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

Abstract

New level schemes of odd-Z 99,101Y (Z ≤ 39) and 101,105Nb (Z ≤ 41) are established based on the measurement of prompt gamma rays from the fission of 252Cf at Gammasphere. Bands of π5/2 +[422], π5/2 -[303] and π3/2 -[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 γ values ranging from -19° to -13° for the 5/2 +[422] ground-state bands of 101,103,105Nb and of -5° for the two negative-parity bands in 101Nb. 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. © 2005 IOP Publishing Ltd.