Evaluation of a Faraday cup‐style detector as a beam diagnostic system for ultra‐high dose rate (FLASH) electron beams

BACKGROUND: Beam diagnostic systems are essential for the development and quality assurance of ultra-high dose rate (UHDR) radiation sources to enable reliable delivery of FLASH radiotherapy (RT). Critically, suitable beam diagnostic systems for electron FLASH sources are lacking. PURPOSE: In this study, we evaluated a Faraday cup-style beam collector (BC), the BC-145-Al, on an UHDR electron linac for FLASH-RT applications. METHODS: The BC performance was benchmarked against a reference alternating-current current transformer (ACCT) across a wide range of beam currents and pulse structures to test the BC's charge linearity and signal resolution capabilities. Two termination impedances were tested: 200 kΩ and 13 Ω. RESULTS: The highly time-resolved signal of the BC was charge-proportional when terminated at 200 kΩ and current-proportional when terminated at 13 Ω, agreeing well with the time-resolved ACCT signal. The cumulative signals from the BC and ACCT showed excellent agreement, maintaining linearity with a deviation of less than ± 0.5% over 50 consecutive pulses (4-µs pulse width, 120-Hz pulse repetition frequency [PRF]). The BC/ACCT signal ratio remained stable within ± 0.5% as the PRF increased from 5 to 120 Hz at constant pulse width. Pulse width measurements from both detectors were also consistent within ± 0.5% across all nominal pulse width values (0.5-4 µs). When the charge per pulse was modulated by either increasing the pulse width or decreasing the source-to-surface distance (from 32.3 to 19.3 cm), the BC signal remained approximately linear with respect to the ACCT signal; however, the BC/ACCT ratio deviated by up to ± 6% across this range. CONCLUSIONS: The BC produced highly time-resolved absolute charge measurements of a pulsed UHDR electron beam across a large range of beam parameters. These results support its potential as a beam diagnostic system for both preclinical studies and clinical applications of electron FLASH radiotherapy.