 Activity is the number of decays per second.  It is measured in Becquerels (Bq).  1 Curie (Ci) = 3.7 x Bq.  Everything emits some kind of radiation and therefore has some kind of activity.

1kg U-238 emits 1.26 x 10 7 α particles each second so it’s activity is:  1.26 x 10 7 Bq = 12.6 MBq  Ci = 3.4 mCi  Every second 1.26 x 10 7 U-238 atoms are converted into 1.26 x 10 7 Th-234 atoms and 1.26 x 10 7 alpha particles are emitted.

 As the atoms decay, the original element changes into a new element.  The time taken for exactly half the original atoms to decay is called the half-life of the isotope.  The higher the activity the shorter the half-life.  The lower the activity the longer the half-life.