The older, non-SI unit of activity is the curie (Ci), which is 3.7×1010 radioactive decays per second.

Another unit of activity is the rutherford, which is defined as 1×106 radioactive decays per second.

The specific activity should not be confused with level of exposure to ionizing radiation and thus the exposure or absorbed dose, which is the quantity important in assessing the effects of ionizing radiation on humans.

Since the probability of radioactive decay for a given radionuclide within a set time interval is fixed (with some slight exceptions, see changing decay rates), the number of decays that occur in a given time of a given mass (and hence a specific number of atoms) of that radionuclide is also a fixed (ignoring statistical fluctuations).

Half-life T1/2 is defined as the length of time for half of a given quantity of radioactive atoms to undergo radioactive decay: Taking the natural logarithm of both sides, the half-life is given by Conversely, the decay constant λ can be derived from the half-life T1/2 as The mass of the radionuclide is given by where M is molar mass of the radionuclide, and NA is the Avogadro constant.

Experimentally measured specific activity can be used to calculate the half-life of a radionuclide.

Where decay constant λ is related to specific radioactivity a by the following equation: Therefore, the half-life can also be described by One gram of rubidium-87 and a radioactivity count rate that, after taking solid angle effects into account, is consistent with a decay rate of 3200 decays per second corresponds to a specific activity of 3.2×106 Bq/kg.

can be calculated by: For instance, on average each gram of potassium contains 117 micrograms of 40K (all other naturally occurring isotopes are stable) that has a

of 1.277×109 years = 4.030×1016 s,[4] and has an atomic mass of 39.964 g/mol,[5] so the amount of radioactivity associated with a gram of potassium is 30 Bq.

The specific activity of radionuclides is particularly relevant when it comes to select them for production for therapeutic pharmaceuticals, as well as for immunoassays or other diagnostic procedures, or assessing radioactivity in certain environments, among several other biomedical applications.