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

Rotating-Anode X-ray Tube

• 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