n Atoms that disintegrate by emission of corpuscular or electromagnetic radiations. The rays most commonly emitted are alpha or beta or gamma rays. The three classes are: Primary, which have half-life times exceeding 108 years. These may be alpha-emitters or beta-emitters. Secondary, which are formed in radioactive transformations starting with U238, U235, or TH232. Induced, having geologically short lifetimes and formed by induced nuclear reactions occurring in nature. All these reactions result in transmutation (Serway RA, Faugh JS, Bennett CV (2005) College physics. Thomas, New York; Weast RC (ed) Handbook of chemistry and physics, 52nd edn. CRC Press, Boca Raton, FL).