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V. A. Plujko

Publications and source records attributed to V. A. Plujko.

14 recordsLinked to original sources
arXiv2026

Systematics of characteristics of pygmy dipole resonances in medium-heavy and heavy atomic nuclei with neutron excess

The systematics of energies and the contribution of pygmy dipole resonance (PDR) to the energy-weighted sum rule of dipole gamma transitions in medium-heavy and heavy nuclei with an excess of neutrons are considered. The modified macroscopic model of Isacker-Nagarajan-Warner was used for calculating PDR energies with the number of surface neutrons proportional to the thickness of the neutron skin according to the Pethick- Ravenhall expression (PR INW approach). Such modification of the macroscopic approach by Isacker-Nagarajan-Warner enables to take into account microscopic evidence of direct relationship between skin thickness and low-energy dipole response. The results are compared with the microscopic calculations for the chains of Ni, Sn and Pb isotopes. It was demonstrated that the dependence of the magnitudes of the energies within the PR INW approach on neutron excess is in rather good agreement with experimental data and microscopic calculations if the absolute value of the strength of the neutron-proton interaction is nearly three times as large as that obtained by Isacker-Nagarajan-Warner by the volume integral of the nucleon-nucleon interaction. While the macroscopic INW PR model can describe the main features of the PDR, above mentioned discrepancy of the strength values doesn't not provide reason enough for the conclusion that PDR is pure collective state. The analytical expressions for the PDR fraction of the energy-weighted sum rule for electric dipole transitions (E1 EWSR) are used. They are based on the "molecular" energy-weighted E1 sum rule considering the number of surface neutrons as a function of the neutron thickness (PR MSR approach). Systematics for the PDR fraction of E1 EWSR are proposed with parameters obtained by fitting the experimental data and microscopic calculations.

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arXiv2022

Reconstruction of high-energy part of gamma-ray spectrum in thermal neutron capture by $^{113}$Cd

The average gamma-ray spectrum $^{114}$Cd after thermal neutron capture in $^{113}$Cd was evaluated in units of mb/MeV. Two approaches are considered for estimation of the average gamma-ray spectrum with normalization of the experimental data: mean spectra for all gamma-energies were found by averaging frequency polygon for experimental data histogram, and mean spectra were estimated as the combination of theoretical values at low gamma-ray energies and averaging experimental data in high-energy range. The experi mental spectra were evaluated from the gamma-intensities presented by Mheemeed et al. [A. Mheemeed et al., Nucl. Phys. A 412 (1984) 113] and Belgya et al [T. Belgya et al., EPJ Web of Conf. 146 (2017) 05009]. They were normalized to the average theoretical spectrum which was calculated using EMPIRE and TALYS codes. The procedure of normalization of the high-energy part of the spectrum was described. Estimated $γ$- spectra for $^{113}$Cd(n,{x$γ$}) reaction induced by thermal neutrons were presented.

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arXiv2018

Giant Dipole Resonance Parameters of Ground-State Photoabsorption: Experimental Values with Uncertainties

Updated values and corresponding uncertainties of Isovector Giant Dipole Resonance (GDR) parameters which are obtained by the least-squares fitting of theoretical photoabsorption cross sections to experimental data are presented. The theoretical photoabsorption cross sections are taken as a sum of the components corresponding to the excitation of the GDR and quasideuteron photodisintegration. The current compilation is an extension and improvement of the earlier compilations of Lorentzian parameters for ground-state photoneutron and photoabsorption cross sections and covers experimental data made available up to June 2017.

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arXiv2016

Shear viscosity of nuclear matter

Shear viscosity $η$ is calculated for the nuclear matter described as a system of interacting nucleons with the van der Waals (VDW) equation of state. The Boltzmann-Vlasov kinetic equation is solved in terms of the plane waves of the collective overdamped motion. In the frequent-collision regime, the shear viscosity depends on the particle-number density $n$ through the mean-field parameter $a$, which describes attractive forces in the VDW equation. In the temperature region $T=15 - 40$~MeV, a ratio of the shear viscosity to the entropy density $s$ is smaller than 1 at the nucleon number density $n =(0.5 - 1.5)\,n^{}_0$, where $n^{}_0=0.16\,$fm$^{-3}$ is the particle density of equilibrium nuclear matter at zero temperature. A minimum of the $η/s$ ratio takes place somewhere in a vicinity of the critical point of the VDW system. Large values of $η/s\gg 1$ are, however, found in both the low-density, $n\ll n^{}_0$, and high-density, $n>2n^{}_0$, regions. This makes the ideal hydrodynamic approach inapplicable for these densities.

