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

Lifetimes of excited states in ${}^{99}$Y , ${}^{101}$Y, ${}^{101}$Nb, ${}^{103}$Nb, and ${}^{105}$Nb 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 ${}^{238}$U beam and a ${}^9$Be target. Prompt γ 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 $\it B(M1)$ and $\it B(E2)$ 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 $\geq$ 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.