The Thorium Decay Chain, often referred to as the Thorium Series, is one of the three major naturally occurring decay series. It begins with the primordial nuclide Thorium-232 () and concludes with stable Lead-208 ().
The chain consists of 11 radionuclides linked by a series of alpha decays and beta decays. Because the mass numbers of all isotopes in this chain are multiples of 4, it is also known as the 4n series.
In mineral sands mining and monazite processing, the Thorium Series represents the dominant radiological risk. The buildup of radium and its progeny in pipes and processing circuits creates external radiation fields that must be managed through specialized monitoring.