History of the metric system levels of the krypton 86 atom as the new standard for the metre. The standard metre artefact was retired. In 1960, Giorgi's proposals were adopted as the basis of the "Système International d'Unités" (International System of Units), the SI. This initial definition of the SI included six base units, the metre, kilogram, second, ampere, degree Kelvin and candela, and sixteen coherent derived units. The evolution of the SI after its publication in 1960 has seen the addition of a seventh base unit, the "mole", and six more derived units, the "pascal" for pressure, the "gray", "sievert" and "becquerel" for radiation,
Specific activity the radionuclide is given by where "m" is mass number of the radionuclide and "N" is the Avogadro constant. Specific radioactivity "a" is defined as radioactivity per unit mass of the radionuclide: In addition, decay constant "λ" is related to the half-life "T" by the following equation: Thus, specific radioactivity can also be described by This equation is simplified by When the unit of half-life converts a year For example, specific radioactivity of radium-226 with a half-life of 1600 years is obtained by This value derived from radium 226 was defined as unit of radioactivity known as Curie (Ci). Experimentally