Abstract: The gamma energy spectrum of 60Co is measured by using NaI （T1） scintillator detector. According to the peak area of the measured 1.173MeV and 1.332MeV gamma rays as well as that of their coincident peak, the activity of radioactive source is obtained, which has a （6.64±0.40）% deviation in comparison with the real value. It is shown that this method can measure the activity of radioactive source.

Key words: NaI （T1） scintillator detector, activity, coincidence