The quantity which expresses the radiation producing potential of a given amount of
radioactive material is called “Activity”. The Curie (Ci) was originally defined as that
amount of any radioactive material that disintegrates at the same rate as one gram of
pure radium. The International System (SI)
unit for activity is the Becquerel (Bq), which is
that quantity of radioactive material in which
one atom is transformed per second. The
radioactivity of a given amount of radioactive
material does not depend upon the mass of
material present. For example, two one-curie
sources of the same radioactive material
might have very different masses depending
upon the relative proportion of nonradioactive atoms present in each source.