Lifetimes and shape-coexisting states of $^{99}Zr$

Lifetimes of intermediate-spin states in two rotational bands of $^{99}Zr$ have been measured. These states were populated following the neutron-induced fission of $^{235}U$ at the PF1B beamline of the Institut Laue-Langevin, Grenoble, during the EXILL-FATIMA campaign. The nucleus $^{99}Zr_{59}$ exhibits shape coexistence and lies precisely on the border of an abrupt change in ground-state deformation when going from N=58 to N=60, making its study interesting for understanding the mechanisms involved in the rapid onset of deformation here. The B(E2) values extracted for decays in the ν3/2[541] band allow quadrupole deformations of $\beta_2$=0.34(1) and 0.26(3) to be determined for the 821.6- and 1236.6-keV members, whereas $\beta_2$=0.32(3) was found for the 850.5-keV member of the ν3/2[411] band. Some of the excited states known in $^{99}Zr$ have been reasonably described with interacting boson-fermion model (IBFM) calculations. Type-II shell evolution is proposed to play a major role in modifying single-particle energies in $^{99}Zr$.