Charge Collection Efficiency in Air-Vented Plane-Parallel Ionisation Chambers at Ultra-High Dose Rates: A Self-Consistent Garfield++ Monte Carlo Model Including Space-Charge Effects and Ion Recombination

Abstract: Ultra-high dose rate (UHDR) irradiation used in FLASH radiotherapy induces strong space-charge effects in plane-parallel ionisation chambers (PPICs), leading to significant reductions in charge collection efficiency (CCE). To investigate these effects, we extended the Garfield++ framework by implementing ion-ion recombination and self-consistent space-charge electric field calculations. The developed Monte Carlo model couples particle transport, electron attachment, recombination processes, and dynamic electric-field distortions. The implementation was validated against analytical and numerical models from the literature, including the works of Fenwick and Kumar, Kranzer et al., and Paz Martín et al., with excellent agreement for the free electron fraction (FEF), CCE, induced current, and electric field evolution. The simulations show that space charge can locally increase the electric field by more than a factor of four or reduce it to nearly zero. The results suggest that CCE reduction under UHDR conditions is mainly driven by the decrease of the FEF caused by electric-field-dependent electron attachment, indicating that recombination may be largely governed by FEF evolution. This opens promising perspectives for improved analytical models and real-time correction methods for ionisation chamber dosimetry under UHDR conditions.