and the target
• This causes electrons in the cloud around the cathode to be accelerated to a high velocity towards the target, which they strike, creating X-rays

Other Components

• X-rays are produced in all directions, so the tube is surrounded by lead shielding
  • This is to ensure the safety of the operators and recipients of the X-rays
  • An adjustable window allows a concentrated beam of X-rays to escape and be controlled safely

• The anode and cathode are housed inside a vacuum chamber
  • This is to ensure that the electrons do not collide with any particles on their way to the metal target