Light Neutron-rich Nuclei Research Articles

Role ofρNNtensor coupling and2s1/2occupation in light exotic nuclei

We study the properties of light neutron-rich exotic nuclei with relativistic-mean-field models. The nonlinear isoscalar-isovector term that is not constrained by the present data is considered. The {rho}NN tensor coupling is included. The data-to-data correlation between the neutron radius of {sup 208}Pb and that of light exotic oxygen, fluorine, and neon isotopes is enhanced by the inclusion of the {rho}NN tensor coupling subject to the 2s{sub 1/2} occupation of the outlayer neutrons. This enhancement is determined by the single-particle property rather than by the bulk-matter property. The role of the {rho}NN tensor coupling and the pairing correlation in reducing the model dependence of the neutron thickness in light exotic nuclei is investigated. The influence of the isoscalar-isovector term on halo neutrons, characteristic of the 2s{sub 1/2} occupation, is discussed.

Studies of light neutron-rich nuclei near the drip line

Coulomb breakup of neutron-rich Be, B, C, and O isotopes at relativistic energies (400–600 A MeV) was studied with kinematically complete measurements. The ground-state properties of these nuclei were deduced by comparing experimental data for Coulomb-dissociation cross-sections with directbreakup model calculations. The dominant ground-state configurations of 15C, 11Be, 14B, and 23O were found to be 14Cgs(0+) ⊗ v s, 10Begs(0+) ⊗ v s, 13Bgs(3/2+) ⊗ v s,d, and 22Ogs(0+) ⊗ v s, respectively, and 16C(2+) ⊗ v s,d, for 17C. The capture cross-section for 14C(n,λ) 15C relevant in astrophysical scenarios was measured indirectly through Coulomb dissociation.

Light-Mass Cluster Reactions in Supernova R-Process

In explosive circumstances of the neutrino-driven winds in gravitational core-collapse Type II supernovae (SNe), not only heavy neutron-rich nuclei but also light unstable nuclei play the significant roles. We here study the efficiency and sensitivity of the SN r-process nucleosynthesis to many relevant nuclear reaction rates, and discuss quantitatively that several specific nuclear reactions on light neutron-rich nuclei take very critical keys to the final nucleosynthesis yields of r-process elements [Sasaqui, Kajino, T., T., Otsuki, K., Mathews, G. J. and Nakamura, T., Astrophys. J.(2004), submitted].

Heavy element synthesis through disk winds in a collapsar

We investigate heavy element synthesis through thermally driven winds launched from an inner neutron-rich region of an accretion disk in a collapsar. Various heavy elements are found to be produced through the wind. There exist three types of winds characterized by their abundant nuclei: (i) 56 Ni, (ii) light ( Z = 30 − 45 ) neutron-rich nuclei, and (iii) light ( Z ⩽ 44 ) p-nuclei. We find that Y e , NSE > 0.49 , Y e , NSE ⩽ 0.45 , and 0.45 Y e , NSE ⩽ 0.49 for winds of types (i), (ii), and (iii), respectively, where Y e , NSE is the electron fraction of the wind at T = 9 × 10 9 K . The collasar winds may be a lighter element primary process, suggested from Galactic evolution of Sr, Y, and Zr. The p-nuclei, 92 Mo and 94 Mo, which are underproduced in oxygen layers in a supernova, can be abundantly synthesized in the collapsar winds.

Nuclear structure at the dripline

The neutron dripline has been reached for the time being for the lightest nuclei up to the element oxygen only. In this region of light neutron-rich nuclei, scattering experiments are feasible even for dripline nuclei by utilizing high-energy secondary beams produced by fragmentation. In the present article, reactions of high-energy radioactive beams will be exemplified discussing recent experimental results derived from measurements of breakup reactions performed at the LAND and FRS facilities at GSI.

