Evolution of structure in exotic nuclei

Nuclear structure in exotic nuclei is likely to be quite different than in the nuclei we have been able to access near the valley of stability. With the development of advanced radioactive-beam facilities, we will, for the first time, have access to long isotopic chains of nuclei. A key facet of exotic nuclei therefore will be the study of the evolution of structure with N, Z, and A to an extent never before imagined. Yet, the beam intensities will be very low (relative to stable beams); hence, we will need to extract more physics from less data. We will discuss several aspects of structural evolution and its elucidation in exotic nuclei.     

Universal Aspects of Neutron Halos in Light Exotic Nuclei

The theoretical few-body aspects associated with universal properties of weakly-bound neutron-rich light nuclei close to the drip line will be reviewed briefly, considering recent theoretical and experimental works. We will address low-energy properties of the one- and two-neutron halo of light exotic nuclei, which are dominated by s-wave short-range two-body interactions. In view of recent experiments with light neutron-rich nuclei, we will discuss properties of exotic nuclei as ¹¹Li, ¹⁴Be, ²⁰C and ²²C, within a three-bodyneutron–neutron-core model. Particular emphasis will be given to model independent properties associated to halo neutrons, which obey universal scaling laws. We discuss how the scaling laws for the s-wave observables of two-neutron halo will be identified with limit-cycles and Thomas–Efimov effect in a zero-range three-body model.     

Gas-cell-based setup for the production and study of neutron rich heavy nuclei

The present limits of the upper part of the nuclear map are very close to stability while the unexplored area of heavy neutron-rich nuclides along the neutron closed shell N = 126 is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleosynthesis. This area of the nuclear map can be reached neither in fusion–fission reactions nor in fragmentation processes widely used nowadays for the production of exotic nuclei. A new way was recently proposed for the production of these nuclei via low-energy multi-nucleon transfer reactions. The estimated yields of neutron-rich nuclei are found to be significantly high in such reactions and several tens of new nuclides can be produced, for example, in the near-barrier collision of ¹³⁶Xe with ²⁰⁸Pb. A new setup is proposed to produce and study heavy neutron-rich nuclei located along the neutron closed shell N = 126.     

Giant resonances in exotic spherical nuclei within the RPA approach with the Gogny force

Theoretical results for giant resonances in the three doubly magic exotic nuclei ⁷⁸Ni, ¹⁰⁰Sn and ¹³²Sn are obtained from Hartree-Fock (HF) plus Random Phase Approximation (RPA) calculations using the D1S parameterization of the Gogny two-body effective interaction. Special attention is paid to full consistency between the HF field and the RPA particle-hole residual interaction. The results for the exotic nuclei, on average, appear similar to those of stable ones, especially for quadrupole and octupole states. More exotic systems have to be studied in order to confirm such a trend. The low energy of the monopole resonance in ⁷⁸Ni suggests that the compression modulus in this neutron-rich nucleus is lower than the one of stable ones.     

From kaonic atoms to kaonic nuclei: A search for antikaon-mediated bound nuclear systems

Strong interaction processes were intensively studied at low energy with exotic atoms, touching one of the fundamental problems in hadron physic today — the still unsolved question of how hadron masses are generated. The question of the origin of the large hadron masses made up of light quarks, the current mass of the up (u) and down (d) quarks (m_u,d≈5 MeV) is two orders of magnitude smaller than a typical hadron mass of about 1 GeV, will be discussed in the context with the experimental work done in the field of exotic atoms expanded to exotic nuclei.An overview of the properties of exotic hydrogen atoms made of pions and kaons are presented, using high precision experiments unrevealing the nature of strong interaction physics at low energy. A new field which contributes to the understanding of the origin of the large hadron mass is the study of the mass modification in a nuclear medium. Antikaon mediated bound nuclear systems would be an ideal tool, if they exist, to look for chiral restoration at high density and zero temperature or even more exotic to look for phase transitions.     

