Bulletin of the American Physical Society
65th Annual Gaseous Electronics Conference
Volume 57, Number 8
Monday–Friday, October 22–26, 2012; Austin, Texas
Session PR3: Positrons and Collision Processes in Plasmas |
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Chair: Tom Kirchner, York University Room: Classroom 202 |
Thursday, October 25, 2012 10:00AM - 10:30AM |
PR3.00001: On the modelling of positron transport in gases and soft-condensed biomaterials Invited Speaker: Ron White The understanding and optimization of positron technologies such as Positron Emission Tomography requires a fundamental understanding of the underpinning physical processes involved, including accurate knowledge of the input positron scattering cross sections, material structure, and transport theories/simulations. The availability of new accurate and more complete sets of positron interaction cross sections has facilitated new theoretical investigations of positron transport. In this presentation we will highlight recent attempts to validate the accuracy and completeness of cross-sections through comparison with available swarm data. Comparisons between electron and positron transport will be presented and the importance of the positronium formation process on transport highlighted. As we progress towards a model of transport in soft-condensed systems (e.g. human tissue), we will discuss the theoretical formalism and present preliminary results for the transport and relaxation of positrons in dense gaseous and soft-condensed systems in spherical geometry. In particular a multi-term Boltzmann equation solution and associated eigenfunction treatment will be compared to a Monte-Carlo simulation. In these systems, the structure of the medium and effects of coherent scattering plays a significant role and we will highlight how this theory will need to be modified to consider biological polar liquids such as liquid water. [Preview Abstract] |
Thursday, October 25, 2012 10:30AM - 10:45AM |
PR3.00002: Search for Positron Bound States in the Doubly Excited Region of the Helium Atom Roisin Boadle, Joshua Machacek, Emma Anderson, James Sullivan, Stephen Buckman Positron-atom binding has been the subject of many theoretical calculations in recent years. In these systems, a positron becomes temporarily bound to the atom, either through polarisation of the electronic charge cloud or formation of positronium (an e$^{-}$e$^{+}$ pair) which is weakly bound to the atom. There is now theoretical evidence of numerous positron-atom bound states, including for the helium atom. Ground state helium is incapable of binding a positron; however, recent calculations [1] have indicated that excited metastable states and doubly excited states may do so. These bound states might be expected to manifest themselves as structure in the energy dependence of the cross sections for processes such as total scattering, positronium formation, or ionization. We have carried out an experimental search for these positronic helium states in the doubly-excited region near 58 eV, using our high-resolution, trap-based positron beam. Results from this study will be presented and their ramifications discussed. \\[4pt] [1] M.J. Bromley and J. Mitroy Private Communication (2012) [Preview Abstract] |
Thursday, October 25, 2012 10:45AM - 11:00AM |
PR3.00003: Recent progress in understanding positron annihilation on molecules A.C.L. Jones, J.R. Danielson, M.R. Natisin, C.M. Surko Annihilation at positron energies in the range of the molecular vibrational modes is dominated by large-amplitude vibrational Feshbach resonances (VFR) in which the positron attaches to the molecule.\footnote{G. F. Gribakin, J. A. Young, C. M. Surko, Rev. Mod. Phys. {\bf 82}, 2557 (2010).} Recently, a broad spectrum of enhanced annihilation has been discovered and is observed in the spectra of many, if not most, molecules.\footnote{A. C. L. Jones, et al., Phys. Rev. Lett., \textbf{108}, 093201 (2012).} This spectral component, known as statistical multimode resonant annihilation (SMRA), dominates the spectra in small molecules with relatively large binding energies, such as CCl$_4$ and CBr$_4$. Incorporation of an SMRA spectral component has allowed for a more accurate probe of VFR magnitudes and is providing insight into the process of intramolecular vibrational redistribution (IVR), through which VFRs can be either enhanced or suppressed. [Preview Abstract] |
Thursday, October 25, 2012 11:00AM - 11:30AM |
PR3.00004: Positron plasma techniques and the production of a positronium gas Invited Speaker: David Cassidy The use of buffer gas positron traps has led to numerous advances in the field of positron physics, including improved beams for measurements of positron scattering from atoms and molecules, the production of antihydrogen and the generation of high-density positron pulses. The latter has been used to study dense positronium (Ps), in particular Ps-Ps scattering and the formation of molecular positronium, Ps$_{2}$. The ability to create non-neutral positron plasmas has played a key role in such experiments; using the rotating wall technique in the strong drive regime [Danielson and Surko, PRL \textbf{94}, 035001] allows for precise control of the positron density as well as very long confinement times. Here I shall outline the methods we have used to produce intense bursts of dense Ps and consider what we can do using these techniques in the future. [Preview Abstract] |
Thursday, October 25, 2012 11:30AM - 11:45AM |
PR3.00005: Electron attachment to halogenated alkenes and alkanes, 300-600 K Thomas M. Miller, Jeffrey F. Friedman, Nicholas S. Shuman, Albert A. Viggiano Rate coefficients (k$_{a})$ and ion product distributions have been measured for 14 alkenes and alkanes with bromine, fluorine, and iodine substituents over the temperature range T = 300-600 K using a flowing-afterglow Langmuir-probe apparatus (FALP), most for the first time. Among these are 3 isomers of C$_{3}$F$_{5}$Br and 2 isomers of C$_{3}$F$_{7}$I. Four dibromide compounds yield Br$_{2}^{-}$ in addition to Br$^{-}$. The results follow the expected trends: k$_{a}$ values near the capture limit decrease slightly with T according to Vogt-Wannier theory, while k$_{a}$ increase with T for molecules which have small k$_{a}$ at 300 K. The results are analyzed using a statistical kinetic modeling approach, which is able to reproduce k$_{a}$ values and product branching within experimental uncertainty. The modeling indicates that factor of 2 differences in k$_{a}$ for the isomeric species can be explained by subtle variations in the potential surfaces. [Preview Abstract] |
Thursday, October 25, 2012 11:45AM - 12:00PM |
PR3.00006: Collisional dissociative recombination in helium-hydrogen afterglow plasmas Rainer Johnsen The puzzling dependence of electron-ion recombination in helium-hydrogen afterglows on neutral\footnote{Glos\'{\i}k et al., Phys. Rev.A \textbf{79}, 052707 (2009)} and electron\footnote{Gougousi et al., Int. J. Mass Spec. Ion Proc. \textbf{149-150}, 131 (1995)} densities is shown to be compatible with the ``Collisional Dissociative Recombination'' mechanism, originally proposed by Collins,\footnote{Collins, Phys. Rev.A \textbf{140}, 1850 (1965)} in which three-body capture of electrons into molecular high Rydberg states of H$_{3}^{+}$ leads to predissociation of the molecular core. While both electrons and neutrals play a role in the three-body capture, their effects on recombination do not add in a simple manner, which makes it difficult to distinguish three-body and binary dissociative recombination. Collision-induced angular momentum mixing ($l$-mixing), invoked in earlier models, also occurs but does not provide the rate-limiting step that controls the overall recombination rate. [Preview Abstract] |
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