Radioactivity was discovered in 1896 by Henri Becquerel, a French scientist who was very interested in the recent discovery of X- rays. Becquerel was experimenting with fluorescent minerals, minerals that give off visible light after being exposed to sunlight. He wondered if fluorescent minerals emitted X rays in addition to visible light. From previous work with X-rays, Becquerel knew that they would penetrate a wrapped, light-tight photographic plate, exposing it as visible light exposes an unprotected plate. Thus, Becquerel decided to place a fluorescent uranium mineral on the plate when it was developed. Believing the uranium mineral emitted X- rays, he continued his studies until the weather turned cloudy. Storing a wrapped, protected photographic plate and the uranium mineral together during the cloudy weather, Becquerel returned to the materials later and developed the photographic plate to again find an image of the mineral. He concluded that the mineral was emitting an “invisible radiation” that was not induced by sunlight. Becquerel named the emission of invisible radiation radioactivity. Materials that have the property of radioactivity are called radioactive materials.
Becquerel’s discovery led to the beginnings of the modern atomic theory and to the discovery of new elements. Ernest Rutherford studied the nature of radioactivity and found that there are three kinds, which are today known by the first three letters of the Greek alphabet-alpha (a), beta (b), and gamma. These Greek letters were used at first before the nature of the radiation was known. Today, an alpha particle (sometimes called an alpha ray) is known to be the nucleus of a helium atom, that is, two protons and two neutrons. A beta particle (or betaray) is a high-energy electron. A gamma ray is electromagnetic radiations, as is light, but of very short wavelength.
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