In the present volume and in the preceding one we have stretched our normal pattern of reviews by including articles of more major proportions than any we have published before. As a consequence each of these two vol umes contains only three review articles. From the beginning of this series it has been our aim, as editors, to achieve variation in the scope, style, and length of individual articles sufficient to match the needs of the individual topic, rather than to restrain the authors within rigid limits. We feel that the two major articles of Vols. 5 and 6 are entirely justified and do not repre sent unnecessary exuberance on the part of the authors. The article by Michaudon on fission is the first comprehensive account of the developments in this subject, which have placed it in the center of the stage of nuclear physics during the past few years. The discovery of fission isomerism and its dramatic manifestations in the intermediate structure of the neutron cross sections for fissionable isotopes are among the most im portant and interesting events to occur in nuclear physics. These events came as a surprise, and reaffirmed that the strength of nuclear physics lies in the combination of ingenious experiments with simple ideas.

With the appearance of Volume 3 of our series the review articles them selves can speak for the nature of the series. Our initial aim of charting the field of nuclear physics with some regularity and completeness is, hopefully, beginning to be established. We are greatly indebted to the willing coopera tion of many authors which has kept the series on schedule. By means of the "stream" technique on which our series is based - in which articles emerge from a flow of future articles at the convenience of the authors-the articles appear in this volume without any special coordination of topics. The topics range from the interaction of pions with nuclei to direct reactions in deformed nuclei. There is a great number of additional topics which the series hopes to include. Some of these are indicated by our list of future articles. Some have so far not appeared on our list because the topics have been reviewed re cently in other channels. Much of our series has originated from the sug gestions of our colleagues. We continue to welcome such aid and we continue to need, particularly, more suggestions about experimentalists who might write articles on experimental topics."

The three articles of the present volume clearly exhibit a wide scope of articles, which is the aim of this series. The article by Kahana and Baltz lies in the main flow of the large stream of work currently in progress with heavy-ion accelerators. A related article by Terry Fortune on "Multinuclear Transfer Reactions with Heavy Ions" is scheduled to appear in the next volume. The article by Whitehead, Watt, Cole, and Morrison pertains to the nuclear-shell model for which a number of articles have appeared in our series. Our very first volume had an article on how SU(3) techniques can, with great elegance, enable one to cope with the sizable number of states within a configuration. But the actual nuclear force is not exactly that yielded by the elegant techniques, and so interest continued in dealing with the large number of states by brute force. Then the Glasgow school of Whitehead et al. discovered that mathematical techniques existed for coping more simply with the lowest eigenvalues of large matrices. The present ar ticle aims generally to make accessible to nuclear physicists the methods developed at Glasgow. The final article by Baer, Crowe, and Truol on radiative pion capture describes a new field of importance because of the advent of the meson factories. More and more pions and muons will become standard tools in nuclear physics."

As much by chance as by design, the present volume comes closer to having a single theme than any of our earlier volumes. That theme is the properties of nuclear strength functions or, alternatively, the problem of line spreading. The line spreading or strength function concepts are essential for the nucleus because of its many degrees of freedom. The description of the nucleus is approached by using model wave functions-for example, the shell model or the collective model-in which one has truncated the number of degrees of freedom. The question then is how closely do the model wave functions correspond to the actual nuclear wave functions which enjoy all the degrees of freedom of the nuclear Hamiltonian? More precisely, one views the model wave functions as vectors in a Hilbert space and one views the actual wave functions as vectors spanning another, larger Hilbert space. Then the question is: how is a single-model wave function (or vector) spread among the vectors corresponding to the actual wave functions? As an example we consider a model state which is a shell-model wave function with a single nucleon added to a closed shell. Such a model state is called a single-particle wave function. At the energy of the single-particle waVe function one of the actual nuclear wave functions may resemble the single-particle wave function closely.

