Gamma rays are produced when energy changes occur within the nucleus of atoms. Gamma rays are electromagnetic radiation of high frequency and energy, and they are produced by nuclear reactions, such as radioactive decay or nuclear fusion. When there is a change in the nucleus of an atom, for example, when a nucleus emits a particle or undergoes a rearrangement of its protons and neutrons, it can release gamma rays. These gamma rays are highly energetic and can penetrate through many materials, making them useful in a variety of fields, including medicine and industry.