Decay chain

A decay chain, also called a radioactive series, is a sequence of nuclides in which each nuclide transforms into the next by radioactive decay until a stable nuclide is reached. There are three "classical" decay chains, which describe the decay of the naturally-occuring actinoids; a fourth long decay chain has become extinct in natural sources, but is known from artificially-produced radionuclides. Shorter decay chains describe the decay of the transfermium elements and lighter non-actinoid radionuclides.

The principle of a decay chain comes from the radioactive displacement law, deduced in 1913 by Fajans, Soddy and Russell. The original version of the law, which describes the most common forms of radioactive decay, is that

• alpha decay leads to a nuclide with an atomic number two lower than the decaying nuclide, and a mass number four lower;
• beta decay leads to a nuclide with the same mass number as the decaying nuclide but with an atomic number one higher.

Actinoid decay chains

Actinium (4n+3) series

Uranium-235 (α, 7.04 × 108 a)
Thorium-231 (β−, 25.52 h)
Protactinium-231 (α, 3.276 × 104 a)
Actinium-227 (21.772 a)
α, 1.38%	β−, 98.62%
Francium-223 (22.00 min)	Thorium-227 (α, 18.68 d)