Quasi-γ Band Research Articles

Emerging γ-soft-like spectrum in 196Pt in the SU3-IBM (I)* *This research is supported by the Educational Department of Jilin Province, China (JJKH20210526KJ), C.-X. Z. gratefully acknowledges support from the Project Supported by Scientific Research Fund of Hunan University of Art and Science (23ZZ04)

Recently, studies have argued that a spherical-like spectrum emerges in the SU3-IBM, thus creating new approaches to understanding γ-softness in realistic nuclei. In a previous study, γ-softness with degeneracy of the ground and quasi-γ bands was observed. In this paper, another special point connected to the middle degenerate point is discussed. It is found to be related to the properties of 196Pt. This emergent γ-softness is also shown to be important for understanding the prolate-oblate asymmetric shape phase transition. The low-lying spectra, values, and quadrupole moments in 196Pt are discussed, and we show that the new model can account for several observed features. This is the first part of the discussion on the γ-soft-like spectrum of 196Pt.

New γ-soft rotation in the interacting boson model with SU(3) higher-order interactions * *Supported by Science and Technology Research Planning Project of Education Department of Jilin Province (JJKH20210526KJ)

The interacting boson model with higher-order interactions offers a new route to enhance our understanding on γ-soft rotation. In this paper, -like and -like new γ-softness are observed, in which the corresponding energy levels in the ground and quasi-γ bands can be exactly degenerate and have a partial dynamical symmetry. The spherical-like γ-softness is not related to the classical dynamical symmetry. The transitional behaviors of values of the low-lying levels and quadrupole moment of the state are also discussed. Spherical-like γ-softness can be used to explain the low-lying spectra and values in 110Cd normal states.

Structure Evolution in the Even-Even 124–134Xe with IBM2

We have investigated the structure evolution of the 124–134Xe isotopic chain in the framework of the proton-neutron interacting model IBM2. The positive parity spectra of the ground state, quasi-β and quasi-γ bands are reproduced well. The staggering in 124–130Xe are almost completely removed and the γ band agree well with the experiment data, even for the high-spin quasi-γ states. The key quantities of the collective structure evolution, including level energies, the B(E2) transition branching ratios, and the M1 excitations to 1+1 mixer-symmetry states are analyzed by comparing with the experimental data. The parameters for representation of the Oπν(6) and SU*πν(3) features in isotopes are examined. Both experimental data and theoretical results show that the shape phase transition of 124–134Xe isotopic chain is from the SU*πν(3) (triaxial rotation) to the Uπν(5) (vibration motion) with a considerable constituent of the Oπν(6) symmetry (γ-unstable rotation), where the shape phase transition rapidly takes place between the neutron number N = 76 and N = 78.

Recent results of decay spectroscopy at RIBF

The neutron-rich nuclei 109Nb and 110Mo have been investigated by means of β-γ and isomer spectroscopy following in-flight fission of a 238U beam at 345 MeV/nucleon at the RIBF facility. A T1/2 = 150(30) ns isomer at 313 keV has been identified in 109Nb. The low-lying levels in 109Nb have been also populated following the β-decay of 109Zr. The observed hindrances of the electromagnetic transitions deexciting the isomeric state are discussed in terms of possible shape coexistence. For 110Mo, spectroscopic information on the low-lying levels of the quasi-γ band built on the 22+ state at 494 keV has been obtained for the first time. The experimental finding of the 22+ state being lower than the 41+ level suggests that axially-asymmetric γ softness is substantially enhanced in this nucleus. The experimental results are compared with model calculations based on the general Bohr Hamiltonian method. A new project of decay spectroscopy at RIBF, named EURICA, is also introduced.

Development of axial asymmetry in the neutron-rich isotope 110Mo

The neutron-rich nucleus 110Mo has been investigated by means of γ-ray spectroscopy following the β-decay of 110Nb, produced using in-flight fission of a 238U beam at 345 MeV/nucleon at the RIBF facility. In addition to the ground-band members reported previously, spectroscopic information on the low-lying levels of the quasi-γ band built on the second 2+ state at 494 keV has been obtained for the first time. The experimental finding of the second 2+ state being lower than the yrast 4+ level suggests that axially-asymmetric γ softness is substantially enhanced in this nucleus. The experimental results are compared with model calculations based on the general Bohr Hamiltonian method. The systematics of the low-lying levels in even-even A ≈ 110 nuclei is discussed in comparison with that in the neutron-rich A ≈ 190 region, by introducing the quantity Es/E(2+1), Es = E(2+2) − E(4+1) as a global signature of the structural evolution involving axial asymmetry.

