Oleksandr Gorbachenko - Academia.edu (original) (raw)

Papers by Oleksandr Gorbachenko

arXiv: Nuclear Theory, 2016

The results of the study of gamma-transition description in fast neutron capture and photofission... more The results of the study of gamma-transition description in fast neutron capture and photofission are presented. Recent experimental data were used, namely, the spectrum of prompt gamma-rays in the energy range 2÷18 MeV from 14-MeV neutron capture in natural Ni and isomeric ratios in primary fragments of photofission of the isotopes of U, Np and Pu by bremsstrahlung with end-point energies EeE_eEe= 10.5, 12 and 18 MeV. The data are compared with the theoretical calculations performed within EMPIRE 3.2 and TALYS 1.6 codes. The mean value of angular momenta and their distributions were determined in the primary fragments 84^{84}84Br, 97^{97}97Nb, 90^{90}90Rb, 131,133^{131,133}131,133Te, 132^{132}132Sb, 132,134^{132,134}132,134I, 135^{135}135Xe of photofissions. An impact of the characteristics of nuclear excited states on the calculation results is studied using different models for photon strength function and nuclear level density.

arXiv: Nuclear Theory, 2020

Mass-angle correlation of fission fragments has been understood as manifestation of quasifission.... more Mass-angle correlation of fission fragments has been understood as manifestation of quasifission. We show that this is not so: the effect can originate from correlation between fusion-fission amplitudes with different total spins signifying matter-wave interference in compound nucleus processes. This resolves the well-known puzzle with the mass-angle correlation in the complete fusion sub-barrier reaction 16^{16}16O+$^{238}$U. Our finding is important for more reliable predictions of production cross sections for superheavy elements. Matter-wave interference also produces quantum-classical transition to the time-orientation localization of the coherently rotating dinucleus in quasifission.

We extend a conventional description of the fusion-fission fragment angular distributions by intr... more We extend a conventional description of the fusion-fission fragment angular distributions by introducing the correlation between compound nucleus states carrying different total angular momenta. This correlation results in the strong anisotropy and mass-angle correlation of fission fragments for compact saddle-point nuclear shapes for which the conventional description predicts almost isotropic angular distributions. The spin off-diagonal phase relaxation timescale, simeq10−19\simeq 10^{-19}simeq10−19 sec, obtained from analysis of anomalous fission fragment angular distributions in 12^{12}12C+$^{236}$U, 16^{16}16O+$^{232}$Th and 16^{16}16O+$^{238}$U collisions at the sub-barrier energies is three orders of magnitude longer than the timescale of the compound nucleus thermalization. Expression for the angle-dependent time power spectrum for quasifission is also presented.

EPJ Web of Conferences, 2020

The closed-form expressions for the photon strength functions (PSF) are tested using the gamma-de... more The closed-form expressions for the photon strength functions (PSF) are tested using the gamma-decay data of OSLO group. The theoretical calculations are performed for the Lorentzian models of PSF for electric and magnetic dipole gamma-rays. The criteria of minimum of least-square value as well as the root-mean-square deviation factor are used. It is shown that a rather good agreement is obtained within the Simple Modified Lorentzian model for E1 PSF modelling.

AIP Conference Proceedings, 2009

The comparisons of different phenomenological methods for calculation of vibrational and rotation... more 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.

AIP Conference Proceedings, 2009

Photoabsorption cross sections and gamma-decay strength functions are calculated and compared wit... more Photoabsorption cross sections and gamma-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.

Nuclear Physics at Border Lines - Proceedings of the International Conference, 2002

ABSTRACT The collisional relaxation times of collective vibrations are studied using both the tra... more ABSTRACT The collisional relaxation times of collective vibrations are studied using both the transport approach and doorway state mechanism with hierarchy of particle-hole configurations in heated nuclei. It is shown that 1) the relaxation times of the kinetic method are rather slowly dependent on multipolarity of the Fermi surface distortion and mode of the collective motion; 2) 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.

The retardation and temperature effects in two-body collisions are studied. The collision integra... more 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.

The cross sections of the neutron reactions at E n = 14.6 MeV on the isotopes of Dy, Er, Yb with ... more The cross sections of the neutron reactions at E n = 14.6 MeV on the isotopes of Dy, Er, Yb with emission of neutrons, proton and alpha-particle were studied by the use of new experimental data and different theoretical approaches. New and improved experimental data were obtained by the neutronactivation technique. Present experimental results and evaluated nuclear data from EXFOR, TENDL, ENDF data libraries were compared with different systematics and calculations within codes of EMPIRE 3.0 and TALYS 1.2. Contribution of pre-equilibrium decay was studied. The recommendations on validity of different systematics for estimations of cross-sections of considered reactions are given.

