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(E 0) and B(E 2) 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 2 γ + 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 0 2 + 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 0 2 + bands have wave functions with large components in a triaxial superdeformed minimum. In the vicinity of 152 Sm, the present calculations predict a soft potential in the β direction but do not find two coexisting minima. This is reminiscent of 152 Sm exhibiting an X (5) behavior. The model also predicts that the 0 3 + bands are of two-phonon nature, having an energy twice that of the 0 2 + band. This is in contradiction with the data and implies that other excitation modes must be invoked to explain their origin.
The structure of 33 Si was studied by a one-neutron knockout reaction from a 34 Si beam at 98.5 MeV/u incident on a 9 Be target. The prompt γ-rays following the de-excitation of 33 Si were detected using the GRETINA γ-ray tracking array while the reaction residues were identified on an eventby-event basis in the focal plane of the S800 spectrometer at NSCL (National Superconducting Cyclotron Laboratory). The presently derived spectroscopic factor values, C 2 S, for the 3/2 + and 1/2 + states, corresponding to a neutron removal from the 0d 3/2 and 1s 1/2 orbitals, agree with shell model calculations and point to a strong N = 20 shell closure. Three states arising from the more bound 0d 5/2 orbital are proposed, one of which is unbound by about 930 keV. The sensitivity of this experiment has also confirmed a weak population of 9/2 − and 11/2 − 1,2 final states, which originate from a higher-order process. This mechanism may also have populated, to some fraction, the 3/2 − and 7/2 − negative-parity states, which hinders a determination of the C 2 S values for knockout from the normally unoccupied 1p 3/2 and 0f 7/2 orbits.
Recent high-precision mass measurements and shell model calculations [Phys. Rev. Lett. 108, 212501 (2012)] have challenged a longstanding explanation for the requirement of a cubic isobaric multiplet mass equation for the lowest A = 9 isospin quartet. The conclusions relied upon the choice of the excitation energy for the second T = 3/2 state in 9 B, which had two conflicting measurements prior to this work. We remeasured the energy of the state using the 9 Be( 3 He, t) reaction and significantly disagree with the most recent measurement. Our result supports the contention that continuum coupling in the most proton-rich member of the quartet is not the predominant reason for the large cubic term required for A = 9 nuclei.
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