束流在270°偏转磁铁系统输运过程中的损失计算

Calculation of the Beam Loss in the 270°Bending Magnet System of the Medical Accelerator

为计算医用加速器中束流经过270°偏转磁铁系统电子损失所造成的辐射剂量问题,将束流传输相应的计算公式和蒙特卡罗抽样方法相结合,在一阶近似条件下计算了电子在偏转系统中的输运过程,分析了不同初始条件对电子输运和电子损失的影响;模拟结果表明能散是产生电子损失的主要因素之一.计算得到了损失电子所处位置、能量和飞行方向等信息,把计算得到的信息作为蒙特卡罗程序的输入源,进一步计算出束流损失所产生的辐射剂量分布,从而能更完善地设计医用加速器照射头的屏蔽.文中给出在电子束初始半径为1mm、散角为5mrad、能散为10%条件下电子损失率为13.5%,损失电子主要是向加速器照射头部上方辐射出去.

The transportation process of the electron beam in the 270° bending magnet system of the medical accelerator is studied under the one-order approximation, and the influence of the initial condition to the beam transportation and loss is analyzed for the calculation of the radiation dose. The theory of the beam transportation, combined with the Monte Carlo sampling method is applied. The simulation result shows that the energy spread is the main factor resulted in the beam loss. The data on the position, energy and direction of the lost electrons obtained through the simulation can be supplied as input to deal with the radiation problem. The result gives that the loss rate of the electron beam is 13.5%, and the direction of the main loss is toward the upside of the irradiating head of the accelerator with the radius lmm, angle spread 5mrad, and energy spread 10% of the initial beam.