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The halo structure originated from nuclear physics but is now encountered more widely. It appears in loosely bound, clustered systems where the spatial extension of the system is significantly larger than that of the binding potentials. A review is given on our current understanding of these structures, with an emphasis on how the structures evolve as more cluster components are added and on the experimental situation concerning halo states in light nuclei.

The nuclear shell structure that has served as a fundamental framework for understanding the arrangement of nucleons exhibits dramatic changes as the neutron to proton ratio in nuclei increases. This paper describes how reaction spectroscopy of the neutron- and proton-rich nuclei has brought about a new revolution towards a more global view of nuclear shells. The relationship between changes in shell structure and the discovery of exotic forms of nuclei such as nuclear halo and skin is discussed. It is shown that the well-known shell gaps (magic numbers) N = 8 and 20 disappear. The discovery of a new magic number at N = 16 at the limit of nuclear binding is discussed.

The evolution of the N = 28 shell closure is investigated far from stability. Using the latest results obtained from various experimental techniques, we discuss the main properties of the N = 28 isotones, as well as those of the N = 27 and 29 isotones. Experimental results are compared with various theoretical predictions. These studies pinpoint the effects of several terms of the nucleon–nucleon interaction, such as the central, the spin–orbit (SO), the tensor and the three-body force components, to account for the modification of the N = 28 shell gap and SO splittings. Analogies between the evolution of the N = 28 shell closure and other magic numbers originating from the SO interaction are proposed (N = 14,50 and 82). More generally, questions related to the evolution of nuclear forces towards the drip line, in bubble nuclei and for nuclei involved in the r-process nucleosynthesis are proposed and discussed.

The often surprising properties of neutron-rich nuclei have prompted extensive experimental and theoretical studies aimed at identifying the driving forces behind the dramatic changes encountered in the exotic regime. Challenging discovery experiments that search for new isotopes in the quest to delineate the limits of nuclear existence provide fundamental benchmarks for nuclear theory, addressing the question what combinations of protons and neutrons can be made into nuclei. As will be discussed here, the existence of certain nuclear species and their production cross sections can provide a first glimpse at nuclear structure effects in the most exotic regime. Striving for more detail on the evolution of nuclear structure, in-beam nuclear spectroscopy with fast beams and thick reaction targets—where γ-ray spectroscopy is used to tag the final state—provides information on the single-particle structure as well as on collective degrees of freedom in nuclei that are available for experiments at beam rates of only a few ions/s. This article will outline—with the example of shell evolution along the N = 40 isotone line—how in-beam experiments measure complementary observables, starting from the nuclear existence and production cross sections to the collective and single-particle properties of the lowest-lying excited states in the vicinity of the very neutron-rich Ti, Cr and Fe N = 40 isotones. The interplay of experimental results and theory will be emphasized at the intersection of nuclear structure and reactions in the joined quest of unraveling the driving forces of shell evolution.

Taking advantage of the resolving power of modern gamma-ray spectrometers in combination with different types of nuclear reactions, it has been possible to investigate to fairly high-spin neutron-rich nuclei in a number of regions of the nuclear chart. The primary motivations for such studies are to characterize changes in shell structure as a function of the neutron-to-proton ratio and to document the impact of a large neutron excess on global properties, such as the nuclear shape. Recent data on nuclei close to the 'island of inversion' near ³²Mg are discussed first. They highlight challenges in describing the observations with modern effective interactions. Subsequently, the nature of excitations in neutron-rich fpg-shell nuclei between Ca and Ni is reviewed. A neutron sub-shell closure at N = 32 has been attributed to the monopole tensor force. Furthermore, the presence of collective excitations at moderate spin in neutron-rich Cr and Fe isotopes illustrates the role of the g_9/2 orbital in driving the nuclear shape. The observations suggest that mixing between 'deformed' and 'shell-model' states needs to be considered. Finally, recent results in the direct vicinity of ⁶⁸Ni indicate that the impact of the N = 40 neutron shell closure is rather modest.

After a discussion of the early steps in the period 1930–1950 that disclosed the presence of both independent particle and collective effects in the atomic nucleus, we highlight the major advances describing the atomic nucleus within a shell-model context as well as starting from a mean-field approach. We also discuss the importance of symmetries in recognizing the most important degrees of freedom in the nuclear many-body system as well as connecting shell-model and collective model approaches.

