N. FEATHER / SCIENCE PHOTO LIBRARY N. FEATHER / SCIENCE PHOTO LIBRARY
Cloud chamber photograph of the tracks of alpha particles emitted from a source of the radioactive material thorium C' (polonium-212). Radioactive materials emit alpha particles at one or more specific energies. Most of the alphas in this photograph have the same energy & therefore travel the same distance (8. A single alpha is emitted with higher energy & travels 11. Alpha particles are nuclei of helium atoms & they are one of the three forms of "radiation" - along with beta & gamma rays - emitted by radioactive materials. The photo was taken by English physicist N. Feather in the 1920s.
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