Measurements by NIST on the solution were performed by three methods: viz., liquid scintillation (LS) standardizations (with 3H-standard efficiency tracing); with triple-to-double-coincidence ratio (TDCR) method; and with by 4ηβ(LS)-γ(NaI) live-timed anticoincidence (LTAC) measurements. Impurities determination was performed by HPGe γ-ray spectrometry and 2πα spectrometry using Si surface barrier detector (α-SPECT). The determination of the 99Tc massic activity for the NPL solution was performed by live-timed 4πβ(LS)-γ(NaI) anti-coincidence counting (LTAC) with 60Co efficiency tracing. LS hemisphere sources were prepared containing different 60Co to 99Tc activity ratios. Also, 60Co-only sources, sources containing no 99Tc and a blank were prepared. The β - γ anti-coincidence data from each mixed-nuclide source and 60Co-only was fit with a linear fit and with a quadratic fit over an efficiency range of 0.58 to 0.93. The 60Co tracer activity was standardized (within 0.04 % agreement with previous standardization) and was subtracted from intercepts of the mixed sources. The standard deviation of the massic activities determined from the 3 mixed sources was 0.18 % and 0.32 % for linear and quadratic fits, respectively. The difference between the average activity determined by linear and quadratic fits was 0.02%. Therefore, the final massic activity was determined by averaging the linear and quadratic fit results. Confirmation was obtained by direct measurements of the NPL solution by CNET and TDCR within the respective method's uncertainty. The previously standardized 99Tc (SRM 4288B) was measured at the same time as the NPL 99Tc by LTAC, TDCR and CNET. The result obtained agreed with the previous standardization to within 0.6 %, 0.4 % and 0.01% for each method, respectively.
Graph: Methods Compairson in 99Tc international comparison. Value by LTAC was the reported value.