Application of TDCR-Geant4 modeling to standardization of 63Ni

As an alternative to the classical TDCR model applied to liquid scintillation (LS) counting, a stochastic approach based on the Geant4 toolkit is presented for the simulation of light emission inside the dedicated three-photomultiplier detection system. To this end, the Geant4 modeling includes a comprehensive description of optical properties associated with each material constituting the optical chamber. The objective is to simulate the propagation of optical photons from their creation in the LS cocktail to the production of photoelectrons in the photomultipliers. First validated for the case of radionuclide standardization based on Cerenkov emission, the scintillation process has been added to a TDCR-Geant4 modeling using the Birks expression in order to account for the light-emission nonlinearity owing to ionization quenching. The scintillation yield of the commercial Ultima Gold LS cocktail has been determined from double-coincidence detection efficiencies obtained for 60Co and 54Mn with the 4π(LS)β−γ coincidence method. In this paper, the stochastic TDCR modeling is applied for the case of the standardization of 63Ni (pure β−-emitter; Emax=66.98 keV) and the activity concentration is compared with the result given by the classical model. - Highlights: ► A TDCR model based on Geant4 is described in the case of standardization of 63Ni. ► All the optical effects (refraction, reflection) in the TDCR counter are simulated. ► The problem of the resolving time duration is also investigated. ► Activity measurements are compared with the classical TDCR model.