Shortly after the discovery of x-rays, another form of penetrating rays was discovered. In 1896, French scientist Henri Becquerel discovered natural radioactivity. Many scientists of the period were working with cathode rays, and other scientists were gathering evidence on the theory that the atom could be subdivided. Some of the new research showed that certain types of atoms disintegrate by themselves. Henri Becquerel discovered this phenomenon while investigating the properties of fluorescent minerals. One of the minerals Becquerel worked with was a uranium compound. Uranium ore produces naturally occurring gamma radiation. Becquerel's discovery was, unlike that of the x-rays, virtually unnoticed by laymen and scientists alike. Only a few scientists were interested in Becquerel's findings. It was not until the discovery of radium by the Curies two years later that interest in radioactivity became widespread.
While working in France at the time of Becquerel's discovery, Polish scientist Marie Curie became very interested in his work. Marie and her husband, French scientist Pierre Curie studied radioactive materials, particularly pitchblende, the ore from which uranium was extracted. They noticed that pitchblende was strangely more radioactive than the uranium extracted from it. They deduced that the pitchblende must contain traces of an unknown radioactive substance far more radioactive than uranium.
Through several years of work, they progressively concentrated the radioactive substances of several tons of pitchblende ore. Their work resulted in the identification of two new chemical elements. The first element, they named "polonium," after Marie's native country, Poland. The other element they named "radium," for its intense radioactivity. Radium became the initial industrial gamma ray source. The material allowed radiographs of castings up to 10 to 12 inches thick to be produced.
The couple became well known for their work, but they also became victims of radiation poisoning. When early scientists were working with naturally occurring radioactive materials, the effects of radiation on the human body were little understood, or were ignored in the haste to learn more about this new substance. By 1929, industrial radiation sources were becoming available for radiographing extremely thick materials. Exposure times were long, and often radiographers were exposed to excessive doses of radiation.
During World War II and the race to produce a nuclear weapon, much was discovered about radioactive materials, and manmade isotopes became available. These sources were smaller, and considerably stronger than the naturally occurring radioactive material. Manmade sources were developed to penetrate even thicker materials, however, they also cause more damage to persons exposed to the radiation. Many deaths and amputations occurred in this era of early experimentation and use of isotopes.