## 4 Results and Discussion  
Worldwide-used commercial devices for Rn detection, based on electrostatic collection, have been simulated with our developed procedure by using Geant4+COMSOL. The resultswill be compared with the experimental ones provided in literature and/or stated by the manufacturer company. The simulated devices are:  
* RAD7 (Durrige Company, USA), likely the most known and used, consisting of a hollow hemispherical metallic chamber of 700 \(\pm\) 5 cm\({}^{3}\) volume, with a silicon surface barrier detector, having a 300 \(\pm\) 1 mm\({}^{2}\) circular active area, centered on the bottom [26]. * RaMonA (University of Naples and University of Campania-Caserta, Italy), consisting of a 785 \(\pm\) 8 cm\({}^{3}\) hollow cylindrical metallic chamber, 10.0 \(\pm\) 0.1 cm high and 10.0 \(\pm\) 0.1 cm diameter, with a silicon surface barrier detector, having a 50.0 \(\pm\) 0.4 mm\({}^{2}\) circular active area, centered on the top [27]. * Radim3A (Tesla company, Czech Republic), consisting of a hollow hemispherical metallic chamber of 830 \(\pm\) 5 cm\({}^{3}\) volume, and a silicon surface barrier detector, having a 450 \(\pm\) 2 mm\({}^{2}\) circular active area, centered 1 cm below on the bottom [10]. The sampling of the measuring air takes place by diffusion of \({}^{222-220}\)Rn inside the chamber in Radim3A; inside RaMonA the air is pumped; while RAD7 works in both ways. The high voltage power circuit of the three systems charges the metallic chamber to a potential of 2500 V for RAD7, 2000 V for Radim3A, and 3500 V for RaMonA, creating an electric field throughout the volume that propels \({}^{222-220}\)Rn positively charged particles onto the detector [8; 9; 10]. In all systems, the detector is insulated in a plastic holder. The systems have an experimentally estimated detection efficiency of \({}^{218}\)Po (about the same assumed for \({}^{216}\)Po [23]) of: 0.007 \(\pm\) 0.001 cpm\(\cdot\)m\({}^{3}\cdot\)Bq\({}^{-1}\) (0.11 \(\pm\) 0.03 cps\(\cdot\)L\(\cdot\)Bq\({}^{-1}\)) for RAD7, 0.8 \(\pm\) 0.1 count\(\cdot\)m\({}^{3}\cdot\)Bq\({}^{-1}\cdot\)h\({}^{-1}\) (0.22 \(\pm\) 0.02 cps\(\cdot\)L\(\cdot\)Bq\({}^{-1}\)) for Radim3A, and 0.047 \(\pm\) 0.005 cps\(\cdot\)L\(\cdot\)Bq\({}^{-1}\) for RaMonA [26; 27]. Considering the volume of the three chambers, the systems have \({}^{218-216}\)Po detection efficiency of: 16 \(\pm\) 4% for RAD7, 27 \(\pm\) 3% for Radim3A, and 6 \(\pm\) 1% for RaMonA. Geant4+COMSOL simulation reproduce the geometries of the three investigated measurement systems, with the specific features of the electric field generated inside the chamber. The thicknesses of the metallic chamber, the detector insulator holder and the detector sensitive surface are set at 1 mm, 2 mm and 500 \(\upmu\)m, respectively (not affecting the simulation). In Figure 1 are reported the 3D-model geometry of the three Rn measurement systems in Geant4, together with the correspondent simulated electric potential spatial distributions and electric field flow lines inside the metallic chamber by COMSOL. An example of spectrum of mixed atmospheres of \({}^{222}\)Rn + \({}^{220}\)Rn from Geant4+COMSOL simulation in the RaMonA device is reported in Figure 2, compared with an experimental one [23; 28]. The simulated efficiency, obtained by the spectrum from Geant4+COMSOL, is computed as the ratio between the alpha particles detected on the Silicon detector and the number of alpha primary starting particles generated in the detection volume of chamber. The efficiency uncertainty comes from the propagation error of both elements of the ratio [19], where the numerator error is the square root of the counts sum. The efficiencies of the three measurement systems obtained by Geant4+COMSOL are the following: 16 \(\pm\) 2% for RAD7, 5 \(\pm\) 2% for RaMonA, and 28 \(\pm\) 3% for Radim3A.