Faraday cup for commissioning and quality assurance for proton pencil beam scanning beams at conventional and ultra-high dose rates.

Recently, proton therapy treatments delivered with ultra-high dose rates have been of high scientific interest, and the Faraday cup is a promising dosimetry tool for such experiments. Different institutes use different Faraday cup designs, and either a high voltage guard ring, or the combination of an electric and a magnetic field is employed to minimize the effect of secondary electrons. The authors first investigate these different approaches for beam energies of 70 MeV, 150 MeV, 230 MeV and 250 MeV, magnetic fields between 0 mT and 24 mT and voltages between -1000V to 1000V. When applying a magnetic field, the measured signal is independent of the guard ring voltage, indicating that this setting minimizes the effect of secondary electrons on the reading of the Faraday cup. Without magnetic field, applying the negative voltage however decreases the signal by an energy dependent factor up to 1.3% for the lowest energy tested and 0.4% for the highest energy, showing an energy dependent response. Next, the study demonstrates the application of the Faraday cup up to ultra-high dose rates. Faraday cup measurements with cyclotron currents up to 800nA (dose rates of up to approximately 1000 Gy/s) show that the Faraday cup is indeed dose rate independent. Then, the Faraday cup is applied to commission the primary gantry monitor for high dose rates. Finally, short-term reproducibility of the monitor calibration is quantified within single days, showing a standard deviation of 0.1% (one sigma). In conclusion, the Faraday cup is a promising, dose rate independent tool for dosimetry up to ultra-high dose rates. Caution is however necessary when using a Faraday cup without magnetic field, as a guard ring with high voltage alone can introduce an energy dependent signal offset.