Application of Faraday Cup and Multi-Layer Faraday Cup for Conformal Uhdr Proton Pbs Flash Radiotherapy
Abstract
Purpose
Proton pencil-beam scanning (PBS) FLASH radiotherapy requires dosimetry systems capable of accurate and reliable operation under ultra high dose rate (UHDR) conditions. Faraday cups (FC) and multi-layer Faraday cups (MLFC) are dose-rate-independent charge collectors suitable for UHDR beam characterization. This work evaluates the complementary use of FC and MLFC for proton PBS FLASH delivery verification.
Methods
A commercial MLFC comprising 128 copper layers was used to measure spot-by-spot depth charge deposition, which can be analyzed to extract beam range information. A FC was used to measure integrated beam charge and intra-spot beam current. Both detectors were coupled to fast electrometers operating at 4 kHz sampling rate. As the ultra-high beam current makes it impractical to measure the beam range using large area parallel plate ion chamber scanned in a water tank, range measurements were conducted under low dose rate conditions. The MLFC was used to confirm the consistency in the pristine beam range under both low dose rates and UHDR.
Results
Both the FC and MLFC resolved individual proton spots and reproduced planned spot delivery timing at 4 kHz sampling rate. Integrated FC charge demonstrated strong linearity with delivered monitor units (R² > 0.999) and enabled direct measurement of intra-spot beam current. MLFC measurements showed stable, reproducible signal response under UHDR delivery, with consistent spot timing and signal magnitude across sequential spot deliveries without saturation at UHDR.
Conclusion
FC and MLFC provide fast, dose-rate-independent measurements of proton beam charge, current, and spot-resolved signal response under UHDR conditions. Although insensitive to scanned beam position, these detectors address a critical instrumentation need for proton PBS FLASH radiotherapy and provide a practical pathway for implementing UHDR quality assurance workflows aligned with established clinical proton therapy practice.