The Compton effect:
This second process is the most common way for gamma photons with mid-level energies to interact with matter. It resembles the photoelectric effect in that gamma rays collide with atomic electrons, but in addition to expelling the electron the photon does not vanish and is replaced by an outgoing photon with a lower energy. The Compton effect predominates when the incoming photon's energy is far greater than that binding the colliding electron to its atom. The process gets its name from the American physicist Arthur Compton.