Saturation currents and collection efficiencies in ionization chambers exposed to pulsed megavoltage photon and electron beams are determined assuming a linear relationship between 1/I and 1/V in the extreme near-saturation region, with I and V the chamber current and polarizing voltage, respectively. Careful measurements of chamber current against polarizing voltage in the extreme near-saturation region reveal a current rising faster than that predicted by the linear relationship. This excess current combined with conventional "two-voltage" technique for determination of collection efficiency may result in an up to 0.7% overestimate of the saturation current for standard radiation field sizes of 10X10 cm2. The measured excess current is attributed to charge multiplication in the chamber air volume and to radiation-induced conductivity in the stem of the chamber (stem effect). These effects may be accounted for by an exponential term used in conjunction with Boag's equation for collection efficiency in pulsed beams. The semiempirical model follows the experimental data well and accounts for both the charge recombination as well as for the charge multiplication effects and the chamber stem effect.
The procedure recommended by radiation dosimetry protocols for determining the collection efficiency f of an ionization chamber assumes the predominance of general recombination and ignores other charge loss mechanisms such as initial recombination and ionic diffusion. For continuous radiation beams, general recombination theory predicts that f can be determined from a linear relationship between 1/Q and 1/V2 in the near saturation region (f > 0.7), where Q is the measured charge and V the applied chamber potential. Measurements with Farmer-type cylindrical ionization chambers exposed to cobalt-60 gamma rays reveal that the assumed linear relationship between 1/Q and 1/V2 breaks down in the extreme near-saturation region (f > 0.99) where Q increases with V at a rate exceeding the predictions of general recombination theory. A comprehensive model is developed to describe the saturation characteristics of ionization chambers. The model accounts for dosimetric charge loss (initial recombination, ionic diffusion, and general recombination) and nondosimetric charge multiplication in an ionization chamber, and suggests that charge multiplication plays a significant role under typical chamber operating conditions (300 V) used in radiation dosimetry. Through exclusion of charge multiplication from the measured chamber signal Q, the model predicts the breakdown of the 1/Q vs 1/V2 relationship and shows that the final approach to saturation is governed by initial recombination and ionic diffusion which are characterized by a linear relationship between 1/Q and 1/V. Collection efficiencies calculated with this model differ by up to 0.4% from those determined through a rigorous application of general recombination theory alone.
This manuscript has been reproduced trom the microfilm master. UMI films the text directly trom the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face. white others may be trom any type of computer printer.The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct prim, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reprodudion.ln the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright materia1had to be removed, a note will indicate the deletion.Oversize matarials (e.g' l maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand corner and continuing trom left to right in equal sections with small overtaps.Photographs included in the original manuscript have been reproduced xerographically in this copy.Higher quality 6" x 9" black and white photographie prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. The author bas granted a nooexclusive licence allowing the NationaJ Library of Canada to reproduce, loan, distribute or sell copies of this thesis in microform, paper or electronic fonnats.The author retains ownership of the copyright in this thesis. Neither the thesis nor substantial extracts from it may he printed or otherwise reproduced without the author' s penmSSlon.L'auteur a accordé une licence non exclusive permettant à la Bibliothèque nationale du Canada de reproduire, prêter, distribuer ou vendre des copies de cette thèse sous la forme de microfiche/film, de reproduction sur papier ou sur fonnat électronique.L'auteur conserve la propriété du droit d'auteur qui protège cette thèse.Ni la thèse ni des extraits substantiels de celle-ci ne doivent être imprimés ou autrement reproduits sans son autorisation. A method for the detennination of the collection efficiency which includes the effects of general recombination, initial recombination, and diffusion loss, in addition to charge multiplication in the chamber volume has been described in detail. Initial recombination, diffusion loss, and charge multiplication have an observable effect on charge measurements at chamber potentials typically used for clinical dosimetry. We demonstrate tbis fact, and develop a technique to separate the parameters of initial recombination, general recombination, and diffusion 10ss from the non·dosimetric contribution of charge iv • multiplication and extract the correct saturation charge, and hence the chamber collection efficiency, from measured data.We are the tirst group to attempt to measure the output of a clinical proton beam with an extrapolation chamber. Given the excellent perfonnance of our phantomembedded extrapolation chamber in high energy photon and electron beams, we are confident that with further study, a p...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.