## 3 ELIMAIA Dosimetry Description
### Detectors for Relative and Absolute Dosimetry
#### 3.2.3 Faraday Cup  
As pointed out before, in general, ion recombination effects in ionization chambers are relevant for these extremely high dose rates per pulse and, consequently, large corrections should be applied to properly account for these effects if ionization chambers should be used for absolute dosimetry of laser-driven beams. Although the use of in-transmission chambers to monitor the beam can be still feasible, provided that the adoption of alternative approaches, as the described double-gap configuration, can further optimize the final response, a detector independent as much as possible of beam intensity per pulse should be considered as the optimal choice for absolute dosimetry. This is important in order to properly cross-calibrate the in-transmission detectors with a reliable absolute detector, whose response is not sensibly influenced by high dose rate and the shot-to-shot non reproducibility. On this concern, the solution adopted at ELIMAIA relies on the use of a Faraday cup in order to determine the number of incident protons at the irradiation point along the in-air final section of the beamline. Knowing the incident proton energy and measuring the beam effective area is thus possible to determine the dose to water, once the stopping power values of protons in waters are retrieved from tables, aiming to obtain an overall uncertainty within 5% when beams are well-controlled and reproducible. FCs are well known devices, their response is independent of the dose rate and, furthermore, they have been initially implemented in clinical proton facilities for absolute dosimetry, prior other methods based on the use of ionization chambers were definitively established and suggested in the protocols [40; 41]. By the way, to determine the dose with an acceptable uncertainty, beyond the collected charge, all the mentioned quantities have to be measured with high level of accuracy. In particular, the collected charge in the cup, which finally provides the total number of incident protons, has to be optimized using an additional electrode (sometimes called electron suppressor), which aims to minimize undesired contribution in the charge, coming from secondary electrons. They are typically produced by the interaction of primary protons with either the entrance window used for maintaining vacuum conditions within the FC (electrons to be suppressed by the electric field) or with the cup itself, where the beam is dumped (electrons to be collected, because of escaping from the cup). The FC designed and realized for ELIMAIA is 400 mm long, with an internal radius of 20 mm, and is made of aluminum. The main components are as follows: a 50 \(\upmu\)m thick kapton entrance window to maintain the vacuum inside the cup, whose thickness has been chosen in order to minimize the beam perturbation; an additional electrode (suppressor) composed by a 180 mm steel cylinder; and a 50 mm thick aluminum cup where the charge is actually collected. Moreover, because the ionization of the residual air could affect the FC measurements, a vacuum of about \(10^{5}\) mbar is required as working condition. Figure 10 shows a picture of the FC installed in the ELIMAIA beamline. Figure 10: Picture of the FC installed along the in-air section of the ELIMAIA beamline. A preliminary Monte Carlo study has been carried out with the aim of optimizing the size, according to the typical beam spot parameters expected at ELIMAIA, and of choosing the proper material that provides the best compromise between secondary electron production and mechanical handling [42]. To measure the beam effective area and the energy spectrum of the incident protons, radiochromic films (RCF) will be used at ELIMAIA. They are one of the most commonly used detectors for relative dosimetry and their independence on dose-rate has contributed to their wide use in the laser environment [43]. They are easy to handle and the physical information contained in the films is not lost during the reading process, allowing post-processing for additional data analysis.