Deformed nuclear halos

Deformation properties of weakly bound nuclei are discussed in the deformed single-particle model. It is demonstrated that in the limit of a very small binding energy the valence particles in specific orbitals, characterized by a very small projection of single-particle angular momentum onto the symmetry axis of a nucleus, can give rise to the halo structure which is completely decoupled from the rest of the system. The quadrupole deformation of the resulting halo is completely determined by the intrinsic structure of a weakly bound orbital, irrespective of the shape of the core.     

Effects of nuclear deformation in quasi-elastic electron scattering

We discuss effects due to nuclear deformation that can be observed in quasi-elastic electron scattering from deformed nuclei. To simplify the discussion we restrict ourselves to the plane wave impulse approximation (PWIA) and compare the form of the spectral function for axially symmetric deformed nuclei with that for spherical nuclei in the independent particle model. We point out that a strong dependence on deformation can be expected for momentum distributions measured in narrow missing energy ranges. This is illustrated taking ²⁸Si as an example.     

Pair-Aligned Intrinsic States in Many-j Configurations

With the aid of a pair aligned model in many-j configurations and the Hamiltonian including monopole, quadrupole-pair forces and the quadrupole deformed field, the intrinsic states of deformed nuclei are determined by a variational method. The quadrupole moment is calculated and the effect of deformation on the intrinsic states is investigated. The results show that angular momentum (J=0,2,4) nucleon-pairs are dominant in the intrinsic states, so that, s, d, g bosons must be used in the study of deformed nuclei.     

RMF理论中Zr同位素链壳结构的形变依赖性

基于球形与轴对称形变的相对论平均场（Relativistic Mean Field, 简称RMF）理论模型, 分别计算了Zr同位素链的基态总能量, 并根据其差值提取了形变修正能后发现, Zr同位素链丰中子区的核具有大的长椭形变, 对应的形变修正能可达到10 MeV。 利用RMF理论计算的基态能量, 在扣除液滴模型计算的结合能后, 得到了Zr同位素链的壳修正能。 通过对壳修正能的分析后发现, 形变使N=50壳效应显著减弱。 特别是在丰中子区, 大形变导致了N=50壳结构的消失。 The total binding energy of nuclei for Zr isotopic chain is calculated by the spherical and axial deformed relativistic mean field(RMF) theory respectively, and the energy contribution due to the deformation(i.e., deformation correction energy) is extracted. It is found that the neutron rich nuclei in the isotopic chain have large prolate deformation, and corresponding deformation correction energy can be up to 10 MeV. The shell correction energy is obtained by the difference between the binding energies calculated by the liquid model and those by the RMF calculations. Detailed analysis indicates that the deformation weakens the shell effect of N=50 remarkably. Especially for the neutron rich nuclei, large deformation leads to disappearance of the N=50 shell structure.     

Transition from spherical to deformed shapes of nuclei in the Monte Carlo shell model

The transition from spherical to deformed shapes is studied in terms of large-scale shell-model calculations for Ba isotopes as a function of valence nucleon number with fixed single-particle space and Hamiltonian. A new version of the Monte Carlo shell model is introduced so as to incorporate pairing correlations efficiently, by utilizing condensed pair bases. The energy levels and electromagnetic matrix elements are described in agreement with experiments throughout the transitional region. The orbital M1 sum rule is calculated as a measure of the deformation evolution, and the Q-phonon picture is shown to be reasonable from spherical to deformed nuclei.     

Magnetic rotation and chiral symmetry breaking

The deformed mean field of nuclei exhibits various geometrical and dynamical symmetries which manifest themselves as various types of rotational and decay patterns. Most of the symmetry operations considered so far have been defined for a situation wherein the angular momentum coincides with one of the principal axes and the principal axis cranking may be invoked. New possibilities arise with the observation of rotational features in weakly deformed nuclei and now interpreted as magnetic rotational bands. More than 120 MR bands have now been identified by filtering the existing data. We present a brief overview of these bands. The total angular momentum vector in such bands is tilted away from the principal axes. Such a situation gives rise to several new possibilities including breaking of chiral symmetry as discussed recently by Frauendorf. We present the outcome of such symmetries and their possible experimental verification. Some possible examples of chiral bands are presented.     

