Purpose: To characterize the beam delivery capabilities and dose rates achievable on the new ultrahigh dose-rate (UHDR) 10MV x-ray irradiation platform at TRIUMF.
Methods: At the TRIUMF Advanced Rare Isotope Laboratory (ARIEL) e-linac, the recently installed x-ray FLASH irradiation platform is being characterized ahead of planned FLASH radiobiological experiments. The UHDR x-ray source comprises a water-cooled electron-to-photon conversion target designed to provide dose rates >40Gy/s. To verify the UHDR capabilities of the system, the absolute dose delivered over a prescribed pulse length (<1s) has been evaluated using radiochromic films while a lead-doped fiber-optic scintillator provided online measurements of relative beam output and irradiation time. Films were irradiated at the surface, and depths of 9mm and 18mm in a solid-water phantom placed on a vertical motion stage within a 1x1cm2 collimated beam to simulate delivery to a small animal. In all cases, a 10MeV, 1kW (0.1mA) continuous electron beam was delivered to the converter using pulse lengths of up to 400ms.
Results: Film doses and temporal information from the scintillator together allowed for calculating the delivered dose rate. Radiochromic films irradiated in the phantom located at an 8.5cm source-to-surface distance in the single (FLASH) pulsed mode yielded doses of 11.6 and 13.8 and 9.5Gy at water-equivalent depths of 0, 0.9 and 1.8-cm, respectively, for a 5mm square ROI. Scintillator measurements verified a 165ms pulse length, from which the corresponding dose-rates were calculated to be 70.3, 83.6 and 57.6Gy/s. Improved beam spot centering on the target will be necessary to improve beam profile symmetry. Field flattening using 3D-printed compensators may be applied to improve dose homogeneity.
Conclusion: Beam commissioning and dosimetry have been conducted on the ARIEL x-ray FLASH irradiation platform. Measured dose rates support that the 10MV x-ray beam may be used as a UHDR source compatible with FLASH radiobiological experiments.
Funding Support, Disclosures, and Conflict of Interest: This work was partially funded by the National Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant, Canada Research Chairs program and the New Frontiers in Research Fund (NFRF)