Interaction of Low-Energy Positrons with Atoms and Molecules

University Of California-San Diego, La Jolla CA

Investigators

Abstract

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.