Interaction of Low-Energy Positrons with Atoms and Molecules

普通观众摘要：本研究项目的重点是正电子（电子的反粒子）科学。正电子在基础科学，包括原子、等离子体和天体物理学、生物学和医学中发挥着重要作用。它们也有许多技术应用，如质谱新技术，使用正电子和正电子发射断层扫描（PET）研究代谢过程和药物设计的材料表征。虽然正电子与物质相互作用的各个方面已经很好地理解，但仍有许多悬而未决的问题。当正电子和电子碰撞时，它们可以湮灭，能量转化为伽马射线。这些过程是理解物质和反物质相互作用的物理和化学的重要方面。这个研究小组已经表明，正电子与许多分子结合（湮灭前的寿命小于一亿分之一秒）。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，从实验的规划到研究成果的传播，研究将积极让各级学生和青年研究人员参与。该项目涉及小规模实验，是科学和技术人员的一个极好的培训场所。 这项工作将被广泛传播，不仅是物理学的观众，而且还涉及到那些参与等离子体，化学和材料科学，以及气体电子学研究。实验室中的反物质是一个重要的科学前沿。 它被非专业观众视为相当迷人;因此，它有助于帮助更广泛的社会阶层欣赏物理科学的基础研究。技术观众摘要：在以前的研究中，该小组表明正电子与分子结合。两个重要的智力挑战与这一现象有关，以及由此产生的湮灭模式，是发展处理电子-正电子相关性的方法，以及更好地理解当正电子接近中性物质时，分子动力学在正电子湮灭过程中的作用。在这个奖项下进行的研究谈到了这两个问题。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东西。第二个目标是更好地了解分子中正电子湮灭的机制，这是通过共振过程进行的，其中湮灭率可以比基于简单碰撞的预期大几个数量级。虽然认识到分子振动在这种增强中起重要作用，但细节并不清楚。 这些实验将使用最先进的、可调谐的、高能量分辨率的正电子束进行，该正电子束是研究小组最近开发的，现在正在全面投入使用。这种新的光束的优良的能量分辨率，预计将发挥重要作用，在详细的湮没谱测量作为入射正电子能量的函数。这可以提供更精确的测量正电子结合能和关键的洞察分子动力学在产生大大提高湮灭率中发挥的关键作用。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.