Identification of Hg and shape coexistence in odd-A Hg isotopes

In-beam γ-ray transitions in ¹⁸¹Hg, the lightest odd-A Hg isotope known thus far, have been identified from fragment mass-γ and γ-γ coincidence measurements. Five prolate deformed rotational bands were placed in the level scheme. A decoupled band built on the strongly prolate deformed 1/2⁻[521] ground state was observed up to 29/2⁻. A 5/2⁻[512] configuration is suggested for a pair of strongly coupled bands displaying no signature splitting. The other two bands are also signature partner bands. They are populated with the largest intensity and exhibit splitting. They have been associated with the mixed neutron i_13/2 orbitals and are proposed to decay to an i_13/2 isomeric state associated with an oblate state.     

Evidence for shape coexistence in186Pt

High-spin states in¹⁸⁶Pt have been populated by the¹⁸⁸Os (α, 6n) reaction and were investigated with the OSIRIS spectrometer. A shape coexistence at high spins was established in the nucleus¹⁸⁶Pt, which lies on the border between light prolate and heavy oblate Pt nuclei. Two bands corresponding to predominantly prolate shapes and one band of predominantly oblate shape have been observed. For prolate shapes a (π h _9/2)² alignment and for oblate shapesa (vi _13/2)² alignment has been found.     

Evidence for shape coexistence at medium spin in Rb

Four previously known rotational bands in ⁷⁶Rb have been extended to moderate spins using the Gammasphere and Microball γ ray and charged particle detector arrays and the ⁴⁰Ca(⁴⁰Ca, 3pn) reaction at a beam energy of 165 MeV. The properties of two of the negative-parity bands can only readily be interpreted in terms of the highly successful Cranked Nilsson-Strutinsky model calculations if they have the same configuration in terms of the number of g9/2 particles, but they result from different nuclear shapes (one near-oblate and the other near-prolate). These data appear to constitute a unique example of shape coexisting structures at medium spins.     

Evidence for shape coexistence in 151Eu

Although the ground states of ¹⁵¹Eu and ¹⁵³Eu both have $\text{I}{}^{\mathrm{\pi }}\text{=}\text{5}\text{2}{}^{\mathrm{+}}$, the (p, t) transition connecting these states is extremely weak. A level at 261 keV with a large (p, t) cross section is assigned as the deformed $\text{5}\text{2}{}^{\mathrm{+}}$[413] state in the “spherical” nucleus ¹⁵¹Eu.     

Shape transitions and shape coexistence in the Ru and Hg chains

The General Collective Model (GCM) is revised and applied to the chain of^182–196Hg- and^96–108Ru-isotopes. Calculated energy spectra, absolute B(E2) values and Quadrupole moments are presented and compared to experiment. For many of these nuclei a coexistence of spherical and triaxial shapes was found. Additionally we investigated the shape transition within the Ru-chain from prolate (⁹⁶Ru) over spherical (⁹⁸Ru) to triaxial (A ≧100). The results are compared to microscopic and IBA calculations.     

Implications of deformation and shape coexistence for the nuclear shell model

The successes of the nuclear shell model in explaining the stability properties of magic nuclei are challenged by the observation of rotational bands for which the sequential filling of single-particle energy levels of the spherical shell model are not respected. This Letter proposes criteria for identifying the shell-model configurations appropriate for describing such bands of states.     

Evidence for shape coexistence in the N=Z nucleus 7236Kr36

Gamma rays associated with the decay of states in the N=Z nucleus ⁷²Kr have been identified following the ¹⁶O(⁵⁸Ni, 2n) ⁷²Kr reaction at a mean beam energy of 170 MeV. Identification was made using the Daresbury Recoil Separator. The first excited state was found to be at 709.1 ± 0.3 keV and to be populated with a cross section of 60 ± 25 υb. The pattern of gamma rays associated with ⁷²Kr indicates the co-existence of nuclear shapes.     

Shape coexistence in187Hg

Excited states of¹⁸⁷Hg have been investigated via the¹⁶⁴Dy (²⁸Si,5n) reaction by means of in- beam gamma-ray spectroscopy using the multidetector 4π away “Ckāteau de. Cristal”. Different bands were populated up to spin I- 45/2. Coexistence of both prolate and oblate structures is observed for the first time in an odd- A Hg isotope. The crossing of aligned and strongly coupled sequences of levels built on the vi13/2 orbital infers the near degeneracy of oblate and prolate structures in¹⁸⁷Hg.     

Variational approach to shape coexistence in Se

Results are presented on shape coexistence and neutron–proton correlations at low and high spins in the N=Z nucleus ⁶⁸Se obtained within the complex version of the Excited Vampir variational approach. Effects of possible contaminations due to the center of momentum motion on the lowest two positive-parity bands are discussed.     

Shape change and shape coexistence in134Pr

The band structure of doubly odd nucleus¹³⁴Pr has been studied by means of the reaction¹¹⁹Sn(¹⁹F,4n) at 87 MeV bombarding energy. Rotational bands built on πh_11/2?vh_11/2 and πh_7/2?vh_11/2 configurations has been observed. A band consisting of quadrupole transitions, probably involving the vi_13/2[600]1/2⁺ orbital was also weakly populated.     

