The activity of a radioactive source is the number of ionising particles it emits each second. Over time, fewer nuclei are left in the source to decay, so the activity drops. The time taken for half the radioactive atoms to decay is called the half-life. The activity of a radioactive source is measured in bequerels (Bq).
Starting with a pure sample of radioactive nuclei, after one half-life half the nuclei will have decayed. The remaining undecayed nuclei still have the same chance of decaying as before, so after a second half -life half of the remaining nuclei will have decayed. After two half-lives a quarter of the nuclei will remain undecayed.
This is an average time for half of the atoms in a given sample are to be decayed or average count rate reduced to half of the original counts.
For different radioactive elements half-life varies from millionths of second to millions of years. For radium it is 1620 years and for carbon-14 it is 5700 years. The table shows the half-lives of some of the common radioactive elements.