A Khan*, J Radtke, L DeWerd, Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison

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  A Khan

MO-F-BRC-4 (Monday, 7/11/2022) 1:45 PM - 2:45 PM [Eastern Time (GMT-4)]

Ballroom C

Purpose: To characterize an etched printed circuit board (PCB)-based extrapolation chamber (EC) with a segmented guard to measure absolute absorbed dose to water from unsealed alpha radionuclides and ⁹⁰Sr/⁹⁰Y beta ophthalmic applicators.

Methods: The guard electrode surrounding the circular collecting electrode was divided into four isolated sectors. A highly accurate parallel alignment between the EC and the source electrode can be achieved by minimizing the differential capacitance of the opposing sector electrodes along both lateral directions while rotating the source electrode. The behavior of the electric field lines near the collector-guard interfaces was investigated using COMSOL electrostatic simulations. The increase in backscatter due to gold electrodes, compared to D400 polystyrene-equivalent electrodes, was studied using TOPAS Monte Carlo (MC) simulations for a commercial ⁹⁰Sr/⁹⁰Y beta ophthalmic applicator and a ²¹⁰Po point source. The k(backscatter) correction factor, as a function of air gap between the EC and source electrodes, was determined by replacing the detector electrode materials with water.

Results: According to the COMSOL simulations, the sensitive volume of the air cavity was found to be defined by 0.5(r(collector)+r(guard inner)), where r is the radius. For the ²¹⁰Po alpha source, the k(backscatter) correction was found to decrease with increasing air gap ranging from 0.85-0.90 and 0.87-0.90 for the PCB and D400 ECs, respectively. However, the opposite trend was observed for the PCB k(backscatter) for the beta source with a range of 0.82-0.93. The D400 k(backscatter), with a magnitude of ~3%, was found to be independent of the air gap for the beta ophthalmic applicator.

Conclusion: A PCB-based EC was constructed to allow superior parallel alignment between the source and collection electrodes. Such a detector was deemed more suited for absorbed dose measurements from alpha sources than beta sources based on the large backscatter correction observed for the PCB EC.

Keywords

Absolute Dosimetry, Ionization Chamber, Extrapolation Chamber

Taxonomy

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

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