V. V. Pashkevich

On the asymmetric deformation of fissioning nuclei

Abstract The potential energy of a nucleus at large (scission) deformation is calculated by the shell- correction method due to Strutinsky. The shape of the nucleus in the zeroth-order approximation is taken to be the Cassinian ovaloid, the deviation being expanded in a series of Legendre polynomials. Asymmetric shapes of the nucleus are studied; a definition of the Woods-Saxon type nuclear potential is given; the single-particle spectrum, shell correction, liquid-drop energy and total potential energy are calculated. It is shown that the transuranium nuclei are symmetric at the first barrier and in the second minimum of the potential energy. The second saddle point is situated at a large asymmetric deformation. The nuclear asymmetry near the scission point is in qualitative agreement with experimental data on the asymmetry in the mass distribution of the fission fragments. The agreement between the theoretical results and experimental data on the fission barriers is improved when the asymmetry is taken into account.

Shell energies of rapidly rotating nuclei

Abstract A theory for the shell corrections to the deformation energy of rapidly rotating nuclei is worked out and applied in a study of the energy landscapes of eighteen rare earth nuclei with spins up to the limit for stability against centrifugal fission. Triaxial ellipsoidal shapes are considered. In addition to the yrast configuration one or two local minima in the deformation energy are often obtained. These equilibrium configurations are generally not axially symmetric. The calculations suggest that cold nuclei are able to carry a larger amount of angular momentum than that expected from the classical model of a charged liquid drop. The implications of the calculated energy landscapes for the structure of the yrast γ-decay are discussed.

The energy of non-axial deformation of heavy nuclei

Abstract The total energy of nuclei in the actinide region was calculated according to non-axial deformation by the Strutinsky method. Stability to γ-deformation is shown for both first and second minima of the potential energy curve. Stiffness of γ-vibrations is considerably larger for the second minimum than for the first one. The saddle point between the two minima is found to be shifted to γ ≈ 5–10°.

Prescission shapes of symmetrically fissioning very heavy nuclei

Abstract It is shown that in the theoretical description of the fission process in the nucleus 264 Fm there turn out to be three valleys on the potential-energy surface in the region of the scission point. One valley corresponds to the compact configuration of two nearly spherical fragments; another, to more separated strongly elongated fragments. There still exists one more valley, in which the nuclear shape is close to a combination of spherical and elongated fragments. Some properties of the fragment mass and energy distributions are considered.

Transport coefficients for shape degrees in terms of Cassini ovaloids

Previous computations of the potential landscape with the shapes parameterized in terms of Cassini ovaloids are extended to collective dynamics at finite excitations. Taking fission as the most demanding example of large scale collective motion, transport coefficients are evaluated along a fission path. We concentrate on those for average motion, namely stiffness C, friction \gamma and inertia M. Their expressions are formulated within a locally harmonic approximation and the help of linear response theory. Different approximations are examined and comparisons are made both with previous studies, which involved different descriptions of single particle dynamics, as well as with macroscopic models. Special attention is paid to an appropriate definition of the deformation of the nuclear density and its relation to that of the single particle potential. For temperatures above 3 MeV the inertia agrees with that of irrotational flow to less than a factor of two, but shows larger deviations below, in particular in its dependence on the shape. Also friction exhibits large fluctuations along the fission path for small excitations. They get smoothed out above 3 - 4 MeV where \gamma attains values in the range of the wall formula. For T>(or=) 2 MeV the inverse relaxation time \beta = \gamma /M turns out to be rather insensitive to the shape and increases with T.

Theory of macroscopic fission dynamics

Abstract Recent advances in the theory of macroscopic fission dynamics are outlined with special emphasis on the diffusion model with realistic inertia and friction parameters, used to describe the fission fragment distributions.

Manifestation of clustering in the 252Cf(sf) and 249Cf(nth,f) reactions

Abstract A comparative analysis of the high-statistics mass-energy distributions of the fission fragments formed in the 252 Cf(sf) and 249 Cf(n th ,f) reactions is performed on the basis of the potential energy surface calculations. The available experimental and theoretical results provide evidence for the existence of fission modes due to the clustering of the fissioning nucleus. In 252 Cf one of the modes can be treated as a heavy-cluster decay involving the formation of two fragments close to the magic nuclei of Sn. A sharp drop of the proton odd-even effect is observed at an excitation energy above E ∗ ≈ 40 MeV at the scission point, which is presumably associated with the complete clusterization of the fissioning nucleus.

Fission barriers of odd-mass nuclei and odd nuclei with 100 ⩽ Z ⩽ 111

Abstract The height and structure of fission barriers for odd-mass nuclei and odd nuclei with 100 ⩽ Z ⩽ 111 are calculated by the Strutinsky method and using a realistic Woods-Saxon potential. Non-axial variations of the nuclear shape are taken into account. The influence of the blocking effect and of taking into account the conservation of spin and parity on the height of the barriers is considered. As in the adjacent even-even nuclei, in heavy isotopes of the heavy elements (Z = 107–111, N = 160–165) the occurrence of a shape isomer is possible. In the nuclei considered the fission barrier heights do not show a tendency of decreasing with increasing mass number.

Manifestation of fine structures in the fission fragment mass-energy distribution of the 233U(nth,f) reaction

Methods of identifying structure peculiarities in two-dimensional distributions of experimental observables are discussed. Structures different from that produced by proton odd-even staggering were revealed for the first time in the mass-energy distribution of fission fragments in the 233 Uðnth; fÞ reaction. The new structures could presumably be linked with fission modes and collective vibrations of the fissioning system. r 2002 Elsevier Science B.V. All rights reserved.

Gross shell structure at high spin in heavy nuclei

Experimental nuclear moments of inertia at high spins along the yrast line have been determined systematically and found to differ from the rigid-body values. The difference is attributed to shell effect and these have been calculated microscopically. The data and quantal calculations are interpreted by means of the semiclassical Periodic Orbit Theory. From this new perspective, features in the moments of inertia as a function of neutron number and spin, as well as their relation to the shell energies can be understood. Gross shell effects persist up to the highest angular momenta observed.