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arXiv2008

Verification of Models for Calculation of E1 Radiative Strength

Photoabsorption cross sections and gamma-decay strength function are calculated and compared with experimental data to test the existing models of dipole radiative strength functions (RSF) for the middle-weight and heavy atomic nuclei. Simplified version of the modified Lorentzian model are proposed. New tables of giant dipole resonance (GDR) parameters are given. It is shown that the phenomenological closed-form models with asymmetric shape can be used for overall estimates of the dipole RSF in the gamma -ray energy region up to about 20 MeV when GDR parameters are known or the GDR systematics can be adopted. Otherwise, the HFB-QRPA microscopic model and the semi-classical approach with moving surface appear to be more adequate methods to estimate the dipole photoabsorption RSF.

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arXiv2002

Shape of Dipole Radiative Strength Function for Asymmetric Nuclei

The semiclassical method for description of the radiative strength function is used for asymmetric nuclei with $N \ne Z$. The theory is based on the linearized Vlasov-Landau equations in two-component finite Fermi liquid. The dependence of the shape $E1$ strength on the coupling constant between proton and neutron subsystems was studied.

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arXiv2002

Non-Markovian Collision Integral in Fermi-systems

The non-Markovian collision integral is obtained on the base of the Kadanoff-Baym equations for Green functions in a form with allowance for small retardation effects. The collisional relaxation times and damping width of the giant isovector dipole resonances in nuclear matter are calculated. For an infinite Fermi liquid the dependence of the relaxation times on the collective vibration frequency and the temperature corresponds to the Landau's prescription.

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arXiv2001

Two-body relaxation times in heated nuclei

The retardation and temperature effects in two-body collisions are studied. The collision integral with retardation effects is obtained on the base of the Kadanoff- Baym equations for Green functions in a form with allowance for reaching the local equilibrium system. The collisional relaxation times of collective vibrations are calculated using both the transport approach and doorway state mechanism with hierarchy of particle-hole configurations in heated nuclei. The relaxation times of the kinetic method are rather slowly dependent on multipolarity of the Fermi surface distortion and mode of the collective motion. The dependence of the relaxation times on temperature as well as on frequency of collective vibrations is considered and compared. It is shown that variations of the in-medium two-body cross-sections with energy lead to non-quadratic dependence of the collisional relaxation time both on temperature and on collective motion frequency.

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arXiv1999

Testing and improvements of gamma-ray strength functions for nuclear model calculations of nuclear data

A closed-form thermodynamic pole approach,TPA, is developed for average description of the E1 radiative strength functions using the microcanonical ensemble for initial states. A semiclassical description of the collective excitation damping in this method is based on modern physical notion on the relaxation processes in Fermi systems.The TPA model gives rather accurate means of simultaneous description of the gamma- decay and photoabsorption strength functions in the medium and heavy nuclei. It is able to cover a relatively wide energy interval, ranging from zeroth gamma-ray energy to values above GDR peak energy, as compared with the others closed-form models for calculation of the E1 strength.

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arXiv1999

Collisional relaxation of collective motion in a finite Fermi liquid

Finite size effects in the equilibrium phase space density distribution function are taken into account for alculations of the relaxation of collective motion in finite nuclei. Memory effects in the collision integral and the diffusivity and the quantum oscillations of the equilibrium distribution function in momentum space are considered. It is shown that a smooth diffuse (Fermi-type) equilibrium distribution function leads to a spurious contribution to the relaxation time. The residual quantum oscillations of the equilibrium distribution function eliminates the spurious contribution. It ensures the disappearance of the gain and loss terms in the collision integral in the ground state of the system and strongly reduces the internal collisional width of the isoscalar giant quadrupole resonances.

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arXiv1998

An investigation of interplay between dissipation mechanisms in heated Fermi systems by means of radiative strength functions

A simple analytical expression for the gamma-decay strength function is derived with microcanonical ensemble for initial excited states. The approach leads to both a non-zero limit of the strength function for vanishing gamma-ray energy and a partial breakdown of Brink hypothesis. It is shown that the low energy behaviour of the gamma-decay strength functions is governed by the energy behavior of the damping width. It may provides a new tool for study of the interplay between different relaxation mechanisms of the collective excitations.

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arXiv1998

Relaxation of fast collective motion in heated nuclei

The damping of the collective vibrations in hot nuclei is studied within the semiclassical Vlasov-Landau kinetic theory. The extention of the method of independent sources of dissipation is used to allow for irreversible energy transfer by chaos weighted wall formula. The expressions for the intrinsic width of the giant multipole resonances are obtained. The interplay between the one-body and the two-body channels which contribute to the formation of the intrinsic width in nuclei is discussed.

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