The photoelectric effect:
The photoelectric effect is the most common way for low-energy photons to interact with matter. It also takes place with gamma rays. A gamma collides with an electron orbiting an atom, expelling the electron tfrom its atomic orbit. The electron explusion is followed by a rearrangement of the atom's electronic cohort to fill the gap. In the process the photon vanishes. When an outermost electron take the place of an interal expelled electron, energy is released in the form of a fluorescence photon not been shown in the picture above.