Shell Effect of Superheavy Nuclei in Self-consistent Mean-Field Models

We analyze in detail the numerical results of superheavy nuclei in deformed relativistic mean-field model and deformed Skyrme-Hartree-Fock model. The common points and differences of both models are systematically compared and discussed. Their consequences on the stability of superheavy nuclei are explored and explained. The theoreticalresults are compared with new data of superheavy nuclei from GSI and from Dubna and reasonable agreement is reached.Nuclear shell effect in superheavy region is analyzed and discussed. The spherical shell effect disappears in some cases due to the appearance of deformation or superdeformation in the ground states of nuclei, where valence nucleons occupysignificantly the intruder levels of nuclei. It is shown for the first time that the significant occupation of vaJence nucleons on the intruder states plays an important role for the ground state properties of superheavy nuclei. Nuclei are stable in the deformed or superdeformed configurations. We further point out that one cannot obtain the octupole deformation of even-even nuclei in the present relativistic mean-field model with the σ,ω and ρ mesons because there is no parityviolating interaction and the conservation of parity of even-even nuclei is a basic assumption of the present relativistic mean-field model.     

A New Formula for the Spectra of Axial Symmetric and Transition Nuclei and a Quantity to Identify the Nuclear γ Deformation

The analytical limits of a speacial potential of the Bohr Hamiltonian are discussed in detail.A new formula for the axial symmetric,triakial deformed and the γ-soft nuclei is presented.The nuclear γ deformation can be identified with this formula.The calculations of some nuclear yrast lines seproduce the experiment al results very Well,and the degree of the nuclear γ deformation is obtained.     

On the role of the second well of the deformation potential energy in nuclear fission in the lead region

The influence of the deformation-dependent attenuation of the rotational enhancement of the level density on the fission probability is discussed. It is shown that the difference in fissility between spherical and deformed nuclei can be explained by taking into account neutron emission by the nucleus in the second well of the deformation potential energy.     

Clustering aspects of nuclei in highly deformed states

The potential energies, the moments of inertia, and the quadrupole and octupole moments of dinuclear systems are compared with corresponding values for the highly deformed nuclear states. The idea is advocated that hyperdeformed states of nuclei are close to near-symmetric dinuclear systems. The superdeformed states are considered as asymmetric dinuclear systems. The cluster superdeformed and hyperdeformed states have quite a large octupole deformation. Measurement of octupole deformations of highly deformed nuclei can answer the question of whether these nuclei exist in cluster configurations.     

Alpha-induced reaction cross-section for Sm,U, Np targets: influence of hexadecapole deformation and deformed surface diffuseness

Alpha-induced reactions on~(154)Sm,~(233,235,236,238)U, and ~(237)Np deformed nuclei are studied theoretically.The effects of hexadecapole deformation, deformed surface diffuseness parameter, and orientation on barrier height and position, fusion cross-section at any angle, and fusion cross-section have been investigated. Both hexadecapole deformation and deformed surface diffuseness can affect barrier characteristics and enhance fusion cross-section. Good agreement between experimental data and theoretical calculations with quadrupole and hexadecapole deformation and deformed surface diffuseness were observed for the ~4He+~(154)Sm,~(235)U,~(237)Np reactions.     

On the shell model and its application to the deformation energy of heavy nuclei

The technical realisation of the shell model with arbitrary fields is presented in detail, with special emphasis of the unusual and large deformations of the nuclear shape as they may occur in the fission process. We discuss how realistic parametrisations of the nuclear shape and the potential well can be developed and how the parameters of the average fields can be determined. We restrict ourselves to wells with a Woods-Saxon distribution in the radial coordinate. By means of Strutinsky's shell correction approach, the single particle energies deserve to calculate the potential part of a collective Hamiltonian. Its behaviour with varying deformation is discussed both qualitatively and quantitatively. Considered are the deformation types of elongation (1), necking (2), reflection (3) and axial (4) asymmetry of the nuclear shape. Evidence is given for geometrical symmetries which can be correlated to normal modes in finite nuclei. The transition from spherical to deformed nuclei is presented in detail for the radium isotopes, revealing the importance of hexadecapole deformations. Finally, we give an extensive and systematic presentation of the energies and of the deformations at the various stationary points of the deformation energy for the nuclei in the actinide region and for the hypothetical superheavy nuclei.     

Tilted rotation of weakly deformed and triaxial nuclei

The Tilted Axis Cranking theory is reviewed. It is used to describe the appearance of magnetic rotation in weakly deformed nuclei. The possibility of aplanar solutions and their experimental signature are discussed.     

