Session MW1: Collisions Involving Positron and Positronium

Live

Positrons have increasing importance in medicine, through the use of Positron Emission Tomography (PET), where a radioisotope is injected into the body, typically attached to a glucose molecule. These molecules accumulate in regions of high metabolic activity, and once there a positron is emitted. After thermalising, these positrons annihilate, emitting back-to-back gamma rays, which can be used to locate and image tumors, for instance.\\ At the Australian National University, we use a low energy positron beamline [1] to investigate the interactions of positrons with molecules that can be used to start building a model of the environment in which these positrons thermalise [2]. In particular, positrons can induce damage, as well as generating a host of secondary electrons which can cause damage of their own. To better understand these processes, high accuracy measurements are required to verify theoretical calculations and models of these processes. \\ This talk will present recent results on the interaction of positrons with furan, one of a class of molecules that in used to model elements of the DNA structure. The measurements in include elastic and inelastic scattering, as well as positronium formation, and are compared to recent theories from colleagues using the Independent Atom Model and the Schwinger Multichannel approach. \\ In addition, the outline of a new experimental effort will be presented, which aims to make measurements of positron thermalisation in liquid water [3]. This will be an important step in verifying many of the assumptions made in using gas phase cross sections to model positron interactions in liquids, and should lead to new tests of recent model approaches that aim to build a complete picture of the positron thermalisation process. \\ ~[1] J. P. Sullivan et al., Rev. Sci. Instr. 79, (2008) 113105\\ ~[2] Tattersall et al., J. Chem. Phys. 140, 044320 (2014)\\ ~[3] F. Blanco et al., J. Phys. B 49, 145001 (2016)\\   

Live

Positron annihilation is one of the most interesting and essential processes in studies of positron transport in media. Calculation of positron annihilation rates is complicated due to its high sensitivity to the accuracy of the scattering wavefunction. As a result, there are not many calculations of annihilation rates even for the simplest case of positron-hydrogen system. The convergent close-coupling method (CCC) has been very successful in obtaining accurate scattering data on a wide range of collision energies. In this report, we present application of the CCC method to calculations of positron annihilation. Detailed studies of convergence combined with internal consistency of large single- and two-centre expansions indicate a high accuracy of the results.   

Live

Electron exchange and correlations play an important role in low-energy positronium (Ps) collisions with atom and molecules since the static potential for Ps interaction with a neutral system is zero. In particular the electron exchange is responsible for the observed similarity between electron and Ps collision cross sections when they are plotted as functions of the projectile velocity [1]. We have performed Ps scattering calculations for several atomic and molecular targets using the free electron gas model for exchange and correlation potentials [2]. Our results confirm experimental observations, although at low energies theoretical collision cross sections are often higher than the experimental results. The similarity between electron and Ps scattering extends to low-energy resonances observed in electron and Ps collisions with nitrogen molecules [3,4]. This work was done in collaboration with Robyn Wilde. [1] S. J. Brawley, S. Armitage, J. Beale, D. E. Leslie, A. I. Williams, and G. Laricchia, Science {\bf 330}, 789 (2010). [2] I. I. Fabrikant and R. S. Wilde, Phys. Rev. A {\bf 97}, 052707 (2018). [3] M. Shipman, S. J. Brawley, L. Sarkadi, and G. Laricchia, Phys. Rev. A {\bf 95}, 032704 (2017). [4] R. S. Wilde and I. I. Fabrikant, Phys. Rev. A {\bf 97}, 052708 (2018).   

Live

Positrons can attach to many molecules through Feshbach-resonant excitation of fundamental vibrational modes which leads to greatly enhanced annihilation rates. The shape of the energy resolved annihilation spectra is expected to be determined by the positron beam energy distribution (FWHM $\sim$ 36 meV), which is found to be true for some molecules, including small chain alkanes. Here we present recent data that shows extra spectral features which broaden the resonance for many large ring and chain alkanes. A high-resolution cryogenic beam (FWHM $\sim$ 22 meV) has resolved this feature for cyclopentane, as a sharp, isolated resonance at a location far from any known fundamental vibrational mode. Details of this new resonant feature will be presented and results will be compared with the infra-red absorption spectra. Implications for other molecular targets will be discussed.   

On Demand

A technique has been developed for calculating the electron-impact ionization single differential cross section directly from the integrated cross sections of positive-energy pseudostates occuring in close-coupling methods. Using the cross sections arising in the convergent close-coupling method, the approach is first tested against the existing benchmark theoretical and experimental data for electron scattering on hydrogen and helium. It is then applied to electron scattering on molecular hydrogen yielding excellent agreement with experimental data when they are normalized to the total ionization cross section.