Interaction of Low-Energy Positrons with Atoms and Molecules

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.