Sensitivity and uncertainty analysis of the simulation of 123I and 54Mn gamma and X-ray emissions in a liquid scintillation vial

Radiation transport simulations of the most probable gamma- and X-ray emissions of 123I and 54Mn in a three photomultiplier tube liquid scintillation detector have been carried out. A Geant4 simulation was used to acquire energy deposition spectra and interaction probabilities with the scintillant, as required for absolute activity measurement using the triple to double coincidence ratio (TDCR) method. A sensitivity and uncertainty analysis of the simulation model is presented here. The uncertainty in the Monte Carlo simulation results due to the input parameter uncertainties was found to be more significant than the statistical uncertainty component for a typical number of simulated decay events. The model was most sensitive to changes in the volume of the scintillant. Estimates of the relative uncertainty associated with the simulation outputs due to the combined stochastic and input uncertainties are provided. A Monte Carlo uncertainty analysis of an 123I TDCR measurement indicated that accounting for the simulation uncertainties increases the uncertainty of efficiency of the logical sum of double coincidence by 5.1%.