Photoelectric effect or photoelectric absorption (PEA) is a form of interaction of x-ray or gamma photon with the matter. A low energy photon interacts with the electron in the atom and removes it from its shell.
The probability of this effect is maximum when
- the energy of the incident photon is equal to or just greater than the binding energy of the electron in its shell (absorption or k edge) and
- the electron is tightly bound (as in K shell)
The electron that is removed is then called a photoelectron. The incident photon is completely absorbed in the process. Hence it forms one of the reasons for attenuation of X-ray beam as it passes through the matter.
To stabilize the atom, an outer-shell electron fill the vacancy in the inner shell and emits the excess energy as characteristic radiation (x-ray) or as an Auger electron, which has a binding energy less than the emitted energy.
PEA is related to the atomic number of the attenuating medium (Z), the energy of the incident photon (E) and the physical density of the attenuating medium (p) by: Z³ p / E³.
Therefore, if Z doubles, PEA will increase by a factor of 8 (because 2³ is 8) and if E doubles, PEA will reduce by 8. As small changes in Z can have quite profound changes in PEA this has practical implications in the field of radiation protection and is why materials with a high Z such as lead (Z = 82) are useful shielding materials. Conversely, the dependence of PEA on Z and E means that it only dominates up to approximately 30 keV when human tissues (Z = 7.4) are irradiated, above which the Compton effect dominates.
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