开放存取 开放存取  受限制的访问 ##reader.subscriptionAccessGranted##  受限制的访问 订阅存取

卷 79, 编号 6 (2016)

Nuclei

Dual origin of pairing in nuclei

Idini A., Potel G., Barranco F., Vigezzi E., Broglia R.

摘要

The pairing correlations of the nucleus 120Sn are calculated by solving the Nambu–Gor’kov equations, including medium polarization effects resulting from the interweaving of quasiparticles, spin and density vibrations, taking into account, within the framework of nuclear field theory (NFT), processes leading to self-energy and vertex corrections and to the induced pairing interaction. From these results one can not only demonstrate the inevitability of the dual origin of pairing in nuclei, but also extract information which can be used at profit to quantitatively disentangle the contributions to the pairing gap Δ arising from the bare and from the induced pairing interaction. The first is the strong 1S0 short-range NN potential resulting from meson exchange between nucleons moving in time reversal states within an energy range of hundreds of MeV from the Fermi energy. The second results from the exchange of vibrational modes between nucleons moving within few MeV from the Fermi energy. Short- (vpbare) and long-range (vpind) pairing interactions contribute essentially equally to nuclear Cooper pair stability. That is to the breaking of gauge invariance in open-shell superfluid nuclei and thus to the order parameter, namely to the ground state expectation value of the pair creation operator. In other words, to the emergent property of generalized rigidity in gauge space, and associated rotational bands and Cooper pair tunneling between members of these bands.

Physics of Atomic Nuclei. 2016;79(6):807-810
pages 807-810 views

Spherical nuclei near the stability line and far from it

Isakov V.

摘要

Results of microscopic and semiphenomenological calculations of features of spherical nuclei lying near the stability line and far from it are presented. The reason why the nuclei being considered are spherical is that they are magic at least in one nucleon sort. The present analysis is performed for Z = 50 and Z = 28 isotopes and for N = 50 isotones, the region extending from neutron-rich to neutron-deficient nuclei being covered. The isotopic dependence of the mean-field spin–orbit nuclear potential is revealed; systematics of energies of levels and probabilities for electromagnetic transitions is examined; and root-mean-square radii of nuclei are calculated, along with the proton- and neutron-density distributions in them. Nuclei in the vicinity of closed shells are considered in detail, and the axial-vector weak coupling constant in nuclei is evaluated. A systematic comparison of the results of calculations with experimental data is performed.

Physics of Atomic Nuclei. 2016;79(6):811-834
pages 811-834 views

Random matrix analysis of the monopole strength distribution in 208Pb

Severyukhin A., Åberg S., Arsenyev N., Nazmitdinov R., Pichugin K.

摘要

We study statistical properties of the 0+ spectrum of 208Pb in the energy region Ex ≤ 20 MeV. We use the Skyrme interaction SLy4 as our model Hamiltonian to create a single-particle spectrum and to analyze excited states. The finite-rank separable approximation for the particle–hole interaction enables us to perform the calculations in large configuration spaces. We show that while the position of the monopole resonance centroid is determined by one-phonon excitations of 0+, the phonon–phonon coupling is crucial for the description of the strength distribution of the 0+ spectrum. In fact, this coupling has an impact on the spectral rigidity Δ3(L) which is shifted towards the random matrix limit of the Gaussian orthogonal ensembles.

Physics of Atomic Nuclei. 2016;79(6):835-841
pages 835-841 views

Toroidal resonance: Relation to pygmy mode, vortical properties, and anomalous deformation splitting

Reinhard P., Nesterenko V., Kvasil J., Repko A., Kleinig W.

摘要

We review a recent progress in investigation of the isoscalar toroidal dipole resonance (TDR). A possible relation of the TDR and low-energy dipole excitations (also called a pygmy resonance) is analyzed. It is shown that the dipole strength in the pygmy region can be understood as a local manifestation of the collective vortical toroidalmotion at the nuclear surface. Application of the TDR as a measure of the nuclear dipole vorticity is discussed. An anomalous splitting of the TDR in deformed nuclei is inspected.

Physics of Atomic Nuclei. 2016;79(6):842-850
pages 842-850 views

The interacting quasiparticle–phonon picture and odd–even nuclei. Overview and perspectives

Mishev S., Voronov V.

摘要

The role of the nucleon correlations in the ground states of even–even nuclei on the properties of low-lying states in odd–even spherical and transitional nuclei is studied. We reason about this subject using the language of the quasiparticle–phonon model which we extend to take account of the existence of quasiparticle⊗phonon configurations in the wave functions of the ground states of the even–even cores. Of paramount importance to the structure of the low-lying states happens to be the quasiparticle–phonon interaction in the ground states which we evaluated using both the standard and the extended random phase approximations. Numerical calculations for nuclei in the barium and cadmium regions are performed using pairing and quadrupole–quadrupole interaction modes which have the dominant impact on the lowest-lying states’ structure. It is found that states with same angular momentum and parity become closer in energy as compared to the predictions of models disregarding the backward amplitudes, which turns out to be in accord with the experimental data. In addition we found that the interaction between the last quasiparticle and the ground-state phonon admixtures produces configurations which contribute significantly to the magnetic dipolemoment of odd-A nuclei. It also reveals a potential for reproducing their experimental values which proves impossible if this interaction is neglected.

