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Ion recombination in liquid ionization chambers: development of an experimental method to quantify general recombination
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics.
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 [en]
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: urn:nbn:se:umu:diva-68942ISBN: 978-91-7459-607-6 (print)ISBN: 978-91-7459-608-3 (print)OAI: oai:DiVA.org:umu-68942DiVA: diva2:618929
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
List of papers
1. Application of the two-dose-rate method for general recombination correction for liquid ionization chambers in continuous beams
Open this publication in new window or tab >>Application of the two-dose-rate method for general recombination correction for liquid ionization chambers in continuous beams
2011 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 56, no 2, 299-314 p.Article in journal (Refereed) Published
Abstract [en]

A method to correct for the general recombination losses for liquid ionization chambers in continuous beams has been developed. The proposed method has been derived from Greening's theory for continuous beams and is based on measuring the signal from a liquid ionization chamber and an air filled monitor ionization chamber at two different dose rates. The method has been tested with two plane parallel liquid ionization chambers in a continuous radiation x-ray beam with a tube voltage of 120 kV and with dose rates between 2 and 13 Gy min-1. The liquids used as sensitive media in the chambers were isooctane (C8H18) and tetramethylsilane (Si(CH3)4). The general recombination effect was studied using chamber polarizing voltages of 100, 300, 500, 700 and 900 V for both liquids. The relative standard deviation of the results for the collection efficiency with respect to general recombination was found to be a maximum of 0.7 % for isooctance and 2.4 % for tetramethylsilane. The results are in excellent agreement with Greening's theory for collection efficiencies over 90 %. The measured and corrected signals from the liquid ionization chambers used in this work are in very good agreement with the air filled monitor chambers with respect to signal to dose linearity.

National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:umu:diva-38507 (URN)10.1088/0031-9155/56/2/001 (DOI)
Available from: 2010-12-16 Created: 2010-12-16 Last updated: 2017-12-11Bibliographically approved
2. On the property of measurements with the PTW microLion chamber in continuous beam
Open this publication in new window or tab >>On the property of measurements with the PTW microLion chamber in continuous beam
2012 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 39, no 8, 4775-4787 p.Article in journal (Refereed) Published
Abstract [en]

Purpose: The performance of liquid ionization chambers, which may prove to be useful tools in the field of radiation dosimetry, is based on several chamber and liquid specific characteristics. The present work investigates the performance of the PTW microLion liquid ionization chamber with respect to recombination losses and perturbations from ambient electric fields at various dose rates in continuous beams.

Methods: In the investigation, experiments were performed using two microLion chambers, containing isooctane (C8H18) and tetramethylsilane (Si(CH3)4) as the sensitive media, and a NACP-02 monitor chamber. An initial activity of approximately 250 GBq 18F was employed as the radiation source in the experiments. The initial dose rate in each measurement series was estimated to 1.0 Gy min-1 by Monte Carlo simulations and the measurements were carried out during the decay of the radioactive source. In the investigation of general recombination losses, employing the two-dose-rate method for continuous beams, the liquid ionization chambers were operated at polarizing voltages 25, 50, 100, 150, 200 and 300 V. Furthermore, measurements were also performed at 500 V polarizing voltage in the investigation of the sensitivity of the microLion chamber to ambient electric fields.

Results: The measurement results from the liquid ionization chambers, corrected for general recombination losses according to the two-dose-rate method for continuous beams, had a good agreement with the signal to dose linearity from the NACP-02 monitor chamber for general collection efficiencies above 70%. The results also displayed an agreement with the theoretical collection efficiencies according to the Greening theory, except for the liquid ionization chamber containing isooctane operated at 25 V. At lower dose rates, perturbations from ambient electric fields were found in the microLion chamber measurement results. Due to the perturbations, measurement results below an estimated dose rate of 0.2 Gy min-1 were excluded from the present investigation of the general collection efficiency. The perturbations were found to be more pronounced when the chamber polarizing voltage was increased.

