Activity (source strength): The activity of a radioactive substance is given by the number of atoms of the substance which disintegrate per second. This is measured in Becquerels (Bq), 1 Becquerel corresponds to 1 disintegration per second(1 Bq = 1/s).
Specific activity: The specific activity of a radioactive source is the activity of this substance per weight unit, expressed in Bq/g.
Specific gamma-ray emission factor (k-factor): The k-factor is the generally used unit for radiation output of a source and is defined as the activity measured at a fixed distance. It indicates the specific gamma-emission (gamma constant) measured at 1 metre distance. The higher the k-factor, the smaller the source can be for a particular source strength. A source of small dimensions will improve the sharpness of a radiograph. Table 1-4 shows the various k-factors and half-life values.
Half-life of a radioactive source: Of an Iridium192 source with an activity of 40 GBq for example 10 GBq will remain after two half-lives (148 days), 5 GBq after three half-lives (222 days) etc.