Cross section
How is the probability of photoelectric effect  occurring calculated?
Photoelectric effect
The photoelectric effect is one interaction mechanism between photons and atoms. It is one of 12 theoretically possible interactions. At the high photon energies comparable to the electron rest energy of , Compton scattering, another process, may take place. Above twice this () pair production may take place. Compton scattering and pair production are examples of two other competing mechanisms. Indeed, even if the photoelectric effect is the favoured reaction for a particular single-photon bound-electron interaction, the result is also subject to statistical processes and is not guaranteed, albeit the photon has certainly disappeared and a bound electron has been excited (usually K or L shell electrons at gamma ray energies). The probability of the photoelectric effect occurring is measured by the cross section of interaction, σ. This has been found to be a function of the atomic number of the target atom and photon energy. A crude approximation, for photon energies above the highest atomic binding energy, is given by:formula_19Here Z is atomic number and "n" is a number which varies between 4 and 5. (At lower photon energies a characteristic structure with edges appears, K edge, L edges, M edges, etc.) The obvious interpretation follows that the photoelectric effect rapidly decreases in significance, in the gamma ray region of the spectrum, with increasing photon energy, and that photoelectric effect increases steeply with atomic number. The corollary is that high-Z materials make good gamma-ray shields, which is the principal reason that lead (Z = 82) is a preferred and ubiquitous gamma radiation shield.