Wickman G, Johansson B, Bahar-Gogani J, Holmström T, Grindborg J E
Radiation Physics Department, Umeå University, Sweden.
Med Phys. 1998 Jun;25(6):900-7. doi: 10.1118/1.598268.
Two new liquid ionization chamber (LIC) designs, consisting of cylindrical and plane-parallel configurations, are presented. They are designed to be suitable for high-precision measurements of absorbed dose-to-water at dose rates and photon energies typical for LDR intermediate photon energy brachytherapy sources. The chambers have a sensitive liquid layer thickness of 1 mm and sensitive volumes of 7 mm3 (plane-parallel) and 20 mm3 (cylindrical). The liquids used as sensitive media in the chambers are either isooctane (C8H18), tetramethylsilane (Si(CH3)4) or mixtures of these two liquids in the approximate proportions 2 to 1. A chamber filled with such a liquid mixture and with a polarizing voltage of 300 V, provides a volume sensitivity of about 10(-9)C Gy(-1) mm(-3) for absorbed dose measurements in water in an x-ray radiation field with an effective photon energy of 120 keV. In the interval 30 to 140 keV, the relative change in sensitivity is less than +/- 2.5%. The leakage current of the chambers is low and stable, which implies that absorbed dose measurements can be done with good reproducibility at dose-rates as low as 50 microGy min-1 (sigma < 3%). The long-term calibration stability was tested for a set of five chambers over a period of more than 1 year. No systematic change in their sensitivity could be observed. The general recombination at a polarizing voltage of 300 V is less than 2% for dose-rates up to about 100 mGy min-1. The temperature dependence at room temperature is 0.5% per degree C. The response is almost independent of the direction of the radiation for the plane-parallel LIC.
本文介绍了两种新型液体电离室（LIC）设计，分别为圆柱形和平行板结构。它们旨在适用于低剂量率近距离放射治疗中能光子源典型的剂量率和光子能量下对水的吸收剂量进行高精度测量。这些电离室的灵敏液体层厚度为1mm，灵敏体积分别为7mm³（平行板结构）和20mm³（圆柱形）。用作电离室灵敏介质的液体为异辛烷（C8H18）、四甲基硅烷（Si(CH3)4）或这两种液体按约2比1的近似比例混合而成的混合物。一个充满这种液体混合物且极化电压为300V的电离室，在有效光子能量为120keV的X射线辐射场中对水中吸收剂量测量的体积灵敏度约为10(-9)C Gy(-1) mm(-3)。在30至140keV的能量区间内，灵敏度的相对变化小于±2.5%。电离室的漏电流低且稳定，这意味着在低至50μGy min-1的剂量率下（σ<3%）也能以良好的重复性进行吸收剂量测量。对一组五个电离室进行了为期一年多的长期校准稳定性测试。未观察到它们的灵敏度有系统性变化。在极化电压为300V时，剂量率高达约100mGy min-1时的一般复合率小于2%。室温下的温度依赖性为每摄氏度0.5%。对于平行板LIC，响应几乎与辐射方向无关。
