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⁺