Neutron Drip Research Articles

Global performance of covariant energy density functionals: Ground state observables of even-even nuclei and the estimate of theoretical uncertainties

Covariant density functional theory is a modern theoretical tool for the description of nuclear structure phenomena. The current investigation aims at the global assessment of the accuracy of the description of the ground state properties of even-even nuclei. We also estimate theoretical uncertainties defined here as the spreads of predictions within four covariant energy density functionals in known regions of the nuclear chart and their propagation towards the neutron drip line. Large-scale axial relativistic Hartree-Bogoliubov calculations are performed for all $Z\ensuremath{\le}104$ even-even nuclei between the two-proton and two-neutron drip lines with four modern covariant energy density functionals such as NL3*, DD-ME2, DD-ME$\ensuremath{\delta}$, and DD-PC1. The physical observables of interest include the binding energies, two-particle separation energies, charge quadrupole deformations, isovector deformations, charge radii, neutron skin thicknesses, and the positions of the two-proton and two-neutron drip lines. The predictions for the two-neutron drip line are also compared in a systematic way with the ones obtained in nonrelativistic models.

Surface diffuseness correction in global mass formula

By taking into account the surface diffuseness correction for unstable nuclei, the accuracy of the macroscopic–microscopic mass formula is further improved. The rms deviation with respect to essentially all the available mass data falls to 298 keV, crossing the 0.3 MeV accuracy threshold for the first time within the mean-field framework. Considering the surface effect of the symmetry potential which plays an important role in the evolution of the “neutron skin” toward the “neutron halo” of nuclei approaching the neutron drip line, we obtain an optimal value of the symmetry energy coefficient J=30.16 MeV. With an accuracy of 258 keV for all the available neutron separation energies and of 237 keV for the α-decay Q-values of super-heavy nuclei, the proposed mass formula is particularly important not only for the reliable description of the r process of nucleosynthesis but also for the study of the synthesis of super-heavy nuclei.

Single-particle neutron characteristics of Cd isotopes with N of 50 to 82

Experimental data on single-particle neutron energies of Cd isotopes are analyzed within a dispersive optical model. Parameters of the potential are extrapolated to the region of unstable isotopes with a neutron excess. The evolution of calculated single-particle spectra and occupation probabilities of single-particle orbits corresponds to formation of magic features in Cd isotopes with N = 50, 82. The results from calculations agree with the concept that the 174Cd isotope with N = 126 forms the end of the peninsula at the neutron drip line.

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.

Dark-matter admixed white dwarfs

We study the equilibrium structures of white dwarfs with dark matter cores formed by non-self-annihilating dark matter (DM) particles with masses ranging from 1 GeV to 100 GeV, which are assumed to form an ideal degenerate Fermi gas inside the stars. For DM particles of mass 10 GeV and 100 GeV, we find that stable stellar models exist only if the mass of the DM core inside the star is less than $O({10}^{\ensuremath{-}3}){M}_{\ensuremath{\bigodot}}$ and $O({10}^{\ensuremath{-}6}){M}_{\ensuremath{\bigodot}}$, respectively. The global properties of these stars, and in particular the corresponding Chandrasekhar mass limits, are essentially the same as those of traditional white dwarf models without DM. Nevertheless, in the 10 GeV case, the gravitational attraction of the DM core is strong enough to squeeze the normal matter in the core region to densities above neutron drip, far above those in traditional white dwarfs. For DM with a particle mass of 1 GeV, the DM core inside the star can be as massive as $\ensuremath{\sim}0.1{M}_{\ensuremath{\bigodot}}$ and affects the global structure of the star significantly. In this case, the radius of a stellar model with DM can be about two times smaller than that of a traditional white dwarf. Furthermore, the Chandrasekhar mass limit can also be decreased by as much as 40%. Our results may have implications on the extent to which type Ia supernovae can be regarded as standard candles\char22{}a key assumption in the discovery of dark energy.

