The quantity which expresses the degree of radioactivity or the radiation producing potential of a given amount of radioactive material is activity. The curie was originally defined as that amount of
any radioactive material that disintegrates at the same rate
as one gram of pure radium. The curie has since been defined more
precisely as a quantity of radioactive material in which 3.7 x
1010 atoms disintegrate per second. 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 Cs-137 might have very different masses depending
upon the relative proportion of non-radioactive atoms present
in each source. Radioactivity is expressed as the number of curies
or becquerels per unit mass or volume.
The concentration of radioactivity, or the relationship between the mass of radioactive material and the activity, is called "specific activity." Specific activity is expressed as the number of curies or becquerels per unit mass or volume. Each gram of cobalt-60 will contain approximately 50 curies. Iridium-192 will contain 350 curies for every gram of material. The shorter half-life, the less amount of material that will be required to produce a given activity or curies. The higher specific activity of iridium results in physically smaller sources. This allows technicians to place the source in closer proximity to the film while maintaining geometric unsharpness requirements on the radiograph. These unsharpness requirements may not be met if a source with a low specific activity were used at similar source to film distances.