Relativistic QRPA Research Articles

Full Self-Consistent Study of Isobaric Analog Resonances

Fully self-consistent framework for isobaric-analog resonances is presented. The chains of parent neutron-rich Ca, Ni, Sn nuclei around the neutron shells at $$N=20$$ , $$28$$ , $$34$$ , $$50$$ , $$82$$ , as well as the nuclei on the neutron-deficient side of these chains are treated in the Density Functional plus Continuum Quasi particle Random Phase Approximation (DF $$+$$ CQRPA). The recently established new Fayans functional DF3-f is applied for calculations in long isotopic chains including non-magic nuclei with pairing in both neutron and proton sectors. A comparison with the relativistic QRPA and SAMi $$+$$ RPA shows a robust performance and flexibility of the DF $$+$$ CQRPA. Constraining the strength of exchange Coulomb contribution from the binding energy splitting of the doublets and triplets of the mirror nuclei allows for better description of the IAR energies.

Self-consistent approach to β-decay and delayed multi-neutron emission

The beta-decay half-lives and delayed multi- neutron emission branchings for the nuclei near the new neutron shell N = 34 are treated within self-consistent Density Functional + Continuum QRPA model (DF + CQRPA). A comparison with the recent self-consistent calculations from relativistic QRPA and standard (semi-microscopic) FRDM is performed.

Self-consistent approach to beta decay and delayed neutron emission

A brief overview of the recent self-consistent studies of nuclear beta decay is given including the relativistic quasi-particle random-phase approximation or QRPA and Finite Amplitude Method. The results of our self-consistent continuum QRPA model based on the density functional description of the ground states are presented. They are in a good agreement with the recent experimental beta-decay half-lives and delayed neutron emission branchings for the nuclei approaching (and beyond) the neutron closed shells N = 50 near 78Ni and N = 82 near 132Sn. A comparison with the recent calculations from relativistic QRPA model, Finite Amplitude Method and semi-microscopic finite-range droplet model is performed. An importance of the quasi-particle phonon coupling is stressed for the description of the beta decay and delayed multi-neutron emission rates. A strategy of extending our approach to the deformed nuclei and the open problems are discussed.

Gamma-ray strength at low energies using relativistic QRPA with exact coupling to the continuum

Continuum-quasiparticle random-phase Approximation (CQRPA) within the relativistic point-coupling model with density-dependent coupling constants is applied to investigate collective excitations in spherical nuclei. In particular we study the impact of the exact continuum on the giant-dipole and pygmy resonance of several Sn isotopes as well as the radiative neutron capture rates of importance for astrophysical calculations.

Neutrino and antineutrino cross sections in 12C

We extend our formalism of weak interaction processes, obtaining new expressions for the transition rates, which greatly facilitate numerical calculations, of neutrino (ν) and antineutrino (ν̄)-nucleus reactions and the muon capture. We have done a thorough study of exclusive (ground state) properties of 12B and 12N within the Projected Quasiparticle Random Phase Approximation (PQRPA). Good agreement with experimental data is achieved in this way. The inclusive ν/ν̄-nucleus reactions 12C(ν, e−)12N and 12C(ν̄, e+)12B are calculated within both the PQRPA, and the Relativistic QRPA (RQRPA). It is found that the magnitudes of the resulting cross-sections: i) are close to the sum-rule limit at low energy, but significantly smaller than this limit at high energies both for ν and ν̄, ii) steadily increase when the size of the configuration space is augmented, and particulary for ν/ν̄ energies > 200 MeV, and iii) converge for sufficiently large configuration spaces and final state angular momenta.

Neutrino and antineutrino charge-exchange reactions onC12

We extend the formalism of weak interaction processes, obtaining new expressions for the transition rates, which greatly facilitate numerical calculations, both for neutrino-nucleus reactions and muon capture. Explicit violation of CVC hypothesis by the Coulomb field, as well as development of a sum rule approach for the inclusive cross sections have been worked out. We have done a thorough study of exclusive (ground state) properties of $^{12}$B and $^{12}$N within the projected quasiparticle random phase approximation (PQRPA). Good agreement with experimental data achieved in this way put in evidence the limitations of standard RPA and the QRPA models, which come from the inability of the RPA in opening the $p_{3/2}$ shell, and from the non-conservation of the number of particles in the QRPA. The inclusive neutrino/antineutrino ($\nu/\tilde{\nu}$) reactions $^{12}$C($\nu,e^-)^{12}$N and $^{12}$C($\tilde{\nu},e^+)^{12}$B are calculated within both the PQRPA, and the relativistic QRPA (RQRPA). It is found that the magnitudes of the resulting cross-sections: i) are close to the sum-rule limit at low energy, but significantly smaller than this limit at high energies both for $\nu$ and $\tilde{\nu}$, ii) they steadily increase when the size of the configuration space is augmented, and particulary for $\nu/\tilde{\nu}$ energies $> 200$ MeV, and iii) converge for sufficiently large configuration space and final state spin. The quasi-elastic $^{12}$C($\nu,\mu^-)^{12}$N cross section recently measured in the MiniBooNE experiment is briefly discussed. We study the decomposition of the inclusive cross-section based on the degree of forbiddenness of different multipoles. A few words are dedicated to the $\nu/\tilde{\nu}$-$^{12}$C charge-exchange reactions related with astrophysical applications.

