Photoelectric absorption The probability of a photoelectric absorption per unit mass is approximately proportional to , where is the atomic number and is the energy of the incident photon. This rule is not valid close to inner shell electron binding energies where there are abrupt changes in interaction probability, so called absorption edges. However, the general trend of high absorption coefficients and thus short penetration depths for low photon energies and high atomic numbers is very strong. For soft tissue, photoabsorption dominates up to about 26 keV photon energy where Compton scattering takes over. For higher atomic number substances, this limit is higher. The high amount of calcium () in bones, together with their high density, is what makes them show up so clearly on medical radiographs.
Photoelectric absorption The probability of a photoelectric absorption per unit mass is approximately proportional to , where is the atomic number and is the energy of the incident photon. This rule is not valid close to inner shell electron binding energies where there are abrupt changes in interaction probability, so called absorption edges. However, the general trend of high absorption coefficients and thus short penetration depths for low photon energies and high atomic numbers is very strong. For soft tissue, photoabsorption dominates up to about 26 keV photon energy where Compton scattering takes over. For higher atomic number substances, this limit is higher. The high amount of calcium () in bones, together with their high density, is what makes them show up so clearly on medical radiographs.