基于不稳定核基本性质测量的原子核结构研究

精密激光谱学是通过测量核素原子光谱的超精细结构和同位素移位来研究原子核的基本性质,为原子核自旋、磁矩、电四极矩及电荷均方根半径的确定提供了一种模型独立的测量方式。这些原子核基本性质的测量,能够比较精确地描述原子核微观结构的演化。近年来,随着放射性束流装置的发展,产生远离β-稳定线的丰中子/丰质子核素成为可能,也进一步促进了高分辨和高灵敏度的激光谱技术更加广泛的应用。简单介绍了基于放射性核素超精细结构的激光谱学测量原理,并通过几个经典实例来回顾近年来激光谱学在原子核奇特结构研究领域的独特贡献。主要通过分析几个重要核区原子核的基本性质,结合大尺度壳模型、ab initio理论、密度泛函理论等,来探索丰中子核中展现出来的一些新的奇特现象,如晕结构、幻数演化、形状共存等。High-precision laser spectroscopy technique is used to determine the ground state properties of exotic nuclei by probing its electronic hyperfine structure and isotope shift. It provides a model-independent measurement of nuclear spin, magnetic moment, electric quadrupole moment and charge radii. These nuclear parameters can be used to investigate the nuclear structure evolution and the nuclear shapes. With the development of accelerators and isotope separators, exotic isotopes far from β stability became accessible experimentally, which enhanced the capability of the laser spectroscopy technique being applied in the field of nuclear physics. A brief introduction to experimental principle is given, followed by a review of several typical examples for the experimental investigations in the different regions of nuclear chart. This aims to demonstrate the contributions of ground state properties measurement by using laser spectroscopy technique to the nuclear structure study of exotic isotopes. This discussion involves several different nuclear theory models in order to interpret the exotic phenomena observed in the neutron-rich isotopes, such as halo structure, shell evolution, shape coexistence and so on.     

Mean field with tensor force and shell structure of exotic nuclei

The tensor force is implemented into the mean-field model so that the evolution of nuclear shells can be described for exotic nuclei as well as stable ones. Besides the tensor-force part simulating the meson exchange, the model is an extension of the successful Gogny model. One of the major issues of rare-isotope beam physics is a reduced spin-orbit splitting in neutron-rich exotic nuclei. It will be shown that the effect of the tensor force on this splitting is larger than or about equal to the one due to the neutron skin. We will present predictions for stable and exotic nuclei with comparisons to conventional results and experimental data.     

POLAREX Study of polarized exotic nuclei at millikelvin temperatures

POLAREX (POLARization of EXotic nuclei) is a new facility for the study of nuclear magnetic moments and decay modes of exotic nuclei using the established On-Line Nuclear Orientation (OLNO) method. A radioactive beam of interest is implanted into a ferromagnetic host foil held at a temperature of order 10mK in a ³He - ⁴He dilution refrigerator. The foil is magnetized by an applied magnetic field and the nuclear spins become polarized through the internal hyperfine field. The angular distribution of decay products from the polarized sample is measured. Accurate values of nuclear moment are obtained by NMR. The new facility will have access to neutron-rich nuclides produced at the ALTO facility (Linear Accelerator at Orsay Tandem) by fission induced by electrons from the linear electron accelerator. Basic concepts and initial tests are outlined.     

Towards the determination of the charge radius of 11Li by laser spectroscopy

Isotope shift measurements by means of laser resonance ionization spectroscopy are a unique tool to determine the charge radii of halo nuclei. The most prominent halo nucleus ¹¹Li is at the same time the best accessible candidate for such studies. The experimental method to determine the charge radius of this exotic nucleus and first test results on ⁷Li will be presented in this paper. This revised version was published online in August 2006 with corrections to the Cover Date.     

Sensitivity of reaction cross sections to halo nucleus density distributions

In order to clear up the sensitivity of the nucleus-nucleus reaction cross sections σ _R to the nuclear matter distributions in exotic halo nuclei, we have calculated the values of σ _R for scattering of ⁶He, ¹¹Li, and ¹⁹C nuclei on several nuclear targets at the energy of 0.8 GeV/nucleon. The calculations were performed in the “rigid target” approximation to the Glauber theory, different shapes of the nuclear density distributions in ⁶He, ¹¹Li, and ¹⁹C being assumed.     

