The Trial Manufacture of a Ultra-low Energy Positron Diffraction Apparatus

作为表面分析技术，LEED在早期阶段被频繁使用。目前，由于LEED数据的理论处理困难，RHEED得到了有效的应用。本课题旨在研制一种理论上容易处理、对表面结构比LEED更敏感的超低能正电子衍射装置。与LEED相比，ULEPD具有多重散射低、无交换作用、内能接近于零等上级优点。在实验的初始阶段，我们计算了各种情况下正电子的运动轨迹，并设计了静电聚焦透镜系统。该聚焦透镜系统由带圆柱静电环的双管透镜、对称单透镜和反对称单透镜组成。对于正电子的提取，我们利用了由美国科学家开发的透镜系统。 ...更多信息 Soa这种SOA枪在很宽的范围内显示出优异的聚焦性能。轨道计算采用电子光学中广泛应用的变换矩阵法。分别计算了第一支Soa枪到CMA入口、CMA出口到第一个改性晶体、第二支Soa枪到第二个改性晶体、第三支Soa枪到最后一个变焦透镜、变焦透镜出口到样品位置的轨迹。通过计算，我们确定了35个静电环的最佳外加电压值。对于这种布置，在1.15度的色散角下，在lev处获得1.7mm的束直径，在10eV处获得0.5mm的束直径，在100V处获得0.17mm的束直径。作为超低正电子衍射的初步测量，我们用BaF_2闪烁体和光电倍增管组件测量了正电子在Cu（100）面湮灭产生的γ射线计数率与入射正电子能量的关系。计数率在对应于布拉格条件的能量处显示出最小值。这保证了该设备的有效运行。少

As surface analysis technique, LEED was frequently used at an early stage. At present, RHEED is efficiently used because og the difficulty in the theoretical treatment of LEED data. In the present ptoject, We aimed to develop a ultra-low energy positron diffraction apparatus, which seems to be easily treated by theoretical aspects and be more sensitive to surface strcutures than LEED. As compared with LEED, ULEPD is superior in the following points; low multiple scatteting, no exchange interaction, nearly zero inner potential and so on. In this project, we manufactured trially a ulta-low energy positron diffraction apparatus. At an initial stage, we calculated the trajectory of positrons for various conditions and designed electrostatic docussing lens system. This focussing lens system was composed of two-tube lenses with cylindtrical electrostatic rings, symmetric Einzel lenses and anti-symmetric Einzel lenses. For the extraction of positrons, we utilized the lens system developed by … More Soa. This Soa gun showed an excellent focussing property in a very wide range. Trajectory calculations were made by the method of transformation matrices which is widely used in the electron optics. We calculated trajectories from the first Soa gum to the entrance to CMA, from the exit fo CMA to the first remoderatoe crystal, from the secong Soa gun to the second remoderator crystal, the third Soa gun to the final zoom lens, and from the exist of zoom lens to teh specimen position, respectively. From this calculation, we determined the optimum values of applied voltages for 35 electrostatic rings. For this arrangement, the beam diameter of 1.7mm at lev, 0.5mm at 10eV and 0.17mm at 100V was obtained at the dispersed angle og 1.15 degree. as a preliminaly measurement of ultra-low positron diffraction, we measured the counting rate of gamma rays from annihilation of positrons in the Cu (100) surface by an assembly of BaF2 scintillator and a photomultiplier tube as a function of incident positron energy. The counting rate showed minima at energies correpsonding to Bragg conditions. This garanteed the efficient operation of this apparatus. Less