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

Objective: Ultra-high dose rate (UHDR) irradiation used in FLASH radiotherapy can lead to significant reductions in charge collection efficiency (CCE) in plane-parallel ionisation chambers (PPICs) due to enhanced ion recombination, while the high charge densities involved can also induce strong space-charge-induced electric-field distortions. This work aims to develop a Monte Carlo framework for investigating these coupled effects. Approach: We extended the Garfield++ Monte Carlo particle-transport framework by implementing ion-ion recombination and self-consistent space-charge electric field calculations. The resulting 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. Main results: Excellent agreement was obtained for the free electron fraction (FEF), CCE, induced current, and electric field evolution under UHDR conditions. 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 in some regions of the chamber. The results further suggest that the decrease in the FEF caused by electric-field-dependent electron attachment plays a major role in the reduction of the CCE under UHDR conditions. Significance: These findings highlight the importance of accurately describing the evolution of the FEF for modelling recombination under UHDR conditions, opening up prospects for improved analytical models and real-time correction methods for ionisation chamber dosimetry. More broadly, this work provides a flexible and self-consistent Monte Carlo framework for investigating recombination phenomena and improving dosimetry models for FLASH radiotherapy.