molar mass of the radionuclide, and N_A is the Avogadro constant. Practically, the mass number A of the radionuclide is within a fraction of 1 % of the molar mass expressed in g/mol and can be used as an approximation.

Specific radioactivity a is defined as radioactivity per unit mass of the radionuclide:

Thus, specific radioactivity can also be described by

This equation is simplified to

When

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