Charge collection efficiency, underlying recombination mechanisms, and the role of electrode distance of vented ionization chambers under ultra-high dose-per-pulse conditions

Kranzer, Rafael; Schüller, A.; Rodríguez, Faustino Gómez; Weidner, Jan; Paz-Martín, Jose; Looe, Hui Khee; Poppe, Björn

doi:10.1016/j.ejmp.2022.10.021

Cited by 16 publications

(31 citation statements)

References 31 publications

(49 reference statements)

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“…However, a separate simulation study from Kranzer et al. reports instantaneous dose rate independence for sub‐mm ion chamber designs suggesting that such phenomenon arises from a static state in the electric field that is achieved in the ion chamber above a certain PW for a given electrode spacing, which is governed by the time of the slowest charge carriers to traverse the distance between the electrodes 29 . The same simulation study reports a less than 0.15% change in charge collection efficiency for different instantaneous dose rates (simulated as high as 3 MGy/s) 29 .…”

Section: Discussionmentioning

confidence: 99%

“…reports instantaneous dose rate independence for sub‐mm ion chamber designs suggesting that such phenomenon arises from a static state in the electric field that is achieved in the ion chamber above a certain PW for a given electrode spacing, which is governed by the time of the slowest charge carriers to traverse the distance between the electrodes 29 . The same simulation study reports a less than 0.15% change in charge collection efficiency for different instantaneous dose rates (simulated as high as 3 MGy/s) 29 . These simulation studies highlight the need for a comprehensive experimental evaluation investigating the PW dependence of ion chambers at different DPPs and instantaneous dose rates, especially for chambers where the ion collection time is on par or shorter than the pulse duration.…”

Section: Discussionmentioning

confidence: 99%

“…Since the SuperMAX electrometer has the capability to provide bias voltages as high as 1000 V, sub‐mm ion chamber design would allow the user to examine different electric field strengths suitable to minimizing charge recombination as an avenue to designing a chamber with minimal saturation and recombination effects. Others have reported on designs of prototype parallel plate ion chambers with sub‐mm (≤1 mm) electrode spacing that have utilized or simulated higher electric field gradients (going as high as 1200 V/mm), and these yielded improved charge collection efficiencies at higher DPP values 28,29 . However, it is important to note that given the same electric field gradient, chambers with a smaller electrode spacing were reported to show improved charge collection efficiencies as a function of DPP, emphasizing the influence that electrode spacing has over bias voltage of the chamber design 29 .…”

Section: Discussionmentioning

confidence: 99%

“…Others have reported on designs of prototype parallel plate ion chambers with sub‐mm (≤1 mm) electrode spacing that have utilized or simulated higher electric field gradients (going as high as 1200 V/mm), and these yielded improved charge collection efficiencies at higher DPP values 28,29 . However, it is important to note that given the same electric field gradient, chambers with a smaller electrode spacing were reported to show improved charge collection efficiencies as a function of DPP, emphasizing the influence that electrode spacing has over bias voltage of the chamber design 29 . This is shown in comparing the charge collection efficiency at 1000 V/mm measured by the TPP with those measured by sub‐mm prototype chambers produced in previous papers 28,29 .…”

Section: Discussionmentioning

confidence: 99%

“…Unfortunately, such comprehensive evaluations are difficult to perform due to variability in machine performance when operating at these extreme dose rates and a lack of a “ground truth” to evaluate the detectors against. Currently, ion chambers evaluated for their response in UHDR electron beamlines include the PTW Advanced Markus chamber, 8,25 PTW Roos, 25,26 and a variety of custom‐designed chambers such as end‐window‐chambers 27 with 0.5–2 mm electrode separation; ultra‐thin parallel plate ion chambers 28 with 0.22 and 0.27 mm air gaps; and an ultra‐thin parallel plate ionization chamber with a 0.25 mm air gap 29 . These reports have indicated through measurement or simulations that the charge collection efficiency, which includes the effects of saturation as well as ion recombination effects due to ion‐ion and ion‐electron recombination processes, depends strongly on DPP, and that modifications to ion chambers such as a higher bias voltage or a smaller electrode separation can improve charge collection efficiency and reduce the polarity effect exhibited by ion chambers in UHDR electron beamlines.…”

Section: Introductionmentioning

confidence: 99%

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Evaluation of ion chamber response for applications in electron FLASH radiotherapy

Liu,

Holmes,

Hooten

et al. 2023

Medical Physics

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Ion chambers are required for calibration and reference dosimetry applications in radiation therapy (RT). However, exposure of ion chambers in ultra‐high dose rate (UHDR) conditions pertinent to FLASH‐RT leads to severe saturation and ion recombination, which limits their performance and usability. The purpose of this study was to comprehensively evaluate a set of commonly used commercially available ion chambers in RT, all with different design characteristics, and use this information to produce a prototype ion chamber with improved performance in UHDR conditions as a first step toward ion chambers specific for FLASH‐RT. The Advanced Markus and Exradin A10, A26, and A20 ion chambers were evaluated. The chambers were placed in a water tank, at a depth of 2 cm, and exposed to an UHDR electron beam at different pulse repetition frequency (PRF), pulse width (PW), and pulse amplitude settings on an IntraOp Mobetron. Ion chamber responses were investigated for the various beam parameter settings to isolate their dependence on integrated dose, mean dose rate and instantaneous dose rate, dose‐per‐pulse (DPP), and their design features such as chamber type, bias voltage, and collection volume. Furthermore, a thin parallel‐plate (TPP) prototype ion chamber with reduced collector plate separation and volume was constructed and equally evaluated as the other chambers. The charge collection efficiency of the investigated ion chambers decreased with increasing DPP, whereas the mean dose rate did not affect the response of the chambers (± 1%). The dependence of the chamber response on DPP was found to be solely related to the total dose within the pulse, and not on mean dose rate, PW, or instantaneous dose rate within the ranges investigated. The polarity correction factor (Ppol) values of the TPP prototype, A10, and Advanced Markus chambers were found to be independent of DPP and dose rate (± 2%), while the A20 and A26 chambers yielded significantly larger variations and dependencies under the same conditions. Ion chamber performance evaluated under different irradiation conditions of an UHDR electron beam revealed a strong dependence on DPP and a negligible dependence on the mean and instantaneous dose rates. These results suggest that modifications to ion chambers design to improve their usability in UHDR beamlines should focus on minimizing DPP effects, with emphasis on optimizing the electric field strength, through the construction of smaller electrode separation and larger bias voltages. This was confirmed through the production and evaluation of a prototype ion chamber specifically designed with these characteristics.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Evaluation of ion chamber response for applications in electron FLASH radiotherapy

