北京慢正电子强束流运行性能测试

慢正电子强束流采用高能脉冲电子束流轰击金属钽靶, 以产生正负电子对的方式提供正电子, 作为慢正电子束流方法学研究和薄膜材料缺陷研究的束流基础. 本文是在该装置实现运行后, 对慢正电子束流的强度、能散、形貌等运行性能的测试工作的介绍, 以及慢正电子湮没多普勒测量系统的调试和标准样品的测量结果.     

22Na放射源慢正电子束流插入装置的研制

北京慢正电子强束流是利用北京正负电子对撞机电子直线加速器电子打靶产生的高强度低能单色正电子束流. 为了提高强束流的机时利用效率和节省强束流用于新建实验站的调试机时, 设计了一套基于²²Na放射源的慢正电子束流装置插入到强束流输运线上. ²²Na放射源慢正电子束流插入装置主要包括²²Na放射源及慢化体、E×B能量选择器、多级静电加速管、磁场输运系统、真空系统、高压绝缘和辐射防护措施等.     

研究堆慢正电子源构建中的关键机理问题

研究堆慢正电子源是获得高强度慢正电子束流的有效方式,国际上己建成多座装置并获得广泛应用.与常规同位素慢正电子源相比,研究堆慢正电子源的物理过程复杂,影响末端束流强度的因素众多,对其进行深入研究与合理建模是未来在中国绵阳研究堆(CMRR)上构建慢正电子源的基础.本文厘清了研究堆慢正电子产生的关键过程与物理机理,建立了预测末端正电子束流强度的理论模型,找到了影响其末端强度的主要物理量:快正电子体产生率、慢化体有效表面积、慢化体扩散距离、慢正电子从表面被提取到靶环末端的效率、及束流系统提取效率.用多种实验结果对模型进行校验,包括多个同位素慢正电子源的效率测量值,以及PULSTAR研究堆慢正电子源测量结果,充分验证了模型正确性.根据模型对各物理量的影响因素进行了分析,找到了需着重关注的影响因素,对未来源/靶结构的设计给出建议.     

北京慢正电子强束流磁场输运系统设计研究

针对基于北京正负电子对撞机的慢正电子强束流系统对输运磁场的设计要求,本文对不同规格的磁场输运线圈模型、长螺线管端口处磁场的补偿以及地磁场和弯管道对正电子束流的影响等进行了计算,提出适用于本系统传输慢正电子束流的输运磁场分布、补偿线圈、调整线圈的加工参数,计算表明,系统的总体磁场不均匀度小于10髎.实际运行束流测试表明,所设计的磁场系统能够很好的将慢正电子束流输运到约16m远的样品测量室,慢正电子束斑尺寸基本没有变化,满足慢正电子束流系统的设计要求.     

低能单色正电子束装置的原理、研制及应用

简要介绍了一种新的灵敏核探针－－低能单色正电子（慢正电子）束流装置的原理及应用。同时介绍了北京慢正电子束流装置的建设进展情况。     

六、核分析技术

北京慢正电子强束流依托于北京正负电子对撞机(BEPC)电子直线加速器(LINAC)，利用其完成注入后的剩余时间，产生高强度、高亮度、低能单色正电子束流，应用于材料科学尤其是材料表面特性的研究。该装置的建成弥补了我国现有的基于放射性同位素的慢正电子束流装置正电子强度较弱的不足，拓展了慢正电子技术在材料科学和微观核探针方法学中的应用领域，为进一步建立慢正电子湮没寿命测量、正电子诱导俄歇电子能谱测量以及低能正电子衍射和正电子显微镜等方法奠定了基础。     

多脉冲强流电子束在线测试数据处理

在多脉冲强流直线感应加速器(LIA)的测控和诊断系统中,电子束的在线测试数据处理是重要的组成部分。多脉冲强流电子束在线测试数据处理系统是集测试仪器、计算机和工作站等组成的工业以太网,以及实现测试网络配置、测试仪器的通信及控制、数据采集、束流强度和束质心位置计算、显示以及存储等功能为一体的专用软件。多脉冲强流电子束采用基于电阻环束流位置检测器(BPM)的束流测试线路,根据束线上每个BPM测试得到的四个象限点的原始电压信号波形,以及测试线路的结构参数,计算束线上每个PBM位置的束流强度以及束质心位置随时间变化的波形,根据随时间变化的波形数据计算每个BPM位置多个脉冲束流的强度以及束质心位置的统计数据。数据的存储采用SQL和文件系统相结合的方式。这套多脉冲强流电子束在线数据处理系统在多脉冲强流LIA的调试实验中稳定可靠运行,为实验提供有效可靠的数据。     

