Author: Mervat A. Alharbi, Wesley S. Culberson 👨🔬
Affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison, Medical Physics Department, School of medicine and Public Health, University of Wisconsin-Madison 🌍
Purpose: The charge collection using ionization chambers is challenging at ultra-high dose rates (UHDR) due to the high ion recombination The Exradin A30 prototype parallel-plate ion chamber manufactured by Standard Imaging incorporates a very small electrode spacing of 0.3 mm. This study evaluates the recombination response of the A30 prototype in UHDR electron beams using the IntraOp® Mobetron®.
Methods: The A30 was placed in a water-equivalent plastic phantom below the entrance window of the IntraOp® Mobetron® with a 4.5 cm air gap at a depth of dmax for 9 MeV and 6 MeV beams (2.2cm and 1.4cm, respectively), with 10 cm of plastic backscatter. A SuperMax electrometer was used to measure charge readings at voltages of 200 and 400V. Three pulses per delivery were delivered at 6 and 9 MeV in UHDR mode with various pulse widths (PW). To assess the proportion of ion pairs successfully collected, the ratio of the charge measured at low voltage to that measured at high voltage is calculated
Results: For a pulse width of 0.5 microseconds, the Pion value for the 6 MeV UHDR beam is 1.025, while for the 9 MeV UHDR beam is 1.026. To evaluate the charge collection, the ratio of the reading at low voltage to that at high voltage is used as an indicator. For pulse widths ranging from 0.5 to 4 microseconds, the ratio is 97% to 82% for the 6 MeV UHDR beam and from 97% to 85% for the 9 MeV UHDR beam
Conclusion: The A30 ion chamber indicates a significant charge collection effect at pulse widths larger than 0.5 microseconds for 6 and 9 MeV UHDR beams.