Activity of a radionuclide is defined as the rate of disintegration of the radionuclide. It is also called count rate and is represented by A. The activity of the sample at any instant is directely proportional to the number of atom left undecayed in the sample at that time. A plot of activity of the sample with time is as shown in fig. Obviously, activity varies exponentially with time and is represented as
SI unit of activity is Becquerel (Bq).
`1Bq=1"decay"//sec`.
