Bound state properties of Borromean halo nuclei: 6He and 11Li

Energy levels of A = 21−44 nuclei (V)

Gaussian expansion method for few-body systems

This article provides an overview of the basic principles of the physics of quantum halo systems, defined as bound states of clusters of particles with a radius extending well into classically forbidden regions. Exploiting the consequences of this definition, the authors derive the conditions for occurrence in terms of the number of clusters, binding energy, angular momentum, cluster charges, and excitation energy. All these quantities must be small. The article discusses the transitions between different cluster divisions and the importance of thresholds for cluster or particle decay, with particular attention to the Efimov effect and the related exotic states. The pertinent properties can be described by the use of dimensionless variables. Then universal and specific properties can be distinguished, as shown in a series of examples selected from nuclear, atomic, and molecular systems. The neutron dripline is especially interesting for nuclei and negative ions for atoms. For molecules, in which the cluster division comes naturally, a wider range of possibilities exists. Halos in two dimensions have very different properties, and their states are easily spatially extended, whereas Borromean systems are unlikely and spatially confined. The Efimov effect and the Thomas collapse occur only for dimensions between 2.3 and 3.8 and thus not for 2. High-energy reactions directly probe the halo structure. The authors discuss the reaction mechanisms for high-energy nuclear few-body halo breakup on light, intermediate, and heavy nuclear targets. For light targets, the strong interaction dominates, while for heavy targets, the Coulomb interaction dominates. For intermediate targets these processes are of comparable magnitude. As in atomic and molecular physics, a geometric impact-parameter picture is very appropriate. Finally, the authors briefly consider the complementary processes involving electroweak probes available through beta decay, electromagnetic transitions, and capture reactions.

Structure and reactions of quantum halos

The experimental and theoretical evidence for intrinsic reflection-asymmetric shapes in nuclei is reviewed. The theoretical methods discussed cover a wide spectrum, from mean-field theory and its extensions to algebraic and cluster approaches. The experimental data for nuclear ground states and at low and high spin, cited as evidence for reflection asymmetry, are collected and categorized. The extensive data on electric dipole transition moments and their theoretical interpretation are surveyed, along with available data on electric octupole moments. The evidence for reflection-asymmetric molecular states in light nuclei is summarized. The application of reflection-asymmetric theories to descriptions of the fission barrier, bimodal fission, superdeformation, and hyperdeformations is reviewed, and some other perspectives in the wider context of nuclear physics are also given. [S0034-6861(96)00102-X]

Intrinsic reflection asymmetry in atomic nuclei

Chapter II. Comprehensive Study of Alpha-Nuclei

l) Included among these are some neutron dripline nuclei, e.g., l1Li, 14Be and 17B. All of these nuclei have been shown to have extremely large root mean square radii. The large radius is clearly related to the very small two-neutron separation energy: This suggests that several valence nucleons are coupled to a core very weakly. In those nuclei with neutron or proton excess it is tempting to explore a new type of motion in which the core part and the loosely bound excess part play a characteristic role. Let us consider the electric dipole mode as an example. The Goldhaber-Teller (GT)2) and Steinwedel-Jensen (SJ)3) models are known as macroscopic models for the giant dipole resonance (GDR). The GT model treats a displacement mode of the bulk of protons against the neutrons assuming that the proton and neutron fluids are incompressible, while the SJ model considers the GDR as the acoustic mode of density oscillations assuming that both the fluids are compressible but the total fluid is incompressible. Both of the GT and SJ models consider the out-of-phase density oscillation of all the protons and all the neutrons, and relate the restoring force against the vibration to the energy of the GDR. It appears possible, however, that the neutrons inside the core of neutron-rich nuclei move together with the protons because of their tight binding but the excess neutrons move against the protons leaving the center-of-mass at rest. The restoring force corresponding to this kind of dipole vibration is expected to be smaller than that of the GDR. This dipole oscillation, which we call pygmy dipole resonance (PDR), will thus appear at lower excitation energy, although more detailed examination is needed to be definitive because the mass paramet~r of the vibration also becomes smaller. Hansen and J onson 4 ) speculated about the existence of a similar mode, "soft dipole mode", in an attempt to discuss the possible enhancement of Coulomb dissociation cross sections in a loosely bound nucleus such as l1Li.

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New Type of Dipole Vibration in Nuclei

A systematic investigation of ^6_ΛHe, ^6_ΛLi, ^7_ΛLi ^8_ΛLi and ^9_ΛBe hypernuclei is carried out within the framework of the microscopic α+χ+Λ cluster-model dynamics (χ=n, p, d, t, or α). The positive and negative parity energy spectra are analysed in detail and are classified into several characteristic “bands” according to the underlying structures. The E2, E1 and M1 γ-ray transition probabilities and magnetic dipole moments are calculated. The intra-band B(E2)’s are obtained to be several times enhanced in comparison with the shell-model values. The level widths of the strong peaks observed at lower B_Λ are roughly evaluated on the basis of the calculated reduced width amplitudes. The existing data can be well explained by the present model. The cluster-model estimates of the effective neutron number Neff(θ=0°) for the (K^-, π^-) reaction are consistent with the experiments.

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Light p-Shell Λ-Hypernuclei by the Microscopic Three-Cluster Model

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Chapter I. \ Introduction to Comprehensive Nuclear Structure Study Based on Cluster Correlations and Molecular Viewpoint

A model is proposed in which we take full advantage of both the shell and cluster models in order to describe a system of valence nucleons weakly coupled to a core. This is realized by introducing radius vectors of the valence nucleons relative to the core, new coordinates of independent particle nature. The model is applied to the neutron-rich He isotopes, $^{6\mathrm{\ensuremath{-}}8}\mathrm{He}$, with reasonable success.

Kiyomi Ikeda

Papers

Chapter II. Comprehensive Study of Alpha-Nuclei

New Type of Dipole Vibration in Nuclei

Light p-Shell Λ-Hypernuclei by the Microscopic Three-Cluster Model

Chapter I. \ Introduction to Comprehensive Nuclear Structure Study Based on Cluster Correlations and Molecular Viewpoint

Cluster-orbital shell model and its application to the He isotopes