Evidence For Possible Shape Transitions In Neutron-Rich Ru Isotopes: Spectroscopy Of Ru-109,Ru-110,Ru-111,Ru-112

The spectroscopy of neutron-rich Ru-109,Ru-110,Ru-111,Ru-112 nuclei was studied by measuring the prompt gamma rays that originate from fission fragments, produced by the U-238( alpha, f) fusion-fission reaction, in coincidence with the detection of both fragments. For Ru-109,Ru-111, 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 gamma-vibrational bands of Ru-110,Ru-112 also were extended to higher spin, allowing observation of the second band crossing at the rotational frequency of approximate to 450 keV in Ru-112, which is approximate to 50 keV above the first band crossing. At a similar rotational frequency, the first band crossing for the h11/2 band in Ru-111 was observed, which is absent in Ru-109. 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 pi g(9/2) orbitals may be evidence of a triaxial shape transition from prolate to oblate occurring in Ru-111. The data together with a comparison of cranked shell-model predictions are presented.