Absolute standardizations of 99m Tc and 57 Co by 4π electron-gamma liquid scintillation coincidence counting for SIRTI and SIR comparisons.

The radionuclides 99m Tc and 57 Co were standardized via absolute liquid scintillation counting techniques. We provide the first technical report on the absolute standardization of 99m Tc using 4π(LS)e-γ coincidence counting. The low detection efficiency of low-energy conversion electrons translates into a large efficiency extrapolation range. A simulation indicated that the γ-ray interacting with the liquid scintillator introduces curvature to the count rate vs. efficiency relationship, the approximation of the functional form used for extrapolation providing the main measurement uncertainty for 99m Tc. A detection efficiency analysis for both radionuclides is presented. Results from the standardizations, and SIRTI and SIR comparison exercises are reported.