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A study of recombination losses in liquid ionization chambers
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In the present work, recombination losses in liquid ionization chambers are studied by employing theories derived for gas-filled ionization chambers. A new and robust approximate relation for the general collection efficiency is derived, which is theoretically and experimentally investigated alongside existing theory. Ionized liquids can be strongly affected by both initial and general recombination, and these effects have therefore been disassociated by the two-dose-rate method in order to evaluate the soundness of modelling recombination losses in liquids according to theories for gases, where initial recombination is negligible. The results indicate that liquids and gases have very similar general recombination characteristics, where liquids mimic the behaviour expected in gases. However, it is shown that the disassociation of initial and general recombination in the two-dose-rate method is not trivial since initial recombination depends on the spatial variation of the collecting electric field, which in turn is employed to describe the general collection efficiency. The problem of initial recombination in liquids is also shown to be further complicated by a dependence on the radiation quality, as well as the type of liquid employed as the sensitive media.

Keyword [en]
General recombination, initial recombination, liquid ionization chamber, radiation dosimetry, isooctane, tetramethylsilane
National Category
Other Physics Topics Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:umu:diva-68941OAI: oai:DiVA.org:umu-68941DiVA: diva2:618905
Available from: 2013-04-30 Created: 2013-04-30 Last updated: 2013-05-08Bibliographically approved
In thesis
1. Ion recombination in liquid ionization chambers: development of an experimental method to quantify general recombination
Open this publication in new window or tab >>Ion recombination in liquid ionization chambers: development of an experimental method to quantify general recombination
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

An experimental method (the two-dose-rate method) for the correction of general recombination losses in liquid ionization chambers has been developed and employed in experiments with different liquids and radiation qualities. The method is based on a disassociation of initial and general recombination, since an ionized liquid is simultaneously affected by both of these processes.

The two-dose-rate method has been compared to an existing method for general recombination correction for liquid ionization chambers, and has been found to be the most robust method presently available.

The soundness of modelling general recombination in liquids on existing theory for gases has been evaluated, and experiments indicate that the process of general recombination is similar in a gas and a liquid. It is thus reasonable to employ theory for gases in the two-dose-rate method to achieve experimental corrections for general recombination in liquids. There are uncertainties in the disassociation of initial and general recombination in the two-dose-rate method for low applied voltages, where initial recombination has been found to cause deviating results for different liquids and radiation qualities.

Sensitivity to ambient electric fields has been identified in the microLion liquid ionization chamber (PTW, Germany). Experimental data may thus be perturbed if measurements are conducted in the presence of ambient electric fields, and the sensitivity has been found to increase with an increase in the applied voltage. This can prove to be experimentally limiting since general recombination may be too severe for accurate corrections if the applied voltage is low.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2013. 76 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1567
Keyword
General recombination, initial recombination, liquid ionization chamber, radiation dosimetry
National Category
Other Physics Topics Radiology, Nuclear Medicine and Medical Imaging
Research subject
radiofysik
Identifiers
urn:nbn:se:umu:diva-68942 (URN)978-91-7459-607-6 (ISBN)978-91-7459-608-3 (ISBN)
Public defence
2013-05-31, Sal 260, by 3A, Norrlands universitetssjukhus, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2013-05-08 Created: 2013-04-30 Last updated: 2014-12-19Bibliographically approved

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Andersson, JonasTölli, Heikki

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