Abrupt shape transition at neutron number N = 60: B(E2) values in ^94,96,98Sr from fast γ-γ timing

Lifetimes of low-lying yrast states in neutron-rich ^94,96,98Sr have been measured by Germanium-gated γ - γ fast timing with LaBr₃(Ce) detectors using the EXILL-FATIMA spectrometer at the Institut Laue-Langevin. Sr fission products were generated using cold-neutron-induced fission of ²³⁵U and stopped almost instantaneously within the thick target. The experimental B(E2) values are compared with results of Monte Carlo shell-model calculations made without truncation on the occupation numbers of the orbits spanned by eight proton and eight neutron orbits and show good agreement. Similarly to the Zr isotopes, the abrupt shape transition in the Sr isotopes near neutron number N = 60 is identified as being caused by many-proton excitations to its g_9/2 orbit. (authors)