Cluster Structure of Atomic Nuclei

Do nucleons form intermediate clusters, for instance alpha-particle clusters? The question is almost as old as nuclear physics! An important step in the direction of alpha clusters in light self conjugate nuclei came from outside nuclear physics. Sir F. Hoyle, an astrophysicist, predicted in 1953 that the triple alpha reaction must go through a long-lived resonance state in 12C, otherwise the great abundance of carbon in the universe could not be explained. Surely Fowler and co-workers found the second 0+ state at 7.65 MeV only a few years later. Still to day this state keeps us busy being regarded as a proto-type of alpha cluster state at low density in lighter nuclei. In the first chapter of the present review book on cluster structures in atomic nuclei, the quartetting of nucleons to form clusters of alpha-particles is, thus, discussed in detail. New theoretical descriptions of this widely investigated phenomenon are outlined. Parallels to nuclear pairing are put forward and the concept of alpha-particle condensation is introduced. This is in contrast to the more conventional point of view which favored a crystalline aspect of alpha-particle clustering. Chapter 3 shows that there is no dispute between the cluster picture of the nucleus and the well established shell model approach. The former may guide the researchers into fruitful physical insights where the latter is still too heavy for obtaining predictive ab initio results, the above mentioned Hoyle state being an illustrious example. In chapter 4, the interest is focused on states containing non-alpha clusters like triton and di-neutrons. As a result candidates of cluster states in 11C, 11B, and 8He are suggested. Although theoretical considerations still dominate the discussions, new experimental results, as presented in the chapter 2, point to the fact that progress is still to come in nuclear experimental cluster physics in the near future. In that chapter resonances observed in alpha-particle scattering from n-alpha nuclei are shown to be related to the interference of cluster states and a statistical background. Theoretical predictions concern alpha-particles orbiting an inert core to form a vortex state. Fission of heavy elements and their formation by fusion is one of the aspects of chapter 5. Nuclei with magic numbers of protons or neutrons offer a rich variety of possibilities by which long-lived super heavy elements may be produced way above the present day knowledge. Interesting options are outlined in this chapter such as the possible buckyball structure of a superheavy nucleus with 60 alpha-particles plus 60 neutrons. The contribution of W. von Oertzen to this book was planned but could not, unfortunately, be realized. In an article (Eur. Phys. J. A. 29,133, 2006) he has discussed the conditions for a phase change with the formation of an alpha-particle condensate, starting from systematic for binding energies per alpha particle in N=Z nuclei. The article is recommended as a complement to this book. This book has allowed the authors to expand their topics on more pages than commonly accepted in conference proceedings and journals. The headings of the chapters were chosen to cover the recent hot topics of nuclear clusters. Fortunately outstanding experts were interested to participate in writing the book after slight modifications of the original outline. The book aims at an up to date description for the use of researchers and students. An important goal was moreover the widening of the scopes, as much as possible, by cross-references between theory and experiment. Some of the ideas presented are speculative. This should be considered an advantage in a field where many approaches are needed to stimulate the process of maturing. The authors of the first chapter thank P. Nozières for his interesting comments on quartet condensation, and are grateful to