b) Attenuation

c) x-ray attenuation mechanisms; simple scatter, photoelectric effect, Compton effect, and pair production

attenuation = the decrease in intensity of EM radiation as it passes through matter

attenuation mechanism

• simple scatter (1-20keV)

  • x-ray photon collides with an electron

  • photon has less energy than required to remove the electron

  • photon is scattered elastically

• photoelectric effect (<100keV)

  • x-ray photon collides with an electron

  • photon is absorbed by the electron

  • electron gains energy

  • electron escapes the atom

  • (most common in x-ray scanning)

• Compton scattering (0.5-5MeV)

  • x-ray photon collides with an electron

  • electron gains energy

  • electron is removed from the atom

  • photon is scattered with reduced energy

  • (energy + momentum are conserved in the collision)

• pair production (>=1.02MeV)

  • x-ray photon collides with a nucleus

  • photon disappears

  • energy of the photon produces an electron and a positron

I = I0e-µx

• transmitted intensity (W/m-2) = initial intensity (Wm-2) x e^attenuation coefficient of the substance (m-1) x thickness of the substance (m)