## 3 Results  
### Correction of the Readout of a LIC: Experimental Data and Simulations  
In Figure 1 we present the readout current versus time in a certain pixel of the LIC used to verify the lymph node tumor treatment delivered at 600 MU/min presented in Section 2.4 (_sequence mode_ acquisition). The variation of dose rate in the pixel is easily noticeable, with segments _missing_ the pixel and delivering no dose other than scatter, and other segments clearly irradiating the pixel. Fluctuations in the readout signal associated to each segment are easily noticeable in Figure 1. Such fluctuations are caused by the collection time of the ionized charge rather than by intrinsic fluctuations of the dose per pulse (Section 2.1.3). The spikes noticed at the beginning of several segments may be an exception. The origin of such spikes is not clear: it may be an overshoot of the LINAC at the start of the delivery, which is then adjusted by the feedback circuit, or it may be caused by the response of the readout electronics. In order to minimize such fluctuations, a moving average was used. The correction method was applied to this set of data. Firstly, the dependence of the collection efficiency with the dose per pulse was investigated [14; 25], in order to obtain the parameters needed to apply the correction method of Equation (12). It is important to notice that when irradiating the LIC at 600 MU/min, the Boag condition of no superposition of charge carriers ionized by different pulses does not hold (this is noticeable in the signal structure of Figure 1). However, as discussed in Section 2.1.3, we can still apply the correction formalism presented in this work. The reference dose per pulse for these calculations was 1 mGy, which results in a value \(u_{ref}=0.090\) (collection efficiency 0.958). The effect of the correction is shown in Figure 2. We present the 2D map of the integrated signal for the treatment delivered at 600 MU/min (panel A) to show the shape of the dose distribution. The correction method is applied to each pixel, and it leads to an increase of the readout current, especially noticeable when the dose per pulse is high, as shown in panel C for a pixel of the detector. The overall effect of the correction method is an increase of the integrated signal up a \(\sim\)3%, which is consistent with the recombination loss at this dose rate in this detector [25]. This is illustrated in panel B for a profile of the integrated signal. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{4}{c}{**Detectors**} \\ \cline{2-4}
**Parameter** & **LIC** & **PTW729** & **PTW1500** \\ \hline Pixel gap (mm) & 0.5 & 1 & 1 \\ Polarization voltage (V) & 1000–1500 & 400 & 1000 \\ Ion mobilities (m\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\)) & 2.3–3.5 \(\times\) 10\({}^{-8}\) & 1.18–1.44 \(\times\) 10\({}^{-4}\) & 1.18–1.44 \(\times\) 10\({}^{-4}\) \\ Electronic mobilities (m\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\)) & - & 0.1504 & 0.0751 \\ G\({}_{fi}\) (ionization pairs/(100 eV)) & 0.69 (@1000 V)–0.88 (@1500 V) & 2.94 & 2.94 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main characteristics and physical parameters of the simulated detectors. LIC corresponds with the isooctane-filled liquid ionization chamber presented in [13], while PTW729 and PTW1500 correspond with the pixel characteristics of the air-filled PTW “729” and “1500” arrays [9]. The effect of recombination in LICs was further explored with simulations. In particular, the spatiotemporal dose-rate profile of the verified treatment was converted to simulate a delivery at a monitor unit rate of 2400 MU/min (2.8 ms between consecutive pulses) and a maximum dose per pulse of 2 mGy (roughly corresponding to a delivery at 10 MV-FFF in a TrueBeam linac). In order to speed up the simulation, the time distance between segments (with no irradiation) was reduced to 0.1 s. In Figure 3 we show the profile of the readout signal for a particular pixel operating at 1000 V, as well as the corrected signal according to the online recombination method (Equations (12) and (13)).