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Interaction of Low-Energy Positrons with Atoms and Molecules

低能正电子与原子和分子的相互作用

基本信息

批准号：
1702230
负责人：
Clifford Surko
金额：
$ 56.47万
依托单位：
University of California-San Diego
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2017
资助国家：
美国
起止时间：
2017-09-01 至 2020-08-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1702230&HistoricalAwards=false
关键词：
Interaction Low Energy Positrons Atoms

项目摘要

The laws of physics indicate that for every matter particle there is a "twin" particle of antimatter. When an electron and its antiparticle (the positron) collide, they can convert into two photons, each carrying off the particle rest energy. The universe contains relatively little antimatter, and this asymmetry is currently not understood. Nevertheless, positrons play an important role in many areas of science and technology, including atomic and plasma physics, astrophysics, materials science, biology and medicine. An important medical application is Positron Emission Tomography (PET) used to image internal organs and measure metabolic activity. In many of these contexts, positron interactions with molecules are significant. This project studies how positrons bind (or "stick") to most molecules, albeit for short times, less than about a billionth of a second. Even though transient, these bound states can have important consequences for medical effects and imaging technologies. This project seeks to understand the nature of positron binding and its potential implications for the fate of positrons in biology and material systems, and also in settings of astrophysical relevance. The Principal Investigator and collaborators have recently developed a new cryogenic, trap-based positron beam with energy resolution of 7 meV FWHM resolution, a factor of 5 better than the previous state of the art. This device will be used to study the interaction of low-energy positrons with molecules with unprecedented precision. Of interest is the excitation of vibrational Feshbach resonances in two-body, positron-molecule collisions; a process in which a positron excites a vibrational mode and becomes trapped on the molecule. While positrons bind to atoms, there are no low-lying excitations to absorb excess energy, and so such resonances cannot occur in two-body collisions in the same manner as for molecules. One objective of the project is to conduct new experiments to gain an understanding of the greatly enhanced molecular annihilation rates as a function of incident positron energy. While a successful theory exists for the excitation of dipole-allowed fundamental resonances, multi-mode effects appear to be important. Their role in annihilation spectra is less well studied to date and less well understood theoretically. The second objective is to make more precise measurements of positron-molecule binding energies. Prediction of these binding energies presents a considerable challenge to theory, particularly in treating accurately electron-positron correlation effects. The new high-resolution beam is expected to provide more precise measurements of binding energies and hence more precise tests of theory.

物理定律表明，每个物质粒子都有一个反物质“孪生”粒子。当电子与其反粒子（正电子）碰撞时，它们可以转化为两个光子，每个光子都带走粒子的静止能量。宇宙中含有相对较少的反物质，这种不对称性目前尚不清楚。尽管如此，正电子在许多科学技术领域发挥着重要作用，包括原子和等离子体物理学、天体物理学、材料科学、生物学和医学。一个重要的医学应用是正电子发射断层扫描 (PET)，用于对内脏器官进行成像并测量代谢活动。在许多这样的情况下，正电子与分子的相互作用是重要的。该项目研究正电子如何与大多数分子结合（或“粘附”），尽管时间很短，不到十亿分之一秒。即使是短暂的，这些束缚态也可能对医疗效果和成像技术产生重要影响。该项目旨在了解正电子结合的本质及其对生物学和材料系统以及天体物理相关环境中正电子命运的潜在影响。首席研究员和合作者最近开发了一种新型低温、基于陷阱的正电子束，其能量分辨率为 7 meV FWHM 分辨率，比之前的技术水平提高了 5 倍。该装置将用于以前所未有的精度研究低能正电子与分子的相互作用。令人感兴趣的是二体正电子分子碰撞中振动费什巴赫共振的激发；正电子激发振动模式并被捕获在分子上的过程。虽然正电子与原子结合，但没有低位激发来吸收多余的能量，因此这种共振不会以与分子相同的方式发生在二体碰撞中。该项目的目标之一是进行新的实验，以了解分子湮灭率随入射正电子能量的变化而大大提高。虽然存在激发偶极子允许的基本共振的成功理论，但多模效应似乎很重要。迄今为止，它们在湮灭光谱中的作用还没有得到很好的研究，并且在理论上也没有得到很好的理解。第二个目标是对正电子分子结合能进行更精确的测量。这些结合能的预测对理论提出了相当大的挑战，特别是在准确处理正电子相关效应方面。新的高分辨率光束有望提供更精确的结合能测量，从而提供更精确的理论测试。