Hot nuclei — Effective shapes and entrance channel effect

The γ-decay of the Giant Dipole Resonance in hot rotating compound nuclei has provided valuable information on the nuclear structure at finite temperature. A number of experimental results showing that the nuclear shapes change with temperature and angular momentum are here reviewed. In particular we concentrate on the temperature interval from 1 to 2 MeV, rotational frequencies from 0.2 to 1.5 MeV and on nuclei in the mass regionsA≈160–170 andA≈110, characterized by the prolate-oblate and spherical-oblate phase transitions, respectively. The possibility to study the shapes involved in the compound nucleus formation is also discussed. For this purpose long formation times are required and the nucleus¹⁷⁰W formed with the reaction⁶⁰Ni+¹¹⁰Pd, here studied, seems to be a good candidate. The gamma and particle decays were compared to those of the reaction⁴⁸Ti+¹²²Te. The comparison shows that in average the energy of the α-particles is larger for the more symmetric reaction, consistent with longer formation times and larger deformations in the pre-equilibrium stage.     

The EMC effect — status and perspectives

Recent experimental results on the EMC effect are presented. The ratios of structure functions for nuclei and deuterium measured by the two muon experiments at CERN show a clear enhancement of a few percent forx<0.25. Atx below 0.05 substantial shadowing with littleQ ² dependence has been observed by a dedicated low angle experiment of the EMC. No significant nuclear mass dependence ofR=αL/αT has been seen in the SLAC experiment E 140. There are several indications that theQ ² evolution of nuclear structure functions deviates from the expectations of perturbative QCD and that the gluon distribution in nuclei is harder than in free nucleons. This is possibly caused by nucleon-nucleon correlations on the quark-gluon level. Many aspects of the EMC effect are presently being investigated by the high energy muon experiment of the NMC collaboration at CERN and a Drell-Yan experiment at FNAL. First results can be expected soon. They will help to develop a better understanding of nuclear effects in quark and gluon distributions.     

Nuclear structure studies on halo nuclei by direct reactions with radioactive beams

The investigation of direct reactions with exotic beams in inverse kinematics gives access to a wide field of nuclear structure studies in the region far off stability. The basic concept and the methods involved are briefly discussed. The present contribution will focus on the investigation of light neutron-rich halo nuclei. Such nuclei reveal a new type of nuclear structure, namely an extended neutron distribution surrounding a nuclear core. An overview on this phenomenon, and on the various methods which gave first evidence and qualitative confirmation of our present picture of halo nuclei, is given. To obtain more quantitative information on the radial shape of halo nuclei, elastic proton scattering on neutron-rich light nuclei at intermediate energies was recently investigated for the first time. This method is demonstrated to be an effective means for studying the nuclear matter distributions of such nuclei. The results on the nuclear matter radii of ⁶He and ⁸He, the deduced nuclear matter density distributions, and the significance of the data on the halo structure is discussed. The present data allow also a sensitive test of theoretical model calculations on the structure of neutron-rich helium isotopes. A few examples are presented. The investigation of few-nucleon transfer reactions in inverse kinematics may provide new and complementary information on nuclear structure, as well as astrophysical questions. The physics motivation and the experimental concept for such experiments, to be performed due to momentum matching reasons at low incident energies around 5–20 MeV/u at the new generation low energy radioactive beam facilities SPIRAL, PIAFE, etc., is briefly discussed.     

Research program for the radioactive beams of the ACCULINNA-2 separator

The ACCULINNA-2 separator is intended to provide beams of exotic nuclei with Z = 1?30, energies of 10?C50 MeV/amu, and intensities that allow one to study excitation spectra and decay dynamics of neutron- and proton-drip nuclei. Examples from studies of the ¹⁰He and ⁶Be nuclei are presented.     

Gamma spectroscopy with RIBs from RISING to AGATA

Nuclear spectroscopy using radioactive isotope beams requires dedicated set-ups. State-of-the-art Ge arrays recently started to provide valuable γ spectroscopic data. At the SIS/FRS facility at GSI exotic beams at relativistic energies were employed for Coulomb excitation and secondary fragmentation experiments with the fast beam RISING set-up. Shell evolution far off stability, pn-pairing, symmetries and nuclear shapes were studied in nuclei ranging from ³⁶Ca to ¹³⁶Nd. The observation of a I = 27 ħ state demonstrated that high spin states can be reached in massive fragmentation reactions. This and the large sensitivity of relativistic in-beam experiments opens a rich ground for advanced nuclear structure studies. Combining RISING with AGATA γ-tracking detectors and improved particle detection is planned for future experimental investigations.     

