Changes

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.