# Characterization of Ultra-High-Dose Rate Electron Beams with ElectronFlash [PERSON]
## 4 Discussion  
In this paper, we reported the methods and the results obtained in the characterization of the electron beam with the relative dose properties, used for the FLASH radiobiological experiments at the Institut Curie. For this purpose, the ElectronFlash Linac was used. The linac, operating at 5 and 7 MeV, can irradiate in conventional and FLASH modalities, by changing the pulse width, the pulse repetition frequency, and the number of pulses. Thanks to the PMMA applicators, it was possible to irradiate different field sizes with an extremely homogeneous beam spot. This is very important for the irradiation of animals (mice) and cells in culture. Because of the high dose per pulse and high dose rate, it was necessary to use an energy-independent and dose-rate-independent dosimeter to measure the depth-dose curves and beam profiles. To this end, EBT-XD Gafchromic films have been used. A beam monitoring system based on a Faraday cup was used to measure the total charge per pulse at the end of the applicators. The results of charge measurements were related to the absorbed dose at the applicator exit for several values of the pulse widths.  
The dose per pulse was changed by varying the applicator diameter between 30 mm and 120 mm or by modifying the time pulse width for the same applicator. The dose delivered downstream the PMMA applicator A-d120 was 3 Gy/pulse at the energy of 7 MeV, corresponding to a UHDR of \(0.75\times 10^{6}\) Gy/s, and 2 Gy/pulse at 5 MeV, corresponding to a UHDR of approximately \(0.5\times 10^{6}\) Gy/s. With the PMMA A-d30, the dose was 19 Gy/pulse, corresponding to a UHDR of approximately \(5\times 10^{6}\) Gy/s at 7 MeV, and 12.3 Gy/pulse, corresponding to \(3.07\times 10^{6}\) Gy/s at 5 MeV. Therefore, with the applicator A-d30, we are always in the FLASH regime (>1 Gy/pulse) with one single pulse and time duration between 1 us and 4 us. For the applicator A-d120, we are in the FLASH regime with a pulse width between 2 us and 4 us.  
As a consequence, the beam parameters are definitively suitable for the investigation of the mechanism underlying the FLASH effect for in vivo and in vitro experiments. The field radiation sizes and the electron beam penetration make the system suitable to perform total body irradiation of small animals (e.g., mice), or the superficial irradiation of cell cultures. Finally, the comparisons between Monte-Carlo simulations with the PDD and dose profile measurements showed a very good agreement with more than 95% of the curve points under the chosen thresholds 3% 3 mm.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Irradiation Settings** & **R100 \(\gamma\)-Index** & **R80 \(\gamma\)-Index** & **R50 \(\gamma\)-Index** \\ \hline A-d120 at 7 MeV & 99.00\% & 100\% & 96.29\% \\ A-d120 at 5 MeV & 95.00\% & 95.57\% & 95.00\% \\ A-d30 at 7 MeV & 98.40\% & 99.60 \% & 99.60\% \\ A-d30 at 5 MeV & 96.01\% & 100\% & 96.41\% \\ \hline \end{tabular}
\end{table}
Table 6: Gamma index analysis for transverse dose profiles at R100, R80, and R50, performed with 3% 3 mm threshold.  
\begin{table}
\begin{tabular}{c c} \hline
**Irradiation Settings** & **PDD\(\gamma\) Index** \\ \hline A-d120 at 7 MeV & 95.68\% \\ A-d120 at 5 MeV & 99.00\% \\ A-d30 at 7 MeV & 98.67\% \\ A-d30 at 5 MeV & 97.01\% \\ \hline \end{tabular}
\end{table}
Table 5: Gamma index analysis for PDD, performed with 3% 3 mm threshold.