Finite temperature calculation of angular velocities and moments of inertia in rotating nuclei

The rotational properties of nuclei in the statistical region are predicted on the basis of a simple BCS model. A back-bending, which disappears as the temperature increases, is observed in the plot of the moment of inertia versus the square of the angular velocity.     

QUASIPARTICLE SPECTRUM IN A ROTATING PETENTIAL AND THE BACK-BENDING PHENOMENA

The condition for back-bending phenomena to occur in rare-earth region is in-vestigated with quasiparticle spectrum in a rotating potential. The first and the se-cond "back-bending" of nuclei ¹⁵⁸Er are explained.     

THREE BAND INTERSECTION MODEL DESCRIBING THE HIGH SPIN STATES OF EVEN-EVEN NUCLEI

In this paper a three bands intersection model describing the high spin states ofeven-even nuclei have been proposed. This model has the ability to explain the back-bending behaviour in the plot of the moments of inertia versus the rotational angularvelocity, i.e. l-ω² curve, of some even-even nuclei.     

Band coupling and crossing in nuclei

A model of coupled rotational bands, including three types of phonons, β, γ and S(K^π = 1⁺ or 0⁺), is proposed and applied to a number of even-even rare earth back-bending nuclei. It reproduces the most complicated experimentally known multiple-band crossings in ¹⁵⁴Gd, ¹⁵⁶Dy, ¹⁶⁴Er and the clockwise circling of the yrast B(E2) values (versus ω²) in back-bending nuclei. The direct coupling strengths, derived from a fit to experimental data, are discussed in detail.     

Macroscopic description of collective motion in fast-rotating nuclei

The distorted Fermi-surface model is generalized to study rotating nuclei. The Chandrasekhar-Lebovitz method is used to solve the equations of nuclear hydrodynamics. A complete analysis of possible stationary ellipsoidal configurations is presented showing such phenomena as giant back-bending and shape isomerism. The normal oscillation modes of quadrupole symmetry are analysed: it is shown that besides the modes corresponding to the quadrupole giant resonance (QGR) split due to the centrifugal and Coriolis forces, the model predicts the appearance of two additional low-lying modes. The nonvibrational modes are analysed, and the corresponding inertia parameters are calculated. The electric quadrupole transitions in rotating nuclei are studied; according to the model the E2 transitions follow very closely the yrast-line both in the axial noncollectively-rotating nuclei and in the three-axial nuclei.     

THE CURRENT DISTRIBUTION IN THE BACK-BENDING REGION

The current distribution in the back-bending region is calculated based on the particlerotor model with particle number conservation. The results show that the sudden increase of aligned angular momentum in the backbending region corresponds to a drastic change of the current distribution.     

Change of nuclear radii due to rotation: Calculation of Mössbauer and muonic isomer shifts

Mössbauer and muonic isomer shifts of 2⁺ rotational states are calculated for rare-earth nuclei in good agreement with experiment using Migdal's effective p-h and p-p interactions. The second-order cranking equations are developed in the framework of the theory of finite Fermi systems. In contrast to the pairing-plus-quadrupole model, the results of this work show a small stretching contribution for most nuclei, but a dominating role for the Coriolis antipairing effect. Shrinking charge radii are found for Dy and Os isotopes and some other nuclei as a consequence of the CAP mechanism. Detailed information is given about the redistribution of protons and neutrons due to rotation. It is seen that only a few levels within the range of the diffuse Fermi edge take part in the redistribution and determine the isomer shifts. Shrinking m.s. proton radii and, at the same time, increasing m.s. neutron radii are obtained for the “back-bending” nuclei ^{158, 160, 162}Dy ^{160, 162}Er and ^{164, 166}Yb.     

