Physics Basics


Flattening Filter

  • To compensate for the lack of scatter at the edge of the field by deliberately designing a profile that increases toward the edges.

Radiation Detectors

All detectors make use of ionization and excitation processes.

Gas Ionization detectors

  • ionization chambers
  • Geiger-Mueller(G-M) counters
  • Proportional counters

Basic parts!

  • A chamber with fixed volume of gas
    • gas can be air, methane
    • chamber=probe
  • Two electrodes ( positive and negative )
  • When photons pass through the chamber ==> ion pairs produced ==> ionization current produced
  • Because chamber is polarized ==> ions travel to the oppposite charged electrode
  • Some ions recombine
    • Collection efficiency is the fraction of charges actually collected
    • Above 300volts —> ionization chamber region ( efficiency almost 100% )


Maximum Depth Dose

Try to imagine what happens when electron hit surface of any thing :

  • At a very first layer —> Photon excite an electron
  • This electron travels through its path which has a known range according to the energy it has acquired from the photon
    • This electron range is the the depth of Dmax
    • Let's say it has the energy of the photon
  • As the electron travels though it's path, it excite secondary electrons…
  • Until the end of the range of the primary electron, it reaches to it's equilibrium
    • Electron equilibrium happens in a point/level in which energy loss is compensated by energy gained.

See Dmax