Tensor correlation in neutron halo nuclei

We investigate the effect of the tensor correlation on the light neutron-rich nuclei. We extend the model space of 4He core with the shell model approach to incorporate the tensor correlation, and π-like 0− particle-hole correlation is strongly favored. This affects the LS splitting in 5He and the 0+2 state of 6He. We also investigate that the tensor correlation is suppressed in 11Li for p2 configuration of last two neutrons, which naturally mixes the s2 component in the ground state and produces the halo structure.

Exotic cluster structure in light neutron-rich nuclei

The cluster structure of C isotopes is investigated using a microscopic α+α+α+n+n… model based on the molecular orbit (MO) model. The stability of the linear chain of 3α with respect to the breathing mode and the bending mode for various neutron configurations is investigated. The combination of the valence neutrons in the π- and σ-orbits is promising to stabilize for these modes, and the excited states of 16C with the (3/2π-)2(1/2σ-)2 configuration for the four valence neutrons are one of the most promising candidates for such structure. Furthermore, the equilateral-triangular shape of 3α surrounded by valence neutrons is suggested for 14C. The 3-, 4-, and 5- members of this rotational band appear around the 10Be+α threshold, and these calculated states correspond to the experimentally observed 3- state (9.80 MeV) and 4- state (11.67 MeV). A positive-parity rotational band (0+, 2+, 4+) also arises around this threshold energy, and these results suggest that the picture of inversion doublet structure works also in neutron-rich nuclei.

Measurement of nuclear moments in the region of light neutron-rich nuclei

Measurement of nuclear moments in the region of light neutron-rich nuclei

One-neutron removal reactions on light neutron-rich nuclei

A study of high energy (43--68 MeV/nucleon) one-neutron removal reactions on a range of neutron-rich psd-shell nuclei (Z = 5--9, A = 12--25) has been undertaken. The inclusive longitudinal and transverse momentum distributions for the core fragments, together with the cross sections have been measured for breakup on a carbon target. Momentum distributions for reactions on tantalum were also measured for a subset of nuclei. An extended version of the Glauber model incorporating second order noneikonal corrections to the JLM parametrisation of the optical potential has been used to describe the nuclear breakup, whilst the Coulomb dissociation is treated within first order perturbation theory. The projectile structure has been taken into account via shell model calculations employing the psd-interaction of Warburton and Brown. Both the longitudinal and transverse momentum distributions, together with the integrated cross sections were well reproduced by these calculations and spin-parity assignments are thus proposed for $^{15}$B, $^{17}$C, $^{19-21}$N, $^{21,23}$O, $^{23-25}$F. In addition to the large spectroscopic amplitudes for the $\nu2$s$_{1/2}$ intruder configuration in the N=9 isotones,$^{14}$B and $^{15}$C, significant $\nu2$s$_{1/2}^2$ admixtures appear to occur in the ground state of the neighbouring N=10 nuclei $^{15}$B and $^{16}$C. Similarly, crossing the N=14 subshell, the occupation of the $\nu2$s$_{1/2}$ orbital is observed for $^{23}$O, $^{24,25}$F. Analysis of the longitudinal and transverse momentum distributions reveals that both carry spectroscopic information, often of a complementary nature. The general utility of high energy nucleon removal reactions as a spectroscopic tool is also examined.

Studies of the behaviour of shell closures far from stability

The structure of light neutron-rich nuclei around N=14,16,20,28 has been investigated at GANIL. In-beam gamma spectroscopy using fragmentation reactions of36S and48Ca beams on a Be target has allowed to obtain interesting results. γ-decay of excited states have been measured for the first time in nuclei around44S. Level schemes are discussed for a large number of these neutron-rich nuclei. The PARRNe2 set-up installed at IPN-Orsay allowed the use of the Tandem as a deuteron driver in order to produce mass separated fission fragment beams. The measured fission fragment yields proved to be high enough to undertake a series of β-decay experiments dedicated at studying the structure of neutron-rich nuclei close to N=50.

