What occurs during Compton scatter?

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Compton scatter happens when a high-energy photon collides with an outer-shell electron. The photon transfers part of its energy to that electron, which is ejected as a recoil electron, and the photon is deflected in a new direction with reduced energy. The amount of energy lost by the photon depends on the scattering angle—the bigger the angle, the more energy is transferred and the longer the photon’s wavelength becomes. This interaction is a major contributor to image fog and patient dose in diagnostic radiology because scattered photons can reach the film or detector from many directions. The outcome is a scattered photon plus a recoil electron. In contrast, the photoelectric effect involves the photon being completely absorbed and ejecting an inner-shell electron; coherent (Rayleigh) scattering involves elastic scattering with no ionization and essentially no energy loss; and pair production requires photon energy well above diagnostic ranges and results in an electron-positron pair rather than two photons.