Interaction of Low-Energy Positrons with Atoms and Molecules

这个研究项目的重点是正电子（电子的反粒子）科学。正电子在原子、等离子体和天体物理学、生物学和医学等基础科学中发挥着重要作用。正电子也有许多技术应用，如质谱、材料表征和正电子发射断层扫描（PET）的新技术，后者用于研究代谢过程和药物设计。虽然正电子与物质相互作用的某些方面已被很好地理解，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”——也就是说，它们原本以质量形式存在的能量被转换成伽马射线。该研究小组已经证明，正电子在经历湮灭之前，会短暂地——通常不到一亿分之一秒——与大多数分子结合。本项目将对正电子在各种分子上的结合进行更详细的研究。这些正电子结合的研究与正电子在材料表面的状态以及正电子在PET和星际介质中的归宿有关。在科学教育和培训方面，从实验的策划到研究成果的传播，各级学生和青年研究人员积极参与研究。该项目涉及小型实验，是科学和技术人员的极好训练场。这本书的读者广泛，不仅包括物理学的读者，也包括等离子体、化学、材料科学和气体电子学的研究人员。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智力挑战是发展处理电子-正电子相关关系的方法，以及更好地理解振动动力学在正电子湮灭过程中的作用。该项目的一个重点是更好地理解正电子与分子的结合、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理解分子中正电子湮灭的机制。这一过程是通过振动共振进行的，其湮灭速率比在简单碰撞中预期的要大几个数量级。实验将使用由研究小组开发的可调的、最先进的、高能量分辨率的正电子束进行，该正电子束现在已经达到了完全的能力。该装置有望在高分辨率测量中发挥重要作用，作为入射正电子能量的函数。它有望为分子动力学在共振正电子湮灭中的关键作用提供新的见解。该研究小组的实验工作激发了世界各地小组的补充理论工作，目标是获得对共振正电子湮灭和正电子-分子结合更完整和基本的理解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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. Positrons also have many technological applications such as new techniques for mass spectroscopy, the characterization of materials, and positron emission tomography (PET), the latter to study metabolic processes and for drug design. While some aspects of positron interactions with matter are well understood, there are many outstanding questions. When positrons and electrons collide, they can ‘annihilate’ – i.e. their energy, originally in the form of mass, is converted into gamma rays. The research group has shown that positrons briefly – typically for less than one hundred millionth of a second - bind to most molecules before undergoing annihilation. The present project will carry out a more detailed study of the binding of positrons on various molecules. These positron binding studies are relevant to positron surface states on materials and the fate of positrons in PET and in the interstellar medium. In terms of scientific education and training, the research actively involves 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 is communicated broadly, not only to physics audiences but also to those involved in plasma, chemical, and materials science, and gaseous electronics research. In previous research, the group showed that positrons bind to molecules. Two important intellectual challenges related to this phenomenon are to develop methods to treat electron-positron correlations and to better understand the role of vibrational dynamics in positron annihilation processes. One focus of the project is to gain a better understanding of positron binding to molecules, the magnitudes of binding energies, and what they depend upon. A second objective is to gain a better understanding of mechanisms for positron annihilation in molecules. This process proceeds by means of vibrational resonances and results in annihilation rates orders of magnitude greater than expected in simple collisions. The experiments will be conducted with a tunable, state-of-the-art, high-energy-resolution positron beam, developed by the research team, that is now coming to full capability. This device is expected to play an important role in making high-resolution measurements as a function of incident positron energy. It is expected to provide new insights into the critical role of molecular dynamics in resonant positron annihilation. The research group’s experimental work has motivated complementary theoretical work by groups around the world with the goal of gaining a more complete and fundamental understanding of resonant positron annihilation and positron-molecule binding.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.