Abstract. For precise measurements of High Purity Germanium (HPGe) detector and in order to estimate the Full Energy Peak Efficiency (FEPE), a theoretical procedure using the Monte Carlo simulation method was established in the Central Laboratory for Environmental Radioactivity Measurements Inter-comparison and Training (CLERMIT), as a continuation of previous work. A set of standard point sources (¹³³Ba, ¹³⁷Cs, ⁶⁰Co and ²²Na) was used, employing a 8 cm distance from the detector and appropriate detection geometry. MCNP5 input file was constructed and the detector was simulated. The detector efficiency curve was normalized to the volume efficiency curve using a KCl solution. Experimental results were compared with those simulated by MCNP5. Uncertainty analyses were performed to determine the effect of each operational and design parameter on the efficiency curve. Another set of point sources with different activities was used for method verification. For the method validation, an IAEA-326 certified reference sample was used for verification. A good agreement between the two methods was achieved, with discrepancies of less than 5% (ranging from -1.10 to 2.63%). Some selected environmental samples with different matrices were measured and the natural radioactivity concentration for ²³⁸U-series, ²³²Th-series and ⁴⁰K were determined.

Key words: Monte Carlo simulation, MCNP5 model, n-type HPGe detector, radioactivity.