Conventional and ultra-high dose rate proton beam energy and output characterization using a high-resolution multi-layer Faraday cup.

Objective.Proton radiotherapy requires accurate quality assurance (QA) of beam range and output. Conventional QA methods using water phantoms and ionization chambers are time-consuming and may suffer from dose-rate dependence, limiting their use for FLASH radiation therapy at ultra-high dose rates (UHDRs). This work evaluated a multi-layer Faraday cup (MLFC) as a detector for proton therapy QA, capable of high-resolution energy verification, beam charge measurements, and operation at both conventional and UHDR.Approach.A commercial 128-layer MLFC made of thin copper layers separated by Kapton and equipped with a MicroHexTMenergy filter was used to measure conventional and UHDR proton pencil beams from a IBA ProteusPlus system. A TOPAS Monte Carlo model was developed to benchmark depth-charge readout. Energy calibration of the pencil beams was performed by Gaussian fitting charge peaks and comparing to nominal ranges in water. Delivered charge was determined by integrating the collected charge across all MLFC channels and comparing it with a reference Faraday cup. Its application to UHDR FLASH radiotherapy was evaluated using spread-out Bragg Peak Conformal FLASH plan deliveries.Main results.The MLFC provided a reproducible high-resolution energy calibration within 1 mm water equivalent depth resolution, with the MicroHexTMfilter broadening charge deposition peaks and improving measurement precision and accuracy. Integrated charge was linear with monitor units and agreed with Faraday cup measurements within 5% for all measured energies (100-228 MeV). The 4 kHz sampling rate enabled spot-by-spot energy and charge output measurements at UHDR.Significance.The MLFC enables simultaneous energy and charge output verification in a compact, dose-rate-independent detector, providing a practical solution for routine QA in both conventional and FLASH proton therapy.