Answer:
Gamma rays are extremely high energy electromagnetic radiation. The source of gamma rays is nuclear "change" like radioactive decay. Radioisotopes, which are either the natural or artificial radioactive isotopes of elements, occur in river deltas, granite, some older paints, some older ceramics, nuclear waste, and specifically irradiated materials that have been made radioactive to be applied to medical devices (like 60Co "pencils" used for medical treatment) or industrial imaging or irradiation sterilization equipments.