Gamma irradiation is a short wave length high energy form of ionizing radiation, it is an effective sterilization utilizing the industrialized radioisotope Cobalt 60.

Gamma rays are photons, they travel at the speed of light and have zero charge or mass.  Gamma rays have enough energy to detach electrons from atoms or molecules in the matter in through which they pass, therefore ionizing the atom causing a reaction with the cells of organic matter. 

Cobalt 60 is created by charging natural Cobalt 59 slugs with a single neutron, this is done by introducing the slugs into a nuclear power reactor for up to 24 months where the Cobalt absorbs the additional neutron.  As an unstable atom, Cobalt 60 emits electrons and gamma rays.  This energy release is not high enough to become reactive, yet strong enough to be an effective sterilizer.