Neutron Halo Nuclei Research Articles

Elastic scattering and breakup reactions with light proton- and neutron-rich exotic nuclei

A microscopic analysis of the optical potentials (OPs) and cross sections of elastic scattering of 8B on 12C, 58Ni, and 208Pb targets at energies 20 < E < 170 MeV and 12,14Be on 12C at 56 MeV/nucleon is carried out. The real part of the OP is calculated by a folding procedure and the imaginary part is obtained on the base of the high-energy approximation (HEA). The density distributions of 8B evaluated within the variational Monte Carlo (VMC) model and the three-cluster model (3CM) are used to construct the potentials. The 14Be densities obtained in the framework of the the generator coordinate method (GCM) are used to calculate the optical potentials, while for the same purpose both the VMC model and GCM densities of 12Be are used. In the hybrid model developed and explored in our previous works, the only free parameters are the depths of the real and imaginary parts of OP obtained by fitting the experimental data. The use of HEA to estimate the imaginary OP at energies just above the Coulomb barrier is discussed. In addition, cluster model, in which 8B consists of a p-halo and the 7Be core, is applied to calculate the breakup cross sections of 8B nucleus on 9Be, 12C, and 197Au targets, as well as momentum distributions of 7Be fragments. A good agreement of the theoretical results with the available experimental data is obtained. It is concluded that the reaction studies performed in this work may provide supplemental information on the internal spatial structure of the proton- and neutron-halo nuclei.

Natural orbital description of the halo nucleus 6He

Ab initio calculations of nuclei face the challenge of simultaneously describing strong short-range internucleon correlations and the long-range properties of weakly-bound halo nucleons. Natural orbitals, which diagonalize the one-body density matrix, provide a basis which is better matched to the physical structure of the many-body wave function. We demonstrate that the use of natural orbitals significantly improves convergence for ab initio no-core configuration interaction calculations of the neutron halo nucleus 6He, relative to the traditional oscillator basis.

Halo-induced large enhancement of soft dipole excitation of 11Li observed via proton inelastic scattering

Proton inelastic scattering off a neutron halo nucleus, 11Li, has been studied in inverse kinematics at the IRIS facility at TRIUMF. The aim was to establish a soft dipole resonance and to obtain its dipole strength. Using a high quality 66 MeV 11Li beam, a strongly populated excited state in 11Li was observed at Ex=0.80±0.02 MeV with a width of Γ=1.15±0.06 MeV. A DWBA (distorted-wave Born approximation) analysis of the measured differential cross section with isoscalar macroscopic form factors leads us to conclude that this observed state is excited in an electric dipole (E1) transition. Under the assumption of isoscalar E1 transitions, the strength is evaluated to be extremely large amounting to 30∼296 Weisskopf units, exhausting 2.2%∼21% of the isoscalar E1 energy-weighted sum rule (EWSR) value. The large observed strength originates from the halo and is consistent with the simple di-neutron model of 11Li halo.

Examining possible neutron-halo nuclei heavier than Mg37

The even-Z odd-N neutron-halo nuclei, which are the possible lightest neutron-halo nuclei heavier than $^{37}$Mg, are explored by studying the shell-structure unique in weakly-bound neutrons for spherical or deformed shape. It is pointed out that due to the narrowed N=50 spherical energy-gap and a few resulting close-lying neutron one-particle levels, 1g$_{9/2}$, 3s$_{1/2}$, and 2d$_{5/2}$, for spherical shape, nuclei with some weakly-bound neutrons filling in those levels may be deformed and have a good chance to show deformed s-wave halo. Promising candidates are $^{71}_{24}$Cr$_{47}$, $^{73}_{24}$Cr$_{49}$, $^{75}_{24}$Cr$_{51}$ and $^{77}_{26}$Fe$_{51}$ in the case that those nuclei lie inside the neutron drip-line. An interesting possibility of the deformed p-wave or s-wave halo is suggested also for the nucleus $^{53}_{18}$Ar$_{35}$.

