Low-Lying electric dipole transitions in tin isotopic chain within the RPA model

A nuclear structuremodel based on a finite rank approximation of Skyrme interaction is applied to calculate the distribution of dipole strength in tin isotopes. The model is based on the quasiparticle random phase approximation. The results obtained with the three types of parametrizations of the Skyrme forces (SLy4, SkM*, and SIII) are compared. The low-lying part of dipole strength distribution reveals the existence of a group of slightly collective states, and the corresponding E1 transition strength increases with the enlargement of neutron excess. The group is associated with the pygmy resonance. The text was submitted by the authors in English.     

Separable Skyrme interactions and quasiparticle RPA

A finite rank separable approximation for the quasiparticle random phase approximation with Skyrme interactions is applied to study the low-lying quadrupole and octupole states in some S isotopes and giant resonances in some spherical nuclei. It is shown that characteristics calculated within the suggested approach are in good agreement with available experimental data.     

A possible proton pygmy resonance in 17Ne

The low-lying electric dipole strengths in proton-rich nuclei ¹⁷F and ¹⁷Ne, which can be produced at HIRFL-CSR in Lanzhou, are investigated. In the framework of the covariant density functional theory the self-consistent relativistic Hartree Bogoliubov model and the relativistic quasiparticle random phase approximation with the NL3 parameter set and Gogny pairing interaction are adopted in the calculations. A pronounced dipole peak appears below 10 MeV in ¹⁷Ne, but does not occur in ¹⁷F. The properties of this low-lying E1 excitation in ¹⁷Ne are studied, which may correspond to a proton pygmy resonance with different characteristics from those of giant dipole resonance.     

Effect of pairing correlation on low-lying quadrupole states in Sn isotopes

We examined the low-lying quadrupole states in Sn isotopes in the framework of fully self-consistent Hartree-Fock+BCS plus QRPA.We focus on the effect of the density-dependence of pairing interaction on the properties of the low-lying quadrupole state.The SLy5 Skyrme interaction with surface,mixed,and volume pairings is employed in the calculations,respectively.We find that the excitation energies and the corresponding reduced electric transition probabilities of the first 2~+ state are different,given by the three pairing interactions.The properties of the quasiparticle state,two-quasiparticle excitation energy,reduced transition amplitude,and transition densities in~(112)Sn are analyzed in detail.Two different mechanisms,the static and dynamical effects,of the pairing correlation are also discussed.The results show that the surface,mixed,and volume pairings indeed affect the properties of the first 2~+ state in the Sn isotopes.     

Isospin character of the pygmy dipole resonance in 124Sn

The pygmy dipole resonance has been studied in the proton-magic nucleus 124Sn with the (α, α'γ) coincidence method at Eα=136 MeV. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structure: one group of states which is excited in (α, α'γ) as well as in (γ, γ') reactions and a group of states at higher energies which is only excited in (γ, γ') reactions. Calculations with the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.     

Collective excitations in the unitary correlation operator method and relativistic QRPA studies of exotic nuclei

The collective excitation phenomena in atomic nuclei are studied in two different formulations of the random-phase approximation (RPA): (i) RPA based on correlated realistic nucleon-nucleon interactions constructed within the unitary correlation operator method (UCOM) and (ii) relativistic RPA derived from effective Lagrangians with density-dependent meson-exchange interactions. The former includes the dominant interaction-induced short-range central and tensor correlations by means of unitary transformation. It is shown that UCOM-RPA correlations induced by collective nuclear vibrations recover a part of the residual long-range correlations that are not explicitly included in the UCOM Hartree-Fock ground state. Both RPA models are employed in studies of the isoscalar giant monopole resonance in closed-shell nuclei across the nuclide chart, with an emphasis on the sensitivity of its properties on the constraints for the range of the UCOM correlation functions. Within the relativistic quasiparticle RPA (RQRPA) based on the relativistic Hartree-Bogolyubov model, the occurrence of pronounced low-lying dipole excitations is predicted in nuclei towards the proton drip line. From the analysis of the transition densities and the structure of the RQRPA amplitudes, it is shown that these states correspond to the proton pygmy dipole resonance. The text was submitted by the authors in English.     

