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

<i>Objective.</i>This study aims to investigate the responses of prototype diamond detectors under pulsed ultra-high dose rates (UHDRs) pencil-beam-scanning (PBS) protons from a compact proton synchrocyclotron (IBA Proteus®ONE) for small-field UHDR dosimetry.<i>Approach</i>. flashDiamond detectors (fDs) 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 (mDs) for cross-reference. Cross-calibrations were performed against an ADCL-calibrated PPC05 plane-parallel ionization chamber with 59.23 cGy nC^-1calibration 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 1.5 × 1.5-3.0 × 3.0 cm²square fields. Nominal absolute UHDR dosimetry was performed at 3 × 3 cm²field sizes with relative responses at smaller fields benchmarked against it.<i>Main results</i>. fD had 28.6 ± 0.1 cGy nC^-1sensitivities 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), 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 treatment planning system calculations. There is negligible partial volume averaging with fDs.<i>Significance</i>. 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.