GEANT4 code for simulation of a germanium gamma-ray detector and its application to efficiency calibration

The GEANT4 software was developed by RD44, a world-wide collaboration of national institutes, laboratories and large High-Energy Physics experiments. GEANT4 is a public software package composed of tools which can be used to accurately simulate the passage of particles through matter. In this article, the first attempt to use GEANT4 to model a reverse electrode germanium detector (REGe), and to improve also its efficiency calibration procedure, is presented. A variance reduction algorithm based on a directional bias scheme is implemented into GEANT4 in order to accelerate the efficiency computations. A fast optimisation method to model the detector geometry using standard point sources is also presented and validated for point, Marinelli and air filter sources. The simulated full-energy peak efficiencies agreed with the measured values to within 1% between 36 and 1460 keV for these three counting geometries.