Studies of the behaviour of shell closures far from stability

The structure of light neutron-rich nuclei around N=14,16,20,28 has been investigated at GANIL. In-beam gamma spectroscopy using fragmentation reactions of 36S and 48Ca beams on a Be target has allowed to obtain interesting results. γ-decay of excited states have been measured for the first time in nuclei around 44S. Level schemes are discussed for a large number of these neutron-rich nuclei. The PARRNe2 set-up installed at IPN-Orsay allowed the use of the Tandem as a deuteron driver in order to produce mass separated fission fragment beams. The measured fission fragment yields proved to be high enough to undertake a series of β-decay experiments dedicated at studying the structure of neutron-rich nuclei close to N=50.

Dipole excitations of neutron-proton asymmetric nuclei

Dipole excitations of unstable short-lived nuclei has been investigated experimentally by utilizing the electromagnetic-excitation process with high-energy secondary beams. From an exclusive measurement of the neutron-decay channels, differential cross sections with respect to excitation energy, which are directly related to the photo-absorption cross section and accordingly to the dipole-strength function, have been derived. Light neutron-rich nuclei in the mass range fromA = 11 toA = 23 with mass-over-charge ratios up toA/Z≈ 2.8 have been investigated systematically. Much in contrast to stable nuclei, low-lying dipole excitations well below the giant dipole resonance region have been observed as a general phenomenon for these neutron-proton asymmetric nuclei. For the neutron-rich oxygen isotopes, for instance, low-lyingE1 strength has been observed exhausting about 10% of the classical dipole sum rule below 15 MeV excitation energy. A quantitative analysis of low-lying threshold strength for loosely bound nuclei indicates that the characteristics of the dipole strength is directly related to the ground-state single-particle structure of the valence nucleon in the projectile.

Dipole excitations of neutron-proton asymmetric nuclei

Dipole excitations of unstable short-lived nuclei has been investigated experimentally by utilizing the electromagnetic-excitation process with high-energy secondary beams. From an exclusive measurement of the neutron-decay channels, differential cross sections with respect to excitation energy, which are directly related to the photo-absorption cross section and accordingly to the dipole-strength function, have been derived. Light neutron-rich nuclei in the mass range fromA = 11 toA = 23 with mass-over-charge ratios up toA/Z≈ 2.8 have been investigated systematically. Much in contrast to stable nuclei, low-lying dipole excitations well below the giant dipole resonance region have been observed as a general phenomenon for these neutron-proton asymmetric nuclei. For the neutron-rich oxygen isotopes, for instance, low-lyingE1 strength has been observed exhausting about 10% of the classical dipole sum rule below 15 MeV excitation energy. A quantitative analysis of low-lying threshold strength for loosely bound nuclei indicates that the characteristics of the dipole strength is directly related to the ground-state single-particle structure of the valence nucleon in the projectile.

Dipole excitations of neutron-proton asymmetric nuclei

Dipole excitations of unstable short-lived nuclei has been investigated experimentally by utilizing the electromagnetic-excitation process with high-energy secondary beams. From an exclusive measurement of the neutron-decay channels, differential cross sections with respect to excitation energy, which are directly related to the photo-absorption cross section and accordingly to the dipole-strength function, have been derived. Light neutron-rich nuclei in the mass range from A = 11 to A = 23 with mass-over-charge ratios up to A/Z≈ 2.8 have been investigated systematically. Much in contrast to stable nuclei, low-lying dipole excitations well below the giant dipole resonance region have been observed as a general phenomenon for these neutron-proton asymmetric nuclei. For the neutron-rich oxygen isotopes, for instance, low-lying E1 strength has been observed exhausting about 10% of the classical dipole sum rule below 15 MeV excitation energy. A quantitative analysis of low-lying threshold strength for loosely bound nuclei indicates that the characteristics of the dipole strength is directly related to the ground-state single-particle structure of the valence nucleon in the projectile.

