Shell effects on rotational damping in superdeformed nuclei

Damping of collective rotational motion in A ∼ 190 and A ∼ 150 superdeformed nuclei is studied by means of the cranked shell model with two-body residual force. Numerical calculations predict that in a typical A ∼ 190 superdeformed nucleus, ¹⁹²Hg, the rotational damping width is significantly small, Γ_rot ∼ 30 keV, and that the number of superdeformed bands in the offyrast region amounts up to 150 at a given rotational frequency. These features are quite different from the prediction for A ∼ 150 superdeformed nuclei and rare-earth normally deformed nuclei. It is shown that the single-particle alignments of the cranked Nilsson orbits have strong shell oscillation. It affects significantly the properties of rotational damping in superdeformed ¹⁹²Hg.     

Detailed spectroscopy of superdeformed actinide nuclei

Progress in the experimental techniques used to investigate superdeformed fission isomers in the actinides allowed for detailed spectroscopic results of collective properties as well as for the identification of the rotational structure of multiphonon vibrational excitations. A novel approach could be established to determine the depth of the second potential well.     

Magic gaps and intruder levels in triaxially superdeformed nuclei

Nuclear shell model calculations based on a modified harmonic-oscillator potential result in amazingly stable triaxial nuclear shapes. Major gaps in the single-particle energy spectra at proton number 71 and neutron number 94 combine constructively at low and intermediate rotational frequencies. At high frequencies, gaps at proton number 72 and neutron number 97 combine in an equally favourable way. The sizes of the gaps may be as large as 35% of the values for the gaps at the classical magic numbers 50 and 82 at spherical shape. The dependence on the positions of the intruder levels in forming the gaps is discussed. Experimentally observed rotational bands in lutetium (Z = 71) and hafnium (Z = 72) appear in isotopes and frequency ranges, which are consistent with the gaps in the theoretical single-particle energy spectra.     

A microscopic study on shape transition and shape coexistence in superdeformed nuclei

Superdeformed nuclei at high-spin states in several mass regions are investigated within a microscopic approach using cranked Nilsson-Strutinsky formalism to explore the equilibrium deformations in the ground state and their evolution with spin. Shape transition from normal deformed to superdeformed states with increasing spin is studied and a clear picture of shape coexistence is provided. Detailed information on spin, rotational energy, dynamical moment of inertia, and rotational frequency of superdeformed rotational bands is presented and the general features of superdeformed bands in certain mass regions are outlined. Rotational energy and dynamical moment of inertia are compared with available experimental data and the impact of temperature and pairing on superdeformed configuration are discussed.     

The spin-orbit field in superdeformed nuclei: a relativistic investigation

Relativistic Mean Field (RMF) theory is used to investigate the behavior of the spin orbit potential,V _ls, in nuclear states of very large deformation and high angular velocity. As a by-product we present a set of parameters for an approximation of the relativistic scalar- and vector-potentialsS andV in the Dirac equation in terms of Saxon-Woods shapes. These reproduce more or less the same single particle specta as a full selfconsistent relativistic mean field calculation.Dedicated to Prof. Dr. H.J. Mang on the occasion of his 60th birthday     

On the fine structure of the rotational bands of superdeformed nuclei

It is shown that the staggering of the energy levels of the superdeformed rotational bands can be explained from first principles, using the standard microscopic Hamiltonian with the pairing, quadrupole, and hexadecapole residual interactions, as resulting from rotation-induced nonadiabatic distortion of the mean field in the nucleus. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 231–236 (25 August 1996)     

Dependence of the first saddle-point energy on temperature and spin in superdeformed rare-earth nuclei

The first saddle-point energies (i.e., the heights of the barriers separating “super-” and “normal-” deformed minima) are calculated as functions of spin and nuclear temperature in a few dozen rare-earth nuclei. Calculations predict systematic growth in the barriers especially at low (I∼20) spins when Z decreases, the strongest effect occuring in samarium and neodymium nuclei with 84⪅N⪅88. Consequences for observability of superdeformed states are discussed.     

How to measure the spreading width for the decay of superdeformed nuclei

A new expression for the branching ratio for the decay via the E1 process in the normal-deformed band of superdeformed nuclei is given within a simple two-level model. Using this expression, the spreading or tunneling width gamma (downward arrow) for superdeformed decay can be expressed entirely in terms of experimentally known quantities. We show how to determine the tunneling matrix element V from the measured value of gamma (downward arrow) and a statistical model of the energy levels. The accuracy of the two-level approximation is verified by considering the effects of the other normal-deformed states.     

Triplet cooper pairing in nuclear matter and rotational states of superdeformed nuclei

One hundred and sixty-one rotational bands of superdeformed states in nuclei are considered on the basis of a model that admits triplet Cooper pairing in superfluid nuclear matter. The behavior of the dynamical moment of inertia for such states is investigated within this model, which is shown to comply well with available experimental data and to describe successfully the rotational spectra of superdeformed states.     

Isospectral superdeformed bands in the N = 46 nuclei 88Mo and 89Tc

Superdeformed bands in ⁸⁸Mo and ⁸⁹Tc were populated using ⁴⁰Ca-induced fusion-evaporation reactions on ⁵⁸Ni at a beam energy of 185MeV. Gamma-rays emitted in the reactions were detected using the Gammasphere spectrometer, in coincidence with charged particles detected by the Microball array. A new superdeformed band was assigned to the nucleus ⁸⁸Mo, leading to a revisit of earlier configuration assignments for superdeformed structures in this nucleus. In particular, the theoretical interpretation of a pair of identical (isospectral) superdeformed bands in ⁸⁸Mo/⁸⁹Tc is discussed. The configurations that are assigned to the four SD bands belonging to ⁸⁸Mo have properties that are predicted to be significantly affected by pair correlations.Received: 1 October 2003, Revised: 30 January 2004, Published online: 21 September 2004PACS: 21.10.Re Collective levels - 21.60.Cs Shell model - 23.20.Lv transitions and level energies - 27.50. + e K. Lagergren: Present address: Department of Physics, Florida State University, Tallahassee, FL 32306, USA;A. Görgen: Present address: DAPNIA/SPhN, CEA Saclay, F-91191 Gif-sur-Yvette Cedex, France.     

Probing the order-to-chaos region in superdeformed 151Tb and 196Pb nuclei with continuum gamma transitions

The gamma decay associated with the warm rotation of the superdeformed nuclei 151Tb and 196Pb has been measured with the EUROBALL IV array. Several independent quantities provide a stringent test of the population and decay dynamics in the superdeformed well. A Monte Carlo simulation of the gamma decay based on microscopic calculations gives remarkable agreement with the data only assuming a large enhancement of the B(E1) strength for 1-2 MeV gamma rays, which may be related to the evidence for octupole vibrations in both mass regions.