考虑磁透镜边缘场的质子成像系统优化设计

高能质子照相系统由四极磁透镜和准直器组成,实际透镜的边缘场将影响成像系统的性能.本文将含边缘场的磁场梯度用贝尔函数近似,提出了一种含边缘场的成像系统优化方法.通过Geant 4程序模拟了能量为1.6 GeV的质子成像系统,并通过优化方法给出了考虑边缘场的优化后的系统参数.研究了考虑边缘场时的成像系统参数对准直器孔径的影响.通过对比理想成像系统和优化前后的成像系统在使用准直器时的客体通量分布,研究了边缘场对质子通过客体的通量影响.结果表明,优化后的成像系统可以减小质子通过客体后的通量误差,并且积分差值在10^–2量级时,准直器的孔径参数变化亦在10^–2量级.

Optimization of proton imaging system including fringe field of magnetic lens

The proton imaging system is composed of four quadrupole magnetic lenses and a collimator.The quadrupole magnetic lenses can realize point-to-point imaging,and the collimator can improve image quality by controlling proton flux and realize material diagnosis.The magnetic field gradient of an ideal quadrupole lens becomes zero at the edge.Inside the lens,the magnetic field gradient is constant along the axis,while the magnetic field boundary of the actual lens extends outward.In the proton imaging system,the fringing field will affect the proton transport state and the performance of the imaging system as well.In this paper,a method to optimize the system is presented when the fringe field is considered.A proton imaging system of 1.6 GeV is established with the Geant 4 program,in which the magnetic field gradient distribution of the actual lens is approximated by the Bell function.In an ideal imaging system,the external drift length is 1.2 m,the internal drift length is 0.5 m,the length of the magnet is 0.8 m,and the magnetic field gradient is 8.09 T/m.The parameters of the practical imaging system can be obtained by using the optimization method:when the integral difference in magnetic field gradient distribution between the actual lens and the ideal lens is equal to zero,the outer drift length of the imaging system is 1.203 m and the inner drift length is 0.506 m;when the integral difference in the magnetic field gradient distribution between the actual lens and the ideal lens is equal to 1%,the outer drift length is 1.208 m and the inner drift length is 0.516 m.In the numerical simulation,a1 mm-thick copper plate and a concentric ball are chosen as the objects,and the influence of the fringing field on the collimator aperture and that on the proton flux error are studied.The results show that the optimized imaging system can reduce the flux error of protons passing through the object,and the difference in the aperture of collimator is on the order of 10^–2 when the integral difference is on the order of 10^–2 in magnitude.