Halo structure of the island of inversion nucleus 31Ne

Magicity versus Superfluidity around ^{28}O viewed from the Study of ^{30}F

The neutron-rich unbound fluorine isotope ^{30}F_{21} has been observed for the first time by measuring its neutron decay at the SAMURAI spectrometer (RIBF, RIKEN) in the quasifree proton knockout reaction of ^{31}Ne nuclei at 235  MeV/nucleon. The mass and thus one-neutron-separation energy of ^{30}F has been determined to be S_{n}=-472±58(stat)±33(sys)  keV from the measurement of its invariant-mass spectrum. The absence of a sharp drop in S_{n}(^{30}F) shows that the "magic" N=20 shell gap is not restored close to ^{28}O, which is in agreement with our shell-model calculations that predict a near degeneracy between the neutron d and fp orbitals, with the 1p_{3/2} and 1p_{1/2} orbitals becoming more bound than the 0f_{7/2} one. This degeneracy and reordering of orbitals has two potential consequences: ^{28}O behaves like a strongly superfluid nucleus with neutron pairs scattering across shells, and both ^{29,31}F appear to be good two-neutron halo-nucleus candidates.

Discovery of ^{39}Na

The new isotope ^{39}Na, the most neutron-rich sodium nucleus observed so far, was discovered at the RIKEN Nishina Center Radioactive Isotope Beam Factory using the projectile fragmentation of an intense ^{48}Ca beam at 345  MeV/nucleon on a beryllium target. Projectile fragments were separated and identified in flight with the large-acceptance two-stage separator BigRIPS. Nine ^{39}Na events have been unambiguously observed in this work and clearly establish the particle stability of ^{39}Na. Furthermore, the lack of observation of ^{35,36}Ne isotopes in this experiment significantly improves the overall confidence that ^{34}Ne is the neutron dripline nucleus of neon. These results provide new key information to understand nuclear binding and nuclear structure under extremely neutron-rich conditions. The newly established stability of ^{39}Na has a significant impact on nuclear models and theories predicting the neutron dripline and also provides a key to understanding the nuclear shell property of ^{39}Na at the neutron number N=28, which is normally a magic number.

Studies of Deformed Halo Structures of 39Na and 42Mg

Background: The recent experimental discovery of drip-line nucleus 39Na has attracted great interest in theoretical studies of exotic nuclear structures in this mass region. Methods: We solve the Skyrme–Hartree–Fock–Bogoliubov (Skyrme-HFB) equation within deformed coordinate-spaces. The present approach is suitable for descriptions of weakly bound deformed nuclei with continuum effects and deformed halo structures. Results: The systematical two-neutron separation energies are obtained with the SkMext1* and UNEDF0ext1 forces for Na and Mg isotopes close to the neutron drip line. The density distributions show that 39Na and 42Mg have deformed halo structures. Furthermore, there are significant influences of various pairing interactions on halo shapes at large distances. Conclusions: Both 39Na and 42Mg are very weakly bound with well prolate deformed cores. However, their surface halo structures are dependent on the choices of pairing interactions. The volume-type pairing interaction tends to predict a prolate deformed halo, while the halo deformations at large distances are reduced by adopting the surface pairing. We demonstrate that 39Na and 42Mg are promising candidates for two-neutron deformed halo nuclei.

Cross-Shell Excitation in F and Ne Isotopes around N = 20

Within the framework of the configuration–interaction shell model, the present work applies three effective interactions to investigate the effects of the cross-shell excitation on F and Ne isotopes around N = 20, which are significantly proton–neutron asymmetric, and have different properties compared with the proton–neutron symmetric nuclei. It is shown that cross-shell excitation is necessary in order to reproduce separation energies, neutron drip lines, and low-energy levels of these isotopes. Furthermore, the cross-shell excitation of (0–5)ħω is suggested to be important in the description of 29F and 30Ne. However, the three interactions are insufficient in describing the bound structure of 29,31Ne, and provide inconsistent shell structures and evolutions in the target nuclei. Their cross-shell interactions are suggested to be improved.

Rotating deformed halo nuclei and shape decoupling effects

To explore the rotational excitation of deformed halo nuclei, the angular momentum projection (AMP) has been implemented in the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc), in which both the mean field and collective wave functions are expanded in terms of Dirac Woods-Saxon basis. The DRHBc + AMP approach self-consistently describes the coupling between single particle bound states and the continuum not only in the ground state but also in rotational states. The rotational modes of deformed halos in ^42,44Mg are investigated by studying properties of rotational states such as the excitation energy, configuration, and density distribution. Our study demonstrates that the deformed halo structure persists from the ground state in the intrinsic frame to collective states. Especially, the typical behavior of shape decoupling effects in rotating deformed halo nuclei is revealed.