Prolate-Oblate Shape Coexistence and Enhanced $\gamma$ Softness in Neutron-Rich Nb and Mo Isotopes with $A \approx 110$

The neutron-rich nuclei Nb and Mo have been investigated by means of β-γ and isomer spectroscopy following in-flight fission of a U beam at 345 MeV/nucleon at the RIBF facility. A T1/2 = 150(30) ns isomer at 313 keV has been identified in Nb. The lowlying levels in Nb have been also populated following the β-decay of Zr. The observed hindrances of the electromagnetic transitions deexciting the isomeric state are discussed in terms of possible shape coexistence. For Mo, spectroscopic information on the low-lying levels of the quasi-γ band built on the 2+2 state at 494 keV has been obtained for the first time. The experimental finding of the 2+2 state being lower than the 4 + 1 level suggests that axiallyasymmetric γ softness is substantially enhanced in this nucleus. The experimental results are compared with model calculations based on the general Bohr Hamiltonian method.

Development of axial asymmetry in the neutron-rich nucleus 110Mo

The neutron-rich nucleus 110Mo has been investigated by means of γ-ray spectroscopy following the β-decay of 110Nb, produced using in-flight fission of a 238U beam at 345 MeV/nucleon at the RIBF facility. In addition to the ground-band members reported previously, spectroscopic information on the low-lying levels of the quasi-γ band built on the second 2+ state at 494 keV has been obtained for the first time. The experimental finding of the second 2+ state being lower than the yrast 4+ level suggests that axially-asymmetric γ softness is substantially enhanced in this nucleus. The experimental results are compared with model calculations based on the general Bohr Hamiltonian method. The systematics of the low-lying levels in even–even A≈110 nuclei is discussed in comparison with that in the neutron-rich A≈190 region, by introducing the quantity ES/E(21+), ES=E(22+)−E(41+), as a global signature of the structural evolution involving axial asymmetry.

A calculation of the position of the quasi-β and quasi-γ bands for the transitional nucleus 154Dy with Monte Carlo methods

A calculation of the excitation energy of the 0+ states and of the 2+ states is performed using Monte Carlo methods for the nucleus 154Dy. The Hamiltonian is assumed to be a monopole+quadrupole pairing+quadrupole with the parameters fixed by the spectroscopic Monte Carlo method so as to reproduce the experimental excitation energies of the yrast states up to J = 8 within the 50–82 and 82–126 proton and neutron major shells. The resulting Hamiltonian has been diagonalized in the J = 0 and J = 2 subspaces using the quantum Monte Carlo method. The size of the basis is fixed by comparing the yrast energies obtained with the basis-independent spectroscopic Monte Carlo method, and those obtained with the quantum Monte Carlo method. The excitation energy of the 0+2 is much higher than the experimental value. The structure of the 0+2,3 and of the 2+2,3 eigenstates is discussed in terms of fluctuating intrinsic states and resolved in terms of the deformation variables.

Signature inversion in odd-odd nuclei in the interacting boson-fermion model

The signature inversion in odd-odd nuclei is studied in the interacting boson-fermion model (IBFM). Energy levels and electromagnetic matrix elements are calculated with the SU(3) and the O(6) IBM2 cores. When the particles occupy high- j orbits, the signature inversion occurs with the O(6) core by strong exchange interactions between fermions and the O(6) core. The essential mechanism of the inversion is the mixing of the members of the quasi-γ band into the unfavored states due to the exchange interactions.

Low-spin non-yrast states and collective excitations in 174Os, 176Os, 178Os, 180Os, 182Os and 184Os

Excited states in the range of isotopes 174, 176, 178, 180, 182, 184Os, populated in the β-decay of 174, 176, 178, 180, 182, 184Ir parent activities have been identified using γ-ray singles and coincidence techniques, utilising the high efficiency of a Compton-suppressed array, and conversion electron techniques. Many new states at low excitation energies have been identified, complementing the level schemes previously established from in-beam studies. The new states include excited 0 + states in 174Os, 176Os 178Os and 180Os. A large body of data on decay properties, spins and parities, and relative E2 and E0 matrix elements has been obtained. The systematics of the quasi-β and quasi-γ bands is discussed. Some detailed analyses in terms of the schematic band-mixing model are presented, incorporating reproduction of the yrare states in the fits. In particular, the proposed interpretation of the anomalous low-frequency alignment gains in the yrast positive-parity states, as a consequence of mixing with a low-lying intruder band, is confronted. The B(E0)/ B(E2) ratios are over-estimated by this model although their form (spin and isotope dependence) is reproduced. Better agreement is obtained within the IBM, which uses however reduced effective charges to match experiment. Absolute measurements of the B(E0) and B(E2) values may be necessary to distinguish between the models and test conjectures of shape differences.