I overview new aspects of the structure of exotic nuclei as compared to stable nuclei, focusing on several characteristic effects of nuclear forces. The shell structure of nuclei has been proposed by Mayer and Jensen, and has been considered to be kept valid basically for all nuclei, with well-known magic numbers, 2, 8, 20, 28, 50, .... Nuclear forces were shown, very recently, to change this paradigm. It will be presented that the evolution of shell structure occurs in various ways as more neutrons and/or protons are added, and I will present basic points of this shell evolution in terms of the monopole interaction of nuclear forces. I will discuss three types of nuclear forces. The first one is the tensor force. The tensor force is one of the most fundamental nuclear forces, but its first-order effect on the shell structure has been clarified only recently in studies on exotic nuclei. The tensor force can change the spin–orbit splitting depending on the occupation of specific orbits. This results in changes of the shell structure in many nuclei, and consequently some of Mayer–Jensen's magic numbers are lost and new ones emerge, in certain nuclei. This mechanism can be understood in an intuitive way, meaning that the effect is general and robust. The second type of nuclear forces is central force. I will show a general but unknown property of the central force in the shell-model Hamiltonian that can describe nuclear properties in a good agreement with experiment. I will then demonstrate how it can be incorporated into a simple model of the central force, and will discuss how this force works in the shell evolution. Actually, by combining this central force with the tensor force, one can understand and foresee how the same proton–neutron interaction drives the shell evolution, for examples such as Sn/Sb isotopes, N = 20 nuclei and Ni/Cu isotopes. The distribution of single-particle strength is discussed also in comparison to (e,e'p) experiment on ⁴⁸Ca. The shell evolution affects shapes of nuclei through Jahn–Teller-type mechanism, and a very interesting example with exotic Si isotopes is discussed. The third type of nuclear force is a three-body force, which originates in the Δ particle excitation as proposed by Fujita and Miyazawa many years ago. This force is shown to produce a repulsive interaction between valence neutrons after averaging effects from the third nucleon in the core. The same three-body force is responsible for neutron stars. By including such effects of the three-body force, one can predict the correct drip line of oxygen isotopes, for instance. Thus, the landscape of atomic nuclei varies in going from stable to exotic nuclei due to particular nuclear forces, leading to a paradigm shift. This paper overviews some basic ideas and selected examples.

Recent advances on the dipole and monopole responses of the atomic nucleus are discussed. The soft dipole response is diversified and its structure depends on the nucleus. The giant dipole resonance is involved in astrophysical processes such as the r-process and in the propagation of ultra-high-energy cosmic rays. A soft monopole response is predicted. The giant monopole resonance is related to the determination of the nuclear incompressibility, but an alternative quantity may be better extracted from measurements.

In this contribution we will review our present understanding of the matter equation of state in the density and temperature conditions where it can be described by nucleonic degrees of freedom. At zero temperature, all the information is contained in the nuclear energy functional in its isoscalar and isovector channels. At finite temperature, particular emphasis will be given to the specificity of the thermodynamics in the nucleonic regime, with the simultaneous presence of long range electromagnetic and short range nuclear interactions. The astrophysical implications of the resulting phase diagram, as well as of different observables of exotic nuclei on the neutron-rich side, will be touched upon.

A review of nuclear astrophysics of light nuclei using radioactive beams or techniques developed for radioactive beams is given. We discuss Big Bang nucleosynthesis, with special focus on the lithium problem, aspects of neutrino-physics, helium-burning and finally selected examples of studies relevant for explosive burning.

Short-lived exotic nuclei play essential roles in the dynamics of many stellar objects and their associated nucleosynthesis, including core-collapse and thermonuclear supernovae, novae, x-ray bursters and mass-accreting neutron stars in binary systems. To produce such nuclei and to study their properties requires radioactive ion-beam facilities. While the short-lived nuclei on the neutron-deficient side of the chart of nuclei are often within reach at current experimental facilities, albeit not necessarily with the beam intensities needed, the extremely neutron-rich nuclei, required for an understanding of the synthesis of heavy elements in the Universe by the astrophysical r-process, have to wait, in most cases, for the next-generation radioactive ion-beam facilities. This paper discusses highlights of recent experimental and theoretical advances in determining the properties of exotic nuclei and the further need for reliable studies of astrophysical objects such as supernovae, novae, x-ray bursters and neutron stars.

