Although the explosion of the atomic bomb over Hiroshima on August 6, 1945 is recognized as the beginning of the nuclear age, the study of nuclear chemistry actually began much earlier, in 1895, when the German physicist Wilhelm Konrad Roentgen accidentally discovered x rays. He found that x rays have a great penetrating power, so great that they can pass through walls.
In 1896, a French physicist by the name of Antoine Becquerel placed some salt that contained uranium on a photographic plate that was wrapped in black paper. When he developed the photographic plate, he found to his surprise that he had the image of the pile of salt. The uranium salt had taken a picture of itself! Becquerel concluded that the uranium was giving off some type of penetrating rays, and that these rays must be very strong to be able to pass through the black paper and expose the photographic plate. However, when he placed a thick barrier of lead between the salt and the photographic plate, the lead absorbed the rays. Becquerel realized that these penetrating rays were probably Roentgen's x rays.
At the same time there was in Paris a young Polish chemist, Marie Curie, who, with her husband Pierre, was working in the laboratories at the Sorbonne. The Curies became interested in Becquerel's problem. It was, in fact, Marie Curie who defined the ability of a substance to produce penetrating rays as Radioactivity. The Curies found that a radioactive substance seems to keep on and on, year after year, emitting these powerful penetrating rays.
In 1898, they discovered that the element thorium is radioactive, and so is polonium. And so, most of all, is radium.
Although early experimenters first thought that radioactive materials produced only x rays, they soon discovered that the situation was more complex. For example, when they allowed the radiation produced by uranium to pass through a magnetic field, they could detect three types of radiation.
The English physicist Lord Rutherford called these three types of radiation alpha rays, beta rays, and gamma rays, from the first three letters of the Greek alphabet.