## 3 ELIMAIA Dosimetry Description
### Detectors for Relative and Absolute Dosimetry
#### 3.2.2 Multi-Gap Ionization Chamber  
In order to address the issue related to the extremely high dose rate per pulse, a ionization chamber equipped with two separate electrodes collecting the charge in two independent gaps has been designed and realized for the ELIMAIA beam line. This approach, initially investigated for accurately determining corrections for ionization chambers used for absolute dosimetry [36], has been extended here for in-transmission monitor chambers. General recombination occurs when ions produced in different tracks recombine each other on their way to the collecting electrodes. The amount of this type of recombination depends on how many ions are created per unit volume and per unit time, consequently, general recombination is dose-rate dependent and, for laser-driven beams, it is expected to be relevant [37; 38]. A solution could be provided by increasing the electric field but, in order to avoid large charge multiplication effects and, not least, a breakdown possibly caused by very high voltages, it is not possible to arbitrarily increase its value.  
The double gap ionization chamber is intended to be a relative monitor detector, which still needs to be calibrated against an absolute dosimeter placed downstream the beamline (a Faraday cup, as described below). By the way, the poor shot-to-shot reproducibility arising large variations in the dose rate per pulse produces non negligible differences in the ion recombination, affecting a ionization chamber for different shots. Therefore, a unique value of collection efficiency cannot be provided if the beam conditions change considerably shot by shot. A real-time procedure to correct for the recombination for each independent shot must be implemented, in order to retrieve the respective collection efficiency and properly correct the chamber response for each shot. This is basically provided by the additional gap, which is used as a monitor for the first one, in order to provide a variable correction to be applied shot by shot to the first gap [39].  
The device has been committed and realized by the DE.TEC.TOR. company and it is basically composed of two adjacent ionization chambers, with a gap (space between the electrodes) of 5 mm and 10 mm, respectively, and independently supplied (different voltage values according to the specific gap). The anode is constituted by a thin layer of 5 um of copper and 2 um of nichel deposited on a 25 um layer of kapton, while the cathode is constituted by a 12 um layer of aluminized mylar. Both of them are fixed by a frame of epoxy glass (Figure 9).  
The two gaps are placed in the center of an aluminum box of 420 \(\times\) 420 \(\times\) 100 mm\({}^{3}\) of size (Figure 9), and an external window of aluminized mylar is present in front. The modularity of the device will give the possibility to eventually add, in future, additional gaps for redundancy.  
Figure 9: An overview of multi-gap ionization chamber (MGIC), the scheme of electrode and the sensors package composed of frames used in stack, in order to separate the cathodes and the anodes and to support the MGIC at the center of the box. In the sensors package (on the right), IC1 is indicated by the red bracket and IC2 by the green bracket.