Signature inversion in odd-odd nuclei in the interacting boson-fermion model

The signature inversion in odd-odd nuclei is studied in terms of the interacting boson-fermi-on model (IBFM). Energy levels and electromagnetic matrix elements are calculated in the model in which a proton and a neutron are coupled to the SU(3) (deformed) and the O(6) (γ-unstable) IBM2 cores. When the particles occupy high- j orbits, the signature inversion is caused in the O(6) limit by strong exchange interactions between fermions and the O(6) core. The essential mechanism of the inversion is the mixing of the members of the quasi-γ band into the unfavored states due to the exchange interactions. In the case of the SU(3) core, the inversion never occurs when the particles are in high- j orbits. It is seen only around the band head when the fermion orbits have small angular momenta.

Study of Er Isotopes in IBA-plus-Two-Fermion-Pairs Model

The high-spin states up to the vicinity of the second backbends of the Er isotopes are studied in the IBA-plus-two-fermion-pairs model. It was found that the energy levels of the quasi-β, quasi-γ bands as well as the yrast state can be reproduced reasonably well. The second backbends can be interpreted as the band crossings of the two-fermion-pair rotational band with the ground state bands. The B(E2) values were calculated and compared with the available experimental data. The drops of the B(E2) values at the backbends can be reproduced. The calculated moments of inertia and B(E2) values change too sharply as compared with the experimental values. Possible improvements of the model are discussed.

Levels in 74Se, 78, 80Kr and 84Sr and systematics of quasi-γ bands

Quasi-band structures in 74Se, 78, 80Kr and 84Sr have been investigated using (p, 2nγ) reactions. The members of the quasi-γ band have been observed up to 5 + in 74Se, 78, 80Kr and up to 3 + in 84Sr. The analyses of the energy systematics of the quasi-γ bands studied in this mass region as well as in other regions make clear the evolution of the quasi-γ band to the γ-band in well-deformed nuclei. In addition to these positive-parity bands, negative-parity levels were observed in 74Se, 80Kr and 84Sr.

High spin levels in 58Fe: Ground-state and quasi-γ bands

The decay properties of high spin states in 58Fe have been investigated by means of the 55Mn(α, γ) 58Fe reaction at E α = 12 to 24 MeV. A detailed decay scheme that includes for the first time seven states with high spin has been constructed from the analysis of γ-γ coincidence measurements at 20 MeV. Values of the lifetimes of these states were extracted from the same γ-γ coincidences, by a lineshape analysis of the Doppler broadened lines. The J π values and mixing ratios were deduced from angular distributions and the DCO method. Values of B( λ) were extracted for many transitions. Members of the quasi-γ band up to 9 + and of the ground-state band up to 8 + have been proposed in 58Fe. The results show pronounced features characteristic of an extremely pure prolate-to-oblate shape transition occurring between 56Fe and 58Fe.

Study of the side band in 188Pt

Low-lying levels in 188pt were studied using the 191Ir(p, 4nγ) 188Pt reaction and higher spin members of the quasi-γ band were proposed. The results resemble a slightly perturbed phonon scheme, from comparisons with macroscopic calculations which would remove the degeneracy of the phonon multiplets. The importance of mass parameter renormalization is pointed out.

Quasi-gamma bands in even-mass Xe AND Ba nuclei

Abstract Low-lying excited states of 124,126 Xe and 132 Ba have been studied by means of in-beam γ-ray spectroscopy in (p, x n y ) reactions. Excitation functions, angular distributions and γ-γ coincidence spectra were obtained. The 2 2 + , 3 + , 4 2 + and 5 + levels were observed at the following excitation energies in keV: 846.4(2 2 + ), 1247.5(3 + ), 1437.3 (4 2 + ), 1836.6(5 + ) in 124 Xe, 879.7(2 2 + ), 1317.3(3 + ), 1488.2(4 2 + ), 1903.1(5 + ) in 126 Xe and 1032.1(2 2 + ), 1511.3(3 + ), 1729.9(4 2 + ) in 132 Ba. The 2214.3 and 2561.7 keV levels in 126 Xe were tentatively assigned as the 6 2 + and 7 + levels, respectively. These 2 2 + , 3 + , 4 2 + , 5 + , 6 2 + and 7 + levels are interpreted as members of a quasi-γ band. The E2/M1 mixing ratios of the 2 2 + → 2 1 + transitions in 124,126 Xe and 132 Ba were obtained as + 6.3 +5.3 −2.0 + 10.8 7.8 −3.2 and + 8.3 +4.9 −2.2 , respectively.