慢正电子束流技术在金属/合金微观缺陷研究中的应用

正电子湮没谱学技术是研究材料微观结构非常有效的一种核谱学分析方法, 主要用于获取材料内部微观结构的分布信息, 特别是微观缺陷结构及其特性等传统表征方法难以获取的微观结构信息. 近年来, 在慢正电子束流技术快速发展的基础上, 正电子湮没谱学技术在薄膜材料表面和界面微观结构的研究中得到了广泛应用. 特别是该技术对空位型缺陷的高灵敏表征能力, 使其在金属/合金材料表面微观缺陷的形成机理、缺陷结构特性及其演化行为等研究方面具有独特的优势. 针对材料内部微观缺陷的形成、演化机理以及缺陷特性的研究, 如缺陷的微观结构、化学环境、电子密度和动量分布等, 正电子湮没谱学测量方法和表征分析技术已经发展成熟. 而能量连续可调的低能正电子束流, 进一步实现了薄膜材料表面微观结构深度分布信息的实验表征. 本文综述了慢正电子束流技术应用研究的最新进展, 主要围绕北京慢正电子束流装置在金属/合金材料微观缺陷的研究中对微观缺陷特性的表征和表面微观缺陷演化行为的应用研究成果展开论述.     

BEPCII直线注入器的尾场效应

BEPCII直线注入器中的强流、短束团的尾场效应将损害束流的性能.用分析解和数值模拟计算的方法,系统地研究了尾场对纵向和径向束流动力学的影响,包括单束团的短程尾场和多束团的长程尾场对束流能量、能散、发射度、轨道和初级电子束在正电子产生靶上束斑尺寸的影响等.研究了有效抑制这些尾场效应的措施     

重复脉冲强流电子束传输技术研究

分析并推导出了环形强流电子束的稳定传输条件，通过静电磁场模拟计算对CHP01强流电子加速器二极管引导磁场的位形分布、幅值大小及磁场电源进行了优化设计.经实验调试及束斑测量，表明设计的1秒磁场满足束流稳定传输条件，能使电压800kV、电流8kA、脉冲宽度40ns、脉冲重复频率100Hz的环形强流电子束稳定传输，并已成功运用于CHP01强流电子加速器束流传输系统. 关键词： 束流传输 引导磁场 二极管 重复脉冲 电子束     

正电子慢化体的研究和进展

正电子湮没技术是一种研究材料的微观缺陷和相变的灵敏工具,在通常的正电子谱仪中,正电子能量为MeV量级,在样品中注入深度比较学（－100μm）,主要研究材料体内的平均缺陷密度,慢正电子束方法把正电子的能量降低为keV量级（而且可以调节）,注入比较浅（－μm）,所以是研究表面缺陷的探测手段,正电子慢化体是产生慢正电子的关键设备,对其研究有重要意义,文章综述了慢化体研究的历史和现状,从物理概念出发介绍使正电子慢化的四种可能方法和当今慢化体的五种几何排列方法,其中应用最广泛的是钨慢化体和百叶窗式的排列方式,效率最高的是惰性气体固体慢化体,而加电场慢化体是有待开发的高效慢化体。     

Photon and positron generation by ultrahigh intensity laser interaction with electron beams

This study investigates the generation of high energy photons and positrons using focused ultrahigh intensity femtosecond laser pulses on a relativistic electron beam with a set of two-dimensional particle-in-cell simulations. We consider circularly and linearly polarized, single and spatially separated double laser pulses. We model both 500 MeV and 1 GeV electron beams. Higher positron production is obtained using circularly polarized laser pulses. Using double pulses, the focusing effect of the ponderomotive force confines the electrons to a small volume, generating additional energetic photons and positrons. The positron spectral distributions are effectively modified by these variations. When the electron beam energy is doubled, the number of positrons increased, while the cutoff energy remained nearly constant.     

BEPCⅡ Positron Source

BEPCⅡ— an upgrade project of the Beijing Electron Positron Collider (BEPC) is a factory type of e^＋e^－ collider. The fundamental requirements for its injector linac are the beam energy of 1.89GeV for on-energy injection and a 40mA positron beam current at the linac end with a low beam emittance of 1.6μm and a low energy spread of ±0.5% so as to guarantee a higher injection rate (≥50mA/min) to the storage ring. Since the positron flux is proportional to the primary electron beam power on the target, we will increase the electron gun current from 4A to 10A by using a new electron gun system and increase the primary electron energy from 120MeV to 240MeV. The positron source itself is an extremely important system for producing more positrons, including a positron converter target chamber, a 12kA flux modulator, the 7m focusing module with DC power supplies and the support. The new positron production linac from the electron gun to the positron source has been installed into the tunnel. In what follows, we will emphasize the positron source design, manufacture and tests.     