Beyond the Neutron Drip-Line

One of the major challenge in experimental and theoretical nuclear physics is the understanding of the properties of nuclei with extreme neutron-to-proton ratios. The most extreme asymmetric nuclei are located at and beyond the neutron drip-line. The first question arising here is the location of the drip-line itself, i.e., where are the limits of nuclear stability. Experimentally, this question is answered today only for the lightest elements up to oxygen [1] with 24O (Z = 8) being the heaviest bound isotope. For fluorine (Z = 9), the drip-line lies considerably further out, with the heaviest bound isotope observed experimentally being 31F [2]. Figure 1 gives an overview on the nuclear chart for the light nuclides for elements up to the fluorine isotopes. Besides the stable (black) and bound (red and blue) isotopes, unbound nuclei that have been observed experimentally as resonance states are indicated as well by the green fields. The apparent anomalous behavior of the location of the drip-line for the oxygen isotopes has been recently explained by the impact of three-nucleon (3N) forces, which contribute repulsively to the interaction among the valence neutrons [3, 4].

Production cross section of neutron-rich isotopes with radioactive and stable beams

The production cross section of neutron-rich isotopes of Ca, Zn, Te, Xe, and Pt are predicted in the diffusive multi-nucleon transfer reactions with stable and radioactive beams. With these isotopes one can treat the neutron shell evolution beyond N = 28, 50, 82, and 126. Because of the small cross sections, the production of nuclei near the neutron drip line requires the optimal choice of reaction partners and bombarding energies.

Symplectic no-core shell-model approach to intermediate-mass nuclei

We present a microscopic description of nuclei in the intermediate-mass region, including the proximity to the proton drip line, based on a no-core shell model with a schematic many-nucleon long-range interaction with no parameter adjustments. The outcome confirms the essential role played by the symplectic symmetry to inform the interaction and the winnowing of shell-model spaces. We show that it is imperative that model spaces be expanded well beyond the current limits up through 15 major shells to accommodate particle excitations, which appear critical to highly deformed spatial structures and the convergence of associated observables.

Near-barrier fusion of proton rich systems

Recent near-barrier measurements for fusion of the proton-halo system 8B + 58Ni (Aguilera E F et al. 2011 Pys. Rev. Lett. 107 092701) showed unexpected results. In contrast to previous results for neutron-halo systems, for instance, the respective fusion cross sections are enhanced for both, below- and above-barrier energies. The charged nature of the halo could thus have an effect on the fusion mechanism. Within this context, in the present work the fusion cross sections for several proton rich systems are compared to each other. Three of these systems correspond to the same target, 58Ni (with projectiles of 3He, 7Be, and 8B), but data for 17F + 208Pb also are included. All these projectiles lie near the proton drip line. An appropriate scaling of the data is made to do the comparison and the results are discussed.

Ab Initio Nuclear Structure Theory: From Few to Many

We summarize recent advances in ab initio nuclear structure theory, aiming to connect few- and many-body systems in a coherent theoretical framework. Starting from chiral effective field theory to construct the nuclear Hamiltonian and the similarity renormalization group to soften it, we address several many-body approaches that have seen major developments over the past few years. We show that the domain of ab initio nuclear structure theory has been pushed well beyond the p-shell and that quantitative predictions connected to QCD via chiral effective field theory are becoming possible all the way from the proton to the neutron drip line up into the medium-mass regime.

Blurring the Boundaries: Decays of Multiparticle Isomers at the Proton Drip Line

A multiparticle spin-trap isomer has been discovered in the proton-unbound nucleus (73)(158)Ta85 . The isomer mainly decays by γ-ray emission with a half-life of 6.1(1) μs. Analysis of the γ-ray data shows that the isomer lies 2668 keV above the known 9+ state and has a spin 10ℏ higher and negative parity. This 19- isomer also has an 8644(11) keV, 1.4(2)% α-decay branch that populates the 9+ state in (154)Lu. No proton-decay branch from the isomer was identified, despite the isomer being unbound to proton emission by 3261(14) keV. This remarkable stability against proton emission is compared with theoretical predictions, and the implications for the extent of observable nuclides are considered.