Signature splitting of the πh11/2 band and high-spin structures in 155Tb

Seven experimentally observed bands of 155Tb are analyzed in detail, using the particle-number-conserving method for treating the cranked shell model with monopole and quadrupole pairing interactions. We satisfactorily reproduce the experimental alignments and especially focus on the microscopic mechanism of the second back-bending and the influence of pair interaction on ultrahigh spins. Our calculated results show that the πi13/2 orbitals are too high to give a contribution to the moment of inertia below hω≈0.7 MeV. Instead, the crossing between the π[541]l/2 and other proton orbitals is responsible for the second back-bending. We assign a possible configuration to the decoupled band found in 155Tb and predict eleven bands which are experimentally unobserved.     

The back-bending phenomenon in 158Er

The results of a variation after angular momentum projection calculation with axial Nilsson-BCS-like intrinsic wave functions and a cranked Hartree-Fock-Bogoliubov calculation for the back-bending region in ¹⁵⁸Er are reported. The moment-of-inertia values in the former are closer to the experimental results.     

A systematic investigation of the Coriolis antipairing effect in the rare earth region including projection onto exact particle number and angular momentum

The Coriolis antipairing effect is investigated for the rare earth nuclei. A new two-parameter expression for the pairing strength is introduced which is adjusted to the even-odd mass differences. Good agreement with the experimental results concerning the linear part of the moment of inertia as a function of the square of the angular velocity can be reached if projection onto exact particle number is taken into account. The latter causes an essential stabilization of the pair correlations. The critical angular velocity for the transition from the suprafluid to the normal state is defined and calculated. It is substantially increased by projection onto exact particle number whereas it remains almost unchanged by projection onto exact angular momentum. The critical angular velocity at which the neutron pairing vanishes is systematically larger than the angular velocity at which back-bending is observed. From this it is concluded that backbending cannot be caused by a collapse of pairing.     

Alpha decay and cluster decay of some neutron-rich actinide nuclei

Nuclei in the actinide region are good in exhibiting cluster radioactivity. In the present work, the half-lives of α-decay and heavy cluster emission from certain actinide nuclei have been calculated using cubic plus Yukawa plus exponential model (CYEM). Our model has a cubic potential for the overlapping region which is smoothly connected by a Yukawa plus exponential potential for the region after separation. The computed half-lives are compared with those of other theoretical models and are found to be in good agreement with each other. In this work, we have also studied the deformation effects on half-lives of cluster decay. These deformation effects lower the half-life values and it is also found that the neutron-rich parent nuclei slow down the cluster decay process. Geiger–Nuttal plots for various clusters are found to be linear and most of the emitted clusters are α-like nuclei.     

Light nuclei near neutron and proton drip lines in relativistic mean-field theory

We have made a detailed study of the ground-state properties of nuclei in the light-mass region with atomic numbers Z = 10–22 in the framework of the relativistic mean-field (RMF) theory. The nonlinear σω model with scalar self-interaction has been employed. The RMF calculations have been performed in an axially deformed configuration using the force NL-SH. We have considered nuclei about the stability line as well as those close to proton and neutron drip lines. It is shown that the RMF results provide good agreement with the available empirical data. The RMF predictions also show reasonably good agreement with those of the mass models. It is observed that nuclei in this mass region are found to possess strong deformations and exhibit shape changes all along the isotopic chains. The phenomenon of shape coexistence is found to persist near the stability line as well as near the drip lines. It is shown that the magic number N = 28 is quenched strongly, thus enabling the corresponding nuclei to assume strong deformations. Nuclei near the neutron and proton drip lines in this region are also shown to be strongly deformed.     

Low-energy collective E1 mode in nuclei

A theory is formulated for the collective enhancement of low-energy E1 transitions that has been observed in certain nuclei. The idea is to calculate an adiabatic isovector as well as isoscalar deformation. When this theory is applied to radium and light thorium nuclei, shell effects on the isovector E1 moment are found to explain the peculiar systematics of the E1 mode in this region.     

