Medium-spin structure of 96,97Sr and 98,99Zr nuclei and the onset of deformation in the A ∼ 100 region

The N = 58, 59 nuclei Sr-96.97 and Zr-98,Zr-99 produced in the spontaneous fission of Cm-248 were studied using the EUROGAM2 array. Regular rotational bands have been found in Sr-97 and Zr-99, firmly establishing shape coexistence in both nuclei. Contrary to earlier suggestions, no rotational bands were found on top of 0(3)(+) states in the N = 58 nuclei Sr-96 and Zr-98. It is likely, however. that the N = 58 nuclei undergo a shape change along the yrast line from spherical ground state to weakly deformed medium-spin states. Numerous single-particle excitations of negative parity, observed in S-97 and (99)[loc=pre]Zr, suggest the important role of the h(11/2) shell in creating deformation in these nuclei. Quadrupole moments were determined for rotational bands in the N = 58. 59, 60, 62 and 64 Sr and Zr nuclei. Deformation parameters, which increase gradually from beta (2) similar to 0.1 at N = 56, through beta (2) similar to 0.2 at N = 58 to beta (2) similar to 0.4 at N = 64, suggest that in strontium and zirconium isotopes the shape change happens gradually between N = 56 and N = 62, and is most likely due to an occupancy of three or more deformation-driving orbitals of h(11/2) parentage. (C) 2001 Elsevier Science B.V. All rights reserved.