Fisica Sanitaria, Azienda Ospedaliero Universitaria Pisa AOUP, ed.18 via Roma 67, Pisa, Italy; Centro Pisano ricerca e implementazione clinica Flash Radiotherapy (CPFR@CISUP), Presidio S. Chiara, ed. 18 via Roma 67, Pisa, Italy; INFN, Sezione di Pisa, Largo B. Pontecorvo 3, I-57127 Pisa, Italy.
Centro Pisano ricerca e implementazione clinica Flash Radiotherapy (CPFR@CISUP), Presidio S. Chiara, ed. 18 via Roma 67, Pisa, Italy.
Phys Med. 2022 Nov;103:175-180. doi: 10.1016/j.ejmp.2022.11.001. Epub 2022 Nov 9.
The free electron fraction is the fraction of electrons, produced inside the cavity of an ionization chamber after irradiation, which does not bind to gas molecules and thereby reaches the electrode as free electrons. It is a fundamental quantity to describe the recombination processes of an ionization chamber, as it generates a gap of positive charges compared to negative ones, which certainly will not undergo recombination. The free electron fraction depends on the specific chamber geometry, the polarizing applied voltage and the gas thermodynamic properties. Therefore, it is necessary to evaluate such fraction in an accurate and easy way for any measurement condition. In this paper, a simple and direct method for evaluating the free electron fraction of ionization chambers is proposed. We first model the capture process of the electrons produced inside an ionization chamber after the beam pulse; then we present a method to evaluate the free electron fraction based on simple measurements of collected charge, by varying the applied voltage. Finally, the results obtained using an Advanced Markus chamber irradiated with a Flash Radiotherapy dedicated research Linac (ElectronFlash) to estimate the free electron fraction are presented. The proposed method allows the use of a conventional ionization chamber for measurements in ultra-high-dose-per-pulse (UHDP) conditions, up to values of dose-per-pulse at which the perturbation of the electric field due to the generated charge can be considered negligible.
自由电子分数是指在照射后，电离室内腔中产生的、不与气体分子结合并因此以自由电子形式到达电极的电子分数。它是描述电离室复合过程的基本量，因为它产生了正电荷与负电荷之间的间隙，这些正电荷肯定不会发生复合。自由电子分数取决于特定的腔几何形状、施加的极化电压和气体热力学性质。因此，对于任何测量条件，都需要以准确和简单的方式评估这种分数。本文提出了一种简单直接的方法来评估电离室的自由电子分数。我们首先对电离室内产生的电子在束脉冲后的捕获过程进行建模；然后，我们提出了一种基于收集电荷的简单测量来评估自由电子分数的方法，通过改变施加的电压。最后，使用专门用于研究的 Flash 放射治疗的电子 Flash 对先进的 Markus 电离室进行照射，以估计自由电子分数，结果表明。该方法允许在超高剂量脉冲（UHDP）条件下使用常规电离室进行测量，直到由于产生的电荷而导致的电场扰动可以被认为可以忽略不计的脉冲剂量值。
