b) Attenuation

• define attenuation

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

• state the four attenuation mechanisms

• simple scatter

• photoelectric effect

• Compton scattering

• pair production

• describe simple scatter

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

• photon has less energy than required to remove the electron

• photon is scattered elastically

• describe photoelectric effect

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

• photon is absorbed by the electron

• electron gains energy

• electron escapes the atom

• describe Compton scattering

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

• electron gains energy

• electron is removed from the atom

• photon is scattered with reduced energy

• energy + momentum conserved in collision

• describe pair production

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

• photon disappears

• energy of the photon produces an electron and a positron

• state the most common attenuation coefficient in x-ray scanning

• photoelectric effect

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

• I = I0e^-lambdax

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