A variable air-volume, parallel-plate, extrapolation chamber forming an integral part of a Solid-WaterTM phantom was built to determine the absorbed dose in Solid-WaterTM directly. The sensitive air-volume of the extrapolation chamber is controlled through the movement of the chamber piston by means of a micrometer mounted to the phantom body. The relative displacement of the piston is monitored by a mechanical distance travel indicator with a precision on the order of 0.002 mm. Irradiations were carried out with cobalt-60 gamma. rays, x-ray beams ranging from 4 to 18 MV, and electron beams between 6 and 22 MeV. The absorbed dose at a given depth in Solid-WaterTM is proportional to the ionization gradient measured in the Bragg-Gray cavity region with a Solid-WaterTM embedded extrapolation chamber. Measured charge is corrected for ion recombination and ion diffusion in the chamber air volume according to a comprehensive model for charge loss in an ionization chamber. The discrepancies between doses determined with our uncalibrated phantom-embedded extrapolation chamber and doses obtained with calibrated Farmer-type cylindrical ionization chambers following the AAPM-TG21 and AAPM-TG25 dosimetry protocols are less than 1% for photon and electron beams at all clinical megavoltage energies. Uncalibrated extrapolation chambers thus offer a simple and practical alternative to other techniques used in output measurements of megavoltage photon and electron beams.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.35424 |
| Date | January 1997 |
| Creators | Zankowski, Corey E. |
| Contributors | Podgorsak, E. B. (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Physics.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001615719, proquestno: NQ44642, Theses scanned by UMI/ProQuest. |
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