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Design Considerations for a Medium-Energy Free-Air Ionization Chamber

E King*, J Hull, D Anderson, L DeWerd, Dept of Med Phys, School of Medicine and Public Health, Univ of Wisc-Madison


(Tuesday, 7/16/2019) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 3

Purpose: To design a variable-length free-air chamber to directly measure x-ray exposure up to 300 keV to be used for characterizing radiobiology x-ray sources.

Methods: There were three main design considerations for this free-air chamber (FAC): the size, the collector-guard system, and shielding. The diameter of 30 cm and length of 30 cm were based on Frank Attix’s proposed FAC design and the diameter was confirmed by a model in MCNP6 (Monte Carlo N-Particle 6), where the energy deposited in the air reached its maximum. The collector-guard system was modeled in COMSOL Multiphysics to ensure no corona discharge will take place. Finally, shielding design was done also using MCNP6, by minimizing the lead thickness that would allow in-scattered photons, with a model of the x-ray vault.

Results: The collecting volume of the FAC was determined to require a 30 cm diameter and a 30 cm length. The electrons produced in air by photons up to 300 keV deposit 100% of their energy within this volume as confirmed by MCNP6. Any contribution of the walls to scatter and electron loss, will be corrected by correction factors determined by Monte Carlo. For the collector-guard system, it was determined that guards at ground and high voltage plates need to be separated by 1.5 cm and the 0.25 inch high-voltage plates need to be radiused to at least 0.1 inch. Through the use of the flux tally in MCNP6, the shielding was determined to be sufficient with a 2 cm front lead plate and 2 mm lead plates on the sides, top and back.

Conclusion: A FAC was designed to be suitable for x-ray energies up to 300 keV, with no corona discharge, and with shielding that allows for little contribution from scattered x-rays within the vault.


Absolute Dosimetry, Calibration, X Rays


TH- Radiation dose measurement devices: Development (new technology and techniques)

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