Ion collection efficiency of ionization chambers in ultra-high dose-per-pulse electron beams
Ion collection efficiency of ionization chambers in ultra-high dose-per-pulse electron beams
复制标题
DOI：
10.1002/mp.14620
复制
发表时间：
2021-01-03
期刊：
影响因子：
3.8
通讯作者：
Poppe, Bjoern
中科院分区：
文献类型：
--
作者：
Kranzer, Rafael;Poppinga, Daniela;Poppe, Bjoern
目的研究超高剂量/脉冲(DPP)电子束中排气电离室的离子收集效率。方法研究了一种先进的马库斯电离室和三种特殊设计的平行板充气电离室(EWC：End Window Chamber)，电极间距分别为0.5 mm、1 mm和2 mm。用两种方法实验测定了它们的离子收集效率：Jaffe图的外推法和与DPP无关的参比检测器的比较。后者是通过将电流互感器与丙氨酸剂量计进行校准来实现的。所有测量都是在24 MeV的电子束中进行的，DPP值在0.01~3Gy.此外，Gotz等人介绍的数值方法。在考虑到这些超高DPP的空间电荷效应的情况下实施。结果所研究的电离室的离子收集效率在超高DPP范围内显著下降。这种下降的程度取决于电极距离、施加的腔室电压以及敏感风量中的场强。对于高级马库斯室，实验、数值和Petersson等人的结果之间有很好的一致性。可能会显示出来。使用三种不同电极间距的EWCs，离子收集效率随电极间距的减小而提高，极性效应随电极间距的减小而减小。此外，从Jaffe图和双电压法测量离子收集效率通常低估了高剂量/脉冲(3~130 mGy)区域的离子收集效率，而高估了超高剂量/脉冲(~gt；1 Gy/脉冲)的离子收集效率。结论对不同方法和电离室测定的离子收集效率进行了比较和讨论。正如预期的那样，电离室中电场的增加，无论是通过施加更高的偏置电压还是减小电极距离，都可以提高离子收集效率，同时也减少了极性效应。对于先进的马库斯室，通过与参考文献的比较，实验结果与数值解符合得很好。根据这些结果，通过适当设计的电离室，似乎可以将复合损失保持在小于或等于5%，最高可达每脉冲3GY的剂量，这与传统放射剂量学协议中可接受的水平相对应。
Purpose The ion collection efficiency of vented ionization chambers has been investigated in an ultra-high dose-per-pulse (DPP) electron beam. The role of the chamber design and the electric field strength in the sensitive air volume have been evaluated.Methods An advanced Markus chamber and three specially designed parallel plate air-filled ionization chambers (EWC: End Window Chamber) with varying electrode distance of 0.5, 1, and 2 mm have been investigated. Their ion collection efficiencies were determined experimentally using two methods: extrapolation of Jaffe plots and comparison against a DPP-independent reference detector. The latter was achieved by calibrating a current transformer against alanine dosimeters. All measurements were performed in a 24 MeV electron beam with DPP values between 0.01 and 3 Gy. Additionally, the numerical approach introduced by Gotz et al. was implemented taking into account space charge effects at these ultra-high DPPs. The method has been extended to obtain time-resolved and position-dependent electric field distortions within the air cavity.Results The ion collection efficiency of the investigated ionization chambers drops significantly in the ultra-high DPP range. The extent of this drop is dependent on the electrode distance, the applied chamber voltage and thus the field strength in the sensitive air volume. For the Advanced Markus chamber, a good agreement between the experimental, numerical and the results of Petersson et al. could be shown. Using the three EWCs with different electrode spacing, an improvement of the ion collection efficiency and a reduction of the polarity effect with decreasing electrode distance could be demonstrated. Furthermore, the results revealed that the determination of the ion collection efficiency from the Jaffe plots and therefore also from two-voltage method typically underestimate the ion collection efficiency in the region of high dose-per-pulse (3 to 130 mGy) and overestimate the ion collection efficiency at ultra-high dose-per-pulse (>1 Gy per pulse).Conclusions In this work, the ion collection efficiency determined with different methods and ionization chambers have been compared and discussed. As expected, an increase of the electric field in the ionization chamber, either by applying a higher bias voltage or a reduction of the electrode distance, improves the ion collection efficiency and also reduces the polarity effect. For the Advanced Markus chamber, the experimental results obtained by comparison against a reference agree well with the numerical solution. Based on these results, it seems possible to keep the recombination loss less than or equal to 5% up to a dose-per-pulse of 3 Gy with an appropriately designed ionization chamber, which corresponds to the level accepted in conventional radiotherapy dosimetry protocols.