Isomeric yield ratios of fission fragments 133 Te, 134 I, 135 Xe in (γ, f), (γ, nf) reactions on ... more Isomeric yield ratios of fission fragments 133 Te, 134 I, 135 Xe in (γ, f), (γ, nf) reactions on 235 U were measured. The contributions to the isomeric yields from nuclei of isobaric chains due to β-decay were taken into consideration. The sample was irradiated by bremsstrahlung of microtron M-30 (Institute of Electron Physics, Uzhgorod, Ukraine) with 17 MeV end point energy. The average angular momenta of the primary fragments were determined with the use of these data. The codes TALYS 1.4 and EMPIRE 3.2 were applied in the calculations. The effect of different expressions for the radiative strength functions and nuclear level densities on average angular momenta was studied.

Nuclear Physics and Atomic Energy

GAMMA-RAYS FROM nat Sn AND nat C INDUCED BY FAST NEUTRONS The cross-sections of prompt gamma-ray ... more GAMMA-RAYS FROM nat Sn AND nat C INDUCED BY FAST NEUTRONS The cross-sections of prompt gamma-ray production from nat Sn and nat C elements induced by 14.1-MeV neutrons were measured. The time-of-flight technique was used for n-γ discrimination. The experimental results were compared with theoretical calculations performed by Empire 3.2 and Talys 1.6 codes using different models for photon strength function and nuclear level density.

The European Physical Journal A

The description of photoabsorption cross-sections of cold nuclei by closed-form Lorentzian models... more The description of photoabsorption cross-sections of cold nuclei by closed-form Lorentzian models of photon strength functions for photoexcitation by electric dipole gamma-rays is considered. Systematics of the GDR parameters are given and input parameters of different analytical models are discussed The experimental data are compared with theoretical calculations for even-even nuclei using criteria of minimum of both least-square value and root-mean-square deviation factor. Simple extensions of the models with energy-dependent widths to high gamma-ray energies 30MeV which hold the energy-weighted sum rule for E1 gamma-transitions in good approximation are proposed and tested.

EPJ Web of Conferences

Analytical expression for the E1 photon strength functions (PSF) is modified to account for the l... more 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.

AIP Conference Proceedings, 2004

Bulletin of the Russian Academy of Sciences: Physics, 2009

ABSTRACT The differential cross sections of γ-rays from the nat Fe(n, xγ) and 209Bi(n, xγ) reacti... more ABSTRACT The differential cross sections of γ-rays from the nat Fe(n, xγ) and 209Bi(n, xγ) reactions with a neutron energy of 14.1 MeV are unfolded from amplitude spectra using the regularization procedure. The cross section errors are estimated. The experimental results are compared with the theoretical ones obtained within the Hauser-Feshbach statistical model. The sensitivity of the theoretically calculated γ-spectra to characteristics of the excited nuclear states is analyzed.

Nuclear Data Sheets, 2014

For nuclear level densities, a modification of an enhanced generalized superfluid model with diff... more For nuclear level densities, a modification of an enhanced generalized superfluid model with different collective state enhancement factors is studied. An effect of collective states on forming the temperature is taken into account. The ready-to-use tables for the asymptotic value of a-parameter of level density as well as for addition shift to excitation energy are prepared using the chi-square fit of the theoretical values of neutron resonance spacing and cumulative number of low-energy levels to experimental values. The systematics of these parameters as a function of mass number and neutron excess are obtained. The collective state effect on gamma-ray spectra and excitation functions of neutron-induced nuclear reactions is investigated by the use of EMPIRE 3.1 code with modified enhanced generalized superfluid model for nuclear level density. I.

Nuclear Data Sheets, 2014

Renewed systematics for giant dipole resonance (GDR) parameters is given. New version of the modi... more Renewed systematics for giant dipole resonance (GDR) parameters is given. New version of the modified Lorentzian approach for radiative strength function is proposed. The gamma-decay strength functions are calculated using renewed GDR parameters. The results are compared with experimental data. The calculations of the neutron induced reaction observables defined by the different RSF models were compared with experimental data. It is demonstrated that closed-form approaches with asymmetric shape of the gamma strength provide the most reliable simple method for description of gamma-decay processes.

