Standardization of 68Ge/68Ga using the 4πβ−γ coincidence method based on Cherenkov counting

In the framework of an international BIPM comparison (Bureau International des Poids et Mesures), the activity standardization of 68Ge in a solution of 68Ge/68Ga in equilibrium provided by NIST was carried out at LNHB. This exercise was organized to meet the growing interest in 68Ga as a radiopharmaceutical in nuclear medicine services (e.g. as a surrogate of 18F for PET imaging). Due to the volatility of germanium, the activity standardization of 68Ge was investigated at LNHB by means of 4πβ−γ coincidence counting based on Cherenkov measurements. This technique was applied to take advantage of the Cherenkov threshold (~ 260 keV in aqueous solutions) in order to prevent counting from electron-capture events associated with 68Ge disintegrations. Cherenkov counting was performed using glass and polyethylene vials and the resulting activity concentrations were compared with 4πβ−γ coincidence measurements based on liquid scintillation. The efficiency-extrapolation curve obtained with Cherenkov measurements in glass vials was compared to Monte Carlo simulations based on the Geant4 code.