Radionuclides of natural origin
Radionuclides that occur naturally on Earth. They include 3H, 7Be, 14C, 40K and radionuclides in the decay chains of the primordial radionuclides. The primordial radionuclide decay chains are: the thorium series, headed by 232Th, the most abundant of all naturally occurring radionuclides and constituted mainly by 228Ra, 228Ac, 228Th, 224Ra, 220Rn, 216Po, 212Pb, 212Bi, 212Po, and 208Tl; the uranium series, headed by 238U and constituted mainly by 234Th, 234mPa, 234U, 230Th, 226Ra, 222Rn, 218Po, 214Pb, 214Bi, 214Po, 210Pb, 210Bi, and 210Po; and, less important in the context of this term, the actinium series, headed by 235U and constituted mainly by 231Th, 231Pa, 227Ac, 227Th, 223Fr, 223Ra, 219Rn, 215Po, 211Pb, 211Bi, and 207Tl.
ICRP Glossary entry - June 2019
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