The impact of nuclear shape on the emergence of the neutron dripline

Atomic nuclei are composed of a certain number of protons Z and neutrons N. A natural question is how large Z and N can be. The study of superheavy elements explores the large Z limit^1,2, and we are still looking for a comprehensive theoretical explanation of the largest possible N for a given Z-the existence limit for the neutron-rich isotopes of a given atomic species, known as the neutron dripline³. The neutron dripline of oxygen (Z = 8) can be understood theoretically as the result of single nucleons filling single-particle orbits confined by a mean potential, and experiments confirm this interpretation. However, recent experiments on heavier elements are at odds with this description. Here we show that the neutron dripline from fluorine (Z = 9) to magnesium (Z = 12) can be predicted using a mechanism that goes beyond the single-particle picture: as the number of neutrons increases, the nuclear shape assumes an increasingly ellipsoidal deformation, leading to a higher binding energy. The saturation of this effect (when the nucleus cannot be further deformed) yields the neutron dripline: beyond this maximum N, the isotope is unbound and further neutrons 'drip' out when added. Our calculations are based on a recently developed effective nucleon-nucleon interaction⁴, for which large-scale eigenvalue problems are solved using configuration-interaction simulations. The results obtained show good agreement with experiments, even for excitation energies of low-lying states, up to the nucleus of magnesium-40 (which has 28 neutrons). The proposed mechanism for the formation of the neutron dripline has the potential to stimulate further thinking in the field towards explaining nucleosynthesis with neutron-rich nuclei.

Location of the Neutron Dripline at Fluorine and Neon

A search for the heaviest isotopes of fluorine, neon, and sodium was conducted by fragmentation of an intense ^{48}Ca beam at 345  MeV/nucleon with a 20-mm-thick beryllium target and identification of isotopes in the large-acceptance separator BigRIPS at the RIKEN Radioactive Isotope Beam Factory. No events were observed for ^{32,33}F, ^{35,36}Ne, and ^{38}Na and only one event for ^{39}Na after extensive running. Comparison with predicted yields excludes the existence of bound states of these unobserved isotopes with high confidence levels. The present work indicates that ^{31}F and ^{34}Ne are the heaviest bound isotopes of fluorine and neon, respectively. The neutron dripline has thus been experimentally confirmed up to neon for the first time since ^{24}O was confirmed to be the dripline nucleus nearly 20 years ago. These data provide new keys to understanding the nuclear stability at extremely neutron-rich conditions.

Observation of a p-wave one-neutron halo configuration in (37)Mg

Cross sections of 1n-removal reactions from the neutron-rich nucleus (37)Mg on C and Pb targets and the parallel momentum distributions of the (37)Mg residues from the C target have been measured at 240  MeV/nucleon. A combined analysis of these distinct nuclear- and Coulomb-dominated reaction data shows that the (37)Mg ground state has a small 1n separation energy of 0.22(-0.09)(+0.12)  MeV and an appreciable p-wave neutron single-particle strength. These results confirm that (37)Mg lies near the edge of the "island of inversion" and has a sizable p-wave neutron halo component, the heaviest such system identified to date.

Deformation-driven p-wave halos at the drip line: 31Ne

The halo structure of 31Ne is studied using 1n-removal reactions on C and Pb targets at 230  MeV/nucleon. A combined analysis of the cross sections of these nuclear and Coulomb dominated reactions that feed directly the 30Ne ground-state reveals 31Ne to have a small neutron separation energy, 0.15(-0.10)(+0.16)  MeV, and spin-parity 3/2-. Consistency of the data with reaction and large-scale shell-model calculations identifies 31Ne as deformed and having a significant p-wave halo component, suggesting that halos are more frequent occurrences at the neutron drip line.

Interplay between valence and core excitation mechanisms in the breakup of halo nuclei

The phenomenon of core excitation in the breakup of a two-body halo nucleus is investigated. We show that this effect plays a significant role in the reaction dynamics and, furthermore, its interference with the valence excitation mechanism has sizable and measurable effects on the breakup angular distributions. These effects have been studied in the resonant breakup of (11)Be on a carbon target, populating the resonances at 1.78 MeV (5/2(+)) and 3.41 MeV (3/2(+)). The calculations have been performed using a recent extension of the distorted-wave Born approximation method, which takes into account the effect of core excitation in both the structure of the halo nucleus and in the reaction mechanism. The calculated angular distributions have been compared with the available data [Fukuda et al., Phys. Rev. C 70, 054606 (2004).]. Although each of these resonances is dominated by one of the two considered mechanisms, the angular patterns of these resonances depend in a very delicate way on the interference between them. This is the first clear evidence of this effect but the phenomenon is likely to occur in other similar reactions.

Direct mass measurements of 19B, 22C, 29F, 31Ne, 34Na and other light exotic nuclei

We report on direct time-of-flight based mass measurements of 16 light neutron-rich nuclei. These include the first determination of the masses of the Borromean drip-line nuclei (19)B, (22)C, and (29)F as well as that of (34)Na. In addition, the most precise determinations to date for (23)N and (31)Ne are reported. Coupled with recent interaction cross-section measurements, the present results support the occurrence of a two-neutron halo in (22)C, with a dominant ν2s(1/2)(2) configuration, and a single-neutron halo in (31)Ne with the valence neutron occupying predominantly the 2p(3/2) orbital. Despite a very low two-neutron separation energy the development of a halo in (19)B is hindered by the 1d(5/2)(2) character of the valence neutrons.

Determination of the structure of {31}ne by a fully microscopic framework

We perform the first quantitative analysis of the reaction cross sections of {28-32}Ne by {12}C at 240  MeV/nucleon, using the double-folding model with the Melbourne g matrix and the deformed projectile density calculated by antisymmetrized molecular dynamics. To describe the tail of the last neutron of {31}Ne, we adopt the resonating group method combined with antisymmetrized molecular dynamics. The theoretical prediction excellently reproduces the measured cross sections of {28-32}Ne with no adjustable parameters. The ground state properties of {31}Ne, i.e., strong deformation and a halo structure with spin parity 3/2{-}, are clarified.