分类： 物理学 >> 核物理学 提交时间： 2017-07-30
摘要： A large number of complete fusion excitation functions of reactions including the breakup channel were measured in recent decades, especially in the last few years. It allows us to investigate the systematic behavior of the breakup effects on the complete fusion cross sections. To this end, we perform a systematic study of the breakup effects on the complete fusion cross sections at energies above the Coulomb barrier. The reduced fusion functions F(x) are compared with the universal fusion functions which are used as a uniform standard reference. The complete fusion cross sections at energies above the Coulomb barrier are suppressed by the breakup of projectiles. This suppression effect for reactions induced by the same projectile is independent of the target and mainly determined by the lowest energy breakup channel of the projectile. There holds a good exponential relation between the suppression factor and the energy corresponding to the lowest breakup threshold.
分类： 物理学 >> 核物理学 提交时间： 2017-07-30
摘要： Applying a macroscopic reduction procedure on the improved quantum molecular dynamics (ImQMD) model, the energy dependences of the nucleus-nucleus potential, the friction parameter, and the random force characterizing a one-dimensional Langevin-type description of the heavy-ion fusion process are investigated. Systematic calculations with the ImQMD model show that the fluctuation-dissipation relation found in the symmetric head-on fusion reactions at energies just above the Coulomb barrier fades out when the incident energy increases. It turns out that this dynamical change with increasing incident energy is caused by a specific behavior of the friction parameter which directly depends on the microscopic dynamical process, i.e., on how the collective energy of the relative motion is transferred into the intrinsic excitation energy. It is shown micro- scopically that the energy dissipation in the fusion process is governed by two mechanisms: One is caused by the nucleon exchanges between two fusing nuclei, and the other is due to a rearrangement of nucleons in the intrinsic system. The former mechanism monotonically increases the dissipative energy and shows a weak dependence on the incident energy, while the latter depends on both the relative distance between two fusing nuclei and the incident energy. It is shown that the latter mechanism is responsible for the energy dependence of the fusion potential and explains the fading out of the fluctuation-dissipation relation.
分类： 物理学 >> 核物理学 提交时间： 2017-07-30
摘要： Background: Potential energy surfaces (PES’s) of actinide nuclei are characterized by a two-humped barrier structure. At large deformations beyond the second barrier the occurrence of a third one was predicted by macroscopic-microscopic model calculations in the 1970s, but contradictory results were later reported by number of studies that used different methods. Purpose: Triple-humped barriers in actinide nuclei are investigated in the framework of covariant density func- tional theory (CDFT). Methods: Calculations are performed using the multidimensionally-constrained relativistic mean field (MDC- RMF) model, with the nonlinear point-coupling functional PC-PK1 and the density-dependent meson exchange functional DD-ME2 in the particle-hole channel. Pairing correlations are treated in the BCS approximation with a separable pairing force of finite range. Results: Two-dimensional PES’s of 226,228,230,232Th and 232,234,236,238U are mapped and the third minima on these surfaces are located. Then one-dimensional potential energy curves along the fission path are analyzed in detail and the energies of the second barrier, the third minimum, and the third barrier are determined. The functional DD-ME2 predicts the occurrence of a third barrier in all Th nuclei and 238U. The third minima in 230,232Th are very shallow, whereas those in 226,228Th and 238U are quite prominent. With the functional PC- PK1 a third barrier is found only in 226,228,230 Th. Single-nucleon levels around the Fermi surface are analyzed in 226Th, and it is found that the formation of the third minimum is mainly due to the Z = 90 proton energy gap at β20 ≈ 1.5 and β30 ≈ 0.7. Conclusions: The possible occurrence of a third barrier on the PES’s of actinide nuclei depends on the effective interaction used in multidimensional CDFT calculations. More pronounced minima are predicted by the DD-ME2 functional, as compared to the functional PC-PK1. The depth of the third well in Th isotopes decreases with increasing neutron number. The origin of the third minimum is due to the proton Z = 90 shell gap at relevant deformations.
分类： 物理学 >> 核物理学 提交时间： 2017-07-30
摘要： The covariant density functional theory with a few number of parameters has been widely used to describe the ground-state and excited-state properties for the nuclei all over the nuclear chart. In order to describe exotic properties of unstable nuclei, the contribution of the continuum and its coupling with bound states should be treated properly. In this Topical Review, the development of the covariant density functional theory in continuum will be introduced, including the relativistic continuum Hartree-Bogoliubov theory, the relativistic Hartree-Fock-Bogoliubov theory in continuum, and the deformed relativistic Hartree-Bogoliubov theory in continuum. Then the descriptions and predictions of the neutron halo phenomena in both spherical and deformed nuclei will be reviewed. The diffuseness of the nuclear potentials, nuclear shapes and density distributions, and the impact of the pairing correlations on nuclear size will be discussed.
点击量
待评议
点击量
下载量
评论
