Caption: Cloud chamber photograph of the track of a fast beta ray traversing the chamber from top to bottom. Beta rays were originally discovered by Rutherford as one of the three forms of radiation emitted by radioactive materials (the others being alpha & gamma rays), but they were soon identified as ordinary, though energetic, electrons. The short thick tracks in the picture are not caused by the beta ray electron; they are due to other, less energetic electrons which have been knocked from atoms in the gas filling the chamber by invisible X-rays. The photograph was taken by C.T.R. Wilson, the inventor of the cloud chamber.
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