Shape transition and shape coexistence in odd-mass nuclei around ¹⁰⁰Zr

The A = 100 mass region is of special interest due to a rapid shape transition from nearly spherical to strongly deformed shapes by going from neutron number 58 to 60, especially pronounced in the Sr and Zr isotopes. The sudden change of the ground-state deformation can be explained by an evolution of the single particle energies with increased proton and neutron occupation and an additional spin orbital partner interaction which leads to a change of the order of the single particle energies. Another explanation is given by the interplay between normal spherical and well deformed intruder configurations which lead to shape coexistence and sudden inversion of the ground-state configuration at N = 60. To further examine this intricate phenomenon, lifetimes of low-lying excited states in the nuclei 99 Zr and 99 Nb were determined using the γ - γ fast-timing techniques and an experimental setup consisting of four LaBr 3 (Ce) detectors. Neutron rich A = 99 fragments were produced in neutron-induced fission and delivered by the LOHENGRIN fission-fragment separator at the Institut Laue-Langevin in Grenoble, France. The experimental results are compared to two different calculations in the framework of the interacting boson-fermion model and discussed in the context of shape transition and shape coexistence.