Definition: | of an amount of a radionuclide in a particular energy state at a given time, the quotient of dN by dt, where dN is the expectation value of the number of spontaneous nuclear transitions from that energy state in the time interval dt:
NOTE – The SI unit of activity is the reciprocal second (s−1) and is given the special name becquerel (Bq). The earlier special unit of activity, still in temporary use, was the curie (Ci). 1 Bq = 1 s−1 = 2,703 × 10−11 Ci approximately.
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