LABORATORI NAZIONALI DI LEGNARO
Neutron-rich nuclei in the A~100 mass region have been under extensive investigation in the last four decades, from both the theoretical and experimental points of view, due to the observation of a rapid onset of deformation when going from N=58 to N=60. This effect was initially observed in mass measurements and later confirmed by laser spectroscopy studies of ground state quadrupole moments (e.g. in the Rb isotopes) as well as by significant amount of experimental data on properties of low-lying excited states in neutron-rich Sr and Zr isotopes. Recent mass measurements and Coulomb excitation experiments on Kr isotopes have shown a more gradual shape transition in this isotopic chain as compared to what is observed for heavier elements, including neighbouring Rb nuclei. Collective excited states in 97,99Rb were probed for the first time in a low-energy Coulomb excitation experiment at REX-ISOLDE. In both nuclei clear rotational structures were established, which confirm the rapid shape transition at N=60. Transition probabilities between the observed states, determined using the GOSIA code from the measured Coulomb excitation cross sections, further support a strong deformation (beta > 0.3) of 97,99Rb nuclei. The shape transition in neutron-rich Kr isotopes, although much more gradual than what is observed for Z=37-40 nuclei, is expected to give rise to shape coexistence of states corresponding to different configurations. The possibility to explore these structures in low-energy Coulomb excitation at SPES will be shortly discussed.

DATA: 01-12-2016