We have developed a methodology for calibrating (68)Ge radioactivity content in a commercially available calibration source for activity calibrators in a way that is traceable to the national standard. Additionally, the source was cross-calibrated for equivalent (18)F content by direct comparison with the national standard for (18)F in the same geometry.
Methods: Sources containing standardized (68)GeCl(4) or (18)F-FDG solutions were prepared at the National Institute of Standards and Technology (NIST) with mock syringe blanks used in the construction of a commercially available epoxy-based (68)Ge calibration source. These sources and several NIST-constructed epoxy-based (68)Ge mock syringes were then used as artifact standards to determine calibration factors for NIST-maintained activity calibrators and secondary standard ionization chambers to enable calibration of the actual commercial sources. A direct comparison between the solution-based (68)Ge sources and the (18)F-FDG sources allowed for an empiric determination of the relative response for these radionuclides in several commercial activity calibrators. Potential measurement effects due to differences between the solution composition and the epoxy and theoretic (68)Ge-to-(18)F response ratios were studied by Monte Carlo simulation.
Results: The calibration factors developed in this study enabled NIST to calibrate epoxy-based mock syringe sources with a relative combined standard uncertainty of 0.52%. The direct comparisons of the (68)Ge and (18)F standards in the various ionization chambers allowed the activity to be expressed in terms of equivalent (18)F activity with a relative combined standard uncertainty of about 0.9%.
Conclusion: The ability for NIST to calibrate these epoxy-based mock syringes enabled, for the first time to our knowledge, the direct traceability to the national (68)Ge standard to be established for this type of source. Through a direct comparison with the NIST (18)F standard, the determination of the relative response ratios in activity calibrators enabled the equivalent (18)F activity to be determined in a way that was also traceable to the national (18)F activity standard.