V.A. Plujko

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

Abstract A simple analytical expression for the γ-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 γ - 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.

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 prescription.

Dependence of E1 Radiative Strength Function on Neutron Excess in Heavy Nuclei

The electric dipole radiative strength function (isovector dipole response) of neutron‐rich spherical nuclei is studied using a semiclassical method based on the Landau‐Vlasov kinetic equation for finite two‐component systems. It is shown that, by taking into account the surface degree of freedom, it is possible to obtain an exact treatment of the centre of mass motion. It is also shown that a prescription used to subtract the spurious strength in RPA calculations gives the correct semiclassical strength function for the systems with fixed surface. We focus on the influence of the neutron excess on the isovector dipole strength distribution.

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 calculations 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.

Improvements and testing practical expressions for photon strength functions of E1 gamma-transitions

Analytical expression for the E1 photon strength functions (PSF) is modified to account for the low-energy enhancement due to nuclear structure effects (presence of pygmy dipole resonance (PDR)). A closed-form expression of the E1 PSF function includes response of two nuclear states – PDR and giant dipole resonance (GDR). Expression for the nuclear response function on electromagnetic field is based on a model of excitation of two coupled damped states. This approach is tested for different data sets for spherical nuclei. Impact on the PSF shape of coupling between the PDR and GDR excitations is considered.

Collective State Enhancement of Nuclear Level Density

The comparisons of different phenomenological methods for calculation of vibrational and rotational enhancement factors of nuclear level density in spherical and deformed nuclei are performed. A practical method for the calculation of vibrational enhancement factor and level density parameters is recommended. The asymptotic level density parameters and additional energy shifts are obtained for 291 atomic nuclei. The systematics of these parameters are given.

Closed‐Form E1 Radiative Strength Functions for Gamma‐Decay and Photoabsorption

Photoabsorption cross sections and γ‐decay strength functions are calculated and compared with experimental data to test simple phenomenological models of E1 gamma‐strength description in the middle‐weight and heavy atomic nuclei. Radiative strength functions with energy asymmetric shape are recommended for overall estimation of averaged gamma‐strengths. Systematics for giant dipole resonance (GDR) parameters are given.

Effect of Vibrational States on Nuclear Level Density

Simple methods to calculate a vibrational enhancement factor of a nuclear level density with allowance for damping of collective state are considered. The results of the phenomenological approach and the microscopic quasiparticle—phonon model are compared. The practical method of calculation of a vibrational enhancement factor and level density parameters is recommended.