Interaction of Low-Energy Positrons with Atoms and Molecules

物理定律表明，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，每个光子都带走了粒子的静止能量。宇宙中含有相对较少的反物质，而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），用于内脏成像和测量代谢活动。在许多情况下，正电子与分子的相互作用是重要的。这个项目研究正电子如何与大多数分子结合（或“粘”），尽管时间很短，不到十亿分之一秒。即使是短暂的，这些束缚状态也会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和材料系统中正电子命运的潜在影响，以及与天体物理相关的设置。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，比以前的技术水平好5倍。该装置将以前所未有的精度用于研究低能正电子与分子的相互作用。我们感兴趣的是两体正电子-分子碰撞中振动费什赫共振的激发；正电子激发振动模式并被分子捕获的过程。当正电子与原子结合时，没有低空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。该项目的一个目标是进行新的实验，以了解大大增强的分子湮灭率作为入射正电子能量的函数。虽然存在一个成功的理论来激发偶极子允许的基本共振，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，理论上也没有得到很好的理解。第二个目标是更精确地测量正电子-分子结合能。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，从而提供更精确的理论测试。

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.