Radioactivity Discovered
In 1896, the French physicist Henri Becquerel found that uranium salts give off a penetrating radiation without any outside source of energy. The discovery, made almost by accident, revealed that atoms are not the stable, unchanging units that nineteenth-century science had assumed. It opened the way to nuclear physics, modern medicine's use of radiation, and eventually nuclear power and weapons.
The background lay in the excitement over X-rays. In late 1895 Wilhelm Röntgen announced that a cathode-ray tube produced mysterious rays that passed through flesh and exposed photographic plates. The glowing glass of the tube led some scientists to wonder whether other glowing, or phosphorescent, materials might also emit such rays. Becquerel, who came from a family of physicists at the Muséum national d'Histoire naturelle in Paris and had studied phosphorescence, set out to test the idea using uranium compounds that his father and he had examined before.
His plan was to expose a uranium salt to sunlight, then place it on a photographic plate wrapped in black paper. If the plate darkened, the salt had emitted penetrating rays. In February 1896 he reported just that. Then, according to the traditional account, cloudy weather in Paris interrupted his experiments, and he put a prepared plate and uranium sample away in a drawer. When he developed the plate anyway in early March, he found a strong image. The uranium had affected the plate without being exposed to sunlight. Historians note that the story is neat and may be simplified, but the central conclusion is secure: the radiation was a property of uranium itself, not a response to light.
At first the finding drew far less attention than X-rays. The next major step came from Marie Curie, who made the study of these "uranium rays" the subject of her doctoral research. She showed that the intensity of the radiation depended on the amount of uranium present, suggesting it came from the atoms themselves, and coined the term "radioactivity." With Pierre Curie she then identified two new radioactive elements, polonium and radium. In 1903 Becquerel and the Curies shared the Nobel Prize in Physics.
The consequences were profound. Ernest Rutherford and others showed that radioactive atoms transform into different elements, and that the radiation comes in distinct types, later named alpha, beta and gamma. This led to the discovery of the atomic nucleus and, in the late 1930s, to nuclear fission. Radioactivity also gave medicine radiation therapy, gave geology radiometric dating, and eventually brought both nuclear energy and the dangers of radiation. The unit of radioactivity, the becquerel, preserves his name today.