1.School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China 2.School of Nuclear Engineering and Technology, North China Electric Power University, Beijing 102206, China 3.RIKEN Nishina Center, RIKEN, Wako 351-0198, Japan 4.Department of Physics, University of Gothenburg, SE-41296 Gothenburg, Sweden
Fund Project:the National Natural Science Foundation of China (Grant Nos. U1732269, 11475075) and the Swedish Foundation for International Cooperation in Research and Higher Education (Grant No. IB2018-8071)
Received Date:16 June 2019
Accepted Date:17 February 2020
Published Online:05 May 2020
Abstract: The transmission of 10-keV Cl– ions through Al2O3 insulating nanocapillaries is studied both by experiment and simulation. The double-peak structure in the transmitted angular distribution is found to be the same as our previous result. The peak around the direction of the primary beam is caused mainly by the directly transmitted Cl–, and the other peak around the tilt angle of Al2O3 nanocapillaries is mainly induced by Cl+ and Cl0. The intensity of transmitted Cl– decreases with the tilt angle increasing, which is in accord with the geometrically allowed transmission. Beyond the geometrically allowed angle, the transmitted projectiles are mainly Cl+ ions and Cl0 atoms. The ratio of transmitted Cl+ ion to Cl0 atom drops as tilt angle increases, and it turns more obvious when the tilt angle is larger than the limit of the geometrical transmission. A detailed physics process was developed within Geometry and Tracking 4 (Geant4) to perform the trajectory simulation, in which the forces from the deposited charges and the image charges, the scattering from the surfaces as well as the charge exchange are taken into consideration. The transmissions at the tilt angle of 1.6o are simulated for the cases without and with deposited charges of –100 e/capillary. For the deposition charge quantity of –100 e/capillary, the majority of the transmitted projectiles are mainly the directly transmitted Cl– ions exiting to the direction of tilt angle, and the transmitted Cl0 and Cl+ account for a very small portion. While for the case with no deposited charges, the simulation results agree well with the experimental results. The dependence of the scattering process on the tilt angle, which results in the different features in the transmitted projectiles, is studied in detail by the simulation. It is found that the transmitted Cl0 atoms exit through single to multiple scattering, and most of transmitted Cl0 atoms exit through single and double scattering, and are centered along the axis of nanocapillaries, while Cl+ ions mainly exit by single scattering, which results in the fact that the intensity of the transmitted Cl0 atoms drops slower than that of the transmitted Cl+ ions with the increase of the tilt angle, leading the ratio of the transmitted Cl+ to Cl0 to decrease as the tilt angle increases in experiment. Our results describe the physical mechanism of low-energy ions through insulating nanocapillaries in detail, i.e. how the scattering process dominates the final transmission. It is found that the transmission of the negative ions in the energy range above 10 keV is caused by the scattering and the charge exchange process. Keywords:Cl– ions/ Al2O3 insulating nanocapillaries/ scattering
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2.1.实验方法
本次实验在兰州大学核科学与技术学院的2 × 1.7 MeV串列加速器上进行. 实验所用Cl–离子束由串列加速器上的铯溅射离子源提供, 经过两级间隔75 cm的四极狭缝准直之后, 形成束斑大小为3 mm × 3 mm, 角发散0.5°, 束流强度为几十个pA/mm2的Cl–离子束. Al2O3纳米微孔膜安装在超高真空靶室的中心处. 微孔膜倾角ψ定义为微孔与初束垂直方向的夹角, 探测角?定义为出射束流与初束之间的夹角, 本次实验采用一维微通道板探测器(1D-MCP)来探测穿透粒子, 可以在探测角方向上分辨束流的穿透角分布. 沿着束流方向, 在微孔膜的后方安装了静电分析器, 用于分析穿透粒子的电荷态组成成分. 探测器的位置信号采用多参数数据获取系统进行提取. 实验装置简图见图1. 实验要求靶室的真空好于2.5 × 10–5 Pa. 图 1 实验装置和探测角示意图 Figure1. Schematic diagram of experimental setup and the observation angle ?
在不同倾角下, 10 keV的Cl–离子穿过厚度为7 μm的Al2O3微孔膜的穿透粒子角分布如图3(b)所示. 随着倾角的增大, 穿透粒子的计数率在下降, 并且角分布也随之展宽呈现出双峰的结构. 穿透粒子角分布的两个峰, 一个峰的峰位在0°附近, 另一个峰的峰位与微孔膜的倾斜角基本一致. 并且随着倾角的增大, 0°附近的峰所占比例在减小, 而与微孔膜倾斜角一致的峰所占比例在增大. 为了探究穿透粒子角分布的峰的组成成分, 我们在静电分析器加上了静电场, 得到了图4(b)所示的电荷态分布. 中性粒子保持原有角分布不变, 负离子向负角度方向移动了4°左右, 正离子向正角度方向移动了4°左右. 由此可以分析得到, 峰位在0°附近的峰成分主要为Cl–离子, 峰位与倾角一致的峰主要成分为Cl0和Cl+. 其中Cl–离子随倾斜角的增大所占比例在减小, 而Cl0和Cl+所占比例则在增大. 图 3 (a)不同倾角ψ下10 keV的Cl–穿透角分布的计算结果(黑色为无沉积电荷的结果, 红色为沉积电荷为–100 e/capillary的结果); (b)不同倾角ψ下10 keV的Cl–穿透角分布的实验结果 Figure3. (a) Calculated transmitted angular distributions for 10 keV-Cl– ions at various tile angles ψ (black lines for no deposited charge and red line for deposited charge of –100 e/capillary); (b) the experimental transmitted angular distributions for 10 keV-Cl– ions at various tile angles ψ.
