Standardization of 99mTc

The radioactivity of 99mTc was standardized by the 4πPC-γ coincidence method with two different modes. One is using coincidences between (119.5-142.6) keV conversion electrons and K X-rays, and the other is coincidences between the 2.13 keV conversion electrons and 140.5 keV γ-rays. The background of the K X-ray peak and the sensitivity of the proportional counter (PC) to 140 keV γ-rays were the main sources of uncertainties in the first case and low detection efficiency for conversion electrons in the second case. General coincidence equations were written, with specific forms, for the three measurement variants, including literature variant. Comparison with the ionization chamber calibration is reported