Two-neutron Halo Nuclei Research Articles

Density distributions of11Li deduced from reaction cross-section measurements

We measured the reaction cross sections of the two-neutron halo nucleus ${}^{11}$Li with solid hydrogen and carbon targets at around 31 and 41 MeV/nucleon. The neutron density distribution of ${}^{11}$Li was deduced for the first time by the Glauber model calculation based on the optical limit approximation. The uncertainty of the matter density of ${}^{11}$Li was improved, compared with earlier measurements. The present root-mean-square radius of the proton distribution agrees with the previous one derived from an optical isotope shift measurement. The present root-mean-square radii reproduce theoretical calculations by the tensor optimized shell model by assuming core excitation. This consistency suggests the possibility that ${}^{9}$Li in ${}^{11}$Li is excited and the disappearance of the $N=8$ shell gap of ${}^{11}$Li is caused by correlations originating from the nucleon force, such as the tensor and the pairing.

Mechanisms of Coulomb breakup reactions of 6He and 11Li

We investigate the three-body Coulomb breakup reactions of two-neutron halo nuclei and discuss the effect of binary subsystem correlations such as the of core-n and n-n. Furthermore, we calculate the invariant mass spectra. It is found that the final-state interactions of core-n and n-n binary subsystems dominantly determine the observed structures of the breakup cross sections, such as the low-lying enhancements.

Multi-cluster problems: resonances, scattering and condensed states

This talk is mainly concerned with many-body resonances in nuclear physics. We extensively discuss the structure and reactions of multi-cluster systems using the complex scaling method (CSM). We expound three interesting problems in recent studies of multi-cluster systems. First, we discuss four- and five-body resonances in A=7 (7He, 7B) and A=8 (8He, 8C) systems, respectively. The observed states are well explained and many additional states are predicted. Second, the Coulomb breakup reactions of two-neutron halo nuclei and the α-d scattering are investigated using a three-body model with CSM. Finally, we discuss the α-condensate-like states and their symplectic excitation properties in three- and four-α models for 12C and 16O, respectively.

Coulomb breakup reactions of11Li in the coupled-channel9Li+n+nmodel

We investigate the three-body Coulomb breakup of a two-neutron halo nucleus ${}^{11}$Li. We use the coupled-channel ${}^{9}$Li+$n$+$n$ three-body model, which includes the coupling between last neutron states and the various two-particle--two-hole ($2\text{p}$-$2\text{h}$) configurations in ${}^{9}$Li due to the tensor and pairing correlations. The three-body scattering states of ${}^{11}$Li are described by using the combined methods of complex scaling and the Lippmann-Schwinger equation. The calculated breakup cross section successfully reproduces the experiments. The large mixing of the $s$ state in the halo ground state of ${}^{11}$Li is shown to play an important role in the explanation of shape and strength of the breakup cross section. In addition, we predict the invariant mass spectra for binary subsystems of ${}^{11}$Li. It is found that the two kinds of virtual $s$ states of ${}^{9}$Li-$n$ and $n$-$n$ systems in the final three-body states of ${}^{11}$Li largely contribute to make low-lying peaks in the invariant mass spectra. On the other hand, in the present analysis, it is suggested that the contributions of the $p$-wave resonances of ${}^{10}$Li are hardly confirmed in the spectra.

Coulomb Breakup Reactions of Two-Neutron Halo Nuclei 6He and 11Li Using Complex-Scaled Solutions of the Lippmann–Schwinger Equation

We investigate the three-body Coulomb breakup reactions of two-neutron halo nuclei and discuss the effect of binary subsystem correlations such as of core–n and n–n. We furthermore calculate the invariant mass spectra. It is found that the final-state interactions of core–n and n–n binary subsystems dominantly determine the observed structures of the breakup cross sections, such as the low-lying enhancements.

