This project focuses on experimental studies of the interaction of low-energy positrons with atoms and molecules using a high-resolution, trap-based positron beam and specially designed techniques to study scattering and annihilation processes. The work is expected to have fundamental impacts on important facets of atomic physics, including the development of methods to treat electron-positron correlations and understanding positron binding to atoms and molecules. The results are relevant to many applications of low energy positrons including the development of new methods to characterize materials and modeling processes of astrophysical interest. The positron beam to be used in these studies has an energy resolution ~ 40 meV, FWHM and is tunable in energy from 50 meV upwards. New experimental capabilities include the ability to heat and cool atomic and molecular targets (from 100 - 800 K). A higher energy resolution beam is currently under development. Recent results on positron annihilation in positron-molecule collisions provide new topics for study, including making precise tests of theoretical predictions for positron-molecule binding. Key themes of the research include understanding quantitatively positron binding to ordinary matter and the annihilation of positrons on large molecules when intramolecular vibrational energy redistribution is operative. Other planned research topics include studies of new targets including large alkane and polycyclic aromatic molecules, carbon-60, and metal atoms.
While this project focuses on aspects of the low-energy interaction of antimatter (positrons) with ordinary matter, in the broader view, it seeks to establish aspects of a quantitative physical and chemical description of the interaction of antimatter with matter. Consequently, the results are expected to have further impacts in other areas of science and in a wide variety of technological applications. Fundamental-physics examples include the formation of antihydrogen, the creation in the laboratory of positronium molecules and Bose-condensed gases of positronium atoms, and astrophysical processes involving positrons. The work will also help to enable technological applications of low energy positrons, such as developing new methods to ionize large molecules and in providing a deeper understanding of positron-based tools to study materials.
This project also has additional impacts in other areas. The research actively involves students and post-doctoral 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. The researchers involved in this project continue to foster active dialogue with experimentalists and with theorists working on problems of mutual interest, both in the U. S. and in a number of other countries. Members of the team frequently advise academic and industrial researchers on practical implications of the scientific results and other technical information gained during the course of this research.
Intellectual merit. The fundamental laws of physics, to the extent that they are currently understood, predict that, for every particle of matter created in our universe, there will be a corresponding antiparticle. Examples of matter particles and their (antimatter) twins are the proton (antiproton) and electron (positron). One facet of our universe not currently understood is that antiparticles are far less numerous than particles of matter. Nevertheless, antimatter is important in many scientific and technological contexts. One example is positron emission tomography (PET) to study metabolic processes in humans and other organisms and for use in drug design. The overall objective of the research done in this project was to understand at a more fundamental level the interactions of positrons ("antielectrons") with ordinary matter in the form of atoms and molecules, so that this knowledge can be exploited for scientific and technical applications. On a more specific level, this work had two goals. One was to improve our understanding of the process by which positrons bind to ordinary matter in the form of molecules and atoms. An attached positron can remain on the molecule for times ~ 1 - 10 nanoseconds before annihilating (i.e., a positron and a molecular electron disappear with the energy carried off by two gamma rays). A second goal was to better understand the process by which positrons annihilate in such bound states. Progress was made in both areas. New experiments were conducted to measure positron binding to a variety of molecules and careful comparisons were made between positron-molecule binding and the analogous cases of electron binding. This work provided new insights into the mechanism by which positrons attach to matter. New experiments were also conducted to understand positron annihilation on molecules, and a simple mathematical model of this process was constructed. Finally progress was made on the development of a tunable-energy positron beam with much improved energy resolution for future studies. Broader Impacts. While this project focuses on aspects of the low-energy interaction of antimatter (positrons) with ordinary matter, in the broader view, it seeks to establish aspects of a quantitative physical and chemical description of the interaction of antimatter with matter. Consequently, the results are expected to have further impacts, either directly or indirectly, in other areas of science and in developing a variety of technological applications. Fundamental-physics examples include the formation and study of antihydrogen (stable, neutral antimatter) and astrophysical processes involving positrons. The work will also help to enable technological applications of low energy positrons, such as providing a deeper understanding of positron-based tools to study materials. This project has additional broader impacts in other areas. The project involved small-scale experiments that are an excellent training ground for scientific and technical personnel. The research actively involved students and post-doctoral researchers at all levels, from the planning of experiments to the dissemination of research results. 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. The researchers involved in this project continued to foster active dialogue with experimentalists and with theorists working on problems of mutual interest, both in the U. S. and in a number of other countries. Members of the team have frequently advised academic and industrial researchers on practical implications of the scientific results and other technical information gained during the course of this research.
