核物质液-气相变和热核稳定性

简述了研究核物质液－气相变的理论计算及研究热核稳定性的二相平衡模型．讨论了相变的临界温度及其对核物质大小、不对称度及库仑相互作用的依赖性．比较了用各种核力或模型算得的热核极限温度及实验结果． A brief introduction of the theoretical studies of liquid gas phase transition in nuclear matter and a two phase equilibrium model for studying stability of hot nuclei is presented. The critical temperature of the phase transition and its dependence on size and asymmetry of nuclear matter and Coulomb interaction are discussed. The limiting temperatures of hot nuclei calculated with various nuclear forces or models and experimental results are compared.     

Skyrme Forces and Their Applications in Low Energy Nuclear Physics

This paper reviews the present status of Skyrme forces and their applications in the field of low energy nuclear physics as an effective nucleon-nucleon interaction. Their applications in the following five domains are presented:1). Hartree-Fock (HF), selfconsistent semiclassical (SCSC) calculations and nuclear ground state properties;2). random phase approximation (RPA), sum rule approach and properties of nuclear giant resonances;3). calculations of microscopic nucleon-nucleus optical potentials and related quantities;4). calculations of nucleus-nucleus optical potentials and fusion barriers;5). multifragmentation, liquid-gas phase transition and instability of hot/compressed nuclei.     

Nuclear interface energy at finite temperatures

The thermodynamic properties at finite temperatures of the plane interface between two phases of nuclear matter in equilibrium are examined theoretically, and explored numerically. The microscopic hamiltonian, the Skyrme I′ interaction, is used in the Thomas-Fermi approximation to obtain the finite-temperature extensions of earlier zero-temperature results which used the Hartree-Fock and Thomas-Fermi methods. Approximate analytic fits are given to the χ_i (proton fraction on the dense-matter side) dependence of the critical temperature, and to the T and χ_i dependences of the surface thermodynamic potentials, the density of surface neutrons, the surface entropy and the neutron and proton chemical potentials at phase equilibrium. These fits are an ingredient in a compressible liquid-drop nuclear model, the basis of an equation of state for hot, dense matter needed in certain astrophysical applications.The liquid-drop model is used here to construct an isolated, low-T nucleus, whose properties are compared with the original zero-T Hartree-Fock calculations which lead to the Skyrme I interaction, and with other mass formulae. The low-temperature expansion of the surface energy is compared with that obtained in other calculations. The nuclear level density at the Fermi surface, related to the low-T expansion of the entropy of the whole nucleus, is also discussed.     

Quantum calculations of Coulomb reorientation for sub-barrier fusion

Classical mechanics and time dependent Hartree-Fock (TDHF) calculations of heavy ions collisions are performed to study the rotation of a deformed nucleus in the Coulomb field of its partner. This reorientation is shown to be independent of the charges and relative energy of the partners. It only depends upon the deformations and inertias. TDHF calculations predict an increase by 30% of the induced rotation due to quantum effects while the nuclear contribution seems negligible. This reorientation modifies strongly the fusion cross section around the barrier for light deformed nuclei on heavy collision partners. For such nuclei a hindrance of the sub-barrier fusion is predicted.     

On the Liguid-Gas Phase Transition and the Instability of Hot Nuclei

Based on a simple semi-empirical energy density, the free energy density for a hot nucleon system is derived, from which the equations of state P(T, p) and chemical potentials μ_q(T, p) for both finite nucleus and infinite nuclear matter are acquired. The liquid-gas phase transition and the instability of hot nuclei are analysed with these ingredients within the framework of thermodynamic's. Both critical temperature T_c for infinite nuclear matter and limiting temperature T_lim, for finite nuclei are predicted and compared with previous calculations.     

The determination of the Coulomb energy of a nuclear system at the scission point upon fission

The dependence of the Coulomb energy of deformed nuclei on the shape and the nuclear matter density distribution is considered. For an ellipsoidal shape of the nucleus and a homogeneous distribution of nuclear matter, the result was obtained in a simple analytical form convenient for further use. Separate attention was paid to the consideration of a Fermi-like distribution of nuclear matter using different shapes of the nucleus that reflect several different kinds of collective motions. After these considerations, the dependence of the Coulomb energy of the fission products at the scission point of binary fission reactions on configurations, shapes and nuclear matter distributions was investigated. Calculation of the dependence of the shape of the nuclei deformation parameters at the scission point from their total kinetic energies was made in detail. Finally, the influence of shell effects on the mass yield of fission products is discussed. The text was submitted by the authors in English.     

