Centre for Proton Therapy, Paul Scherrer Institute, Switzerland.
Physics Department, ETH Zurich, Switzerland.
Phys Med Biol. 2021 Jun 8;66(12). doi: 10.1088/1361-6560/abfbf2.
Recently, proton therapy treatments delivered with ultra-high dose rates have been of high scientific interest, and the Faraday cup (FC) is a promising dosimetry tool for such experiments. Different institutes use different FC designs, and either a high voltage guard ring, or the combination of an electric and a magnetic field is employed to minimize the effect of secondary electrons. The authors first investigate these different approaches for beam energies of 70, 150, 230 and 250 MeV, magnetic fields between 0 and 24 mT and voltages between -1000 and 1000 V. When applying a magnetic field, the measured signal is independent of the guard ring voltage, indicating that this setting minimizes the effect of secondary electrons on the reading of the FC. Without magnetic field, applying the negative voltage however decreases the signal by an energy dependent factor up to 1.3% for the lowest energy tested and 0.4% for the highest energy, showing an energy dependent response. Next, the study demonstrates the application of the FC up to ultra-high dose rates. FC measurements with cyclotron currents up to 800 nA (dose rates of up to approximately 1000 Gy s) show that the FC is indeed dose rate independent. Then, the FC is applied to commission the primary gantry monitor for high dose rates. Finally, short-term reproducibility of the monitor calibration is quantified within single days, showing a standard deviation of 0.1% (one sigma). In conclusion, the FC is a promising, dose rate independent tool for dosimetry up to ultra-high dose rates. Caution is however necessary when using a FC without magnetic field, as a guard ring with high voltage alone can introduce an energy dependent signal offset.
最近，采用超高剂量率的质子治疗引起了科学界的极大兴趣，而法拉第杯（FC）是此类实验中很有前途的剂量测量工具。不同的研究所使用不同的 FC 设计，要么采用高压保护环，要么采用电场和磁场的组合来最小化二次电子的影响。作者首先研究了这些不同的方法，用于 70、150、230 和 250 MeV 的束能，0 到 24 mT 的磁场以及-1000 到 1000 V 的电压。施加磁场时，测量信号与保护环电压无关，表明这种设置可将二次电子对 FC 读数的影响最小化。没有磁场时，施加负电压会使信号降低，对于测试的最低能量，降低幅度高达 1.3%，对于最高能量，降低幅度为 0.4%，表明存在能量依赖性响应。接下来，该研究展示了 FC 在超高剂量率下的应用。高达 800 nA 的回旋加速器电流（剂量率高达约 1000 Gy s）的 FC 测量表明，FC 确实与剂量率无关。然后，FC 被用于对高剂量率的初级龙门监测器进行校准。最后，在单个天内量化了监测器校准的短期可重复性，标准偏差为 0.1%（一个标准差）。总之，FC 是一种很有前途的、与剂量率无关的工具，可用于高达超高剂量率的剂量测量。但是，在没有磁场的情况下使用 FC 时需要谨慎，因为单独使用高压保护环可能会引入能量相关的信号偏移。