Conclusions: By using the two-dose-rate method for continuous beams, comparable corrected ionization currents from experiments in low- and medium energy photon beams can be achieved. However, the valid range of general collection efficiencies has been found to vary in a comparison between experiments performed in continuous beams of 120 kVp x-ray, and the present investigation of 511 keV annihilation photons. At very high dose rates in continuous beams, there are presently no methods that can be used to correct for general recombination losses and at low dose rates the microLion chamber may be perturbed by ambient electric fields. Increasing the chamber polarizing voltage, which diminishes the general recombination effect, was found to increase the microLion chamber sensitivity to ambient electric fields. Prudence is thus advised when employing the microLion chamber in radiation dosimetry, as ambient electric fields of the strength observed in the present work may be found in many common situations. Due to uncertainties in the theoretical basis for recombination losses in liquids, further studies on the underlying theories for the initial and general recombination effect are needed if liquid ionization chambers are to become a viable option in high precision radiation dosimetry.

Place, publisher, year, edition, pages
American Association of Physicists in Medicine, 2012
Keyword
General recombination, continuous beam, two-dose-rate method, liquid ionization chamber, radiation dosimetry, isooctane, tetramethylsilane, perturbation, ambient electric field
National Category
Other Physics Topics
Research subject
radiofysik
Identifiers
urn:nbn:se:umu:diva-57472 (URN)10.1118/1.4736804 (DOI)
Available from: 2012-08-07 Created: 2012-07-30 Last updated: 2017-12-07Bibliographically approved
3. A comparison of different experimental methods for general recombination correction for liquid ionization chambers
Open this publication in new window or tab >>A comparison of different experimental methods for general recombination correction for liquid ionization chambers
Show others...
2012 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 57, no 21, 7161-7175 p.Article in journal (Refereed) Published
Abstract [en]

Radiation dosimetry of highly modulated dose distributions requires a detector with a high spatial resolution. Liquid filled ionization chambers (LICs) have the potential to become a valuable tool for the characterization of such radiation fields. However, the effect of an increased recombination of the charge carriers, as compared to using air as the sensitive medium has to be corrected for. Due to the presence of initial recombination in LICs, the correction for general recombination losses is more complicated than for air-filled ionization chambers. In the present work, recently published experimental methods for general recombination correction for LICs are compared and investigated for both pulsed and continuous beams. The experimental methods are all based on one of two approaches, either measurements at two different dose rates (two-dose-rate methods), or measurements at three different LIC polarizing voltages (three-voltage methods). In a comparison with the two-dose-rate methods, the three-voltage methods fail to achieve accurate corrections in several instances, predominantly at low polarizing voltages and dose rates. However, for continuous beams in the range of polarizing voltages recommended by the manufacturer of the LICs used, the agreement between the different methods is generally within the experimental uncertainties. For pulsed beams, the agreement between the methods is poor. The inaccuracies found in the results from the three-voltage methods are associated with numerical difficulties in solving the resulting equation systems, which also make these methods sensitive to small variations in the experimental data. These issues are more pronounced for the case of pulsed beams. Furthermore, the results suggest that the theoretical modelling of initial recombination used in the three-voltage methods may be a contributing factor to the deviating results observed.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2012
Keyword
General recombination, initial recombination, liquid ionization chamber, radiation dosimetry, two-dose-rate method, three-voltage method, isooctane, tetramethylsilane
National Category
Other Physics Topics Radiology, Nuclear Medicine and Medical Imaging
Research subject
radiofysik
Identifiers
urn:nbn:se:umu:diva-60453 (URN)10.1088/0031-9155/57/21/7161 (DOI)
Available from: 2012-10-19 Created: 2012-10-13 Last updated: 2017-12-07Bibliographically approved
4. A study of recombination losses in liquid ionization chambers
Open this publication in new window or tab >>A study of recombination losses in liquid ionization chambers
(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
General recombination, initial recombination, liquid ionization chamber, radiation dosimetry, isooctane, tetramethylsilane
National Category
Other Physics Topics Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:umu:diva-68941 (URN)
Available from: 2013-04-30 Created: 2013-04-30 Last updated: 2013-05-08Bibliographically approved

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