Purpose: To investigate the feasibility of online monitoring of irradiation and scan time for FLASH beam using a pixelated semiconductor detector.
Methods: Measurements of the time structure of FLASH irradiations were performed using fast, pixelated detectors, AdvaPIX-TPX3 and Minipix-TPX3. The latter has a fraction of its sensor coated with a material to increase neutron sensitivity. With little or no dead time and an ability to resolve events that are closely spaced in time (tens of nanoseconds), both detectors can accurately determine irradiation times, provided no pile-ups. To avoid pile-ups, we placed the detectors well beyond the Bragg peak or at a large scattering angle. We acquired prompt gammas and secondary neutrons and calculated irradiation times based on timestamps of the first charged species and the last charged species. We also measured scan times in x, y, and diagonal directions. We performed these measurements for a single spot, for a standard field, for an irregular field, and for an experiment using an anthropomorphic phantom to demonstrate in vivo online monitoring of irradiation time. All measurements were compared to vendor log files.
Results: Differences between measurements and log files for a single spot, a standard, and an irregular field were 0.4%, -0.02%, and 0.9%, respectively. In vivo monitoring of irradiation times (95-270 ms) was accurate within 0.001% for AdvaPIX-TPX3 and within 5.6% for Minipix-TPX3. The scan times in x, y, and diagonal directions were 4.0, 3.7, and 4.0 ms, respectively.
Conclusion: High accuracy of our measurements indicates that prompt gammas are a good surrogate for primary protons. The Minipix-TPX3 showed a somewhat higher discrepancy, suggesting a need for further investigation. The scan times in the y-direction were slightly less than in the x-direction, confirming the faster scanning speed of the Y magnets. Diagonal scan speed was limited by the slower X magnets.