Relativistic Quasiparticle Time-blocking Approximation Research Articles

Electric dipole response of neutron-rich calcium isotopes in relativistic quasiparticle time blocking approximation

New results for electric dipole strength in the chain of even-even Calcium isotopes with the mass numbers A = 40 - 54 are presented. Starting from the covariant Lagrangian of Quantum Hadrodynamics, spectra of collective vibrations (phonons) and phonon-nucleon coupling vertices for $J \leq 6$ and normal parity were computed in a self-consistent relativistic quasiparticle random phase approximation (RQRPA). These vibrations coupled to Bogoliubov two-quasiparticle configurations (2q$\otimes$phonon) form the model space for the calculations of the dipole response function in the relativistic quasiparticle time blocking approximation (RQTBA). The results for giant dipole resonance in the latter approach are compared to those obtained in RQRPA and to available data. Evolution of the dipole strength with neutron number is investigated for both high-frequency giant dipole resonance (GDR) and low-lying strength. Development of a pygmy resonant structure on the low-energy shoulder of GDR is traced and analyzed in terms of transition densities. A dependence of the pygmy dipole strength on the isospin asymmetry parameter is extracted.

Nature of low-lying electric dipole resonance excitations inGe74

Isospin properties of dipole excitations in 74 Ge are investigated using the ({\alpha},{\alpha}'{\gamma}) reaction and compared to ({\gamma},{\gamma}) data. The results indicate that the dipole excitations in the energy region of 6 to 9 MeV adhere to the scenario of the recently found splitting of the region of dipole excitations into two separated parts: one at low energy, being populated by both isoscalar and isovector probes, and the other at high energy, excited only by the electromagnetic probe. Relativistic quasiparticle time blocking approximation (RQTBA) calculations show a reduction in the isoscalar E1 strength with an increase in excitation energy, which is consistent with the measurement.

Nuclear response theory for spin-isospin excitations in a relativistic quasiparticle-phonon coupling framework

A new theoretical approach to spin-isospin excitations in open-shell nuclei is presented. The developed method is based on the relativistic meson-exchange nuclear Lagrangian of Quantum Hadrodynamics and extends the response theory for superfluid nuclear systems beyond relativistic quasiparticle random phase approximation in the proton-neutron channel (pn-RQRPA). The coupling between quasiparticle degrees of freedom and collective vibrations (phonons) introduces a time-dependent effective interaction, in addition to the exchange of pion and $\rho$-meson taken into account without retardation. The time-dependent contributions are treated in the resonant time-blocking approximation, in analogy to previously developed relativistic quasiparticle time blocking approximation (RQTBA) in the neutral (non-isospin-flip) channel. The new method is called proton-neutron RQTBA (pn-RQTBA) and applied to Gamow-Teller resonance in a chain of neutron-rich Nickel isotopes $^{68-78}$Ni. A strong fragmentation of the resonance along with quenching of the strength, as compared to pn-RQRPA, is obtained. Based on the calculated strength distribution, beta-decay half-lives of the considered isotopes are computed and compared to pn-RQRPA half-lives and to experimental data. It is shown that a considerable improvement of the half-life description is obtained in pn-RQTBA because of the spreading effects, which bring the lifetimes to a very good quantitative agreement with data.

Dipole excitations via isoscalar probes: The splitting of the pygmy dipole resonance in124Sn

Inelastic $\ensuremath{\alpha}$-scattering excitation cross sections are calculated for electric-dipole excitations in ${}^{124}\mathrm{Sn}$ based on the transition densities obtained from the relativistic quasiparticle time-blocking approximation (RQTBA) in the framework of a semiclassical model. The calculation provides the missing link to directly compare the results from the microscopic RQTBA calculations to recent experimental data measured via the $(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction, which show a structural splitting of the low-lying $E1$ strength often denoted as pygmy dipole resonance (PDR). The experimentally observed splitting is reproduced by the cross-section calculations, which allows us to draw conclusion on the structure of the PDR.

