## 3 Elements of the Beam Line Prototype
### On-line Beam Diagnostics and Dosimetry
An issue related to the total charge measurements is represented by the emission of secondary particles, usually electrons, produced by the interaction of the protons with the entrance foil and the cup material itself. The emitted secondary electrons may be either collected or leave the cup, affecting the collected charge measurements. The secondary electrons produced in the vacuum window former may lead to a total charge underestimation, while those produced within the cup material, if produced with sufficient energy, may leave the collecting cup and cause a charge overestimation. In order to maximize the charge collection measurement accuracy by increasing the probability of capturing the secondary electrons emitted, shape, sizes, materials and electric field characteristics have been modeled and determined, with the help of the GEANT4 MC simulations and electrostatic ion trajectory software, such as the COMSOL FEM (Finite Element Method) [30] and the Simion FEA (Finite Element Analyse). These studies aimed to increase the efficiency and reliability of the FC design as a charged particle current detector. Figure 9 shows the final FC prototype configuration with all the geometrical details. The FC prototype technical details and the simulation results on the electric field generated can be found in ref. [27]. Typical FC detectors have cylindrically symmetric components and employ coaxial electrostatic fields to recapture the ejected electrons. In the FC prototype, the electric field results by the combined effect of two coaxial electrodes. The external electrode is a metallic hollow cylinder and the internal one, follows a similar design presented by [PERSON] _et al._[29], is a peculiar beveled cylinder. The addition of this element breaks the electrostatic coaxial symmetry of the field, provided solely by the external electrode, by introducing a relevant transverse electric field component, as it can be seen in Figure 10, which allows recapturing secondary electrons improving the deflection of the secondary electrons emitted by both the entrance window and the cup. In order to investigate the FC prototype response, an experimental campaign has been performed using the 62 MeV proton beams delivered by the superconducting cyclotron (CS) at the CATANA protontherapy facility [31] at the INFN-LNS (Catania, Italy). A complete characterization of the FC prototype charge measurement varying the bias applied to the suppression electrodes, the proton beam incident current and the dose delivered has been also performed. The FC prototype has been placed about 9 cm downstream from the final collimator along the beam-line, a 20 mm diameter collimator has been  
Figure 9: Technical drawing of the FC prototype realized at the INFN-LNS. used for the measurements. The FC operating vacuum was 10\({}^{-5}\) mbar. The FC current signal as a function of the collecting time was measured using of the electrometer Keithley 6517A. In order to compare the secondary electron suppression efficiency using the external electrode alone and the two coaxial electrodes together, the characterization of the FC prototype charge collection as a function of the applied voltage has been performed using both configurations. The voltage has been varied from \(-\)4000 Volt to 4000 Volt. Each charge measurement has been performed delivering a given dose of 15 Gy. The dose released has been monitored using the online transmission ionizations chambers. Figure 11 shows some preliminary results of the collected charge as a function of the applied voltage. Each charge measurement has been obtained integrating the current signal for the time interval required to deliver 15 Gy. As one can see, a typical flat voltage response plateau in the negative voltage range between \(-\)2400 V and \(-\)400 V can be observed; on the other hand, for positive voltages a collected charge dependence increasing with the voltage applied is evident.