Three of the attenuation mechanisms change or destroy the X-ray photon.Photoelectric effect (below about 100 keV): the photon is completely absorbed by an inner electron of an atom, and that electron is ejected. The photon disappears. The chance of this rises steeply with proton number, so bone (containing calcium) absorbs much more strongly than soft tissue. This is the main mechanism that makes X-ray images.Compton effect (about 0.5 to 5 MeV): the photon interacts with an outer electron. The electron is ejected and the photon is scattered with less energy, so it has a longer wavelength and travels in a new direction. Energy and momentum are shared between the electron and the scattered photon.Pair production (above 1.02 MeV): a photon passing close to a nucleus turns into an electron and a positron. The rest energy of each particle is 0.511 MeV, so the photon needs at least 2 × 0.511 = 1.02 MeV. Any extra energy becomes kinetic energy of the pair. The positron later annihilates with an electron.
Worked example: find the minimum energy and maximum wavelength for pair production.Emin = 2mec2 = 2 × 9.11 × 10−31 × (3.00 × 108)2 = 1.64 × 10−13 J
λmax = hc / Emin = (6.63 × 10−34 × 3.00 × 108) / 1.64 × 10−13 = 1.21 × 10−12 m Photons with a longer wavelength than this cannot produce an electron-positron pair.
Worked example: find the minimum energy and maximum wavelength for pair production.