I.N. Mikhailov

Microstructure of excited nuclear states in the vicinity of an yrast line

Abstract The microscopic model describing the wobbling motion in fast rotating nuclei is presented and the connection of the parameters describing it with the Coriolis effects at low spins is discussed.

THE SHAPE OF THE HEATED FAST-ROTATING NUCLEI

A model is formulated for studying the shape of fast-rotating heated nuclei. Systematic calculations of the Gibbs-Routhian function FR, the Gibbs function F, and the energy E have been performed for rare-earth nuclei with 60 t,,.

The microscopic description of the isovector dipole excitations at high spins

Abstract The strength functions of the γ-spectrum obtained from the decay of fast rotating nuclei by deexcitation of the isovector dipole modes are calculated in a microscopic model.

Equilibrium properties of fast-rotating headed nuclei

Abstract We report on calculations of the equilibrium deformation in excited heated rotating nuclei. At A ∼ 150–200 and temperature t > t c ≈ 1.2 MeV the shell effects turn out to be small to compete with the variations of the liquid drop component of the energy. The transition from the shape of a “cool” nucleus to that of a “hot” nucleus takes place at t c and in deformed nuclei resembles a phase transition. The stiffness parameter with respect to shape variations at t c is anomalously low.

On the determination of the axial and non-axial deformability of rotating heavy nuclei

Abstract The consequences of a possible existence of non-axial deformation in excited rotational states of heavy nuclei is considered. Some estimates are given to exemplify the influence of this effect on the quadrupole moment matrix elements as well as on their relation to the r.m. charge radii.

On octupole alignment in actinides

Abstract The alignment of the octupole angular momentum in the rotational states of the K π = 0 − band is analysed in the microscopic model. The model describes qualitatively the branching ratio for the E1-transitions from these states to the ground band states.

Theory of coupled rotational bands in nuclei: (I). General

Abstract An RPA type approach to nuclear rotation and its coupling to other modes is proposed, giving both energies and transition rates in deformed nuclei. It is based on a general expansion of any multipole operator, in terms of earlier introduced elementary transition operators, acting in and between nuclear rotational bands. The expansion is formally obtained by using symmetry properties only. The matrix element of any expansion term is calculated from the algebraic properties of the transition operators. Applied to the Hamiltonian it gives a phenomenological model, accounting for vibrations and rotation with coupling between these modes, together with the prescription for evaluation of the model parameters. Applied to a multipole transition inducing field, it leads to a model-independent derivation of the spin (I)-dependent corrections to adiabatic unified model transition rates. A microscopic procedure for calculating the model-dependent expansion coefficients is proposed. The latter are expressed in terms of the elementary transition operators, or in terms of an extended density matrix involving these operators, giving their quasiparticle structure. The procedure is based on a generalized equations-of-motion method for the density matrix. The method is illustrated by the example of coupled ground-state (g), beta (β)-state and gamma (γ)-state rotational bands in even-even nuclei. Results for E2 non-adiabatic transition rate corrections inside the g-band and for transitions from the γ- and β-bands to the g-band are obtained. This example is to be developed in the following paper towards numerical applications and an extension of the description to high-spin states.

Theory of coupled rotational bands in nuclei: (II). Application to even-even nuclei

Abstract The method developed in the preceding paper is applied to study the basic parameters of the states belonging to the ground, β- and γ-rotational bands in even-even deformed nuclei. The parameters of the model Hamiltonian and the expansion coefficients of the physical operators are calculated within a microscopic model, including pairing and quadrupole-quadrupole interactions in an average field of the Woods-Saxon type. One obtains the energies of the states and the non-adiabatic corrections to the different multipole moments in the charge and density distributions of nuclei, and electromagnetic transition probabilities. The agreement of the theoretical results with the known experimental data is generally good. Some conclusions are drawn concerning the deformability of the rotating nuclei in the region of angular momenta critical for the exisience of pairing correlations.

Macroscopic description of the giant quadrupole resonance in rotating nuclei

Abstract The distorted Fermi-surface model is generalized to study the giant quadrupole resonance behaviour in rotating nuclei. The mathematical problems of the model are solved with the help of the tensor virial method of Chandrasekhar and Lebovitz.

CALCULATIONS OF LOW-LYING COLLECTIVE EXCITATION ENERGIES IN 168yb AT HIGH ANGULAR MOMENTA

Abstract The excitation energy of the lowest I -odd states in 168 Yb is calculated in a wide range of spins ( I ) by using the microscopic model suggested earlier by two of the authors (D.J. and I.M.). This family of states has close relation to the γ-vibrational states at low spins and to the one-phonon precessional excitations when I is large.