图 4 加静电场后, (a)不同倾角ψ下10 keV的Cl–穿透粒子的电荷态分布的计算结果(黑色为无沉积电荷的结果, 红色为沉积电荷为–100 e/capillary的结果); (b)不同倾角ψ下10 keV的Cl–穿透粒子的电荷态分布的实验结果 Figure4. Exerting electrostatic field, (a) thecalculated charge state distributions of transmitted projectiles for 10 keV-Cl– at various tilt angles ψ (black line for no deposited charge and red line for deposited charge of –100 e/capillary); (b) the experimental charge state distributions of transmitted projectiles for 10 keV-Cl– at various tilt angles ψ.
在实验数据的电荷态分布 (图4(b))中可以看出Cl0峰的峰位随倾角变化而改变, 并与倾角基本形成图5所示的线性关系. 图5中实验结果的Cl0峰位与 Y = X 的直线只有微小歧离, X为倾角, Y是Cl0峰的峰位. 我们分析了不同角度下的模拟计算的Cl0出射峰位和实验结果的Cl0出射峰位角度与倾角的关系, 计算结果与实验结果基本符合, Cl0峰位以微孔轴向为中心分布. 不同角度下Cl–和Cl0相对于0°的相对穿透强度与倾角的关系如图6(a) 所示. 由此可见穿透粒子的强度随倾角增大而减小. 可以看到, 与Cl0和Cl+相比, Cl–的穿透强度随倾角下降得要快很多. Cl–的穿透强度在小于1.2°时快速下降, 大于1.2°时, 其穿透相对强度与0°时相比保持在1.0 × 10–3基本不变. 而Cl+和Cl0的穿透强度在大于1.2°时, 仍存在下降趋势. 主要原因在于, Cl–穿透过程为几何穿透, 其穿透强度变化与沿束流方向的光学穿透率一致. 而Cl+和Cl0经过电荷交换产生, 因此需要经过一次或多次近距离碰撞后才能从微孔中出射. 为了清晰地看出Cl+和Cl0变化趋势的差别, 将其相对强度用以2为底的对数坐标表示在图6(b) 中. 结果发现Cl+较Cl0下降稍快. 图 5 实验与计算结果的中性穿透粒子(Cl0)角分布的峰位置随倾角的变化(实线是线性函数Y = X) Figure5. Peak position of experimental and simulated angular distribution of transmitted neutrals (Cl0) as a function of the tilt angle. The solid line is the linear function that shows the peak position of transmitted neutral shifts according to the tilt angle.
图 6 (a)穿透的Cl–, Cl0, Cl+粒子相对强度随倾角ψ变化; (b)穿透的Cl0和Cl+粒子相对强度随倾角ψ变化的对数坐标图 Figure6. (a) Relative intensity of transmitted Cl–, Cl0 and Cl+ vs. the tilt angle ψ for 10 keV-Cl– ions; (b) the logarithm scale of the relative intensity of transmitted Cl0 and Cl+ as a function of the tilt angle ψ
离子穿越微孔时可能的几何穿透角${\sigma _{{\rm{geot}}}} = $$\sqrt {{\sigma ^2}_{{\rm{asp}}} + {\sigma ^2}_{{\rm{beam}}} + {\sigma _{{\rm{axis}}}}} \approx {\rm{1}}{\rm{.}}{{\rm{2}}^ \circ }$, 其中, σasp, σbeam和σaxis分别代表微孔几何张角, 束流发散角和微孔轴向发散角[20]. 穿透粒子中Cl+和Cl0的比值与倾角的关系如图7所示. 在倾角小于1.2°时, Cl+/Cl0的比值在0.16附近波动; 当倾角大于1.2°时, Cl+/Cl0的比值迅速减小. 实验的1.2°转折点与几何穿透角基本一致, 与之前16 keV的工作相似[19]. 然而在相同角度下10 keV的Cl+/Cl0的比值小于16 keV的Cl–的穿透结果. 图 7 在不同倾角ψ下10 keV的Cl– 穿透的Cl+/Cl0的比值(红色实心圆是实验结果, 黑色实心矩形是计算结果, 蓝色虚线代表几何穿透角) Figure7. Intensity ratio of transmitted Cl+ to Cl0 vs. the tilt angle ψ for the incident ions of 10 keV-Cl–. The red solid circle corresponds to the experimental results; black solid square corresponds to the simulation results; blue dash line indicates the angle within which the geometrical transmission occurs.
其中ψ是入射角度, θ是散射角度. 图8展示了在入射角为0.6°时根据Firsov散射公式计算的散射角分布, 可以看到散射粒子概率最大的出射角与入射角基本相同, 这具有镜面反射特征. 图 8 入射角为0.6°时, Firsov公式计算的散射粒子角分布 Figure8. Scattered angular distribution at the incident angle of 0.6° to the surface given by Firsov formula.
23.3.电荷态交换 -->
3.3.电荷态交换
我们构筑了一个电荷态交换的唯像模型来定量地获得穿透粒子的电荷态分布[19]. 在实验中, 微孔的倾斜角度比较小, 所以碰撞过程以一次和两次碰撞为主体, 还有部分Cl–离子直接穿过微孔, 如图9所示. 当Cl–离子与微孔内部发生碰撞时, 会发生图10中所示意的电荷态交换. 图 9 Cl–离子穿过纳米微孔的原理简图(绿线为离子直接穿透的轨迹简图, 红线为一次碰撞散射的轨迹简图, 黑线为二次碰撞散射的简图) Figure9. Schematic diagram of Cl– ions transmitted through a nanocapillary. The green line is a schematic diagram of the direct transmission of ions, the red line is a schematic diagram of ions transmitted by single scattering, and the black line is a schematic diagram of ions transmitted by double scattering.
图 10 传输过程的电荷交换简图 Figure10. Schematic diagram of charge state exchange during transmission.