Positron spectroscopy for materials characterization
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I.
INTRODUCTION
POSITRONS have been used to study solids in various ways for more than 30 years. As the antimatter equivalent of the electron, the positron turns out to be an extremely sensitive probe of both structural and electronic properties of the solid lattice. In addition, the intensities typically required for most useful experiments are sufficiently low so that probe-induced radiation damage is negligibly small. The benefits of this high sensitivity are, in part, contrasted by the fact that the positron-solid interaction is often too complicated to describe completely, and many of the experiments done necessarily result in only semiquantitative conclusions or interpretation. Nevertheless, the past three decades have seen tremendous advances in positron/solid-state research, not only in the development of new techniques and applications, but also in the understanding of some of the fundamental aspects of positron-solid interactions. In the present paper, we will briefly discuss the most common experimental techniques and review some of their applications to studies of materials (with particular emphasis on metals). One of the more distinguished areas of positron research is the study of defects, primarily because there are few other nondestructive techniques that can compete with the sensitivity and selectivity of positrons. We will, therefore, emphasize this work in the following discussion. We will also separate the text roughly into two halves. The first part will feature a general discussion of positron interactions with solids and the so-called "bulk" techniques which have evolved to study these. The second part of the text will focus on the relatively new technique of producing monoenergetic positron beams and a few examples of studies similar to those described in the first part. More detailed descrip-
PETER J. SCHULTZ, Professor of Physics, is with the Department of Physics, The University of Western Ontario, London, ON, N6A 3K7 Canada. C.L. SNEAD, Jr., Division Head, is with the Neutral Beam Division, Department of Nuclear Energy, Brookhaven National Laboratory, Upton, NY 11973. This paper is based on a presentation made in the symposium "Irradiation-Enhanced Materials Science and Engineering" presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD. METALLURGICAL TRANSACTIONS A
tions of the experimental techniques and more diverse accounts of the different types of research can be found in the books and reviews by Hautojiirvi, t~] Brandt and Dupasquier, t2] Jain et al., t3] and Schultz and Lynn. t41 II. BULK MATERIAL STUDIES WITH POSITRONS
The traditional experiment with positrons uses a relatively weak positron emitting radioisotope (e.g., tens of/xCi of 22Na) located near the sample to be studied. Positrons are emitted from the source with the usual/3 energy distribution that is continuous from zero up to the endpoint energy
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