243Am is a very important nuclide as a chemical yield tracer for the radiochemical determination of 241Am in environmental, food and other samples. For the study of absolute activity measurement of 243Am samples, efficiency extrapolation method by 4πα,β(PC)-γ(NaI(TI)) counting system is introduced in this paper. A solution containing 243Am in equilibrium with its daughter 239Np was standardized by this coincidence system, and the activity concentration of 243Am was just half of the gross activity concentration based on the theory of radioactivity equilibrium. In the experiment, the efficiencies of 243Am and 239Np were changed by altering the applied HV step by step, and the gross activity was obtained while the total efficiency of α and β was extrapolated to 100%. The final result for the efficiency extrapolation method is 5116.1(1±0.89%) Bq/g, which is the mean of the results obtained for the eight VYNS sources. As comparison, the result for ionization chamber is 5141.2(1±0.39%)Bq/g which is the mean of the results obtained for the six glass sources. The results show that there is good consistency between both methods adopted, and the agreement between results by both expereiments was 0.49%. The reference time is January 1 2012 0h.

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