A study of the nuclear response function

We examine the excitation properties of spherical nuclei in the Random Phase Approximation using the Green's function method. The calculations are done with interactions of the Skyrme type for nuclei from ¹⁶O to ²⁰⁸Pb. Different Skyrme interactions can have the same predictions for ground state Hartree-Fock properties, but give quite different predictions for the dynamic response. Our calculations favor a mild velocity-dependence in the interaction, such as given by Skyrme I. The level of agreement with empirical properties is as follows: energies of low-lying states, ≈25%; positions of giant resonances, ≈10%; transition rates of low states, factor of 2 typical. Inelastic scattering of electrons is reasonably accounted for by the model, but nucleon inelastic scattering has difficulties with the noncollective strength.     

Proton electric pygmy dipole resonance

The evolution of the low-lying E1 strength in proton-rich nuclei is analyzed in the framework of the self-consistent relativistic Hartree-Bogoliubov model and the relativistic quasiparticle random-phase approximation (RQRPA). Model calculations are performed for a series of N=20 isotones and Z=18 isotopes. For nuclei close to the proton drip line, the occurrence of pronounced dipole peaks is predicted in the low-energy region below 10 MeV excitation energy. From the analysis of the proton and neutron transition densities and the structure of the RQRPA amplitudes, it is shown that these states correspond to the proton pygmy dipole resonance.     

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.     

Effects of mean-field and pairing correlations on the Bogoliubov quasiparticle resonance

The quasiparticle resonances are investigated by examining three kinds of quasiparticle spectra, i.e., the density of quasiparticle states, the occupation number density, and the pair number density in the continuum Skyrme Hartree-Fock-Bogoliubov theory with the Green's function method. Taking the weakly bound nucleus ~(66)Ca as an example, the quasiparticle resonant energies and widths extracted from these three kinds of quasiparticle spectra are compared. For the narrow resonances, the extracted resonant energy and the width are consistent with each other. However, it is difficult to use the density of quasiparticle states to identify the broad resonances due to the background of nonresonant continuum. By switching off the pairing potential and/or the Hartree-Fock(HF) potential respectively in the calculation of these quasiparticle spectra, the roles of HF mean-field and pairing correlations in the quasiparticle resonances are demonstrated clearly. It turns out that all the quasiparticle resonances corresponding to the deeply bound, weakly bound and positive-energy single-particle resonant states, are mainly contributed by the HF potential. The pairing potential helps to slightly increase the resonant energy and the width. However, the pairing potential is important to make the nucleons occupy the low-lying nonresonant continuum states near the threshold and take part in the pairing correlations here,especially for the partial waves with small angular momentum ?.     

Description of low-lying state structures with Skyrme interaction

We use the quasiparticle random phase approximation to study properties of the low-lying 2⁺ states in the even-even nuclei around ¹³²Sn. Starting from a Skyrme interaction in the particle-hole channel and a density-dependent zero-range interaction in the particle-particle channel, the calculation within the finite-rank separable approximation for the residual interaction is performed.     

对关联对核基态和集体激发态性质的影响

基于相对论平均场和BCS理论,研究了共振连续对奇特核对关联性质的影响. 利用S矩阵方法，通过设定合理的散射态边界条件来得到单粒子共振态的能量和宽度. 通过引入连续态能级密度的方法来处理共振态宽度对核对关联的贡献. 计算结果显示合理地处理共振态对对关联性质的贡献在研究滴线附近核性质时很重要. 它可以影响中子的对隙、费米能级、对关联能以及总结合能. 其次,基于RMF+BCS基态,采用线性响应理论给出了描述开壳核集体激发态性质的准粒子相对论无规位相近似理论. 并且将该方法应用于开壳核120Sn的各种同位旋标量集体激发态性质的研究中. 研究表明：对关联对核的集体激发性质的影响主要表现在低能集体激发态上,考虑对关联后的相对论无规位相近似理论能够很好地再现低能集体激发的实验结果.     