One- and two-neutron removal reactions from the most neutron-rich carbon isotopes

The structure of 19,20,22C has been investigated using high-energy (around 240 MeV/nucleon) one- and two-neutron removal reactions on a carbon target. Measurements were made of the inclusive cross sections and momentum distributions for the charged residues. Narrow momentum distributions were observed for one-neutron removal from 19C and 20C and two-neutron removal from 22C. Two-neutron removal from 20C resulted in a relatively broad momentum distribution. The results are compared with eikonal-model calculations combined with shell-model structure information. The neutron removal cross sections and associated momentum distributions are calculated for transitions to both the particle-bound and particle-unbound final states. The calculations take into account the population of the mass A−1 reaction residues A−1C and, following one-neutron emission after one-neutron removal, the mass A−2 two-neutron removal residues A−2C. The smaller contributions of direct two-neutron removal, that populate the A−2C residues in a single step, are also computed. The data and calculations are shown to be in good overall agreement and consistent with the predicted shell-model ground-state configurations and one-neutron overlaps with low-lying states in 18−21C. These suggest significant νs1/22 valence neutron configurations in both 20C and 22C. The results for 22C strongly support the picture of 22C as a two-neutron halo nucleus with a dominant νs1/22 ground-state configuration.Received 30 November 2011DOI:https://doi.org/10.1103/PhysRevC.86.054604©2012 American Physical Society

Unbound nuclei studied by projectile fragmentation

The level ordering in the unbound nucleus 13Be is firmly established by 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. Thus, from the shell ordering, the neutron–core interaction is determined, which is needed by any three-body model of Borromean nuclei. As a consistency test, we show that the energy spectra of some beryllium isotopes obtained using the particle–particle random phase approximation predict the same shell ordering as those extracted from the reaction model versus data analysis.

Matter and charge radius of6He in the hyperspherical-harmonics approach

We present ab-initio calculations of the binding energy and radii of the two-neutron halo nucleus 6He using two-body low-momentum interactions based on chiral effective field theory potentials. Calculations are performed via a hyperspherical harmonics expansion where the convergence is sped up introducing an effective interaction for non-local potentials. The latter is essential to reach a satisfactory convergence of the extended matter radius and of the point-proton radius. The dependence of the results on the resolution scale is studied. A correlation is found between the radii and the two-neutron separation energy. The importance of three-nucleon forces is pointed out comparing our results and previous calculations to experiment.

Nuclear structure of 14Be nucleus

We investigated in this work the two-neutron halo nuclei of 14Be. The cluster model was used in the present work to study the properties of a three-body system. The three-body system described depended on the Jacobi coordinates using two configurations: the T-configuration and the Y-configuration. In this work, the binding energy, the root-mean-square (r.m.s.) radius and effect of deformation of the core (12Be) on the properties of two-neutron halo were studied. The calculations confirmed that the core had a prolate-shaped deformation, which affected the structure of the three-body system.

STRUCTURE OF TWO-NEUTRON HALO NUCLEI, 11Li

The two-neutron halo nucleus 11 Li has been investigated in this work. Jacobi coordinates is used to describe the three-body system, using two configurations that are the T-configuration and Y-configuration. The calculations have been calculated based on a cluster model. The 11 Li nucleus is considered as three-body system core (9 Li )+n+n. The core here has been considered as a deformed core. The calculations confirmed that the core has some deformation and has an oblate shape which in turn has effects on the structure of three-body system.

Binary Subsystem Correlations in Coulomb Breakup Reactions of Two-Neutron Halo Nuclei

Binary Subsystem Correlations in Coulomb Breakup Reactions of Two-Neutron Halo Nuclei

TWO-NEUTRON CORRELATIONS IN HALO NUCLEI VIA COULOMB BREAKUP REACTIONS

We investigate the three-body Coulomb breakup of a two-neutron halo nucleus, 6 He . We calculate the breakup cross section and the invariant mass spectra by using the complex-scaled solutions of the Lippmann-Schwinger equation, and discuss the relations between the structures in these observables and the n-n and α-n correlations in 6 He .

Two-neutron halo nuclei in one dimension: dineutron correlation and breakup reaction

We propose a simple schematic model for two-neutron halo nuclei. In this model, the two valence neutrons move in a one-dimensional mean field, interacting with each other via a density-dependent contact interaction. We first investigate the ground-state properties and demonstrate that the dineutron correlation can be realized with this simple model due to the admixture of even- and odd-parity single-particle states. We then solve the time-dependent two-particle Schrödinger equation under the influence of a time-dependent one-body external field, in order to discuss the effect of dineutron correlation on nuclear breakup processes. The time evolution of two-particle density shows that the dineutron correlation enhances the total breakup probability, especially for the two-neutron breakup process, in which both the valence neutrons are promoted to continuum scattering states. We find that the interaction between the two particles definitely favors a spatial correlation of the two outgoing particles, which are mainly emitted in the same direction.

