反射三极管的轫致靶优化

S. Swanekamp, B. Weber, S. Stephanakis, D. Mosher
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引用次数: 11

摘要

对反射三极管进行了耦合粒子池(PIC)和蒙特卡罗模拟,其中钽箔厚度在2.5 mum (1 MeV时CSDA范围的0.0056倍)到250 mum (1 MeV时CSDA范围的0.56倍)之间变化。PIC/Monte Carlo模拟结果与gamerii上1mv, 1ma的反射三极管实验结果吻合较好。实验测量和模拟结果均表明,当膜厚约为20 μ m时,剂量最大。对于厚度大于20 μ m的箔片,分析表明10-100 keV的光子较少逃离箔片,从而降低了剂量。对于薄于20 μ m的箔片,剂量的减少是由于箔片失去了电子约束,允许电子向外径向漂移并撞击低原子序数的箔片托架,从而导致剂量的减少。对电子轨道的检查表明,对于所有厚度的箔,在强自磁场的影响下,电子最初呈径向向内流动。如果箔很厚,那么电子在每次与箔相互作用时都会损失大量的能量,并且在它们最初与箔相互作用的地方被吸收。如果箔很薄,电子在每次通过时损失很少的能量。对于非常薄的箔,模拟表明,每通过一次,电子向外移动的半径大约是拉莫尔半径的两倍。因此,对于薄箔,有有限数量的通过电子可以使移动出二极管之前,他们击中箔持有人。基于这些结果,导出了一个公式,该公式能够很好地预测最佳剂量的阳极厚度。
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Bremmstrahlung target optimization for reflex triodes
Coupled particle-in-cell (PIC) and Monte Carlo simulations of the reflex triode have been performed with tantalum foil thicknesses varying between 2.5 mum (0.0056 times the CSDA range at 1 MeV) to 250 mum (0.56 the CSDA range at 1 MeV). The PIC/Monte Carlo simulations are in good agreement with reflex triode experiments on Gamble II at 1 MV, 1 MA. Experimental measurements and simulations both show that the dose is maximized for a foil thickness of about 20 mum. For foils thicker than 20 mum, the analysis shows that fewer of the 10-100 keV photons escape the foil reducing the dose. For foils thinner than 20 mum, the dose decrease is due to a loss of electron confinement to the foil allowing electrons to drift radially outward and strike a low-atomic-number foil holder which causes the dose to decrease. An examination of the electron orbits shows that for all foil thicknesses electrons initially flow radially inward under the influence of the strong self-magnetic field. If the foil is thick, then electrons lose a significant amount of energy with each interaction with the foil and are absorbed close to the point where they initially interact with the foil. If the foil is thin, electrons lose very little energy with each pass. For very thin foils, the simulations show that, with each pass, the electrons move outward in radius a distance of approximately twice the Larmor radius. Therefore, for thin foils, there are a limited number of passes the electrons can make before moving out of the diode where they strike the foil holder. Based on these results, a formula is derived that is able to predict fairly well the anode thickness that optimizes the dose.
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