Investigating the behavior of hydrogenated amorphous silicon under UHDR proton beam

Abstract

Radiation-based therapies are crucial treatments in the fight against malignant cancers. Ultra-high dose rate (UHDR) proton therapy promises a higher radiobiological advantage and increased effectiveness in targeting tumor cells while sparing healthy cells by providing the total dose in a short time at a rate higher than 40 Gy/s. This work aims to characterize a hydrogenated amorphous silicon (a-Si:H) pixel under UHDR proton beams. A-Si:H gains particular interest due to its capacity to produce large-scale and high-resolution flat panel detectors at an affordable cost with high radiation hardness. A test board of a 2.4 × 2.4 mm2 a-Si:H pixel array was acquired and bonded to test at MedAustron in Wiener Neustadt. The test plan was to irradiate using a single spot of constant dose at different spill lengths and spill numbers to test the behavior from conventional to flash regimes for dose rate dependence measurements. Additionally, an ionization chamber for proton measurements (IBA PPC05) was placed at the isocenter for dose and spill length verification. The chamber was connected to the IBA DoseX electrometer equipped with an API measuring at a rate of 1 kHz. To analyze dose rate dependence, measurements were normalized twice by the dose measured via the PPC05 and the average of the specific measurement group. The integration time was kept the same at 10 s for all measurements. At higher dose rates and the same doses, the normalized charge increases until it reaches a plateau, then saturates in the flash regime. This is expected since, at higher current, charge traps get filled the higher the dose rate, until all the charge traps saturate. To test for radiation hardness, dose sensitivity was measured before and after heavy irradiation of 3 kGy using a clinical map. The sensitivity was reduced by 10%. The results were reproducible and linear with dose.