| Download | - View final version: Evaluation of the two‐voltage method for parallel‐plate ionization chambers irradiated with pulsed beams (PDF, 3.1 MiB)
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| DOI | Resolve DOI: https://doi.org/10.1002/mp.17814 |
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| Author | Search for: Paz‐Martín, José; Search for: Schüller, Andreas; Search for: Bourgouin, Alexandra1; Search for: Gago‐Arias, Araceli; Search for: González‐Castaño, Diego M.; Search for: Gómez‐Fernández, Nicolás; Search for: Pardo‐Montero, Juan; Search for: Gómez, Faustino |
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| Affiliation | - National Research Council of Canada. Metrology Research Centre
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| Funder | Search for: Xunta de Galicia |
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| Format | Text, Article |
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| Subject | recombination; saturation factor; two-voltage method |
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| Abstract | Background: Air-vented ionization chambers exposed to clinical radiationbeams may suffer from recombination during the drift of the charge carrierstowards the electrodes. Thus, dosimetry protocols recommend the use of acorrection factor, usually denominated saturation factor (ksat ), to correct the ion-ization chamber readout for the incomplete collection of charge.The two-voltagemethod (TVM) is the recommended methodology for the calculation of the sat-uration factor, however, it is based on the early Boag model, which only takesinto account the presence of positive and negative ions in the ionization cham-ber and does not account for the electric field screening or the free electroncontribution to the signal.
Purpose: To evaluate the impact of a more realistic approach to the saturationproblem that accounts for the free electron fraction.
Methods: The saturation factor of four ionization chambers (two AdvancedMarkus and two PPC05) was experimentally determined in the ultra-highdose per pulse reference beam of the German National Metrology Institute(Physikalisch-Technische Bundesanstalt [PTB]) for voltages ranging from 50 to400 V and pulse durations between 0.5 and 2.9 μs. Several analytical modelsand a recently developed numerical model are used to calculate the satura-tion factor as a function of the dose per pulse and compare it to the obtainedexperimental data.Parameterizations of the saturation factor against the ratio ofcharges at different voltages are given for parallel plate ionization chamber witha distance between electrodes of 0.6 and 1 mm in pulsed beams for differentpulse durations.
Results: The saturation factors calculated using the different Boag analyticalmodels do not agree neither with each other nor with the numerical simula-tion even at the lowest dose per pulse of the investigated range (< 30 mGy).A recently developed analytical model by Fenwick and Kumar agrees withthe numerical simulation in the low dose per pulse regime but discrepanciesare observed when the dose becomes larger (i.e., > 40 mGy for AdvancedMarkus) due to the electric field perturbation. The numerical simulation is in agood agreement with the experimentally determined charge collection efficiency(CCE) with an average discrepancy of 0.7% for the two PPC05 and 0.5% forthe two Advanced Markus. The saturation factor obtained with the numerical simulation of the collected charge has been fitted to a third-order polynomialfor different voltage ratios and pulse duration. This methodology provides apractical way for ksat evaluation whenever ksat < 1.05.
Conclusions: The numerical simulation shows a better agreement with theexperimental data than the current analytical theories in terms of CCE.The clas-sical TVM, systematically overestimates the saturation factor, with differencesincreasing with dose per pulse but also present at low dose per pulse. Theseresults may have implications for the dosimetry with ionization chambers in ther-apy modalities that use a dose per pulse higher than conventional radiotherapysuch as intraoperative radiotherapy but also in conventional dose per pulse forionization chambers that suffer from significant charge recombination. |
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| Publication date | 2025-04-11 |
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| Publisher | Wiley |
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| Licence | |
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| In | |
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| Language | English |
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| In press | Yes |
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| Peer reviewed | Yes |
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| Export citation | Export as RIS |
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| Report a correction | Report a correction (opens in a new tab) |
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| Record identifier | e35f93f2-96b3-4cbf-8393-23ee9895df9b |
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| Record created | 2025-05-01 |
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| Record modified | 2025-05-01 |
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