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 Ci. Iridium-192 will contain 350 Ci for every gram of material.
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 the
sharpness of the radiograph.