D.O. Eremenko

Stochastic model of tilting mode in nuclear fission

A model of induced nuclear fission was developed with consideration of thermodynamically fluctuating orientation degree of freedom (tilting) of deformed nuclei. This model was applied to analysis of the experimental angular anisotropy of fission fragments in the 16O + 232Th, 238U, 248Cm, 208Pb, 209Bi; 12C + 236U; 19F + 208Pb; and 11B + 237Np reactions. Information on the equilibrating time of the tilting mode was obtained.

Fragment angular distribution as a probe of fission dynamics in nuclei with two classes of excited states

The fragment angular distribution from the multichance fission of excited nuclei with a double humped fission barrier is considered. It is shown that the fragment angular distribution is determined by contributions related to both the saddle points. The relative weights of these contributions are calculated in the framework of the diffusion model. The possibility of determining the nuclear dissipation is discussed. A comparison of the experimental data with the calculations leads to a value of the reduced dissipation coefficient for the Pu isotopes produced in the reaction at = 37 and 43 MeV.

Statistical description of the decay of nuclei having two classes of excited states

The decay of heavy nuclei is discussed in the framework of the statistical theory using the double-humped model for the fission barrier. It is shown that the existence of two classes of excited states in such nuclei is reflected in many observables, such as the decay law, the cross sections, the partial decay probabilities and the time dependence of decay yields. It is demonstrated that: (i) the decay time in the fission channel is different from that in any other channel; (ii) the partial decay probabilities depend on the initial conditions under which the compound nucleus is formed; (iii) the lifetime of the second-well states influences the evaporation spectra. It is also shown that the decay time characteristics of nuclei having two classes of excited states contain information on the strongly deformed excited states in the second well, like their density, and the depth of the second well itself.

Lifetime measurement of fissionable nuclei produced in the development of neutron emission (III) Lifetime of protactinium isotopes

Abstract The lifetimes of the fissionable plutonium isotopes 238Pu, 240Pu, 241Pu have been measured using the crystal-blocking technique. These isotopes were produced by α-particle bombardment of 235UO2 and 238UO2 single crystals with following emission of neutrons. Analysis of the experimental results in the framework of the statistical model of nuclear reactions was performed.

Investigation of shell effects for heavy fissionable nuclei by the blocking technique

Abstract The crystal blocking technique has been used to measure delay times in the fission channel for excited nuclei produced in the 28 Si + nat Pt reaction. Experimental results range from 10 −17 to 10 −18 s at bombarding energies from 140 to 170 MeV. It is shown that fission barriers keep their double-humped structure for temperature up to 1.7–1.8 MeV, and that the lifetimes of excited states in the second potential well contribute essentially to the observed delay times in the fission channel.

Fission time in the 28Si + natPt reaction

The experimental fission times are analyzed for excited nuclei produced in the 28Si + natPt reaction. Experimental lifetimes obtained by the crystal-blocking technique range between 10−17 and 10−18 s at bombarding energies between 140 and 170 MeV, respectively. Experimental data are analyzed within the statistical theory of nuclear reactions and the double-humped-fission-barrier model with allowance for preequilibrium processes and the nuclear-dissipation phenomenon. It is shown that fission barriers retain their double-humped structure for nuclear temperatures up to about 1.7–1.8 MeV and that the lifetimes of excited strongly deformed states in the second potential well contribute substantially to the observed delay times in the fission decay channel.

THE PROBABILITY OF POPULATING THE SECOND POTENTIAL WELL STATES IN FISSION DYNAMICS

We study the probability of populating the second well states in the de-excitation of the compound nucleus during the competition between fission and emission of light particles. We consider the cases of underdamped and overdamped fission processes caused by a different influence of nuclear dissipation. The probability dependencies on the second well depth and the excitation energy are also analyzed.

Stochastic Model of the Tilting Mode in Nuclear Fission

A model of induced nuclear fission is developed with consideration of thermodynamically fluctuating orientational degree of freedom of deformed nuclei. This model was applied in analysis of the experimental angular anisotropy of fission fragments in the 16O + 232Th, 238U and 248Cm reactions at the oxygen energies from 90 to 160 MeV. Information on the equlibrating time of the orientational mode was obtained.

PRE-EQUILIBRIUM EFFECTS IN THE SECONDARY PARTICLE SPECTRA IN THE REACTIONS WITH HEAVY IONS

Theoretical description of the experimentally obtained spectra for protons and α-particles and model calculations for the neutron spectra in the reactions with heavy ions has been presented. The hybrid model of non-equilibrium processes was used. Equilibrium evaporation process was analyzed in the framework of the statistical theory of nuclear reactions with Monte-Carlo simulation including certain dynamical and kinematical characteristics. This approach was included in PACE code, which permits to simulate Monte-Carlo de-excitation nuclear process. The Fermi-gas model and level-density phenomenological model for the variation of the nuclear level density parameters was used. In this approach data on 16O+116Sn reaction with Ebeam = 130, 250 MeV were analyzed. Double-differential light charged particle spectra for this reaction were measured using the GARFIELD apparatus in coincidence with evaporation residues. The experimental data were collected in four angular ranges from 29 to 41, 41 to 53, 53 to 67 and 67 to 82 degrees in the laboratory system. The results of the calculations are shown and discussed for these four angular ranges. The contributions from the evaporative and pre-equilibrium processes were analyzed in connection with different nucleus equilibration mechanisms.

Influence of shell effects on the angular distributions of fission fragments

A dynamical-statistical model is used to analyze the experimental angular distributions of fission fragments in the reactions α + 238U, 237Np at Eα = 20–100 MeV, as well as to determine the Am isotope fission probabilities and the shape isomer yields in the reactions d + 242,240Pu at Ed = 20–30 MeV. Manifestations of shell effects are found in the fission barrier structure up to the excitation energies of 50–60 MeV.