Relativistic two-phonon model for the low-energy nuclear response

A two-phonon version of the relativistic quasiparticle time-blocking approximation introduces a new class of many-body models for nuclear structure calculations based on the covariant energy density functional. As a fully consistent extension of the relativistic quasiparticle random phase approximation, the relativistic two-phonon model implies fragmentation of nuclear states over two-quasiparticle and two-phonon configurations coupled to each other. In particular, we show how the lowest two-phonon ${1}^{\ensuremath{-}}$ state, identified as a member of the $[{2}^{+}\ensuremath{\bigotimes}{3}^{\ensuremath{-}}]$ quintuplet, emerges from the coherent two-quasiparticle pygmy dipole mode in vibrational nuclei. The inclusion of the two-phonon configurations into the model space allows a quantitative description of the positions and the reduced transition probabilities of the lowest ${1}^{\ensuremath{-}}$ states in tin isotopes ${}^{112,\phantom{\rule{0.16em}{0ex}}116,\phantom{\rule{0.16em}{0ex}}120,\phantom{\rule{0.16em}{0ex}}124}$Sn as well as the low-energy fraction of the dipole strength below the giant dipole resonance without any adjustment procedures. The model is applied to the low-lying dipole strength in neutron-rich ${}^{68,\phantom{\rule{0.16em}{0ex}}70,\phantom{\rule{0.16em}{0ex}}72}$Ni isotopes. Recent experimental data for ${}^{68}$Ni are reproduced fairly well.

Structure of the pygmy dipole resonance in124Sn

Background: In atomic nuclei, a concentration of electric dipole strength around the particle threshold, commonly denoted as pygmy dipole resonance, may have a significant impact on nuclear structure properties and astrophysical scenarios. A clear identification of these states and the structure of this resonance is still under discussion.Purpose: We present an experimental and theoretical study of the isospin character of the pygmy dipole resonance and investigation of a splitting of the electric dipole strength previously observed in experiments on $N=82$ nuclei.Method: The pygmy dipole resonance has been studied in the semi-magic $Z=50$ nucleus ${}^{124}$Sn by means of the $(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\gamma})$ coincidence method at ${E}_{\ensuremath{\alpha}}=136\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ using the Big-Bite Spectrometer at the Kernfysisch Versneller Instituut in Groningen, The Netherlands.Results: A splitting of the low-energy part of the electric dipole strength was identified in ${}^{124}$Sn by comparing the differential cross sections measured in $(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\gamma})$ to results stemming from $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ photon-scattering experiments. While an energetically lower-lying group of states is observed in both kinds of experiments, a higher-lying group of states is only excited in the $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ reaction. In addition, theoretical calculations using the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle-phonon model have been performed. Both calculations show a qualitative agreement with the experimental data and predict a low-lying isoscalar component that is dominated by neutron-skin oscillations as expected for the pygmy dipole resonance. Furthermore, the states at higher energies show a pronounced isovector component and a different radial dependence of the corresponding transition densities as expected for the tail of the giant dipole resonance.Conclusions: An experimental signature of the neutron-skin oscillation of the pygmy dipole resonance has been corroborated. The combination of the presented reactions might make it possible to identify states of this resonance.

Isospin Character of the Pygmy Dipole Resonance inSn124

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.

Relativistic quasiparticle time blocking approximation. II. Pygmy dipole resonance in neutron-rich nuclei

Theoretical studies of low-lying dipole strength in even-even spherical nuclei within the relativistic quasiparticle time blocking approximation (RQTBA) are presented. The RQTBA developed recently as an extension of the self-consistent relativistic quasiparticle random-phase approximation (RQRPA) enables one to investigate the effects of the coupling of two-quasiparticle excitations to collective vibrations within a fully consistent calculation scheme based on covariant energy density functional theory. Dipole spectra of even-even $^{130}\mathrm{Sn}\text{\ensuremath{-}}^{140}\mathrm{Sn}$ and $^{68}\mathrm{Ni}\text{\ensuremath{-}}^{78}\mathrm{Ni}$ isotopes calculated within both RQRPA and RQTBA show two well-separated collective structures: the higher lying giant dipole resonance and the lower lying pygmy dipole resonance, which can be identified by the different behavior of the transition densities of states in these regions.

Low-lying dipole response in the relativistic quasiparticle time blocking approximation and its influence on neutron capture cross sections

We have computed dipole strength distributions for nickel and tin isotopes within the Relativistic Quasiparticle Time Blocking Approximation (RQTBA). These calculations provide a good description of data, including the neutron-rich tin isotopes 130,132Sn. The resulting dipole strengths have been implemented in Hauser–Feshbach calculations of astrophysical neutron capture rates relevant for r-process nucleosynthesis studies. The RQTBA calculations show the presence of enhanced dipole strength at energies around the neutron threshold for neutron rich nuclei. The computed neutron capture rates are sensitive to the fine structure of the low lying dipole strength, which emphasizes the importance of a reliable knowledge of this excitation mode.