Evaluation of the Exradin A30 Parallel Plate Ion Chamber as a Reference Dosimeter in Ultra-High Dose Rate (UHDR)...

Establishing reliable reference dosimetry for ultra-high dose-rate (UHDR) beams (≥ 40 Gy/s) is challenging with conventional reference ionization chambers (IC) due to saturation effects arising from ion recombination. The Exradin A30 IC was introduced to overcome these challenges by utilizing an ultra-thin electrode spacing of 0.3 mm to enhance charge-collection efficiency (CCE).

The aim of this study is to investigate the commercial A30 IC as a reference dosimeter for UHDR electron beams. It explores the dosimetric properties of the A30 IC, including leakage current, CCE, polarity, and beam quality correction factors.

Several measurements at the IntraOp^® Mobetron^® were acquired at a 1.5 cm distance from a medical UHDR electron accelerator head to achieve a maximum dose per pulse (DPP) of 9 Gy, and the instantaneous (or intra-pulse) dose rate (IDR) was 2.25 MGy/s using a 9 MeV electron beam. The measurements were obtained in a grounded water-equivalent plastic phantom, distilled water, and saline water. DPP values were adjusted by varying the SSD at a fixed pulse width (4 μs) while the pulse repetition frequency (PRF) was varied between 5 and 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 P_pol were also measured as a function of DPP and PRF in distilled and saline water. Beam quality correction factors, $k_Q$ , were calculated using Monte Carlo and measured in electron beams from a TrueBeam linear accelerator.

The A30 IC exhibited a leakage current of less than 2 fA. Both P_pol and CCE decreased with increasing DPP, with CCE remaining in the 90-99% range across all three phantoms while polarity corrections decreased from 0.99 to 0.981 in both liquid and virtual water, respectively. Both CCE and P_pol were observed to be independent of the PRF, ranging from 5-90 Hz, in distilled and saline water. Measured $k_Q$ values agreed with calculation to within 0.8% for all energies except 9 MeV, which showed a 2% discrepancy.

The commercial A30 ionization chamber exhibited 5% recombination with DPP of up to 5 Gy in both distilled and saline water. The IC signal in a solid phantom was found to be dependent on charge buildup effects, which can be mitigated by utilizing grounded phantoms. When appropriate CCE corrections are applied, the A30 IC is a suitable reference dosimeter for UHDR electron beams.