A photographic emulsion impregnated with a radio- active thorium salt reveals the tracks of alpha particles when developed. As Ernest Rutherford & Frederick Soddy found in 1900-1902, the decay of thorium initiates a chain of decays. First one radioactive element is formed, then that decays to another, & so on. This is why the emulsion picture shows the tracks of two or more alpha particles emerging from the same spot: they are the result of successive stages in the decay of a single nucleus. (The length of the alpha tracks in this negative picture is about 0.03 mm).