Speaker
Description
A national primary standard for Tc-99m activity was established at NRSL/INER in Taiwan using a self-developed 4πex–γ coincidence counting system. The system integrates a compact flow-type 4π proportional counter with a thin NaI(Tl) detector optimized for the detection of low-energy internal conversion electrons and 140 keV γ-rays. Detection efficiency was systematically varied through controlled self-absorption and extrapolated to unity efficiency, yielding a specific activity of 8.945 MBq g⁻¹ ± 0.49% (k = 1).
Independent verification was performed using the CIEMAT/NIST liquid scintillation counting (CN) method. After dilution correction and normalization, the CN method yielded 8.957 MBq g⁻¹ ± 0.66% (k = 1). The ratio between these two techniques was 1.001 ± 0.82% (k = 1), corresponding to a relative difference of 0.1%, well within the combined uncertainty. A weighted mean specific activity of 8.949 MBq g⁻¹ ± 0.40% (k = 1) was adopted for traceable standard transfer.
Calibration of an ISOCAL IV 4πγ ionization chamber resulted in calibration factors of 1.245 pA MBq⁻¹ ± 0.50% (5 c.c. ampoule) and 1.233 pA MBq⁻¹ ± 0.50% (10 c.c. vial), consistent with NPL reference values within 1%. A commercial CRC-15R dose calibrator exhibited deviations of −1.7% and −1.3% relative to the weighted mean prior to correction.
To evaluate implementation at the clinical level, a nationwide survey of 153 dose calibrators was conducted. Geometry-dependent deviations of approximately ±2% were observed, and variability in long-term quality control practices was identified.
The innovation of this work lies in the realization of a domestically developed 4πex–γ primary system with controlled efficiency extrapolation, dual-technique validation, and demonstrated traceability transfer to national and clinical measurement systems, establishing a complete metrological chain for Tc-99m activity standardization.