Radioactive Activity
The number of nuclear disintegrations occurring in a sample per unit time.
Formula / equation
A = λN
Unit
Bq
In depth
Activity is proportional to the number of undecayed nuclei present, so it falls exponentially with exactly the same half-life as the nuclide itself. It measures the rate of emission, not the biological harm caused, which depends on the type and energy of the radiation.
Examples in the real world
A source falling from 8000 Bq to 1000 Bq over three half-lives.