The strength of a radioactive source is called its activity, which is defined as the rate at which the isotope decays. Specifically, it is the number of atoms that decay and emit radiation in one second.
Activity of a given amount of radioactive material is calculated as the decay constant (related to the half-life T) multiplied by the number of radioactive nuclei. One kilogram of a pure radioactive isotope with half-life T[sec] has activity R[Bq/kg]:
where N = 6.023´1026 /kmol is the Avogadro number, A[kg/kmol] is the mass number, ln 2 = 0.693, and T is the half-life in seconds.