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. .
Flores-Mancera et al. (Fri,) studied this question.