Bulletin of the Russian Academy of Sciences: Physics, 2014

arXiv: Nuclear Theory, 2016

The results of the study of gamma-transition description in fast neutron capture and photofission... more The results of the study of gamma-transition description in fast neutron capture and photofission are presented. Recent experimental data were used, namely, the spectrum of prompt gamma-rays in the energy range 2÷18 MeV from 14-MeV neutron capture in natural Ni and isomeric ratios in primary fragments of photofission of the isotopes of U, Np and Pu by bremsstrahlung with end-point energies EeE_eEe= 10.5, 12 and 18 MeV. The data are compared with the theoretical calculations performed within EMPIRE 3.2 and TALYS 1.6 codes. The mean value of angular momenta and their distributions were determined in the primary fragments 84^{84}84Br, 97^{97}97Nb, 90^{90}90Rb, 131,133^{131,133}131,133Te, 132^{132}132Sb, 132,134^{132,134}132,134I, 135^{135}135Xe of photofissions. An impact of the characteristics of nuclear excited states on the calculation results is studied using different models for photon strength function and nuclear level density.

arXiv: Nuclear Theory, 2020

Mass-angle correlation of fission fragments has been understood as manifestation of quasifission.... more Mass-angle correlation of fission fragments has been understood as manifestation of quasifission. We show that this is not so: the effect can originate from correlation between fusion-fission amplitudes with different total spins signifying matter-wave interference in compound nucleus processes. This resolves the well-known puzzle with the mass-angle correlation in the complete fusion sub-barrier reaction 16^{16}16O+$^{238}$U. Our finding is important for more reliable predictions of production cross sections for superheavy elements. Matter-wave interference also produces quantum-classical transition to the time-orientation localization of the coherently rotating dinucleus in quasifission.

We extend a conventional description of the fusion-fission fragment angular distributions by intr... more We extend a conventional description of the fusion-fission fragment angular distributions by introducing the correlation between compound nucleus states carrying different total angular momenta. This correlation results in the strong anisotropy and mass-angle correlation of fission fragments for compact saddle-point nuclear shapes for which the conventional description predicts almost isotropic angular distributions. The spin off-diagonal phase relaxation timescale, simeq10−19\simeq 10^{-19}simeq10−19 sec, obtained from analysis of anomalous fission fragment angular distributions in 12^{12}12C+$^{236}$U, 16^{16}16O+$^{232}$Th and 16^{16}16O+$^{238}$U collisions at the sub-barrier energies is three orders of magnitude longer than the timescale of the compound nucleus thermalization. Expression for the angle-dependent time power spectrum for quasifission is also presented.

EPJ Web of Conferences, 2020

The closed-form expressions for the photon strength functions (PSF) are tested using the gamma-de... more The closed-form expressions for the photon strength functions (PSF) are tested using the gamma-decay data of OSLO group. The theoretical calculations are performed for the Lorentzian models of PSF for electric and magnetic dipole gamma-rays. The criteria of minimum of least-square value as well as the root-mean-square deviation factor are used. It is shown that a rather good agreement is obtained within the Simple Modified Lorentzian model for E1 PSF modelling.

AIP Conference Proceedings, 2009

The comparisons of different phenomenological methods for calculation of vibrational and rotation... more 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.

AIP Conference Proceedings, 2009

Photoabsorption cross sections and gamma-decay strength functions are calculated and compared wit... more Photoabsorption cross sections and gamma-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.

Nuclear Physics at Border Lines - Proceedings of the International Conference, 2002

ABSTRACT The collisional relaxation times of collective vibrations are studied using both the tra... more ABSTRACT The collisional relaxation times of collective vibrations are studied using both the transport approach and doorway state mechanism with hierarchy of particle-hole configurations in heated nuclei. It is shown that 1) the relaxation times of the kinetic method are rather slowly dependent on multipolarity of the Fermi surface distortion and mode of the collective motion; 2) 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.

The retardation and temperature effects in two-body collisions are studied. The collision integra... more 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.

The cross sections of the neutron reactions at E n = 14.6 MeV on the isotopes of Dy, Er, Yb with ... more The cross sections of the neutron reactions at E n = 14.6 MeV on the isotopes of Dy, Er, Yb with emission of neutrons, proton and alpha-particle were studied by the use of new experimental data and different theoretical approaches. New and improved experimental data were obtained by the neutronactivation technique. Present experimental results and evaluated nuclear data from EXFOR, TENDL, ENDF data libraries were compared with different systematics and calculations within codes of EMPIRE 3.0 and TALYS 1.2. Contribution of pre-equilibrium decay was studied. The recommendations on validity of different systematics for estimations of cross-sections of considered reactions are given.

