PTW Freiburg, Freiburg, Germany.
University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany.
Biomed Phys Eng Express. 2020 Dec 3;7(1). doi: 10.1088/2057-1976/abcae5.
The aim of this work is the dosimetric characterization of a plane parallel ionization chamber under defined beam setups at the CERN Linear Electron Accelerator for Research (CLEAR). A laser driven electron beam with energy of 200 MeV at two different field sizes of approximately 3.5 mm FWHM and approximately 7 mm FWHM were used at different pulse structures. Thereby the dose-per-pulse range varied between approximately 0.2 and 12 Gy per pulse. This range represents approximately conventional dose rate range beam conditions up to ultra-high dose rate (UHDR) beam conditions. The experiment was based on a water phantom which was integrated into the horizontal beamline and radiochromic films and an Advanced Markus ionization chamber was positioned in the water phantom. In addition, the experimental setup were modelled in the Monte Carlo simulation environment FLUKA. In a first step the radiochromic film measurements were used to verify the beamline setup. Depth dose distributions and dose profiles measured by radiochromic film were compared with Monte Carlo simulations to verify the experimental conditions. Second, the radiochromic films were used for reference dosimetry to characterize the ionization chamber. In particular, polarity effects and the ion collection efficiency of the ionization chamber were investigated for both field sizes and the complete dose rate range. As a result of the study, significant polarity effects and recombination loss of the ionization chamber were shown and characterized. However, the work shows that the behavior of the ionization chamber at the laser driven beam line at the CLEAR facility is comparable to classical high dose-per-pulse electron beams. This allows the use of ionization chambers on the CLEAR system and thus enables active dose measurement during the experiment. Compared to passive dose measurement with film, this is an important step forward in the experimental equipment of the facility.
本工作旨在对 CERN 线性电子加速器研究（CLEAR）中定义的光束设置下的平行平面电离室进行剂量学特性研究。使用了两种不同的场大小（约 3.5mm FWHM 和约 7mm FWHM）的激光驱动电子束，具有 200MeV 的能量，具有不同的脉冲结构。因此，每个脉冲的剂量范围在约 0.2 到 12Gy 之间。这个范围代表了传统剂量率范围内的束条件，直到超高剂量率（UHDR）束条件。实验基于集成在水平束线上的水模体和辐射色胶片，以及位于水模体中的先进 Markus 电离室。此外，实验装置在 Monte Carlo 模拟环境 FLUKA 中进行了建模。在第一步中，使用辐射色胶片测量来验证束线设置。通过辐射色胶片测量的深度剂量分布和剂量分布与 Monte Carlo 模拟进行了比较，以验证实验条件。其次，辐射色胶片用于参考剂量学，以对电离室进行特性描述。特别是，研究了两种场大小和整个剂量率范围内的电离室的极性效应和离子收集效率。研究结果表明，电离室存在显著的极性效应和离子复合损失。然而，该工作表明，CLEAR 设施激光驱动束线上的电离室行为与经典高剂量率电子束相似。这使得在 CLEAR 系统上使用电离室成为可能，并在实验过程中实现了主动剂量测量。与使用胶片的被动剂量测量相比，这是设施实验设备的一个重要进步。