Simulation study of indirect positron generation by an ultra-short laser

Positron generation by an ultra-short ultra-intense laser in an indirect manner has been studied with two-dimensional particle-in-cell (PIC) and Monte Carlo (MC) simulations. In this generation scheme, positrons are produced with energetic electrons accelerated by an ultra-shot laser pulse propagating through an underdense plasma. The dependence of the positron beam properties on the plasma length and secondary target (converter) thickness was investigated in detail. The simulation results reveal that the positron yield is strongly correlated with the total energy of laser-accelerated electrons; both the temperature and divergence of the positron beam are sensitive to the plasma length; and the positron beam has a pulse duration comparable to the incident electron beam. In addition, it is indicated that even with the optimal converter thickness, only a small fraction (11.4%) of positrons can escape out and most of the detected positrons originate from the back edge of the converter.     

Construction and timing system of the EPOS beam system

The Forschungszentrum Dresden-Rossendorf provides an intense pulsed 40 MeV electron beam with high brilliance and low emittance (ELBE). The pulse has a length of 1-10 ps and a repetition time of 77 ns, or in slow mode 616 ns. The EPOS system (ELBE Positron Source) generates by pair production on a tungsten converter and a tungsten moderator an intense pulsed beam of mono-energetic positrons. To transport the positrons to the laboratory (12 m) we constructed a magnetic beam guidance system with a longitudinal magnetic field of 75 G. In the laboratory outside the cave, the positron beam is chopped and bunched according to the time structure, because the very sharp bunch structure of the electron pulses is broadened for the positron beam due to transport and moderation.     

一种新的灵敏核探针——慢正电子束流装置

简要介绍了广泛应用于表面科学的灵敏核探针——慢正电子束设备的原理、构造和应用 ,阐述了北京慢正电子束流装置的设计原理和性能 ,讨论了北京慢正电子束流装置今后的研究前景. A novel and compact slow positron beam line——Beijing Slow Positron Beam Line was described. The basic physical concepts of the slow positron beam and its applications were reviewed. Recent experimental results showed the converting effcicency for the total positrons from the radioisotope to slow positrons was 8×10 -5 and the moderation efficiency was 1.7×10 -4 . The future development of Beijing Intense Slow Positron Beam, based on the BEPC LINAC, was also briefly discussed.     

Rectifying barrier at GaN/SiC hetero-junction studied with positron annihilation spectroscopy

Positron annihilation spectroscopy on GaN films grown on SiC substrate with MBE are presented. It is shown that the GaN/SiC interface is rectifying towards positrons, such that positrons can only travel from SiC to GaN and not vice versa. Potential steps seen by the positron at the GaN/SiC interface are calculated from experimental values of electron and positron work function. This “rectifying” effect has been successfully mimicked by inserting a thin region of very high electric field in the Variable Energy Positron Fit （VEPF） analysis. The built-in electric field is attributed to different positron affinities, dislocation and/or interface defects at the GaN/SiC interface.     

Transmission positron microscopes

Immediate and near-future plans for transmission positron microscopes being built at KEK, Tsukuba, Japan, are described. The characteristic feature of this project is remolding a commercial electron microscope to a positron microscope. A point source of electrons kept at a negative high voltage is changed to a point source of positrons kept at a high positive voltage. Positional resolution of transmission microscopes should be theoretically the same as electron microscopes. Positron microscopes utilizing trapping of positrons have always positional ambiguity due to the diffusion of positrons.     

Efficient propagation of polarization from laser photons to positrons through compton scattering and electron-positron pair creation

We have demonstrated for the first time the production of highly polarized short-pulse positrons with a finite energy spread in accordance with a new scheme that consists of two-quantum processes, such as inverse Compton scattering and electron-positron pair creation. Using a circularly polarized laser beam of 532 nm scattered off a high-quality, 1.28 GeV electron beam, we have obtained polarized positrons with an intensity of 2 x 10(4) e+ /bunch. The magnitude of positron polarization has been determined to be 73 +/- 15(stat) +/- 19(syst)% by means of a newly designed positron polarimeter.