NATURAL AND ARTIFICIAL RADIOACTIVITY.
Nuclear reactions which occur spontaneously are said to be an example of natural radioactivity. There are three naturally occurring radioactive series among the elements in the periodic table. These are known as the uranium series, the actinium series and the thorium series, each named after the element at which the series start (except the actinium series which starts with a different uranium isotope). Each series decays through a number of unstable nuclei by means of alpha and beta emmission, until each series end on a different stable istope of lead.
Not all nuclear reactions are spontaneous. These reactions occur when
stable isotopes are bombarded with particles such as neutrons. This
method of inducing a nuclear reaction to proceed is termed artificial
radioactivity. This meant new nuclear reactions, which wouldn't have
been viewed spontaneously, could now be observed. Since about 1940, a
set of new elements with atomic numbers over 92 (the atomic number of
the heaviest naturally occurring element, Uranium) have been
artificially made. They are called the transuranium elements.