Stephen J Gilbert

New source of ultra-cold positron and electron beams

Abstract We have developed a new and versatile source of cold ( ΔE≈0.018 eV ) , low-energy ( E≈0−9 eV ) , magnetized positrons and electrons. Particles are extracted from thermalized, room-temperature, single species plasmas confined in a Penning trap. Typically, the trap contains from 107 to 109 particles, which can be released either in the form of a quasi steady-state beam or as a pulsed beam. Of the order of 100 pulses of 105 positrons, each 10 μs in duration, have been achieved. Size, duration and frequency of pulses within a pulse train are easily variable over a wide range. Cold quasi steady-state electron beams with a diameter of ≈3 mm have also been achieved. Limited only by the total charge contained in the trap, electron currents of 0.1 μA for several milliseconds have been generated routinely. This source represents a combination of attractive features not previously available using any single technique.

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.

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.