Γ-soft Nuclei Research Articles

Applications of IBM-3 to the Z ∼ N ∼ 40 nuclei

The nuclei with N ∼ Z ∼ 40 are characterized by large deformation but γ-soft spectra with low values of the energy ratio E 4 1 + ∗ E 2 1 + ∗ ∼ 2.5 and E 4 1 + ∗ ∼ E 2 γ + ∗ . Since N ∼ Z, we have applied the Interacting Boson Model with Isospin (IBM-3) to these nuclei. Using a complete diagonalization program for IBM-3 hamiltonians, we have investigated the shape transitions from A = 80 to 90 of the Sr and Zr isotopes. To describe the γ-soft features, we assume the O(5) symmetry for the hamiltonian. As the deformation increases with A→80, the calculated energy levels are in good agreement with the experimental data. In particular, B( E2: J 1 + → ( J − 2) 1 +) of the ground band as well as the energy spectra of the ground and the γ bands of 80Sr are close to the predictions of the O(6) limit of the IBM. The splitting of the scissors motions by isospin is also discussed. We have shown that IBM-3 is applicable to the strongly deformed γ-soft nuclei at A ∼ 80.

New results from GASP in the A=130 mass region

The GASP array has been used to study the high spin structures of the 134Nd and 136Nd nuclei populated in the 110Pd(28Si,4n) and 110Pd(30Si,4n) reactions at 130 MeV and 125 MeV, respectively. Several deexcitation pathways have been established for the yrast and excited highly deformed (HD) bands in 134Nd. The decay out process in this nucleus has been understood in terms of mixing between the normal deformed and HD states, which is triggered by the crossing between the vi13/2 and vd5/2 orbitals. The study of the high spin excitations in 134Nd and 136Nd revealed two different mechanisms for generating high angular momentum: the stepwise occupation of the N = 6 i13/2 intruders in the more rigid nucleus 134Nd and the alignment of successive pairs of h11/2 protons and neutrons coupled to a stable small-deformation triaxial core in the γ-soft nucleus 136Nd.

A EUROBALL module at the S-DALINAC — Investigation of elementary and magnetic excitation modes

Results from the first application of a EUROBALL Cluster module in (γ, γ′) experiments at the S-DALINAC are reported. The scissors mode was observed for the first time in a γ-soft nucleus, 196Pt. A combination of detailed spectroscopy of 165Ho and 169Tm with a fluctuation analysis technique demonstrates that the total scissors mode strength in odd-mass nuclei is comparable to that in the even neighbours, but fragmentation is so strong that large parts are below detection thresholds. The search for the spin M1 resonance in the heavy deformed nucleus 154Sm gives a hint on rather large cancellations of spin and orbital contributions. The fine structure of the E1 response in the semimagic 140Ce below particle threshold is fully resolved and successfully interpreted in the microscopic quasiparticle-phonon model.

Competing mechanism for generating high spin excitations in γ-soft nuclei: the 136Nd case

High-spin structures in 136Nd have been investigated via the 110Pd( 30Si,4n) reaction at 125 MeV using the GASP array. Several bands consisting of stretched quadrupole transitions were identified. Two of them reach spins comparable to those of the highly-deformed (HD) band in 136Nd, which is based on a neutron i 13 2 - h 9 2 f 7 2 configuration. Total routhian surface (TRS) calculations indicate that regular high-spin excitations in this γ-soft nucleus, can be also generated through successive pairs of h 11 2 protons and neutrons coupled to a stable small-deformation triaxial core. The previously reported γ-ray transitions of an excited HD band, identical to the yrast HD band, are not confirmed by our data.

Stretched alignment due to pairing correlation between the normal and abnormal parity orbits for the gamma -soft nuclei in the light rare-earth region.

For the \ensuremath{\gamma}-soft rotational motion, a two-parameter model is constructed considering the pair correlation between the normal and abnormal parity states based on the fermion dynamical symmetry model. The numerical calculation of this model shows that the data of ground bands of ten typical \ensuremath{\gamma}-soft rotational nuclei in the light rare-earth region are fitted in a very high quality. \textcopyright{} 1996 The American Physical Society.

The Mottelson-Valatin effect in the SO(5) scheme

It is shown in this paper that the Mottelson-Valatin effect can also occur in the 5-dimentional surface oscillations, namely the SO(5) scheme. This method is used to discuss the violation of the SO(6) symmetry properties in the γ-soft nuclei.

A pairing effect in γ-soft nuclei

A new regularity in the level structure of γ-soft nuclei is suggested by using the O(6) limit of the interacting boson model. This regularity, which will be referred to as τ-compression, is the compression of level spacing between neighboring O(5) multiplets for higher τ values. It is shown that the pairing interaction is the origin of the τ-compression. The spectra of 126–130Xe are discussed as prominent examples of the τ-compression.

The shape effect of rotating high-j orbits in γ-soft nuclei

The rotational aligned quasiparticle in a high-j shell exerts a strong polarizing effect on γ-degree of freedom of the γ-soft nuclei. The Krypton region where nuclei are expected to be γ-soft is a important test ground for the effect. The calculated results of Krypton indicate a strong influence of the rotating g 9 2 orbits on the γ-deformation. The disappearance of the signature splitting of the quasi-neutron bands of 81Kr is well reproduced by the model calculation and the conclusion is relatively insensitive to the quadrupole radail deformation ε 2.

Triaxial shapes of rotating nuclei with neutron numbers 88, 89, 90

The tendency of high- j quasiparticles to drive rotating nuclei towards triaxial shapes is studied by means of the Cranked Shell Model. Depending on the position of the chemical potential within the j-shell different γ-values are prefered by the quasiparticles, which may induce significant configuration dependent γ-deformations in γ-soft nuclei. As an example the rotational spectra of Z = 66, 67, N = 88, 89, 90 nuclids are studied. Equilibrium γ-values are found by minimizing the sum of phenomenological expressions for the ground state deformation- and rotational energies and the quasiparticle energies. The competition between ν i 1 3 2 (driving to γ > 0) and h 1 1 2 (driving to γ < 0) quasiparticles causes a γ-flip-flop between the configurations observed as changes of the signature splitting and E2-transition probabilities, which may be related to the prolate-oblate difference of the deformation energy.