SUMMARY

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 ? and ? 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.

1. 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 start a chain. Note that many radionuclides occur in chains, e.g., ¹⁰⁰Sn ?6 EC/ ? ⁺ ? ¹⁰⁰Ru, but early members of the chains have short half-lives and disappear before the chain is completed. The distinguishing characteristic of the natural chains is that they begin with a radionuclide with a long enough half-life that some of it has survived since the Earth was formed.

There is considerable variety in the modes of decay of heavy elements; if spontaneous fission is considered a...