Spontaneous emission of ionizing radiation as a consequence of a nuclear reaction, or directly from the breakdown of an unstable nucleus.
In 1896, the French physicist Antoine Henri Becquerel accidentally found that a uranium-rich mineral called pitchblende emitted invisible, penetrating rays that could darken a photographic plate enclosed in an opaque envelope. While working on phosphorescent materials, he happened to place the pitchblende on black paper that he had used to cover a piece of ﬁlm. When he looked more carefully, he noticed that the ﬁlm had lots of patches on it, and that this did not happen when other elements were placed on the paper. He eventually concluded that some rays must be coming out of the uranium crystals to produce this eﬀect.
This means that rays carry energy, but pitchblende emits them continuously without any energy input. This is an apparent violation of the law of conservation of energy. It was soon evident that Becquerel's rays originated in the nuclei of the atoms.
The emission of these rays is called nuclear radioactivity, or simply radioactivity. The rays are called nuclear radiation. A nucleus that spontaneously destroys part of its mass to emit radiation is said to decay. A substance or object that emits nuclear radiation is said to be radioactive. Becquerel found radiation to be associated with certain elements, such as uranium. Uranium is radioactive whether it is in the form of an element or compound. In addition, radiation does not vary with temperature, pressure, or ionization state of the uranium atom. Since all of these factors affect electrons in an atom, the radiation cannot come from electron transitions, as atomic spectra do.
Marie and Pierre Curie
In 1898, Marie Curie began her doctoral study of Becquerel's rays. She and her husband, Pierre, soon discovered two new radioactive elements, which she named polonium, after her native land of Poland, and radium, because it radiates. These two new elements filled holes in the periodic table and displayed much higher levels of radioactivity than uranium. Over four years, working under poor conditions and spending their own funds, the Curies processed more than a ton of uranium ore to isolate a mere gram of radium salt.
Radium became highly desirable because it was about two million times as radioactive as uranium. The Curies' radium salt glowed visibly from the radiation. This radiation took its toll on them and other unaware researchers, though.
Shortly after Marie completed her PhD, both Curies and Becquerel shared the 1903 Nobel Prize in Physics for their work on radioactivity. Awarded the 1911 Nobel Prize in Chemistry for her discovery of two new elements, Curie remains the only person to win Nobel Prizes in both physics and chemistry. Marie's radioactive fingerprints on some pages of her notebooks can still expose film.
Marie suffered from radiation-induced lesions and died of leukemia, likely caused by radiation. She was active in research almost until her death in 1934. The following year, her daughter and son-in-law, Irene and Frederic Joliot-Curie, were awarded the Nobel Prize in Chemistry for their discovery of artificially induced radiation.