Extracting three-body breakup observables from continuum-discretized coupled-channels calculations with core excitations

Core-excitation effects in the scattering of two-body halo nuclei have been investigated in previous works. In particular, these effects have been found to affect in a significant way the breakup cross sections of neutron-halo nuclei with a deformed core. To account for these effects, appropriate extensions of the continuum-discretized coupled-channels (CDCC) method have been recently proposed. We aim to extend these studies to the case of breakup reactions measured under complete kinematics or semi-inclusive reactions in which only the angular or energy distribution of one of the outgoing fragments is measured. We use the standard CDCC method as well as its extended version with core excitations (XCDCC), assuming a pseudo-state basis for describing the projectile states. Two- and three-body observables are computed by projecting the discrete two-body breakup amplitudes, obtained within these reaction frameworks, onto two-body scattering states with definite relative momentum of the outgoing fragments and a definite state of the core nucleus. The presented method provides a tool to compute double and triple differential cross sections for outgoing fragments following the breakup of a two-body projectile, and might be useful to analyze breakup reactions with other deformed weakly-bound nuclei, for which core excitations are expected to play a role. We have found that, while dynamical core excitations are important for the proton target at intermediate energies, they are very small for the Zn target at energies around the Coulomb barrier.

Effect of Coulomb breakup on the elastic cross section of theB8proton-halo projectile on a heavy,Pb208target

We investigate the role of the breakup channel in the elastic and breakup cross sections, in collisions of proton-halo nuclei. For this purpose, we perform continuum discretized couple channel (CDCC) calculations for the $^{8}\mathrm{B}+\phantom{\rule{0.16em}{0ex}}^{208}\mathrm{Pb}$ system and evaluate polarization potentials. One-channel calculations including the polarization potential are shown to reproduce very well the elastic cross sections obtained by CDCC calculations. We also study the individual contributions of the Coulomb and the nuclear couplings to the cross sections. To complement our study, we compare the effects of the breakup channel in proton-halo and neutron-halo nuclei, performing calculations treating $^{8}\mathrm{B}$ as a $^{7}\mathrm{B}+n$ core-nucleon system, with an artificially low breakup threshold. When only the nuclear breakup is considered, this approach can reasonably describe the elastic scattering.

The effect of proton halo on fusion reactions

In a recent brief report [Kumar et al., Phys. Rev. C 89, 027601 (2014)] we proposed a method to include the effect of a large break up in fusion reactions with neutron and proton halo nuclei. A stronger enhancement on the total fusion cross section was found for the proton case. However, neither complete nor incomplete fusion was studied and only one target was considered. Here, we revisit the problem in order to address the importance of complete and incomplete fusion. The sensitivity of the cross section to the target is also explored.

Halo nucleiHe6andHe8with the Coulomb-Sturmian basis

The rapid Gaussian falloff of the oscillator functions at large radius makes them poorly suited for the description of the asymptotic properties of the nuclear wave function, a problem which becomes particularly acute for halo nuclei. We consider an alternative basis for ab initio no-core configuration interaction (NCCI) calculations, built from Coulomb-Sturmian radial functions, allowing for realistic exponential falloff. NCCI calculations are carried out for the neutron halo nuclei 6,8He, as well as the baseline case 4He, with the JISP16 nucleon-nucleon interaction. Estimates are made for the root-mean-square radii of the proton and matter distributions.

Hyperfine structure constant of the neutron halo nucleus (11)Be(+).

The hyperfine splittings of ground state Be+11 have been measured precisely by laser-microwave double resonance spectroscopy for trapped and laser cooled beryllium ions. The ions were produced at relativistic energies and subsequently slowed down and trapped at mK temperatures. The magnetic hyperfine structure constant of Be+11 was determined to be A11=-2677.302 988(72) MHz from the measurements of the mF-mF'=0-0 field independent transition. This measurement provides essential data for the study of the distribution of the halo neutron in the single neutron halo nucleus Be11 through the Bohr-Weisskopf effect.