Sensitivity of cross sections for elastic nucleus-nucleus scattering to halo nucleus density distributions

In order to clear up the sensitivity of the nucleus-nucleus scattering to the nuclear matter distributions in exotic halo nuclei, we have calculated differential cross sections for elastic scattering of the ⁶He and ¹¹Li nuclei on several nuclear targets at the energy of 0.8 GeV/nucleon with different assumed nuclear density distributions in ⁶He and ¹¹Li.     

Three-Body Forces and Neutron-Rich Exotic Nuclei

Physics of three-body forces in connection to exotic nuclei will be discussed. Three-nucleon forces (3NF), especially Fujita–Miyazawa 3NF, are shown to be the key for the solution to the long-standing problem of oxygen drip line, which deviates from basic trend towards the stability line. Three-body forces produce repulsive modifications to effective interaction between valence neutrons, and make the ground states less bound. The oxygen drip line is then located at the right place. The relation to the neutron matter is presented. Applications to Ca isotopes will be presented. The shell evolution due to the 3NF depicts the raising of single-particle energies and the widening of splitting among the orbits. This is in contrast to the so-called shell quenching. The 2⁺ levels are calculated for Ca isotopes, suggesting about the same sub-magic structure for N = 32 and 34.     

Nuclear longitudinal form factors for axially deformed charge distributions expanded by nonorthogonal basis functions

In this paper,the nuclear longitudinal form factors are systematically studied from the intrinsic charge multipoles.For axially deformed nuclei,two different types of density profiles are used to describe their charge distributions.For the same charge distributions expanded with different basis functions,the corresponding longitudinal form factors are derived and compared with each other.Results show the multipoles C_λ of longitudinal form factors are independent of the basis functions of charge distributions.Further numerical calculations of longitudinal form factors of~(12)C indicates that the C_0 multipole reflects the contributions of spherical components of all nonorthogonal basis functions.For deformed nuclei,their charge RMS radii can also be determined accurately by the C_0 measurement.The studies in this paper examine the model-independent properties of electron scattering,which are useful for interpreting electron scattering experiments on exotic deformed nuclei.     

Double-beta decay in gauge theories

Neutrinoless double-beta decay is a very important process both from the particle and nuclear physics point of view. From the elementary particle point of view, it pops up in almost every model, giving rise among others to the following mechanisms: (a) the traditional contributions like the light neutrino mass mechanism as well as the j _L –j _R leptonic interference (λ and η terms), (b) the exotic R-parity-violating supersymmetric (SUSY) contributions. Thus, its observation will severely constrain the existing models and will signal that the neutrinos are massive Majorana particles. From the nuclear physics point of view, it is challenging, because (1) the nuclei, which can undergo double-beta decay, have complicated nuclear structure; (2) the energetically allowed transitions are suppressed (exhaust a small part of all the strength); (3) since in some mechanisms the intermediate particles are very heavy one must cope with the short distance behavior of the transition operators (thus novel effects, like the double-beta decay of pions in flight between nucleons, have to be considered; in SUSY models, this mechanism is more important than the standard two-nucleon mechanism; and (4) the intermediate momenta involved are quite high (about 100 MeV/c). Thus one has to take into account possible momentum-dependent terms of the nucleon current, like modification of the axial current due to PCAC, weak magnetism terms, etc. We find that, for the mass mechanism, such modifications of the nucleon current for light neutrinos reduce the nuclear matrix elements by about 25%, almost regardless of the nuclear model. In the case of heavy neutrino, the effect is much larger and model-dependent. Taking the above effects into account, the needed nuclear matrix elements have been obtained for all the experimentally interesting nuclei A=76, 82, 96, 100, 116, 128, 130, 136, and 150. Then, using the best presently available experimental limits on the half-life of the 0νββ decay, we have extracted new limits on the various lepton-violating parameters. In particular, we find 〈m _ν〉 < 0.3 eV/c ², and, for reasonable choices of the parameters of SUSY models in the allowed SUSY parameter space, we get a stringent limit on the R-parity-violating parameter λ′₁₁₁<4.0×10^?4.