Shape trends and triaxiality in neutron-rich odd-mass Y and Nb isotopes

New level schemes of odd-Z Y-99,Y-101 (Z = 39) and Nb-101,Nb-105 (Z = 41) are established based on the measurement of prompt gamma rays from the fission of Cf-252 at Gammasphere. Bands of pi 5/2(+)[422], pi 5/2(-)[303] and pi 3/2(-)[301] are observed and extended to provide spectroscopic information concerning nuclear shapes in this important odd-Z region. In combination with the level structure of the odd-Z Tc (Z = 43), Rh (Z = 45) and the neighbouring even-Z isotopes the level systematics, signature splittings and kinematic and dynamic moments of inertia of the bands in the Y and Nb isotopes are discussed in terms of shape transition and triaxiality. The pronounced difference observed in the signature splittings between Y and Tc, Rh isotopes is interpreted as evidence of the axially symmetric deformed shape in the Y isotopes, and, as previously reported, large and near maximum triaxiality in Tc-Rh isotopes. The likely lowering of crossing frequencies of the ground-state bands in Tc and Rh isotones in comparison with those in Y isotones also implies a shape transition from axially symmetric deformed shapes in Y nuclei to triaxiality in Tc and Rh isotones. Triaxial-rotor-plus-particle model calculations strongly support a pure axially symmetric shape with large quadrupole deformation in Y isotopes. The model calculations yielded gamma values ranging from -19 degrees to -13 degrees for the 5/2(+)[422] ground-state bands of Nb-101,Nb-103,Nb-105 and of -5 degrees for the two negative-parity bands in Nb-101. The fact that Nb isotopes have intermediate values of signature splitting and band crossing frequencies between those of Y and Tc, Rh isotopes is interpreted as that the Nb isotopes are transitional nuclei with regard to triaxial deformation. A correlation of quadrupole deformations and of triaxiality is seen in the neutron-rich nuclei with Z = 39-45.