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Polarity effect in plane-parallel ionization chambers using air or a dielectric liquid as ionization medium
1Radiation Physics Department, University of Umeå, Sweden.
Medical Physics
|May 1, 1992
Summary
This study introduces a novel liquid ionization chamber using tetramethylsilane, demonstrating minimal polarity effects even in challenging radiation fields. This advancement offers improved accuracy for radiation dosimetry.
Area of Science:
- Medical Physics
- Radiation Detection and Measurement
- Dosimetry
Background:
- Traditional plane-parallel ionization chambers face limitations in specific radiation fields, such as the build-up region and near the practical range of electrons.
- These limitations are often due to polarity effects, which can significantly impact measurement accuracy in these critical areas.
- Existing chambers struggle with accurate measurements where electronic equilibrium is not established.
Purpose of the Study:
- To describe a novel plane-parallel ionization chamber utilizing a liquid dielectric medium (tetramethylsilane) instead of air.
- To investigate and minimize unwanted currents arising from the chamber components and dielectric material.
- To evaluate the chamber's performance, specifically the polarity effect, in challenging radiation field regions.
Main Methods:
- Design and construction of a plane-parallel ionization chamber with a 2 mm³ sensitive volume using tetramethylsilane.
- Meticulous attention to design factors that could induce spurious currents in the cable, stem, and dielectric.
- Experimental testing of the liquid ionization chamber's polarity effect in photon build-up regions and near the practical electron range.
Main Results:
- The liquid ionization chamber exhibited negligible polarity effects, not exceeding a few tenths of a percent.
- These results were observed in radiation field positions where conventional chambers show significant polarity effects (5% to 30%).
- The chamber's small ionization volume did not compromise its accuracy in challenging dosimetry scenarios.
Conclusions:
- The developed liquid ionization chamber offers superior performance regarding polarity effects compared to conventional gas-filled chambers.
- This technology holds promise for accurate radiation measurements in regions previously challenging for standard dosimetry equipment.
- The study discusses strategies for minimizing spurious currents, applicable to both liquid and gas-filled ionization chambers.