Bahar-Gogani J, Grindborg J E, Johansson B E, Wickman G
Radiation Physics Department, Umeå University, Sweden.
Phys Med Biol. 2001 Mar;46(3):729-40. doi: 10.1088/0031-9155/46/3/309.
The long-term sensitivity and calibration stability of liquid ionization chambers (LICs) has been studied at a local and a secondary standards dosimetry laboratory over a period of 3 years. The chambers were transported several times by mail between the two laboratories for measurements. The LICs used in this work are designed for absorbed dose measurements in the dose rate region of 0.1-100 mGy min(-1) and have a liquid layer thickness of 1 mm and a sensitive volume of 16.2 mm3. The liquids used as sensitive media in the chambers are mixtures of isooctane (C8H18) and tetramethylsilane (Si(CH3)4) in different proportions (about 2 to 1). Operating at a polarizing voltage of 300 V the leakage current of the chambers was stable and never exceeded 3% of the observable current at a dose rate of about 1 mGy min(-1). The volume sensitivity of the chambers was measured to be of the order of 10(-9) C Gy(-1) mm3. No systematic changes in the absorbed dose to water calibration was observed for any of the chambers during the test period (sigma < 0.2%). Variations in chamber dose response with small changes in the polarizing voltage as well as sensitivity changes with accumulated absorbed dose were also investigated. Measurements showed that the LIC response varies by 0.15% per 1% change in applied voltage around 300 V. No significant change could be observed in the LIC sensitivity after a single absorbed dose of 15 kGy. The results indicate that the LIC can be made to serve as a calibration transfer instrument and a reference detector for absorbed dose to water determinations providing good precision and long-term reproducibility.
在3年的时间里，一个地方标准剂量学实验室和一个二级标准剂量学实验室对液体电离室（LIC）的长期灵敏度和校准稳定性进行了研究。这些电离室在两个实验室之间通过邮件运输了好几次用于测量。本工作中使用的LIC设计用于在0.1 - 100 mGy min⁻¹的剂量率区域进行吸收剂量测量，液体层厚度为1 mm，灵敏体积为16.2 mm³。电离室中用作灵敏介质的液体是不同比例（约为2比1）的异辛烷（C₈H₁₈）和四甲基硅烷（Si(CH₃)₄）的混合物。在300 V的极化电压下运行时，电离室的漏电流稳定，在剂量率约为1 mGy min⁻¹时从未超过可观测电流的3%。测量得到电离室的体积灵敏度约为10⁻⁹ C Gy⁻¹ mm³。在测试期间，未观察到任何一个电离室在水吸收剂量校准方面有系统变化（σ < 0.2%）。还研究了极化电压小变化时电离室剂量响应的变化以及累积吸收剂量时灵敏度的变化。测量结果表明，在300 V左右，LIC响应随施加电压每变化1%变化0.15%。在单次吸收剂量为15 kGy后，未观察到LIC灵敏度有显著变化。结果表明，LIC可作为校准传递仪器和用于水吸收剂量测定的参考探测器，具有良好的精度和长期可重复性。
