A well defined range
This historic photograph shows the trajectories of 9 MeV alpha emitted from a radioactive source of thorium-C (polonium-212) inside a Wilson cloud chamber. The ‘shaving-brush’ aspect is due to the fact that alpha, issued with the same energy, have a well defined range and travel no farther than 11.5 cm in the chamber. The curve on the right illustrates the same effect is a different way : the percentage of alpha particles crossing a given thickness of matter, remains at 100% before falling sharply to 0 when this range is reached.