Physics of Atomic Nuclei. 2016;79(6):851-857
pages 851-857 views

Particle-vibration coupling: Recent advances in microscopic calculations with the Skyrme Hamiltonian

Colò G., Baldo M., Bortignon P., Rizzo D., Bocchi G.

摘要

In this contribution, we report some recent progress in our understanding of particle-vibration coupling (PVC) in nuclei. In particular, we first review the formal development that has allowed some of us to deduce the PVC equations within the Green’s functionmethod. Applications are then discussed, both in the case of single-particle states and giant resonances in magic nuclei. We also present a new model that extends the PVC ansatz and is meant to account for the complete low-lying spectra of odd nuclei.

Physics of Atomic Nuclei. 2016;79(6):858-867
pages 858-867 views

The phonon-coupling model for Skyrme forces

Lyutorovich N., Tselyaev V., Speth J., Krewald S., Reinhard P.

摘要

A short review on the self-consistent RPA based on the energy-density functional of the Skyrme type is given. We also present an extension of the RPA where the coupling of phonons to the single-particle states is considered. Within this approach we present numerical results which are compared with data. The self-consistent approach is compared with the Landau–Migdal theory. Here we derive from the self-consistent ph interaction, the Landau–Migdal parameters as well as their density dependence. In the Appendix a new derivation of the reduced matrix elements of the ph interaction is presented.

Physics of Atomic Nuclei. 2016;79(6):868-884
pages 868-884 views

Energy–density functional plus quasiparticle–phonon model theory as a powerful tool for nuclear structure and astrophysics

Tsoneva N., Lenske H.

摘要

During the last decade, a theoretical method based on the energy–density functional theory and quasiparticle–phonon model, including up to three-phonon configurations was developed. The main advantages of themethod are that it incorporates a self-consistentmean-field and multi-configuration mixing which are found of crucial importance for systematic investigations of nuclear low-energy excitations, pygmy and giant resonances in an unified way. In particular, the theoretical approach has been proven to be very successful in predictions of new modes of excitations, namely pygmy quadrupole resonance which is also lately experimentally observed. Recently, our microscopically obtained dipole strength functions are implemented in predictions of nucleon-capture reaction rates of astrophysical importance. A comparison to available experimental data is discussed.

Physics of Atomic Nuclei. 2016;79(6):885-903
pages 885-903 views

E2 transitions between excited single-phonon states: Role of ground-state correlations

Kamerdzhiev S., Voitenkov D.

摘要

The probabilities for E2 transitions between low-lying excited 3 and 5 single-phonon states in the 208Pb and 132Sn magic nuclei are estimated on the basis of the theory of finite Fermi systems. The approach used involves a new type of ground-state correlations, that which originates from integration of three (rather than two, as in the random-phase approximation) single-particle Green’s functions. These correlations are shown to make a significant contribution to the probabilities for the aforementioned transitions.

Physics of Atomic Nuclei. 2016;79(6):904-909
pages 904-909 views

Self-consistent approach to beta decay and delayed neutron emission

Borzov I.

摘要

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.

Physics of Atomic Nuclei. 2016;79(6):910-923
pages 910-923 views

Unitary version of the particle–hole dispersive optical model

Gorelik M., Tulupov B., Urin M.

摘要

For the particle–hole dispersive optical model developed recently, a method is proposed for restoring unitarity weakly violated by a phenomenological description of the spreading effect. The method is implemented by performing a quantitative analysis of the energy-averaged isoscalar monopole double transition density and strength functions over a broad energy range including the isoscalar giant monopole resonance and its overtone in the 208Pb nucleus.

Physics of Atomic Nuclei. 2016;79(6):924-928
pages 924-928 views

Charge-exchange resonances and restoration of Wigner’s supersymmetry in heavy and superheavy nuclei

Lutostansky Y., Tikhonov V.

摘要

Various facets of the question of whether Wigner’s supersymmetry [SU(4) symmetry] may be restored in heavy and superheavy nuclei are analyzed on the basis of a comparison of the results of calculations with experimental data. The energy difference between the giant Gamow–Teller resonance and the analog resonance (the difference of EG and EA) according to calculations based on the theory of finite Fermi systems is presented for the case of 33 nuclei for which experimental data are available. The calculated difference ΔEG–A of EG and EA tends to zero in heavier nuclei, showing evidence of the restoration of Wigner’s SU(4) symmetry. Also, the isotopic dependence of the Coulomb energy difference between neighboring isobaric nuclei is analyzed within the SU(4) approach for more than 400 nuclei in the mass-number range of A = 5–244. The restoration of Wigner’s SU(4) symmetry in heavy nuclei is confirmed. It is shown that the restoration of SU(4) symmetry is compatible with the possible existence of the stability island in the region of superheavy nuclei.

