Radioactivity was originally a natural phenomenon in which unstable nuclei spontaneously emitted rays. The discovery of radioactivity rays has revealed to scientists a clearer picture of the composition of the nuclei of atoms, and has opened up vast new fields of nuclear and atomic physics.
A French scientist, Henri Becquerel, discovered radioactivity in February 1896, a few months after the discovery of X-rays. He was investigating whether phosphorescent substances emit X-rays. A phosphorescent substance is one, which glows after the incident radiation is cut off. He was experimenting with a compound of uranium, which was considered as a worthless metal at that time.
He wanted to expose the crystals to sunlight to study the phosphorescent effect. Since the weather was dull, he wrapped the crystals in black paper and kept them in a drawer. There were photographic plates in the drawer which were also wrapped in black paper.
Since the weather remained dull, he developed the photographic plates out of curiosity and, to his surprise , discovered that the plates were fogged and had the outline of the crystals on them. He traced the effect to be due to a radiation naturally emitted by the uranium crystals. The rays were originally known as Becquerel rays. In 1903, Becquerel was awarded the Nobel Prize for his discovery.
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Madame Curie and her husband, Pierre Curie, became interested in the phenomenon and in 1898 isolated two highly radioactive elements from pitchblende, an ore of uranium. The first was called ‘polonium’ and the second ‘radium’. Radium is a million times more radioactive than uranium. The curies rename the phenomenon ‘radioactivity’.