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

Abstract 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.