Feasibility of prototype diamond detectors for pulsed UHDR PBS small-field proton dosimetry for proton FLASH experiments

Objective. This study aims to investigate the responses of prototype diamond detectors under pulsed ultra-high dose rates (UHDR) pencil-beam-scanning (PBS) protons from a compact proton synchrocyclotron (IBA Proteus®ONE) for small-field UHDR dosimetry. Approach. flashDiamond detectors (fD) were cross-calibrated with their relative proton responses characterized at conventional dose rates (CONV). Then, absolute UHDR dosimetry was performed and small-field response assessed. These experiments were also conducted with Razor Diode and microdiamond detectors (mD) for cross-reference. Cross-calibrations were performed against an ADCL-calibrated PPC05 plane-parallel ionization chamber with 59.23 cGy/nC calibration coefficients. fD’s linearity, dose-rate, energy, and linear-energy-transfer (LET) responses were assessed under CONV protons. Pulsed UHDR PBS protons of 228 MeV were produced from a medical proton synchrocyclotron (IBA Proteus®ONE) for 1.5x1.5-3.0x3.0 cm2 square fields. Nominal absolute UHDR dosimetry was performed at 3x3 cm2 field sizes with relative responses at smaller fields benchmarked against it. Main results. fD had 28.6±0.1 cGy/nC sensitivities under CONV protons and were linear in response with dose-rate independence within ±0.50%. fD were similar to mD in proton energy and LET responses. However, there is an over-response of approximately 5.49%, 6.51% and 13.7% at the 226, 150 and 70 MeV Bragg peaks respectively. Under pulsed proton UHDR irradiation (0.80% s.t.d, 32.6±0.5 cGy dose-per-pulse, DPP), fD responded within ±1% as PPC05 with negligible saturation. fD agreed within ±1% with other comparable small-field detectors under small-field UHDR beams and within ±2% of RayStation TPS calculations. There is negligible partial volume averaging with fDs. Significance. Novel fD detectors did not saturate under pulsed UHDR PBS proton irradiation. Their miniscule active crystals make them suitable for small-field dosimetry but render them relatively insensitive compared to mDs. When cross-calibrated, they are suitable for absolute small-field UHDR dosimetry or for relative exit dosimetry monitoring purposes during UHDR radiobiological experiments.