Speaker
Description
Monte Carlo Simulation of Plane-Parallel Ionization Chambers at Ultra-High Dose Rates
Background and Motivation
Air-filled plane-parallel ionisation chambers (PPICs) are widely used in medical dosimetry due to their simplicity and low cost. However, their performance under ultra-high dose rates (UHDR), such as those encountered in FLASH radiotherapy or high-intensity particle beams, is compromised by space charge effects and ion-ion recombination.
The space charge phenomena distort the electric field, reduce charge collection efficiency (CCE), and challenge the validity of traditional analytical models (e.g., Boag or Fenwick’s theory). A detailed, physics-based simulation framework is thus essential to understand detector limitations and optimize designs for extreme conditions.
Methods
We developed a fully 3D Monte Carlo simulation using Garfield++, a C++ toolkit for, among other things, gaseous detector modeling. Key extensions were implemented to accurately describe
- Ion-ion recombination,
- space charge accumulation and its feedback on the electric field, solved directly in Garfield++ via Poisson’s equation (iterative Gauss-Seidel with Successive over-relaxation(SOR) acceleration).
Results
The simulation was validated against Fenwick’s analytical model (Fig. 1) for recombination without space charge, showing good agreement for CCE.
Evolution of the free electron fraction (FEF) and the CCE with the dose per pulse was compared to numerical solutions of J. Paz martin et al. [1] and also shows good agreement (Fig. 2-3) for different gap sizes and bias voltages.
The response of an IBA 1.0 mm air gap PPIC was then simulated for 10 µs proton pulses representative of the IBA S2C2 230 MeV beam with dose per pulse (DPP) up to 0.5 Gy. The simulation results are in good agreement with experimental data (Fig. 4), and space charge was confirmed to be the dominant mechanism: disabling the space charge solver leads to a clear disagreement, while its inclusion restores agreement.
Conclusions
This work demonstrates that space charge effects are essential to accurately model air-filled ionization chambers at UHDR. The good agreement with experimental data confirms the validity of the approach and highlights the limitations of models neglecting space charge. This framework provides a flexible and general tool for detector optimization and reliable dosimetry in UHDR conditions, supporting different particle types, gases, detector geometries, and beam configurations.
References
[1] Paz-Martín, Jose, Andreas Schüller, Alexandra Bourgouin, et al. « Numerical Modeling of Air-Vented Parallel Plate Ionization Chambers for Ultra-High Dose Rate Applications ». Physica Medica 103 (novembre 2022): 147‑56. https://doi.org/10.1016/j.ejmp.2022.10.006.
| Name of the speaker | Pierre Gérard Ortega |
|---|---|
| Eligible for the Georges Charpak Young Scientist Award. | yes |