James Sullivan

Low energy positron scattering and annihilation studies using a high resolution trap-based beam

Abstract A high resolution positron beam, generated from a Penning–Malmberg trap, has been used for a range of low energy scattering and annihilation studies on atoms and molecules. We describe measurements of total scattering, differential elastic scattering and integral vibrational and electronic excitation cross sections for a number of atoms and molecules using this beam, and compare the absolute cross sections that are obtained with data from electron impact. The first study of annihilation on atoms and molecules as a function of positron energy is described. The results in molecules indicate large resonant enhancements of the annihilation rates at energies corresponding to those of the molecular vibrations.

Low-energy positron scattering from atoms and molecules using positron accumulation techniques

Positron scattering from atoms and molecules is studied at low values of incident positron energy. The experiments use a cold magnetized positron beam formed in a positron accumulator. A discussion of positron scattering in a magnetic field is presented along with diAerential cross-sections (DCS) for positron‐atom collisions and positron‐ molecule total vibrational excitation cross-sections. Absolute values of the DCS for elastic scattering from argon and krypton are measured at energies ranging from 0.4 to 2.0 eV. The first low-energy positron‐molecule vibrational excitation cross-sections have been measured (i.e., for carbon tetrafluoride at energies ranging from 0.2 to 1 eV). Using information gained from these experiments a second generation scattering apparatus is described, which was designed and built specifically for scattering experiments using a magnetized positron beam. This apparatus has a number of improvements, including an order of magnitude higher throughput, better energy separation between elastic and inelastic scattering events, and improved measurement of the absolute pressure of the test-gases. Analysis techniques for the data taken in these experiments using a retarding potential energy analyzer and possible extensions of these experiments are also discussed. ” 2000 Elsevier Science B.V. All rights reserved.

Scattering and Annihilation Experiments Using a Trap-Based Beam

The buffer gas trap technique for positron trapping and beam formation has made available a low energy, high resolution positron beam for the first time. This has opened the way for a range of new studies in positron-atom(molecule) scattering, including measurements of annihilation cross sections (Zeff) as a function of energy and processes such as excitation by low energy positron impact. New light is now being shed on the interactions of positrons with ordinary matter, including the first observation of positron-molecule resonances, and the latest progress in these studies is summarised here. The application of these techniques to the studies of larger systems, such as clusters, is also briefly discussed.

Low-Energy Positron-Matter Interactions Using Trap-Based Beams

We present an overview of nonneutral plasma techniques developed to study the interaction of low-energy positrons with atoms and molecules. Both scattering and positron annihilation experiments are described. The scattering experiments provide the first state-resolved cross sections for both vibrational excitation of molecules and electronic excitation of atoms and molecules by positron impact. The annihilation experiments provide the first energy-resolved measurements of positron annihilation. Extensions of these techniques are briefly discussed, including work to create a new generation of positron beams with millielectron volt energy resolution and the development of methods to study atomic clusters and dust grains.