Pulsed beam monitoring for electron FLASH

Safe implementation and translation of FLASH radiotherapy to the clinic requirehs development of beam monitoring devices capable of high temporal resolution with wide dynamic ranges. Ideal detectors should be able to monitor LINAC pulses, withstand high doses and dose rates, and provide information about the beam output, energy/range, and profile.

Two novel detectors have been designed and tested for ultra-high dose-rate (UHDR) monitoring: a multilayer nano-structured 3-layer high-energy-current (HEC3) detector, and a segmented large area, 4-section flat (S4) detector with the goal of exploring their properties for a future combined design.

A Novalis-TX LINAC was converted to produce a 10 MeV electron-FLASH beam. Pulses were monitored using both HEC3 and S4 detectors. The HEC3 detector structure consisted of three electrode layers separated by a nanoporous aerogel (Aero): Al(50 µm)-Aero(100 µm)-Ta(10 µm)-Aero(100 µm)-Al(50 µm). The S4 structure was comprised of three layers: Cu(100 nm)-air(1 mm)-Al(100 nm) with contact potential for charge collection. Both detectors are self-powered as they do not require an external voltage bias for charge collection. The beam was also characterized using a photodiode, Gafchromic EBT-XD Film, OSLDs, and an Advanced Markus Chamber.

The electron-FLASH beam displayed a Gaussian-like profile with 15 cm FWHM at isocenter. Electron-FLASH dose rates up to an average of 260 Gy/s were measured on the surface of a solid water phantom at isocenter with an instantaneous dose rate of 1.8 × 10⁵ Gy/s and a dose per pulse of up to 1 Gy/pulse. Both HEC3 and S4 detectors could record individual pulses for repetition rates of 360 Hz with a 4 µs pulse-width. The HEC3 detector signal increased linearly with dose, MU, number of pulses, and dose rate up to 850 Gy/s with no loss of functionality at high doses or dose rates. The S4 detector showed linearity with MU and number of pulses at each of the four channels independently showing potential for spatial information and steering but lacked dose rate independence.

Two novel detectors, HEC3 and S4, successfully measured electron-FLASH pulses and hence can be considered capable of electron-FLASH beam monitoring in different capacities. HEC3 detector technology is suitable for monitoring high-dose and UHDR beams with high temporal resolution required for pulse counting. We envision the combination of the HEC3 internal structure with the S4 piece-wise design for real-time monitoring of the temporal structure, spatial profiles, energy, and dosimetric properties of UHDR beams.