radiation emitted in one of the three types of natural radioactivity. It is the most energetic form of electromagnetic radiation, with a very short wavelength of less than 10−10 m. Gamma rays are essentially very energetic X-RAYS emitted by excited nuclei. They often accompany alpha or beta particles, because a nucleus emitting those particles may be left in an excited (higher-energy) state. Gamma-ray sources are used in medicine to treat cancer, for diagnostic purposes, and to sterilize equipment and supplies, in industry in the inspection of castings and welds, and in food processing to kill microorganisms and retard spoilage. Gamma radiation in large doeses even over time can be very harmful and even cause cancer and/or death.