At the present time, high DPP electron beams are often clinically used in intra operative radiation therapy (IORT). The latter effect is known to be more dominant at high dose rates or doses-per-pulse (DPP) compared to the other two. Generally, the recombination loss can be grouped into three categories: initial recombination, recombination by diffusion and the volume recombination. To consider the loss of charge carriers through recombination processes, the so-called saturation correction factor is defined as the inverse of the charge collection efficiency. However, the measured signal can be also influenced by ion recombination and polarity effects so that further corrections are required. Additionally, the energy dependence of the water-to-air stopping power ratio according to the cavity theory and detector’s perturbation factors between 60Co beam and the beam quality under investigation is considered by the beam quality correction factor. In this case, to obtain the absorbed dose-to-water, the measured signal is multiplied by the calibration coefficient. Ideally, the measured signal shall represent the total charge carriers liberated within the sensitive air volume by the radiation field. The methodology to determine the absorbed dose-to-water using an ionization chamber is described in dosimetry protocols like TRS-398, 1 TG-51 2 or DIN 6800-2 3 based on a calibration performed using 60Co source that can be traced back to a primary or secondary standard laboratory. Vented ionization chambers are used as the standard dosimeter for clinical reference dosimetry. Based on these results, it seems possible to keep the recombination loss less than or equal to 5% up to a dose-per-pulse of 3 Gy with an appropriately designed ionization chamber, which corresponds to the level accepted in conventional radiotherapy dosimetry protocols. For the Advanced Markus chamber, the experimental results obtained by comparison against a reference agree well with the numerical solution. As expected, an increase of the electric field in the ionization chamber, either by applying a higher bias voltage or a reduction of the electrode distance, improves the ion collection efficiency and also reduces the polarity effect. In this work, the ion collection efficiency determined with different methods and ionization chambers have been compared and discussed. Furthermore, the results revealed that the determination of the ion collection efficiency from the Jaffé plots and therefore also from two-voltage method typically underestimate the ion collection efficiency in the region of high dose-per-pulse (3 to 130 mGy) and overestimate the ion collection efficiency at ultra-high dose-per-pulse (>1 Gy per pulse). Using the three EWCs with different electrode spacing, an improvement of the ion collection efficiency and a reduction of the polarity effect with decreasing electrode distance could be demonstrated. For the Advanced Markus chamber, a good agreement between the experimental, numerical and the results of Petersson et al. The extent of this drop is dependent on the electrode distance, the applied chamber voltage and thus the field strength in the sensitive air volume. The ion collection efficiency of the investigated ionization chambers drops significantly in the ultra-high DPP range. The method has been extended to obtain time-resolved and position-dependent electric field distortions within the air cavity. was implemented taking into account space charge effects at these ultra-high DPPs. Additionally, the numerical approach introduced by Gotz et al. All measurements were performed in a 24 MeV electron beam with DPP values between 0.01 and 3 Gy. The latter was achieved by calibrating a current transformer against alanine dosimeters. Their ion collection efficiencies were determined experimentally using two methods: extrapolation of Jaffé plots and comparison against a DPP-independent reference detector. MethodsĪn advanced Markus chamber and three specially designed parallel plate air-filled ionization chambers (EWC: End Window Chamber) with varying electrode distance of 0.5, 1, and 2 mm have been investigated. The role of the chamber design and the electric field strength in the sensitive air volume have been evaluated. The ion collection efficiency of vented ionization chambers has been investigated in an ultra-high dose-per-pulse (DPP) electron beam.
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