Positron Beams and Their Applications

The following sections are included:

• PREFACE

• TABLE OF CONTENTS

The forty-year history of low-energy positron beams is a model example of how an experimental technique can grow and flourish through persistence, ingenuity, and a small measure of good fortune. From modest beginnings the field has witnessed orders of magnitude increase in beam intensities, the development of positron beams which rival or even exceed the specifications of their state-of-the-art electron equivalents, and a burgeoning of applications in almost every field of scientific endeavour. This chapter is a short personal account of some of the milestones in positron beam research since a Princeton graduate student's thesis set the ball rolling - albeit slowly - in 1958.

This chapter will follow the positron from source to target. The variety of primary positron sources and moderation methods will be reviewed. Magnetic and electrostatic transport and focusing of positron beams, including velocity filtering, acceleration, brightness enhancement, bunching, and timing, are described and discussed. The chapter ends with the detection of positrons at the end of their flight path, both directly and via their annihilation radiation.

In recent years there have been considerable advances in studies of atomic and molecular physics with positrons and positronium. This chapter is devoted to a discussion of the fundamental properties of these particles, their interactions with atoms and molecules and possibilities for the formation of low-energy antihydrogen.

This Chapter summarizes the basic processes for positrons entering condensed matter. The energy loss mechanisms and slowing-down of energetic positrons are discussed, and the key features of the ensuing implantation distributions are presented. This includes a discussion of the elementary excitations and scattering processes in the medium. The ballistic, diffusive and quantum regimes of positron motion are then described. The Chapter also discusses the physics of positronion interactions and electron-positron correlation. Their effects on the thermalised positron wave functions are presented, with the view of quantitative predictions of positron energetics (affinities, positron-lattice coupling etc.) and annihilation characteristics (annihilation rates, momentum distributions). The main features of positron trapping at and detrapping from bulk and surface defects are summarized. Finally, the different scenarios for positrons in the vicinity of surfaces and interfaces are discussed.

Positron beam technology has developed in tandem with our understanding of positron-surface interactions to enable the positron to be used in novel and unique ways to study the physical and chemical properties of surfaces. Most notable among the new positron spectroscopies are Low-Energy Positron Diffraction (LEPD) and Positron Annihilation-induced Auger Electron Spectroscopy (PAES) and these, along with a number of other applications of positron beams to the investigation of surface phenomena, are described and discussed in this chapter.

In positron beam analysis, mono-energetic positrons are implanted into a solid surface and the characteristics of the positron annihilation process are interpreted in terms of implantation depth, positron transport, and trapping at defects or at interface sites. In principle, all observation methods used for bulk studies with unmoderated positrons can be used also for beam studies, but the majority of them rely on Doppler broadening techniques. A few beam analysis systems have been designed to carry out depth sensitive positron lifetime analysis or analysis with 2D-ACAR. This chapter describes the observation techniques, including the recent refinement of the Doppler broadening method by employing two-detectors in coincidence techniques, and the use of a set of line-shape parameters (S,W) related to valence and core-electrons, respectively. Examples are given of analysis for point defects and defect clusters, precipitates, and layered structures with planar interfaces. Aspects related to the charged nature of the positron are described for electrically biased capacitor (e.g., MOS) systems.

The considerable potential for novel surface and near-surface imaging by positron re-emission microscopy, and for extending the capability of traditional one-dimensional positron implantation studies by rastering a microbeam of controllable energy across a sample, is discussed. The advantages and limitations of the techniques are presented, as are the possibilities of future developments.

The following sections are included:

• Introduction

• The Setting Up of an MeV Positron Beam

• Beam-Based Positron Lifetime Measurements

• Age–Momentum Correlation

• Selected Experiments

• Acknowledgements

• References

The following sections are included:

• Introduction

• The principles of the spin–polarized positron set-up

• Determination of the positron–spin polarization

• Positron–spin relaxation (e⁺SR) experiments

• e⁺SR experiments on α iron

• Magnetic quenching of the positronium hyperfine interaction

• e⁺SR experiments on samples in which positronium may be formed

• Summary and future prospects of the spin–polarized positron–beam technique

• Acknowledgements

• References

There is a critical need by all positron experiments for high intensity positron beams. Attempts to fill this need have been successful and beams with intensities as high as 10¹⁰e⁺s^-1 are available. These beams are necessarily located in association with nuclear reactors or high power electron linacs. New facilities under development offer the promise of even higher positron currents.

The following sections are included:

• INDEX