The most complicated and therefore probably the most interesting decay
scheme of all is the decay of uranium to lead, discovered well over half
a century ago and still intensively studied. There are several reasons
for the interest.
First, uranium and lead are geochemically separated to a high degree,
not only on the small scale of an ore deposit but also on the scale of
the earth as a whole. Second, natural uranium has two isotopes with
half-lives that are neither too long nor too short to be useful (the
greater half-life almost exactly equaling the age of the earth), and
these half-lives differ from each other by a factor of about 6.3. That
leads to very important consequences, as we shall see. Third, uranium
and lead are both common, and techniques are available for extracting
them in measurable quantities from almost any natural material.
As a consequence of these happy circumstances, the study of uranium and
lead has contributed a great deal to understanding the earth’s history
and the processes that go on inside it. F. G. Houtermans, one of the
great pioneers in this study, jokingly called the method PLUMBOLOGY, and
it seems a useful name.