Two-neutron Halo Nuclei Research Articles

10–11Li and 13–14Be studied by projectile fragmentation and pp-RPA

The level ordering in the unbound nuclei 10Li and 13Be is established on a firm basis using a time dependent projectile fragmentation model, comparing to experimental data and using structure inputs obtained from a semi-phenomenological core-vibration coupling model of two-neutron halo nuclei. The information on the shell ordering corresponds to the understanding of the neutron-core interaction. This is a building block of any three body model of borromean nuclei. As a consistency test we show that the energy spectra of some Beryllium isotopes obtained by using pp-RPA predict the same shell ordering as extracted from the reaction model vs. data analysis and that the well known structure of 11Li is reproduced.

Breakup reactions of two-neutron halo nuclei

Investigation of reactions initiated in collisions of beams of unstable nuclei with nuclear targets has resulted in the discovery of a new type of nuclear structure, the halo, in some light nuclei at the boundary of nucleon stability. A survey of different reactions with two-neutron halo nuclei and their use for gaining information on the structure of exotic nuclei is presented. Kinematically complete breakup reactions, which provide the possibility of obtaining quite reliable information on the structure of the ground state and the continuum of nuclei, are considered in detail. The microscopic four-body model of breakup reactions for two-neutron halo nuclei with account of characteristic specific features of their structure is formulated. The model is based on the distorted wave approximation and is applicable for analysis of low excitations of the continuum near the breakup threshold, the region most sensitive to manifestation of specific features of the halo structure. The described approach enables the calculation of all observables of kinematically complete experiments in nucleus-nucleus collisions at intermediate energies for which one-step processes dominate, and creates the basis for the spectroscopy of continua via successive analysis of various correlation cross sections available in kinematically complete experiments.

A MICROSCOPIC HYPER-SPHERICAL MODEL: APPLICATION TO 6He

We have developed a microscopic cluster model for two-neutron halo nuclei with special attention paid to long distance correlations between the core and the valence neutrons. The emphasis put on a good treatment of the asymptotic region is necessary for nuclear reaction calculations involving these nuclei. In this contribution, we present the first results of our model applied to the ground state of 6 He bound by an effective central Minnesota N-N interaction, and compare with experiment and other theoretical models.

Time-dependent description for nuclear reaction dynamics in the continuum

We have recently developed time-dependent few-body approaches to describe reactions involving halo nuclei. In this report, we demonstrate usefulness of the time-dependent description taking two applications as examples: the fusion reaction of halo nuclei, and the dipole strength function of two-neutron halo nuclei.

FOLDING MODEL ANALYSIS OF 6He + 12C ELASTIC SCATTERING

Elastic scattering of the two-neutron halo nucleus, 6 He , on 12 C target at 38.3 and 41.6 MeV/nucleon has been analyzed in the framework of the double-folding optical model. Real double-folded potentials based on the realistic density-dependent DDM3Y and JLM effective nucleon–nucleon interactions are generated using different forms of the nuclear matter density distribution of 6 He . The imaginary optical potentials are taken in the conventional Woods–Saxon form. The bare (unnormalized) real folded potentials derived from the JLM interaction are more successful in reproducing the data at both energies than those derived from the DDM3Y interaction. The effect of contribution of the dynamic polarization potential is also studied. A semimicroscopic approximation is proposed to simulate this potential by introducing a repulsive real part extracted from the generated folded potential. Fits to data have been slightly improved by considering this approximation.

Direct reactions with exotic nuclei, nuclear structure and astrophysics

Intermediate energy Coulomb excitation and dissociation is a useful tool for nuclear structure and astrophysics studies. Low-lying strength in nuclei far from stability was discovered by this method. The effective range theory for low-lying strength in one-neutron halo nuclei is summarized and extended to two-neutron halo nuclei. This is of special interest in view of recent rather accurate experimental results on the low-lying electric dipole strength in 11Li. Another indirect approach to nuclear astrophysics is the Trojan-horse method. It is pointed out that it is a suitable tool to investigate subthreshold resonances.

