Study of 0+ States at iThemba LABS

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Spectroscopy of low-spin states in 157Dy : Search for evidence of enhanced octupole correlations

CITATION: Majola, S. N. T., et al. 2019. Spectroscopy of low-spin states in 157Dy : Search for evidence of enhanced octupole correlations. Physical Review C, 100(6):034322, doi:10.1103/PhysRevC.100.034322.The original publication is available at https://journals.aps.org/prcLow-spin states of ¹⁵⁷Dy have been studied using the JUROGAM II array, following the ¹⁵⁵Gd (α, 2n) reaction at a beam energy of 25 MeV. The level scheme of ¹⁵⁷Dy has been expanded with four new bands. Rotational structures built on the [523]5/2⁻ and [402]3/2⁺ neutron orbitals constitute new additions to the level scheme as do many of the inter- and intraband transitions. This manuscript also reports the observation of cross I⁺ →(I–1) ⁻ and I⁻ →(I–1)⁺ E1 dipole transitions interlinking structures built on the [523]5/2⁻ (band 5) and [402]3/2⁺ (band 7) neutron orbitals. These interlacing band structures are interpreted as the bands of parity doublets with simplex quantum number s=–i related to possible octupole correlations.https://journals.aps.org/prc/abstract/10.1103/PhysRevC.100.034322Publisher's versio

DSAM lifetime measurements for the chiral pair in 194Tl

Most important for the identification of chiral symmetry in atomic nuclei is to establish a pair of bands that are near-degenerate in energy, but also in B(M1) and B(E2) transition probabilities. Dedicated lifetime measurements were performed for four bands of 194Tl, including the pair of four-quasiparticle chiral bands with close near-degeneracy, considered as a prime candidate for best chiral symmetry pair. The lifetime measurements confirm the excellent near-degeneracy in this pair and indicate that a third band may be involved in the chiral symmetry scenario

β and γ bands in N = 88 , 90, and 92 isotones investigated with a five-dimensional collective Hamiltonian based on covariant density functional theory : vibrations, shape coexistence, and superdeformation

CITATION: Majola, S. N. T. et al. 2019. β and γ bands in N=88, 90, and 92 isotones investigated with a five-dimensional collective Hamiltonian based on covariant density functional theory: Vibrations, shape coexistence, and superdeformation. Physical Review C, 100(4). doi:10.1103/PhysRevC.100.044324.The original publication is available at https://journals.aps.org/prc/A comprehensive systematic study is made for the collective β and γ bands in even-even isotopes with neutron numbers N = 88 to 92 and proton numbers Z = 62 (Sm) to 70 (Yb). Data, including excitation energies, B(E0) and B(E2) values, and branching ratios from previously published experiments are collated with new data presented for the first time in this study. The experimental data are compared to calculations using a five-dimensional collective Hamiltonian (5DCH) based on the covariant density functional theory (CDFT). A realistic potential in the quadrupole shape parameters V (β,γ ) is determined from potential energy surfaces (PES) calculated using the CDFT. The parameters of the 5DCH are fixed and contained within the CDFT. Overall, a satisfactory agreement is found between the data and the calculations. In line with the energy staggering S(I) of the levels in the 2γ + bands, the potential energy surfaces of the CDFT calculations indicate γ -soft shapes in the N = 88 nuclides, which become γ rigid for N = 90 and N = 92. The nature of the 02 + bands changes with atomic number. In the isotopes of Sm to Dy, they can be understood as β vibrations, but in the Er and Yb isotopes the 02 + bands have wave functions with large components in a triaxial superdeformed minimum. In the vicinity of 152Sm, the present calculations predict a soft potential in the β direction but do not find two coexisting minima. This is reminiscent of 152Sm exhibiting an X(5) behavior. The model also predicts that the 03 + bands are of two-phonon nature, having an energy twice that of the 02 + band. This is in contradiction with the data and implies that other excitation modes must be invoked to explain their origin.https://journals.aps.org/prc/abstract/10.1103/PhysRevC.100.044324Publisher’s versio

First candidates for γ vibrational bands built on the [505] 11/2− neutron orbital in odd-A Dy isotopes:

