Antoine Henri Becquerel

French physicist

• Born: December 15, 1852; Paris, France
• Died: August 25, 1908; Le Croisic, France

In his experiments with uranium, French physicist and Nobel laureate Antoine Henri Becquerel discovered a source of incredible energy that required no human interaction to produce: radioactivity. His discovery opened up a new field of science called nuclear physics.

Primary field: Physics

Specialties: Nuclear physics; electromagnetism

Early Life

Antoine Henri Becquerel, called Henri for most of his life, was born on December 15, 1852, in Paris, France. Both his father, Alexandre Edmond Becquerel (1820–1891), and his grandfather, Antoine César Becquerel (1788–1878), were renowned physicists. As a result, he was aware of the importance of education early in life. He attended the Lycée Louis-le-Grand secondary school, and later, like many aspiring French scientists, the École Polytechnique (Polytechnic School).

Becquerel graduated from the Polytechnique in 1874, the same year he married Lucie Zoé Marie Jamin. He entered the government-sponsored École des Ponts et Chaussées (School of Bridges and Highways) later that year. When Becquerel graduated, he took a job as an engineer, and eventually became chief engineer of the École des Ponts et Chaussées in 1894.

During his career as a civil servant, Becquerel continued his scientific research. Becquerel’s son Jean was born in 1878, the same year that Becquerel became an assistant at the Muséum National d’Histoire Naturelle (Museum of Natural History). Becquerel also became the chair of applied physics at the Conservatoire des Arts et Métiers (Conservatory of Arts and Crafts), a position he took over when his father died in 1891. He succeeded his father’s position as chair of physics at the museum as well, and became a professor at the École Polytechnique in 1895. Jean Becquerel eventually became a respected physicist in his own right, taking over the assistant position at the museum after his father’s death. Bequerel remarried in 1890 after his wife’s death. He and his second wife, Louise Désirée Lorieux, had no children.

Life’s Work

Like his father, Becquerel was especially interested in phosphorescence, which is light emitted by a substance without an observable chemical reaction, such as combustion, that would normally result in light. Becquerel experimented with the polarization of light, as well as phosphorescence, in his early years. He also studied the Earth’s magnetism, specifically the effects of magnetic fields on light, and infrared radiation. Becquerel’s doctoral thesis from the École Polytechnique, which he completed in 1888, was concerned with crystals and their absorption of light.

Becquerel’s father had performed many experiments with uranium (in the form of a uranium salt, potassium uranyl sulfate), which phosphoresces when exposed to sunlight, and he always had several pieces of uranium on hand for experiments. Those uranium fragments would eventually play an important role in his son’s most famous discovery. One year after receiving his doctorate, Henri Becquerel was awarded membership in the prestigious Académie Royale des Sciences (French Academy of Sciences) in 1889, like his father and grandfather before him. He was a highly respected physicist, with particular proficiency in phosphorescence, photography, and laboratory technique, all of which would be instrumental in his discovery of radioactivity several years later.

German physicist Wilhelm Conrad Röntgen attracted widespread attention in late 1895 with his discovery of X-rays. Becquerel was interested in the phosphorescent properties of the rays, and set out to discover more about the phenomenon. He wanted to know whether the phosphorescence was related to natural phosphorescence, and whether naturally phosphorescent materials would have the same effects as X-rays on a photographic plate.

The initial hypothesis on which Becquerel based his experimentation was that X-rays were related to the phosphorescence that certain substances attained after being exposed to sunlight. Becquerel thought that, after being exposed to sunlight, these substances would affect a photographic plate in the same way as X-rays. He tested this hypothesis in 1896 using his father’s uranium. Becquerel exposed some of the substance to sunlight, and found that it did leave a mark on a photographic plate, though he still did not know why.

When he attempted to confirm his results with a second experiment, Becquerel decided to see whether the phosphorescent uranium would create a shadow of a copper cross placed between the uranium and the photographic plate. However, there was very little sunshine during Becquerel’s second experiment, and the uranium never achieved a phosphorescent glow. Becquerel put his uranium and the photographic plate away together in a dark closet. Eventually, he decided to take out the photographic plate and develop it anyway, expecting it to show, at best, a very hazy image of the cross. Much to his surprise, the plate had a clear, bright shadow of the copper cross projected onto it. Becquerel knew that he had stumbled onto something unknown.

X-rays and phosphorescence were not responsible for this outcome, meaning that some inherent quality in the uranium was causing the result. Becquerel surmised that the uranium caused the air around it to become an electrical superconductor. Eventually he also learned that, unlike X-rays, the mysterious energy emitted by uranium could be deflected by electric or magnetic fields. The specific particles that caused the reaction later became known as alpha particles. Becquerel published seven papers on the phenomenon in 1896, but none of them drew much attention, nor did the two he published the following year.

Of the few scientists who recognized the distinction between other kinds of radiation and that discovered by Becquerel, Marie and Pierre Curie would make some of the most important discoveries in this new field. Marie Curie began to experiment with other substances to see if they produced the same effect, which she called radioactivity. In 1898, when Curie discovered the radioactive properties of another element, thorium, Becquerel’s discovery was recognized as part of an important new scientific discipline. His discovery also contributed to the mounting evidence against the indivisibility of the atom, a theory that was disproved when J. J. Thomson discovered the electron in 1897.

It would be several years before the benefits—and dangers—of radioactivity were discovered. Becquerel redoubled his research in the field after 1899, and eventually was able to measure the deflection of another kind of radioactive particle, called beta particles, and prove that they were identical to Thomson’s electrons. Becquerel made another discovery in 1901, when he noticed that he had been burned by a piece of the radioactive element radium that was in his vest pocket. Becquerel surmised that this property of radioactive elements could be useful in medical treatments.

In 1900, Becquerel was made an officer of the French Légion d’honneur (Legion of Honor). He was also a member of other prestigious scientific societies, including the Italian Lincean Academy and the Royal Academy of Berlin. In 1903, Becquerel was awarded the Nobel Prize in Physics, which he shared with Marie Curie and her husband Pierre.

Becquerel died in Le Croisic, France, on August 25, 1908.

Impact

Becquerel’s discovery of radioactivity began a new field of science called nuclear physics and opened up the related fields of nuclear chemistry and radiochemistry. When the International System of Units (SI) was developed in the 1960s, the unit of radioactive decay was named a becquerel (Bq). Following Becquerel’s discoveries, American biologist Samuel Prescott discovered that irradiation, or exposing an object to radiation, could kill harmful bacteria present in many food products. By 1930, both France and the United States were irradiating food such as meat, produce, and grains, and the practice has continued. Although exposure to high levels of radioactivity can be dangerous—as seen most dramatically in the radiation poisoning caused by the atomic bombs dropped on Japan during World War II—many hundreds of practical applications for the radioactivity first identified by Becquerel and his colleagues have been developed, from nuclear power to nuclear medicine.

Bibliography

Malley, Marjorie C. Radioactivity: A History of a Mysterious Science. New York: Oxford UP, 2011. Print. A historical and scientific account of the history of radioactivity. Topics range from the discovery of radioactivity and related phenomena, to applications of radioactivity and the ethical issues tied to its use.

Segrè, Emilio. From X-rays to Quarks: Modern Physicist and Their Discoveries. 1980. Mineola, NY: Dover, 2007. Print. Includes a detailed chapter on the discoveries made by Becquerel and the Curies.

Strutt, Robert John. The Becquerel Rays and the Properties of Radium. 1904. Whitefish, MT: Kessinger, 2007. Print. Reprint of the 1904 publication on the science behind radioactivity, written at a time when the field of physics was rapidly transforming.