The NATO Advanced Research Workshop on Atomic Physics with Positrons, which was held at University College London during 15-18 July 1987, was the fourth meeting in a series devoted to the general theme of positron colli­ sions in gases. Previous meetings have been held at York University, Toronto (1981); Royal Holloway College, Egham (1983) and Wayne State Uni­ versity, Detroit (1985). Recent very significant improvements in positron beam currents, due to the development of more efficient moderators and the use of more intense positron sources, are making possible an increasingly sophisticated range of experiments in atomic collision physics. Whereas a few years ago only total scattering cross sections could be determined, measurements can now be made of various partial and differential cross sections. Intense positron beams are also being used to produce positronium beams and already, as reported here, preliminary investigations have been made of collisions of positronium with several target systems. These experimental developments have stimulated, and been stimulated by, steady, if somewhat less spectacu1ar,progress in associated theoretical studies. Both aspects of the field are well represented in these Proceed­ ings.

Table of Contents:
Invited Papers.- Positron-Impact Ionization and Positronium Formation.- Inelastic Positron Collisions in Gases.- Positron-Atom Differential Scattering Measurements.- Total and Inelastic Positron Scattering Cross Sections.- Studies of Inelastic Positron Scattering Using 2.3 and 3 M Spectrometers.- Recent Developments in the Theory of Fast Positron-Atom Collisions.- New Understanding of Atomic Physics Using High Velocity Positrons.- Calculations of Scattering Cross Sections and Annihilation Rates in Low Energy Collisions of Positrons with Molecular Hydrogen.- The AB Initio Inclusion of Polarisation Effects in Low-Energy Positron-Molecule Collisions Using the R-Matrix Method.- Positron-Molecule Collisions: Elastic, Inelastic and Differential Cross Section Calculations.- Field Theory of Electron and Positron Scattering from Atomic and Molecular Systems.- The High Brightness Beam at Brandeis.- A High Intensity Positron Beam at the Brookhaven Reactor.- Field Assisted Moderators.- Production of Short-Lived Positron Sources.- Electrostatic Lenses: How to Roll Your Own.- Theoretical Aspects of Positronium Collisions.- Electron Capture from Solids by Positrons.- On the Production of a Timed Positronium Beam by Positron-Gas Scattering.- A Positronium Beam and Positronium Reflection from LiF.- Monoenergetic Ps Created in a Gas and Ps-He Collision Cross Section Measurements.- Positron- (and Electron-)Alkali Atom Total Scattering Measurements.- Theoretical Studies of Low-Energy Positron-Alkali Atom Scattering.- Precision Measurement of the Triplet Positronium Decay Rate in Gases.- The Brookhaven Positron-Hydrogen Scattering Experiment: Motivation and General Scope.- The Brookhaven Positron-Hydrogen Scattering Experiment: Experimental Setup.- Synthesis of Antihydrogen.- Antihydrogen:Production and Applications.- On Antihydrogen Production.- ?+ Charge Exchange, Muonium Formation and Depolarization in Gases.- Slow Muon Physics.- Contributed Papers.- The Inclusion of the Contributions to Low Energy e +H2 Scattering and Annihilation from the Lowest ?u+ and ?u Waves.- Positronium Formation in Positron-Hydrogen Scattering at Intermediate Energies.- Epithermal Positron Effects in Surface Measurements.- A Proposed Source of Atomic Hydrogen Suitable for the Measurement of Total and Partial Scattering Cross Sections.- Angular Correlation Studies of Positron and Positronium Annihilation in Gases.- Formation of Antihydrogen in Excited States in Antiproton-Positronium Collisions.- The Effect of an Electric Field on Free Positron Annihilation in Atomic and Molecular Gases.- Slow Positron Setup at the Giessen 65 MeV Linac.- Interactions between p and H at Intermediate Antiproton Energies.- Formation of Antihydrogen and Destruction of Positronium in $$\bar p$$ + Ps Collisions in the Antiproton Energy Range 2–100 keV.- Comparison of Impact-Ionisation and Charge-Transfer Cross Sections for Positron and Proton Scattering on Helium.- Measurement of the Impact Ionisation and Positronium Formation Cross Sections for Positron Scattering on Molecular Hydrogen.- Auto-Dissociating Resonant States of Positronium Molecules.- Positron-Hydrogen Resonances Associated with the n = 4 Hydrogen and n = 3 Positronium Thresholds.- Calculation of Ps Distribution from Solid Surfaces.- Elastic Scattering of Fast Positrons by Sodium and Potassium Atoms.- Excitation of Helium by Positron Impact and a Comparison with Electron Data.- Differential Cross Sections for Electron and Positron Scattering from Argon.- Ionization of Positronium by Electrons.- Positronium Formation at CondensedAtomic Semilayers of Krypton and Carbon.- Positron-Na Scattering Using CCA.- Elastic Differential Positron Scattering from Argon.- Angular Differential Cross Sections and Moderation Effects.- Positronium Formation in the Excited States (2s, 2p) in Positron-Hydrogen Atom Collisions.- Ionisation of Atomic Hydrogen by Positron Impact.- Cross Sections for .$${e^ + } + {H^ - } \to Ps\left( {n\ell } \right) + H\left( {ls} \right)$$. in Fock-Tani Representation.- Elastic Scattering of Positrons from argon.- Positronium Formation from Atomic Hydrogen.- Positron-Atomic Hydrogen Scattering at Medium to High Energies.- Low Energy Positron Transmission Measurement from Thin Single-Crystal W Foil.