When photon energies are as high as the electron rest energy of 511 keV, yet another process, Compton scattering, may occur. Above twice this energy, at 1.022 MeV, pair production is also more likely. Compton scattering and pair production are examples of two other competing mechanisms. Even if the photoelectric effect is the favoured reaction for a particular interaction of a single photon with a bound electron, the result is also subject to quantum statistics and is not guaranteed. The probability of the photoelectric effect occurring is measured by the cross section of the interaction, σ. This has been found to be a function of the atomic number of the target atom and photon energy. In a crude approximation, for photon energies above the highest atomic binding energy, the cross section is given by: