Evolution of nuclear shapes in odd-mass yttrium and niobium isotopes from lifetime measurements following fission reactions

Lifetimes of excited states in Y-99, Y-101, Nb-101, Nb-103, and Nb-105 were measured in an experiment using the recoil distance Doppler shift method at GANIL (Grand Accelerateur National d'Ions Lourds). The neutron-rich nuclei were produced in fission reactions between a U-238 beam and a Be-9 target. Prompt gamma rays were measured with the EXOGAM array and correlated with fission fragments that were identified in mass and atomic number with the VAMOS++ spectrometer. The measured lifetimes, together with branching ratios, provide B(M1) and B(E2) values for the strongly coupled rotational bands built on the [422] 5/2(+) ground state in the Y and Nb nuclei with neutron number N >= 60. The comparison of the experimental results with triaxial particle-rotor calculations provides information about the evolution of the nuclear shape in this mass region.