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V. Yu. Ponomarev

Publications and source records attributed to V. Yu. Ponomarev.

At least 55 records · Page 3Linked to original sources
arXiv2009

Gamow-Teller strength distributions at finite temperatures and electron capture in stellar environments

We propose a new method to calculate stellar weak-interaction rates. It is based on the Thermo-Field-Dynamics formalism and allows the calculation of the weak-interaction response of nuclei at finite temperatures. The thermal evolution of the GT$_+$ distributions is presented for the sample nuclei $^{54, 56}$Fe and ~$^{76,78,80}$Ge. For Ge we also calculate the strength distribution of first-forbidden transitions. We show that thermal effects shift the GT$_+$ centroid to lower excitation energies and make possible negative- and low-energy transitions. In our model we demonstrate that the unblocking effect for GT$_+$ transitions in neutron-rich nuclei is sensitive to increasing temperature. The results are used to calculate electron capture rates and are compared to those obtained from the shell model.

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arXiv2009

Skyrme-Rpa Description of Spin-Flip M1 Giant Resonance

The spin-flip M1 giant resonance is explored in the framework of Random Phase Approximation on the basis of the Skyrme energy functional. A representative set of eight Skyrme parameterizations (SkT6, SkM*, SLy6, SG2, SkO, SkO', SkI4, and SV-bas) is used. Light and heavy, spherical and deformed nuclei ($^{48}$Ca, $^{158}$Gd, $^{208}$Pb, and $^{238}$U) are considered. The calculations show that spin densities play a crucial role in forming the collective shift in the spectrum. The interplay of the collective shift and spin-orbit splitting determines the quality of the description. None of the considered Skyrme parameterizations is able to describe simultaneously the M1 strength distribution in closed-shell and open-shell nuclei. It is found that the problem lies in the relative positions of proton and neutron spin-orbit splitting. Necessity to involve the tensor and isovector spin-orbit interaction is called for.

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arXiv2009

Excitation energy and strength of the pygmy dipole resonance in stable tin isotopes

The $^{112,120}$Sn$(γ,γ')$ reactions have been studied at the S-DALINAC. Electric dipole (E1) strength distributions have been determined including contributions from unresolved strength extracted by a fluctuation analysis. Together with available data on $^{116,124}$Sn, an experimental systematics of the pygmy dipole resonance (PDR) in stable even-mass tin isotopes is established. The PDR centroid excitation energies and summed strengths are in reasonable agreement with quasiparticle-phonon model calculations based on a nonrelativistic description of the mean field but disagree with relativistic quasiparticle random-phase approximation predictions.

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arXiv2008

Test of a modified BCS theory performance in the Picket Fence Model

Analyses of a modified BCS (MBCS) theory performance at finite temperatures in the Picket Fence Model (PFM) for light and heavy systems is presented. Both symmetric, $Ω=N$ ($N$ particles on $Ω$ twice-degenerate levels), and asymmetric, $Ω\neq N$, versions of the PFM are considered. The quantities known exactly from particle-hole symmetry of the $Ω=N$ PFM are calculated. Starting from very low temperatures, these quantities are found in dramatic deviation from the exact values in MBCS results. Consequences of the MBCS prediction that heating generates a thermal constituent of the pairing gap, are discussed. Thermodynamical inconsistency of the MBCS is also addressed.

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arXiv2006

Excitation of low-lying state by E3 transition in reaction with real photons

The yield of the isomeric state 117mSn (Eiso = 314.58 keV) has been measured in (gamma, gamma') reaction by activation method with the bremsstrahlung end-point energy from 2.1 to 3.0 MeV. Only one intermediate state (IS) responsible for the isomer feeding has been found. The excitation energy of the IS (2.25 +/- 0.05) MeV and photoproduction integral cross section (0.022 +/- 0.002) eV b have been deduced. Microscopic calculations within the Quasiparticle-phonon model have been performed to learn on the IS structure. We conclude that the IS is excited in the present experiment by the E3-transition.

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arXiv2005

Two-phonon 1- state in 112Sn observed in resonant photon scattering

Results of a photon scattering experiment on 112Sn using bremsstrahlung with an endpoint energy of E_0 = 3.8 MeV are reported. A J = 1 state at E_x = 3434(1) keV has been excited. Its decay width into the ground state amounts to Gamma_0 = 151(17) meV, making it a candidate for a [2+ x 3-]1- two-phonon state. The results for 112Sn are compared with quasiparticle-phonon model calculations as well as the systematics of the lowest-lying 1- states established in other even-mass tin isotopes. Contrary to findings in the heavier stable even-mass Sn isotopes, no 2+ states between 2 and 3.5 MeV excitation energy have been detected in the present experiment.

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arXiv2005

Test of modified BCS model at finite temperature

A recently suggested modified BCS (MBCS) model has been studied at finite temperature. We show that this approach does not allow the existence of the normal (non-superfluid) phase at any finite temperature. Other MBCS predictions such as a negative pairing gap, pairing induced by heating in closed-shell nuclei, and ``superfluid -- super-superfluid'' phase transition are discussed also. The MBCS model is tested by comparing with exact solutions for the picket fence model. Here, severe violation of the internal symmetry of the problem is detected. The MBCS equations are found to be inconsistent. The limit of the MBCS applicability has been determined to be far below the ``superfluid -- normal'' phase transition of the conventional FT-BCS, where the model performs worse than the FT-BCS.

