Abstract: This paper derived the ¹²⁵I activity calculation formula based on the coincidence summing characteristics of ¹²⁵I nuclide X-ray and γ ray. The formula only included the net counting rate of the full-energy peaks (27.3, 35.5, 31.2 keV), the net counting rate of the sum-peaks (58.5, 66.7 keV) and the net counting rate of the total spectrum. The point source, activated carbon cartridge source and liquid source were verified by Broad-energy and N-type germanium spectrometer. When the distance from point source to detector was less than 5cm, the relative deviation between the calculated results of the absolute activity and the reference values was within ±1.2%. The self-absorption effect of ¹²⁵I nuclide in activated carbon cartridge could be ignored, and the absolute activity calculation formula was not affected by the distribution form of ¹²⁵I nuclide. The relative deviation of front and rear measurements for activated carbon cartridge source was within ±1.4%. The relative deviation between the measurement results and reference values of liquid source with volume less than 40mL was within ±2.6%. The sample height should not be too high during the measurement of liquid source, so as to reduce the weakening effect of self-absorption on sum-peaks. The experimental results show that the absolute activity calculation formula derived in this paper can be used to accurately measure the activity of ¹²⁵I nuclide without relying on standard substances and nuclide parameters.

Key words: ¹²⁵I nuclide, coincidence summing, absolute activity, HPGe spectrometer