The present volume reaffirms nuclear physics as an experimental science since the authors are primarily experimentalists and since the treatment of the topics might be said to be "experimental." (This is no reflection on the theoretical competence of any of the authors.) The subject of high-spin phenomena in heavy nuclei has grown much beyond the idea of "backbending" which gave such an impetus to its study five years ago. It is a rich, new field to which Lieder and Ryde have contributed greatly. The article "Valence and Doorway Mechanisms in Resonance Neutron Capture" is, in contradistinction, an article pertaining to one of the oldest branches of nuclear physics-and it brings back one of our previous authors. The Doppler-shift method, reviewed by Alexander and Forster, is one of the important new experimental techniques that emerged in the previous decade. This review is intended, deliberately, to describe thoroughly a classic technique whose elegance epitomizes much of the fascination which nuclear physics techniques have held for a generation of scientists. This volume concludes the work on the Advances in Nuclear Physics series of one of the editors (M. Baranger), whose judgment and style characterize that which is best in the first ten volumes. Many of our readers and most of our authors will be grateful for the high standards which marked his contributions and which often elicited extra labor from the many authors of the series.

The aim of Advances in Nuclear Physics is to provide review papers which chart the field of nuclear physics with some regularity and completeness. We define the field of nuclear physics as that which deals with the structure and behavior of atomic nuclei. Although many good books and reviews on nuclear physics are available, none attempts to provide a coverage which is at the same time continuing and reasonably complete. Many people have felt the need for a new series to fill this gap and this is the ambition of Advances in Nuclear Physics. The articles will be aimed at a wide audience, from research students to active research workers. The selection of topics and their treatment will be varied but the basic viewpoint will be pedagogical. In the past two decades the field of nuclear physics has achieved its own identity, occupying a central position between elementary particle physics on one side and atomic and solid state physics on the other. Nuclear physics is remarkable both by its unity, which it derives from its concise boundaries, and by its amazing diversity, which stems from the multiplicity of experimental approaches and from the complexity of the nucleon-nucleon force. Physicists specializing in one aspect of this strongly unified, yet very complex, field find it imperative to stay well-informed of the other aspects. This provides a strong motivation for a comprehensive series of reviews.

In both the present volume of Advances in Nuclear Physics and in the next volume, which will follow in a few months' time, we have stretched our normal pattern of reviews by including articles of more major proportions than any we have published before. As a result we have only three review articles in Volume 5. From the beginning of this series it has been our aim, as editors, to achieve variation in the scope, style, and length of individual articles sufficient to match the needs of the individual topic, rather than to restrain authors within rigid limits. It has not been our experience that this flexibility has led to unnecessary exuberance on the part of the authors. We feel that the major articles now entering the series are entirely justified. The article by Professor Delves on "Variational Techniques in the Nuclear Three-Body Problem" is an authoritative, definitive article on a subject which forms a cornerstone of nuclear physics. If we start with two body interactions, then the three-nucleon system is, perhaps, the only many nucleon system whose exact description may lie within the scope of human ingenuity. In recent years some new techniques of scattering theory, origi nating mostly in particle physics, have led to a great deal of new interest in the nuclear three-body problem. In this series we have had two articles (by Mitra and by Duck) on the new approaches."

The aim of Advances in Nuclear Physics is to provide review papers which chart the field of nuclear physics with some regularity and completeness. We define the field of nuclear physics as that which deals with the structure and behavior of atomic nuclei. Although many good books and reviews on nuclear physics are available, none attempts to provide a coverage which is at the same time continuing and reasonably complete. Many people have felt the need for a new series to fill this gap and this is the ambition of Advances in Nuclear Physics. The articles will be aimed at a wide audience, from research students to active research workers. The selection of topics and their treatment will be varied but the basic viewpoint will be pedagogical. In the past two decades the field of nuclear physics has achieved its own identity, occupying a central position between elementary particle physics on one side and atomic and solid state physics on the other. Nuclear physics is remarkable both by its unity, which it derives from its concise boundaries, and by its amazing diversity, which stems from the multiplicity of experimental approaches and from the complexity of the nucleon-nucleon force. Physicists specializing in one aspect of this strongly unified, yet very complex, field find it imperative to stay well-informed of the other aspects. This provides a strong motivation for a comprehensive series of reviews.