A gamma-gamma ( \({{\upgamma }}-{{\upgamma }}\) ) coincidence spectrometer was modelled using the Geant4 toolkit and validated against experimental measurements. Incorporation of a decay-time estimation method enabled the generation of time-stamped list mode (T-List) data, facilitating a quantitative assessment of the performance advantages of high-resolution \({{\upgamma }}-{{\upgamma }}\) coincidence spectrometry relative to conventional high-resolution \({{\upgamma }}\) -spectrometry. The detection limit sensitivity for specific radionuclides of interest in the presence of a specified contaminant were investigated and the relative benefits of coincidence analysis discussed. The detection limit for \(^{\text {134}}\) Cs, using the 605-795 keV signature, is shown to be independent of \(^{\text {137}}\) Cs activity but sensitive to \(^{\text {132}}\) I due to the presence of abundant high-energy coincident photons. Similarly, the potential acceleration of analytical timelines through the combination of partial radiochemical separation and \({{\upgamma }}-{{\upgamma }}\) coincidence spectrometry is explored. For example, \(^{\text {156}}\) Eu is predicted to be detectable in fission product matrices containing other lanthanide radionuclides using \({{\upgamma }}\) – \({{\upgamma }}\) coincidence spectrometry, despite remaining below conventional \({{\upgamma }}\) -spectrometric detection limits, with quantifiable results obtainable several hours earlier than via full radiochemical separation followed by standard \({{\upgamma }}\) -spectrometry. In the analysis of compositionally complex samples, this approach also enables the identification of radionuclides that adversely affect detection limits, thereby highlighting candidates for removal by radiochemical separation.
