The lifetime of a positron inside a solid is normally less than a fraction of nanosecond. This is a very short time on a human scale, but is long enough to enable the positron to visit an extended region of the material, and to sense the atomic and electronic structure of the environment. Thus we can inject a positron in a sample to draw from it some signal giving us information on the microscopic properties of the material. This idea has been successfully developed in a number of positron-based techniques of physical analysis, with resolution in energy, momentum, or position. The complex of these techniques is what we call now positron spectroscopy of solids.

The field of application of the positron spectroscopy extends from advanced problems of solid-state physics to industrial applications in the area of characterization of high-tech materials. In the latest years, we are witnessing an increasing interest for applications of the positron spectroscopy in industrial laboratories: Indeed, a large amount of data gathered in practical situations is giving full evidence of the usefulness of the method. We feel, however, that the contribution of experts in positron physics, working side by side with material scientists, is still important for choosing the most interesting objectives, for defining the most effective conditions of use and for increasing the accuracy of the quantitative determinations.

There are still many fronts, also potentially interesting for technological applications, where the interest of positron spectroscopy has been clearly demonstrated by pioneer studies, but the stage of industrial exploitation has yet to come. We need to know more, or we just need more powerful or more refined equipment. There is presently an important effort for building better instrumentation, sometimes with costs unusually large for this field.

Technical and experimental progresses are well matched with the progress of theory. It may be said that we have now the key for interpreting the outcome of the measurements. The data on the electron momentum distributions, that one can obtain from measurements of the angular correction of the annihilation radiation, can be compared to accurate model predictions. The relation between the lifetime of a positron and the chemical and morphological properties of the local environment can be easily predicted by powerful calculation methods. Problems concerning slowing-down and transport of positrons in matter are being solved with increasing reliability by computer simulations. All this makes positron spectroscopy a really quantitative methodology. Most important, the theory tells us where to address our attention for future discoveries.

If rather frequent meetings of restricted or enlarged scope are an effective channel for keeping the experts in tune with the progress of a field in rapid expansion, there is sometimes also the need of looking back at the paces made in the recent past. We need to unravel the main road from many wandering paths, in order to be able to guide the newcomer in the field. For a long time this has not been done in our area of study: the last time it was with another Fermi Summer School in 1981 and with the volume of the Proceedings of that School (Positron Solid-State Physics, W. Brandt and A. Dupasquier, editors, 1983). It was a useful initiative, but now twelve years have passed. With the encouragement of many colleagues, we have volunteered to repeat the experience. This time, the School has been combined with a NATO Workshop on Advances with Positron Spectroscopy of Solids and Surfaces, in order to bring the participants in contact not only with well-digested and organized material, but also with research work in progress. We have been able to convince to lecture in Varenna, and to contribute at this volume, an outstanding group of experts of solid-state spectroscopies. The task we assigned to them was to focus the attention on the physics that can be learned from positron-based methods, but also to frame those methods in a wider context including other experimental approaches. This is exactly what was done at the School, and is also reflected in this volume. Our ambition was to put together a true textbook on positron spectroscopy of solids, the sort of book that the newcomer takes for his approach to this field, but also a useful research tool for the expert. Our readers will judge how close we get to our objective.

This volume contains two kind of contributions: the lectures of the E. Fermi School and the written version of the seminars presented at the NATO Workshop. In general, the lectures are of wider scope and more systematic character than the seminars, which, on the contrary, are sharply focused on a special subject, normally reflecting work in progress. The sequence of the lectures in the index of this volume, however, ignores this shade of difference. The priority has been given to connections between the subjects, grouped under the headings Electronic Structure, Surface Studies and Defect Probing. Contributions that fit into more or less than a single heading are assigned arbitrarily to one of them. As an appendix, the volume contains also the abstracts of the communications presented at a poster session open to all participants to the NATO Workshop. This appendix is the only part of the book that has not been revised by a referee.

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We are indebted with the Italian Physical Society for having given us the possibility of organizing the Course on Positron Spectroscopy of Solids, and with the NATO Scientific Affairs Division for supporting the Workshop Advances with Positron Spectroscopy of Solids and Surfaces as a satellite meeting. The participation of several students without independent financial support has been made possible by the generosity of different organizations: eleven scholarships have been assigned by the Italian Physical Society to students of nine different countries, two scholarships have been offered to Italian students by the GNSM (Gruppo Nazionale di Struttura della Materia of the Italian National Council of Research). The Scientific and Technical Research Council of Turkey (Tübitak) has supported the participation of a Turkish student to the NATO Workshop.

We are extremely grateful to the Secretary of the Italian Physical Society Mrs. E. Mazzi and to her staff for the perfect practical organization of the two combined meetings, and for the very warm hospitality in Varenna that has made the two weeks spent at Villa Monastero a most pleasant experience for many of us. We also thank Mrs. C. Vasini and Mr. P. Papali of the editorial office of the Italian Physical Society, with their expert collaborators, for the production of this volume.

Our special gratitude goes to the colleagues that have accepted to postpone many important duties for lecturing in Varenna and for contributing to this volume, with a true spirit of service for the scientific community and, in particular, for the young generation that is coming aboard.

A. Dupasquier and A.P. Mills jr.