The natural radioactivity of rocks:
We find the majority of naturally-occurring radioactive elements in rocks. This section of the nucleus graph shows the successive radioactive disintegrations that lead from uranium-238 to radon. The ‘descendants’ of uranium-238, present in trace amounts in these minerals, emit alpha and beta rays which remain trapped inside the rocks. Gamma rays, on the other hand, are able to escape. The sixth descendant of uranium – radon, as shown above – is a gas. Large amounts of it are therefore able to escape from the ground, thereby releasing radiation into the atmosphere.
IN2P3