有限核物质的饱和性质与液气相变

在核物质Hartree-Fock理论框架下,应用SKM^*,SKb和SGⅡ三种Skyrme等效核力参数与带屏蔽的二体库仑势,顾及到核的有限尺度效应,得到了合理的有限核零温饱和性质,发现有限核物质的液气相变临界温度约为12MeV,比无限对称核物质的约低2—3MeV。     

Physical properties of hot,dense matter: The general case

The thermodynamic properties of hot, dense matter are examined in the density range 10^?5 fm^?3 ? n ? 0.35 fm^?3 and the temperature range 0 ? T ? 21 MeV, for fixed lepton fractions Y_? = 0.4, 0.3 and 0.2 and for matter in β-equilibrium with no neutrinos. Three phases of the matter are considered: the nuclei phase is assumed to consist of Wigner-Seitz cells with central nuclei surrounded by a nucleon vapor containing also α-particles; in the bubbles phase the cell contains a central spherical bubble of nucleon vapor surrounded by dense nuclear matter; the third phase is that of uniform nuclear matter. All are immersed in a sea of leptons (electrons and neutrinos) and photons. The nuclei and bubbles are described by a compressible liquid drop model which is self-consistent in the sense that all of the constituent properties — bulk, surface, Coulomb energies and other minor contributions — are calculated from the same nuclear effective hamiltonian, in this case the Skyrme 1' interaction. The temperature dependence of all of these energies is included, for bulk and surface energies by direct calculation, for the Coulomb energy by combining in a plausible way the usual electrostatic energy and the numerical results pertaining to a hot Coulomb plasma. Lattice contributions to the Coulomb energy are an essential ingredient, and lattice modifications to the nuclear translational energy are included. A term is constructed to allow also for the reduced density of excited states of light nuclei. All of these modifications incorporate necessary physical effects which modify significantly the matter properties in some regions.     

奇Z超重核的禁戒α衰变(英文)

采用密度依赖的结团模型研究了奇Z 超重核的禁戒α衰变, 粒子与子核之间的微观核势通过双折叠模型对M3Y 核子-核子相互作用势以及 粒子与子核的密度积分给出。 粒子与子核之间的库仑相互作用也通过 粒子与子核的电荷密度积分给出。计算发现,由于非零角动量带来的禁戒效应和小的α粒子预形成几率,奇Z 超重核的α衰变寿命会明显变长。We investigate the α-transition of odd-Z superheavy nuclei by the density-dependent cluster model (DDCM). The microscopic nuclear potential between the -particle and the daughter nucleus is evaluated numerically from the double-folding model with the standard M3Y nucleon-nucleon interaction. The Coulomb potential is also obtained from the double-folding integral of the proton-proton Coulomb interaction with the charge density distributions of α-particle and daughter nucleus. From our calculations, enhanced stability againstα-decays is found for the odd-Z superheavy nuclei due to the hindrance effect of non-zero angular momentum and the small preformation factor of the -particle.     

Weibel Instability Growth Rate in Magnetized Plasmas with Quasi-Relativistic Distribution Function

The mechanism of the Weibel instability is investigated for dense magnetized plasmas. As we know, due to the electron velocity distribution, the Coulomb collision effect of electron-ion and the relativistic properties play an important role in such study. In this study an analytical expression for the growth rate and the condition of restricting the Weibel instability are derived for low-frequency limit. These calculations are done for the oscillation frequency dependence on the electron cyclotron frequency. It is shown that, the relativistic properties of the particle lead to increasing the growth rate of the instability. On the other hand the collision effects and background magnetic field try to decrease the growth rate by decreasing the temperature anisotropy and restricting the particles movement.     

Gamow-Teller resonance in hot nuclei and astrophysical applications

A formalism based on thermo field dynamics is described. It allows the effect of the temperature on the strength distribution of charge-exchange transitions in hot nuclei to be taken into account. Numerical calculations with the pair correlations in the BCS approximation and the schematic στ interaction are carried out for Gamow-Teller transitions in the ⁵⁶Fe nucleus. The electron capture and β^? decay rates are calculated for this nucleus at temperatures and densities corresponding to the advanced stage of the evolution of massive stars.     

Weibel Instability Growth Rate in Magnetized Plasmas with Quasi-Relativistic Distribution Function

The mechanism of the Weibel instability is investigated for dense magnetized plasmas. As we know, due to the electron velocity distribution, the Coulomb collision effect of electron-ion and the relativistic properties play an important role in such study. In this study an analytical expression for the growth rate and the condition of restricting the Weibel instability are derived for low-frequency limit. These calculations are done for the oscillation frequency dependence on the electron cyclotron frequency. It is shown that, the relativistic properties of the particle lead to increasing the growth rate of the instability. On the other hand the collision effects and background magnetic field try to decrease the growth rate by decreasing the temperature anisotropy and restricting the particles movement.     