Nuclear territory predicted with a newly developed hyperon-nucleon interaction

The boundary of the nuclear chart is studied in the relativistic continuum Hartree-Bogoliubov theory with a newly developed effective \( \Lambda\)-nucleon interaction PK1-Y1. Compared with the linear prediction from the generalized mass formula, the neutron drip line is shifted outwards in light single-\( \Lambda\) hypernuclei and exhibits a pronounced shell structure in the heavy-mass region. The existence of a neutron magic number at \( N = 184\) is suggested. Systematic trend in the heavier region shows a very weak \( \Lambda\Lambda\) interaction. With \( \sigma^{*}\) , \( \phi\) mesons and \( \omega\Lambda\Lambda\) tensor coupling, the bond energy of Lambdas is sharply reduced.

A nuclear physics program at the Rare Isotope Beams Accelerator Facility in Korea

This paper outlines the new physics possibilities that fall within the field of nuclear structure and astrophysics based on experiments with radioactive ion beams at the future Rare Isotope Beams Accelerator facility in Korea. This ambitious multi-beam facility has both an Isotope Separation On Line (ISOL) and fragmentation capability to produce rare isotopes beams (RIBs) and will be capable of producing and accelerating beams of wide range mass of nuclides with energies of a few to hundreds MeV per nucleon. The large dynamic range of reaccelerated RIBs will allow the optimization in each nuclear reaction case with respect to cross section and channel opening. The low energy RIBs around Coulomb barrier offer nuclear reactions such as elastic resonance scatterings, one or two particle transfers, Coulomb multiple-excitations, fusion-evaporations, and direct capture reactions for the study of the very neutron-rich and proton-rich nuclides. In contrast, the high energy RIBs produced by in-flight fragmentation with reaccelerated ions from the ISOL enable to explore the study of neutron drip lines in intermediate mass regions. The proposed studies aim at investigating the exotic nuclei near and beyond the nucleon drip lines, and to explore how nuclear many-body systems change in such extreme regions by addressing the following topics: the evolution of shell structure in areas of extreme proton to neutron imbalance; the study of the weak interaction in exotic decay schemes such as beta-delayed two-neutron or two-proton emission; the change of isospin symmetry in isobaric mirror nuclei at the drip lines; two protons or two neutrons radioactivity beyond the drip lines; the role of the continuum states including resonant states above the particle-decay threshold in exotic nuclei; and the effects of nuclear reaction rates triggered by the unbound proton-rich nuclei on nuclear astrophysical processes.

Exotic decay of hot rotating nuclei near proton drip line

Hot and rotating exotic 124Ce nucleus near proton drip line has been populated through fusion evaporation reaction of 32S and 92Mo. This exotic nucleus was de-excited by evaporating p, n, α and/or light nuclei etc and several exotic nuclei have been populated. The experimentally obtained relative population of those exotic nuclei have been compared with the statistical model calculation. Agreement between experimental and statistical model calculation have been observed for most of the evaporation channels. Huge enhancement in comparison to statistical model calculation have been observed fora few channels related to multiple proton evaporation which could not be explained by using default and modified input parameters in statistical calculation.

