Radioactivity is discovered accidentally by French physicist Antoine Henri Becquerel.
Becquerel was investigating German colleague Wilhelm Roentgen’s work (see here) on phosphorescence in uranium salts when he made his discovery.
While conducting an experiment using photographic plates, Becquerel found plates that were already fully exposed before being subjected to bright sunlight. After further investigation, he concluded that fluorescent uranium salts that had been placed next to the photographic plates (which were wrapped in thick black paper) emitted their own nuclear radiation. The uranium did not depend on the sun or other external light source to excite it.
Radioactivity is triggered by the spontaneous disintegration of atomic nuclei, resulting in radiant energy in the form of alpha, beta, or gamma rays.
Becquerel, the scion of a distinguished scientific family, shared the 1903 Nobel Prize in Physics with Pierre and Marie Curie (see here) for their combined work in the field of radioactivity. He was also elected permanent secretary of l’Académie des Sciences in 1908, the year he died. He’s remembered today in a metric unit: the becquerel is the radiation caused by one nuclear disintegration per second, or approximately twenty-seven picocuries.—TL