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Over the past decades, research in Radiotherapy (RT) has been focused on the goal of reducing damages from additional dose to healthy tissues whilst retaining high tumor control probability. Recent preclinical studies suggest that a new approach could be adopted by exploiting beams with ultrahigh dose-rates (up to 10⁷ Gy/s instantaneous dose-rate in a single pulse) to deliver the whole planned dose in a very short time (<200 ms), thus triggering what has been called FLASH effect, consisting of a tumor control probability comparable to standard RT and reduced toxicity to healthy tissues. However, the biological in vivo validation of FLASH effect is hindered by many open issues. One of the most urgent ones is the lack of a beam control system able to perform real time monitoring of beam intensity and direction since standard detectors (such as gas-filled ionization chambers) undergo saturation and discharges in FLASH regime. The Flash Detector beam Counter (FlashDC) project aims to develop an innovative and economical detection system capable of overcoming these limitations.

The FlashDC monitor is based on the physical principle of air fluorescence of electrons, never exploited before for this application. The main advantages of using fluorescence with respect to other luminescence-based phenomena are: a) a simple light collection system due to isotropic emission of photons; b) the absence of an energy threshold; c) photon yield per electron almost constant (4-5 fluorescence ph./m) in a wide kinetic energy range (10-1000 MeV), allowing for a linear response as a function of the beam intensity. So far two prototypes have been built and tested with two electron linacs provided by the SIT company (Aprilia, Italy), consisting of two cuboids (with different dimensions optimized for the machine characteristics) in PVC with black walls, filled with air, with light sensors on the opposite edges, assembled and positioned as shown in Fig. 1.

The first qualitative results in terms of beam position sensitivity and signal linearity as a function of the linac current are encouraging. The monitor with volume 7x7x90 cm³ was able to discriminate the in-beam/off-beam configurations, and showed a variation of signal as the beam was shifted along its longitudinal direction, hinting at an intrinsic spatial resolution. The monitor with volume 2x2x60 cm³ was irradiated with electron FLASH currents in the range 70-130 mA, and kept a remarkable signal linearity as the density of electrons traveling through the active volume increased (Fig. 2).

The analysis of preliminary data taken with FLASH electron beams has provided an encouraging indication that the FlashDC monitor can fulfill the expected requirements. With the help of a FLUKA MC simulation the detector design, capable of making a 2D map of the beam intensity, will be finalized. Further measurements will be made to verify the possible installation and performances of SiPM devices in the light collection system.