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Physical analysis of the radiation shielding for the medical accelerators

机译：医用加速器辐射屏蔽的物理分析

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摘要

Radiation safety standards today require comprehensive shielding protection schemes for all particle accelerators. The original shielding system of BJ-20 (BeiJing-20 MeV), the high-energy medical electron linac, was designed only for the 18 MeV level. And the dose caused by the lost electrons in the 270° bending magnet system was neglected. In this paper, the leakage dose of BJ-20 is carefully analyzed. The radiation leakage dose distribution of the photons coming from the accelerator head is obtained for energy levels of 6, 12, 14, and 18 MeV. The dose of the photoneutrons is especially analyzed for the 18 MeV level. The result gives that even neglecting the dose from the 270° bending magnet system, the shielding system is still not enough for the energy levels lower than 18 MeV. The radiation leakage produced by electrons that are lost in the 270° bending magnet system has been particularly studied. Using beam transport theory and Monte Carlo sampling methods, which have been combined in calculations, we have obtained the distribution of the energy, position, and direction of the lost electrons. These data were then further processed by the Monte Carlo N-particle (MCNP) code as input data. The results show that when the electron loss rate in the 270° bending magnet system is 13.5%, the radiation leakage dose of the photons generated by the lost electrons is 0.1% higher than that at the isocenter, and the corresponding relative leakage dose of the photoneutrons reaches 0.045% around an angle of 170° at 18 MeV level. Both of these parameters exceed radioprotection safety standards for medical accelerators. The original shielding design is therefore not suitable and is also incomplete since the radiation produced by the electrons being lost in the 270° bending magnet system was neglected and the leakage dose for the low-energy levels was not considered in the original design. Our calculations provide a very useful tool for further optimization and design improvement that will enable this radiation shielding to conform to present day safety standards. © 2009 American Institute of Physics Article Outline INTRODUCTION THE MODEL AND ANALYSIS METHOD The model of the shielding system Analysis of the electron beam loss in the 270° bending magnet system CALCULATION OF THE LEAKAGE DOSE FOR THE SHIELDING SYSTEM The radiation dose not considering the lost electrons in the 270° bending magnet system The leakage dose of the photons The leakage dose of the photoneutrons The radiation dose generated by the electrons lost in the 270° bending magnet system The leakage dose of the photons The leakage dose of the photoneutrons CONCLUSIONS
机译：当今的辐射安全标准要求所有粒子加速器都具有全面的屏蔽保护方案。 BJ-20（Beijing-20 MeV）的原始屏蔽系统，即高能医用电子直线加速器，仅设计用于18 MeV的水平。并忽略了在270°弯曲磁体系统中由电子损失引起的剂量。本文仔细分析了BJ-20的泄漏剂量。对于6、12、14和18 MeV的能级，获得了来自加速器头的光子的辐射泄漏剂量分布。特别针对18 MeV水平分析了光中子的剂量。结果表明，即使忽略来自270°弯曲磁体系统的剂量，屏蔽系统仍然不足以使能量水平低于18 MeV。由电子产生的辐射泄漏在270°弯曲磁体系统中损失了，对此进行了专门研究。使用结合计算的束传输理论和蒙特卡洛采样方法，我们获得了损失电子的能量，位置和方向的分布。然后，这些数据通过蒙特卡洛N粒子（MCNP）代码作为输入数据进行进一步处理。结果表明，当270°弯曲磁体系统中的电子损失率为13.5％时，由损失的电子产生的光子的辐射泄漏剂量比等中心处的辐射泄漏剂量高0.1％，相应的相对泄漏剂量为在18 MeV水平下，光中子在170°角处达到0.045％。这两个参数都超过了医疗加速器的辐射防护安全标准。因此，原始的屏蔽设计不合适，并且也不完整，因为在270°弯曲磁体系统中电子丢失所产生的辐射被忽略了，并且在原始设计中未考虑低能级的泄漏剂量。我们的计算为进一步优化和设计改进提供了非常有用的工具，使该辐射屏蔽符合当今的安全标准。 ©2009美国物理研究所文章大纲模型和分析方法简介屏蔽系统的模型270°弯曲磁体系统中电子束损耗的分析屏蔽系统的泄漏剂量计算辐射剂量未考虑丢失的电子270°弯曲磁体系统中的光子泄漏剂量光子中子的泄漏剂量270°弯曲磁体系统中的电子损失所产生的辐射剂量，光子的泄漏剂量，光中子的泄漏剂量结论

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