NEUTRON-NEUTRON CORRELATIONS IN HALO NUCLEI

Work aimed at probing the spatial configuration of the valence neutrons in two-neutron halo nuclei using the technique of intensity interferometry is described. Following a brief review of the method and its application to our first measurements of the breakup of 6 He , 11 Li and 14 Be , the results of the analysis of a high statistics data set for 6 He are presented.

Coulomb Breakup of 11Li

A new kinematically complete measurement of the Coulomb breakup of twoneutron halo nucleus 11Li on a Pb target has revealed a strong soft-E1 transition strength peaking at Ex∼0.6 MeV. This soft E1 strength amounts to 1.42±0.18 e2fm2, corresponding to 4.5±0.6 Weisskopf units(W.u.). The observed B(E1) distribution has been found in excellent agreement with a three-body calculation with strong two-neutron correlation and final state interactions.

C22: Ans-wave two-neutron halo nucleus

A dripline nucleus $^{22}\mathrm{C}$ is studied in a Borromean three-body model of $^{20}\mathrm{C}+n+n$. The valence neutrons, interacting via a realistic potential, are constrained to be orthogonal to the occupied orbits in $^{20}\mathrm{C}$. We obtain ample results supporting that $^{22}\mathrm{C}$ is an ideal $s$-wave two-neutron halo nucleus: The ground state is bound by 390--570 keV, the root mean square neutron and proton radii are 4.0 and 2.4 fm, and the two neutrons are predominantly in $({s}_{1/2}){}^{2}$ orbits. The binding mechanism of $^{22}\mathrm{C}$ is discussed. One- and two-body density distributions elucidate the halo character as well as the correlated motion of the neutrons. The reaction cross sections of $^{22}\mathrm{C}+^{12}\mathrm{C}$ collisions are predicted.

Continuum spectroscopy of Borromean two-neutron halo nuclei

Energy and angular correlation distributions of the three fragments in $^{6}\mathrm{He}$ breakup on $^{208}\mathrm{Pb}$ at a collision energy of 240 MeV/nucleon are discussed within the microscopic four-body distorted wave model and compared with experimental data. The nuclear structure of the ground state and low-energy three-body continuum of $^{6}\mathrm{He}$ is calculated by the method of hyperspherical harmonics within the three-body cluster model. Reflections of the fundamental permutation symmetry of the halo neutrons in angular and energy correlations are pointed out. The calculations describe the experimental data for fragment correlations near breakup threshold rather well, and the physics is contained in a few elementary modes; but with increasing excitation energy of $^{6}\mathrm{He}$, some striking deviations from experimental distributions are encountered. Possible reasons for this are discussed.

The Complex Scaling Method for Many-Body Resonances and Its Applications to Three-Body Resonances

Resonance phenomena in quantum physics are very familiar in many fields of physics, but we have not yet obtained a complete physical understanding, mathematical description or computational treatment, especially in the case of many-body resonances. Recently, in experimental developments concerning unstable nuclear physics and heavy-ion nuclear reactions, much interest has been concentrated on many-body resonance problems. In the last quarter century, theoretical and mathematical treatments of many-body resonances have experienced great development through application of the complex scaling method (CSM). We can now treat resonant states of three-body systems in the same way as those of twobody systems. In this article, starting from the definition of a resonant state and discussion of its norm, we present a summary of recent studies of CSM to treat many-body resonances and applications to three-body resonant states in two-neutron halo nuclei and three-cluster systems.

Observation of Strong Low-LyingE1Strength in the Two-Neutron Halo NucleusLi11

An exclusive measurement has been made of the Coulomb dissociation of the two-neutron halo nucleus 11Li at 70 MeV/nucleon at RIKEN. Strong low-energy (soft) E1 excitation is observed, peaked at about Ex = 0.6 MeV with B(E1) = 1.42(18) e2fm2 for Erel < or = 3 MeV, which was largely missed in previous measurements. This excitation represents the strongest E1 transition ever observed at such low excitation energies. The spectrum is reproduced well by a three-body model with a strong two-neutron correlation, which is further supported by the E1 non-energy-weighted cluster sum rule.

