Specific activity

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Activity
Ra 226 radiation source. Activity 3300 Bq (3.3 kBq)
Common symbols	A
SI unit	becquerel
Other units	rutherford, curie
In SI base units	s−1

Specific activity (symbol a) is the activity per unit mass of a radionuclide and is a physical property of that radionuclide.^[1][2] It is usually given in units of becquerel per kilogram (Bq/kg), but another commonly used unit of specific activity is the curie per gram (Ci/g).

In the context of radioactivity, activity or total activity (symbol A) is a physical quantity defined as the number of radioactive transformations per second that occur in a particular radionuclide.^[3] The unit of activity is the becquerel (symbol Bq), which is defined equivalent to reciprocal seconds (symbol s^-1). 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).