Saturation Effect Study and Ion Recombination Correction on Ionization Chamber for Ultra-high Dose Rate (FLASH) of Carbon Ions

Background and Purpose: The ionization chamber produces significant space-charge and ion recombination effects at ultra-high dose rates, posing a challenge for dose monitoring. In addition, there is no generally ac- cepted ion correction model for dosimetry in FLASH radiotherapy, making it crucial to monitor the dose at ultra-high dose rates accurately and in real time. Methods: In this study, the air pressure of the ionization chamber was reduced to perform real-time beam monitoring, and a Faraday cup was used for calibration for active dosimetry. To study the saturation effect of the ionization chamber, the drift, attachment, recombination, and diffusion processes of the electron-ion pairs were modeled using finite-element analysis based on physi- cal phenomenological principles, and the correction factor was calculated. Results: The experimental results showed that the FLASH ionization chamber measures good dose linearity at a dose rate of approximately 0.2 Gy/s. When the air pressure of the chamber was adjusted to 10 mbar, the response of the FLASH ionization chamber was linear at a dose rate of approximately 50 Gy/s, with the residuals within 2 %. Furthermore, by using physical phenomenology to resolve the process of electron-ion pair motion in the sensitive volume of the ionization chamber, the analytical model better describes the saturation effect of carbon ions at ultra-high dose rates. The maximum deviation in the calculated correction factor is less than 10 %. Conclusion: We studied the saturation effect in dose measurement, achieving accurate and fast dose and profile position measurement across different dose rates in a wide range based on the Heavy Ion Research Facility in Lanzhou.

背景与目的：电离室（ionization chamber）在超高剂量率下会产生显著的空间电荷与离子复合效应，给剂量监测带来挑战。此外，当前FLASH放射治疗（FLASH radiotherapy）的剂量学领域尚无公认的离子校正模型，因此精准实时监测超高剂量率下的剂量至关重要。
方法：本研究通过降低电离室气压开展实时束流监测，并采用法拉第杯（Faraday cup）进行校准以实现主动剂量学。为探究电离室的饱和效应，研究基于物理现象学原理，采用有限元分析对电子-离子对（electron-ion pairs）的漂移、附着、复合与扩散过程进行建模，并计算校正因子。
结果：实验结果表明，当剂量率约为0.2 Gy/s时，FLASH电离室展现出良好的剂量线性响应。当将电离室气压调整至10 mbar时，其在约50 Gy/s的剂量率下仍保持线性响应，残差小于2%。此外，通过物理现象学解析电离室敏感体积内电子-离子对的运动过程，所构建的解析模型可更好地描述碳离子在超高剂量率下的饱和效应，计算得到的校正因子最大偏差小于10%。
结论：本研究针对剂量测量中的饱和效应展开研究，依托兰州重离子研究装置（Heavy Ion Research Facility in Lanzhou），实现了宽剂量率范围内不同剂量率下的精准、快速剂量与轮廓位置测量。

搜集汇总

背景与挑战

背景概述

该数据集研究超高速碳离子束（FLASH）放疗中电离室的饱和效应与离子复合校正问题，通过降低气压和有限元分析建模电子-离子对运动，开发了校正模型，实验结果显示在高达50 Gy/s的剂量率下响应线性良好，校正因子偏差小于10%，为准确剂量监测提供支持。