Effect of the geometrical parameters of an HPGe detector on efficiency calculations using Monte Carlo methods

This paper describes a procedure for constructing a Monte Carlo model of an HPGe detector used to measure activity. The purpose of this parametric study is to obtain a single model that effectively reproduces the detection efficiency for both small (< 1 cm) and large (> 40 cm) source-detector distances. We therefore investigated the effect of the HPGe detector's geometrical parameters on the Monte Carlo efficiency calculations. The results highlight detector parameters with the greatest effect on the calculations within a wide range of gamma energy levels (60-1900 keV) and source-detector distances (1 to 50 cm): the air gap, the dead layers thicknesses and the crystal height (see Figure 1). Analysis of these results has made it possible to determine the key parameters, together with the optimal energy and distance ranges, significantly reducing the computational time required. A broad range of calibrated point-like radioactive sources were used to ensure a high level of experimental efficiency when constructing the detector model. A single Monte Carlo model for this detector was thus optimised by minimising the difference between the experimental efficiency and the calculations. The single HPGe model resulting from this optimisation process led to an average deviation between experimental and simulated efficiency of 3% for both small and large source-detector distances.