MRI: Development of an Intense Positron Annihilation Spectrometry System for Nanophase Characterization

北卡罗来纳州立大学(NCSU)、橡树岭国家实验室(ORNL)和密歇根大学(UM)已结成合作伙伴，在NCSU校园内的PULSTAR反应堆开发强正电子(反物质电子)设备。正电子是电子的反粒子，当注入到正常物质中时，它们会与电子一起消失--也就是说，根据爱因斯坦著名的公式E=mc2，它们的所有结合质量都会转化为电磁辐射脉冲。正电子湮没是作为国家纳米技术倡议的一部分而开发的先进材料的理想探测器。纳米技术的许多新应用包括理解和操纵材料中空洞的原子尺度微结构。然而，由于传统技术(如电子显微镜)不足以在真正的纳米尺度上表征这种空洞，对空洞在增强或降低材料性能方面的作用的基本了解往往受到阻碍。物质中的正电子寻找并直接探测这种开放的结构。在名为(NC)3的新设施中，北卡罗来纳州国家纳米相表征中心将把在NCSU核反应堆核心附近产生的正电子输送到几个目标站，来自世界各地的学生、教师和工业合作者可以在那里监测新开发的材料的湮灭过程。通过极大地加强研究人员对反物质正电子束的强烈、聚焦的访问，下一代材料研究将受益于一种“看到”它们内部结构的新方法。北卡罗来纳州立大学(NCSU)、橡树岭国家实验室(ORNL)和密歇根大学(UM)已结成合作伙伴，在NCSU校园内的PULSTAR反应堆开发强流正电子束设备和相关光谱仪。正电子正在成为纳米尺度上一种理想的物质探测器。带正电的正电子由于其对物质中开放结构的亲和力，在作为纳米结构的探测器和理解工程纳米空洞对宏观性质的作用方面已经获得了重要的作用。这主要是因为通过微调和设计材料的微观结构，提高了定制材料宏观性能的能力。强正电子束将在转换器中通过使用反应堆堆芯和(n，？)产生的裂变伽马射线对产生而产生。在转化器组件周围的镉覆层中的反应。正电子束将驱动两个互补的正电子/正电子能谱仪：第一个是“下一代”正电子PALS光谱仪，其束流直径为1毫米，能够对纳米孔薄膜和图案化微电子器件进行高精度的寿命研究；第二个是时间聚束正电子PALS光谱仪，将被引导用于研究金属和半导体中的湮灭。最终，我们的目标是建立北卡罗来纳州纳米相表征国家中心(NC)3。(NC)3的核心装置是PULSTAR反应堆上的强正电子束线，辅以现有的PULSTAR和NCSU中子散射系统和电子显微镜。正电子束设施将在材料科学和工程、生物科学/生物医学工程、化学工程、电气工程、环境科学和工程、物理和化学等领域具有适用性。其主要技术重点将集中在纳米材料的三个方面：(A)用于改善材料性能的工程纳米孔，(B)纳米空洞/缺陷结合在材料性能退化中的作用，以及(C)相之间的界面效应。在美国的一所大学校园里开发这一设施代表着一个重大的研究和教育机会。正电子设备还将并入PULSTAR在线教育网络。

North Carolina State University (NCSU), Oak Ridge National Laboratory (ORNL), and the University of Michigan (UM) have formed a collaboration to develop an intense positron (antimatter electrons) facility at the PULSTAR reactor on the campus of NCSU. Positrons are the antiparticles of electrons and when injected into normal matter they "annihilate" with electrons-that is they both disappear with all of their combined mass being converted into a pulse of electromagnetic radiation according to Einstein's famous formula E=mc2. Positron annihilation is emerging as an ideal probe of advanced materials being developed as part of the nation's Nanotechnology Initiative. Many new applications of nanotechnology involve understanding and manipulating the atomic-scale microstructure of voids within a material. However, fundamental understanding of the role of voids in enhancing or degrading materials properties is often hampered by the inadequacy of traditional techniques (such as electron microscopes) to characterize such voids at the true nanometer scale. Positrons in matter seek out and directly probe such open structures. In the new facility called (NC)3, the North Carolina National Center for Nanophase Characterization, positrons produced near the core of the NCSU nuclear reactor will be piped out to several target stations where students, faculty, and industrial collaborators from around the world can monitor the annihilation process in newly developed materials. By vastly enhancing the access of researchers to an intense, focused beam of antimatter positrons next-generation materials research will benefit from a new way to "see" the structure inside them. The center's research impact is highly leveraged through many applications in science and engineering with broad societal impacts including miniaturized and faster computing, higher strength and longer lasting materials, cleaner energy production, and corrosion- and radiation-resistant coatings.North Carolina State University (NCSU), Oak Ridge National Laboratory (ORNL), and The University of Michigan (UM) have formed a collaboration to develop an intense positron beam facility and associated spectrometers at the PULSTAR reactor on the campus of NCSU. Positrons are emerging as an ideal probe of matter on the nanoscale. The positively charged positron, with its affinity for open structures in matter, has been gaining an important role as a probe of nanostructure and in understanding the role of engineered nanovoids on macroscopic properties. This is mainly due to the increased ability to tailor the macroscopic properties of a material by fine-tuning and engineering its microstructure. The intense positron beam will be created in a converter by pair-production using fission gamma rays produced in the reactor core and by (n,?) reactions in a cadmium cladding surrounding the converter assembly. The positron beam will drive two complementary positron/positronium spectrometers: First, a "next generation" positronium PALS spectrometer (Ps-PALS), with a 1-mm diameter beam, that is capable of performing high accuracy lifetime studies on nanoporus thin films and patterned microelectronic devices, and second, a time-bunched positron PALS spectrometer (e+-PALS) that will be directed for studying annihilation in metals and semiconductors. Ultimately, our goal is to establish (NC)3, the North Carolina National Center for Nanophase Characterization. The core apparatus of (NC)3 is the intense positron beam-line at the PULSTAR reactor, complemented by existing PULSTAR and NCSU systems for neutron scattering, and electron microscopy. The positron beam facility will have applicability in fields such as materials science and engineering, bioscience/biomedical engineering, chemical engineering, electrical engineering, environmental science and engineering, physics and chemistry to name a few. Its main technical focus will be on three aspects of nanophase materials: (a) engineered nanoporosity for improved material properties, (b) the role of nanovoid/defect coalescence in the degradation of materials properties, and (c) interface effects between phases. The development of this facility on a university campus in the United States represents a major research and educational opportunity. The positron facility will also be integrated into the PULSTAR on-line educational network.