Atmospheric pressure influence on the charge collection efficiency of air-vented ionization chambers in ultra-high dose per pulse electron beams for FLASH radiotherapy

This work studies the dependency of the charge collection efficiency (CCE) on the atmospheric pressure of commercial ionization chambers (ICs) using an ultra-high dose-per-pulse (DPP) electron beam and proves a theoretical model of this phenomenon. Approach: A custom-made PMMA water phantom, water- and pressure-tight sealed, was connected to a vacuum/pressure pump to vary its inside pressure from 900 hPa to 1100 hPa. ICs were placed at different depths in water and irradiated with ultra-high DPP electron beams (20 MeV, 1.2 μs and 2.0 μs pulse duration). Chamber signals were air-density- and polarity-corrected as a function of DPP (0.1 Gy - 6.5 Gy) at different pressures. The actual DPP was determined and monitored using a calibrated PTW flashDiamond and a current transformer, respectively. Experimental CCEs were compared with numerical calculations, describing the charge transport inside ICs, including free electrons and electric field distortion. Main results: The CCE decreased with increasing pressure (-6%/100 hPa and -8%/100 hPa for the Advanced Markus, and Roos IC, respectively) and followed "logistic functions" with DPP. The CCE0 at a given pressure P0 can be obtained from a CCE1 at different pressure P1 (scaling rule) as: CCE0(P0,DPP) = CCE1(P1,DPP*(P0/P1)^2). Our theoretical model predicted the relative variation of the CCE with pressure, with residuals <3%. The effect can be corrected using the scaling rule even at small changes in pressure (~35 hPa), which can cause a 2% deviation on the CCE, without adding significant CCE uncertainty (~0.1%) for a DPP up to 6.5 Gy per pulse. Significance: This work proposes a scaling rule to correct for recombination losses dependent on atmospheric pressure in ultra-high DPP electron beams. The proposed scaling rule provides a simple, low-uncertainty correction that can be applied to empirically predetermined CCE functions, improving the accuracy of commercial IC-based dosimetry in ultra-high DPP electron beams. .