Muir B R, McEwen M R, Rogers D W O
Measurement Science and Standards, National Research Council of Canada, Ottawa, ON, K1A 0R6, Canada.
Phys Med Biol. 2014 Oct 7;59(19):5953-69. doi: 10.1088/0031-9155/59/19/5953. Epub 2014 Sep 11.
A method is presented to obtain ion chamber calibration coefficients relative to secondary standard reference chambers in electron beams using depth-ionization measurements. Results are obtained as a function of depth and average electron energy at depth in 4, 8, 12 and 18 MeV electron beams from the NRC Elekta Precise linac. The PTW Roos, Scanditronix NACP-02, PTW Advanced Markus and NE 2571 ion chambers are investigated. The challenges and limitations of the method are discussed. The proposed method produces useful data at shallow depths. At depths past the reference depth, small shifts in positioning or drifts in the incident beam energy affect the results, thereby providing a built-in test of incident electron energy drifts and/or chamber set-up. Polarity corrections for ion chambers as a function of average electron energy at depth agree with literature data. The proposed method produces results consistent with those obtained using the conventional calibration procedure while gaining much more information about the behavior of the ion chamber with similar data acquisition time. Measurement uncertainties in calibration coefficients obtained with this method are estimated to be less than 0.5%. These results open up the possibility of using depth-ionization measurements to yield chamber ratios which may be suitable for primary standards-level dissemination.
本文提出了一种通过深度电离测量来获取电子束中相对于次级标准参考电离室的电离室校准系数的方法。研究结果是作为深度以及来自加拿大国家研究委员会（NRC）医科达精确直线加速器的4、8、12和18兆电子伏电子束深度处平均电子能量的函数而得到的。对PTW Roos、扫描电子学NACP - 02、PTW高级马库斯和NE 2571电离室进行了研究。讨论了该方法的挑战和局限性。所提出的方法在浅深度处产生有用的数据。在超过参考深度的深度处，定位的微小偏移或入射束能量的漂移会影响结果，从而提供了对入射电子能量漂移和/或电离室设置的内置测试。电离室的极性校正作为深度处平均电子能量的函数与文献数据一致。所提出的方法产生的结果与使用传统校准程序获得的结果一致，同时在相似的数据采集时间内获得了更多关于电离室行为的信息。用该方法获得的校准系数的测量不确定度估计小于0.5%。这些结果开辟了利用深度电离测量来得出可能适用于一级标准水平传播的电离室比值的可能性。
