Abstract: The modified sum-peak method estimates radioactivity by using only the peak and the sum-peak count rates.To verify the modified sum-peak method,the dependence of the full energy peak efficiency on the source-to-detector distance in a high-purity germanium detector system was studied using a Geant4 Monte Carlo simulation.The effect of the dead-layer in the germanium crystal was estimated by reference to experiments on ²⁴¹Am and the relative efficiency of the detector.The peak efficiency dependence on the source-to-detector distance was compared between the simulation and measurements.The modified sum-peak method is discussed with respect to these peak efficiencies.

Key words: Geant4 Monte Carlo simulation, sum-peak method, peak efficiency, HPGe, ⁶⁰Co