Detailed Investigation of the Faraday Cup, a Dosimetry Device for Proton Therapy Up to Ultra-High Dose Rates
C Winterhalter*1,2, J Ehwald1,2, M Togno1, A Lomax1,2, D C Weber1,3, S Safai1 (1) Paul Scherrer Institute, Villigen PSI, CH (2) Department of Physics, ETH Zurich, CH (3) Radiation Oncology Department of University Hospital of Bern and Zurich, CH
Presentations
WE-G-206-3 (Wednesday, 7/13/2022) 2:45 PM - 3:45 PM [Eastern Time (GMT-4)]
Room 206
Purpose: Experiments using high-dose-rate proton beams are of increased scientific interest, as these could substantially shorten treatment times, and might lead to increased tissue sparing (FLASH effect). Accurate measurements of absolute doses are crucial for these experiments, and as such the dose rate independent Faraday cup (FC) is an important dosimetric device. This study investigates and quantifies the absolute efficiency of the Faraday cup.
Methods: A detailed model of the FC, which consists of an absorber block, a vacuum tube and shielding, has been simulated using TOPAS_3.6 (Geant4_10.06.p03). The magnetic field within the FC, which aims to shield the absorber block from secondary electrons, has been measured and been included in the simulation. A range of charge/phase-space scorers has been defined to investigate the contribution of secondary particles to the absolute efficiency of the FC for incoming beam energies of 70MeV/150MeV/228MeV.
Results: For magnetic fields from 0mT to 24mT, efficiency of the FC varies by 0.30%/0.19%/0.05% (70MeV/150MeV/228MeV), with an absolute efficiency of 99.96%/100.11%/100.22% for the highest magnetic field. Without magnetic field/with maximum magnetic field, secondary electrons created within the vacuum window impact the efficiency by up to 1.00%/0.15%, whereas secondary electrons backscattered from the absorber block shift the efficiency by up to 0.78%/0.30%. Secondary electrons leaving the absorber block at its rear side, a contribution which has so far not been mentioned in literature, degrade the efficiency by up to 0.37%. This effect is only marginally suppressed by the magnetic field within the FC.
Conclusion: When used with a magnetic field, the FC is an accurate dosimetry device for high-dose-rate proton therapy. To account for small remaining contributions of secondary electrons not suppressed by the magnetic field, energy dependent correction factors might need to be implemented.
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