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Natural Radioactive Decay Chains

Abstract

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.

Introduction

Radioactivity was discovered (1896) from observations of radioactivity occurring in natural radioactive chains. There are other instances of natural radioactivity, such as 14C, 40K, and 187Re, but none of these nuclides leads to a radioactive product and therefore starts a chain. Note that many radionuclides occur in chains, e.g., 100Sn → 6 EC/β+