Radioactivity is the spontaneous disintegration of atomic nuclei. This phenomenon was first reported in 1896 by the French physicist Henri Becquerel. Marie Curie and her husband Pierre Curie contributed further to the understanding of radioactivity. Their research led to the discovery of two new radioactive elements, polonium and radium, and forced scientists to change their ideas about the structure of the atom.
Radioactivity is the result of an atom trying to reach a more stable nuclear configuration. The process of radioactive decay, can be achieved via three primary methods; a nucleus can change one of its neutrons into a proton with the simultaneous emission of an electron (beta decay), by emitting a helium nucleus (alpha decay), or by spontaneous fission (splitting) into two fragments. Often associated with these events is the release of high energy photons or gamma rays. There are some other method of radioactive decay, but they are more exotic in nature.
Each individual radioactive substance has a characteristic decay period or half-life. A half-life is the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay. The radioactive isotope cobalt 60, which is used in radiation cancer therapy, has, for example, a half-life of 5.26 years. Thus after that interval, a sample originally containing 16 grams of cobalt 60 would contain only 8 grams of cobalt 60 and would emit only half as much radiation. After another interval of 5.26 years, the sample would contain only 4 grams of cobalt 60. Half-lives can range from thousands of years to milliseconds.
Sometimes after undergoing radioactive decay, the new atom is still left in a radioactive form. This means that the atom will decay again as it attempts to reach a stable nuclear state.