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

Highlights: • Absolute standardizations of ^99mTc via 4π(LS)e-γ and ⁵⁷Co via 4π(LS)(e,X)-γ coincidence counting. • Preparation at NMISA for SIRTI and SIR comparisons of ^99mTc and ⁵⁷Co, respectively. • First technical report on the standardization of ^99mTc via liquid scintillation coincidence counting. • Count rate vs. efficiency simulation for ^99mTc. - Abstract: The radionuclides ^99mTc and ⁵⁷Co were standardized via absolute liquid scintillation counting techniques. We provide the first technical report on the absolute standardization of ^99mTc 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 ^99mTc. A detection efficiency analysis for both radionuclides is presented. Results from the standardizations, and SIRTI and SIR comparison exercises are reported.