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Evaluation of a Faraday cup-style detector as a beam diagnostic system for ultra-high dose rate (FLASH) electron
Alan Lopez1,2, Alexander Baikalov1,3,4, Kevin Liu1,2
1Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas, USA.
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.
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