Role Of Triaxiality Research Articles

Influence of Triaxiality on the Description of Low-Energy Excitation Spectrum of 96Zr

The observed properties of the low-lying collective states of $^{96}$Zr are investigated within the geometrical collective model. The quadrupole-collective Bohr Hamiltonian with the potential having spherical and axially-symmetric deformed minima is applied. The role of triaxiality is investigated by rotating the potential in $\gamma$-degree of freedom so that the deformed minimum occurs at various axially asymmetric shapes. The change of excitation energies and reduced matrix elements of quadrupole transitions with increase of triaxiliaty is analyzed.

Role of triaxiality in deformed halo nuclei

It is known that nuclear deformation plays an important role in inducing the halo structure in neutron-rich nuclei by mixing several angular momentum components. While previous theoretical studies on this problem in the literature assume axially symmetric deformation, we here consider non-axially symmetric deformations. With triaxial deformation, the $\Omega$ quantum number is admixed in a single-particle wave function, where $\Omega$ is the projection of the single-particle angular momentum on the symmetric axis, and the halo structure may arise even when it is absent with the axially symmetric deformation. In this way, the area of halo nuclei may be extended when triaxial deformation is considered. We demonstrate this idea using a deformed Woods-Saxon potential for nuclei with neutron number N=13 and 43.

Role of triaxiality in the structure of chiral partner bands

The spectrum and wave-functions of chiral partner band states are obtained by treating semiclassically a particle-rotor Hamiltonian with rigid quasiparticle alignments and arbitrary triaxial deformation. The results for valence h11/2 proton particle and h11/2 neutron hole alignments are investigated as a function of total angular momentum and the magnitude of the triaxial deformation. It is shown that the chiral vibration persists in more angular momentum states with decreasing triaxial deformation. At sufficiently small triaxial deformation, the emergence of static chirality is observed even for low spin states which compete with the chiral vibration component.

Extension of the nuclear landscape to hyperheavy nuclei

The properties of hyperheavy nuclei and the extension of nuclear landscape to hyperheavy nuclei are extensively studied within covariant density functional theory. Axial reflection symmetric and reflection asymmetric relativistic Hartree-Bogoliubov (RHB) calculations are carried out. The role of triaxiality is studied within triaxial RHB and triaxial relativistic mean field + BCS frameworks. With increasing proton number beyond Z~130 the transition from ellipsoidal-like nuclear shapes to toroidal ones takes place. The description of latter shapes requires the basis which is typically significantly larger than the one employed for the description of ellipsoidal-like shapes. Many hyperheavy nuclei with toroidal shapes are expected to be unstable towards multifragmentation. However, three islands of stability of spherical hyperheavy nuclei have been predicted for the first time in Ref. [1]. Proton and neutron densities, charge radii, neutron skins and underlying shell structure of the nuclei located in the centers of these islands have been investigated in detail. Large neutron shell gaps at N=228, 308 and 406 define approximate centers of these islands in neutron number. On the contrary, large proton gap appear only at Z=154 in the (Z~156, N~310) island. As a result, this is the largest island of stability of spherical hyperheavy nuclei found in the calculations. The calculations indicate the stability of the nuclei in these islands with respect of octupole and triaxial distortions. Fission barriers in neutron-rich superheavy nuclei are studied in triaxial RHB framework; the impact of triaxiality on the heights of fission barriers is substantial in some parts of this region. Based on the results obtained in the present work, the extension of nuclear landscape to hyperheavy nuclei is provided.

Calculation of multidimensional potential energy surfaces for even-even transuranium nuclei: systematic investigation of the triaxiality effect on the fission barrier**Supported by National Natural Science Foundation of China (11675148, 11505157), the Project of Youth Backbone Teachers of Colleges and Universities of Henan Province (2017GGJS008), the Foundation and Advanced Technology Research Program of Henan Province (162300410222), the Outstanding Young Talent Research Fund

Static fission barriers for 95 even-even transuranium nuclei with charge number Z = 94–118 have been systematically investigated by means of pairing self-consistent Woods-Saxon-Strutinsky calculations using the potential energy surface approach in multidimensional (β2, γ, β4) deformation space. Taking the heavier 252Cf nucleus (with the available fission barrier from experiment) as an example, the formation of the fission barrier and the influence of macroscopic, shell and pairing correction energies on it are analyzed. The results of the present calculated β2 values and barrier heights are compared with previous calculations and available experiments. The role of triaxiality in the region of the first saddle is discussed. It is found that the second fission barrier is also considerably affected by the triaxial deformation degree of freedom in some nuclei (e.g., the Z=112–118 isotopes). Based on the potential energy curves, general trends of the evolution of the fission barrier heights and widths as a function of the nucleon numbers are investigated. In addition, the effects of Woods-Saxon potential parameter modifications (e.g., the strength of the spin-orbit coupling and the nuclear surface diffuseness) on the fission barrier are briefly discussed.

