Natural Radioactive Decay Chains
Shell effects on nuclear stability have created an island of relative stability for nuclides near A = 230–240 and Z = 90–92. Three nuclides, 232Th, 238U, and 235U, have half-lives long enough for significant amounts to have survived since the heavy elements in the Earth’s crust were created. When one of these nuclides decays, it starts a journey that ends with an isotope of lead (Z = 82, A ≈ 208). The predominant steps in this journey are α and β decays, so that each of the long-lived parents heads a distinct chain. Each chain, as well as a fourth one that is extinct, is described.