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

New level schemes of $^{ 99,101}$Y (Z = 39) and $^{101,105}$Nb (Z = 41) are established from prompt $\gamma -\gamma -\gamma $ coincidences from the fission of $^{252}$Cf at Gammasphere. Bands of $\pi $5/2$^{+}$[422], $\pi $5/2$^{-}$[303] and $\pi $3/2$^{-}$[301] are extended to provide information on nuclear shapes in this odd-Z region. With the Tc (Z = 43), Rh (Z = 45) data and neighboring even-Z data, 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 indicates an axially-symmetric deformed shape in the Y isotopes, and, large and near maximum triaxiality in Tc-Rh isotopes. 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 from -19\r{ } to -13\r{ } for the 5/2$^{+}$ [422] ground-state bands of $^{101,103,105}$Nb and -5\r{ } for the two negative-parity bands in $^{101}$Nb.