Isomeric yield ratios of fission fragments 133 Te, 134 I, 135 Xe in (γ, f), (γ, nf) reactions on ... more Isomeric yield ratios of fission fragments 133 Te, 134 I, 135 Xe in (γ, f), (γ, nf) reactions on 235 U were measured. The contributions to the isomeric yields from nuclei of isobaric chains due to β-decay were taken into consideration. The sample was irradiated by bremsstrahlung of microtron M-30 (Institute of Electron Physics, Uzhgorod, Ukraine) with 17 MeV end point energy. The average angular momenta of the primary fragments were determined with the use of these data. The codes TALYS 1.4 and EMPIRE 3.2 were applied in the calculations. The effect of different expressions for the radiative strength functions and nuclear level densities on average angular momenta was studied.

Nuclear Physics and Atomic Energy

GAMMA-RAYS FROM nat Sn AND nat C INDUCED BY FAST NEUTRONS The cross-sections of prompt gamma-ray ... more GAMMA-RAYS FROM nat Sn AND nat C INDUCED BY FAST NEUTRONS The cross-sections of prompt gamma-ray production from nat Sn and nat C elements induced by 14.1-MeV neutrons were measured. The time-of-flight technique was used for n-γ discrimination. The experimental results were compared with theoretical calculations performed by Empire 3.2 and Talys 1.6 codes using different models for photon strength function and nuclear level density.

The European Physical Journal A

The description of photoabsorption cross-sections of cold nuclei by closed-form Lorentzian models... more The description of photoabsorption cross-sections of cold nuclei by closed-form Lorentzian models of photon strength functions for photoexcitation by electric dipole gamma-rays is considered. Systematics of the GDR parameters are given and input parameters of different analytical models are discussed The experimental data are compared with theoretical calculations for even-even nuclei using criteria of minimum of both least-square value and root-mean-square deviation factor. Simple extensions of the models with energy-dependent widths to high gamma-ray energies 30MeV which hold the energy-weighted sum rule for E1 gamma-transitions in good approximation are proposed and tested.

EPJ Web of Conferences

Analytical expression for the E1 photon strength functions (PSF) is modified to account for the l... more 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.

AIP Conference Proceedings, 2004

Bulletin of the Russian Academy of Sciences: Physics, 2009

ABSTRACT The differential cross sections of γ-rays from the nat Fe(n, xγ) and 209Bi(n, xγ) reacti... more ABSTRACT The differential cross sections of γ-rays from the nat Fe(n, xγ) and 209Bi(n, xγ) reactions with a neutron energy of 14.1 MeV are unfolded from amplitude spectra using the regularization procedure. The cross section errors are estimated. The experimental results are compared with the theoretical ones obtained within the Hauser-Feshbach statistical model. The sensitivity of the theoretically calculated γ-spectra to characteristics of the excited nuclear states is analyzed.

Nuclear Data Sheets, 2014

For nuclear level densities, a modification of an enhanced generalized superfluid model with diff... more For nuclear level densities, a modification of an enhanced generalized superfluid model with different collective state enhancement factors is studied. An effect of collective states on forming the temperature is taken into account. The ready-to-use tables for the asymptotic value of a-parameter of level density as well as for addition shift to excitation energy are prepared using the chi-square fit of the theoretical values of neutron resonance spacing and cumulative number of low-energy levels to experimental values. The systematics of these parameters as a function of mass number and neutron excess are obtained. The collective state effect on gamma-ray spectra and excitation functions of neutron-induced nuclear reactions is investigated by the use of EMPIRE 3.1 code with modified enhanced generalized superfluid model for nuclear level density. I.

Nuclear Data Sheets, 2014

Renewed systematics for giant dipole resonance (GDR) parameters is given. New version of the modi... more Renewed systematics for giant dipole resonance (GDR) parameters is given. New version of the modified Lorentzian approach for radiative strength function is proposed. The gamma-decay strength functions are calculated using renewed GDR parameters. The results are compared with experimental data. The calculations of the neutron induced reaction observables defined by the different RSF models were compared with experimental data. It is demonstrated that closed-form approaches with asymmetric shape of the gamma strength provide the most reliable simple method for description of gamma-decay processes.

Bulletin of the Russian Academy of Sciences: Physics, 2014