A method is described for determination of 123 I activity with a radionuclide activity meter.In 123 I sample often exists 124 I as a radioactive impurity.The activity ratio of 124 I to 123 I is changed with time because the two nuclides have different half lifetime.Based on the measured value in the activity meter and the “activity ratio vs.time”relationship
the real 123 I activity can be read directly by adjusting the nuclide factor in the activity meter.The measuring error is less 5%.