Physics of Atomic Nuclei. 2016;79(6):929-942
pages 929-942 views

Internal bremsstrahlung of strongly interacting charged particles

Kurgalin S., Tchuvil’sky Y., Churakova T.

摘要

A universal theoretical model intended for calculating internal-bremsstrahlung spectra is proposed. In this model, which can be applied to describing nuclear decays of various type (such as alpha decay, cluster decay, and proton emission), use is made of realistic nucleus–nucleus potentials. Theoretical internal-bremsstrahlung spectra were obtained for the alpha decay of the 214Po nucleus, as well as for the decay of the 222Ra nucleus via the emission of a 14C cluster and for the decay of the 113Cs nucleus via proton emission, and the properties of these spectra were studied. The contributions of various regions (internal, subbarrier, and external) to the internal-bremsstrahlung amplitude were analyzed in detail. It is shown that the contribution of the internal region to the amplitude for internal bremsstrahlung generated in nuclear decay via proton emission is quite large, but that this is not so for alpha decay and decay via cluster emission. Thus, a process in which strong interaction of nuclear particles affects the internal-bremsstrahlung spectrum if found.

Physics of Atomic Nuclei. 2016;79(6):943-950
pages 943-950 views

Manifestation of the structure of heavy nuclei in their alpha decays

Adamian G., Antonenko N., Bezbakh A., Malov L.

摘要

Low-lying one- and two-quasiparticle states of heavy nuclei are predicted. Alpha-decay chains, including those that proceed through isomeric states, are examined on the basis of the predicted properties of superheavy nuclei.

Physics of Atomic Nuclei. 2016;79(6):951-962
pages 951-962 views

Description of alternating-parity bands within the dinuclear-system model

Shneidman T., Adamian G., Antonenko N., Jolos R., Lenske H., Scheid W.

摘要

A cluster approach is used to describe ground-state-based alternating-parity bands in even–even nuclei and to study the band-termination mechanism. A method is proposed for testing the cluster nature of alternating-parity bands.

Physics of Atomic Nuclei. 2016;79(6):963-977
pages 963-977 views

Application of a folding-model optical potential to analyzing inelastic pion–nucleus scattering and the in-medium effect on a pion–nucleon amplitude

Lukyanov V., Zemlyanaya E., Lukyanov K., Abdul-Magead I.

摘要

The folding-model optical potential is generalized in such a way as to apply it to calculating the cross sections for inelastic scattering of π±-mesons on 28Si, 40Ca, 58Ni, and 208Pb nuclei at the energies of 162, 180, 226, and 291 MeV leading to the excitation of the 2+ and 3 collective states. In doing this, use is made of known nucleon-density distributions in nuclei and the pion–nucleon scattering amplitude whose parameters were obtained previously by fitting the elastic scattering cross sections for the same nuclei. Thus, the values of quadrupole (β2) and octupole (β3) deformations of nuclei appear here as the only adjustable parameters. The scattering cross section is calculated by solving the relativistic wave equation, whereby effects of relativization and distortion in the entrance and exit scattering channels are taken exactly into account. The cross sections calculated in this way for inelastic scattering are in good agreement with respective experimental data. The importance of the inclusion of in-medium effects in choosing parameters of the pion–nucleon amplitude is emphasized.

Physics of Atomic Nuclei. 2016;79(6):978-986
pages 978-986 views

Formation of Hyper Hydrogen isotopes in light p-shell nuclei

Jolos R., Majling L., Majlingova O.

摘要

Recent experiment by A1 Collaboration at Mainz, namely identification of Λ4H hyperfragment from primary Λ9Li hypernucleus produced in reaction (e, eK+), stirred renewed interest in baryonic decay of hypernuclei. The important role of s−1sΛ excitation in p-shell hypernuclei as well as the existence of selection rules connected with [f] (Young diagram) was noted earlier within Translational Invariant Shell Model (TISM). The particular conditions of the present experiment (reaction is not selective and produces highly excited states) dictate extension of the simplest TISM: expansion of the harmonic oscillator basis space and compression of multiplets (to only (λμ) for L and [f] for S, T). Such modified TISM explains abundance production of Λ4H hyperfragment and predicts production of another Hyper Hydrogen Λ6H.

Physics of Atomic Nuclei. 2016;79(6):987-994
pages 987-994 views

Quantum signatures of chaos or quantum chaos?

Bunakov V.

