Performance evaluation of ESR dosimeters under proton beam FLASH irradiation conditions

Two chemical dosimeters, lithium formate monohydride (LFM) and L-alanine (ALA), were first evaluated under ultra-high dose rate (UHDR) proton irradiation conditions, known as 'FLASH', which has the potential to reduce the impact on normal tissue while effectively killing tumors, using electron spin resonance (ESR) spectroscopy. Both ALA and LFM demonstrated a significant linear increase in ESR peaks that correlated with the physical dose when comparing conventional radiation (CONV) to UHDR radiation. The relative effectiveness (RE: the ratio of the amount of free radicals produced by each type of proton irradiation to the amount produced by 60Co γ-ray irradiation) was determined for CONV, UHDR-Plateau and UHDR-Peak, yielding RE values of 0.849, 0.731 and 0.661 for LFM and 0.834, 0.692 and 0.624 for ALA, respectively. The decrease in RE values was likely due to the combination of UHDR and the increase of linear energy transfer (LET) to facilitate the recombination of radicals formed within the crystal during CONV and UHDR of proton beams. When using the height of the ESR central peak as an indicator of sensitivity, LFM was assessed to be ~20% more sensitive than ALA.