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Development of a Known-Volume Spherical Ionization Chamber

E King*, L DeWerd, Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To quantify the volume of a commercial spherical ionization chamber using CT imaging techniques.

Methods: Two Exradin A3 ionization chambers were imaged using a Siemens miroCT/microPET hybrid scanner with 50 µm resolution. The Siemens Inveon Research Workplace imaging software was used to contour the images to determine the volume of the chambers. The collection volume of the spherical chamber differs from its physical volume due to a portion of the electric field terminating in the guard instead of the collecting electrode. The electric field was modeled in COMSOL Multiphysics to quantify this discrepancy. The volume of the electric field that terminates in the guard was subtracted from the physical volume. A magnification factor was applied to this volume and compared to the volume calculated from each chamber's air kerma calibration in Co-60.

Results: Through the imaging techniques it was determined that the chambers had volumes of 3.57 cm³ and 3.38 cm³ with an uncertainty of 0.53% at k=2. In comparison, the calibration determined the volumes were 3.52 cm³ and 3.36 cm³, respectively, with an uncertainty of 1.5% at k=2. A physical measurement of the ionization chambers with calipers gave volumes of 3.53 cm³ and 3.37 cm³, respectively.

Conclusion: This work shows that the volume of an ionization chamber can be determined through imaging techniques within the uncertainty of volume determination from calibration, with the imaging method providing a smaller uncertainty. This indicates the potential for commercial ionization chambers to be used for absolute dosimetry at facilities outside of primary standards laboratories.

Funding Support, Disclosures, and Conflict of Interest: Larry DeWerd has a partial interest in Standard Imaging, Inc. that manufactured the ionization chambers used in this work.


Absolute Dosimetry, Ionization Chamber, X Rays


TH- Radiation Dose Measurement Devices: ion chamber: air cavity

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