Application of TDCR-Geant4 modeling to activity standardization

The LNE-LNHB, in charge of the French standards for radionuclide metrology, develops primary measurement methods intended for the calibration of secondary measuring instruments, such as used in hospitals, to determine the activity of radiopharmaceuticals injected into patients. The triple-to-double coincidence ratio (TDCR) method, based on liquid scintillation counting, is widely implemented for radionuclides which are pure beta and electron capture emitters. This technique requires the mixing of the radioactive solution with a scintillator cocktail. When preparing samples, the existence of physicochemical processes such as chemi-luminescence can require to delay the measurements to achieve the necessary stability conditions, which is not always compatible for short halflife radionuclides. To avoid this constraint, a new approach using Cerenkov radiation as a method of light production is presented in this paper, applied to the standardization of the short-lived radiopharmaceutical C (T1/2 ~ 20 min). The method is based on comprehensive modeling of the TDCR counter (the experimental equipment) using a Geant4 toolkit in order to calculate the detection efficiency.