Stem effect-free (Y,Yb)AG-based detectors for ultra-high dose rate electron beam dosimetry

FLASH radiotherapy has the potential to radically change the approach to treating tumours by providing unaltered therapeutic efficacy with enhanced healthy tissue sparing. However, beam monitoring and dosimetry at the ultra-high dose and dose rates typical of FLASH pose a serious challenge to its clinical translation and require the development of dedicated innovative solutions. An interesting candidate is inorganic scintillators, which lack water-equivalence but have higher radiation hardness than their organic counterpart. In this work, fibre-coupled inorganic scintillator detectors manufactured with Yb-admixed yttrium aluminum garnet (YAG) crystals were developed and characterised with 9 MeV electron FLASH beams. Thanks to the near-infrared (NIR) emission of Yb3 +, they offer the possibility of spectrally separating the scintillation light from the spurious background luminescence (known as the stem effect). The possibility of modifying their scintillation properties by varying the Yb content makes them flexible and adaptable to specific applications. First, the luminescence properties of the crystals were characterized by steady-state and time-resolved photoluminescence and radioluminescence measurements. Second, the response of the dosimeters to ultra-high dose per pulse values with various average dose rates was evaluated, and the stem effect dependence on the dosimeter orientation with respect to the beam was assessed. Although the long measured decay times (above 100 μs) limit the possibility of using these crystals for monitoring the beam pulse time structure, first results showed a linear response up to dose per pulse values of at least 6 Gy (instantaneous dose rate > 106 Gy/s), independence from the average dose rate in the interval 38–925 Gy/s, and negligible stem effect contribution in the NIR emission region of Yb3+, suggesting applicability with FLASH beams.