|Radioactivity indicates the property
of an unstable atomic nucleus changing (decaying) into a different
atomic nucleus and emitting radiation...
other words, decay of the atomic nucleus...
For that reason, the intensity of radioactivity is expressed
as the number of atomic nuclei that decay per second. The
intensity of radioactivity and the intensity of radiation
are not the same.
of 1 nucleus per second = 1 becquerel
Model of decay of a 60Co atomic nucleus