The electric dipole (E1) response of exotic nuclei, with a focus on the soft E1 excitation of halo nuclei and the pygmy dipole resonance (PDR) of neutron-skin nuclei, is reviewed here from an experimental point of view. Since the discovery of halo nuclei in the late 1980s, the particular dipole response of these loosely bound nuclei located at the neutron-drip line resulting in an extraordinary enhancement of the electric dipole strength at low excitation energies (soft E1 excitation) has been one of the phenomena under intense investigation both in experiments and in nuclear theories. We discuss how such phenomena have been recognized as a unique feature of halo nuclei and how it is applied to probe the halo structure in a quantitative way. Another intriguing feature of the E1 response of neutron-rich nuclei is the PDR in neutron-skin nuclei, which is viewed as a dipole vibration of excess neutrons against a core nucleus and which appears in the low-energy part of the E1 strength function below the giant dipole resonance. The status of the experimental evidence for such a redistribution of dipole strength for neutron-rich nuclei compared to stable ones is reviewed. The relation of the dipole response to the symmetry energy in the equation of state of asymmetric nuclear matter and to the neutron-skin thickness is also briefly discussed. Finally, we outline some perspectives for future experimental investigations toward heavier and more neutron-rich systems. The experimental results reviewed here have been accumulated over the last two decades at fragmentation facilities worldwide at GANIL, GSI, MSU and RIKEN.

The β-decay process allows one to understand better the interactions and behaviour of the nucleons inside the nucleus. The study of nuclei at the limits of stability has recently become a central focus of nuclear structure research. The main characteristic of β-decay far from stability is the large number of decay channels open. The decay process is well understood and the interpretation of the data yields a wide variety of spectroscopic information: level energies, spins, parities, widths and level densities. In this brief paper, a summary of the progress achieved over the last few decades in the study and identification of β-delayed particle precursors is given with special emphasis on the achievements in studies of β-delayed charged particles where the detection system allowed for refined spectroscopy studies. For instance, detailed studies have been conducted on many β-delayed proton precursors of the sd shell. The comparison with the best performing shell-model calculations shows that although overall agreement is obtained in B_GT distributions, the density of the observed states is higher than predicted. This points to the need to include higher order configurations in the present sd shell calculations. The extension to heavier systems is desirable. The main challenges for the present and the next decade in this domain are outlined.

For the atomic nuclei located beyond the drip-lines, new forms of radioactivity are possible due to the instability against the emission of particles. At the neutron-deficient limit, single-proton and two-proton radioactivity are characteristic phenomena. While the former is well understood and represents a mature research field of nuclear spectroscopy, the knowledge about the latter is still scarce but growing rapidly. In some areas of the proton drip-line, α decay sets in as the main decay mode. This paper briefly describes the phenomena of particle emission at the proton drip-line and presents some examples illustrating the current status of experimental achievements and the theoretical understanding. The idea of the Gamow state as a unifying concept in the description of particle radioactivity is highlighted. The possibility for neutron radioactivity is briefly discussed.

Isomers and their roles in discovery and as probes of nuclear structure are discussed, with a focus on neutron-rich nuclei in the mass 180–190 region. Long-lived, high-K isomers are predicted in the heavy hafnium nuclei for example, some of which should be accessible by projectile fragmentation and deep-inelastic reactions. Progress with these reactions in reaching a broad range of isotopes to the right of the stability line is summarized. Recent results for isomers in the iridium and osmium isotopes produced in deep-inelastic reactions and studied with time-correlated γ-ray spectroscopy are presented as examples. These studies venture into the border region where well-deformed prolate shapes give way to soft, transitional nuclei. Prolate, oblate and triaxial structures are predicted, but with shapes that depend sensitively on the multi-particle configurations. Dynamical effects such as rotation alignment also play a significant role and produce isomeric states. In some cases, very low-lying multi-quasiparticle intrinsic states are predicted but these are possibly missed with current approaches. The prospects for the identification and characterization of such exotic isomers using new storage ring techniques, that might also provide a window into exotic states populated in their subsequent decay are outlined, as is the potential for studies with beams of isomers themselves.

The study of heavy and superheavy elements has always been one of the cornerstones of nuclear physics studies. These studies are driven by a desire to create new elements and to determine the limits of nuclear stability. Current experiments to synthesize new elements aim at the fabled 'Island of Stability' which should be found in the region of the next 'magic' numbers for protons and neutrons beyond Z = 82 and N = 126 (²⁰⁸Pb). The island is predicted to be around proton number 114–126 and neutron number 184. In recent years, another approach to understanding heavy nuclear systems has gained momentum, whereby nuclei with a much lower proton number of around 100 are studied in detail. The motivation for and results of such studies will be presented.

