C. POWELL, P. FOWLER & D. PERKINS / SCIENCE PHOTO LIBRARY C. POWELL, P. FOWLER & D. PERKINS / SCIENCE PHOTO LIBRARY
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.
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