Medium-spin structure of {sup96,97}Sr and {sup98,99}Zr nuclei and the onset of deformation in the A{similar_to} 100 region

The N=58, 59 nuclei {sup 96,97}Sr and {sup 98,99}Zr produced in the spontaneous fission of {sup 248}Cm were studied using the EUROGAM2 array. Regular rotational bands have been found in {sup 97}Sr and {sup 99}Zr, firmly establishing shape coexistence in both nuclei. Contrary to earlier suggestions, no rotational bands were found on top of 0{sup +}{sub 3} states in the N=58 nuclei {sup 96}Sr and {sup 98}Zr. 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 {sup 97}Sr and {sup 99}[loc=pre]Zr, suggest the important role of the h{sub 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}{sub 2}{approx}0.1 at N=56, through {beta}{sub 2}{approx}0.2 at N=58 to {beta}{sub 2}{approx}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{sub 11/2} parentage.