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Specific activity is the activity per unit mass of a radionuclide and is a physical property of that radionuclide.
Activity is a quantity related to radioactivity, and is defined as the number of radioactive transformations per second that occur in a particular radionuclide. The unit of activity is the becquerel , which is defined as one radioactive decay per second. The older, non-SI unit of activity is the curie , which is 3.7×10 radioactive decay per second.
Since the probability of radioactive decay for a given radionuclide within a set time interval is fixed , the number of decays that occur in a given time of a given mass of that radionuclide is also a fixed.
Thus, specific activity is defined as the activity per unit mass of a particular radionuclide. It is usually given in units of Bq/kg, but another commonly used unit of activity is the curie , allowing specific activity to be given the unit Ci/g. The amount of specific activity should not be confused with level of exposure to ionizing radiation and thus the exposure or absorbed dose. The absorbed dose is the quantity important in assessing the effects of ionizing radiation on humans.