Radon-222

Uranium-238 (238U) is a natural radioactive element that is present in all rocks and soils in various concentrations. Decay of 238U through a series of shorter-lived radionuclides eventually produces radium-226 (226Ra), which has a half-life of 1620 years. Radium-226 decays by alpha-particle emission directly to radon-222 (222Rn), which is short-lived (half-life = 3.82 days). Two other isotopes of radon are formed in natural decay chains, one from thorium-232 and one from uranium-235: radon-220 (220Rn) with a half-life of 56 seconds and radon-219 (219Rn) with a half-life of 4 seconds, respectively. Radon-222 has a long enough half-life to make it useful in geohydrologic studies, whereas the half-lives of 220Rn and 219Rn are too short to make them useful as tracers in environmental investigations.