Nondestructive Sensitive Volume Determination Using Micro-CT of a Cavity Ionization Chamber
Abstract
Purpose
Absolute measurements offer the highest metrological quality. Known-volume ionization chambers are absolute dosimeters that determine air-kerma for photon energies above 300 keV, where free-air chambers are impractical. The precise volume determination of cavity ionization chambers has traditionally required chamber disassembly or other external measurements. This methodology restricts such measurements to primary standards laboratories. Micro-CT enables non-destructive determination of chamber geometric volume, which yields the effective sensitive volume after electric field corrections. This work develops an Exradin® A3 known-volume chamber to obtain absolute measurements of 60Co and 137Cs air-kerma rates and compares these results to NIST physical standards.
Methods
The sensitive volume of an Exradin® A3 ionization chamber was determined using micro-CT imaging, with corrections for scaling effects and electric field inhomogeneities in COMSOL Multiphysics. EGSnrc Monte Carlo simulations and experiments were performed to determine physics-, setup-, and detector-based correction factors. Using these values, the known-volume chamber measured the air-kerma rate of 60Co and 137Cs irradiators at 1 meter in a 10x10 cm2 field. Results were compared to traceable reference measurements using NIST-calibrated Exradin® A12 and A4 ionization chambers for 60Co and 137Cs, respectively.
Results
Air-kerma rates measured with the known-volume A3 and physical standards agreed within their uncertainties, with the known-volume chamber yielding lower relative standard uncertainties. For 60Co, the air-kerma rates were 11.097 mGy/s ± 0.37% (A3) versus 11.099 mGy/s ± 0.74% (A12), corresponding to a 0.02% difference. For 137Cs, air-kerma rates were 0.1566 mGy/s ± 0.39% (A3) versus 0.1564 mGy/s ± 0.74% (A4), with a 0.13% difference.
Conclusion
The precise sensitive volume of a cavity ionization chamber can be established using a non-destructive, micro-CT-based methodology, enabling absolute air-kerma measurements for high-energy photon sources such as 60Co and 137Cs. This work demonstrates potential for developing an absolute measurement-based primary standard for high-dose-rate 192Ir brachytherapy in the U.S.