A quantal Hamiltonian Ĥ coll expressed in terms of the five collective quadrupole coordinates is built for eight nuclei ( 190,192,194 Hg, 192,194,196 Pb, and 196,198 Po͒ which display secondary minima at large elongation in their potential energy surface. These surfaces as well as the tensor of inertia entering Ĥ coll are deduced from constrained Hartree-Fock-Bogoliubov calculations based on Gogny force. A two-center basis method employed to solve Ĥ coll is presented. The stability of predicted collective spectra is discussed. Yrast and vibrational ϭϩ superdeformed ͑SD͒ bands are predicted together with collective bands at normal deformation ͑ND͒. The predicted yrast SD bands at low spin display properties which compare favorably with experimental information. Quite good agreement is in particular obtained for the isomeric energies of nuclei for which the link between SD and ND levels is experimentally known. Among the excited SD bands which are here predicted, those built on top of  vibrations are lower in energy. Only for the 196,198 Po isotopes are these excitation energies falling in the low energy range Eϳ0.8-1.0 MeV. These properties should favor an experimental discovery of -vibrational SD bands in the Aϳ190 mass region. ͓S0556-2813͑99͒03709-7͔
Background: Neutron-rich nuclei with protons in the fp shell show an onset of collectivity around N = 40. Spectroscopic information is required to understand the underlying mechanism and to determine the relevant terms of the nucleon-nucleon interaction that are responsible for the evolution of the shell structure in this mass region. Methods: We report on the lifetime measurement of the first 2 + and 4 + states in 70,72,74 Zn and the first 6 + state in 72 Zn using the recoil distance Doppler shift method. The experiment was carried out at the INFN Laboratory of Legnaro with the AGATA demonstrator, first phase of the Advanced Gamma Tracking Array of highly segmented, high-purity germanium detectors coupled to the PRISMA magnetic spectrometer. The excited states of the nuclei of interest were populated in the deep inelastic scattering of a 76 Ge beam impinging on a 238 U target. Results: The maximum of collectivity along the chain of Zn isotopes is observed for 72 Zn at N = 42. An unexpectedly long lifetime of 20 +1.8 −5.2 ps was measured for the 4 + state in 74 Zn. Conclusions: Our results lead to small values of the B(E2; 4 + 1 → 2 + 1 )/B(E2; 2 + 1 → 0 + 1 ) ratio for 72,74 Zn, suggesting a significant noncollective contribution to these excitations. These experimental results are not reproduced by state-of-the-art microscopic models and call for lifetime measurements beyond the first 2 + state in heavy zinc and nickel isotopes.
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