The basic unit of measure for describing the activity (radioactivity) of a quantity of radioactive material is the curie, named after Marie Curie. A quantity of radioactive material is considered to have an activity of 1 curie or 1 C, when 37 billion of its atoms decay (disintegrate) in one second. In scientific terms, this is expressed by the equation: 1C = 3.7 X 1010 disintegrations/sec. Remember that we said each isotope has its own decay pattern. If the rate of decay is greater than 37 billion atoms in one second, then the source would have an activity greater than one curie, and if that source had fewer than 37 billion atoms decaying in one second, its activity would be less than one curie.
This leads us to another distinction among radioactive materials. That is, one disintegration in a radioactive source does not necessarily result in the same amount of radiation emission.
Let's look at an example
In a Cobalt-60 source (Co-60) each atom decays by emitting a beta particle. Almost immediately additional energy adjustments are made in the atom and two gamma rays are emitted. Each of the gamma rays possess a certain energy level that is always the same. Therefore, when a CO-60 source decays, each disintegration of an atom results in two gamma rays. Another example of characteristic decay patterns is that of Thulium-170 (Tm-170). When TM-170 decays, approximately ¼ of its atoms emit beta particles and an associated gamma ray. The other ¾ of the atoms are beta emitters with no associated gamma emission. This decay pattern is always the same for TM-170. The point here is that even though the activities of different isotopes may be the same, the number of gamma rays resulting from decay of their atoms can be different.