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