Rotational structures have been measured using the Jurogam II and GAMMASPHERE arrays at low spin following the 155Gd(α,2n)157Dy and 148Nd(12C,5n)155Dy reactions at 25 and 65 MeV, respectively. We report high-K bands, which are conjectured to be the first candidates of a Kπ=2+γ vibrational band, built on the [505]11/2− neutron orbital, in both odd-A155,157Dy isotopes. The coupling of the first excited K=0+ states or the so-called β vibrational bands at 661 and 676 keV in 154Dy and 156Dy to the [505]11/2− orbital, to produce a Kπ=11/2− band, was not observed in both 155Dy and 157Dy, respectively. The implication of these findings on the interpretation of the first excited 0+ states in the core nuclei 154Dy and 156Dy are also discussed

“Stiff” deformed nuclei, configuration dependent pairing and the

We review the current experimental data on collective structures within the pairing gap of even-even deformed nuclei, with emphasis on nuclei near mass number $A \sim 150$. The essential physics that determines the characteristics of the first excited 0+ (02 +) state in these nuclei has been in dispute for several decades. Interpretation of these states in terms of surface $\beta$ quadrupole vibrations has often been challenged. We examine the role that configuration dependent pairing can play in these levels particularly at the onset of deformation as major shells fill. In all deformed nuclei rotational bands are found experimentally, starting at a state with spin 2+ with excitation energies near the middle of the pairing gap. These rotational bands, with quantum number $K^{\pi} = 2^{+}$, are usually referred to as $\gamma$ bands and have been identified with quadrupole surface vibrations in the plane perpendicular to the major axis of deformation. However $K^{\pi} = 2^{+}$ bands can also arise due to the breaking of axial symmetry of the quadrupole shape. We discuss data that can help with these different interpretations

In-beam spectroscopy of

High-spin states in the nucleus 72Ge were investigated via the 70Zn($\alpha$, 2n)72Ge reaction at a beam energy of 30MeV, using the AFRODITE spectrometer. One aim of the study was to search for tetrahedral states. There was no evidence for such states in our coincidence data. The existing decay scheme was substantially revised and extended. Several $\gamma$ -ray placements and level spin-parities were changed, and some 30 new transitions were added to the level scheme. One new negative-parity rotational band was identified. The new band is likely the unfavoured signature partner of the band built on the previously known \ensuremath I^{\pi}=3^{-} state at 2515keV. The two negative-parity bands are interpreted as involving an aligned octupole vibration which evolves to a four-quasiparticle structure at higher spins. The upbend in the yrast band is interpreted as the AB neutron alignment. The band structures are discussed with reference to Cranked Shell Model calculations, the aligned angular momenta, experimental routhians, and moments of inertia

Yrare low-spin positive-parity states in N = 88

Low-spin positive-parity yrare states of 66 154 Dy88 were established using the 155Gd(3He,4n) reaction at a beam energy of $E_{lab} = 37.5$ MeV. The AFRODITE spectrometer array at iThemba LABS was used to record $\gamma\gamma$ coincidences and measure DCO ratios and polarisation asymmetries. The $K^{\pi}=2^{+}$ $\gamma$ band has been observed up to spin $13^{+}$ in the odd spins and to $12^{+}$ in the even spins. The staggering parameter S(I) of the $\gamma$ band is compared to that found in other N = 88 isotones. Different behaviour of S(I) with increasing spin is observed for each of the isotones. We conjecture that the variation in S(I) is mainly due to mixing of the even-spin states with the same spin and parity states in neighbouring rotational bands. A second $K^{\pi}=2^{+}$ band has been established up to a spin of $12^{+}$ in the even spins. We suggest that this is a $\gamma$ band based on the $J^{\pi} = 0^{+}$ state at $E_{x} = 660.6$ keV

Spin-parity assignments and extension of the 0

Low and medium spin collective structures in 158Er have been studied using the 150Sm(12C,4nγγ) fusion-evaporation reaction at a beam energy of Elab = 65 MeV. A band built on the 02+ excitation has been established and extended to Jπ = 18+ from the analysis of γ-γ coincidence relationships, intensity arguments and DCO ratios. The 02+ band in 158Er presents a similar trend to the 02+ bands in the lighter N = 90 isotones but lies about 125 keV higher. This systematic trend supports a similar configuration for the 02+ bands in the N = 90 isotones