Neutron-halo structure of light nuclei studied with effects of deformations and orientations included

Based on the cluster-core model, the effects of nuclear deformations and orientations on the halo structure of the observed and proposed cases of neutron-halo nuclei is analyzed. The calculations are performed for the deformed structures using quadrupole (β2) deformations with ‘optimum’ orientations as well as including higher multipole deformations (β2-β4) with the ‘compact’ (hot) orientations. The interesting result is that, although potential energy surfaces are modified with the inclusion of deformation effects up to β2, but the 1n- and 2n-halo nature remains intact for almost all cases investigated here, except for 17B and 22C nuclei. However, in some cases (22O, 26F, 27F), the choice of higher multipole deformations up to hexadecapole is shown to be rather sensitive. In addition, the relevance of the use of different nuclear proximity potentials is also explored in the context of the halo nature of neutron-rich light nuclei. The possible role of Q-value and angular momentum effects are also addressed.

Study of fusion probabilities with halo nuclei using different proximity based potentials

Abstract We study fusion of halo nuclei with heavy targets using proximity based potentials due to Aage Winther (AW) 95, Bass 80 and Proximity 2010. In order to consider the extended matter distribution of halo nuclei, the nuclei radii borrowed from cross section measurements are included in these potentials. Our study reveals that the barrier heights are effectively reduced and fusion cross sections are appreciably enhanced by including extended radii of these nuclei. We also find that the extended sizes of halos contribute towards enhancement of fusion probabilities in case of proton halo nuclei, but, contribute to transfer or break-up process rather than fusion yield in case of neutron halo nuclei.

Elastic scattering of the proton drip-line nucleus8B off anatPb target at 170.3 MeV

The elastic scattering of ${}^{8}$B by a ${}^{\mathrm{nat}}$Pb target was measured at an incident energy of 170.3 MeV. Special care was taken with the limited intensity and broad profile of the secondary beam. The measured angular distribution of the differential cross section shows that the Coulomb-nuclear interference peak (CNIP) is not suppressed in this system, in contrast to what was observed in the scattering of neutron halo nuclei by heavy targets at energies around the Coulomb barrier. Analyses of the angular distribution were performed both in terms of the optical model using a single-folding-type potential and the continuum discretized coupled-channels (CDCC) method, which explicitly takes into account the breakup-channel couplings to the elastic scattering. The overall pattern of the differential cross section is well reproduced by the CDCC calculations. The calculations show that the effect of breakup-channel couplings on the elastic scattering is small in the present case.

Theoretical study on neutron skin thickness of Ca isotopes by parity-violating electron scattering

The parity-violating electron scattering (PVS) experiment of Ca-48 has been proposed in Jefferson Lab recently. In this paper, the relation between neutron radius of Ca-48 and corresponding parity-violating asymmetry A(pv) is investigated. With this relation, we can extract the neutron radius of Ca-48 from measurement at only one single momentum transfer. The statistical error for measuring A(pv) of Ca-48 is also calculated at the kinematics of CREx. Besides, with the study of linear dependence between Delta R-np and L, it enable us to constrain the slope L of the symmetry energy. Parity-violating electron scattering off Ca isotopes is also studied because some experimental errors can be canceled by combining PVS measurements of isotopes. To analyze the differences of neutron skin thickness of Ca isotopes, we fit the mean-field densities with 2pF distributions and study the bulk and surface contributions. By comparing the half density radius and surface parameter along Ca isotopes, we note there may exist neutron halo nuclei in the extreme neutron rich range. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.