Study of the lowest [formula omitted] excitations and B(E2) transition strengths in relativistic QRPA for Sn-, and Pb-isotopes

In a relativistic quasiparticle random phase approximation (RQRPA) we have calculated the excitation energies of the first 2+ excited states and reduced B(E2;0+→2+) transition rates for very neutron-defficient to neutron-rich long chain of Sn-isotopes with even mass numbers A=100–136. In RQRPA approach this is the first such attempt and a very satisfactory agreement with the available experimental data is obtained. Furthermore, we predict a substantial rise of the B(E2) values at neutron magic numbers 50 and 82, where the E2 energy also goes up, rather substantially, compared to that for the immediate neighbours. This feature is quite similar to the already experimentally known behaviour of E2 and B(E2) around 208Pb, which are also well reproduced in the present calculation.

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 (RRPA) 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 an 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 monopole resonance (ISGMR) 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 Relativistic Hartree-Bogoliubov 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.

Self-consistent relativistic QRPA studies of soft modes and spin-isospin resonances in unstable nuclei

The excitation phenomena in unstable nuclei are investigated in the framework of the relativistic quasiparticle random-phase approximation (RQRPA) in the relativistic Hartree-Bogoliubov model (RHB) which is extended to include effective interactions with explicit density-dependent meson-nucleon couplings. The properties of the pygmy dipole resonance (PDR) are examined in 132Sn and within isotopic chains, showing that already at moderate proton-neutron asymmetry the PDR peak energy is located above the neutron emission threshold. A method is suggested for determining the size of the neutron skin within an isotopic chain, based on the measurement of the excitation energies of the Gamow-Teller resonance relative to the isobaric analog state. In addition, for the first time the relativistic RHB + RQRPA model, with tensor ω meson-nucleon couplings, is employed in calculations of β-decay half-lives of nuclei of the relevance for the r-process.

Relativistic QRPA description of low-lying dipole strength in neutron-rich nuclei

The relativistic quasi-particle random phase approximation (RQRPA) is applied in the analysis of the evolution of isovector dipole response in nuclei with large neutron excess. In neutron-rich nuclei the dipole response is characterized by the fragmentation of the strength distribution and its spreading into the low-energy region. In light nuclei the onset of dipole strength in the low-energy region is due to non-resonant single-particle excitations of the loosely bound neutrons. In heavier neutron-rich nuclei low-lying dipole states appear which display a more distributed structure of the RQRPA amplitudes. For these nuclei a single collective dipole state is identified in the low-energy region, and an analysis of the corresponding transition densities and velocity distributions reveals the dynamics of the pygmy resonance.

Relativistic QRPA description of low-lying dipole strength in neutron-rich nuclei

The relativistic quasi-particle random phase approximation (RQRPA) is applied in the analysis of the evolution of isovector dipole response in nuclei with large neutron excess. In neutron-rich nuclei the dipole response is characterized by the fragmentation of the strength distribution and its spreading into the low-energy region. In light nuclei the onset of dipole strength in the low-energy region is due to non-resonant single-particle excitations of the loosely bound neutrons. In heavier neutron-rich nuclei low-lying dipole states appear which display a more distributed structure of the RQRPA amplitudes. For these nuclei a single collective dipole state is identified in the low-energy region, and an analysis of the corresponding transition densities and velocity distributions reveals the dynamics of the pygmy resonance.

Relativistic QRPA description of low-lying dipole strength in neutron-rich nuclei

The relativistic quasi-particle random phase approximation (RQRPA) is applied in the analysis of the evolution of isovector dipole response in nuclei with large neutron excess. In neutron-rich nuclei the dipole response is characterized by the fragmentation of the strength distribution and its spreading into the low-energy region. In light nuclei the onset of dipole strength in the low-energy region is due to non-resonant single-particle excitations of the loosely bound neutrons. In heavier neutron-rich nuclei low-lying dipole states appear which display a more distributed structure of the RQRPA amplitudes. For these nuclei a single collective dipole state is identified in the low-energy region, and an analysis of the corresponding transition densities and velocity distributions reveals the dynamics of the pygmy resonance.