Microscopic analysis of shape transition in neutron-deficient Yb isotopes

The development of nuclear collectivity in even-even $^{152\text{--}170}\mathrm{Yb}$ is studied with three types of mean-field calculations: the nonrelativistic Hartree-Fock plus BCS calculation using the Skyrme SLy4 force plus a density-dependent $\ensuremath{\delta}$ pairing force and the relativistic mean-field calculation using a point-coupling energy functional supplemented with either a density-independent $\ensuremath{\delta}$ pairing force or a separable pairing force. The low-lying states are obtained by solving a five-dimensional collective Hamiltonian with parameters determined from the three mean-field solutions. The energy surfaces, excitation energies, electric multiple transition strengths, and differential isotope shifts are presented in comparison with available data. Our results show that different treatments of pairing correlations have a significant influence on the speed of developing collectivity as the increase of neutron number. All the calculations demonstrate the important role of dynamic shape-mixing effects in resolving the puzzle in the dramatic increase of charge radius from $^{152}\mathrm{Yb}$ to $^{154}\mathrm{Yb}$ and the role of triaxiality in $^{160,162,164}\mathrm{Yb}$.

Role of triaxiality in 76Ge and 76Se nuclei studied with Gogny energy density functionals

The structure of the nuclei 76Ge and 76Se is studied with symmetry conserving configuration mixing methods based on the Gogny D1S interaction. These two nuclei are of key importance in the search for neutrinoless double-beta decay. The energy density functionals used here include symmetry restorations (particle number and angular momentum) and shape mixing within the generator coordinate method. The comparison with the experimental data shows a good qualitative agreement when triaxial shapes are included, revealing the important role played by this degree of freedom in these two nuclei.

Superdeformed and Triaxial States in ^{42}Ca.

Shape parameters of a weakly deformed ground-state band and highly deformed slightly triaxial sideband in ^{42}Ca were determined from E2 matrix elements measured in the first low-energy Coulomb excitation experiment performed with AGATA. The picture of two coexisting structures is well reproduced by new state-of-the-art large-scale shell model and beyond-mean-field calculations. Experimental evidence for superdeformation of the band built on 0_{2}^{+} has been obtained and the role of triaxiality in the A∼40 mass region is discussed. Furthermore, the potential of Coulomb excitation as a tool to study superdeformation has been demonstrated for the first time.

Shape coexistence and evolution in neutron-deficient krypton isotopes**Supported by National Key Basic Research Program of China (2013CB83440) and National Natural Science Foundation of China (11235001, 11320101004)

Total Routhian Surface (TRS) calculations have been performed to investigate shape coexistence and evolution in neutron-deficient krypton isotopes 72,74,76Kr. The ground-state shape is found to change from oblate in 72Kr to prolate in 74,76Kr, in agreement with experimental data. Quadrupole deformations of the ground states and coexisting 0+2 states as well as excitation energies of the latter are also well reproduced. While the general agreement between calculated moments of inertia and those deduced from observed spectra confirms the prolate nature of the low-lying yrast states of all three isotopes (except the ground state of 72Kr), the deviation at low spins suggests significant shape mixing. The role of triaxiality in describing shape coexistence and evolution in these nuclei is finally discussed.

On the role of triaxiality in mode-I resistance curves

Insight into the role of triaxiality in mode-I, plane strain resistance curves of a representative ductile metal has been gained. Growth of a macroscopic crack is simulated as per modified boundary layer formulation for a range of constraint parameter with the fracture process represented by a triaxiality dependent cohesive model. In contrast to the predictions by a fixed cohesive law, the study shows that by including the effect of triaxiality on the work of separation, the stick-slip nature or the non-uniformity in the rate of the crack growth and its manifestations on the plastic wake and fracture surface can be predicted that are closer to trends observed in experimental literature.

