A phantom-embedded extrapolation chamber (PEEC) made of Solid Water(TM) was used for studying the buildup region of megavoltage radiation beams. We investigated the polarity effect produced in the PEEC and found that radiation induced currents, called the Compton current, were the dominant cause of the polarity effect in the PEEC. In the dose build-up region of megavoltage photon beams, the collecting electrode is the primary Compton current source and the magnitude of the current depends on the measurement depth, field size, and photon beam quality. The connecting cable acts as a secondary Compton current source that produces a very small Compton current which depends on the field size and the photon beam quality, becoming the dominant source when the PEEC is placed at depths greater than the depth of maximum dose. A study of the dose build-up region of megavoltage photon beams showed that the percentage depth ionizations obtained from measurements are higher than the percentage depth doses obtained with Monte Carlo (MC) techniques. To validate the MC-calculated percentage depth doses, the design of the PEEC was incorporated in the simulations. While the MC-calculated and measured percentage depth ionizations in the dose build-up region agreed with one another for the 6 MV, a non-negligible difference is observed for the 18 MV x-ray beam. A number of experiments and theoretical studies of various possible effects which could be the source of this discrepancy is investigated. We show that the contribution of contaminating neutrons and protons to the doses in the 18 MV x-ray beam is negligible. Moreover, the MC calculations using the XCOM photon cross sections database and the NIST bremsstrahlung differential cross sections do not explain the discrepancy between the MC calculations and measurement in the dose build-up region for the 18 MV and this discrepancy is yet to be further investigated.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.84456 |
| Date | January 2004 |
| Creators | Abdel-Rahman, Wamied |
| Contributors | Podgorsak, Ervin 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: 002148779, proquestno: AAINQ98189, Theses scanned by UMI/ProQuest. |
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