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
