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，对于每个物质粒子，都有一个“双”反物质粒子。当电子及其反颗粒（正电子）碰撞时，它们可以转换为两个光子，每个光子都携带粒子休息能。宇宙包含的反物质相对较少，目前尚不清楚这种不对称性。然而，正面子在许多科学技术领域（包括原子和血浆物理学，天体物理学，材料科学，生物学和医学）中起着重要作用。重要的医学应用是用于成像内部器官并测量代谢活动的正电子发射断层扫描（PET）。在许多情况下，与分子的正电子相互作用很重要。该项目研究了正上子如何与大多数分子结合（或“棍子”），尽管短时间内，却小于十亿秒。即使短暂，这些约束状态也可能对医疗效应和成像技术产生重要的后果。该项目旨在了解正电子结合的性质及其对生物学和材料系统上正电子命运的潜在影响，以及在天体物理相关性的环境中。首席研究员和合作者最近开发了一种新的低温，基于陷阱的正电子光束，能量分辨率为7 MEV FWHM分辨率，比以前的最新状态要好5倍。该设备将用于研究低能正常与具有前所未有的精度的分子的相互作用。有趣的是激发了两体，正电子 - 分子碰撞中的振动Feshbach共振。正电子引起振动模式并被困在分子上的过程。虽然正电子与原子结合，但没有低洼的激发来吸收多余的能量，因此这种共振不能以与分子相同的方式发生在两体碰撞中。该项目的一个目的是进行新的实验，以了解大大增强的分子歼灭速率，这是事件正电子能量的函数。尽管存在一种成功的理论来激发偶极子允许的基本共振，但多模式效应似乎很重要。迄今为止，它们在歼灭光谱中的作用较少，理论上对他们的理解较少。第二个目标是对正电子分子结合能进行更精确的测量。这些结合能的预测给理论带来了巨大的挑战，尤其是在准确治疗电子旋律相关效应时。期望新的高分辨率梁可提供对结合能的更精确的测量，从而提供更精确的理论测试。