Polarity effect of the thimble-type ionization chamber at a low dose rate

Kim, Yong Kyun; Park, Se Hwan; Kim, Han Soo; Kang, Sang Mook; Ha, Jang Ho; Chung, Chong Eun; Cho, Seung‐Yeon; Kim, J. K.

doi:10.1088/0031-9155/50/21/003

Cited by 7 publications

(3 citation statements)

References 10 publications

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“…Boag 9 and Kim et al 10 demonstrated that a potential difference between the bias of the collecting electrode and guard electrode could distort the electric field lines within the collecting volume and ultimately affect the chamber response. However, little work has been done to investigate the prevalence of this effect in commercial ionization chambers.…”

Section: E Potential Difference Between the Guard And Collecting Ementioning

confidence: 99%

Polarity effects and apparent ion recombination in microionization chambers

et al. 2016

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A potential difference between the guard and collecting electrodes was found to be the primary source of the voltage-dependent polarity effects demonstrated by microchambers. For a given potential difference between electrodes, the relative change in the collecting volume is smaller for larger-volume chambers, illustrating why these polarity effects are not seen in larger-volume chambers with similar guard and collecting electrode designs. Thus, for small-volume chambers, it is necessary to reduce the potential difference between the guard and collecting electrodes in order to reduce polarity effects for reference dosimetry measurements.

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Section: E Potential Difference Between the Guard And Collecting Ementioning

confidence: 99%

Polarity effects and apparent ion recombination in microionization chambers

et al. 2016

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“…3. Kim et al 81 provided the magnitude of polarity effect in thimble ion chamber at low dose rate that also needs to be clearly evaluated. In general, some differences with Ϯ polarity are expected.…”

Section: Iiia1b High Voltage (Bias)mentioning

confidence: 99%

Accelerator beam data commissioning equipment and procedures: Report of the TG‐106 of the Therapy Physics Committee of the AAPM

Das

Cheng

Watts³

et al. 2008

Medical Physics

422

424

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For commissioning a linear accelerator for clinical use, medical physicists are faced with many challenges including the need for precision, a variety of testing methods, data validation, the lack of standards, and time constraints. Since commissioning beam data are treated as a reference and ultimately used by treatment planning systems, it is vitally important that the collected data are of the highest quality to avoid dosimetric and patient treatment errors that may subsequently lead to a poor radiation outcome. Beam data commissioning should be performed with appropriate knowledge and proper tools and should be independent of the person collecting the data. To achieve this goal, Task Group 106 (TG-106) of the Therapy Physics Committee of the American Association of Physicists in Medicine was formed to review the practical aspects as well as the physics of linear accelerator commissioning. The report provides guidelines and recommendations on the proper selection of phantoms and detectors, setting up of a phantom for data acquisition (both scanning and no-scanning data), procedures for acquiring specific photon and electron beam parameters and methods to reduce measurement errors (<1%), beam data processing and detector size convolution for accurate profiles. The TG-106 also provides a brief.discussion on the emerging trend in Monte Carlo simulation techniques in photon and electron beam commissioning. The procedures described in this report should assist a qualified medical physicist in either measuring a complete set of beam data, or in verifying a subset of data before initial use or for periodic quality assurance measurements. By combining practical experience with theoretical discussion, this document sets a new standard for beam data commissioning.

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“…Polarity effects, the difference in charge collected when the polarity of the supplied voltage is inverted, can also be viewed to be a function of beam quality. Polarity effects can be attributed to charges produced outside the chamber collecting volume, charge production in the central electrode and electric field distortions within the chamber [26,28]. P pol is calculated such that P pol M gives the averaged chamber response, removing the polarity dependence which is typically on the order of 0.3 % [16,17].…”

Section: Ionometrymentioning

confidence: 99%

A Novel Metrology Standard for Synchrotron Produced Monochromatic X-ray Beams

El Gamal

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Purpose: X-ray beams produced using synchrotrons have unique properties that are of interest to radiotherapy investigations and the characterization of radiation detectors. To facilitate the use of synchrotron produced photons in such investigations, this project aims to determine the absorbed dose to water at depth for synchrotronproduced monochromatic x-ray beams with a standard uncertainty less than 1 %, significantly lower than current methods. Methods:The absorbed dose to water at a depth of 2 cm, D w, 2cm , was evaluated using a purpose built novel aluminum calorimeter design. Finite element modeling and radiation transport simulations were performed to refine the design, calculate correction factors, and assess influence quantities. Two iterations of the design were constructed to test calorimeter operation and a comparison of the quantity air kerma, using a secondary microDiamond detector, was performed. Results:The combined standard uncertainty in the determination of absorbed dose to water was estimated to be 0.89 %, and was found to be constant for monochromatic energies investigated (80 keV to 140 keV). No dependence on calorimeter design was observed and the comparison with the microDiamond detector showed agreement in the determination of air kerma rate at the 1.5 % level, providing confidence in the new calorimeter standard.Conclusions: This new calorimeter has been shown to accurately determine absorbed dose to water in synchrotron-produced x-ray beams with a substantially lower uncertainty compared to current methods. Routine operation has also been demonstrated, providing a new high-accuracy dosimetric tool for users of synchrotron beams, initially in Canada but potentially at sites worldwide. i Statement of OriginalityThis thesis summarizes the author's research over the course of his doctoral studies.The work has been published in the peer-reviewed papers and conference presentations listed below. This project was completed under the supervision of Dr. MalcolmMcEwen, where he has provided guidance throughout the project and provided comments on the published thesis and peer-reviewed papers. The author performed all of the measurements, design, computational work, analysis of the results and prepared the manuscript. Peer-reviewed papers I. El Gamal I, Dessureault J, McEwen MR. A novel calorimeter for synchrotron produced monochromatic x-ray beams. Med Phys. 2023.

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Polarity effect of the thimble-type ionization chamber at a low dose rate

Cited by 7 publications

References 10 publications

Polarity effects and apparent ion recombination in microionization chambers

Polarity effects and apparent ion recombination in microionization chambers

Accelerator beam data commissioning equipment and procedures: Report of the TG‐106 of the Therapy Physics Committee of the AAPM

A Novel Metrology Standard for Synchrotron Produced Monochromatic X-ray Beams

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