Interaction of Low-Energy Positrons with Atoms and Molecules

普通观众摘要：这个研究项目聚焦于正电子(电子的反粒子)的科学。正电子在原子、等离子体和天体物理、生物学和医学等基础科学中扮演着重要的角色。它们还具有许多技术应用，例如用于质谱学的新技术，使用正电子对材料进行表征，以及用于研究新陈代谢过程和药物设计的正电子发射断层扫描(PET)。虽然正电子与物质相互作用的各个方面都很清楚，但仍有许多悬而未决的问题。当正电子和电子相撞时，它们可能会湮灭，能量转化为伽马射线。这些过程是了解物质和反物质相互作用的物理和化学的重要方面。这个研究小组已经证明，正电子与许多分子结合(湮灭前的寿命不到十亿分之一秒)。该小组将进行的正电子结合研究与材料上的正电子表面状态和正电子在PET中的命运有关。在科学教育和培训方面，研究将积极让各级学生和青年研究人员参与，从实验规划到研究成果的传播。该项目涉及小规模实验，是科技人员的极好培训基地。这项工作将被广泛传播，不仅面向物理学观众，还将涉及等离子体、化学、材料科学和气体电子学研究的人员。实验室中的反物质是一个重要的科学新前沿。它被非专业观众视为相当着迷；因此，它有助于帮助更广泛的社会阶层欣赏物理科学中的基础研究。技术观众摘要：在之前的研究中，该小组表明正电子与分子结合。与这一现象相关的两个重要的智力挑战以及由此导致的湮灭模式，是开发处理电子-正电子关联的方法，并更好地理解当正电子在中性物质附近时，分子动力学在正电子湮没过程中的作用。根据该奖项进行的研究说明了这两个问题。该项目的一个重点是更好地了解正电子与分子的结合，这些结合能的大小，以及它们所依赖的。第二个目标是更好地理解分子中正电子湮灭的机制，这是通过一个共振过程进行的，在这个过程中，湮灭率可以在简单碰撞的基础上比预期的大几个数量级。虽然人们意识到分子振动在这种增强中发挥了重要作用，但细节尚不清楚。这些实验将使用最先进的、可调的、高能量分辨率的正电子束进行，这是研究小组最近开发的，现在已经达到了完全的能力。这种新光束良好的能量分辨率有望在作为入射正电子能量函数的详细湮没谱测量中发挥重要作用。这可以提供更精确的正电子结合能测量和对分子动力学在产生极大增强的湮灭率中所起的关键作用的关键洞察。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

General audience abstract:This research project focuses on science with positrons (the antiparticles of electrons). Positrons play an important role in basic science, including atomic, plasma and astrophysics, biology and medicine. They also have many technological applications such as new techniques for mass spectroscopy, the characterization of materials using positrons, and positron emission tomography (PET) to study metabolic processes and for drug design. While aspects of positron interactions with matter are well understood, there are many outstanding questions. When positrons and electrons collide, they can annihilate with the energy converted to gamma rays. Such processes are important facets in gaining an understanding of the physics and chemistry of the interaction of matter and antimatter. This research group has shown that positrons bind to many molecules (lifetimes before annihilation less than one hundred millionth of a second). The positron binding studies the group will be doing are relevant to positron surface states on materials and the fate of positrons in PET. In terms of scientific education and training, the research will actively involve students and young researchers at all levels, from the planning of experiments to the dissemination of research results. The project involves small-scale experiments that are an excellent training ground for scientific and technical personnel. The work will be communicated broadly, not only to physics audiences but also to those involved in plasma, chemical, and materials science, and gaseous electronics research. Antimatter in the laboratory is an important new frontier of science. It is regarded with considerable fascination by lay audiences; and as such, it aids in helping broader segments of society to appreciate basic research in the physical sciences.Technical audience abstract:In previous research, the group showed that positrons bind to molecules. Two important intellectual challenges related to this phenomenon, and the modes of annihilation that result from it, are to develop methods to treat electron-positron correlations and to better understand the role of molecular dynamics in positron annihilation processes when a positron is in the proximity of neutral matter. The research being conducted under this award speaks to both of these issues. One focus of the project is gaining a better understanding of positron binding to molecules, the magnitudes of these binding energies, and what they depend upon. A second objective is gaining a better understanding of mechanisms for positron annihilation in molecules, which proceeds by a resonant process in which annihilation rates can be orders of magnitude greater than expected on the basis of simple collisions. While it is appreciated that molecular vibrations play an important role in this enhancement, the details are not understood. The experiments will be conducted with a state-of-the-art, tunable, high-energy-resolution positron beam that was developed recently by the research team and is now coming to full capability. The excellent energy resolution of this new beam is expected to play an important role in making detailed annihilation spectral measurements as a function of incident positron energy. This can provide much more precise measurements of positron binding energies and key insights into the critical role molecular dynamics plays in producing greatly enhanced annihilation rates.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.