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Radioactivity

The discovery of natural radioactivity in the late 19th century was a gradual process involving several stages.
On November 8, 1895, the German physicist Wilhelm Röntgen discovered an invisible form of radiation capable of producing an image on a photographic plate. The radiation was so unusual that he called it “X-rays.” By placing his hand in its path, he was able to capture an image of his finger bones. The technique of radiography was born.
On January 20, 1896, the mathematician Henri Poincaré presented Röntgen’s discovery to the French Academy of Sciences. One of its members, Henri Becquerel, wondered where these mysterious, invisible rays originated. Poincaré, himself fascinated with physics, suggested that Becquerel investigate a potential link between the fluorescence accompanying the emission of X-rays and the rays themselves. Fluorescence is the emission of light previously absorbed by certain substances.
After performing unsuccessful tests with various materials, Becquerel tried uranium salt crystals, which he exposed to sunlight on top of a photographic plate encased in black cardboard to shield it from the light. The plate was exposed through the cardboard, and Becquerel concluded that — once excited by light — this salt emitted X-rays.
Yet his most important discovery would come a few days later, by sheer coincidence. Becquerel wanted to duplicate his recent successful experiment with uranium and prepared the necessary equipment, before deciding against it, due to insufficient sunlight in Paris that day. Intending to try again later, he stored the photographic plates and uranium salts in a drawer. On March 1st, for the sake of completeness, he decided to develop the plates, which had remained inside the drawer. Much to his surprise, he discovered that they had been strongly exposed in the dark. This phenomenon was independent from the uranium’s fluorescence — uranium salt emits penetrating rays spontaneously, whether it has been exposed to sunlight or not.
Becquerel had just discovered radioactivity.
From 1898, Pierre and Marie Curie also became interested in the phenomenon and discovered polonium and radium. On March 26, 1900, Becquerel identified the beta radiation of radium as an emission of electrons, thus achieving the first detection of an elementary particle. Then Pierre Curie observed that radium emitted considerable energy, a million times greater than any known combustion energy, becoming the first to recognize nuclear energy.
The health hazards of radioactivity were not immediately understood. On the contrary, many therapeutic properties were attributed to radium and it became a popular tonic, prescribed in the form of powders, creams, beverages, compresses, toothpaste and even talcum powder for babies! A grain of radium was displayed at the 1904 Saint Louis World Fair in Missouri, to satisfy public curiosity.
The biological effects of the energy released by radioactivity were observed accidentally by Becquerel. One day, after carrying a vial of radium in his pocket for a few hours, he noticed a red spot on his skin that transformed into a burn. Pierre and Marie Curie confirmed this observation by experimenting on themselves. This was the birth of what would become radiation therapy. In 1903 Becquerel and the Curies received the Nobel Prize in Physics for their discovery of radioactivity, a breakthrough that paved the way for the future of nuclear physics, nuclear energy, the study of the structure of matter and elementary particle physics. In 1911, Ernest Rutherford established the existence of the atomic nucleus and gave the name “proton” to the hydrogen nucleus. He suspected the existence of a neutral proton, or “neutron,” which was identified by James Chadwick in 1932. In 1934, Frédéric and Irène Joliot-Curie discovered artificial radioactivity, and Enrico Fermi demonstrated that nuclei could capture neutrons. Four years later, Meitner, Hahn and Strassmann discovered nuclear fission. In early 1939, Frédéric Joliot identified the chain reaction phenomenon and along with it, the possibility of producing nuclear energy and nuclear weapons.
On December 2nd, 1942, the world’s first nuclear reactor, built by Fermi, became operational in Chicago.