The anode of the x-ray tube is where x-rays are produced. Most x-ray tube anodes are made of tungsten (Z=74 atomic number), which has a high melting point 3370oC with a low rate of evaporation. The anode is positively charged. The free electrons released from cathode filament (via thermionic emission) are accelerated towards the anode.
When the free-electron hits the tungsten anode target, two main things happen:
- Electron interacts with the nucleus and “breaks/slows down” where it changes direction and loses energy. The energy it loses is released as an x-ray photon known as “Bremsstrahlung radiation”.
- An incident-free electron collides and ejects a K-shell electron of the tungsten anode target, leaving behind a “hole”. An outer shell electron (L-shell or M-shell) fills the vacancy in the K-shell and emits a single x-ray photon called “characteristic radiation”, with an energy equivalent to the binding energy level difference between the K-shell and outer shell electron. Characteristic radiation is emitted at specific energies.
Additional information:
Bremsstrahlung = “braking radiation”
- Free electron is accelerated towards the target anode (Tungsten)
- Electron interacts with the nuclear electrical field and swerves around the nucleus and slows down
- The kinetic energy loss by the electron is emitted as an X-ray photon
- The energy of the photon equals the energy lost by the electron (Law of Conservation of Energy)
- More energy is lost the closer the electron is to the nucleus due to Coulombic forces = hence higher x-ray photon energies
- An electron can lose all of its kinetic energy hence produces a spectrum of x-ray energy *from zero to maximal kinetic energy (kVp)*
- Bremsstrahlung increases with both accelerating voltage (kV) and atomic number (Z) of the target (larger nucleus!)
- Majority 90% x-rays are Bremsstrahlung (in general radiography, fluoroscopy, CT)
Characteristic Radiation
- The incoming electron has enough energy to remove a K-shell electron from tungsten atom i.e. (>69.5keV)
- An outer shell electron will fall into the k-shell and release an x-ray photon equal to the energy difference between the shells of the particular target atom
- Tungsten shell energies
- K shell = 69.5keV
- L shell = 11.5keV
- M shell = 2 keV
- L -> K shell = 69.5 – 11.5 keV = 58 keV (K alpha – adjacent shell)
- M ->K shell = 69.5 – 2 keV = 67.5 keV (K beta – higher energy)
- M -> L shall = 11.5 – 2 keV = 9.5keV (not seen as the filters would absorb such low energy photons)
- Characteristic radiation only occur at discrete energy levels (not continuous)
- K shell characteristic x-rays are always slightly lower than K shell binding energy
- L shell characteristic x-rays are lower energy and absorbed by the glass envelope
- K shell account for 10% of x-ray energies at 100 keV
- For voltages <69.5keV – there are no K-shell characteristic x-rays from tungsten
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