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arXiv2005

Low-energy dipole excitations towards the proton drip-line: doubly magic 48Ni

The properties of the low-energy dipole response are investigated for the proton-rich doubly magic nucleus $^{48}$Ni, in a comparative study of two microscopic models: fully self-consistent Relativistic Random-Phase Approximation(RRPA) based on the novel density-dependent meson-exchange interactions, and Continuum Random-Phase Approximation(CRPA) using Skyrme-type interactions with the continuum properly included. Both models predict the existence of the low-energy soft mode, i.e. the proton pygmy dipole resonance (PDR), for which the transition densities and RPA amplitudes indicate the dynamics of loosely bound protons vibrating against the rest of the nucleons. The CRPA analysis indicates that the escape width for the proton PDR is rather large, as a result of the coupling to the continuum.

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arXiv2004

A microscopic investigation of the transition form factor in the region of collective multipole excitations of stable and unstable nuclei

We have used a self-consistent Skyrme-Hartree-Fock plus Continuum-RPA model to study the low-multipole response of stable and neutron/proton-rich Ni and Sn isotopes. We focus on the momentum-transfer dependence of the strength distribution, as it provides information on the structure of excited nuclear states and in particular on the variations of the transition form factor (TFF) with the energy. Our results show, among other things, that the TFF may show significant energy dependence in the region of the isoscalar giant monopole resonance and that the TFF corresponding to the threshold strength in the case of neutron-rich nuclei is different compared to the one corresponding to the respective giant resonance. Perspectives are given for more detailed future investigations.

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arXiv2004

Nuclear vorticity and the low-energy nuclear response - Towards the neutron drip line

The transition density and current provide valuable insight into the nature of nuclear vibrations. Nuclear vorticity is a quantity related to the transverse transition current. In this work, we study the evolution of the strength distribution, related to density fluctuations, and the vorticity strength distribution, as the neutron drip line is approached. Our results on the isoscalar, natural-parity multipole response of Ni isotopes, obtained by using a self-consistent Skyrme-Hartree-Fock + Continuum RPA model, indicate that, close to the drip line, the low-energy response is dominated by L>1 vortical transitions.

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arXiv2003

"Modified" BCS theory at finite temperature and its flaws

Drawbacks of a recently suggested modified BCS theory at finite temperature in [Phys. Rev. C {\bf 67}, 014304 (2003)] are discussed. We conclude that the thermal behavior of the pairing correlations is not under control within this model.

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arXiv2001

Description of Double Giant Dipole Resonance within the Phonon Damping Model

In a recent Letter [1] an overall agreement with the experimental data for the excitation of the single and double giant dipole resonances in relativistic heavy ion collision in 136Xe and 208Pb nuclei has been reported. We point out that this agreement is achieved by a wrong calculation of the DGDR excitation mechanism. We also argue that the agreement with the data for the widths of resonances is achieved by an unrealistically large value of a model parameter. [1] Nguyen Dinh Dang, Vuong Kim Au, and Akito Arima, Phys. Rev. Lett. 85 (2000) 1827.

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arXiv2000

Anharmonic properties of double giant dipole resonance

A systematic microscopic study of the anharmonic properties of the double giant dipole resonance (DGDR) has been carried out, for the first time, for nuclei with mass number $A$ spanning the whole mass table. It is concluded that the corrections of the energy centroid of the $J^π = 0^+$ and $2^+$ components of the DGDR from its harmonic limit are negative, have a value of the order of few hundred keV and follow an $A^{-1}$ dependence.

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arXiv1999

Population of isomers in decay of the giant dipole resonance

The value of an isomeric ratio (IR) in N=81 isotones ($^{137}$Ba, $^{139}$Ce, $^{141}$Nd and $^{143}$Sm) is studied by means of the ($γ, n)$ reaction. This quantity measures a probability to populate the isomeric state in respect to the ground state population. In ($γ, n)$ reactions, the giant dipole resonance (GDR) is excited and after its decay by a neutron emission, the nucleus has an excitation energy of a few MeV. The forthcoming $γ$ decay by direct or cascade transitions deexcites the nucleus into an isomeric or ground state. It has been observed experimentally that the IR for $^{137}$Ba and $ ^{139}$Ce equals about 0.13 while in two heavier isotones it is even less than half the size. To explain this effect, the structure of the excited states in the energy region up to 6.5 MeV has been calculated within the Quasiparticle Phonon Model. Many states are found connected to the ground and isomeric states by $E1$, $E2$ and $M1$ transitions. The single-particle component of the wave function is responsible for the large values of the transitions. The calculated value of the isomeric ratio is in very good agreement with the experimental data for all isotones. A slightly different value of maximum energy with which the nuclei rest after neutron decay of the GDR is responsible for the reported effect of the A-dependence of the IR.

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