• dosimetry
• FLASH radiotherapy
• ionization chamber
• UHDP

Ultra-High Dose per Pulse (UHDP) beams, essential to induce the FLASH effect, present major challenges, especially in accurate dosimetry, limiting their clinical use. Ionization chambers (ICs), the gold standard in conventional dosimetry, fail under UHDR conditions due to charge recombination effects. To address this, a novel gas-based parallel-plate detector called ALLS has been proposed. By using noble gases instead of air and maintaining a positive electric field, recombination is minimized, but this requires high voltage, risking electrical discharge. Lowering gas pressure can help avoid this issue. The goal of this research was to develop and characterize a practical ALLS-inspired chamber. A Python-based simulation of charge transport was created to study the chamber’s response. Results showed helium at 50 mbar and nitrogen at 10 mbar ensured charge collection efficiency (CCE) stability within 1% up to 12 and 10 Gy/pulse, respectively. A first prototype, built using PEEK and metallized aluminum, was tested with conventional pulsed beams, showing performance comparable to commercial ICs. Due to sealing difficulties, a PMMA vessel was constructed to host the prototype at variable gas pressures. Tests at the ElectronFlash LINAC with nitrogen confirmed agreement with simulations, demonstrating the feasibility of the concept and enabling future studies under FLASH conditions.