Theoretical Study on Properties of New Isotope 265Bh

The properties of nuclei belonging to the newly observed α-decay chain starting from 265Bh have been studied. The axially deformed relativistic mean-field calculation with the force NL-Z2 has been performed in the blockedBCS approximation. Some ground state properties such as binding energies, deformations, spins, and parities, as well as Q-values of the α-decay for this decay chain have been calculated and compared with known experimental data. Good agreement is found. The single-particle spectrum of the nucleus 265 Bh is studied and some new magic numbers are found,while the magnitudes of the shell gaps in superheavy nuclei are much smaller than those of nuclei before the actinium region, and the Fermi surfaces are close to the continuum. Thus the superheavy nuclei are usually not stable. The The methods which give good agreement with the data are selected.     

Properties and structure of N=Z nuclei within relativistic mean field theory

The axially deformed relativistic mean field theory with the force NLSH has been performed in the blocked BCS approximation to investigate the properties and structure of N=Z nuclei from Z=20 to Z=48.Some ground state quantities such as binding energies, quadrupole deformations, one/two-nucleon separation energies, root-mean-square (rms) radii of charge and neutron, and shell gaps have been calculated.The results suggest that large deformations can be found in medium-heavy nuclei with N=Z=38-42.The charge and neutron rms radii increase rapidly beyond the magic number N=Z=28 until Z=42 with increasing nucleon number, which is similar to isotope shift, yet beyond Z=42, they decrease dramatically as the structure changes greatly from Z=42 to Z=43.The evolution of shell gaps with proton number Z can be clearly observed.Besides the appearance of possible new shell closures, some conventional shell closures have been found to disappear in some region.In addition, we found that the Coulomb interaction is not strong enough to breakdown the shell structure of protons in the current region.     

Properties of rotational excitations in odd mass Dy isotopes

The data on high-spin rotational states in¹⁵⁵Dy,¹⁵⁷Dy,¹⁵⁹Dy and¹⁶¹Dy are discussed within a model where a particle is coupled to a rotor with a variable moment of inertia. In this model there is often a definite improvement in the quality and stability of the fits to levels in the positive-parity band below spin 25/2 when the moment of inertia of the core exhibits a linear dependence on the square of the rotational angular velocity as compared to the situation where a linear dependence onI(I+1) is assumed. In the improved fits the empirical matrix elements of the Coriolis force exhibit a smooth and significant increase with decreasing moment of inertia. The rotational energies above spin 25/2 in the positive-parity band and above 19/2 in the 11/2^? [505] band are understood within the present theoretical model if the moment of inertia increases more rapidly than the linear trend established at lower spin. In the 11/2^? [505] band this increase follows rather closely the behaviour in the even nuclei whereas in the positive-parity band the curves are quite flat even beyond the point where the moment of inertia in the even nuclei exhibits a discontinuous or back-bending behaviour. The significance of this observation is discussed in the light of existing theories on nuclear moments of inertia.     

RHIC能区的纵向流分布特征和核阻止特性的研究

首先利用相对论纵向非均匀集体流模型讨论RHIC能区的净质子分布特征,并分别与AGS,SPS的实验结果比较,发现,RHIC能区所产生的粒子系统具有很强的非均匀分布特征,表现为很强的核透明性,并分析了非均匀集体流模型与实验中的核阻止特性的紧密联系,接着分别研究了RHIC能区所产生质子、反质子和净质子的分布,这些研究可以帮助我们深入了解最新的RHIC能区的粒子分布和核阻止特征.     

Properties of nuclei at the third-minimum deformation

Potential-energy surfaces of heavy nuclei in the Ra-Th region are calculated by the macroscopic-microscopic method using the modified oscillator potential. A rather large region of nuclei with a third minimum along the fission trajectory is found. This minimum appears at a very large quadrupole deformation, ε₂ ∼ 0.9, and has a stable octupole deformation with ε₃ ∼ 0.2. It is found that in addition to the deformation parameters ε₂, ε₃, ε₄ and ε₅ it is important to include also ε₆. Particular attention is paid to the Th-isotopes, for which experimental evidence for the existence of a third minimum in the fission barrier has been found. Properties like the moment of inertia, the decoupling parameter and the energy splitting between positive- and negative-parity rotational bands are studied.