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Kluwer Academic Publishers eBooks

From Positron to Positronium Scattering

Gaetana Laricchia, A J Garner, Kirsten Paludan

Published Dec 15, 2005• 0 citations
Publisher

Abstract

Conclusions and OutlookCollisions involving positron and positronium are contributing to the understanding of general atomic physics phenomena as well as illuminating specific processes and interactions such as annihilation and exchange. It is hoped that the considerable effort presently aimed towards the production of large ensembles of cold positrons86 will result in beams of extremely well defined energy (≤kT) which should enable the detailed study of near-threshold phenomena, resonances, annihilation and positronic compounds.

DOI
10.1007/0-306-47464-6_12

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Conclusions and OutlookCollisions involving positron and positronium are contributing to the understanding of general atomic physics phenomena as well as illuminating specific processes and interactions such as annihilation and exchange. It is hoped that the considerable effort presently aimed towards the production of large ensembles of cold positrons86 will result in beams of extremely well defined energy (≤kT) which should enable the detailed study of near-threshold phenomena, resonances, annihilation and positronic compounds.
Positronium (Ps) atoms exist in different spin configurations that determine their fundamental properties, such as energy level structure, decay rate and response to a magnetic field. The nature of interactions between Ps atoms is also strongly influenced by their relative spin states; oppositely polarized Ps atoms may exchange particles and scatter into different states, or join together to form molecular positronium. Conversely, interactions between spin aligned Ps atoms cannot lead directly to changes of spin states or molecule formation, but at high densities/low temperatures a collection of such atoms may undergo a phase transition to form a Bose-Einstein condensate. Here I consider some of the physics involved in such interactions and the positron beam parameters required to conduct such experiments.
The very large annihilation rates observed at room temperature in positron collisions with certain molecules have instigated the development of several models of positron annihilation based on the role of virtual processes, Feshbach resonances and bound states. One of the outcomes of these models has been the interest they have generated in investigations of the energy dependence of the annihilation cross-section in the vicinity of inelastic thresholds. The aim of this article is to provide a review of these recent studies and to highlight outstanding questions.
The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.
A report is presented of the XIIth International Workshop on Positron and Positronium Physics (Sandbjerg, Denmark, 19–21 July 2003). This workshop covered positron and positronium interactions with atoms, molecules and condensed matter systems. One key development reported was the first creation in the laboratory of low-energy antihydrogen atoms. Facets of positron-electron many-body systems were also considered, including the positronium molecule and BEC gases of positronium atoms. Aspects of the future of the field were discussed, including the development of new theoretical and experimental capabilities.