Standardization of $^{68}$Ge/$^{68}$Ga using the 4$\pi \beta-\gamma$ coincidence method based on Cherenkov counting

Abstract : In the framework of an international BIPM comparison (Bureau International des Poids et Mesures), the activity standardization of $^{68}$Ge in a solution of $^{68}$Ge/$^{68}$Ga in equilibrium provided by NIST was carried out at LNHB. This exercise was organized to meet the growing interest in $^{68}$Ga as a radiopharmaceutical in nuclear medicine services (e.g. as a surrogate of $^{18}$F for PET imaging). Due to the volatility of germanium, the activity standardization of $^{68}$Ge was investigated at LNHB by means of 4$\pi \beta -\gamma$ coincidence counting based on Cherenkov measurements. This technique was applied to take advantage of the Cherenkov threshold (similar to 260 keV in aqueous solutions) in order to prevent counting from electron-capture events associated with $^{68}$Ge disintegrations. Cherenkov counting was performed using glass and polyethylene vials and the resulting activity concentrations were compared with 4$\pi \beta -\gamma$ 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.