Submicrosecond isomer in45117Rh72and the role of triaxiality in its electromagnetic decay rate

The neutron-rich nucleus 117Rh was synthesized in the fission of a relativistic 238U beam produced at the GSI laboratory in Darmstadt, Germany. An isomeric state with t1/2=138(17) ns decaying by a single γ ray was observed, providing the first information on the excited states in this nucleus. The experimental data are discussed in terms of systematics and interpreted by using the Woods-Saxon deformed shell model and triaxial-rotor-plus-particle calculations. The origin of the isomer is explained as being due to a hindered E2 transition to the ground state.

Fission barriers in actinides in covariant density functional theory: The role of triaxiality

Relativistic mean-field theory allowing for triaxial deformations is applied for a systematic study of fission barriers in the actinide region. Different pairing schemes are studied in detail and it is shown that covariant density functional theory is able to describe fission barriers on a level of accuracy comparable with nonrelativistic calculations, even with the best phenomenological macroscopic$+$microscopic approaches. Triaxiality in the region of the first saddle plays a crucial role in achieving that.

Role of triaxiality in the ground-state shape of neutron-rich Yb, Hf, W, Os and Pt isotopes

The evolution of the ground-state shape of several isotopes of Yb, Hf, W, Os and Pt along the triaxial landscape is analyzed using the self-consistent Hartree–Fock–Bogoliubov approximation. Two well-reputed interactions (Gogny D1S and Skyrme SLy4) have been used in the study in order to assess to which extent the results are independent of the details of the effective interaction. A large number of even–even nuclei, with neutron numbers from N = 110 up to N = 122, have been considered, covering in this way a vast extension of the nuclear landscape where signatures of oblate–prolate shape transitions have already manifested both theoretically and experimentally.

Evolution of nuclear shapes in medium mass isotopes from a microscopic perspective

The self-consistent Hartree-Fock-Bogoliubov approximation based on the finite-range and density-dependent Gogny interaction (parametrization D1S) has been used to generate potential energy curves in various chains of Pd, Xe, Ba, Nd, Sm, Gd, and Dy isotopes. The evolution of shapes with the number of nucleons is studied and a search for signatures of E(5) and X(5) critical point symmetries is also conducted. The results are compared with those obtained in the framework of other mean-field approximations to the nuclear many-body problem. The role of triaxiality in nuclei where this degree of freedom could be relevant is considered and discussed.

Triaxiality and Fracture of Steel Moment Connections

The connections of welded steel moment frames undergo a complex multiaxial state of stress that leads to high levels of stress triaxiality. As triaxiality increases, the propensity for fracture increases. Classic engineering models of fracture and modern microscale models of fracture mechanisms explicitly consider the role of triaxiality. Nonetheless, triaxiality is generally not directly considered by structural engineers. In this paper, triaxiality is defined as the ratio of the maximum principal stress to the von Mises stress. Triaxiality and maximum principal stress demands are investigated for tests on fractured notched round bars, small-scale tension specimens, and a full-scale moment connection. Based on analysis of the tests, it is proposed that, for fractures driven by triaxiality demands, the maximum principal stress at fracture is a function of the level of triaxiality. Calculation of the triaxiality demands requires 3D nonlinear analysis and depends on the loading, connection geometry, and postyield stress-strain relationships of all parent and weld metals. Examination of a welded steel moment connection indicates particularly high triaxiality demands. The triaxiality demands indicate that fracture of these connections may be governed by triaxiality even when high toughness parent and weld metals are used.

Micromechanics of void growth at high rates

The aim of this work, based on a micromechanical approach, is to caracterise the behavior of porous material under dynamic conditions. The porous media is represented by an aggregate of hollow spheres. With use of a virtual velocity field, an analytical link between the porosity, the macroscopic strain rate and the macroscopic stress is obtained and provides the viscoplastic behavior of the porous media. The results of Carroll and Holt (1972) [1] concerning the dynamic compression of a porous aluminium are retrieved. The present model is not limited to spherical loadings. An application to an axisymmetric tensile path enlights the role of triaxiality on the evolution of voids.

The role of triaxiality in the ground states of even-even neutron-rich Ru isotopes

Partial decay schemes for the very neutron-rich nuclei 108,110,112Ru and 114Ru, about which nothing was previously known, have been determined from the study of prompt, triple-γ coincidences in 248Cm fission fragments. Levels with probable spin up to 10ħ have been observed and γ-ray branching ratios obtained. Ratios of reduced electric quadrupole transition probabilities obtained from the branching ratios are in good agreement with predictions of a simple model of a rigid triaxial rotor.