Photoelectric Effect:
The photoelectric effect occurs when a low energy gamma strikes an orbital electron, as displays in Figure. The total energy of the gamma is expended in ejecting the electron from its orbit. The conclusion is ionization of the atom and expulsion of a high energy electron.
The photoelectric effect is most predominant along with low energy gammas and rarely occurs along with gammas having energy above 1 MeV (million electron volts).
Figure: Photoelectric Effect