Any of four sets of unstable heavy atomic nuclei that undergo a series of alpha decay and beta decay until a stable nucleus is achieved.
The natural series are the thorium series, the uranium series, and the actinium series. These are headed by naturally occurring species of unstable nuclei that have half-lives comparable to the age of the earth. The fourth set, the neptunium series, is headed by neptunium-237, which has a half-life of 2 million years. Its members do not occur naturally but are artificially produced by nuclear reactions and have short half-lives.