at which the nuclei of a radioactive substance disintegrate due to radioactive decay. [P9.2]
is: the rate at which the nuclei of a radioactive substance disintegrate due to radioactive decay. [P9.2]
is also: a measure of the rate of emission of 
-particles, 
-particles or 
-radiation from a radioactive isotope. [P9.2]
is related: to the number 
of unstable nuclei of decay constant 
in a pure sample (containing only a single type of radionuclide) by 
[P9.2]
has as its SI unit: the becquerel (Bq). 1 Bq = 1 decay per second. The non-SI unit of activity, the curie (Ci, 
) is also in common use. [P9.2]
See activity law.
Copyright 1997, The Open University