CT: When an atom of a radioisotope decays, it gives off some of its excess energy as radiation in the form of gamma rays or fast-moving sub-atomic particles. If it decays with emission of an alpha or beta particle, it becomes a new element. One can describe the emissions as gamma, beta and alpha radiation. All the time, the atom is progressing in one or more steps towards a stable state where it is no longer radioactive.
Another source of nuclear radioactivity is when one form of a radioisotope changes into another form, or isomer, releasing a gamma ray in the process. The excited form is signified with an “m” (meta) beside its atomic number, eg technetium-99m (Tc-99m) decays to Tc-99. Gamma rays are often emitted with alpha or beta radiation also, as the nucleus decays to a less excited state.
Apart from the normal measures of mass and volume, the amount of radioactive material is given in becquerel (Bq), a measure which enables us to compare the typical radioactivity of some natural and other materials. A becquerel is one atomic decay per second*, and each disintegration produces some ionising radiation.
S: http://www.world-nuclear.org/info/Safety-and-Security/Radiation-and-Health/Radiation-and-Life/ (last access: 5 June 2015)
N: 1. Henri Becquerel, in full Antoine-Henri Becquerel (born December 15, 1852, Paris, France—died August 25, 1908, Le Croisic), French physicist who discovered radioactivity through his investigations of uranium and other substances. In 1903 he shared the Nobel Prize for Physics with Pierre and Marie Curie.
He was a member of a scientific family extending through several generations, the most notable being his grandfather Antoine-César Becquerel (1788–1878), his father, Alexandre-Edmond Becquerel (1820–91), and his son Jean Becquerel (1878–1953).
2. A becquerel (Bq) is a derived metric SI (System International) measurement unit of radioactivity. One becquerel (1Bq) is defined as the radioactivity in which one nucleus decays per second.
S: 1. EncBrit – http://global.britannica.com/EBchecked/topic/58020/Henri-Becquerel (last access: 5 June 2015). 2. http://www.aqua-calc.com/what-is/radioactivity/becquerel (last access: 5 June 2015).
CR: nuclear energy