Charge Accumulation In Liquid Water Under Electron Flash Irradiation
Abstract
Purpose
Accurate absorbed dose estimation is essential for radiotherapy and research. In ultra-high dose-rate (FLASH) beams, ~Gy doses are delivered in a few pulses, so small perturbations can bias monitoring. Beam current transformers (BCTs) are widely used for FLASH beam monitoring, but charge accumulation in insulating materials can perturb BCT signals. Because reference dosimetry is performed in water (whose resistivity varies widely and is contained in an insulating tank), the impact of liquid water on BCT-based monitoring can be variable. This work quantified the influence of charge accumulation in liquid water phantoms on BCT signals in ultra-high dose-per-pulse (DPP) electron beams and evaluated mitigation strategies.
Methods
Experiments were performed on a Mobetron using 6- and 9-MeV ultra-high DPP electron beams (1–9 Gy/pulse). Two BCTs were installed in the Mobetron head and operated with and without a grounded Faraday shield. Measurements were acquired for media placed below the collimator (air; Virtual Water; distilled, tap, or saline water contained in a PMMA tank), source-to-surface distances (18–50 cm), and DPP values (varied via pulse width).
Results
Unshielded lower BCT (LBCT) signals decreased by up to ~22% in Virtual Water and ~10% in distilled water relative to air, while the upper BCT (UBCT) remained stable, consistent with inherent shielding by conductive scattering foils. The LBCT reduction depended on distance and DPP but remained linear with DPP (r² > 0.9997). Adding a grounded Faraday shield mitigated the LBCT reduction. Increasing water conductivity (NaCl addition) and adding a submerged, grounded electrode restored LBCT stability within Type-A uncertainties, indicating that charge accumulation and slow dissipation in low-conductivity media drive BCT perturbations.
Conclusion
Charge accumulation in insulating phantoms can bias BCT response in ultra-high DPP electron beams. Grounding/shielding and increasing phantom conductivity reduce these effects and improve the reliability of BCT-based monitoring for FLASH dosimetry and quality assurance.