Becquerel, Antoine-Henri

Becquerel, Antoine-Henri

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FRENCH PHYSICIST
1852–1908

Antoine-Henri Becquerel was born the son of the physicist Alexandre-Edmond Becquerel, and the grandson of the physicist Antoine-César Becquerel, and it is not surprising that he followed in their footsteps. It is also not surprising that his research interests centered around solar radiation and phosphorescence, as these are phenomena that his father had investigated. He entered the École Polytechnique, in Paris, in 1872, which he left in 1874 and to which he subsequently returned. Becquerel received a doctorate degree from the Faculty of Sciences of Paris in 1888. In 1892, he was appointed professor of applied physics in the Department of Natural History at the Paris Museum, and in 1895, professor of physics at the École Polytechnique.

Becquerel's early work focused on plane-polarized light, the phenomenon of phosphorescence (in which certain compounds glow after being exposed to direct light), and the absorption of light by crystals. But all of his early research became overshadowed by his discovery of natural radioactivity. Although Becquerel did not initially comprehend what he was observing, his landmark discovery of radioactivity paved the way for a new understanding of the atom and atomic structure.

On February 24, 1896, Becquerel attended a meeting of the French Academy of Science and presented a short paper (one of the quickest methods in France at that time for disseminating results). One can well imagine Becquerel's excitement as he reported his results to the members of the academy.

One wraps a Lumiere 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. (Becquerel Comptes Rendus )

From this simple experiment, Becquerel concluded that the phosphorescent substance had to be emitting a type of ray that was passing through the paper and reducing the silver in the emulsion. This would seem to make sense, as the production of X rays, discovered a few years earlier by Wilhelm Röntgen, is accompanied by a soft glowing spot at the surface of the cathode ray tube. Becquerel decided to probe his unusual rays a little further. One week later, on March 2, 1896, Becquerel was back before the French Academy with the results of his further experiments. He had continued his experiments using a double sulfate salt of uranium and potassium

(potassium uranium sulfate monohydrate), which has a strong but short-lived phosphorescence.

He carefully wrapped his photographic plates in black paper, coating the paper with a crust of the uranium double salt, and upon exposure to sunlight he once again observed the "signature" of the phosphorescenceinduced rays. However, repeating the experiment on Wednesday, February 26, and Thursday, February 27, he was frustrated by two days of only intermittent sunlight. And because the Sun made no appearance on the two days following, on March 1 he developed his plates. Expecting to see only a faint silhouette resulting from the wrapped plates' intermittent exposure to sunlight, he was surprised to see that the silhouettes appeared with great intensity.

Becquerel suspected that the rays that produced the silhouettes emanated from the uranium salt itself, and that the small amount of sunlight was of no consequence. He arranged three more experiments, in which photographic plates were kept completely in the dark but put in direct contact with: (1) the salt; (2) a thin sheet of glass; and (3) a thin sheet of aluminum. He surmised that the glass would eliminate any possibility that a silhouette was the consequence of a chemical reaction, and that the aluminum would block the mysterious rays.

Developing the photographic plates, Becquerel observed an intensely defined silhouette on the first two plates, and a clear but considerably weaker silhouette on the third. Because he had double-boxed his plates inside his dark room and had placed the ensembles inside a drawer that he then closed, he was able to conclude that his mysterious rays were not related to phosphorescence and were not induced by sunlight.

It was another four years before Becquerel's radiation became understood as the production of β -rays (high energy electrons), but by then there was no question that Becquerel had discovered the instability of some atomic nuclei, and that he was richly deserving of the 1903 Nobel Prize that he shared with Pierre and Marie Curie.

see also Curie, Marie Sklodowska; RÖntgen, Wilhelm.

Todd W. Whitcombe

Bibliography

Asimov, Isaac (1989). Asimov's Chronology of Science and Discovery. New York: Harper & Row.

Farber, Eduard (1952). The Evolution of Chemistry: A History of Its Ideas, Methods, and Materials. New York: Ronald Press.

Leicester, Henry M. (1956). The Historical Background of Chemistry. New York: Wiley.

Internet Resources

Becquerel, Antoine H. Comptes Rendus, translated. "On the Rays Emitted by Phosphorescence." Available from <http://webserver.lemoyne.edu/faculty/giunta>.

Becquerel, Antoine H. Comptes Rendus translated. "On the Invisible Rays Emitted by Phosphorescent Bodies." Available from <http://webserver.lemoyne.edu/faculty/giunta>.

Nobel e-Museum. "Antoine Henri Becquerel—Biography." Available from <http://www.nobel.se>.