Abel Niepce De Saint Vector, a photographer, was experimenting with verious chemicals including uranium compounds. He exposed them to sunlight and placed them, along with pieces of photographic paper, in a dark drawer. Upon opening the drawer, he found that some of the chemicals, including Uranium, exposed the photographic papers. Niepce thought he had found some new sort of invisible radiation and reported his findings to the French Academy of Science but no one investigated the effect any further. Almost forty years later a French physicist Henri Becquerel did almost the same thing.
In 1883 Becquerel began studying fluroscence and phosphorescence. Henri was especially interested in Uranium and its compounds. He was also skilled in photography. After knowing about Roentgen's discovery of X-ray in January 1896 at a meeting of the French Academy of Science, Becquerel began looking for a connection between the phosphorescence he had already been investigating and the newly discovered X-rays. Becquerel thought that the phosphorescent Uranium salts he had been studying might absorb sunlight and reemit it as X-rays which turned out to be wrong.
To test this idea Becquerel wrapped photographic plates in black paper so that sunlight couldn't reach them. He then placed the crystals of Uranium salt on top of the wrapped plates and put the whole set-up outside in the sun. When he developed the plates, he saw an outline of the crystals. He also placed objects such as coins or cut out metal shapes between the crystal and the photographic plate, and found that he could produce outlines of those shapes on the photographic plates. Becquerel took this as evidence that his was correct, which is the phosphorescent Uranium salts absorbed sunlight and emitted a penetrating radiation similar to X-rays. He reported this result at the French Academy of Science meeting on February 24, 1896. Seeking further confirmation of what he had found, he planned to continue his experiments. But the weather in paris became cloudy and overcast for the next several days in late February. Thinking he couldn't do any research without bright sunlight, Becquerel put his Uranium crystals and photographic plates away in drawer. On March 1, he opened the drawer and developed the plates, expecting to see a very weak image. Instead the image was amazingly clear. The next day, March 2, Becquerel reported at the French Academy of Science that the Uranium salts emitted radiation without any stimulation from sunlight.
Many people have wondered why Becquerel developed the plates at all on that cloudy March 1, since he didn't expect to see anything. Possibly he was motivated by simple scientific curiosity or perhaps he was under pressure to have something to report at the next day's meeting or may be he was simply impatient. Whatever the reason for developing the plates, Becquerel realized he had observed something significant. He did further tests to confirm that sunlight was indeed unnecessary and the Uranium salts emitted the radiation on their own. At first he thought that the effect was due to particularly long lasting phosphorescence but he soon discovered nonphosphorescent Uranium compound exhibited the same effect. In May he announced that the element Uranium was indeed emitting the radiation.
Becquerel initially believed his rays were similar to X-rays but his further experiments showed that unlike X-rays, which are neutral, his rays could be deflected by electric or magnetic fields. Many in the scientific community were still absorbed in following up on the recent discovery of X-rays. But in 1898 Marie and Pierre Curie in paris began to study the strange Uranium rays.
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| Marie Curie & Piere Curie |
They figured out how to measure the intensity of the radioactivity and soon found other radioactive elements, such as Polonium, Thorium and Radium. Marie Curie coined the term as Radioactivity to describe the new phenomenon. Soon Ernest Rutherford separated the new rays into Alpha, Beta and Gamma radiation. In 1902 Rutherford and Frederick Soddy explained radioactivity as a spontaneus transmutation of elements.
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| Henry Becquerel |
Becquerel and the Curies shared the
1903 Nobel Prize in Physics for their work on radioactivity.
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