PHYSICAL REVIEW C 73, 034312 (2006) Evidence for possible shape transitions in neutron-rich Ru isotopes: Spectroscopy of 109,110,111,112 Ru C. Y. Wu Lawrence Livermore National Laboratory, Livermore, 94551 California, USA H. Hua, D. Cline, A. B. Hayes, R. Teng, and D. Riley Nuclear Structure Research Laboratory, Department of Physics, University of Rochester, Rochester, New York, 14627, USA R. M. Clark, P. Fallon, A. Goergen, A. O. Macchiavelli, and K. Vetter Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA (Received 22 July 2005; published 17 March 2006) The spectroscopy of neutron-rich 109,110,111,112 Ru nuclei was studied by measuring the prompt γ rays that originate from fission fragments, produced by the 238 U(α, f ) fusion-fission reaction, in coincidence with the detection of both fragments. For 109,111 Ru, both the negative-parity (h 11/2 orbitals) and K = 5/2 positive-parity (mainly g 7/2 and d 5/2 orbitals) bands were extended to substantially higher spin and excitation energy than known previously. The ground-state and γ -vibrational bands of 110,112 Ru also were extended to higher spin, allowing observation of the second band crossing at the rotational frequency of ≈450 keV in 112 Ru, which is ≈50 keV above the first band crossing. At a similar rotational frequency, the first band crossing for the h 11/2 band in 111 Ru was observed, which is absent in 109 Ru. These band crossings most likely are caused by the alignment of the g 9/2 proton pair. This early onset of the band crossing for the aligned πg 9/2 orbitals may be evidence of a triaxial shape transition from prolate to oblate occurring in 111 Ru. The data together with a comparison of cranked shell-model predictions are presented. DOI: 10.1103/PhysRevC.73.034312 PACS number(s): 23.20.Lv, 25.70.Jj, 27.60.+j I. INTRODUCTION For neutron-rich A  100 nuclei, the nuclear shape changes rapidly as the valence nucleons fill the g 9/2 proton and h 11/2 neutron orbitals, which is manifest by such phenomena as the sudden onset of quadrupole deformation in Sr-Zr isotopes, the development of triaxial degrees of freedom in Mo-Ru isotopes, and the predicted transition of a triaxial shape from prolate to oblate in Ru-Pd isotopes. The ramifications on the nuclear structure because of various shapes make these neutron-rich nuclei an ideal testing ground for various theoretical models [1–3]. For instance, the exact location where shape transitions occur is very sensitive to the model assumptions. A prolate-to- oblate shape transition for Pd isotopes is predicted to happen at 111 Pd by the finite-range droplet model [1] and at 112 Pd by the relativistic mean-field theory [3]. Both calculations were made for nuclei across the period table but the latter was applied only to even-even nuclei. Calculations using the Nilsson-Strutinsky method with the cranked Woods-Saxon average potential and a monopole pairing residual interaction [2], which were applied to even-even neutron-rich A ≈ 100 nuclei, predict the transition occurring at 116 Pd. Experimental verification of this shape transition has important implications on our understanding of the residual interactions in neutron-rich nuclei. In this article, we discuss the experimental evidence for a prolate-to-oblate shape transition in neutron-rich Ru isotopes resulting from the study of the γ -ray spectroscopy of fission fragments. The preliminary results of this shape transition in Ru-Pd isotopes have been presented in our earlier publications [4,5]. To distinguish a prolate from an oblate quadrupole defor- mation, one needs to measure the sign of the static quadrupole moment for the state of interest. Typically, this can be accomplished using the Coulomb excitation technique. For example, a prolate-to-oblate shape transition was identified in 192 Os- 194 Pt isotones by measuring both the magnitude and sign of static quadrupole moments for their first 2 + states using Coulomb excitation [6–8]. This shape transition was predicted by Kumar and Baranger from solving Bohr’s Hamiltonian using the pairing-plus-quadrupole model [9–11]. However, this experimental technique is difficult to apply for nuclei away from the valley of β stability such as neutron-rich Ru-Pd isotopes. An alternative approach is to recognize processes that may yield a distinct signature to differentiate between two shapes. In this work, we explore one such opportunity to address possible evidence of a triaxial shape transition from prolate to oblate in neutron-rich Ru isotopes by studying the band-crossing phenomenon, which is sensitive to the interplay between the single-particle and shape degrees of freedom. II. EXPERIMENT The neutron-rich Ru isotopes were produced as fission fragments by the 238 U(α, f ) fusion-fission reaction. The experiment was carried out at the 88-inch cyclotron facility of the Lawrence Berkeley National Laboratory by bombarding a ≈300 µg/cm 2 238 U target on a ≈30 µg/cm 2 carbon backing with an α beam at E lab = 30 MeV. Fission fragments were detected by the Rochester 4π , highly segmented heavy-ion detector array, CHICO [12,13], in coincidence with the detection of deexcitation γ rays using Gammasphere. This particle detector has a geometric coverage for scattering angles from 12 ◦ to 85 ◦ and 95 ◦ to 168 ◦ relative to the beam axis 0556-2813/2006/73(3)/034312(10)/$23.00 034312-1 ©2006 The American Physical Society