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Evaluation of the Exradin A26 Ion Chamber in Megavoltage Photon Beams as a Reference Class Instrument

M McEwen

M McEwen1*, (1) National Research Council, Ottawa, ON


SU-E-T-172 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

The Exradin A26 is a new design of micro-ionization ion chamber that externally resembles the Exradin A16 model but has significant internal changes to address measurement issues reported in the literature for the A16. This project involved the characterization of two versions of the A26 chamber in high energy x-rays with particular reference to the performance specification laid out in the imminent Addendum to TG-51.

The Exradin A26 was investigated in a range of megavoltage photon beams (6-25 MV). Investigations looked at chamber settling, ion recombination and polarity. Since it has been previously shown that non-ideal performance is most easily identified through ion recombination measurements, the focus was on the determination of Pion.

i) Chamber settling - the chamber response stabilizes very quickly (within 3 minutes), even after a large change in the polarizing voltage.
ii) The polarity correction was found to be small (within 0.2% of unity)
iii) The chamber showed linear behavior for a Jaffe plot (1/reading vs 1/polarizing voltage) for applied voltages ≤ 200 V.
iv) The recombination correction showed a linear variation with the dose-per-pulse, was not significantly dependent on the polarity of the collecting voltage and was consistent with the chamber dimensions (i.e. agreed with Boag theory).

An initial investigation of the Exradin A26 micro chamber suggests that although its performance exceeds the AAPM specification for a reference-class ion chamber for use in megavoltage photon beams it is a significant improvement over the previous A16 design. Further work is required to evaluate long-term stability and determine kQ factors.

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