Radioactivity 09 : Spontaneous transformation of the nucleus of an atom; this resulting in a new element, generally with the emission of alpha or beta particles often accompanied by gamma rays
U.S. Environmental Protection Agency, Radiation Glossary, Office of Radiation and Indoor Air Radiation Protection Division, Washington, DC, USA 2013. http://www.epa.gov/radiation/glossary/index.html#a , (Dzstr/ Rad Emr - Amer) {823}