Elastic and inelastic scattering of K+ mesons on deformed nuclei

Various aspects of the scattering of medium energy K⁺ mesons on deformed nuclei are investigated in the light of a specific example, viz. the scattering of 800 MeV/c positive kaons on ¹⁵²Sm. The effects of coupled channels are found to be significant despite the fact that the K⁺ meson is a rather weakly interacting hadronic probe. The localization of the reaction mechanism on a deformed target is discussed for both protons and K⁺ mesons to compare the extent of surface and volume probing capability of these two projectiles. The results show the remarkable sensitivity of the elastic and inelastic K⁺ differential cross sections to the differences between the neutron and proton density distributions in deformed nuclei.     

Deformation effects on isospin mixing and isobar analogue resonance for 74−80Kr isotopes

Pyatov’s method has been applied to investigate Fermi beta transitions in deformed ^74–80Kr isotopes. This self-consistent method, which was used to study the isobar analogue states in the spherical odd-odd nuclei, has to date not been applied for the isobar analogue states in deformed nuclei. The nucleon-nucleon residual interaction has been included so that the broken isospin symmetry in the mean field approximation has been restored and the strength parameter of the effective interaction has been taken out to be a free parameter. The energies and wave functions of the isobaric analogue excitations in ^74–80Rb isotopes have been obtained within the framework of the pnQRPA method. The probability of the isospin mixing in the ground states and the centroid energies of the isobar analogue resonance have been presented and the deformation effects on these quantities have been quantified.     

Interaction and fusion of deformed nuclei

It is shown that the height of the barrier and its position, as well the depth of the capture well, are highly sensitive to the relative orientation of colliding strongly deformed nuclei. It is found that the fusion/capture cross sections and the nucleus-nucleus potential for heavy nuclear systems depend greatly on the magnitude and sign of the quadrupole deformation of nuclear surfaces. In order to describe correctly the cross section for the capture of heavy strongly deformed nuclei, it is necessary to perform averaging over all three angles that describe their relative orientation. Allowance for a hexadecapole deformation leads to a significant increase in the capture cross section for very heavy nucleus-nucleus systems.     

The effect of plastic deformation on positron lifetime spectra in alkali halides

Positron lifetime spectra have been measured in plastically deformed KCl and NaCl single crystals. The experiments indicate that a third component of the order of 2 nsec is produced by the deformation, and its relative intensity (2–4%) is only weakly dependent on the degree of deformation.     

奇特核结构与激发的微观研究

原子核是一个从少体到多体过渡的量子体系,展现了很多有趣的集体现象。随着国际上若干大型放射性核束流实验装置的发展,极不稳定奇特核的结构与激发的研究成为当前核物理的前沿热点问题。本工作以形变弱束缚核40Mg为例,基于格点空间连续谱能量密度泛函和自洽的FAM-QRPA对奇特核的基态与激发态跃迁进行了研究。发现弥散的表面密度分布与连续谱对低能共振有很大影响。通过对同位旋矢量偶极激发的研究,说明低能矮共振与巨共振的微观机制有很大差异。此外还进行了大规模拟合,发展针对丰中子核,超重核的新的高精度有效相互作用,以期为相关学科如核天体物理、核裂变能等提供更为可靠的核理论模型。Nuclei are quantum systems in the evolution from few-body to many-body systems, and can exhibit many amazing collective phenomena. With the development of several advanced radioactive-beam facilities, the study of structures and excitations of extreme unstable exotic nuclei has become a hot issue. In this work, we solve the self-consistent FAM-QRPA in large deformed coordinate-spaces to treat continuum effects. We study properties of structures and collective excitations in deformed drip-line nuclei. We found that in weakly bound nuclei 40Mg, the diffuse surface density and pairing density play an important role in low-lying resonance. Through analysis of deformation-induced K-splitting in isovector dipole modes, we see that pygmy resonances have very different mechanism compared to giant resonances. In addition, large-scale fittings are performed to develop highprecision effective interactions, which will provide more reliable theoretical model for related subjects, such as nuclear astrophysics and nuclear fission energy.     

Behavior of shell effects with the excitation energy in atomic nuclei

We study the behavior of shell effects, like pairing correlations and shape deformations, with the excitation energy in atomic nuclei. The analysis is carried out with the finite temperature Hartree-Fock-Bogoliubov method and a finite range density dependent force. For the first time, properties associated with the octupole and hexadecupole deformation and with the superdeformation as a function of the excitation energy are studied. Calculations for the well quadrupole deformed 164Er and 162Dy, superdeformed 152Dy, octupole deformed 224Ra, and spherical 118Sn nuclei are shown. We find, in particular, the level density of superdeformed states to be 4 orders of magnitude smaller than for normal deformed ones.