Specific activity several other biomedical applications. Specific activity Specific activity is the activity per quantity of a radionuclide and is a physical property of that radionuclide. Activity is a quantity related to radioactivity. The SI unit of activity is the becquerel (Bq), equal to one reciprocal second. The becquerel is defined as the number of radioactive transformations per second that occur in a particular radioactive isotope. Its related non-SI unit equivalent is the Curie (Ci) which is transformations per second. Since the probability of radioactive decay for a given radionuclide is a fixed physical quantity (with some slight exceptions, see changing decay
2019 redefinition of SI base units seven base units whose definitions are unconstrained by that of any other unit and another twenty-two named units derived from these base units. The metre was redefined in terms of a wavelength of krypton-86 radiation, making it derivable from universal natural phenomena, but the kilogram remained defined in terms of a physical prototype, leaving it the only artefact upon which the SI unit definitions depend. Although designed for long-term stability, the masses of the kilogram prototype and its secondary copies have shown small variations over time relative to each other, and they are not thought to be adequate for the