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.