Sub-barrier fusion of two-neutron halo nuclei

The tunneling of composite systems, where breakup may occur during the barrier penetration process, is considered in connection with the fusion of halo-like radioactive, neutron- and proton-rich nuclei, on heavy targets. The large amount of recent and new data clearly indicates that breakup hinders the fusion at energies near and below the Coulomb barrier. However, clear evidence for enhancement due to halo properties seems to over ride the breakup hindrance at lower energies, owing, to a large extent, to the extended matter density distribution. In particular we report here that at sub-barrier energies the fusion cross section of the Borromean two-neutron halo nucleus 6He with the actinide nucleus 238U is significantly enhanced as compared to the fusion of a similar projectile with no halo. This conclusion differs from that of the original work, where it was claimed that no such enhancement ensues. This sub-barrier fusion enhancement is also observed in the 6He+209Bi system.

DI-NEUTRON CORRELATION IN 6He THROUGH COULOMB BREAKUP REACTIONS

We investigate the three-body Coulomb breakup of a two-neutron halo nucleus, 6 He . The three-body scattering states of 6 He are described by using the Complex-scaled solutions of the Lippmann-Schwinger equation. We calculate the breakup cross section and the invariant mass spectra, and discuss the relations between the structures in these observables and the n-n and α-n correlations of 6 He .

Two-neutron overlap functions for 6He from a microscopic structure model

A fully antisymmetrized microscopic model is developed for light two-neutron halo nuclei using a hyper-spherical basis to describe halo regions. The many-body wavefunction is optimized variationally. The model is applied to 6He bound by semi realistic Minnesota nucleon–nucleon forces. The two-neutron separation energy and the radius of the halo are reproduced in agreement with experiment. Antisymmetrization effects between 4He and halo neutrons are found to be crucial for binding of 6He. We also properly extract two-neutron overlap functions and find that there is a significant increase of 30%–70% in their normalization due to microscopic effects as compared to the results of three-body models.

Two-neutron correlations inHe6in a Coulomb breakup reaction

We investigate the three-body Coulomb breakup of a two-neutron halo nucleus, $^{6}\mathrm{He}$. Based on the $\ensuremath{\alpha}+n+n$ model, the three-body scattering states of $^{6}\mathrm{He}$ are described by using the combined methods of the complex scaling and the Lippmann-Schwinger equation. We calculate the breakup cross section, the two-dimensional energy distributions, and the invariant mass spectra for the $E1$ transition of $^{6}\mathrm{He}$. We discuss the relations between the structures in these strengths and the $n$-$n$ and $\ensuremath{\alpha}$-$n$ correlations of $^{6}\mathrm{He}$. It is found that the $^{5}\mathrm{He}$ resonance in the final states contributes to make a low-energy enhancement of the strength. The $n$-$n$ final-state interaction also contributes to enhance the strength globally. However, the ground-state correlations of $^{6}\mathrm{He}$, such as a dineutron, are difficult to recognize in the strength because of the dominant effect of the final-state interaction.

Range corrections for two-neutron halo nuclei in effective theory

The range corrections to the universal properties and structure of two-neutron halo nuclei are investigated within an effective quantum mechanics framework. Treating the nucleus as an effective three-body system, we make a systematic improvement upon previous calculations by calculating the linear range corrections at next-to-leading order. Since the effective ranges for the neutron–core interactions are not known, we estimate the effective range to be set by the inverse of the pion mass. We investigate the possibility of excited Efimov states in two-neutron halo nuclei and calculate their mean square radii to next-to-leading order. We find that the effective range corrections are generally small and the leading order predictions are very robust.

Tensor optimized shell model using bare interaction for light nuclei

We propose a new theoretical approach to calculate nuclear structure using bare nuclear interaction, in which the tensor and short-range correlations are described by using the tensor optimized shell model (TOSM) and the unitary correlation operator method (UCOM), respectively. We compare the obtained results using TOSM+UCOM for 4He with the rigorous calculation. We also apply TOSM to the analysis of two-neutron halo nuclei, 6He and 11Li, on the basis of the “core with TOSM”+n+n model. The halo formation of 11Li is explained by inclusion of the tensor correlation in 9Li core, which produces the Pauli-blocking on the p-wave neutrons in 11Li and enhances the s-wave component of halo structure.

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.