Gamma Ray:

The gamma ray is a photon of electromagnetic radiation along with an extremely short wavelength and high energy.  That is emitted from an unstable atomic nucleus and has high penetrating power.

There are three methods of attenuating (reducing the energy level of) gamma-rays: photoelectric effect, compton scattering, and pair production.

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