Gamma ray<span> (also called </span>gamma radiation<span>), denoted by the lower-case Greek letter </span>gamma,<span> is penetrating </span>electromagnetic radiation<span> of a kind arising from the </span>radioactive decay<span> of </span>atomic nuclei<span>. It consists of </span>photons<span> in the highest observed range of </span>photon energy<span>. </span>