Identical transitions in the strongly deformed 99 Sr and 100

Neutron-rich isotopes with N>60 and A.100 are characterized by strong axial deformation. Strontium isotopes are the most deformed nuclei known so far in this region. Quadrupole deformations of b.0.4 have been deduced for Sr, Sr, and Sr from the lifetimes of the first excited states and from mean-square radii measured by collinear laser spectroscopy @1–5#. According to these results, ground-state deformation remains constant after its sudden onset at N 560. This trend might even continue at larger neutron number since neither the Sr level spacings @6# nor the 2-state energy of 126 keV in the N564 isotone Sr @7# indicate any large change of deformation. A peculiar feature of neutron-rich Sr nuclei is the occurrence of identical bands in isotopes with DA52. Transitions in the ground-state bands in the even-even Sr and in the K53/2 bands in the odd-neutron Sr have very close energies. A local trend of strong dependence of moments of inertia on deformation and the fact that all involved Sr isotopes have the same deformation seem to be the origin of these identical bands @3,6#. However, the intricate mechanism is not yet understood. Presently, the nucleus Sr62 is the most neutron-rich even-even Sr isotope for which experiments can yield some structure information. The levels in Sr were first observed at the CERN-ISOLDE facility in a b-decay study of Rb by Azuma et al. who identified the 41→21→01 cascade and performed the first lifetime measurement of the 2 state @8#, establishing large deformation. A more accurate lifetime measurement performed later by our group, lead to the deformation of b50.40 @3#. In the same experiment, a 85 ns lifetime was observed for the 1619 keV level and attributed to K hindrance of the decay to the 4 1 state by the 1202 keV transition @9#. Recently, further members of the ground-state band up to I510 were identified in prompt-fission studies @10#. The large moment of inertia shows very little variation