Development of SIRTI at National Institute of Metrology (NIM), China

To strengthen the Mutual Recognition Arrangement (MRA) for short-lived radionuclides, a SIRTI for the Asia-Pacific Metrology Programme (APMP) region has been developed at the NIM, China. A dual-threshold comparator coupled with an FPGA-based counter was developed to replace the MTR2 module and the ORTEC 994 scaler used in the BIPM SIRTI. A 100 MHz clock serves as a frequency standard, enabling simultaneous counting and dead time measurement. A^93mNb/⁹⁴Nb reference source was prepared with high-purity radioactive solution and housed in a plexiglass holder. The 16.6 keV X-ray peak of ^93mNb is used to set the lower threshold and to evaluate its stability, whereas the count rate of ⁹⁴Nb above the threshold serves as a metric for the system's stability and reproducibility. Measurements performed over two months showed that the maximum drift of the threshold position remained within ±2 channels and the relative standard deviation of the ⁹⁴Nb count rate was 2.7 × 10⁻⁴. The medical radionuclides ^99mTc, ¹⁸F, and ⁶⁸Ga were measured for duration exceeding seven half-lives. Following rigorous dead time and background corrections, the measured count rate exhibited excellent agreement with the theoretically predicted values after decay correction. For the corrected count-rate interval of 2000 - 20,000 s⁻¹, the maximum non-linearity of the system was determined to be 3.6 × 10⁻⁶.