Ionization Chamber Measurements of High Energy Radiation
Date
1953-03
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ORCID
Type
Degree Level
Doctoral
Abstract
This thesis reports some of the results of the research conducted by the writer concerning the physical problems involved in the biological use of high energy radiation. The investigation of ionisation chamber response herein reported has led to the study of a number of allied problems, the results of which have been already published and therefore have not been included in the body of the thesis. Reprints of the papers, however, have been attached in Appendix G.
In developing a theoretical expression for the current produced in an ionisation chamber it appeared useful to know the energy distribution of electrons crossing the air volume of the chamber. A method of determining this energy distribution was developed, involving the determination of the initial energy distribution of photo-. Compton and pair electrons produced by a spectrum of radiation. In addition to the
electron distributions, it is possible to derive the distribution of ion densities of the electron tracks produced in water. These ion density distributions provide a physical basis for comparing the biological effectiveness of radiations of various energies. This determination of electron energies and ion densities is given in the paper by Cormack and Johns, a copy of which will be found in Appendix C.
In order to determine these ion density distributions, a thorough study was made by the use of the Klein-Nishina formula of the initial energy and angular distributions of Compton electrons. Values of the differential cross sections were calculated for a large number of photon energies and are given in the paper by Johns, Cormack, Denesuk, and Whitmore (26).
During 1951, the writer was involved in the physical measurements made during the installation of the Saskatchewan 1000-curie Co60 therapy unit. These results have been reported in several journals, but most completely in the article by Johns, Epp, Cormack, and Fedoruk (25). Several theoretical problems dealing with the cobalt unit mere also undertaken. These included the calculation of the percentage depth dose in water of the cobalt radiation and the calculation of the once-scattered radiation from the treatment head of the unit. This work has not yet been published.
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Degree
Doctor of Philosophy (Ph.D.)
Department
Physics
Program
Physics