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In the late 1800s the German physicist, Wilhelm Röntgen, discovered a strange new ray produced when an electron beam struck a piece of metal. Since these were rays of an unknown nature, he called them "x rays".

Two months after this discovery, the French physicist, Henri Becquerel, was studying fluorescence, when he found that photographic plates were exposed in the presence of some ores, even when the plates were wrapped in black paper. Becquerel realized that these materials, which included uranium, emitted energetic rays without any energy input.

Becquerel's experiments showed that some natural process must be responsible for certain elements releasing energetic x rays. This suggested that some elements were inherently unstable, because these elements would spontaneously release different forms of energy. This release of energetic particles due to the decay of the unstable nuclei of atoms is called radioactivity.