Effects of 2 particle–2 hole configurations on dipole states in neutron-rich <Emphasis Type="Italic">N</Emphasis> = 80–84 isotones

Starting from the Skyrme interaction SLy4 we study the effects of 2 particle–2 hole configurations on the low-energy electric dipole response in ^130–134Sn. It is shown that the pygmy dipole resonance properties are correlated with the neutron skin thickness. The two-phonon configurations give a considerable contribution to the low-lying E1 strength.     

Description of proton-neutron mixed-symmetry states with Skyrme interaction

Starting from an effective Skyrme interaction we study the effects of the phonon-phonon coupling on the M1 transitions between quadrupole states. The finite rank separable approach for the quasiparticle random phase approximation is used. Choosing as an example the nucleus ⁹⁴Mo, we demonstrate an ability of the method to describe the properties of the low-lying states.     

A possible proton pygmy resonance in ~(17)Ne

The low-lying electric dipole strengths in proton-rich nuclei 17F and 17Ne, which can be produced at HIRFL-CSR in Lanzhou, are investigated. In the framework of the covariant density functional theory the self-consistent relativistic Hartree Bogoliubov model and the relativistic quasiparticle random phase approximation with the NL3 parameter set and Gogny pairing interaction are adopted in the calculations. A pronounced dipole peak appears below 10 MeV in17Ne, but does not occur in 17F. The prop erties of t...     

Relativistic RPA and Giant Resonances in β Stable and Unstable Nuclei

The nuclear isoscalar and isovector giant resonances in stable and unstable nuclei are studied in the framework of the relativistic random phase approximation. The classical meson propagators with nonlinear self-interactions are constructed in momentum space from the second variation of the action. The relativistic models with the parameter sets, TMI and NLSH, which provide good account of static ground state properties, can also well describe the collective states of nuclei, such as giant resonances. The isovector giant dipole resonances in the unstable Ar isotope chain are investigated.     

Excitation of collective states in spherical nuclei in high-energy proton interactions

The excitation of collective states in spherical nuclei is investigated in interactions with ≈ 1 GeV protons. Collective states of dipole and quadrupole type in a large energy range, including the isoscalar and isovector giant resonances, are considered. The cross sections for proton-nucleus interactions are found within the single-inelastic-scattering approximation of Glauber theory with the use of semi-microscopic nuclear wave functions. The largest probability of excitation is proved to take place for states of isoscalar type. Isovector states are strongly suppressed. It is shown that 1 GeV proton inelastic scattering can be used for the excitation of giant resonances of isoscalar type. The probability of their excitation is of the same order as that for low-lying collective states.     

Shape mixing dynamics in the low-lying states of proton-rich Kr isotopes

We study the oblate–prolate shape mixing in the low-lying states of proton-rich Kr isotopes using the five-dimensional quadrupole collective Hamiltonian. The collective Hamiltonian is derived microscopically by means of the CHFB (constrained Hartree–Fock–Bogoliubov) + Local QRPA (quasiparticle random phase approximation) method, which we have developed recently on the basis of the adiabatic self-consistent collective coordinate method. The results of the numerical calculation show the importance of large-amplitude collective vibrations in the triaxial shape degree of freedom and rotational effects on the oblate–prolate shape mixing dynamics in the low-lying states of these isotopes.     

Octupole Strength of β-Stable and Drip-Line Nuclei

The octupole excitations of β-stable nucleus 20882 Pb126, a neutron skin nucleus 6200Ca40 and a drip-line nucleus 288O20 are studied by using the self-consistent Hartree-Fock calculation plus the random phase approximation (RPA) with Skyrme interaction. The lowest isoscalar (IS) excitation below threshold for nuclei 20882 Pb126 and 6020Ca40, and the IS and isovector (Ⅳ) giant resonances of nuclei 20882 Pb126, 6020Ca40 and 288 O20 can be well described by collective model. For skin nucleus 2600Ca40 and drip-line nucleus 288 O20, the low-lying unperturbed neutron octupole strength (△N ＝ 1) of transitions to non-resonant states are nearly unaffected and the transitions to bound states are absorbed into collective states by taking into account the RPA correlation.