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 96,97Sr and 98,99Zr produced in the spontaneous fission of 248Cm were studied using the EUROGAM2 array. Regular rotational bands have been found in 97Sr and 99Zr, 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 96Sr and 98Zr. 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 97Sr and 99[loc=pre]Zr, suggest the important role of the h11/2 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 β2∼0.1 at N=56, through β2∼0.2 at N=58 to β2∼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 h11/2 parentage. © 2001 Elsevier Science B.V.