Evaluation of the Exradin A30 Parallel Plate Ion Chamber As a Reference Dosimeter In Ultra-High Dose Rate (UHDR) Electron Beam
Abstract
Purpose
Ultra-high dose rate (UHDR) beams (40Gy/s) compromise the accuracy of conventional reference ion chambers (ICs) due to saturation effects. The Exradin A30 is an ultra-thin parallel-plate IC featuring a 0.3 mm nominal electrode spacing to enhance the charge collection efficiency (CCE). This study aimed to evaluate the commercial A30 IC as a reference dosimeter for UHDR pulsed electron beams.
Methods
This study investigated key characteristics of the A30 IC, including leakage current, CCE, and polarity. The chamber was connected to a SuperMAX electrometer, and all measurements were acquired at the exit window of a medical UHDR electron accelerator head to achieve a maximum dose-per-pulse (DPP) of 9.6 Gy (2.4 MGy/s instantaneous dose rate) using a 9 MeV electron beam. Various phantoms were tested, including virtual water, distilled water, and grounded saline water. DPPs were modulated by varying the pulse width from 0.5 to 4 μs while PRF was varied between 5-90 Hz. The CCE was calculated using EBT-XD radiochromic films under both UHDR and conventional beam conditions at identical dose and energy settings. Both CCE and Ppol were also measured as a function of DPP and PRF.
Results
The A30 IC exhibited a leakage current of 1fA. No statistically significant difference in Ppol was found between the distilled and saline water, whereas an enhanced polarity correction was observed in virtual water. Both Ppol and CCE decreased with increasing DPP, with polarity corrections decreasing to 0.966 while CCE remained in the 92-99% range. Both CCE and Ppol were noted to be independent of the PRF.
Conclusion
The A30 IC exhibited <5% recombination with DPP of up to 5 Gy and a polarity correction factor of 0.995 at the same DPP in water.