Liu,

Holmes,

Hooten

et al. 2023

Medical Physics

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Characterization of a commercial plastic scintillator for electron FLASH dosimetry

Oh,

Hyun,

Gallagher

et al. 2024

J Applied Clin Med Phys

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PurposeThis study investigated the potential of a commercially available plastic scintillator, the Exradin W2, as a real‐time dosimeter for ultra‐high‐dose‐rate (UHDR) electron beams. This work aimed to characterize this system's performance under UHDR conditions and addressed limitations inherent to other conventional dosimetry systems.Methods and materialsWe assessed the W2's performance as a UHDR electron dosimeter using a 16 MeV UHDR electron beam from the FLASH research extension (FLEX) system. Additionally, the vendor provided a beta firmware upgrade to better handle the processing of the high signal generated in the UHDR environment. We evaluated the W2 regarding dose‐per‐pulse, pulse repetition rate, charge versus distance, and pulse linearity. Absorbed dose measurements were compared against those from a plane‐parallel ionization chamber, optically stimulated luminescent dosimeters and radiochromic film.ResultsWe observed that the 1 × 1 mm W2 scintillator with the MAX SD was more suitable for UHDR dosimetry compared to the 1 × 3 mm W2 scintillator, capable of matching film measurements within 2% accuracy for dose‐per‐pulse up to 3.6 Gy/pulse. The W2 accurately ascertained the inverse square relationship regarding charge versus virtual source distance with R2 of ∼1.00 for all channels. Pulse linearity was accurately measured with the W2, demonstrating a proportional response to the delivered pulse number. There was no discernible impact on the measured charge of the W2 when switching between the available repetition rates of the FLEX system (18–180 pulses/s), solidifying consistent beam output across pulse frequencies.ConclusionsThis study tested a commercial plastic scintillator detector in a UHDR electron beam, paving the way for its potential use as a real‐time, patient‐specific dosimetry tool for future FLASH radiotherapy treatments. Further research is warranted to test and improve the signal processing of the W2 dosimetry system to accurately measure in UHDR environments using exceedingly high dose‐per‐pulse and pulse numbers.

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A semi‐analytical procedure to determine the ion recombination correction factor in high dose‐per‐pulse beams

Bancheri,

Seuntjens

2024

Medical Physics

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BackgroundThe conventional theories and methods of determining the ion recombination correction factor, such as Boag theory and the related two voltage method and Jaffé plot extrapolation, do not seem to yield accurate results in FLASH /high dose per pulse (DPP) beams (10 mGy DPP). This is due to the presence of a large free electron fraction that distorts the electric field inside the chamber sensitive volume. To understand the influence of these effects on the ion recombination correction factor and to develop new expressions for it, it is necessary to re‐visit the underlying physics.PurposeTo present a mathematical procedure to develop an analytical expression for the ion recombination correction factor. The expression is the basis for an extrapolation method so the correction factor can be determined in a clinical setting.MethodsA semi‐analytical solution method, the homotopy perturbation method (HPM), is used to solve the partial differential equations (PDEs) describing the charge carrier physics, including space charge and free electrons. The electron velocity and attachment rate are modeled as functions of the electric field strength. An expression for the charge collection efficiency and ion recombination correction factor are developed. A fit procedure based on this expression is used to compare it to measured data from previously published articles. Another fit procedure using a general equation is also proposed and compared to the data.ResultsThe series obtained for the charge collection efficiency and the ion recombination correction factor are determined to be asymptotic series and the optimal truncation established. The ion recombination correction factor exhibits a dependency due to the free electron presence. The fit using this expression agrees well with measured data as long as (1) the DPP is below 1 Gy for chambers with a 1 mm plate separation and (2) when the DPP is below 3 Gy for chambers with a 0.5 mm plate separation. In these DPP ranges, the deviation between measured and fit value did not exceed 6%. In both chamber cases the voltage range where the fit applies decreases as DPP increases. The general equation yielded comparable results.ConclusionsThe HPM was shown to be applicable to a complex system of PDEs and generate meaningful and novel solutions, as they include both space charge and free electrons. The HPM also lends itself to other chamber geometries. The fit procedure was also shown to yield accurate results for the ion recombination correction up to the 1 Gy DPP level.

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Charge collection efficiency, underlying recombination mechanisms, and the role of electrode distance of vented ionization chambers under ultra-high dose-per-pulse conditions

Cited by 16 publications

References 31 publications

Evaluation of ion chamber response for applications in electron FLASH radiotherapy

Evaluation of ion chamber response for applications in electron FLASH radiotherapy

Characterization of a commercial plastic scintillator for electron FLASH dosimetry

A semi‐analytical procedure to determine the ion recombination correction factor in high dose‐per‐pulse beams

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