b) Attenuation

• attenuation

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

• the four attenuation mechanisms

• simple scatter

• the photoelectric effect

• Compton scattering

• pair production

• simple scatter

• an x-ray photon (1-20keV) collides with an electron

• the photon has less energy than required to remove the electron

• the photon is scattered elastically

• the photoelectric effect

• an x-ray photon (<100keV) collides with an electron

• the photon is absorbed by the electron

• the electron gains energy

• the electron escapes the atom

• Compton scattering

• an x-ray photon (0.5-5MeV) collides with an electron

• the electron gains energy

• the electron escapes the atom

• the photon is scattered with reduced energy

• energy and momentum are conserved in the collision

• pair production

• an x-ray photon (>=1.02MeV) collides with a nucleus

• the photon disappears

• its energy is converted into an electron and a positron

• the most common attenuation coefficient in x-ray scanning

• the photoelectric effect

• the formula for the transmitted intensity of an x-ray photon

• I = I0e^(-µx)

  • i = transmitted intensity (Wm^-2)

  • I0 = initial intensity (Wm^-2)

  • µ = attenuation coefficient of the substance (m^-1)

  • x = thickness of the substance (m)