摘要

A critical analysis of the present-day concept of chaos in quantum systems as nothing but a “quantum signature” of chaos in classical mechanics is given. In contrast to the existing semi-intuitive guesses, a definition of classical and quantum chaos is proposed on the basis of the Liouville–Arnold theorem: a quantum chaotic system featuring N degrees of freedom should have M < N independent first integrals of motion (good quantum numbers) specified by the symmetry of the Hamiltonian of the system. Quantitative measures of quantum chaos that, in the classical limit, go over to the Lyapunov exponent and the classical stability parameter are proposed. The proposed criteria of quantum chaos are applied to solving standard problems of modern dynamical chaos theory.

Physics of Atomic Nuclei. 2016;79(6):995-1009
pages 995-1009 views

Raynal–Revai coefficients for a general kinematic rotation

Ershov S.

摘要

In a three-body system, transitions between different sets of normalized Jacobi coordinates are described as general kinematic transformations that include an orthogonal or a pseudoorthogonal rotation. For such rotations, the Raynal–Revai coefficients execute a unitary transformation between three-body hyperspherical functions. Recurrence relations that make it possible to calculate the Raynal–Revai coefficients for arbitrary angular momenta are derived on the basis of linearized representations of products of hyperspherical functions.

Physics of Atomic Nuclei. 2016;79(6):1010-1018
pages 1010-1018 views

The Skyrme-TQRPA calculations of electron capture on hot nuclei in pre-supernova environment

Dzhioev A., Vdovin A., Stoyanov C.

摘要

We combine the thermal QRPA approach with the Skyrme energy density functional theory (Skyrme–TQRPA) for modelling the process of electron capture on nuclei in supernova environment. For a sample nucleus, 56Fe, the Skyrme–TQRPA approach is applied to analyze thermal effects on the strength function of GT+ transitions which dominate electron capture at Ee ≤ 30 MeV. Several Skyrme interactions are used in order to verify the sensitivity of the obtained results to the Skyrme force parameters. Finite-temperature cross sections are calculated and the results are comparedwith those of the other model calculations.

Physics of Atomic Nuclei. 2016;79(6):1019-1029
pages 1019-1029 views

Self-consistent theory of finite Fermi systems and Skyrme–Hartree–Fock method

Saperstein E., Tolokonnikov S.

摘要

Recent results obtained on the basis of the self-consistent theory of finite Fermi systems by employing the energy density functional proposed by Fayans and his coauthors are surveyed. These results are compared with the predictions of Skyrme–Hartree–Fock theory involving several popular versions of the Skyrme energy density functional. Spherical nuclei are predominantly considered. The charge radii of even and odd nuclei and features of low-lying 2+ excitations in semimagic nuclei are discussed briefly. The single-particle energies ofmagic nuclei are examined inmore detail with allowance for corrections to mean-field theory that are induced by particle coupling to low-lying collective surface excitations (phonons). The importance of taking into account, in this problem, nonpole (tadpole) diagrams, which are usually disregarded, is emphasized. The spectroscopic factors of magic and semimagic nuclei are also considered. In this problem, only the surface term stemming from the energy dependence induced in the mass operator by the exchange of surface phonons is usually taken into account. The volume contribution associated with the energy dependence initially present in the mass operator within the self-consistent theory of finite Fermi systems because of the exchange of high-lying particle–hole excitations is also included in the spectroscopic factor. The results of the first studies that employed the Fayans energy density functional for deformed nuclei are also presented.

Physics of Atomic Nuclei. 2016;79(6):1030-1066
pages 1030-1066 views

Shells, orbit bifurcations, and symmetry restorations in Fermi systems

Magner A., Koliesnik M., Arita K.

摘要

The periodic-orbit theory based on the improved stationary-phase method within the phase-space path integral approach is presented for the semiclassical description of the nuclear shell structure, concerning themain topics of the fruitful activity ofV.G. Soloviev. We apply this theory to study bifurcations and symmetry breaking phenomena in a radial power-law potential which is close to the realistic Woods–Saxon one up to about the Fermi energy. Using the realistic parametrization of nuclear shapes we explain the origin of the double-humped fission barrier and the asymmetry in the fission isomer shapes by the bifurcations of periodic orbits. The semiclassical origin of the oblate–prolate shape asymmetry and tetrahedral shapes is also suggested within the improved periodic-orbit approach. The enhancement of shell structures at some surface diffuseness and deformation parameters of such shapes are explained by existence of the simple local bifurcations and new non-local bridge-orbit bifurcations in integrable and partially integrable Fermi-systems. We obtained good agreement between the semiclassical and quantum shell-structure components of the level density and energy for several surface diffuseness and deformation parameters of the potentials, including their symmetry breaking and bifurcation values.

Physics of Atomic Nuclei. 2016;79(6):1067-1123
pages 1067-1123 views