radioactivity: Discovery of Radioactivity
Natural radioactivity was first observed in 1896 by A. H. Becquerel, who discovered that when salts of uranium are brought into the vicinity of an unexposed photographic plate carefully protected from light, the plate becomes exposed. The radiation from uranium salts also causes a charged electroscope to discharge. In addition, the salts exhibit phosphorescence and are able to produce fluorescence. Since these effects are produced both by salts and by pure uranium, radioactivity must be a property of the element and not of the salt. In 1899 E. Rutherford discovered and named alpha and beta radiation, and in 1900 P. Villard identified gamma radiation. Marie and Pierre Curie extended the work on radioactivity, demonstrating the radioactive properties of thorium and discovering the highly radioactive element radium in 1898. Frédéric and Irène Joliot-Curie discovered the first example of artificial radioactivity in 1934 by bombarding nonradioactive elements with alpha particles.
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