Experimental indication of the onset of nuclear deformation in neutron-rich Sr isotopes at mass 98

A partial decay scheme for 0.1 sec 98Rb has been deduced from γ-singles, γ-multispectra and γγ coincidence measurements taken at the OSTIS mass-separator system at the Institut Laue-Langevin. The low-lying levels of 98Sr follow the energy level systematics of the even-A N=60 isotones. The onset of nuclear deformation in even-A neutron-rich Sr isotopes appears to occur at mass 98, as the first 2+ level energy drops from the nearly constant value of about 800 keV for masses 90-96 to 144 keV at mass 98. Energy level systematics indicate that a transition in the nuclear structures of the more neutron-rich nuclei near mass 100 occurs rather sharply at neutron number N=60. (Auth.)