Shape Coexistence, Triaxiality, Chiral Bands in Neutron-Rich Nuclei and Hot Fission Mode

The structure of neutron-rich nuclei in the A=100 region have been investigated via prompt γ-γ-γ coincidences in the spontaneous fission of ²⁵²Cf at Gammasphere. New levels are observed in ^93,95,97Sr, ^99,101Y, ^101,105Nb, ^104,106Mo, ^105,107,109Tc, ^111,113Rh and ^115,117Ag. The level structures show a clear evolution from spherical single particle structures seen in Sr, to symmetric, large prolate deformation in Y, to increasing triaxial shapes with increasing Z in Nb, Mo, Tc, Rh and Ag. Rigid triaxial-plus-rotor calculations were carried out for ¹⁰⁷Tc and ^111,113Rh. Best fits in ¹⁰⁷Tc and ^111,113Rb are for prolate β₂ ~ 0.3 and γ increasing from -22.5° in ¹⁰⁷Tc to near maximum triaxiality, γ = -28° in ^111,113Rh. A K= 1/2 intruder band with symmetric deformation is found to coexist with the triaxial asymmetric bands in the Tc and Rh nuclei. In ¹⁰⁶Mo, two sets of ΔI=1 bands have all the characteristics of chiral doublets. Tilted axis cranking calculations support the chiral assignment and indicate these form a new type of chiral band with a one and two phonon chiral vibrational nature associated essentially with the neutrons. These new type chiral doublets demonstrate the general nature of chirality in nuclei. Binary yields of Mo-Ba and Ru-Xe were determined with higher accuracy. The hot fission mode is seen only in Mo-Ba where it goes via a type of hyperdeformed shape for ^144,145,146Ba with a 4.7% intensity.