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- In photelectric absorption the incident photon interacts with an electron from one of the inner orbiting shells of an atom.
- This electron is displaced from its orbit as a photelectron - and all the incident energy is lost in this process.
- An electron from an outer orbit (k) then drops into the inner orbit (l) to fill the ''vacancy''.
- As this electron moves to a lower energy shell it emits ''characteristic radiation'' which is specific to the absorbing material.
- Photoelectric absorption is highly dependent on the atomic number of the absorbing tissue and is most important at low kV when theCompton effectis reduced.