Decay: Radioactivity

Next Back


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

Two months after this discovery, the French physicist Henri Becquerel was performing an experiment where he wrapped different elements with black-coated photographic plates to measure whether these elements could emit rays. If an element did emit a ray, it would penetrate the black coating and expose the photographic plate. Much to his surprise, Becquerel found that a few elements, including uranium, emitted energetic rays without any energy input.

The significance of Becquerel's experiments is that some natural process was 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 (like x rays) from the decay of unstable atoms is called radioactivity.


Source: Hewitt, Paul. Conceptual Physics. Harper Collins College Publishers (San Francisco, 1993) pp 596.