The possibility of using channeling as a tool for high energy particle physics has now been extensively investigated. Bent crystals have been used as an accelerator extraction element and for particle deflection. Applications as accelerating devices have been discussed but appear remote. The major advantage in using a bent crystal rather than a magnet is the large deflection that can be achieved in a short length. The major disadvantage is the low transmission. A good understanding of dechanneling is important for applications. 43 refs., 1 fig., 3 tabs.

"Nature performs not hing vainly, and makes nothing unnecessary" Aristotle Interest in the passage of charged particles through crystals first appeared at the beginning of this century following experiments on x-ray diffraction in crystallattices, which provided the proof of an ordered distribution of atoms in a crystal. Stark [1] put forward the hypothesis that certain directions in a crystal should be relatively transparent to charged particles. These first ideas on the channeling of charged particles in crystals were forgotten but became topical again in the early 1960s when the channeling effect was rediscovered by computer simulation [2] and in experiments [3] that revealed anomalously long ion ranges in crystals. The orientational ef fects during the passage of charged particles through crystals have been found for a whole range of processes characterized by small impact parameters for collisions between particles and atoms: nuclear reactions, large-angle scatter ing, energy losses. Lindhard explained the channeling of charged particles in crystals [4]. The results of the numerous investigations into the channeling of low-energy (amounting to several MeV) charged particles in crystals have been summarized in several monographs and reviews [5~8l.

Channeling, by its nature, involves a wide and disparate range of disciplines. Crystal preparation, material science, accelerator physics, sophisticated theoretical analysis and, of course, channeling itself all must work in concert in a research program. In spite of the gulfs separating some of these activities, researchers have drawn together over the last decade to carry out remarkable experiments in relativistic channeling and channeling radiation. Several informal workshops on high-energy channeling have been held over ~he years at Aarhus and Fermilab. However, with the vigorous progress in the field in the last several years it became clear that a more formal, comprehensive workshop was needed along with a book that covered the whole spectrum of the new developments, probed the future, and also laid out some of the foundations of the subject. This volume is the outcome of that process. The organization and preparation of both the volume and the workshop owe much to several outstanding scientific committees. The membership of these included J. Andersen (Aarhus), S. Baker (Fermilab), B. Berman (G. Washington), G. Bologna (Torino), E. Bonderup (Aarhus), S. Datz (Oak Ridge), J. Forster (Chalk River), F. Fujimoto (Tokyo), W. Gibson (Albany), I. Mitchell (Chalk River), Y. Ohtsuki (Waseda), R. Pantell (Stanford), S. Picraux (Sandia), J. Remillieux (Lyon), A. Saenz (NRL), V. Schegelsky (Gatchina), C. Sun (Albany), H. tiberall (Catholic U. ), E. Uggerh¢j (CERN), and R. Wedell (Humboldt). Others from across the spectrum of scientific disciplines agreed to serve as session chairmen.

The book is devoted to processes at the interaction of high energy charged particles and photons with crystals. Among them are the creation of electron-positron pair by photon in crystalline field, the radiation of particles in this field and, connected with these effects, the new type of electromagnetic showers in crystals, the channeling of fast particles in crystal and channeling radiation. At high energies, the processes of quantum electrodynamics (QED) in intense external fields play an important role in crystals. The first third of the book contains a new formulation of QED in external fields which is valid for any external field, including an essentially nonuniform one and has vast applications.

The book "Channeling 2008", Charged and Neutral Particles Channeling Phenomena, is formed by the same title conference contributions. This volume includes papers by leading researchers from different world centers. Their recent results on the coherent phenomena of charged and neutral particles propagating through the structures of various sizes and periodicities are included here, along with historical reviews by pioneers of coherent Bremsstrahlung and channeling radiation as well as crystal channeling collimation.

While information exists on high energy negative particle channeling there has been little study of the challenges of negative particle bending and channeling collimation. Partly this is because negative dechanneling lengths are relatively much shorter. Electrons are not particularly useful for investigating negative particle channeling effects because their material interactions are dominated by channeling radiation. Another important factor is that the current central challenge in channeling collimation is the proton-proton Large Hadron Collider (LHC) where both beams are positive. On the other hand in the future the collimation question might reemerge for electon-positron or muon colliders. Dechanneling lengths increase at higher energies so that part of the negative particle experimental challenge diminishes. In the article different approaches to determining negative dechanneling lengths are reviewed. The more complicated case for axial channeling is also discussed. Muon channeling as a tool to investigate dechanneling is also discussed. While it is now possible to study muon channeling it will probably not illuminate the study of negative dechanneling.