March 1, 1896: Radioactivity discovered by Becquerel

William Atkins
Thursday, 28 February 2008 19:01

Science - Energy

On February 26, 1896, Henri Becquerel stored a photographic plate with uranium salts lying on top of it. He intended to later perform an experiment on phosphorescent emissions stimulated by the Sun. However, something unusual happened!    

French scientist  Henri Becquerel (1852-1908) had wrapped together potassium uranyl sulfate and photographic plates within black material in preparation for an experiment with sunlight. However, several days later, on March 1, 1896, he found that the photographic plates were already exposed.

Somehow invisible rays, what he called “uranic rays,” from the uranium had made an imprint on the photographic plates. He forgot about his original experiment and investigated what would eventually be called the spontaneous emission of nuclear radiation.

Becquerel told the French academy of Sciences on January 24, 1896: “One wraps a Lumière photographic plate with a bromide emulsion in two sheets of very thick black paper, such that the plate does not become clouded upon being exposed to the sun for a day. One places on the sheet of paper, on the outside, a slab of the phosphorescent substance, and one exposes the whole to the sun for several hours. When one then develops the photographic plate, one recognizes that the silhouette of the phosphorescent substance appears in black on the negative. If one places between the phosphorescent substance and the paper a piece of money or a metal screen pierced with a cut-out design, one sees the image of these objects appear on the negative. … One must conclude from these experiments that the phosphorescent substance in question emits rays which pass through the opaque paper and reduces silver salts.” [Transcripts]

It took many more years for other scientists to discover the true nature of Becquerel’s discovery: radioactivity.

Radioactivity is the process, often called radioactive decay, by which an unstable nucleus of an atom loses energy by emitting radiation in the form of electromagnetic waves or particles.

New Zealand chemist Ernest Rutherford, in 1899, discovered alpha and beta radiation and, in 1900, French chemist and physicist Paul Villard discovered gamma radiation. Polish-French chemist and physicist Maria Curie and French physicist Pierre Curie demonstrated the radioactive properties of thorium and discovered the radioactive element radium in 1898.

Becquerel and the Curies shared the 1903 Nobel Prize in Physics for their work on radioactivity.

The SI (International System of Units) unit for radioactivity, the Becquerel (Bq), is named after Henri Becquerel.

A biography of Henri Becquerel appears on the Nobel Prize website: http://nobelprize.org/nobel_prizes/physics/laureates/1903/becquerel-bio.html.