The development of ultra-high dose rate (UHDR) platform machines and extensive studies on radiobiological effects have demonstrated a reduction in normal tissue toxicity via FLASH radiotherapy. Very high-energy electrons (VHEE) with energies ranging from 50 to 250 MeV have gained increasing interest to be employed as radiation sources for FLASH radiotherapy due to their ability to penetrate deeply seated targets. The delivery of high doses within sub-seconds (>40 Gy/s), pose significant dosimetric challenges. Conventional detectors suffer from saturation and ion recombination, leading to substantial errors and uncertainties in measurements. Alanine dosimeters can potentially be well suited for such UHDR beams. They are composed of organic crystalline amino acids. Alanine radiation characteristics are similar to those of tissue. Stable free radicals generated in irradiated alanine have unpaired electron which can be measured using electron paramagnetic resonance (EPR) spectrometers. The amplitude of the measured signal is correlated to the energy deposition i.e. dose. Alanine dosimeter is used as a secondary standard dosimeter in radiotherapy by several national metrology laboratories. Alanine is weak energy dependent within the therapeutic energy range (6-25 MeV). Its dose rate independence makes alanine a potential dosimeter for UHDR dosimetry. However, the response of alanine to very high energy electrons has not been reported, which is the chief aim of this research. Alanine pellets calibrated with Co-60 gamma-ray, were irradiated using 100 MeV electron beams from the Pulsed Energetic Electrons for Research (PEER) end station, which serves as the injector for the Australian Synchrotron. The linac can deliver electron pulses with pulse dose rate of 10⁷Gy/s. Six different dose per pulse (DPP) from 6 – 28 Gy per pulse (in single pulse of 200 ns time) were delivered to alanine pellets, with three pallets for each dose. The EPR spectra of irradiated alanine pallets were measured using Bruker EPR spectrometer. The amplitudes of the spectra were converted to absorbed dose to water using a calibration curve for alanine dosimeter irradiated with Co-60 gamma ray. The absorbed dose measurement of the alanine dosimeter irradiated with a 100 MeV VHEE beam is 16 % lower compared to the nominal dose as measured by Faraday cup. The relative response of alanine dosimeter for 100 MeV electron beam was 0.84. This result demonstrates the significant energy dependence of alanine dosimeters when exposed to a 100 MeV VHEE.

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