Neutron Halo Structure Probed by Breakup Reactions

The neutron drip line represents the boundary of nuclei in the neutron-rich side of the nuclear chart. In the vicinity of the neutron drip line, we often observe 'neutron halo' structure. We discuss how the neutron halo nuclei are studied by the breakup reactions at relativistic energies. Coulomb breakup is the dominant process in the breakup with a heavy target, such as Pb. In the Coulomb breakup of halo nuclei, enhancement of the electric dipole strength at low excitation energies (soft E1 excitation) is observed as a unique property for halo nuclei. The mechanism of the soft E1 excitation and its spectroscopic significance is shown as well as the applications of the Coulomb breakup to the very neutron rich 22C and 31Ne, which was measured at 230-240 MeV/nucleon at the new-generation RI beam facility, RIBF(RI Beam Factory), at RIKEN. Evidence of halo structures for these nuclei is provided as enhancement of the inclusive Coulomb breakup, which is a useful tool for the low-intense secondary beam. We also show that the breakup with a light target (C target), where nuclear breakup is a dominant process, can be used to extract the spectroscopic information of the removed neutron. The combinatorial analysis was found very useful to extract more-detailed information such as the spectroscopic factor and the separation energy. Prospects of the breakup reactions on neutron-drip line nuclei at RIBF at RIKEN are also briefly presented.

Outstanding features of reactions with halo nuclei

Recent results for total reaction and fusion excitation functions for halo systems are compiled. The striking similarity in the behavior of total cross sections for proton-halo (8B) and neutron-halo (6He) systems is pointed out. Static and dynamic halo effects are elucidated by comparing data for a neutron-halo nucleus and its core. Fusion cross sections are discussed in terms of a maximum angular momentum for fusion, Lf. A linear energy dependence for Lf is consistent with the data for all the existing measurements so this might actually be a characteristic feature of true halo systems. A qualitative difference between the fusion of neutro-halo and proton-halo systems also is pointed out.

Comparing nonperturbative models of the breakup of neutron-halo nuclei

Breakup reactions of loosely-bound nuclei are often used to extract structure and/or astrophysical information. Here we compare three non-perturbative reaction theories often used when analyzing breakup experiments, namely the continuum discretized coupled channel model, the time-dependent approach relying on a semiclassical approximation, and the dynamical eikonal approximation. Our test case consists of the breakup of 15C on Pb at 68 MeV/nucleon and 20 MeV/nucleon.

Charge radii and neutron correlations in helium halo nuclei

Within the complex-energy configuration-interaction framework, we study correlations of valence neutrons to explain the behavior of charge radii in neutron halo nuclei ${}^{6,8}$He. We find that the experimentally observed decrease of the charge radius between ${}^{6}$He and ${}^{8}$He is caused by a subtle interplay between three effects: dineutron correlations, a spin-orbit contribution to the charge radius, and a core swelling effect. We demonstrate that two-neutron angular correlations in the ${2}_{1}^{+}$ resonance of ${}^{6}$He differ markedly from the ground-state correlations in ${}^{6,8}$He. Finally, we discuss the impact of the neutron threshold position and valence neutron correlation energy on the neutron radius, i.e., the pairing-antihalo effect.

Structure of 6He in the frame of a cluster model

A microscopic cluster model with a fully correlated Gaussian basis is developed. In the model, the stochastic variational method is used in order to calculate the ground-energy and the mean-square radius conveniently. Based on this model, the ground-energy level and radius of the neutron halo nucleus, 6He, are calculated as a α+n+n three-cluster model. The results are in good agreement with the experimental data.

MICROSCOPICAL ANALYSIS OF THE REACTION CROSS-SECTIONS OF 11Li+12C and 22C+12C ELASTIC SCATTERING

Total reaction cross-sections of the two neutron halo nuclei 11Li and 22C elastic scattering from 12C target at E = 30–1000 MeV/nucleon have been analyzed using the eikonal phase shift based, for the first time, on the semi-phenomenological nucleon density. The obtained results reasonably agree with those of previous theoretical calculations as well as the corresponding experimental data.

Observation of a Heavy Neutron-Halo Nucleus 31Ne(Current Topics)

Observation of a Heavy Neutron-Halo Nucleus ³¹Ne(Current Topics)