Nuclear Clusters and Covalently Bound Nuclear Molecules

Nuclear clustering in N = Z nuclei has been studied for many decades. Of particular relevance are states close to the decay thresholds, as described by the Ikeda diagram. Recent interest has focused on loosely bound systems as observed with exotic nuclei. Extreme deformations are simultaneously observed. In the deformed shell model these are referred to as super- and hyper-deformation. Another feature related to clustering is the development of octupole deformations. A possible approach to describe these states is to use explicitly molecular concepts, with neutrons in covalent binding orbits. Examples for molecular structure in beryllium isotopes and in other neutron-rich light nuclei (carbon and neon) are now well established. The predicted chain states in the carbon isotopes are the first example of structures with an axis ratio of 3:1. A threshold diagram for molecular configurations in nuclei with clusters and covalent neutrons can be established.

Exotic Cluster Structure in Light Neutron-Rich Nuclei

The cluster structure of C isotopes is investigated using a microscopic α+α+α+n+n… model based on the molecular orbit (MO) model. The stability of the linear chain of 3α with respect to the breathing mode and the bending mode for various neutron configurations is investigated. The combination of the valence neutrons in the π- and σ-orbits is promising to stabilize for these modes, and the excited states of 16C with the (3/2 π - )2(1/2 σ - )2 configuration for the four valence neutrons are one of the most promising candidates for such structure. Furthermore, the equilateral-triangular shape of 3α surrounded by valence neutrons is suggested for 14C. The 3-, 4-, and 5- members of this rotational band appear around the 10Be+α threshold, and these calculated states correspond to the experimentally observed 3- state (9.80 MeV) and 4- state (11.67 MeV). A positive-parity rotational band (0+, 2+, 4+) also arises around this threshold energy, and these results suggest that the picture of inversion doublet structure works also in neutron-rich nuclei.

Clusters and covalently bound nuclear molecules

Nuclear clustering based on α particles and strongly bound substructures with N=Z has been studied for many decades. Of particular interest are excited states close to the decay thresholds into substructures, as described by the Ikeda diagram. This diagram can be extended to neutron-rich nuclei; in these cases strongly deformed isomeric states consisting of clusters and loosely bound neutrons will appear. A possible approach to describe these states is to use explicitly molecular concepts, with neutrons in covalent binding orbits. Examples for molecular structure in beryllium isotopes and in other neutron-rich light nuclei (carbon and neon) are discussed.

Shell model study on E2 effective charges in light neutron-rich nuclei

Theoretical values of quadrupole moments of neutron-rich nuclei are larger than experimental values if we use E2 effective charges of stable nuclei. We consider that the nucleon excitations to outside model space decrease with neutron excess. Using the shell model, we investigate quadrupole moments and effective charges in the light mass region, from stable nuclei to neutron-rich nuclei.

Probing the halo structure of exotic nuclei by direct reactions with radioactive beams

The investigation of light-ion induced direct reactions using radioactive beams in inverse kinematics gives access to a wide field of nuclear structure studies in the region far off stability. The present contribution is mainly focused on the investigation of light neutron-rich halo nuclei. The method of proton-nucleus elastic scattering at intermediate energies is demonstrated to be an effective means for obtaining accurate and detailed information on the size and radial shape of halo nuclei. Differential cross sections for small-angle scattering were measured at energies near 700 MeV/u for the neutron-rich lithium isotopes 8Li, 9Li and 11Li. The experimental concept and the procedure of the data analysis in terms of the Glauber multiple scattering theory are discussed and the results on the nuclear matter radii, the matter distributions, and the significance of the data for a halo structure are presented. In a very recent experiment data on p 6,8He scattering at higher momentum transfer, covering the region of the first diffraction minimum, were taken. Such data are expected to give more detailed information on the structure of the α-like core in these halo nuclei. First results are presented.

Importance of clustering in light neutron-rich nuclei

We discuss two topics about the cluster structure in light neutron-rich nuclei with the astrophysical interests. One is 9Be: using a microscopic a + a + n model, the photodisintegration cross section is analyzed. It is shown that good agreement is achieved with the recently measured B(El) transition probability from the ground 3 2 − state to the first excited state ( 1 2 + ), which is 2 times larger than the previous measurement. The other topic is the cluster-shell mixing in 12C, which is crucial to explain the B(E2) value from the second 0 + state to the first. 2 + state in triple-α reaction.