## 4 Results and Discussion
As Ramonetto geometry shape is equal to those of RAD7/Radim3A, no figures of Geant4 and COMSOL are shown being the simulations similar. Referring to the work [18] that presents a new reference measurement system (Sabot chamber) mainly focused on the \({}^{220}\)Rn (and also its ionized progeny) direct detection by diffusion, small enough to reduce as much as possible the energy loss of the non-ionized \({}^{220}\)Rn alpha particles before they reach the detector (due to \({}^{220}\)Rn short half-life of 55.8 s), we reproduced experimentally and by simulation that chamber. The Sabot chamber is a 1 cm high metallic chamber with a surface composed of a silicon detector of diameter 4 cm: the active volume is 13.0 \(\pm\) 0.1 cm\({}^{3}\). With 1000 V potential applied to the chamber, the \({}^{220}\)Rn efficiency value is 35 \(\pm\) 2%, by using a Pylon \({}^{228}\)Th source to create a \({}^{220}\)Rn atmosphere [18]. We have made a similar chamber (RadonSmall, Figure 3b) using a hallow parallelepiped metallic mesh chamber, having a volume of 7840 \(\pm\) 8 mm\({}^{3}\). A silicon PIN photodiode detector, having a square active area of 784 \(\pm\) 2 mm\({}^{2}\) and insulated in a plastic holder, acts as a basis. By using \({}^{220}\)Rn atmosphere realized with a calibrated source of \({}^{232}\)Th salts, and by applying a voltage of 200 V on the metallic walls of the chamber, the \({}^{220}\)Rn detection efficiency is 24 \(\pm\) 2%. In addition, for RadonSmall, to choose the best voltage value to be applied to the chamber walls, experimental and simulated tests have been carried out on the \({}^{216}\)Po counts per second (cps) as the applied voltage varied, to reach the maximum  
Figure 2: Experimental (**a**) and Geant4+COMSOL simulated (**b**) spectrum of \({}^{222}\)Rn (6344 \(\pm\) 541 Bq) + \({}^{220}\)Rn (3258 \(\pm\) 356 Bq) mixed atmospheres in the RaMonA measurement system [23]. cps saturation. The \({}^{220}\)Rn efficiency by Geant4+COMSOL simulations (Figure 3(b.1)) gives the values of 23 \(\pm\) 4%, in agreement with the experimental one. It is important to point out that in Sabot chamber and in RadonSmall, the non-ionized \(\alpha\)-particles peak of \({}^{220}\)Rn is obtained with the procedure described in [18,19]: subtracting from the total spectrum (\({}^{220}\)Rn + \({}^{216}\)Po + \({}^{212}\)Bi + \({}^{212}\)Po) the spectrum collected after eliminating the \({}^{220}\)Rn and consisting of \({}^{212}\)Bi + \({}^{212}\)Po that is achieved simply by removing the \({}^{228-232}\)Th source. Strengthened by all previous results, a final investigation has been conducted with the aim of simulating the detection efficiency of \({}^{222-220}\)Rn and their progeny (\({}^{218-216}\)Po) versus different sizes of the three main geometries of the electrostatic collection measurements system (i.e., parallelepiped, cylinder and hemisphere), also varying the applied voltages. The silicon PIN photodiode of 784 \(\pm\) 2 mm\({}^{2}\) square active area is fixed as detector in each simulation, always insulated in a plastic holder. Table 1 shows the obtained results. The main conclusions are: (i) detection efficiency is strongly dependent on the size of the chambers and on the voltage value applied to obtain electrostatic collection; (ii) detection efficiency decreases as chamber's height and/or radius increases, in agreement with the theoretical result that the detection efficiency of a radioactive source decrease as the distance source-detector increases; (iii) \({}^{222-220}\)Rn efficiency depends on the height of the chambers due to the fact that both isotopes do not ionize, contrary to \({}^{218-216}\)Po ions that are electrostatic collected, therefore, small chambers in height ensure the collection of the \(\alpha\)-particles of non-ionized \({}^{220-222}\)Rn nuclei; (iv) the electric field inside the chamber is not strong enough to collect all the ions with low voltage value, while increasing the voltage the electric field intensifies and most of the ions are collected till the efficiency saturation is reached.