Tests of the commercial Exradin A30 ion chamber found that its response varied with dose per pulse and differed between grounded and ungrounded virtual water in ultra-high-dose-rate (UHDR) electron beams. Charge-collection efficiency (CCE) and the polarity correction, Ppol, were lower at higher dose per pulse, and both were approximately 2% lower in ungrounded than grounded virtual water.
The study also reported 5% recombination at dose per pulse up to 5 Gy in distilled and saline water.
The evaluation measured leakage current, CCE, Ppol and the beam-quality correction factor kQ. It used water-equivalent phantoms and Monte Carlo calculations, with kQ also measured in electron beams from a TrueBeam linear accelerator.
How the measurements were set up
The UHDR measurements used a 1.5-centimetre air gap and a 9 MeV electron beam. The maximum dose per pulse was 9 Gy, the instantaneous dose rate was 2.25 MGy/s, pulse width was fixed at 4 microseconds, and pulse-repetition frequency ranged from 5 to 90 hertz. Source-to-surface distance was varied to change dose per pulse.
Measurements used grounded water-equivalent solid phantoms and a one-dimensional water tank filled first with distilled water and later with saline at 5 grams per litre. The comparison also included grounded and ungrounded virtual water.
To determine CCE, researchers compared A30 responses under UHDR and conventional beams at identical dose and energy, using EBT-XD film for dose normalization. Ppol was calculated using a two-polarity voltage equation, while kQ values were calculated with Monte Carlo and measured in electron beams from a TrueBeam linear accelerator.
The response shifted with test conditions
At higher dose per pulse, both CCE and Ppol were lower. CCE remained in the reported 90% to 99% range, while Ppol decreased from 0.99 to 0.981 in liquid and virtual media.
In virtual water, CCE and Ppol were approximately 2% lower in the ungrounded condition than in the grounded condition across all tested dose-per-pulse values.
Across pulse-repetition frequencies from 5 to 90 hertz, no dependence of CCE or Ppol was observed in distilled and saline water.
Measured kQ values agreed with calculations to within 0.8% at the tested electron energies except at 9 MeV, where the discrepancy was 2%.
Leakage current was below 2 fA, and Ppol was 1.0009 under Co-60 reference-calibration conditions.
The result is conditional
The study's stated uncertainty concerns the reference used for the CCE calculations. Those estimates relied on film and BCT dose reporting, which the authors identify as a source of uncertainty, and they recommend additional independent standards.
The authors conclude that the A30 is suitable for UHDR reference dosimetry when appropriate CCE corrections are applied. The measurements therefore support conditional use rather than a single unqualified reading across different dose-per-pulse and phantom-grounding conditions.
Funding and disclosures
The funding statement reports support from the National Cancer Institute and National Institutes of Health, a Phase I Small Business Innovation Research contract with Standard Imaging, and the University Cancer Foundation through the MD Anderson institutional grant program.
Shannon Holmes, Larry DeWerd and Brian Hooten are employees of Standard Imaging Inc.
Research data are stored in an institutional repository and will be shared upon request to the corresponding author.
Paper data and sources
Original title: Evaluation of the Exradin A30 Parallel Plate Ion Chamber as a Reference Dosimeter in Ultra-High Dose Rate (UHDR) Electron Beams
Authors: Mervat Alharbi, Kevin Liu, Brian Hooten et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-25
DOI: Not available
Original paper · Full text