When a [[photon]] ([[gamma ray]]) collides with an [[atom]], it may be completely absorbed and its total energy used to eject one of the orbital electrons from those surrounding the nucleus. Part of the photon's energy is used to overcome the binding energy holding the electron in the atom; the remainder serves to impart a velocity to the recoiling electron. In general, this photoelectric effect is greater for low energy incident gamma rays (below about 100 keV), and occurs at higher energies for atoms of higher atomic number. The rate of absorption varies only with the energy of the incident gamma ray and the nature of the atom.

Photoelectric absorption is the process which produces the high-speed ionizing particle (i.e., electron) which causes the [[scintillation]] to appear in the phosphors of [[scintillation  counter|scintillation detectors]]. Produces an effect in the [[formation]] which influences some [[density log|density logging]] measurements.

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