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Radioactive decay series

When a radioactive nucleus decays another nucleus is formed. This new nucleus (called a daughter product) is often radioactive itself. It then decays again and again until finally a stable nucleus is reached. This process is called a decay series or a decay chain.

One of the most well known is the decay series that starts with uranium 238. It is particularly interesting because it includes radon 222 that is the radioactive gas that is responsible for about 50% of the radioactivity that we receive as background radiation.

Decay series for uranium 238
Isotope Particle emitted Half life
Uranium 238 alpha 4 500 million years
Thorium 234 beta 24 days
Protactinium 234 beta 1.2 minutes
Uranium 234 alpha 250 thousand years
Thorium 230 alpha 80 thousand years
Radium 226 alpha 1620 years
Radon 222 beta 1.2 minutes
Polonium 218 alpha 3.1 minutes
Lead 214 beta 27 minutes
Bismuth 214 beta 20 minutes
Polonium 214 alpha 0.00016 s
Lead 210 beta 19 years
Bismuth 210 alpha 250 thousand years
Polonium 210 beta 5 days
Lead 206   stable

To help you understand the table have a look at radon 222. It has a nucleus containing 86 protons and 136 neutrons. It emits an alpha particle (2 protons and 2 neutrons) and becomes a polonium 218 nucleus (84 protons and 134 neutrons). The polonium then emits an alpha particle to become lead 214 (82 protons and 132 neutrons). This emits a beta minus particle (an electron) to become bismuth 214 (83 protons and 131 neutrons) and so on down the series.


© Keith Gibbs 2020