Abstract: 7110 A. Introduction
1. Occurrence
a. Natural radioactivity: Uranium-238, uranium-235, and thorium-232 are naturally-occurring radioactive elements. These primordial radionuclides were present at the formation of earth and are ubiquitous trace constituents in the earth’s crust. Their radioactive progeny undergo decay by emitting alpha, beta, and gamma radiations until the respective decay chains terminate in stable (non-radioactive) isotopes of lead (206Pb, 207Pb, 208Pb). The decay series include radioisotopes such as 234U; 234Th, 230Th and 228Th;223Ra, 224Ra, 226Ra, and 228Ra; 219Rn, 220Rn and 222Rn; 210Po, 212Po and 214Po; 210Bi, 212Bi and 214Bi; and 210Pb, 212Pb, and 214Pb, and others, mostly members of the actinium decay chain.