Excitation of pygmy dipole resonance in neutron-rich nuclei via Coulomb and nuclear fields

We study the nature of the low-lying dipole strength in neutron-rich nuclei, often associated with the pygmy dipole resonance. The states are described within the Hartree-Fock plus RPA formalism, using different parametrizations of the Skyrme interaction. We show how the information from combined reaction processes involving the Coulomb and different mixtures of isoscalar and isovector nuclear interactions can provide a clue to reveal the characteristic features of these states.     

In-medium two-nucleon properties in high electric fields

The quantum mechanical two — particle problem is considered in hot dense nuclear matter under the influence of a strong electric field such as the field of the residual nucleus in heavy — ion reactions. A generalized Galitskii-Bethe-Salpeter equation is derived and solved which includes retardation and field effects. Compared with the in-medium properties in the zero-field case, bound states are turned into resonances and the scattering phase shifts are modified. Four effects are observed due to the applied field: (i) A suppression of the Pauli-blocking below nuclear matter densities, (ii) the onset of pairing occurs already at higher temperatures due to the field, (iii) a field dependent finite lifetime of deuterons and (iv) the imaginary part of the quasiparticle self-energy changes its sign for special values of density and temperatures indicating a phase instability. The latter effect may influence the fragmentation processes. The lifetime of deuterons in a strong Coulomb field is given explicitly.     

Nuclear shapes and interaction potentials of two colliding208Pb nuclei at finite temperature

The effect of nuclear and Coulomb interactions on the shapes of two colliding²⁰⁸Pb nuclei at finite temperature is investigated. The complex potential energy density derived by Faessler and collaborators and the kinetic energy density and entropy density for two Fermi spheres at finite temperature are used to calculate the free energy of the²⁰⁸Pb +²⁰⁸Pb system in the energy density formalism. Shell corrections are added to the free energy in the framework of the Strutinsky method. The total free energy is minimized with respect to the quadrupole deformation and the diffuseness to determine the density distribution of²⁰⁸Pb nucleus at certain distanceR and temperatureT assuming the deformed Woods-Saxon shape for each nucleus. It is found that the nucleus acquires larger deformation and diffuseness as the temperature increases. The interaction potential between two²⁰⁸Pb nuclei is calculated from the minimized free energy. The total (nuclear + Coulomb) potential is found to decrease with increasing temperature, whereas the real part of the nuclear potential becomes more repulsive as the temperature increases.     

In-medium two-nucleon properties in high electric fields

The quantum mechanical two — particle problem is considered in hot dense nuclear matter under the influence of a strong electric field such as the field of the residual nucleus in heavy — ion reactions. A generalized Galitskii-Bethe-Salpeter equation is derived and solved which includes retardation and field effects. Compared with the in-medium properties in the zero-field case, bound states are turned into resonances and the scattering phase shifts are modified. Four effects are observed due to the applied field: (i) A suppression of the Pauli-blocking below nuclear matter densities, (ii) the onset of pairing occurs already at higher temperatures due to the field, (iii) a field dependent finite lifetime of deuterons and (iv) the imaginary part of the quasiparticle self-energy changes its sign for special values of density and temperatures indicating a phase instability. The latter effect may influence the fragmentation processes. The lifetime of deuterons in a strong Coulomb field is given explicitly.     

Hartree-Fock and Hartree-Bogolyubov calculations inp shell nuclei

Hartree-Fock (HF) and Hartree-Fock-Bogolyubov (HFB) calculations have been performed for the 1p shell nuclei. Nuclear deformations are assumed to be at most axially symmetric. The HFB transformation is restricted to allow forp-p andn-n pairing only.Volkov's force, a soft-core, two-body interaction of semi-realistic nature, is used which does not produce any single-particle spin-orbit splittings. Coulomb force and the usual correction for centre-of-mass motion are taken into account. The calculations are carried out in a single-particle basis including all states up to principle oscillator quantum numberN=3 (in some cases,N=4). Binding energies, rms radii, density distributions, and quadrupole moments are calculated and found to be in reasonable agreement with experiment. Large Hartree-Fock energy gaps are obtained. They prevent the pairing correlations considered from becoming effective in an HFB approach and from changing the HF ground-state properties appreciably. In non-selfconjugate nuclei, the Pauli principle, rather than the Coulomb interaction, yields large differences between the charge and mass distributions. A theorem on selfconsistent symmetries is proved. The coefficients of the HFB transformation turn out to be real, if time-reversal and angular momentum projection flip are selfconsistent symmetries.     

Selfconsistent calculations of highly excited nuclei

Selfconsistent calculations of excitation energies and entropies of highly excited nuclei have been performed using the temperature-dependent constrained Hartree-Fock (HF)method with the Skyrme interaction. The selfconsistent variations of the level spectra as functions of the temperature (at a fixed deformation) are shown to be small and practically do not affect the results for the entropy versus excitationenergy. The disappearance of the shell fluctuations in the potential energy surface of a rare earth nucleus around T ～ 3 MeV is demonstrated.