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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，对于每一个物质粒子，都有一个反物质的“孪生”粒子。当一个电子和它的反粒子（正电子）碰撞时，它们可以转化为两个光子，每个光子都携带粒子的静止能量。宇宙中含有相对较少的反物质，这种不对称性目前还不清楚。然而，正电子在许多科学和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），用于成像内部器官和测量代谢活动。在许多这些情况下，正电子与分子的相互作用是显着的。该项目研究正电子如何结合（或“粘”）到大多数分子上，尽管时间很短，不到十亿分之一秒。即使是短暂的，这些束缚态也会对医学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性质及其对正电子在生物学和材料系统中的命运的潜在影响，以及与天体物理学相关的设置。首席研究员和合作者最近开发了一种新的低温，基于陷阱的正电子束，能量分辨率为7 meV FWHM分辨率，比以前的最先进水平高出5倍。该设备将用于研究低能正电子与分子的相互作用，具有前所未有的精度。感兴趣的是在两体，正电子-分子碰撞中振动Feshbach共振的激发;一个正电子激发振动模式并被捕获在分子上的过程。当正电子与原子结合时，没有低位激发来吸收多余的能量，因此这种共振不能像分子那样在两体碰撞中发生。该项目的一个目标是进行新的实验，以了解作为入射正电子能量函数的大大增强的分子湮灭率。虽然存在一个成功的理论激发偶极允许的基本共振，多模效应似乎是重要的。它们在湮没谱中的作用迄今为止研究得不多，理论上也不太清楚。第二个目标是更精确地测量正电子-分子结合能。预测这些结合能对理论提出了相当大的挑战，特别是在精确处理电子-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，从而更精确地测试理论。