Coulomb breakup of the two-neutron halo nucleus 11Li

Coulomb breakup of the two-neutron halo nucleus 11Li has been investigated in a kinematically complete measurement of three-body breakup of 11Li on a Pb target at approximately 70 MeV/nucleon at RIKEN. Cross-talk events have been excluded without affecting the efficiency very much up to very low relative energy. The resultant preliminary B(E1) spectrum has revealed high (E1) strength at Erel (three-body relative energy) ∼0.3 MeV, which was missing in the previous measurements. This E1 strength with the help of the E1 cluster sum-rule indicates a significant neutron–neutron correlation in the ground state of 11Li.

Coulomb breakup and pairing excitation of the two-neutron halo nucleus 11Li

The Coulomb breakup and pairing excitation of the two-neutron halo nucleus 11Li with the cluster orbital shell model have been studied. For the excited resonances in 11Li, we use the complex scaling method. We get an excited solution of 3/2− in 11Li.

Electron scattering on two-neutron halo nuclei with full inclusion of final state interactions

Electron scattering on two-neutron halo nuclei with full inclusion of final state interactions

Breakup reactions of halo nuclei

Coulomb and nuclear breakup of halo nuclei have been studied at RIKEN. In this contribution, we focus on the cases of Coulomb breakup of the one-neutron halo nucleus 11Be at 69MeV/nucleon with a lead target and the nuclear breakup of 11Be at 67MeV/nucleon with a carbon target. In these studies, we have extracted the angular distributions of 10Be + n c.m. system (inelastic scattering) as well as the relative energy spectra. The angular distributions have been found very important to extract the pure E1 component in the breakup with Pb target, while it has been used to specify the angular momentum of two discrete levels observed for the breakup with C target. We also present preliminary results of Coulomb breakup of the two-neutron halo nucleus 11Li.

Experimental Studies of Unbound Neutron-Rich Nuclei

The three-body description of two-neutron halo nuclei relies on the two-body interactions between the constituents. In order to provide constraints on calculations devoted to $^{14}$Be and $^{17}$B, neutron unbound states of $^{13}$Be and $^{16}$B have been investigated by one-proton knockout. The experimental techniques and results are discussed here.

TWO-BODY CORRELATION CONTRIBUTIONS IN HALO NUCLEI WITH A RELATIVISTIC HARTREE APPROACH

The relativistic density-dependent Hartree framework, where the relativistic two-body correlations are properly incorporated, is developed to study the properties of halo nuclei. The halo nucleon–meson vertex is reconstructed considering the nuclear potentials can be built dominantly from the two-body interactions. The two-neutron halo nucleus 11 Li , together with the one-neutron halo nucleus 19 C , is investigated. Separation energies, root-mean-square (rms) radii, and halo tails of above halo nuclei are nicely reproduced. The correlation contribution which provides essential attractions for halo neutrons is important to guarantee the relation Sn&gt;S2n for 11 Li .

Three-body continuum spatial correlations in Borromean halo nuclei

Spatial correlations in the three-body continuum of Borromean (having no bound binary subsystems) three-body systems are discussed. The hyperspherical harmonics method is used to investigate low-lying resonances and the soft dipole mode in the two-neutron halo nucleus $^{6}\mathrm{He}$, which has only the $\ensuremath{\alpha}+n+n$ continuum for excitation energies below $13\phantom{\rule{0.3em}{0ex}}\text{MeV}$. The spatial correlations reveal characteristic structures for true three-body resonances, a moderate amplification in the interior region for above-barrier resonances and long-range correlations in the cases of three-body ${1}^{\ensuremath{-}}$ virtual and ${0}^{+}$ continuum states.

Pre-asymptotic behavior of single-particle overlap integrals of non-Borromean two-neutron halos

For non-Borromean two-neutron halo nuclei, modifications to the behavior of single-particle overlap integrals will arise due to the correlations of the two interacting nucleons in the halo. An additional contribution to the overlap integral can be obtained using the Feynman diagram approach. This additional term is modeled using a simple local potential model. We show that these modifications may play a role in detailed interpretations of experimental results from single-nucleon knockout, transfer, and other reactions that probe the single-nucleon overlap functions.