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An incident photon may also transfer its energy to an orbital (generally inner‐shell) electron. This process is called the photoelectric effect and the ejected electron is called a photoelectron (Figure 2.4). This electron leaves the atom with an energy equal to the energy of the incident gamma ray diminished by the binding energy of the electron. An outer‐shell electron then fills the inner‐shell vacancy and the excess energy is emitted as an X‐ray.

Table 2.1 lists the predominant photon interactions in some common materials.

Figure 2.1 Predominant type of interaction for various combinations of incident photons and absorber atomic numbers.

Figure 2.2 Compton scattering.