Deformation and shape coexistence in medium mass nuclei

Emerging evidence for deformed structures in medium mass nuclei is reviewed. Included in this review are both nuclei that are ground state symmetric rotors and vibrational nuclei where there are deformed structures at excited energies (shape coexistence). For the first time. Nilsson configurations in odd-odd nuclei within the region of deformation are identified. Shape coexistence in nuclei that abut the medium mass region of deformation is also examined. Recent establishment of a four-particle, four-hole intruder band in the doublesubshell closure nucleus⁹⁶Zr₅₆ is presented and its relation to the Nuclear Vibron Model is discussed. Special attention is given to the N=59 nuclei where new data have led to the reanalysis of⁹⁷Sr and⁹⁹Zr and the presence of the [404 9/2] hole intruder state as isomers in these nuclei. The low energy levels of the N=59 nuclei from Z=38 to 50 are compared with recent quadrupole-phonon model calculations that can describe their transition from near-rotational to single closed shell nuclei. The odd-odd N=59 nuclei are discussed in the context of coexisting shape isomers based on the (p[303 5/2]n[404 9/2])2^– configuration. Ongoing in-beam (t.p conversion-electron) multiparameter measurements that have led to the determination of monopole matrix elements for even-even₄₂Mo nuclei are presented, and these are compared with initial estimates using lBA-2 calculations that allow mixing of normal and cross subshell excitations. Lastly, evidence for the neutron-proton³S₁ force's influence on the level structure of these nuclei is discussed within the context of recent quadrupole-phonon model calculations.Work supported by USDOE contract Nr.W-7405-Eng-48 and NATO Grant Nr.RGO565/82.     

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.     

Evidence for the coexistence of shapes in even-mass Cd nuclei

A quasi-rotational band built on the known proton four-hole, two-particle 0⁺ level at 1473.07-keV in¹¹⁴Cd has been identified, with 2⁺, 4⁺, and 6⁺ members at 1783.44-, 2250.53- and 2876.75-keV respectively. These levels and their properties are compared with those in other even-mass Cd nuclei to show an increasing average deformation in both the 0 ₁ ⁺ (ground-state) and 0 ₂ ⁺ bands when neutron pairs are added beyond¹¹⁰Cd.     

Unveiling the origin of shape coexistence in lead isotopes

Shape coexistence in the nuclei (182-192)Pb is analyzed with the Hartree-Fock-Bogoliubov approach and the Gogny force. Good agreement with the experimental energies is found for the coexisting spherical, oblate, and prolate states. Contrary to the established interpretation, it is found that the low-lying prolate and oblate 0+ states are predominantly characterized by neutron correlations whereas the protons behave, in general, rather as spectators than playing an active role.     

Lifetime measurements in Pt and the shape coexistence picture

Lifetimes for levels in the yrast band of ¹⁸⁴Pt have been measured up to spin 16⁺ using the recoil distance technique. The B(E2) values exhibit a marked increase in going from spin 2 to 10, consistent with a proposal that two bands of different deformations are mixing at low spin. This provides further support for shape coexistence occuring at low excitation energies in this region.     

Evidence for second-phonon nuclear wobbling

The nucleus 163Lu has been populated through the reaction 139La(29Si,5n) with a beam energy of 157 MeV. Three triaxial, strongly deformed (TSD) bands have been observed with very similar rotational properties. The first excited TSD band has earlier been assigned as a one-phonon wobbling excitation built on the lowest-lying (yrast) TSD band. The large B(E2)(out)/B(E2)(in) value obtainable for one of four observed transitions between the second and first excited TSD bands is in good agreement with particle-rotor calculations for a two-phonon wobbling excitation.     

Evidence for the coexistence of spherical and deformed states in38Ar

The³⁸Ar levels at E_x=5350, 7289 and 9341 keV have been investigated using the³⁵Cl(α,pγ) reaction at E_α=l4 and 14.5 MeV. From particle-γ-ray angular correlations the spin assignments J(5350)=4, J(7289)=6,4 and J(9341)=8,6,4 have been obtained. Life-time measurements using the Doppler-shift attenuation method yielded τ(9341)=106±25 fs and τ(7289)=77±30 fs, while the lifetime τ(5350)=200±50 fs was known previously. All levels have positive parity and decay by enhanced E2 transitions. Hence we propose that they are the J^π=4⁺, 6⁺ and 8⁺ members, respectively, of a K^π=0⁺ rotational band which has the E_x=3377keV, J^π=0⁺ and the 3937 keV, J^π=2⁺ levels as further members. The enhancement of inband E2 transitions is 30 _?6 ⁺¹⁰ W.u. (4→2), 35 _?14 ⁺³⁰ W.u. (6→4) and 20–36 W.u. (8 → 6), respectively, yielding an average intrinsic quadrupole moment Q₀=850 _?125 ⁺²⁰⁰ mb. The moment of inertia is given by h²2θ=92 keV. The present data are in good agreement with the predictions of a deformed state in³⁸Ar that coexists with the spherical states.