Lifetimes and lineshapes in superdeformed bands

The effective lifetimes and the intensity of the discrete transitions of the superdeformed band in¹⁵²Dy are described using a statistical model based on a Monte Carlo simulation. The effect of the transition strength in the continuum and the energy dependence of the barrier are investigated. The importance of the mixing of superdeformed and normal states for the resulting feeding times and intensities is emphasized. The calculations are compared to experimental data, and it is shown that the discrete line shapes are sensitive to the transition strength in the continuum.     

Superfluid-to-normal phase transition in superdeformed bands

A semiclassically exact solution for the second inertial parameter B is found for the superfluid and normal phases. An interpolation between these limiting values shows that B changes sign in the transition region at the spin I _c that is critical for the rotational spectrum. A superfluid-to-normal transition reveals itself in a specific variation of B versus the spin I. Experimental data show the existence of a transition for superdeformed bands in the A～80, 130, and 150 mass regions and for some bands characterized by a normal deformation. A transition to the normal phase explains the extreme regularity of superdeformed bands.     

Observation of superdeformed bands in194Hg

Two rotational bands, with energy spacings characteristic of superdeformed shapes, have been observed following bombardment of¹⁵⁰Nd with⁴⁸Ca. The more intensively populated band consists of 18 transitions and is assigned to¹⁹⁴Hg. The depopulation of this band occurs around spin 10. The second band, consisting of at least 16 transitions, was populated less strongly and is tentatively assigned to¹⁹⁴Hg also. The lowest level in this band is assigned spin 8. The energy differences between transitions for both bands decrease from ～40 keV at low rotational frequencies to ～30 keV at the highest observed frequencies. The moments of inertia of the bands are similar to those of the two previously observed superdeformed bands in^191,192Hg. The similarities and differences of the four known bands in the mercury region are discussed.     

Quadrupole moments of superdeformed bands in 193Tl

Lifetimes of states in the two strongest superdeformed (SD) bands in ¹⁹³Tl were measured using the Doppler-shift attenuation method. The reaction ¹⁷⁶Yb(²³Na,6n)¹⁹³Tl at a beam energy of 129 MeV was used and γ-rays were detected by the Gammasphere array. Quadrupole moments of 18.3(10) eb and 17.4(10) eb were extracted for SD bands 1 and 2, respectively, using the fractional Doppler-shifts of the SD transitions. The previously reported linking transitions of these SD bands to normal deformed near yrast levels could not be confirmed. No other candidates for linking transitions could be established. Received: 27 April 1999     

Microscopic approach to transition-energy staggering in superdeformed bands

The current status of the ΔI=4 bifurcation in superdeformed bands is reviewed by making use of a theoretical model based on the interaction of rotation and single-particle nucleon motion in nuclei with an axial deformation. It is shown that the hexadecapole-type distortion of a nuclear shape by rotation is especially important for explaining the phenomenon. The necessary condition for staggering is obtained from an analysis of the nonadiabatic effect of rotation. This criterion is applied to 30 superdeformed bands in the mass region around A ～ 150. An analysis confirms the configuration-dependence effect and allows us to discriminate between single-particle states active and inactive for staggering. The consideration is based on the additivity of the nonaxial hexadecapole moment, which plays a key role in the staggering phenomenon.     

Identical bands: two steps forward

The identical bands phenomenon has been studied in detail both experimentally and theoretically. A first step has been done by analysing the distributions of the fractional changes in the dynamical moments of inertia of superdeformed nuclei. They showthat there is a clear excess of identical bands in superdeformed nuclei compared to normally-deformed nuclei at low spin. Furthermore, this difference can be traced back to some special properties of superdeformed bands. The second step consists in the precise lifetime measurements that have been performed in superdeformed nuclei of the rare earth region. The derived quadrupole moments support the picture that alignments and deformation effects cancel in identical bands.     

Superfluid-to-normal phase transition and extreme regularity of superdeformed bands

We derive an exact semiclassical expression for the second inertial parameter B for superfluid and normal phases. Interpolation between these limiting values shows that the function B(I) changes sign at the spin I _c that is critical for the rotational spectrum. The quantity B proves to be a sensitive measure of the change in static pairing correlations. The superfluid-to-normal transition reveals itself in a specific variation of the ratio B/A versus the spin I with a plateau characteristic of the normal phase. We find this dependence to be universal for normal deformed and superdeformed bands. A long plateau with a small value of B/A ～ A ^?8/3 explains the extreme regularity of superdeformed bands.     

Inertial parameters and superfluid-to-normal phase transition in superdeformed bands

The quasiclassically exact solution for the second inertial parameter ? is found in a self-consistent way. It is shown that superdeformation and nonuniform pairing arising from the rotation-induced pair density significantly reduce this parameter. The new signature for the transition from pairing to normal phase is suggested in terms of the variation ?/A versus spin. Experimental data indicate the existence of such a transition in the three SD mass regions.     

Cranked relativistic mean field description of superdeformed rotational bands

The cranked relativistic mean field theory is applied for a detailed investigation of eight superdeformed rotational bands observed in¹⁵¹Tb. It is shown that this theory is able to reproduce reasonably well not only the dynamic moments of inertiaJ ⁽²⁾ of the observed bands but also the alignment properties of the single-particle orbitals.