Isoscalar giant monopole resonance for drip-line and super heavy nuclei in the framework of relativistic mean field formalism with scaling calculation

AbstractWe study the isoscalar giant monopole resonance for drip-lines and super heavy nuclei in the framework of relativistic mean field theory with a scaling approach. The well known extended Thomas-Fermi approximation in the nonlinear σ-ω model is used to estimate the giant monopole excitation energy for some selected light spherical nuclei starting from the region of proton to neutron drip-lines. The application is extended to the super heavy region for Z=114 and 120, which are predicted by several models as the next proton magic numbers beyond Z=82. We compared the excitation energy obtained by four successful force parameters NL1, NL3, NL3*, and FSUGold. The monopole energy decreases toward the proton and neutron drip-lines in an isotopic chain for lighter mass nuclei, in contrast to a monotonic decrease for super heavy isotopes. The maximum and minimum monopole excitation energies are obtained for nuclei with minimum and maximum isospin in an isotopic chain, respectively.

Superdeformed structures and low Ω parity doublet in Ne-S nuclei near neutron drip-line

AbstractThe structures of Ne, Na, Mg, Al, Si, P and S nuclei near the neutron drip-line region are investigated in the frame-work of relativistic mean field theory and non-relativistic Skyrme Hartree-Fock formalism. The recently discovered nuclei 40Mg and 42Al, which are beyond the drip-line predicted by various mass formulae are located within these models. We find many largely deformed neutron-rich nuclei, whose structures are analyzed. From the structure anatomy, we find that at large deformation low orbits of opposite parities (e.g. $$\frac{1} {2}^ +$$ and $$\frac{1} {2}^ -$$) occur close to each other in energy.

Towards New Horizons in Ab Initio Nuclear Structure Theory

We present a cross-section of recent advances in ab initio nuclear structure theory, which have changed the horizons of this field. Starting from chiral effective field theory to construct the nuclear Hamiltonian and the similarity renormalization group to further soften it, we address several many-body approaches that have seen major developments over the past few years. We show that the domain of ab initio nuclear structure theory has been pushed well beyond the p-shell and that quantitative QCD-based predictions are becoming possible all the way from the proton to the neutron drip line up into the medium-mass regime.

Search for halo nucleus in Mg isotopes through the measurements of reaction cross sections towards the vicinity of neutron drip line

Reaction cross sections ( R) for 24 38 Mg on C targets at the energies of around 240 MeV/ nucleon have been measured precisely at RIBF, RIKEN for the purpose of obtaining the crucial information on the changes of nuclear structure in unstable nuclei, especially around the so-called of region. In the island of inversion region, which includes neutron-rich Ne, Na, and Mg isotopes, the vanishing of the N = 20 magic number for neutrons have been discussed along with nuclear deformation. The present result suggest deformation features of Mg isotopes and shows a large cross section of weakly-bound nucleus 37 Mg, which could be caused by a neutron halo formation.

Measurements of interaction cross sections for22–35Na isotopes

Interaction cross sections ( I) for 22 35 Na isotopes from the stability line to the vicinity of the neutron drip line have been measured at around 240 MeV/ nucleon. The I for 33 35 Na were measured for the first time. Enhancement in cross sections is clearly observed from the systematics for stable nuclei, for isotopes with large mass numbers. From the known values of the nuclear-deformation parameters 2 of 22 31 Na, these en- hancement can be mainly ascribed to the nuclear deformation. Large enhancement in heavier isotopes suggest that these nuclei are strongly deformed. The root-mean-square (RMS) nuclear matter radii were deduced from the I by using Glauber-type calculation. Furthermore, a monotonic growth of the neutron-skin thickness has been deduced with increasing neutron number for Na isotopes.

Separation Between $d_{5/2}$ and $s_{1/2}$ Neutron Single Particle Strength in $^{15}$N

The separation between single particle levels in nuclei plays the dominant role in determining the location of the neutron drip line. The separation also provides a test of current crossed shell model interactions if the experimental data is such that multiple shells are involved. The present work uses the $^{14}$N(d, p)$^{15}$N reaction to extract the 2s$_{1/2}$, and 1d$_{5/2}$ total neutron single particle strengths and then compares these results with a shell model calculation using a p-sd crossed shell interaction to identify the J$^\pi$ of all levels in $^{15}$N up to 12.8 MeV in excitation.