Atomic physics techniques for the determination of ground-state properties of radioactive isotopes are very sensitive and provide accurate masses, binding energies, Q-values, charge radii, spins and electromagnetic moments. Many fields in nuclear physics benefit from these highly accurate numbers. They give insight into details of the nuclear structure for a better understanding of the underlying effective interactions, provide important input for studies of fundamental symmetries in physics, and help to understand the nucleosynthesis processes that are responsible for the observed chemical abundances in the Universe. Penning-trap and storage-ring mass spectrometry as well as laser spectroscopy of radioactive nuclei have now been used for a long time but significant progress has been achieved in these fields within the last decade. The basic principles of laser spectroscopic investigations, Penning-trap and storage-ring mass measurements of short-lived nuclei are summarized and selected physics results are discussed.

The beta decay process offers unique possibilities to search for so-called exotic, non-standard model components in the weak interaction, and for testing its fundamental symmetries. Sensitive candidates for such studies are nuclei at or close to the N = Z line for which nuclear structure-related corrections are well under control. We present here an overview of the current status of this field and discuss prospects for further significant progress in the forthcoming years.

This paper describes the evolution of our understanding of the reaction mechanisms involved in the scattering of exotic nuclei starting from the first measurements of reaction cross sections in the 1980s up to the most recent and advanced exclusive experiments for dripline unbound nuclei. We shall mainly cover nuclear and Coulomb breakup reactions, elastic scattering and total reaction cross sections measurements. Some open problems like kinematical effects on core parallel momentum distributions, the existence of a proton halo, the determination of the ground state of unbound nuclei such as ¹³Be, will be discussed in detail. A general framework for the reaction theory will be used, based on a time dependent perturbation theory approach and when suitable on its eikonal limit.

Transfer reactions are a powerful probe of the properties of atomic nuclei. When used in inverse kinematics with radioactive ion beams they can provide detailed information on the structure of exotic nuclei and can inform nucleosynthesis calculations. There are a number of groups around the world who use these reactions, usually with particle detection in large silicon arrays. Sometimes these arrays are coupled to gamma-ray detectors, and occasionally smaller arrays of silicon detectors are mounted within a solenoid magnet. Modern techniques using transfer reactions in inverse kinematics are covered, with specific examples, many from measurements made with beams from the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory.

Recent studies of nuclei far from the stability line have revealed drastic changes in nuclear orbitals and reported the appearance of new magic numbers and the disappearance of magic numbers observed at the stability line. One of the important reasons for such changes is considered to be because of the effect of tensor forces on nuclear structure. Although the role of tensor forces in binding very light nuclei such as deuterons and ⁴He has been known, direct experimental evidence for the effect on nuclear structure is scarce. In this paper, I review known effects of tensor forces in nuclei and then discuss the recently raised question of s–p wave mixing in a halo nucleus of ¹¹Li. Following these reviews, the development of a new experiment to see the high-momentum components due to the tensor forces is discussed and some of the new data are presented.

A first-principles description of nuclear systems along the drip lines presents a substantial theoretical and computational challenge. In this paper, we discuss the nuclear theory roadmap, some of the key theoretical approaches, and present selected results with a focus on long isotopic chains. An important conclusion, which consistently emerges from these theoretical analyses, is that three-nucleon forces are crucial for both global nuclear properties and detailed nuclear structure, and that many-body correlations due to the coupling to the particle continuum are essential as one approaches particle drip lines. In the quest for a comprehensive nuclear theory, high performance computing plays a key role.

Radioactive ion beam facilities are transforming nuclear science by making beams of exotic nuclei with various properties available for experiments. New infrastructures and development of existing installations enlarges the scientific scope continuously. An overview of the main production, separation and beam handling methods with focus on recent developments is done, as well as a survey of existing and forthcoming facilities world-wide.

The availability of intense secondary beams in conjunction with modern efficient detection setups allows for the production and detailed study of the most extreme nuclear systems, in terms of asymmetry of proton and neutron number, in the continuum. Nuclei close to the drip-lines, exhibiting exotic properties themselves, can be used as a basis in order to populate these even more exotic nuclear systems, e.g. in transfer and knockout reactions. The latter challenge nuclear structure theory by being open quantum systems far from the valley of beta stability as well as reaction aiming at a description of their production mechanisms. Experiments provide data on momentum distributions, while relative energy spectra, and spin alignment during the reaction can be extracted and lead to the observation of energy and angular correlations as well as to dependent quantities such as, e.g., the profile function denoting a momentum width as a function of relative energy. They are determined from reaction products and gamma radiation emerging from the reaction zone. The link to intrinsic properties of these unbound systems has to be explored by gathering precise knowledge of the properties of the seed nuclei and compare them to structures observed in the continuum. In this paper, examples of the above-mentioned methods are presented. The current knowledge about light systems such as ^5,7H, ⁷⁻¹⁰He, ¹⁰⁻¹³Li and the most neutron-rich oxygen systems is reviewed.