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.     

Motional narrowing and ergodic bands in excited superdeformed states of 194Hg

The E(gamma) - E(gamma) coincidence spectra from the electromagnetic decay of excited superdeformed states in (194)Hg reveal surprisingly narrow ridges, parallel to the diagonal. A total of 100-150 excited bands are found to contribute to these ridges, which account for nearly all the unresolved E2 decay strength. Comparison with theory suggests that these excited bands have many components in their wave functions, yet they display remarkable rotational coherence. This phenomenon can be explained in terms of the combination of shell effects and motional narrowing.     

Superdeformed bands in193Hg and194Hg

By analyzing the thick target yield curve of resonant (p,γ) reactions on²³Na and²⁶Mg for various target temperatures within a Monte Carlo approach, we are able to define the Debye and Einstein temperature of the target material in the surface region. The obtained surface temperatures are noticeably higher than the respective calorimetric values.     

Lifetime measurements and decay of superdeformed bands in Hg isotopes

The lifetimes of states in Superdeformed (SD) bands in ^192,194Hg have been measured using the Doppler shift attenuation method. Intrinsic quadrupole moments have been extracted for band 1 of ¹⁹²Hg and for bands 1, 2, and 3 of ¹⁹⁴Hg. It was found that the quadrupole moments for the “identical” SD bands in these nuclei are the same. In the same experiment, one-step transitions linking SD states to yrast and near yrast states for bands 1 and 3 of ¹⁹⁴Hg have been identified. From this observation, excitation energies, spins and probable parities have been assigned to SD states in these bands.     

Identical superdeformed bands

The phenomenon of identical bands is studied by analyzing the distributions of fractional changes in the dynamical moments of inertia of pairs of bands in superdeformed (SD) nuclei. These distributions are found to exhibit a peak with a centroid at nearly zero. Their widths increase in going from the SD bands in the massA～150, to the SD bands in the mass ～190 and to the normally-deformed bands in the rare-earth region. These differences may be attributed to the weaker pairing correlations and the stabilizing role of intruder orbitals on the structures of SD bands. Precise level lifetimes have been measured for various pairs of identical SD bands in Gd and Dy isotopes. By comparing the derived quadrupole moments with calculations performed in the framework of the cranking Skyrme-Hartree-Fock model, it is shown that, independently of the intrinsic configuration and of the proton and neutron numbers, the charge moments calculated with respect to the doubly-magic SD core of¹⁵²Dy can be expressed in terms of independent contributions from the individual hole and particle orbitals.     

Band head spin assignment of superdeformed bands in Hg isotopes through power index formula

The power index formula has been used to obtain the band head spin(I_0) of all the superdeformed(SD) bands in Hg isotopes. A least squares fitting approach is used. The root mean square deviations between the determined and the observed transition energies are calculated by extracting the model parameters using the power index formula. Whenever definite spins are available, the determined and the observed transition energies are in accordance with each other. The computed values of dynamic moment of inertia J~((2)) obtained by using the power index formula and its deviation with the rotational frequency is also studied. Excellent agreement is shown between the calculated and the experimental results for J~((2)) versus the rotational frequency. Hence, the power index formula works very well for all the SD bands in Hg isotopes expect for ~(195)Hg(2, 3, 4).     

New results on the superdeformed band in192Hg

New results on the¹⁹²Hg superdeformed band have been obtained with EUROGAM. The experiment has been performed with the¹⁶⁰Gd(³⁶S,4n) reaction at 159 MeV. Above 800 keV the γ-ray energies differ from the previously published ones. Thus the rise of the dynamical moment of inertia $\mathfrak{F}^{(2)} $ above ?ω=0.4 MeV is no longer observed. This is in better agreement with recent cranked Hartree-Fock-Bogoliubov calculations.     

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.     

Transition quadrupole moments of superdeformed states in194Pb

Lifetimes of superdeformed states in¹⁹⁴Pb were measured by the Doppler-shift attenuation method. Quadrupole moments around 20 eb, that are constant over the observed frequency range, are derived. The results rule out large centrifugal stretching effects.     

Superdeformed bands in194Tl

Two superdeformed (SD) bands have been found in¹⁹⁴Tl using the reactions¹⁷⁶Yb(²³Na,5n) and¹⁸¹Ta(¹⁸O,5n). The mass assignments were made from excitation function data. These are the first SD bands found in the mass-190 region that are not in Hg nuclei. The energies of many of the transitions of one of these new SD bands are similar in energy to those in the SD band of¹⁹²Hg.     

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